path: root/doc/rfc/rfc7609.txt

Independent Submission                                            M. Fox
Request for Comments: 7609                                   C. Kassimis
Category: Informational                                       J. Stevens
ISSN: 2070-1721                                                      IBM
                                                             August 2015


     IBM's Shared Memory Communications over RDMA (SMC-R) Protocol

Abstract

   This document describes IBM's Shared Memory Communications over RDMA
   (SMC-R) protocol.  This protocol provides Remote Direct Memory Access
   (RDMA) communications to TCP endpoints in a manner that is
   transparent to socket applications.  It further provides for dynamic
   discovery of partner RDMA capabilities and dynamic setup of RDMA
   connections, as well as transparent high availability and load
   balancing when redundant RDMA network paths are available.  It
   maintains many of the traditional TCP/IP qualities of service such as
   filtering that enterprise users demand, as well as TCP socket
   semantics such as urgent data.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7609.














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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................5
      1.1. Protocol Overview ..........................................6
           1.1.1. Hardware Requirements ...............................8
      1.2. Definition of Common Terms .................................8
      1.3. Conventions Used in This Document .........................11
   2. Link Architecture ..............................................11
      2.1. Remote Memory Buffers (RMBs) ..............................12
      2.2. SMC-R Link Groups .........................................18
           2.2.1. Link Group Types ...................................18
           2.2.2. Maximum Number of Links in Link Group ..............21
           2.2.3. Forming and Managing Link Groups ...................23
           2.2.4. SMC-R Link Identifiers .............................24
      2.3. SMC-R Resilience and Load Balancing .......................24
   3. SMC-R Rendezvous Architecture ..................................26
      3.1. TCP Options ...............................................26
      3.2. Connection Layer Control (CLC) Messages ...................27
      3.3. LLC Messages ..............................................27
      3.4. CDC Messages ..............................................29
      3.5. Rendezvous Flows ..........................................29
           3.5.1. First Contact ......................................29
                  3.5.1.1. Pre-negotiation of TCP Options ............29
                  3.5.1.2. Client Proposal ...........................30
                  3.5.1.3. Server Acceptance .........................32
                  3.5.1.4. Client Confirmation .......................32
                  3.5.1.5. Link (QP) Confirmation ....................32
                  3.5.1.6. Second SMC-R Link Setup ...................35
                           3.5.1.6.1. Client Processing of ADD LINK
                                      LLC Message from Server ........35
                           3.5.1.6.2. Server Processing of ADD LINK
                                      Reply LLC Message from Client ..36
                           3.5.1.6.3. Exchange of RKeys on
                                      Second SMC-R Link ..............38
                           3.5.1.6.4. Aborting SMC-R and
                                      Falling Back to IP .............38



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           3.5.2. Subsequent Contact .................................38
                  3.5.2.1. SMC-R Proposal ............................39
                  3.5.2.2. SMC-R Acceptance ..........................40
                  3.5.2.3. SMC-R Confirmation ........................41
                  3.5.2.4. TCP Data Flow Race with SMC
                           Confirm CLC Message .......................41
           3.5.3. First Contact Variation: Creating a
                  Parallel Link Group ................................42
           3.5.4. Normal SMC-R Link Termination ......................43
           3.5.5. Link Group Management Flows ........................44
                  3.5.5.1. Adding and Deleting Links in an
                           SMC-R Link Group ..........................44
                           3.5.5.1.1. Server-Initiated ADD
                                      LINK Processing ................45
                           3.5.5.1.2. Client-Initiated ADD
                                      LINK Processing ................45
                           3.5.5.1.3. Server-Initiated DELETE
                                      LINK Processing ................46
                           3.5.5.1.4. Client-Initiated DELETE
                                      LINK Request ...................48
                  3.5.5.2. Managing Multiple RKeys over
                           Multiple SMC-R Links in a Link Group ......49
                           3.5.5.2.1. Adding a New RMB to an
                                      SMC-R Link Group ...............50
                           3.5.5.2.2. Deleting an RMB from an
                                      SMC-R Link Group ...............53
                           3.5.5.2.3. Adding a New SMC-R Link to a
                                      Link Group with Multiple RMBs ..54
                  3.5.5.3. Serialization of LLC Exchanges,
                           and Collisions ............................56
                           3.5.5.3.1. Collisions with ADD
                                      LINK / CONFIRM LINK Exchange ...57
                           3.5.5.3.2. Collisions during
                                      DELETE LINK Exchange ...........58
                           3.5.5.3.3. Collisions during
                                      CONFIRM RKEY Exchange ..........59
   4. SMC-R Memory-Sharing Architecture ..............................60
      4.1. RMB Element Allocation Considerations .....................60
      4.2. RMB and RMBE Format .......................................60
      4.3. RMBE Control Information ..................................60
      4.4. Use of RMBEs ..............................................61
           4.4.1. Initializing and Accessing RMBEs ...................61
           4.4.2. RMB Element Reuse and Conflict Resolution ..........62
      4.5. SMC-R Protocol Considerations .............................63
           4.5.1. SMC-R Protocol Optimized Window Size Updates .......63
           4.5.2. Small Data Sends ...................................64
           4.5.3. TCP Keepalive Processing ...........................65




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      4.6. TCP Connection Failover between SMC-R Links ...............67
           4.6.1. Validating Data Integrity ..........................67
           4.6.2. Resuming the TCP Connection on a New SMC-R Link ....68
      4.7. RMB Data Flows ............................................69
           4.7.1. Scenario 1: Send Flow, Window Size Unconstrained ...69
           4.7.2. Scenario 2: Send/Receive Flow, Window Size
                  Unconstrained ......................................71
           4.7.3. Scenario 3: Send Flow, Window Size Constrained .....72
           4.7.4. Scenario 4: Large Send, Flow Control, Full
                  Window Size Writes .................................74
           4.7.5. Scenario 5: Send Flow, Urgent Data, Window
                  Size Unconstrained .................................77
           4.7.6. Scenario 6: Send Flow, Urgent Data, Window
                  Size Closed ........................................79
      4.8. Connection Termination ....................................81
           4.8.1. Normal SMC-R Connection Termination Flows ..........81
           4.8.2. Abnormal SMC-R Connection Termination Flows ........86
           4.8.3. Other SMC-R Connection Termination Conditions ......88
   5. Security Considerations ........................................89
      5.1. VLAN Considerations .......................................89
      5.2. Firewall Considerations ...................................89
      5.3. Host-Based IP Filters .....................................89
      5.4. Intrusion Detection Services ..............................90
      5.5. IP Security (IPsec) .......................................90
      5.6. TLS/SSL ...................................................90
   6. IANA Considerations ............................................90
   7. Normative References ...........................................91
   Appendix A. Formats ...............................................92
     A.1. TCP Option .................................................92
     A.2. CLC Messages ...............................................92
          A.2.1. Peer ID Format ......................................93
          A.2.2. SMC Proposal CLC Message Format .....................94
          A.2.3. SMC Accept CLC Message Format .......................98
          A.2.4. SMC Confirm CLC Message Format .....................102
          A.2.5. SMC Decline CLC Message Format .....................105
     A.3. LLC Messages ..............................................106
          A.3.1. CONFIRM LINK LLC Message Format ....................107
          A.3.2. ADD LINK LLC Message Format ........................109
          A.3.3. ADD LINK CONTINUATION LLC Message Format ...........112
          A.3.4. DELETE LINK LLC Message Format .....................115
          A.3.5. CONFIRM RKEY LLC Message Format ....................117
          A.3.6. CONFIRM RKEY CONTINUATION LLC Message Format .......120
          A.3.7. DELETE RKEY LLC Message Format .....................122
          A.3.8. TEST LINK LLC Message Format .......................124
     A.4. Connection Data Control (CDC) Message Format ..............125






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   Appendix B. Socket API Considerations ............................129
     B.1. setsockopt() / getsockopt() Considerations ................130
   Appendix C. Rendezvous Error Scenarios ...........................131
     C.1. SMC Decline during CLC Negotiation ........................131
     C.2. SMC Decline during LLC Negotiation ........................131
     C.3. The SMC Decline Window ....................................133
     C.4. Out-of-Sync Conditions during SMC-R Negotiation ...........133
     C.5. Timeouts during CLC Negotiation ...........................134
     C.6. Protocol Errors during CLC Negotiation ....................134
     C.7. Timeouts during LLC Negotiation ...........................135
          C.7.1. Recovery Actions for LLC Timeouts and Failures .....136
     C.8. Failure to Add Second SMC-R Link to a Link Group ..........142
   Authors' Addresses ...............................................143

1.  Introduction

   This document specifies IBM's Shared Memory Communications over RDMA
   (SMC-R) protocol.  SMC-R is a protocol for Remote Direct Memory
   Access (RDMA) communication between TCP socket endpoints.  SMC-R runs
   over networks that support RDMA over Converged Ethernet (RoCE).  It
   is designed to permit existing TCP applications to benefit from RDMA
   without requiring modifications to the applications or predefinition
   of RDMA partners.

   SMC-R provides dynamic discovery of the RDMA capabilities of TCP
   peers and automatic setup of RDMA connections that those peers can
   use.  SMC-R also provides transparent high availability and
   load-balancing capabilities that are demanded by enterprise
   installations but are missing from current RDMA protocols.  If
   redundant RoCE-capable hardware such as RDMA-capable Network
   Interface Cards (RNICs) and RoCE-capable switches is present, SMC-R
   can load-balance over that redundant hardware and can also
   non-disruptively move TCP traffic from failed paths to surviving
   paths, all seamlessly to the application and the sockets layer.
   Because SMC-R preserves socket semantics and the TCP three-way
   handshake, many TCP qualities of service such as filtering, load
   balancing, and Secure Socket Layer (SSL) encryption are preserved, as
   are TCP features such as urgent data.

   Because of the dynamic discovery and setup of SMC-R connectivity
   between peers, no RDMA connection manager (RDMA-CM) is required.
   This also means that support for Unreliable Datagram (UD) Queue Pairs
   (QPs) is also not required.








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   It is recommended that the SMC-R services be implemented in kernel
   space, which enables optimizations such as resource-sharing between
   connections across multiple processes and also permits applications
   using SMC-R to spawn multiple processes (e.g., fork) without losing
   SMC-R functionality.  A user-space implementation is compatible with
   this architecture, but it may not support spawned processes (e.g.,
   fork), which limits sharing and resource optimization to TCP
   connections that originate from the same process.  This might be an
   appropriate design choice if the use case is a system that hosts a
   large single process application that creates many TCP connections to
   a peer host, or in implementations where a kernel-space
   implementation is not possible or introduces excessive overhead for
   "kernel space to user space" context switches.

1.1.  Protocol Overview

   SMC-R defines the concept of the SMC-R link, which is a logical
   point-to-point link using reliably connected queue pairs between
   TCP/IP stack peers over a RoCE fabric.  An SMC-R link is bound to a
   specific hardware path, meaning a specific RNIC on each peer.  SMC-R
   links are created and maintained by an SMC-R layer, which may reside
   in kernel space or user space, depending upon operating system and
   implementation requirements.  The SMC-R layer resides below the
   sockets layer and directs data traffic for TCP connections between
   connected peers over the RoCE fabric using RDMA rather than over a
   TCP connection.  The TCP/IP stack, with its requirements for
   fragmentation, packetization, etc., is bypassed, and the application
   data is moved between peers using RDMA.

   Multiple SMC-R links between the same two TCP/IP stack peers are also
   supported.  A set of SMC-R links called a link group can be logically
   bonded together to provide redundant connectivity.  If there is
   redundant hardware -- for example, two RNICs on each peer -- separate
   SMC-R links are created between the peers to exploit that redundant
   hardware.  The link group architecture with redundant links provides
   load balancing and increased bandwidth, as well as seamless failover.

   Each SMC-R link group is associated with an area of memory called
   Remote Memory Buffers (RMBs), which are areas of memory that are
   available for SMC-R peers to write into using RDMA writes.  Multiple
   TCP connections between peers may be multiplexed over a single SMC-R
   link, in which case the SMC-R layer manages the partitioning of the
   RMBs between the TCP connections.  This multiplexing reduces the RDMA
   resources, such as QPs and RMBs, that are required to support
   multiple connections between peers, and it also reduces the
   processing and delays related to setting up QPs, pinning memory, and
   other RDMA setup tasks when new TCP connections are created.  In a
   kernel-space SMC-R implementation in which the RMBs reside in kernel



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   storage, this sharing and optimization works across multiple
   processes executing on the same host.  In a user-space SMC-R
   implementation in which the RMBs reside in user space, this sharing
   and optimization is limited to multiple TCP connections created by a
   single process, as separate RMBs and QPs will be required for each
   process.

   SMC-R also introduces a rendezvous protocol that is used to
   dynamically discover the RDMA capabilities of TCP connection partners
   and exchange credentials necessary to exploit that capability if
   present.  TCP connections are set up using the normal TCP three-way
   handshake [RFC793], with the addition of a new TCP option that
   indicates SMC-R capability.  If both partners indicate SMC-R
   capability, then at the completion of the three-way TCP handshake the
   SMC-R layers in each peer take control of the TCP connection and use
   it to exchange additional Connection Layer Control (CLC) messages to
   negotiate SMC-R credentials such as QP information; addressability
   over the RoCE fabric; RMB buffer sizes; and keys and addresses for
   accessing RMBs over RDMA.  If at any time during this negotiation a
   failure or decline occurs, the TCP connection falls back to using the
   IP fabric.

   If the SMC-R negotiation succeeds and either a new SMC-R link is set
   up or an existing SMC-R link is chosen for the TCP connection, then
   the SMC-R layers open the sockets to the applications and the
   applications use the sockets as normal.  The SMC-R layer intercepts
   the socket reads and writes and moves the TCP connection data over
   the SMC-R link, "out of band" to the TCP connection, which remains
   open and idle over the IP fabric, except for termination flows and
   possible keepalive flows.  Regular TCP sequence numbering methods are
   used for the TCP flows that do occur; data flowing over RDMA does not
   use or affect TCP sequence numbers.

   This architecture does not support fallback of active SMC-R
   connections to IP.  Once connection data has completed the switch to
   RDMA, a TCP connection cannot be switched back to IP and will reset
   if RDMA becomes unusable.

   The SMC-R protocol defines the format of the RMBs that are used to
   receive TCP connection data written over RDMA, as well as the
   semantics for managing and writing to these buffers using Connection
   Data Control (CDC) messages.









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   Finally, SMC-R defines Link Layer Control (LLC) messages that are
   exchanged over the RoCE fabric between peer SMC-R layers to manage
   the SMC-R links and link groups.  These include messages to test and
   confirm connectivity over an SMC-R link, add and delete SMC-R links
   to or from the link group, and exchange RMB addressability
   information.

1.1.1.  Hardware Requirements

   SMC-R does not require full Converged Enhanced Ethernet switch
   functionality.  SMC-R functions over standard Ethernet fabrics,
   provided that endpoint RNICs are provided and IEEE 802.3x Global
   Pause Frame is supported and enabled in the switch fabric.

   While SMC-R as specified in this document is designed to operate over
   RoCE fabrics, adjustments to the rendezvous methods could enable it
   to run over other RDMA fabrics, such as InfiniBand [RoCE] and iWARP.

1.2.  Definition of Common Terms

   This section provides definitions of terms that have a specific
   meaning to the SMC-R protocol and are used throughout this document.

   SMC-R Link

      An SMC-R link is a logical point-to-point connection over the RoCE
      fabric via specific physical adapters (Media Access Control /
      Global Identifier (MAC/GID)).  The link is formed during the
      "first contact" sequence of the TCP/IP three-way handshake
      sequence that occurs over the IP fabric.  During this handshake,
      an RDMA reliably connected queue pair (RC-QP) connection is formed
      between the two peer SMC hosts and is defined as the SMC-R link.
      The SMC-R link can then support multiple TCP connections between
      the two peers.  An SMC-R link is associated with a single LAN (or
      VLAN) segment and is not routable.

   SMC-R Link Group

      An SMC-R link group is a group of SMC-R links between the same two
      SMC-R peers, typically with each link over unique RoCE adapters.
      Each link in the link group has equal characteristics, such as the
      same VLAN ID (if VLANs are in use), access to the same RMB(s), and
      access to the same TCP server/client.








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   SMC-R Peer

      The SMC-R peer is the peer software stack within the peer
      operating system with respect to the Shared Memory Communications
      (messaging) protocol.

   SMC-R Rendezvous

      SMC-R Rendezvous is the SMC-R peer discovery and handshake
      sequence that occurs transparently over the IP (Ethernet) fabric
      during and immediately after the TCP connection three-way
      handshake by exchanging the SMC-R capabilities and credentials
      using experimental TCP option and CLC messages.

   RoCE SendMsg

      RoCE SendMsg is a send operation posted to a reliably connected
      queue pair with inline data, for the purpose of transferring
      control information between peers.

   TCP Client

      The TCP client is the TCP socket-based peer that initiates a TCP
      connection.

   TCP Server

      The TCP server is the TCP socket-based peer that accepts a TCP
      connection.

   CLC Messages

      The SMC-R protocol defines a set of Connection Layer Control
      messages that flow over the TCP connection that are used to manage
      SMC-R link rendezvous at TCP connection setup time.  This
      mechanism is analogous to SSL setup messages.

   LLC Commands

      The SMC-R protocol defines a set of RoCE Link Layer Control
      commands that flow over the RoCE fabric using RoCE SendMsg, that
      are used to manage SMC-R links, SMC-R link groups, and SMC-R
      link group RMB expansion and contraction.








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   CDC Message

      The SMC-R protocol defines a Connection Data Control message that
      flows over the RoCE fabric using RoCE SendMsg that is used to
      manage the SMC-R connection data.  This message provides
      information about data being transferred over the out-of-band RDMA
      connection, such as data cursors, sequence numbers, and data flags
      (for example, urgent data).  The receipt of this message also
      provides an interrupt to inform the receiver that it has received
      RDMA data.

   RMB

      A Remote (RDMA) Memory Buffer is a fixed or pinned buffer
      allocated in each of the peer hosts for a TCP (via SMC-R)
      connection.  The RMB is registered to the RNIC and allows remote
      access by the remote peer using RDMA semantics.  Each host is
      passed the peer's RMB-specific access information (RMB Key (RKey)
      and RMB element offset) during the SMC-R Rendezvous process.  The
      host stores socket application user data directly into the peer's
      RMB using RDMA over RoCE.

   RToken

      The RToken is the combination of an RMB's RKey and RDMA virtual
      address.  An RToken provides RMB addressability information to an
      RDMA peer.

   RMBE

      The Remote Memory Buffer Element (RMBE) is an area of an RMB that
      is allocated to a specific TCP connection.  The RMBE contains data
      for the TCP connection.  The RMBE represents the TCP receive
      buffer, whereby the remote peer writes into the RMBE and the local
      peer reads from the local RMBE.  The alert token resolves to a
      specific RMBE.

   Alert Token

      The SMC-R alert token is a 4-byte value that uniquely identifies
      the TCP connection over an SMC-R connection.  The alert token
      allows the SMC peer to quickly identify the target TCP connection
      that now has new work.  The format of the token is defined by the
      owning SMC-R endpoint and is considered opaque to the remote peer.
      However, the token should not simply be an index to an RMBE; it
      should reference a TCP connection and be able to be validated to
      avoid reading data from stale connections.




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   RNIC

      The RDMA-capable Network Interface Card (RNIC) is an Ethernet NIC
      that supports RDMA semantics and verbs using RoCE.

   First Contact

      "First contact" describes an SMC-R negotiation to set up the first
      link in a link group.

   Subsequent Contact

      "Subsequent contact" describes an SMC-R negotiation between peers
      who are using an already-existing SMC-R link group.

1.3.  Conventions Used in This Document

   In the rendezvous flow diagrams, dashed lines (----) are used to
   indicate flows over the TCP/IP fabric and dotted lines (....) are
   used to indicate flows over the RoCE fabric.

   In the data transfer ladder diagrams, dashed lines (----) are used to
   indicate RDMA write operations and dotted lines (....) are used to
   indicate CDC messages, which are RDMA messages with inline data that
   contain control information for the connection.

2.  Link Architecture

   An SMC-R link is based on reliably connected queue pairs (QPs) that
   form a "logical point-to-point link" between the two SMC-R peers over
   a RoCE fabric.  An SMC-R link extends from SMC-R peer to SMC-R peer,
   where typically each peer would be a TCP/IP stack and would reside on
   separate hosts.

                            ,,.--..,_
     +----+             _-``         `-,           +-----+
     |QP 8|            -   RoCE         ',         |QP 64|
     |    |          /     VLAN M         .        |     |
     +----+--------+/                     \+-------+-----+
      | RNIC 1     |    SMC-R Link         | RNIC 2     |
      |            |<--------------------->|            |
      +------------+ ,                    /+------------+
              MAC A (GID A)             MAC B (GID B)
                       .                .`
                        `',          ,-`
                           ``''--''``

                       Figure 1: SMC-R Link Overview



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   Figure 1 illustrates an overview of the basic concepts of SMC-R peer-
   to-peer connectivity; this is called the SMC-R link.  The SMC-R link
   forms a logical point-to-point connection between two SMC-R peers via
   RoCE.  The SMC-R link is defined and identified by the following
   attributes:

      SMC-R link = RC QPs
         (source VMAC GID QP + target VMAC GID QP + VLAN ID)

   The SMC-R link can optionally be associated with a VLAN ID.  If VLANs
   are in use for the associated IP (LAN) connection, then the VLAN
   attribute is carried over on the SMC-R link.  When VLANs are in use,
   each SMC-R link group is associated with a single and specific VLAN.
   The RoCE fabric is the same physical Ethernet LAN used for standard
   TCP/IP-over-Ethernet communications, with switches as described in
   Section 1.1.1.

   An SMC-R link is designed to support multiple TCP connections between
   the same two peers.  An SMC-R link is intended to be long lived,
   while the underlying TCP connections can dynamically come and go.
   The associated RMBs can also be dynamically added and removed from
   the link as needed.  The first TCP connection between the peers
   establishes the SMC-R link.  Subsequent TCP connections then use the
   previously established link.  When the last TCP connection
   terminates, the link can then be terminated, typically after an
   implementation-defined idle timeout period has elapsed.  The TCP
   server is responsible for initiating and terminating the SMC-R link.

2.1.  Remote Memory Buffers (RMBs)

   Figure 2 shows the hosts -- Hosts X and Y -- and their associated
   RMBs within each host.  With the SMC-R link, and the associated RKeys
   and RDMA virtual addresses, each SMC-R-enabled TCP/IP stack can
   remotely access its peer's RMBs using RDMA.  The RKeys and virtual
   addresses are exchanged during the rendezvous processing when the
   link is established.  The combination of the RKey and the virtual
   address is the RToken.  Note that the SMC-R link ends at the QP
   providing access to the RMB (via the link + RToken).













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          Host X                                     Host Y
     +-------------------+        ,.--.,_       +-------------------+
     |                   |     .'`       '.     |                   |
     | Protection        |   ,'            `,   |    Protection     |
     | Domain X          |  /                \  |    Domain Y       |
     |            +------+ /                  \ +------+            |
     |       QP 8 |RNIC 1| |   SMC-R Link     | |RNIC 2|  QP 64     |
     |        |   |      |<-------------------->|      |   |        |
     |        |   |      ||                    ||      |   |        |
     |        |   +------+|    VLAN A          |+------+   |        |
     |        |          ||                    ||          |        |
     |        |          | |   RoCE           | |          |        |
     |        |RToken X  | \                  / |RToken Y  |        |
     |        |          |  \                /  |          |        |
     |        V          |   `.            ,'   |          V        |
     | +--------+        |     '._       ,'     |        +--------+ |
     | |        |        |        `''-'``       |        |        | |
     | | RMB    |        |                      |        | RMB    | |
     | |        |        |                      |        |        | |
     | +--------+        |                      |        +--------+ |
     +-------------------+                      +-------------------+

                       Figure 2: SMC-R Link and RMBs

   An SMC-R link can support multiple RMBs that are independently
   managed by each peer.  The number and the size of RMBs are managed by
   the peers based on the host's unique memory management requirements;
   however, the maximum number of RMBs that can be associated to a link
   group on one peer is 255.  The QP has a single protection domain, but
   each RMB has a unique RToken.  All RTokens must be exchanged with the
   peer.

   Each peer manages the RMBs in its local memory for its remote SMC-R
   peer by sharing access to the RMBs via RTokens with its peers.  The
   remote peer writes into the RMBs via RDMA, and the local peer (RMB
   owner) then reads from the RMBs.

   When two peers decide to use SMC-R for a given TCP connection, they
   each allocate a local RMB element for the TCP connection and
   communicate the location of this local RMB element during rendezvous
   processing.  To that end, RMB elements are created in pairs, with one
   RMB element allocated locally on each peer of the SMC-R link.









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                  ---  +------------+---------------+
                  /\   |Eye Catcher |               |
                   |   +------------+               |
                   |   |                            |
         RMB Element 1 |                            |
                   |   |   Receive Buffer           |
                   |   |                            |
                   |   |                            |
                  \/   |                            |
                  ---  +------------+---------------+
                  /\   |Eye Catcher |               |
                   |   +------------+               |
                   |   |                            |
         RMB Element 2 |                            |
                   |   |   Receive Buffer           |
                   |   |                            |
                   |   |                            |
                  \/   |                            |
                  ---  +----------------------------+
                       |            .               |
                       |            .               |
                       |            .               |
                       |            .               |
                       |    (up to 255 elements)    |
                       +----------------------------+

                           Figure 3: RMB Format

   Figure 3 illustrates the basic format of an RMB.  The RMB is a
   virtual memory buffer whose backing real memory is pinned, which can
   support up to 255 TCP connections to exactly one remote SMC-R peer.
   Each RMB is therefore associated with the SMC-R links within a link
   group for the two peers and a specific RoCE Protection Domain.  Other
   than the two peers identified by the SMC-R link, no other SMC-R peers
   can have RDMA access to an RMB; this requires a unique Protection
   Domain for every SMC-R link.  This is critical to ensure integrity of
   SMC-R communications.

   RMBs are subdivided into multiple elements for efficiency, with each
   RMB Element (RMBE) associated with a single TCP connection.
   Therefore, multiple TCP connections across an SMC-R link group can
   share the same memory for RDMA purposes, reducing the overhead of
   having to register additional memory with the RNIC for every new TCP
   connection.  The number of elements in an RMB and the size of each
   RMBE are entirely governed by the owning peer, subject to the SMC-R
   architecture rules; however, all RMB elements within a given RMB must
   be the same size.  Each peer can decide the level of resource-sharing
   that is desirable across TCP connections based on local constraints,



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   such as available system memory.  An RMB element is identified to the
   remote SMC-R peer via an RMB Element Token, which consists of the
   following:

   o  RMB RToken: The combination of the RKey and virtual address
      provided by the RNIC that identifies the start of the RMB for RDMA
      operations.

   o  RMB Index: Identifies the RMB element index in the RMB.  Used to
      locate a specific RMB element within an RMB.  Valid value range is
      1-255.

   o  RMB Element Length: The length of the RMB element's eye catcher
      plus the length of the receive buffer.  This length is equal for
      all RMB elements in a given RMB.  This length can be variable
      across different RMBs.

   Multiple RMBs can be associated to an SMC-R link group, and each peer
   in an SMC-R link group manages allocation of its RMBs.  RMB
   allocation can be asymmetric.  For example, Server X can allocate two
   RMBs to an SMC-R link group while Server Y allocates five.  This
   provides maximum implementation flexibility to allow hosts to
   optimize RMB management for their own local requirements.  The
   maximum number of RMBs that can be allocated on one peer to a link
   group is 255.  If more RMBs are required, the peer may fall back to
   IP for subsequent connections or, if the peer is the server, create a
   parallel link group.

   One use case for multiple RMBs is multiple receive buffer sizes.
   Since every element in an RMB must be the same size, multiple RMBs
   with different element sizes can be allocated if varying receive
   buffer sizes are required.

   Also, since the maximum number of TCP connections whose receive
   buffers can be allocated to an RMB is 255, multiple RMBs may be
   required to provide capacity for large numbers of TCP connections
   between two peers.














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   Separately from the RMB, the TCP/IP stack that owns each RMB
   maintains control data for each RMB element within its local control
   structures.  The control data contains flags for maintaining the
   state of the TCP data (for example, urgent data indicator) and, most
   importantly, the following two cursors, which are illustrated below
   in Figure 4:

   o  The peer producer cursor: This is a wrapping offset into the
      RMB element's receive buffer that points to the next byte of data
      to be written by the remote peer.  This cursor is provided by the
      remote peer in a Connection Data Control (CDC) message, which is
      sent using RoCE SendMsg processing, and tells the local peer how
      far it can consume data in the RMBE buffer.

   o  The peer consumer cursor: This is a wrapping offset into the
      remote peer's RMB element's receive buffer that points to the next
      byte of data to be consumed by the remote peer in its own RMBE.
      The local peer cannot write into the remote peer's RMBE beyond
      this point without causing data loss.  This cursor is also
      provided by the peer using a Connection Data Control message.

   Each TCP connection peer maintains its cursors for a TCP connection's
   RMBE in its local control structures.  In other words, the peer who
   writes into a remote peer's RMBE provides its producer cursor to the
   peer whose RMBE it has written into.  The peer who reads from its
   RMBE provides its consumer cursor to the writing peer.  In this
   manner, the reads and writes between peers are kept coordinated.

   For example, referring to Figure 4, Peer B writes the hashed data
   into the receive buffer of Peer A's RMBE.  After that write
   completes, Peer B uses a CDC message to update its producer cursor to
   Peer A, to indicate to Peer A how much data is available for Peer A
   to consume.  The CDC message that Peer B sends to Peer A wakes up
   Peer A and notifies it that there is data to be consumed.

   Similarly, when Peer A consumes data written by Peer B, it uses a CDC
   message to update its consumer cursor to Peer B to let Peer B know
   how much data it has consumed, so Peer B knows how much space is
   available for further writes.  If Peer B were to write enough data to
   Peer A that it would wrap the RMBE receive buffer and exceed the
   consumer cursor, data loss would result.

   Note that this is a simplistic description of the control flows, and
   they are optimized to minimize the number of CDC messages required,
   as described in Section 4.7 ("RMB Data Flows").






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      Peer A's RMBE Control Info            Peer B's RMBE Control Info
     +--------------------------+          +--------------------------+
     |                          |          |                          |
      /----Peer producer cursor |    +-----+-Peer consumer cursor     |
    /|                          |    |     |                          |
   | +--------------------------+    |     +--------------------------+
   |  Peer A's RMBE                  |
   | +--------------------------+    |
   | |            +------------------+
   | |            |             |
   | |            \/            |
   | |             +------------|
   | |-------------+/////////// |
   | |//RDMA data written by ///|
   | |/// Peer B that is ////// |
   | |/available to be consumed/|
   | |///////////////////////// |
   | |///////// +---------------|
   | |----------+/\             |
   | |            |             |
    \|            |             |
     \           /              |
     |\---------/               |
     |                          |
     |                          |

                          Figure 4: RMBE Cursors

   Additional flags and indicators are communicated between peers.  In
   all cases, these flags and indicators are updated by the peer using
   CDC messages, which are sent using RoCE SendMsg.  More details on
   these additional flags and indicators are described in Section 4.3
   ("RMBE Control Information").


















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2.2.  SMC-R Link Groups

   SMC-R links are logically grouped together to form an SMC-R link
   group.  The purpose of the link group is for supporting multiple
   links between the same two peers to provide for:

   o  Resilience: Provides transparent and dynamic switching of the link
      used by existing TCP connections during link failures, typically
      hardware related.  TCP traffic using the failing link can be
      switched to an active link within the link group, thereby avoiding
      disruptions to application workloads.

   o  Link utilization: Provides an active/active link usage model
      allowing TCP traffic to be balanced across the links, which
      increases bandwidth and also avoids hardware imbalances and
      bottlenecks.  Note that both adapter and switch utilization can
      become potential resource constraint issues.

   SMC-R link group support is required.  Resilience is not optional.
   However, the user can elect to provision a single RNIC (on one or
   both hosts).

   Multiple links that are formed between the same two peers fall into
   two distinct categories:

   1. Equal Links: Links providing equal access to the same RMB(s) at
      both endpoints, whereby all TCP connections associated with the
      links must have the same VLAN ID and have the same TCP server and
      TCP client roles or relationship.

   2. Unequal Links: Links providing access to unique, unrelated and
      isolated RMB(s) (i.e., for unique VLANs or unique and isolated
      application workloads, etc.) or having unique TCP server or client
      roles.

   Links that are logically grouped together forming an SMC-R link group
   must be equal links.

2.2.1.  Link Group Types

   Equal links within a link group also have another "Link Group Type"
   attribute based on the link's associated underlying physical path.
   The following SMC-R link types are defined:

   1. Single link: the only active link within a link group

   2. Parallel link: not allowed -- SMC-R links having the same physical
      RNIC at both hosts



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   3. Asymmetric link: links that have unique RNIC adapters at one host
      but share a single adapter at the peer host

   4. Symmetric link: links that have unique RNIC adapters at both hosts

   These link group types are further explained in the following figures
   and descriptions.

   Figure 2 above shows the single-link case.  The single link
   illustrated in Figure 2 also establishes the SMC-R link group.  Link
   groups are supposed to have multiple links, but when only one RNIC is
   available at both hosts then only a single link can be created.  This
   is expected to be a transient case.

   Figure 5 shows the symmetric-link case.  Both hosts have unique and
   redundant RNIC adapters.  This configuration meets the objectives for
   providing full RoCE redundancy required to provide the level of
   resilience required for high availability for SMC-R.  While this
   configuration is not required, it is a strongly recommended "best
   practice" for the exploitation of SMC-R.  Single and asymmetric links
   must be supported but are intended to provide for short-term
   transient conditions -- for example, during a temporary outage or
   recycle of an RNIC.

          Host X                                     Host Y
     +-------------------+                      +-------------------+
     |                   |                      |                   |
     | Protection        |                      |    Protection     |
     | Domain X          |                      |    Domain Y       |
     |            +------+                      +------+            |
     |       QP 8 |RNIC 1|     SMC-R Link 1     |RNIC 2|  QP 64     |
     |RToken X|   |      |<-------------------->|      |   |        |
     |        |   |      |                      |      |   |RToken Y|
     |       \/   +------+                      +------+  \/        |
     |+--------+         |                      |        +--------+ |
     ||        |         |                      |        |        | |
     || RMB    |         |                      |        | RMB    | |
     ||        |         |                      |        |        | |
     |+--------+         |                      |        +--------+ |
     |       /\   +------+                      +------+  /\        |
     |RToken Z|   |      |     SMC-R Link 2     |      |   |RToken W|
     |        |   |RNIC 3|<-------------------->|RNIC 4|   |        |
     |       QP 9 |      |                      |      |  QP 65     |
     |            +------+                      +------+            |
     +-------------------+                      +-------------------+

                      Figure 5: Symmetric SMC-R Links




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          Host X                                     Host Y
     +-------------------+                      +-------------------+
     |                   |                      |                   |
     | Protection        |                      |    Protection     |
     | Domain X          |                      |    Domain Y       |
     |            +------+                      +------+            |
     |       QP 8 |RNIC 1|     SMC-R Link 1     |RNIC 2|  QP 64     |
     |RToken X|   |      |<-------------------->|      |   |        |
     |        |   |      |                   .->|      |   |RToken Y|
     |       \/   +------+                 .`   +------+  \/        |
     |+--------+         |               .`     |        +--------+ |
     ||        |         |             .`       |        |        | |
     || RMB    |         |           .`         |        | RMB    | |
     ||        |         |         .`SMC-R      |        |        | |
     |+--------+         |       .` Link 2      |        +--------+ |
     |       /\   +------+     .`               +------+            |
     |RToken Z|   |      |   .`                 |      |down or     |
     |        |   |RNIC 3|<-`                   |RNIC 4|unavailable |
     |       QP 9 |      |                      |      |            |
     |            +------+                      +------+            |
     +-------------------+                      +-------------------+

                     Figure 6: Asymmetric SMC-R Links

   In the example provided by Figure 6, Host X has two RNICs but Host Y
   only has one RNIC because RNIC 4 is not available.  This
   configuration allows for the creation of an asymmetric link.  While
   an asymmetric link will provide some resilience (for example, when
   RNIC 1 fails), ideally each host should provide two redundant RNICs.
   This should be a transient case, and when RNIC 4 becomes available,
   this configuration must transition to a symmetric-link configuration.
   This transition is accomplished by first creating the new symmetric
   link and then deleting the asymmetric link with reason code
   "Asymmetric link no longer needed" specified in the DELETE LINK LLC
   message.
















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          Host X                                     Host Y
     +-------------------+                      +-------------------+
     |                   |                      |                   |
     | Protection        |                      |    Protection     |
     | Domain X          |                      |    Domain Y       |
     |            +------+  SMC-R Link 1        +------+            |
     |       QP 8 |RNIC 1|<-------------------->|RNIC 2|  QP 64     |
     |RToken X|   |      |                      |      |   |        |
     |        |   |      |<-------------------->|      |   |RToken Y|
     |       \/   +------+  SMC-R Link 2        +------+  \/        |
     |+--------+   QP 9  |                      | QP 65  +--------+ |
     ||        |    |    |                      |  |     |        | |
     || RMB    |<-- +    |                      |  +---->| RMB    | |
     ||        |         |                      |        |        | |
     |+--------+         |                      |        +--------+ |
     |            +------+                      +------+            |
     |     down or|      |                      |      |down or     |
     | unavailable|RNIC 3|                      |RNIC 4|unavailable |
     |            |      |                      |      |            |
     |            +------+                      +------+            |
     +-------------------+                      +-------------------+

              Figure 7: SMC-R Parallel Links (Not Supported)

   Figure 7 shows parallel links, which are two links in the link group
   that use the same hardware.  This configuration is not permitted.
   Because SMC-R multiplexes multiple TCP connections over an SMC-R link
   and both links are using the exact same hardware, there is no
   additional redundancy or capacity benefit obtained from this
   configuration.  In addition to providing no real benefit, this
   configuration adds the unnecessary overhead of additional queue
   pairs, generation of additional RKeys, etc.

2.2.2.  Maximum Number of Links in Link Group

   The SMC-R protocol defines a maximum of eight symmetric SMC-R links
   within a single SMC-R link group.  This allows for support for up to
   eight unique physical paths between peer hosts.  However, in terms of
   meeting the basic requirements for redundancy, support for at least
   two symmetric links must be implemented.  Supporting more than two
   links also simplifies implementation for practical matters relating
   to dynamically adding and removing links -- for example, starting a
   third SMC-R link prior to taking down one of the two existing links.
   Recall that all links within a link group must have equal access to
   all associated RMBs.






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   The SMC-R protocol allows an implementation to assign an
   implementation-specific and appropriate value for maximum symmetric
   links.  The implementation value must not exceed the architecture
   limit of 8; also, the value must not be lower than 2, because the
   SMC-R protocol requires redundancy.  This does not mean that two
   RNICs are physically required to enable SMC-R connectivity, but at
   least two RNICs for redundancy are strongly recommended.

   The SMC-R peers exchange their implementation maximum link values
   during the link group establishment using the defined maximum link
   value in the CONFIRM LINK LLC command.  Once the initial exchange
   completes, the value is set for the life of the link group.  The
   maximum link value can be provided by both the server and client.
   The server must supply a value, whereas the client maximum link value
   is optional.  When the client does not supply a value, it indicates
   that the client accepts the server-supplied maximum value.  If the
   client provides a value, it cannot exceed the server-supplied maximum
   value.  If the client passes a lower value, this lower value then
   becomes the final negotiated maximum number of symmetric links for
   this link group.  Again, the minimum value is 2.

   During run time, the client must never request that the server add a
   symmetric link to a link group that would exceed the negotiated
   maximum link value.  Likewise, the server must never attempt to add a
   symmetric link to a link group that would exceed the negotiated
   maximum value.

   In terms of counting the number of active links within a link group,
   the initial link (or the only/last) link is always counted as 1.
   Then, as additional links are added, they are either symmetric or
   asymmetric links.

   With regards to enforcing the maximum link rules, asymmetric links
   are an exception having a unique set of rules:

   o  Asymmetric links are always limited to one asymmetric link allowed
      per link group.

   o  Asymmetric links must not be counted in the maximum symmetric-link
      count calculation.  When tracking the current count or enforcing
      the negotiated maximum number of links, an asymmetric link is not
      to be counted.









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2.2.3.  Forming and Managing Link Groups

   SMC-R link groups are self-defining.  The first SMC-R link in a link
   group is created using TCP option flows on the TCP three-way
   handshake followed by CLC message flows over the TCP connection.
   Subsequent SMC-R links in the link group are created by sending LLC
   messages over an SMC-R link that already exists in the link group.
   Once an SMC-R link group is created, no additional SMC-R links in
   that group are created using TCP and CLC negotiation.  Because
   subsequent SMC-R links are created exclusively by sending LLC
   messages over an existing SMC-R link in a link group, the membership
   of SMC-R links in a link group is self-defining.

   This architecture does not define a specific identifier for an SMC-R
   link group.  This identification may be useful for network management
   and may be assigned in a platform-specific manner, or in an extension
   to this architecture.

   In each SMC-R link group, one peer is the server for all TCP
   connections and the other peer is the client.  If there are
   additional TCP connections between the peers that use SMC-R and have
   the client and server roles reversed, another SMC-R link group is set
   up between them with the opposite client-server relationship.

   This is required because there are specific responsibilities divided
   between the client and server in the management of an SMC-R link
   group.

   In this architecture, the decision of whether to use an existing
   SMC-R link group or create a new SMC-R link group for a TCP
   connection is made exclusively by the server.

   Management of the links in an SMC-R link group is also a server
   responsibility.  The server is responsible for adding and deleting
   links in a link group.  The client may request that the server take
   certain actions, but the final responsibility is the server's.















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2.2.4.  SMC-R Link Identifiers

   This architecture defines multiple identifiers to identify SMC-R
   links and peers.

   o  Link number: This is a 1-byte value that identifies an SMC-R link
      within a link group.  Both the server and the client use this
      number to distinguish an SMC-R link from other links within the
      same link group.  It is only unique within a link group.  In order
      to prevent timing windows that may occur when a server creates a
      new link while the client is still cleaning up a previously
      existing link, link numbers cannot be reused until the entire link
      numbering space has been exhausted.

   o  Link user ID: This is an architecturally opaque 4-byte value that
      a peer uses to uniquely define an SMC-R link within its own space.
      This means that a link user ID is unique within one peer only.
      Each peer defines its own link user ID for a link.  The peers
      exchange this information once during link setup, and it is never
      used architecturally again.  The purpose of this identifier is for
      network management, display, and debugging.  For example, an
      operator on a client could provide the operator on the server with
      the server's link user ID if he requires the server's operator to
      check on the operation of a link that the client is having trouble
      with.

   o  Peer ID: The SMC-R peer ID uniquely identifies a specific instance
      of a specific TCP/IP stack.  It is required because in clustered
      and load-balancing environments, an IP address does not uniquely
      identify a TCP/IP stack.  An RNIC's MAC/GID also doesn't uniquely
      or reliably identify a TCP/IP stack, because RNICs can go up and
      down and even be redeployed to other TCP/IP stacks in a
      multiple-partitioned or virtualized environment.  The peer ID is
      not only unique per TCP/IP stack but is also unique per instance
      of a TCP/IP stack, meaning that if a TCP/IP stack is restarted,
      its peer ID changes.

2.3.  SMC-R Resilience and Load Balancing

   The SMC-R multilink architecture provides resilience for network high
   availability via failover capability to an alternate RoCE adapter.

   The SMC-R multilink architecture does not define primary, secondary,
   or alternate roles to the links.  Instead, there are multiple active
   links representing multiple redundant RoCE paths over the same LAN.






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   Assignment of TCP connections to links is unidirectional and
   asymmetric.  This means that the client and server may each choose a
   separate link for their RDMA writes associated with a specific TCP
   connection.

   If a hardware failure occurs or a QP failure associated with an
   individual link occurs, then the TCP connections that were associated
   with the failing link are dynamically and transparently switched to
   use another available link.  The server or the client can detect a
   failure, immediately move their TCP connections, and then notify
   their peer via the DELETE LINK LLC command.  While the client can
   notify the server of an apparent link failure with the DELETE LINK
   LLC command, the server performs the actual link deletion.

   The movement of TCP connections to another link can be accomplished
   with minimal coordination between the peers.  The TCP connection
   movement is also transparent to, and non-disruptive to, the TCP
   socket application workloads for most failure scenarios.  After a
   failure, the surviving links and all associated hardware must handle
   the link group's workload.

   As each SMC-R peer begins to move active TCP connections to another
   link, all current RDMA write operations must be allowed to complete.
   The moving peer then sends a signal to verify receipt of the last
   successful write by its peer.  If this verification fails, the TCP
   connection must be reset.  Once this verification is complete, all
   writes that failed may then be retried, in order, over the new link.
   Any data writes or CDC messages for which the sender did not receive
   write completion must be replayed before any subsequent data or CDC
   write operations are sent.  LLC messages are not retried over the new
   link, because they are dependent on a known link configuration, which
   has just changed because of the failure.  The initiator of an LLC
   message exchange that fails will be responsible for retrying once the
   link group configuration stabilizes.

   When a new link becomes available and is re-added to the link group,
   each peer is free to rebalance its current TCP connections as needed
   or only assign new TCP connections to the newly added link.  Both the
   server and client are free to manage TCP connections across the link
   group as needed.  TCP connection movement does not have to be
   stimulated by a link failure.

   The SMC-R architecture also defines orderly versus disorderly
   failover.  The type of failover is communicated in the LLC
   DELETE LINK command and is simply a means to indicate that the link
   has terminated (disorderly) or link termination is imminent
   (orderly).  The orderly link deletion could be initiated via operator
   command or programmatically to bring down an idle link.  For example,



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   an operator command could initiate orderly shutdown of an adapter for
   service.  Implementation of the two types is based on implementation
   requirements and is beyond the scope of the SMC-R architecture.

3.  SMC-R Rendezvous Architecture

   "Rendezvous" is the process that SMC-R-capable peers use to
   dynamically discover each others' capabilities, negotiate SMC-R
   connections, set up SMC-R links and link groups, and manage those
   link groups.  A key aspect of SMC-R Rendezvous is that it occurs
   dynamically and automatically, without requiring SMC-R link
   configuration to be defined by an administrator.

   SMC-R Rendezvous starts with the TCP/IP three-way handshake, during
   which connection peers use TCP options to announce their SMC-R
   capabilities.  If both endpoints are SMC-R capable, then Connection
   Layer Control (CLC) messages are exchanged between the peers' SMC-R
   layers over the newly established TCP connection to negotiate SMC-R
   credentials.  The CLC message mechanism is analogous to the messages
   exchanged by SSL for its handshake processing.

   If a new SMC-R link is being set up, Link Layer Control (LLC)
   messages are used to confirm RDMA connectivity.  LLC messages are
   also used by the SMC-R layers at each peer to manage the links and
   link groups.

   Once an SMC-R link is set up or agreed to by the peers, the TCP
   sockets are passed to the peer applications, which use them as
   normal.  The SMC-R layer, which resides under the sockets layer,
   transmits the socket data between peers over RDMA using the SMC-R
   protocol, bypassing the TCP/IP stack.

3.1.  TCP Options

   During the TCP/IP three-way handshake, the client and server indicate
   their support for SMC-R by including experimental TCP option 254 on
   the three-way handshake flows, in accordance with [RFC6994] ("Shared
   Use of Experimental TCP Options").  The Experiment Identifier (ExID)
   value used is the string "SMCR" in EBCDIC (IBM-1047) encoding
   (0xE2D4C3D9).  This ExID has been registered in the "TCP Experimental
   Option Experiment Identifiers (TCP ExIDs)" registry maintained
   by IANA.









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   After completion of the three-way TCP handshake, each peer queries
   its peer's options.  If both peers set the TCP option on the
   three-way handshake, inline SMC-R negotiation occurs using CLC
   messages.  If neither peer, or only one peer, sets the TCP option,
   SMC-R cannot be used for the TCP connection, and the TCP connection
   completes the setup using the IP fabric.

3.2.  Connection Layer Control (CLC) Messages

   CLC messages are sent as data payload over the IP network using the
   TCP connection between SMC-R layers at the peers.  They are analogous
   to the messages used to exchange parameters for SSL.

   The use of CLC messages is detailed in the following sections.  The
   following list provides a summary of the defined CLC messages and
   their purposes:

   o  SMC Proposal: Sent from the client to propose that this TCP
      connection is eligible to be moved to SMC-R.  The client
      identifies itself and its subnet to the server and passes the
      SMC-R elements for a suggested RoCE path via the MAC and GID.

   o  SMC Accept: Sent from the server to accept the client's TCP
      connection SMC Proposal.  The server responds to the client's
      proposal by identifying itself to the client and passing the
      elements of a RoCE path that the client can use to perform RDMA
      writes to the server.  This consists of such SMC-R link elements
      as RoCE MAC, GID, and RMB information.

   o  SMC Confirm: Sent from the client to confirm the server's
      acceptance of the SMC connection.  The client responds to the
      server's acceptance by passing the elements of a RoCE path that
      the server can use to perform RDMA writes to the client.  This
      consists of such SMC-R link elements as RoCE MAC, GID, and RMB
      information.

   o  SMC Decline: Sent from either the server or the client to reject
      the SMC connection, indicating the reason the peer must decline
      the SMC Proposal and allowing the TCP connection to revert back to
      IP connectivity.

3.3.  LLC Messages

   Link Layer Control (LLC) messages are sent between peer SMC-R layers
   over an SMC-R link to manage the link or the link group.  LLC
   messages are sent using RoCE SendMsg and are 44 bytes long.  The
   44-byte size is based on what can fit into a RoCE Work Queue Element
   (WQE) without requiring the posting of receive buffers.



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   LLC messages generally follow a request-reply semantic.  Each message
   has a request flavor and a reply flavor, and each request must be
   confirmed with a reply, except where otherwise noted.  The use of LLC
   messages is detailed in the following sections.  The following list
   provides a summary of the defined LLC messages and their purposes:

   o  ADD LINK: Used to add a new link to a link group.  Sent from the
      server to the client to initiate addition of a new link to the
      link group, or from the client to the server to request that the
      server initiate addition of a new link.

   o  ADD LINK CONTINUATION: A continuation of ADD LINK that allows the
      ADD LINK to span multiple commands, because all of the link
      information cannot be contained in a single ADD LINK message.

   o  CONFIRM LINK: Used to confirm that RoCE connectivity over a newly
      created SMC-R link is working correctly.  Initiated by the server.
      Both this message and its reply must flow over the SMC-R link
      being confirmed.

   o  DELETE LINK: When initiated by the server, deletes a specific link
      from the link group or deletes the entire link group.  When
      initiated by the client, requests that the server delete a
      specific link or the entire link group.

   o  CONFIRM RKEY: Informs the peer on the SMC-R link of the addition
      of an RMB to the link group.

   o  CONFIRM RKEY CONTINUATION: A continuation of CONFIRM RKEY that
      allows the CONFIRM RKEY to span multiple commands, in the event
      that all of the information cannot be contained in a single
      CONFIRM RKEY message.

   o  DELETE RKEY: Informs the peer on the SMC-R link of the deletion of
      one or more RMBs from the link group.

   o  TEST LINK: Verifies that an already-active SMC-R link is active
      and healthy.

   o  Optional LLC message: Any LLC message in which the two high-order
      bits of the opcode are b'10'.  This optional message must be
      silently discarded by a receiving peer that does not support the
      opcode.  No such messages are defined in this version of the
      architecture; however, the concept is defined to allow for
      toleration of possible advanced, optional functions.






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   CONFIRM LINK and TEST LINK are sensitive to which link they flow on
   and must flow on the link being confirmed or tested.  The other flows
   may flow over any active link in the link group.  When there are
   multiple links in a link group, a response to an LLC message must
   flow over the same link that the original message flowed over, with
   the following exceptions:

   o  ADD LINK request from a server in response to an ADD LINK from a
      client.

   o  DELETE LINK request from a server in response to a DELETE LINK
      from a client.

3.4.  CDC Messages

   Connection Data Control (CDC) messages are sent over the RoCE fabric
   between peers using RoCE SendMsg and are 44 bytes long.  The 44-byte
   size is based on the size that can fit into a RoCE WQE without
   requiring the posting of receive buffers.  CDC messages are used to
   describe the socket application data passed via RDMA write
   operations, as well as TCP connection state information, including
   producer cursors and consumer cursors, RMBE state information, and
   failover data validation.

3.5.  Rendezvous Flows

   Rendezvous information for SMC-R is exchanged as TCP options on the
   TCP three-way handshake flows to indicate capability, followed by
   inline TCP negotiation messages to actually do the SMC-R setup.
   Formats of all rendezvous options and messages discussed in this
   section are detailed in Appendix A.

3.5.1.  First Contact

   First contact between RoCE peers occurs when a new SMC-R link group
   is being set up.  This could be because no SMC-R links already exist
   between the peers, or the server decides to create a new SMC-R link
   group in parallel with an existing one.

3.5.1.1.  Pre-negotiation of TCP Options

   The client and server indicate their SMC-R capability to each other
   using TCP option 254 on the TCP three-way handshake flows.

   A client who wishes to do SMC-R will include TCP option 254 using an
   ExID equal to the EBCDIC (codepage IBM-1047) encoding of "SMCR" on
   its SYN flow.




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   A server that supports SMC-R will include TCP option 254 with the
   ExID value of EBCDIC "SMCR" on its SYN-ACK flow.  Because the server
   is listening for connections and does not know where client
   connections will come from, the server implementation may choose to
   unconditionally include this TCP option if it supports SMC-R.  This
   may be required for server implementations where extensions to the
   TCP stack are not practical.  For server implementations that can add
   code to examine and react to packets during the three-way handshake,
   the server should only include the SMC-R TCP option on the SYN-ACK if
   the client included it on its SYN packet.

   A client who supports SMC-R and meets the three conditions outlined
   above may optionally include the TCP option for SMC-R on its ACK
   flow, regardless of whether or not the server included it on its
   SYN-ACK flow.  Some TCP/IP stacks may have to include it if the SMC-R
   layer cannot modify the options on the socket until the three-way
   handshake completes.  Proprietary servers should not include this
   option on the ACK flow, since including it on the SYN flow was
   sufficient to indicate the client's capabilities.

   Once the initial three-way TCP handshake is completed, each peer
   examines the socket options.  SMC-R implementations may do this by
   examining what was actually provided on the SYN and SYN-ACK packets
   or by performing a getsockopt() operation to determine the options
   sent by the peer.  If neither peer, or only one peer, specified the
   TCP option for SMC-R, then SMC-R cannot be used on this connection
   and it proceeds using normal IP flows and processing.

   If both peers specified the TCP option for SMC-R, then the TCP
   connection is not started yet and the peers proceed to SMC-R
   negotiation using inline data flows.  The socket is not yet turned
   over to the applications; instead, the respective SMC layers exchange
   CLC messages over the newly formed TCP connection.

3.5.1.2.  Client Proposal

   If SMC-R is supported by both peers, the client sends an SMC Proposal
   CLC message to the server.  It is not immediately apparent on this
   flow from client to server whether this is a new or existing SMC-R
   link, because in clustered environments a single IP address may
   represent multiple hosts.  This type of cluster virtual IP address
   can be owned by a network-based or host-based Layer 4 load balancer
   that distributes incoming TCP connections across a cluster of
   servers/hosts.  For purposes of high availability, other clustered
   environments may also support the movement of a virtual IP address
   dynamically from one host in the cluster to another.  In summary, the
   client cannot predetermine that a connection is targeting the same
   host by simply matching the destination IP address for outgoing TCP



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   connections.  Therefore, it cannot predetermine the SMC-R link that
   will be used for a new TCP connection.  This information will be
   dynamically learned, and the appropriate actions will be taken as the
   SMC-R negotiation handshake unfolds.

   In the SMC-R proposal message, the initiator (client) proposes the
   use of SMC-R by including its peer ID, GID, and MAC addresses, as
   well as the IP subnet number of the outgoing interface (if IPv4) or
   the IP prefix list for the network over which the proposal is sent
   (if IPv6).  At this point in the flow, the client makes no local
   commitments of resources for SMC-R.

   When the server receives the SMC Proposal CLC message, it uses the
   peer ID provided by the client, plus subnet or prefix information
   provided by the client, to determine if it already has a usable SMC-R
   link with this SMC-R peer.  If there are one or more existing SMC-R
   links with this SMC-R peer, the server then decides which SMC-R link
   it will use for this TCP connection.  See Sections 3.5.2 and 3.5.3
   for the cases of reusing an existing SMC-R link or creating a
   parallel SMC-R link group between SMC-R peers.

   If this is a first contact between SMC-R peers, the server must
   validate that it is on the same LAN as the client before continuing.
   For IPv4, the server does this by verifying that it has an interface
   with an IP subnet number that matches the subnet number sent by the
   client in the SMC Proposal.  For IPv6, it does this by verifying that
   it is directly attached to at least one IP prefix that was listed by
   the client in its SMC Proposal message.

   If the server agrees to use SMC-R, the server begins the setup of a
   new SMC-R link by allocating local QP and RMB resources (setting its
   QP state to INIT) and providing its full SMC-R information in an SMC
   Accept CLC message to the client over the TCP connection, along with
   a flag set indicating that this is a first contact flow.  While the
   SMC Accept message could flow over any IP route back to the client
   depending upon Layer 3 IP routing, the SMC-R credentials provided
   must be for the common subnet or prefix between the server and
   client, as determined above.  If the server cannot or does not want
   to do SMC-R with the client, it sends an SMC Decline CLC message to
   the client, and the connection data may begin flowing using normal
   TCP/IP flows.










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3.5.1.3.  Server Acceptance

   When the client receives the SMC Accept from the server, it
   determines whether this is a new or existing SMC-R link, using the
   combination of the following: the first contact flag, its MAC/GID and
   the MAC/GID returned by the server, the VLAN over which the
   connection is setting up, and the QP number provided by the server.

   If it is an existing SMC-R link and the client agrees to use that
   link for the TCP connection, see Section 3.5.2 ("Subsequent Contact")
   below.  If it is a new SMC-R link between peers that already have an
   SMC-R link, then the server is starting a new SMC-R link group.

   Assuming that either (1) this is a first contact between peers or
   (2) the server is starting a new SMC-R link group, the client now
   allocates local QP and RMB resources for the SMC-R link (setting the
   QP state to RTR (ready to receive)), associates them with the server
   QP as learned from the SMC Accept CLC message, and sends an SMC
   Confirm CLC message to the server over the TCP connection with its
   SMC-R link information included.  The client also starts a timer to
   wait for the server to confirm the reliably connected queue pair, as
   described below.

3.5.1.4.  Client Confirmation

   Upon receipt of the client's SMC Confirm CLC message, the server
   associates its QP for this SMC-R link with the client's QP as learned
   from the SMC Confirm CLC message and sets its QP state to RTS (ready
   to send).  The client and the server now have reliably connected
   queue pairs.

3.5.1.5.  Link (QP) Confirmation

   Since setting up the SMC-R link and its QPs did not require any
   network flows on the RoCE fabric, the client and server must now
   confirm connectivity over the RoCE fabric.  To accomplish this, the
   server will send a CONFIRM LINK Link Layer Control (LLC) message to
   the client over the newly created SMC-R link, using the RoCE fabric.
   The CONFIRM LINK LLC message will provide the server's MAC, GID, and
   QP information for the connection, allow each partner to communicate
   the maximum number of links it can tolerate in this link group (the
   "link limit"), and will additionally provide two link IDs:

   o  a 1-byte server-assigned link number that is used by both peers to
      identify the link within the link group and is only unique within
      a link group.





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   o  a 4-byte link user ID.  This opaque value is assigned by the
      server for the server's local use and is provided to the client
      for management purposes -- for example, to use in network
      management displays and products.

   When the server sends this message, it will set a timer for receiving
   confirmation from the client.

   When the client receives the server's confirmation in the form of a
   CONFIRM LINK LLC message, it will cancel the confirmation timer it
   set when it sent the SMC Confirm message.  The client will also
   advance its QP state to RTS and respond over the RoCE fabric with a
   CONFIRM LINK response LLC message that (1) provides its MAC, GID,
   QP number, and link limit, (2) confirms the 1-byte link number sent
   by the server, and (3) provides its own 4-byte link user ID to the
   server.



































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       Host X -- Server                           Host Y -- Client
    +-------------------+                      +-------------------+
    | Peer ID = PS1     |                      |   Peer ID = PC1   |
    |            +------+                      +------+            |
    |       QP 8 |RNIC 1|                      |RNIC 2|  QP 64     |
    |RToken X|   |MAC MA|                      |MAC MB|   |        |
    |        |   |GID GA|                      |GID GB|   |RToken Y|
    |       \/   +------+      (Subnet S1)     +------+  \/        |
    |+--------+         |                      |        +--------+ |
    || RMB    |         |                      |        | RMB    | |
    |+--------+         |                      |        +--------+ |
    |            +------+                      +------+            |
    |            |RNIC 3|                      |RNIC 4|            |
    |            |MAC MC|                      |MAC MD|            |
    |            |GID GC|                      |GID GD|            |
    |            +------+                      +------+            |
    +-------------------+                      +-------------------+

                     SYN TCP options(254,"SMCR")
        <---------------------------------------------------------

                     SYN-ACK TCP options(254,"SMCR")
        --------------------------------------------------------->

                     ACK [TCP options(254,"SMCR")]
        <--------------------------------------------------------

                    SMC Proposal(PC1,MB,GB,S1)
        <--------------------------------------------------------

    SMC Accept(PS1,first contact,MA,GA,MTU,QP8,RToken=X,RMB elem index)
        --------------------------------------------------------->

         SMC Confirm(PC1,MB,GB,MTU,QP64,RToken=Y,RMB element index)
         <--------------------------------------------------------

       CONFIRM LINK(MA,GA,QP8, link lim, server link user ID, linknum)
        .........................................................>

    CONFIRM LINK rsp(MB,GB,QP64, link lim, client link user ID, linknum)
        <........................................................

                           Legend:
                    ------------   TCP/IP and CLC flows
                    ............   RoCE (LLC) flows
           Square brackets ("[ ]") indicate optional information

                 Figure 8: First Contact Rendezvous Flows



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   Technically, the data for the TCP connection could now flow over the
   RoCE path.  However, if this is a first contact, there is no
   alternate for this recently established RoCE path.  Since in the
   current architecture there is no failover from RoCE to IP once
   connection data starts flowing, this means that a failure of this
   path would disrupt the TCP connection, meaning that the level of
   redundancy and failover is less than that provided by IP.  If the
   network has alternate RoCE paths available, they would not be usable
   at this point.  This situation would be unacceptable.

3.5.1.6.  Second SMC-R Link Setup

   Because of the unacceptable situation described above, TCP data will
   not be allowed to flow on the newly established SMC-R link until a
   second path has been set up, or at least attempted.

   If the server has a second RNIC available on the same LAN, it
   attempts to set up the second SMC-R link over that second RNIC.  If
   it only has one RNIC available on the LAN, it will attempt to set up
   the second SMC-R link over that one RNIC.  In the latter case, the
   server is attempting to set up an asymmetric link, in case the client
   does have a second RNIC on the LAN.

   In either case, the server allocates a new QP over the RNIC it is
   attempting to use for the second link and assigns a link number to
   the new link; the server also creates an RToken for the RMB over this
   second QP (note that this means that the first and second QP each
   have their own RToken to represent the same RMB).  The server
   provides this information, as well as the MAC and GID of the RNIC
   over which it is attempting to set up the second link, in an ADD LINK
   LLC message that it sends to the client over the SMC-R link that is
   already set up.

3.5.1.6.1.  Client Processing of ADD LINK LLC Message from Server

   When the client receives the server's ADD LINK LLC message, it
   examines the GID and MAC provided by the server to determine whether
   the server is attempting to use the same server-side RNIC as the
   existing SMC-R link or a different one.

   If the server is attempting to use the same server-side RNIC as the
   existing SMC-R link, then the client verifies that it has a second
   RNIC on the same LAN.  If it does not, the client rejects the
   ADD LINK request from the server, because the resulting link would be
   a parallel link, which is not supported within a link group.  If the
   client does have a second RNIC on the same LAN, it accepts the
   request, and an asymmetric link will be set up.




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   If the server is using a different server-side RNIC from the existing
   SMC-R link, then the client will accept the request and a second
   SMC-R link will be set up in this SMC-R link group.  If the client
   has a second RNIC on the same LAN, that second RNIC will be used for
   the second SMC-R link, creating symmetric links.  If the client does
   not have a second RNIC on the same LAN, it will use the same RNIC as
   was used for the initial SMC-R link, resulting in the setup of an
   asymmetric link in the SMC-R link group.

   In either case, when the client accepts the server's ADD LINK
   request, it allocates a new QP on the chosen RNIC and creates an RKey
   over that new QP for the client-side RMB for the SMC-R link group,
   then sends an ADD LINK reply LLC message to the server providing that
   information as well as echoing the link number that was sent by the
   server.

   If the client rejects the server's ADD LINK request, it sends an ADD
   LINK reply LLC message to the server with the reason code for the
   rejection.

3.5.1.6.2.  Server Processing of ADD LINK Reply LLC Message from Client

   If the client sends a negative response to the server or no reply is
   received, the server frees the RoCE resources it had allocated for
   the new link.  Having a single link in an SMC-R link group is
   undesirable.  The server's recovery is detailed in Appendix C.8
   ("Failure to Add Second SMC-R Link to a Link Group").

   If the client sends a positive reply to the server with
   MAC/GID/QP/RKey information, the server associates its QP for the new
   SMC-R link to the QP that the client provided.  Now, the new SMC-R
   link is in the same situation that the first was in after the client
   sent its ACK packet -- there is a reliably connected queue pair over
   the new RoCE path, but there have been no RoCE flows to confirm that
   it's actually usable.  So, at this point, the client and server will
   exchange CONFIRM LINK LLC messages just like they did on the first
   SMC-R link.

   If either peer receives a failure during this second CONFIRM LINK LLC
   exchange (either an immediate failure -- which implies that the
   message did not reach the partner -- or a timeout), it sends a DELETE
   LINK LLC message to the partner over the first (and now only) link in
   the link group.  This DELETE LINK LLC message must be acknowledged
   before data can flow on the single link in the link group.







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       Host X -- Server                           Host Y -- Client
    +-------------------+                      +-------------------+
    | Peer ID = PS1     |                      |   Peer ID = PC1   |
    |            +------+                      +------+            |
    |       QP 8 |RNIC 1|      SMC-R Link 1    |RNIC 2|  QP 64     |
    |RToken X|   |MAC MA|<-------------------->|MAC MB|   |        |
    |        |   |GID GA|                      |GID GB|   |RToken Y|
    |       \/   +------+                      +------+  \/        |
    |+--------+         |                      |        +--------+ |
    ||        |         |                      |        |        | |
    || RMB    |         |                      |        | RMB    | |
    ||        |         |                      |        |        | |
    |+--------+         |                      |        +--------+ |
    |       /\   +------+                      +------+  /\        |
    |        |   |RNIC 3|      SMC-R Link 2    |RNIC 4|  |         |
    |RToken Z|   |MAC MC|<-------------------->|MAC MD|  |RToken W |
    |       QP 9 |GID GC|      (being added)   |GID GD| QP 65      |
    |            +------+                      +------+            |
    +-------------------+                      +-------------------+

                First SMC-R link setup as shown in Figure 8
            <-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.->

            ADD LINK request(QP9,MC,GC, link number = 2)
            ............................................>

            ADD LINK response(QP65,MD,GD, link number = 2)
            <............................................

            ADD LINK CONTINUATION request(RToken=Z)
            ............................................>

           ADD LINK CONTINUATION response(RToken=W)
            <............................................

         CONFIRM LINK(MC,GC,QP9, link number = 2, link user ID)
            .............................................>

      CONFIRM LINK response(MD,GD,QP65, link number = 2, link user ID)
            <.............................................

                          Legend:
                   ------------   TCP/IP and CLC flows
                   ............   RoCE (LLC) flows

                Figure 9: First Contact, Second Link Setup





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3.5.1.6.3.  Exchange of RKeys on Second SMC-R Link

   Note that in the scenario described here -- first contact -- there is
   only one RMB RKey to exchange on the second SMC-R link, and it is
   exchanged in the ADD LINK CONTINUATION request and reply.  In
   scenarios other than first contact -- for example, adding a new SMC-R
   link to a longstanding link group with multiple RMBs -- additional
   flows will be required to exchange additional RMB RKeys.  See
   Section 3.5.5.2.3 ("Adding a New SMC-R Link to a Link Group with
   Multiple RMBs") for more details on these flows.

3.5.1.6.4.  Aborting SMC-R and Falling Back to IP

   If both partners don't provide the SMC-R TCP option during the
   three-way TCP handshake, the connection falls back to normal TCP/IP.
   During the SMC-R negotiation that occurs after the three-way TCP
   handshake, either partner may break off SMC-R by sending an SMC
   Decline CLC message.  The SMC Decline CLC message may be sent in
   place of any expected message and may also be sent during the CONFIRM
   LINK LLC exchange if there is a failure before any application data
   has flowed over the RoCE fabric.  For more details on exactly when an
   SMC Decline can flow during link group setup, see Appendices C.1
   ("SMC Decline during CLC Negotiation") and C.2 ("SMC Decline during
   LLC Negotiation").

   If this fallback to IP happens while setting up a new SMC-R link
   group, the RoCE resources allocated for this SMC-R link group
   relationship are torn down, and it will be retried as a new SMC-R
   link group next time a connection starts between these peers with
   SMC-R proposed.  Note that if this happens because one side doesn't
   support SMC-R, there will be very little to tear down, as the TCP
   option will have failed to flow on either the initial SYN or the
   SYN-ACK before either side had reserved any local RoCE resources.

3.5.2.  Subsequent Contact

   "Subsequent contact" means setting up a new TCP connection between
   two peers that already have an SMC-R link group between them and
   reusing the existing SMC-R link group.  In this case, it is not
   necessary to allocate new QPs.  However, it is possible that a new
   RMB has been allocated for this TCP connection, if the previous TCP
   connection used the last element available in the previously used
   RMB, or for any other implementation-dependent reason.  For this
   reason, and for convenience and error checking, the same TCP
   option 254, followed by the inline negotiation method described for
   initial contact, will be used for subsequent contact, but the
   processing differs in some ways.  That processing is described below.




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3.5.2.1.  SMC-R Proposal

   When the client begins the inline negotiation with the server, it
   does not know if this is a first contact or a subsequent contact.
   The client cannot know this information until it sees the server's
   peer ID, to determine whether or not it already has an SMC-R link
   with this peer that it can use.  There are several reasons why it is
   not sufficient to use the partner IP address, subnet, VLAN, or other
   IP information to make this determination.  The most obvious reason
   is distributed systems: if the server IP address is actually a
   virtual IP address representing a distributed cluster, the actual
   host serving this TCP connection may not be the same as the host that
   served the last TCP connection to this same IP address.

   After the TCP three-way handshake, assuming that both partners
   indicate SMC-R capability, the client builds and sends the
   SMC Proposal CLC message to the server in exactly the same manner as
   it does in the "first contact" case, and in fact at this point
   doesn't know if it's a first contact or a subsequent contact.  As in
   the "first contact" case, the client sends its peer ID value,
   suggested RNIC MAC/GID, and IP subnet or prefix information.

   Upon receiving the client's proposal, the server looks up the
   provided peer ID to determine if it already has a usable SMC-R
   link group with this peer.  If it does already have a usable SMC-R
   link group, the server then needs to decide whether it will use the
   existing SMC-R link group or create a new link group.  For the case
   of the new link group, see Section 3.5.3 ("First Contact Variation:
   Creating a Parallel Link Group") below.

   For this discussion, assume that the server decides to use the
   existing SMC-R link group for the TCP connection, which is expected
   to be the most common case.  The server is responsible for making
   this decision.  The server then needs to communicate that information
   to the client, but it is not necessary to allocate, associate, and
   confirm QPs for the chosen SMC-R link.  All that remains to be done
   is to set up RMB space for this TCP connection.

   If one of the RMBs already in use for this SMC-R link group has an
   available element that uses the appropriate buffer size, the server
   merely chooses one for this TCP connection and then sends an SMC
   Accept CLC message providing the full RoCE information for the chosen
   SMC-R link to the client, using the same format as the SMC Accept CLC
   message described in Section 3.5.1 ("First Contact") above.







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   The server may choose to use the SMC-R link that matches the
   suggested MAC/GID provided by the client in the SMC Proposal for its
   RDMA writes but is not obligated to do so.  The final decision on
   which specific SMC-R link to assign a TCP connection to is an
   independent server and client decision.

   It may be necessary for the server to allocate a new RMB for this
   connection.  The reasons for this are implementation dependent and
   could include the following:

   o  no available space in existing RMB or RMBs, or

   o  desire to allocate a new RMB that uses a different buffer size
      from the ones already created, or

   o  any other implementation-dependent reason

   In this case, the server will allocate the new RMB and then perform
   the flows described in Section 3.5.5.2.1 ("Adding a New RMB to an
   SMC-R Link Group").  Once that processing is complete, the server
   then provides the full RoCE information, including the new RKey, for
   this connection in an SMC Confirm CLC message to the client.

3.5.2.2.  SMC-R Acceptance

   Upon receiving the SMC Accept CLC message from the server, the client
   examines the RoCE information provided by the server to determine
   whether this is a first contact for a new SMC-R link group or a
   subsequent contact for an existing SMC-R link group.  It is a
   subsequent contact if the server-side peer ID, GID, MAC, and QP
   number provided in the packet match a known SMC-R link, and the first
   contact flag is not set.  If this is not the case -- for example, the
   GID and MAC match but the QP is new -- then the server is creating a
   new, parallel SMC-R link group, and this is treated as a first
   contact.

   A different RMB RToken does not indicate a first contact, as the
   server may have allocated a new RMB or may be using several RMBs for
   this SMC-R link.  The client needs the server's RMB information only
   for its RDMA writes to the server, and since there is no requirement
   for symmetric RMBs, this information is simply control information
   for the RDMA writes on this SMC-R link.

   The client must validate that the RMB element being provided by the
   server is not in use by another TCP connection on this SMC-R link
   group.  This validation must validate the new <rtoken, index> across





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   all known <rtoken, index> on this link group.  See Section 4.4.2
   ("RMB Element Reuse and Conflict Resolution") for the case in which
   the server tries to use an RMB element that is already in use on this
   link group.

   Once the client has determined that this TCP connection is a
   subsequent contact over an existing SMC-R link, it performs an RMB
   allocation process similar to what the server did: it either
   (1) allocates an element from an RMB already associated with this
   SMC-R link or (2) allocates a new RMB, associates it with this SMC-R
   link, and then chooses an element out of it.

   If the client allocates a new RMB for this TCP connection, it
   performs the processing described in Section 3.5.5.2.1 ("Adding a New
   RMB to an SMC-R Link Group").  Once that processing is complete, the
   client provides its full RoCE information for this TCP connection in
   an SMC Confirm CLC message.

   Because an SMC-R link with a verified connected QP already exists and
   is being reused, there is no need for verification or alternate QP
   selection flows or timers.

3.5.2.3.  SMC-R Confirmation

   When the server receives the client's SMC Confirm CLC message on a
   subsequent contact, it verifies the following:

   o  The RMB element provided by the client is not already in use by
      another TCP connection on this SMC-R link group (see Section 4.4.2
      ("RMB Element Reuse and Conflict Resolution") for the case in
      which it is).

   o  The MAC/GID/QP information provided by the client matches an
      active link within the link group.  The client is free to select
      any valid/active link.  The client is not required to select the
      same link as the server.

   If this validation passes, the server stores the client's RMB
   information for this connection, and the RoCE setup of the TCP
   connection is complete.

3.5.2.4.  TCP Data Flow Race with SMC Confirm CLC Message

   On a subsequent contact TCP/IP connection, a peer may send data as
   soon as it has received the peer RMB information for the connection.
   There are no additional RoCE confirmation flows, since the QPs on the
   SMC-R link are already reliably connected and verified.




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   In the majority of cases, the first data will flow from the client to
   the server.  The client must send the SMC Confirm CLC message before
   sending any connection data over the chosen SMC-R link; however, the
   client need not wait for confirmation of this message, and in fact
   there will be no such confirmation.  Since the server is required to
   have the RMB fully set up and ready to receive data from the client
   before sending an SMC Accept CLC message, the client can begin
   sending data over the SMC-R link immediately upon completing the send
   of the SMC Confirm CLC message.

   It is possible that data from the client will arrive at the
   server-side RMB before the SMC Confirm CLC message from the client
   has been processed.  In this case, the server must handle this race
   condition and not provide the arrived TCP data to the socket
   application until the SMC Confirm CLC message has been received and
   fully processed, opening the socket.

   If the server has initial data to send to the client that is not a
   response to the client (this case should be rare), it can send the
   data immediately upon receiving and processing the SMC Confirm CLC
   message from the client.  The client must have opened the TCP socket
   to the client application upon sending the SMC Confirm CLC message so
   the client will be ready to process data from the server.

3.5.3.  First Contact Variation: Creating a Parallel Link Group

   Recall that parallel SMC-R links within an SMC-R link group are not
   supported.  These are multiple SMC-R links within a link group that
   use the same network path.  However, multiple SMC-R link groups
   between the same peers are supported.  This means that if multiple
   SMC-R links over the same RoCE path are desired, it is necessary to
   use multiple SMC-R link groups.  While not a recommended practice,
   this could be done for platform-specific reasons, like QP separation
   of different workloads.  Only the server can drive the creation of
   multiple SMC-R link groups between peers.

   At a high level, when the server decides to create an additional
   SMC-R link group with a client with which it already has an SMC-R
   link group, the flows are basically the same as the normal
   "first contact" case described above.  The following text provides
   more detail and clarification of processing in this case.

   When the server receives the SMC Proposal CLC message from the client
   and, using the MAC/GID information, determines that it already has an
   SMC-R link group with this client, the server can either reuse the
   existing SMC-R link group (detailed in Section 3.5.2 ("Subsequent
   Contact") above) or create a new SMC-R link group in addition to the
   existing one.



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   If the server decides to create a new SMC-R link group, it does the
   same processing it would have done for first contact: allocate QP and
   RMB resources as well as alternate QP resources, and communicate the
   QP and RMB information to the client in the SMC Accept CLC message
   with the first contact flag set.

   When the client receives the server's SMC Accept CLC message with the
   new QP information and the first contact flag set, it knows that the
   server is creating a new SMC-R link group even though it already has
   an SMC-R link group with the server.  In this case, the client will
   also allocate a new QP for this new SMC-R link, allocate an RMB for
   it, and generate an RKey for it.

   Note that multiple SMC-R link groups between the same peers must
   access different RMB resources, so new RMBs will be required.  Using
   the same RMBs that are in use in another SMC-R link group is not
   permitted.

   The client then associates its new QP with the server's new QP and
   sends its SMC Confirm CLC message back to the server providing the
   new QP/RMB information, and then sets its confirmation timer for the
   new SMC-R link.

   When the server receives the client's SMC Confirm CLC message, it
   associates its QP with the client's QP as learned from the SMC
   Confirm CLC message and sends a confirmation LLC message.  The rest
   of the flow, with the confirmation QP and setup of additional SMC-R
   links, unfolds just like the "first contact" case.

3.5.4.  Normal SMC-R Link Termination

   The normal socket API trigger points are used by the SMC-R layer to
   initiate SMC-R connection termination flows.  The main design point
   for SMC-R normal connection flows is to use the SMC-R protocol to
   first shut down the SMC-R connection and free up any SMC-R RDMA
   resources, and then allow the normal TCP connection termination
   protocol (i.e., FIN processing) to drive cleanup of the TCP
   connection that exists on the IP fabric.  This design point is very
   important in ensuring that RDMA resources such as the RMBEs are only
   freed and reused when both SMC-R endpoints are completely done with
   their RDMA write operations to the partner's RMBE.

   When the last TCP connection over an SMC-R link group terminates, the
   link group can be terminated.  Similar to creation of SMC-R links and
   link groups, the primary responsibility for determining that normal
   termination is needed and initiating it lies with the server.





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   Implementations may opt to set timers to keep SMC-R link groups up
   for a specified time after the last TCP connection ends, to avoid
   churn in cases where TCP connections come and go regularly.

   The link or link group may also be terminated as a result of a
   command initiated by the operator.  This command can be entered at
   either the client or the server.  If entered at the client, the
   client requests that the server perform link or link group
   termination, and the responsibility for doing so ultimately lies with
   the server.

   When the server determines that the SMC-R link group is to be
   terminated, it sends a DELETE LINK LLC message to the client, with a
   flag set indicating that all links in the link group are to be
   terminated.  After receiving confirmation from the adapter that the
   DELETE LINK LLC message has been sent, the server can clean up its
   end of the link group (QPs, RMBs, etc.).  Upon receipt of the DELETE
   LINK message from the server, the client must immediately comply and
   clean up its end of the link group.  Any TCP connections that the
   client believes to be active on the link group must be immediately
   terminated.

   The client can request that the server delete the link group as well.
   The client does this by sending a DELETE LINK message to the server,
   indicating that cleanup of all links is requested.  The server must
   comply by sending a DELETE LINK to the client and processing as
   described in the previous paragraph.  If there are TCP connections
   active on the link group when the server receives this request, they
   are immediately terminated by sending a RST flow over the IP fabric.

3.5.5.  Link Group Management Flows

3.5.5.1.  Adding and Deleting Links in an SMC-R Link Group

   The server has the lead role in managing the composition of the link
   group.  Links are added to the link group by the server.  The client
   may notify the server of new conditions that may result in the server
   adding a new link, but the server is ultimately responsible.  In
   general, links are deleted from the link group by the server;
   however, in certain error cases the client may inform the server that
   a link must be deleted and treat it as deleted without waiting for
   action from the server.  These flows are detailed in the sections
   that follow.








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3.5.5.1.1.  Server-Initiated ADD LINK Processing

   As described in previous sections, the server initiates an ADD LINK
   exchange to create redundancy in a newly created link group.  Once a
   link group is established, the server may also initiate ADD LINK for
   other reasons, including:

   o  Availability of additional resources on the server host to support
      an additional SMC-R link.  This may include the provisioning of an
      additional RNIC, more storage becoming available to support
      additional QP resources, operator command, or any other
      implementation-dependent reason.  Note that in order to be
      available for an existing link group a new RNIC must be attached
      to the same RoCE LAN that the link group is using.

   o  Receipt of notification from the client that additional resources
      on the client are available to support an additional SMC-R link.
      See Section 3.5.5.1.2 ("Client-Initiated ADD LINK Processing").

   Server-initiated ADD LINK processing in an established SMC-R link
   group is the same as the ADD LINK processing described in
   Section 3.5.1.6 ("Second SMC-R Link Setup"), with the following
   changes:

   o  If an asymmetric SMC-R link already exists in the link group, a
      second asymmetric link will not be created.  Only one asymmetric
      link is permitted in a link group.

   o  TCP data flow on already-existing link(s) in the link group is not
      halted or otherwise affected during the process of setting up the
      additional link.

   The server will not initiate ADD LINK processing if the link group
   already has the maximum number of links negotiated by the partners.

3.5.5.1.2.  Client-Initiated ADD LINK Processing

   If an additional RNIC becomes available for an existing SMC-R link
   group on the client's side, the client notifies the server by sending
   an ADD LINK request LLC message to the server.  Unlike an ADD LINK
   request sent by the server to the client, this ADD LINK request
   merely informs the server that the client has a new RNIC.  If the
   link group lacks redundancy or has redundancy only on an asymmetric
   link with a single RNIC on the client side, the server must initiate
   an ADD LINK exchange in response to this message, to create or
   improve the link group's redundancy.





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   If the link group already has symmetric-link redundancy but has fewer
   than the negotiated maximum number of links, the server may respond
   by initiating an ADD LINK exchange to create a new link using the
   client's new resource but is not required to do so.

   If the link group already has the negotiated maximum number of links,
   the server must ignore the client's ADD LINK request LLC message.

   Because the server is not required to respond to the client's
   ADD LINK LLC message in all cases, the client must not wait for a
   response or throw an error if one does not come.

3.5.5.1.3.  Server-Initiated DELETE LINK Processing

   Reasons that a server may delete a link include the following:

   o  The link has not been used for TCP connections for an
      implementation-defined time interval, and deleting the link will
      not cause the link group to lack redundancy.

   o  Errors in resources supporting the link occur.  These errors may
      include, but are not limited to, RNIC errors, QP errors, and
      software errors.

   o  The RNIC supporting this SMC-R link is being taken down, either
      because of an error case or because of an operator or software
      command.

   If a link being deleted is supporting TCP connections and there are
   one or more surviving links in the link group, the TCP connections
   are moved to the surviving links.  For more information on this
   processing, see Section 2.3 ("SMC-R Resilience and Load Balancing").

   The server deletes a link from the link group by sending a
   DELETE LINK request LLC message to the client over any of the usable
   links in the link group.  Because the DELETE LINK LLC message
   specifies which link is to be deleted, it may flow over any link in
   the link group.  The server must not clean up its RoCE resources for
   the link until the client responds.

   The client responds to the server's DELETE LINK request LLC message
   by sending the server a DELETE LINK response LLC message.  The client
   must respond positively; it cannot decline to delete the link.  Once
   the server has received the client's DELETE LINK response, both sides
   may clean up their resources for the link.






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   Either a positive write completion or some other indication from the
   RNIC on the client's side is sufficient to indicate to the client
   that the server has received the DELETE LINK response.

         Host X                                     Host Y
    +-------------------+                      +-------------------+
    |            +------+                      +------+            |
    |       QP 8 |RNIC 1|     SMC-R Link 1     |RNIC 2| QP 9       |
    |RToken X|   |Failed|<--X----X----X----X-->|      |            |
    |        |   |      |                      |      |            |
    |       \/   +------+                      +------+            |
    |+--------+         |                      |                   |
    || Deleted|         |                      |                   |
    || RMB    |         |                      |                   |
    ||        |         |                      |                   |
    |+--------+         |                      |                   |
    |       /\   +------+                      +------+            |
    |RToken Z|   |      |     SMC-R Link 2     |      |            |
    |        |   |RNIC 3|<-------------------->|RNIC 4|            |
    |       QP 64|      |                      |      | QP 65      |
    |            +------+                      +------+            |
    +-------------------+                      +-------------------+

          DELETE LINK(request, link number = 1,
                ................................................>
                       reason code = RNIC failure)

          DELETE LINK(response, link number = 1)
               <................................................

           (Note: Architecturally, this exchange can flow over either
                  SMC-R link but most likely flows over Link 2, since
                  the RNIC for Link 1 has failed.)

               Figure 10: Server-Initiated DELETE LINK Flow
















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3.5.5.1.4.  Client-Initiated DELETE LINK Request

   The client may request that the server delete a link for the same
   reasons that the server may delete a link, except for inactivity
   timeout.

   Because the client depends on the server to delete links, there are
   two types of delete requests from client to server:

   o  Orderly: The client is requesting that the server delete the link
      when able.  This would result from an operator command to bring
      down the RNIC or some other nonfatal reason.  In this case, the
      server is required to delete the link but may not do it right
      away.

   o  Disorderly: The server must delete the link right away, because
      the client has experienced a fatal error with the link.

   In either case, the server responds by initiating a DELETE LINK
   exchange with the client, as described in the previous section.  The
   difference between the two is whether the server must do so
   immediately or can delay for an opportunity to gracefully delete the
   link.




























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          Host X                                     Host Y
     +-------------------+                      +-------------------+
     |            +------+                      +------+            |
     |       QP 8 |RNIC 1|     SMC-R Link 1     |RNIC 2| QP 9       |
     |RToken X|   |      |<---X--X--X--X--X--X->|Failed|            |
     |        |   |      |                      |      |            |
     |       \/   +------+                      +------+            |
     |+--------+         |                      |                   |
     || Deleted|         |                      |                   |
     || RMB    |         |                      |                   |
     ||        |         |                      |                   |
     |+--------+         |                      |                   |
     |       /\   +------+                      +------+            |
     |RToken Z|   |      |     SMC-R Link 2     |      |            |
     |        |   |RNIC 3|<-------------------->|RNIC 4|            |
     |       QP 64|      |                      |      | QP 65      |
     |            +------+                      +------+            |
     +-------------------+                      +-------------------+

           DELETE LINK(request, link number = 1, disorderly,
                <...............................................
                       reason code = RNIC failure)

           DELETE LINK(request, link number = 1,
                 ................................................>
                        reason code = RNIC failure)

           DELETE LINK(response, link number = 1)
                <................................................

           (Note: Architecturally, this exchange can flow over either
                  SMC-R link but most likely flows over Link 2, since
                  the RNIC for Link 1 has failed.)

               Figure 11: Client-Initiated DELETE LINK Flow

3.5.5.2.  Managing Multiple RKeys over Multiple SMC-R Links in a
          Link Group

   After the initial contact sequence completes and the number of TCP
   connections increases, it is possible that the SMC peers could add
   more RMBs to the link group.  Recall that each peer independently
   manages its RMBs.  Also recall that an RMB's RToken is specific to a
   QP, which means that when there are multiple SMC-R links in a link
   group, each RMB accessed with the link group requires a separate
   RToken for each SMC-R link in the group.





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   Each RMB that is added to a link must be added to all links within
   the link group.  The set of RMBs created for the link is called the
   "RToken set".  The RTokens must be exchanged with the peer.  As RMBs
   are added and deleted, the RToken set must remain in sync.

3.5.5.2.1.  Adding a New RMB to an SMC-R Link Group

   A new RMB can be added to an SMC-R link group on either the client
   side or the server side.  When an additional RMB is added to an
   existing SMC-R link group, that RMB must be associated with the QPs
   for each link in the link group.  Therefore, when an RMB is added to
   an SMC-R link group, its RMB RToken for each SMC-R link's QP must be
   communicated to the peer.

   The tokens for a new RMB added to an existing SMC-R link group are
   communicated using CONFIRM RKEY LLC messages, as shown in Figure 12.
   The RToken set is specified as pairs: an SMC-R link number, paired
   with the new RMB's RToken over that SMC-R link.  To preserve failover
   capability, any TCP connection that uses a newly added RMB cannot go
   active until all RTokens for the RMB have been communicated for all
   of the links in the link group.






























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          Host X                                     Host Y
     +-------------------+                      +-------------------+
     |            +------+                      +------+            |
     |       QP 8 |RNIC 1|     SMC-R Link 1     |RNIC 2| QP 9       |
     |RToken X|   |      |<-------------------->|      |            |
     |        |   |      |                      |      |            |
     |       \/   +------+                      +------+            |
     |+--------+         |                      |                   |
     || New    |         |                      |                   |
     || RMB    |         |                      |                   |
     ||        |         |                      |                   |
     |+--------+         |                      |                   |
     |       /\   +------+                      +------+            |
     |RToken Z|   |      |     SMC-R Link 2     |      |            |
     |        |   |RNIC 3|<-------------------->|RNIC 4|            |
     |       QP 64|      |                      |      | QP 65      |
     |            +------+                      +------+            |
     +-------------------+                      +-------------------+

           CONFIRM RKEY(request, Add,
                 ................................................>
                      RToken set((Link 1,RToken X),(Link 2,RToken Z)))

           CONFIRM RKEY(response, Add,
                <................................................
                      RToken set((Link 1,RToken X),(Link 2,RToken Z)))

            (Note: This exchange can flow over either SMC-R link.)

                 Figure 12: Add RMB to Existing Link Group

   Implementations may choose to proactively add RMBs to link groups in
   anticipation of need.  For example, an implementation may add a new
   RMB when a certain usage threshold (e.g., percentage used) for all of
   its existing RMBs has been exceeded.

   A new RMB may also be added to an existing link group on an as-needed
   basis -- for example, when a new TCP connection is added to the link
   group but there are no available RMB elements.  In this case, the CLC
   exchange is paused while the peer that requires the new RMB adds it.
   An example of this is illustrated in Figure 13.










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       Host X -- Server                            Host Y -- Client
    +-------------------+                      +--------------------+
    | Peer ID = PS1     |                      |   Peer ID = PC1    |
    |            +------+                      +------+             |
    |       QP 8 |RNIC 1|    SMC-R Link 1      |RNIC 2|  QP 64      |
    |RToken X|   |MAC MA|<-------------------->|MAC MB|   |         |
    |        |   |GID GA|                      |GID GB|   |RToken Y2|
    |       \/   +------+                      +------+  \/         |
    |+--------+         |                      |        +--------+  |
    ||        |         |   Subnet S1          |        | New    |  |
    || RMB    |         |                      |        | RMB    |  |
    |+--------+         |                      |        +--------+  |
    |       /\   +------+                      +------+  /\         |
    |        |   |RNIC 3|    SMC-R Link 2      |RNIC 4|   |RToken W2|
    |        |   |MAC MC|<-------------------->|MAC MD|   |         |
    |       QP 9 |GID GC|                      |GID GD|  QP 65      |
    |            +------+                      +------+             |
    +-------------------+                      +--------------------+

           SYN / SYN-ACK / ACK TCP three-way handshake with TCP option
        <--------------------------------------------------------->

                    SMC Proposal(PC1,MB,GB,S1)
        <--------------------------------------------------------

      SMC Accept(PS1,not 1st contact,MA,GA,QP8,RToken=X,RMB elem index)
        --------------------------------------------------------->

          CONFIRM RKEY(request, Add,
        <........................................................
                  RToken set((Link 1,RToken Y2),(Link 2,RToken W2)))

          CONFIRM RKEY(response, Add,
         ........................................................>
                  RToken set((Link 1,RToken Y2),(Link 2,RToken W2)))

          SMC Confirm(PC1,MB,GB,QP64,RToken=Y2, RMB element index)
        <--------------------------------------------------------

                         Legend:
                  ------------   TCP/IP and CLC flows
                  ............   RoCE (LLC) flows

          Figure 13: Client Adds RMB during TCP Connection Setup







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3.5.5.2.2.  Deleting an RMB from an SMC-R Link Group

   Either peer can delete one or more of its RMBs as long as it is not
   being used for any TCP connections.  Ideally, an SMC-R peer would use
   a timer to avoid freeing an RMB immediately after the last TCP
   connection stops using it, to keep the RMB available for later TCP
   connections and avoid thrashing with addition and deletion of RMBs.
   Once an SMC-R peer decides to delete an RMB, it sends a DELETE RKEY
   LLC message to its peer.  It can then free the RMB once it receives
   a response from the peer.  Multiple RMBs can be deleted in a
   DELETE RKEY exchange.

   Note that in a DELETE RKEY message, it is not necessary to specify
   the full RToken for a deleted RMB.  The RMB's RKey over one link in
   the link group is sufficient to specify which RMB is being deleted.

          Host X                                     Host Y
     +-------------------+                      +-------------------+
     |            +------+                      +------+            |
     |       QP 8 |RNIC 1|     SMC-R Link 1     |RNIC 2| QP 9       |
     |RToken X|   |      |<-------------------->|      |            |
     |        |   |      |                      |      |            |
     |       \/   +------+                      +------+            |
     |+--------+         |                      |                   |
     || Deleted|         |                      |                   |
     || RMB    |         |                      |                   |
     ||        |         |                      |                   |
     |+--------+         |                      |                   |
     |       /\   +------+                      +------+            |
     |RToken Z|   |      |     SMC-R Link 2     |      |            |
     |        |   |RNIC 3|<-------------------->|RNIC 4|            |
     |       QP 9 |      |                      |      |            |
     |            +------+                      +------+            |
     +-------------------+                      +-------------------+

           DELETE RKEY(request, RKey list(RKey X))
                 ................................................>

           DELETE RKEY(response, RKey list(RKey X))
                <................................................

           (Note: This exchange can flow over either SMC-R link.)

                Figure 14: Delete RMB from SMC-R Link Group







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3.5.5.2.3.  Adding a New SMC-R Link to a Link Group with Multiple RMBs

   When a new SMC-R link is added to an existing link group, there could
   be multiple RMBs on each side already associated with the link group.
   There could also be a different number of RMBs on one side than on
   the other, because each peer manages its RMBs independently.  Each of
   these RMBs will require a new RToken to be used on the new SMC-R
   link, and those new RTokens must then be communicated to the peer.
   This requires two-way communication, as the server will have to
   communicate its RTokens to the client and vice versa.

   RTokens are communicated between peers in pairs.  Each RToken pair
   consists of:

   o  The RToken for the RMB, as is already known on an existing SMC-R
      link in the link group.

   o  The RToken for the same RMB, to be used on the new SMC-R link.

   These pairs are required to ensure that each peer knows which RTokens
   across QPs are equivalent.

   The ADD LINK request and response LLC messages do not have enough
   space to contain any RToken pairs.  ADD LINK CONTINUATION LLC
   messages are used to communicate these pairs, as shown in Figure 15.
   The ADD LINK CONTINUATION LLC messages are sent on the same SMC-R
   link that the ADD LINK LLC messages were sent over, and in both the
   ADD LINK and ADD LINK CONTINUATION LLC messages the first RToken in
   each RToken pair will be the RToken for the RMB as known on the SMC-R
   link over which the LLC message is being sent.





















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       Host X -- Server                           Host Y -- Client
    +-------------------+                      +-------------------+
    | Peer ID = PS1     |                      |   Peer ID = PC1   |
    |            +------+                      +------+            |
    |       QP 8 |RNIC 1|    SMC-R Link 1      |RNIC 2|  QP 64     |
    |RKey set|   |MAC MA|<-------------------->|MAC MB|   |RKey set|
    |X,Y,Z   |   |GID GA|                      |GID GB|   |Q,R,S,T |
    |       \/   +------+                      +------+  \/        |
    |+--------+         |                      |        +--------+ |
    || 3 RMBs |         |                      |        | 4 RMBs | |
    |+--------+         |                      |        +--------+ |
    |       /\   +------+                      +------+  /\        |
    |RKey set|   |RNIC 3|    SMC-R Link 2      |RNIC 4|  | RKey set|
    |U,V,W   |   |MAC MC|<-------------------->|MAC MD|  | L,M,N,P |
    |       QP 9 |GID GC|    (being added)     |GID GD| QP 65      |
    |            +------+                      +------+            |
    +-------------------+                      +-------------------+

            ADD LINK request (QP9,MC,GC, link number = 2)
            ............................................>

            ADD LINK response (QP65,MD,GD, link number = 2)
            <............................................

    ADD LINK CONTINUATION req(RToken pairs=((X,U),(Y,V),(Z,W)))
             ............................................>

    ADD LINK CONTINUATION rsp(RToken pairs=((Q,L),(R,M),(S,N),(T,P)))
             <.............................................

           CONFIRM LINK req/rsp exchange on Link 2
            <.............................................>


                          Legend:
                   ------------   TCP/IP and CLC flows
                   ............   RoCE (LLC) flows

   Figure 15: Exchanging RKeys when a New Link Is Added to a Link Group












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3.5.5.3.  Serialization of LLC Exchanges, and Collisions

   LLC flows can be divided into two main groups for serialization
   considerations.

   The first group is LLC messages that are independent and can flow at
   any time.  These are one-time, unsolicited messages that either do
   not have a required response or have a simple response that does not
   interfere with the operations of another group of messages.  These
   messages are as follows:

   o  TEST LINK from either the client or the server: This message
      requires a TEST LINK response to be returned but does not affect
      the configuration of the link group or the RKeys.

   o  ADD LINK from the client to the server: This message is provided
      as an "FYI" to the server to let it know that the client has an
      additional RNIC available.  The server is not required to act upon
      or respond to this message.

   o  DELETE LINK from the client to the server: This message informs
      the server that either (1) the client has experienced an error or
      problem that requires a link or link group to be terminated or
      (2) an operator has commanded that a link or link group be
      terminated.  The server does not respond directly to the message;
      rather, it initiates a DELETE LINK exchange as a result of
      receiving it.

   o  DELETE LINK from the server to the client, with the "delete entire
      link group" flag set: This message informs the client that the
      entire link group is being deleted.

   The second group is LLC messages that are part of an exchange of LLC
   messages that affects link group configuration; this exchange must
   complete before another exchange of LLC messages that affects link
   group configuration can be processed.  When a peer knows that one of
   these exchanges is in progress, it must not start another exchange.
   These exchanges are as follows:

   o  ADD LINK / ADD LINK response / ADD LINK CONTINUATION / ADD LINK
      CONTINUATION response / CONFIRM LINK / CONFIRM LINK response: This
      exchange, by adding a new link, changes the configuration of the
      link group.

   o  DELETE LINK / DELETE LINK response initiated by the server,
      without the "delete entire link group" flag set: This exchange, by
      deleting a link, changes the configuration of the link group.




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   o  CONFIRM RKEY / CONFIRM RKEY response or DELETE RKEY / DELETE RKEY
      response: This exchange changes the RMB configuration of the link
      group.  RKeys cannot change while links are being added or deleted
      (while an ADD LINK or DELETE LINK is in progress).  However,
      CONFIRM RKEY and DELETE RKEY are unique in that both the client
      and server can independently manage (add or remove) their own
      RMBs.  This allows each peer to concurrently change their RKeys
      and therefore concurrently send CONFIRM RKEY or DELETE RKEY
      requests.  The concurrent CONFIRM RKEY or DELETE RKEY requests can
      be independently processed and do not represent a collision.

   Because the server is in control of the configuration of the link
   group, many timing windows and collisions are avoided, but there are
   still some that must be handled.

3.5.5.3.1.  Collisions with ADD LINK / CONFIRM LINK Exchange

   Colliding LLC message: TEST LINK

      Action to resolve: Send immediate TEST LINK reply.

   Colliding LLC message: ADD LINK from client to server

      Action to resolve: Server ignores the ADD LINK message.  When
      client receives server's ADD LINK, client will consider that
      message to be in response to its ADD LINK message and the flow
      works.  Since both client and server know not to start this
      exchange if an ADD LINK operation is already underway, this can
      only occur if the client sends this message before receiving the
      server's ADD LINK and this message crosses with the server's ADD
      LINK message; therefore, the server's ADD LINK arrives at the
      client immediately after the client sent this message.

   Colliding LLC message: DELETE LINK from client to server, specific
   link specified

      Action to resolve: Server queues the DELETE LINK message and
      processes it after the ADD LINK exchange completes.  If it is an
      orderly link termination, it can wait until after this exchange
      continues.  If it is disorderly and the link affected is the one
      that the current exchange is using, the server will discover the
      outage when a message in this exchange fails.

   Colliding LLC message: DELETE LINK from client to server, entire link
   group to be deleted

      Action to resolve: Immediately clean up the link group.




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   Colliding LLC message: CONFIRM RKEY from client

      Action to resolve: Send a negative CONFIRM RKEY response to the
      client.  Once the current exchange finishes, client will have to
      recompute its RKey set to include the new link and then start a
      new CONFIRM RKEY exchange.

3.5.5.3.2.  Collisions during DELETE LINK Exchange

   Colliding LLC message: TEST LINK from either peer

      Action to resolve: Send immediate TEST LINK response.

   Colliding LLC message: ADD LINK from client to server

      Action to resolve: Server queues the ADD LINK and processes it
      after the current exchange completes.

   Colliding LLC message: DELETE LINK from client to server (specific
   link)

      Action to resolve: Server queues the DELETE LINK message and
      processes it after the current exchange completes.  If it is an
      orderly link termination, it can wait until after this exchange
      continues.  If it is disorderly and the link affected is the one
      that the current exchange is using, the server will discover the
      outage when a message in this exchange fails.

   Colliding LLC message: DELETE LINK from either client or server,
   deleting the entire link group

      Action to resolve: Immediately clean up the link group.

   Colliding LLC message: CONFIRM RKEY from client to server

      Action to resolve: Send a negative CONFIRM RKEY response to the
      client.  Once the current exchange finishes, client will have to
      recompute its RKey set to include the new link and then start a
      new CONFIRM RKEY exchange.












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3.5.5.3.3.  Collisions during CONFIRM RKEY Exchange

   Colliding LLC message: TEST LINK

      Action to resolve: Send immediate TEST LINK reply.

   Colliding LLC message: ADD LINK from client to server

      Action to resolve: Queue the ADD LINK, and process it after the
      current exchange completes.

   Colliding LLC message: ADD LINK from server to client (CONFIRM RKEY
   exchange was initiated by the client, and it crossed with the server
   initiating an ADD LINK exchange)

      Action to resolve: Process the ADD LINK.  Client will receive a
      negative CONFIRM RKEY from the server and will have to redo this
      CONFIRM RKEY exchange after the ADD LINK exchange completes.

   Colliding LLC message: DELETE LINK from client to server, specific
   link to be deleted (CONFIRM RKEY exchange was initiated by the
   server, and it crossed with the client's DELETE LINK request)

      Action to resolve: Server queues the DELETE LINK message and
      processes it after the CONFIRM RKEY exchange completes.  If it is
      an orderly link termination, it can wait until after this exchange
      continues.  If it is disorderly and the link affected is the one
      that the current exchange is using, the server will discover the
      outage when a message in this exchange fails.

   Colliding LLC message: DELETE LINK from server to client, specific
   link deleted (CONFIRM RKEY exchange was initiated by the client, and
   it crossed with the server's DELETE LINK)

      Action to resolve: Process the DELETE LINK.  Client will receive a
      negative CONFIRM RKEY from the server and will have to redo this
      CONFIRM RKEY exchange after the ADD LINK exchange completes.

   Colliding LLC message: DELETE LINK from either client or server,
   entire link group deleted

      Action to resolve: Immediately clean up the link group.

   Colliding LLC message: CONFIRM LINK from the peer that did not start
   the current CONFIRM LINK exchange

      Action to resolve: Queue the request, and process it after the
      current exchange completes.



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4.  SMC-R Memory-Sharing Architecture

4.1.  RMB Element Allocation Considerations

   Each TCP connection using SMC-R must be allocated an RMBE by each
   SMC-R peer.  This allocation is performed by each endpoint
   independently to allow each endpoint to select an RMBE that best
   matches the characteristics on its TCP socket endpoint.  The RMBE
   associated with a TCP socket endpoint must have a receive buffer that
   is at least as large as the TCP receive buffer size in effect for
   that connection.  The receive buffer size can be determined by what
   is specified explicitly by the application using setsockopt() or
   implicitly via the system-configured default value.  This will allow
   sufficient data to be RDMA-written by the SMC-R peer to fill an
   entire receive buffer size's worth of data on a given data flow.
   Given that each RMB must have fixed-length RMBEs, this implies that
   an SMC-R endpoint may need to maintain multiple RMBs of various sizes
   for SMC-R connections on a given SMC-R link and can then select an
   RMBE that most closely fits a connection.

4.2.  RMB and RMBE Format

   An RMB is a virtual memory buffer whose backing real memory is
   pinned.  The RMB is subdivided into a whole number of equal-sized RMB
   Elements (RMBEs).  Each RMBE begins with a 4-byte eye catcher for
   diagnostic and service purposes, followed by the receive data buffer.
   The contents of this diagnostic eye catcher are implementation
   dependent and should be used by the local SMC-R peer to check for
   overlay errors by verifying an intact eye catcher with every RMBE
   access.

   The RMBE is a wrapping receive buffer for receiving RDMA writes from
   the peer.  Cursors, as described below, are exchanged between peers
   to manage and track RDMA writes and local data reads from the RMBE
   for a TCP connection.

4.3.  RMBE Control Information

   RMBE control information consists of consumer cursors, producer
   cursors, wrap counts, CDC message sequence numbers, control flags
   such as urgent data and "writer blocked" indicators, and TCP
   connection information such as termination flags.  This information
   is exchanged between SMC-R peers using CDC messages, which are passed
   using RoCE SendMsg.  A TCP/IP stack implementing SMC-R must receive
   and store this information in its internal data structures, as it is
   used to manage the RMBE and its data buffer.





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   The format and contents of the CDC message are described in detail in
   Appendix A.4 ("Connection Data Control (CDC) Message Format").  The
   following is a high-level description of what this control
   information contains.

   o  Connection state flags such as sending done, connection closed,
      failover data validation, and abnormal close.

   o  A sequence number that is managed by the sender.  This sequence
      number starts at 1, is increased each send, and wraps to 0.  This
      sequence number tracks the CDC message sent and is not related to
      the number of bytes sent.  It is used for failover data
      validation.

   o  Producer cursor: a wrapping offset into the receiver's RMBE data
      area.  Set by the peer that is writing into the RMBE, it points to
      where the writing peer will write the next byte of data into an
      RMBE.  This cursor is accompanied by a wrap sequence number to
      help the RMBE owner (the receiver) identify full window size
      wrapping writes.  Note that this cursor must account for (i.e.,
      skip over) the RMBE eye catcher that is in the beginning of the
      data area.

   o  Consumer cursor: a wrapping offset into the receiver's RMBE data
      area.  Set by the owner of the RMBE (the peer that is reading from
      it), this cursor points to the offset of the next byte of data to
      be consumed by the peer in its own RMBE.  The sender cannot write
      beyond this cursor into the receiver's RMBE without causing data
      loss.  Like the producer cursor, this is accompanied by a wrap
      count to help the writer identify full window size wrapping reads.
      Note that this cursor must account for (i.e., skip over) the RMBE
      eye catcher that is in the beginning of the data area.

   o  Data flags such as urgent data, writer blocked indicator, and
      cursor update requests.

4.4.  Use of RMBEs

4.4.1.  Initializing and Accessing RMBEs

   The RMBE eye catcher is initialized by the RMB owner prior to
   assigning it to a specific TCP connection and communicating its RMB
   index to the SMC-R partner.  After an RMBE index is communicated to
   the SMC-R partner, the RMBE can only be referenced in "read-only
   mode" by the owner, and all updates to it are performed by the remote
   SMC-R partner via RDMA write operations.





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   Initialization of an RMBE must include the following:

   o  Zeroing out the entire RMBE receive buffer, which helps minimize
      data integrity issues (e.g., data from a previous connection
      somehow being presented to the current connection).

   o  Setting the beginning RMBE eye catcher.  This eye catcher plays an
      important role in helping detect accidental overlays of the RMBE.
      The RMB owner should always validate these eye catchers before
      each new reference to the RMBE.  If the eye catchers are found to
      be corrupted, the local host must reset the TCP connection
      associated with this RMBE and log the appropriate diagnostic
      information.

4.4.2.  RMB Element Reuse and Conflict Resolution

   RMB elements can be reused once their associated TCP and SMC-R
   connections are terminated.  Under normal and abnormal SMC-R
   connection termination processing, both SMC-R peers must explicitly
   acknowledge that they are done using an RMBE before that element can
   be freed and reassigned to another SMC-R connection instance.  For
   more details on SMC-R connection termination, refer to Section 4.8.

   However, there are some error scenarios where this two-way explicit
   acknowledgment may not be completed.  In these scenarios, an RMBE
   owner may choose to reassign this RMBE to a new SMC-R connection
   instance on this SMC-R link group.  When this occurs, the partner
   SMC-R peer must detect this condition during SMC-R Rendezvous
   processing when presented with an RMBE that it believes is already in
   use for a different SMC-R connection.  In this case, the SMC-R peer
   must abort the existing SMC-R connection associated with this RMBE.
   The abort processing resets the TCP connection (if it is still
   active), but it must not attempt to perform any RDMA writes to this
   RMBE and must also ignore any data sitting in the local RMBE
   associated with the existing connection.  It then proceeds to free up
   the local RMBE and notify the local application that the connection
   is being abnormally reset.

   The remote SMC-R peer then proceeds to normal processing for this new
   SMC-R connection.











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4.5.  SMC-R Protocol Considerations

   The following sections describe considerations for the SMC-R protocol
   as compared to TCP.

4.5.1.  SMC-R Protocol Optimized Window Size Updates

   An SMC-R receiver host sends its consumer cursor information to the
   sender to convey the progress that the receiving application has made
   in consuming the sent data.  The difference between the writer's
   producer cursor and the associated receiver's consumer cursor
   indicates the window size available for the sender to write into.
   This is somewhat similar to TCP window update processing and
   therefore has some similar considerations, such as silly window
   syndrome avoidance, whereby TCP has an optimization that minimizes
   the overhead of very small, unproductive window size updates
   associated with suboptimal socket applications consuming very small
   amounts of data on every receive() invocation.  For SMC-R, the
   receiver only updates its consumer cursor via a unique CDC message
   under the following conditions:

   o  The current window size (from a sender's perspective) is less than
      half of the receive buffer space, and the consumer cursor update
      will result in a minimum increase in the window size of 10% of the
      receive buffer space.  Some examples:

      a. Receive buffer size: 64K, current window size (from a sender's
         perspective): 50K.  No need to update the consumer cursor.
         Plenty of space is available for the sender.

      b. Receive buffer size: 64K, current window size (from a sender's
         perspective): 30K, current window size from a receiver's
         perspective: 31K.  No need to update the consumer cursor; even
         though the sender's window size is < 1/2 of the 64K, the window
         update would only increase that by 1K, which is < 1/10th of the
         64K buffer size.

      c. Receive buffer size: 64K, current window size (from a sender's
         perspective): 30K, current window size from a receiver's
         perspective: 64K.  The receiver updates the consumer cursor
         (sender's window size is < 1/2 of the 64K; the window update
         would increase that by > 6.4K).









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   o  The receiver must always include a consumer cursor update whenever
      it sends a CDC message to the partner for another flow (i.e., send
      flow in the opposite direction).  This allows the window size
      update to be delivered with no additional overhead.  This is
      somewhat similar to TCP DelayAck processing and quite effective
      for request/response data patterns.

   o  If a peer has set the B-bit in a CDC message, then any consumption
      of data by the receiver causes a CDC message to be sent, updating
      the consumer cursor until a CDC message with that bit cleared is
      received from the peer.

   o  The optimized window size updates are overridden when the sender
      sets the Consumer Cursor Update Requested flag in a CDC message to
      the receiver.  When this indicator is on, the consumer must send a
      consumer cursor update immediately when data is consumed by the
      local application or if the cursor has not been updated for a
      while (i.e., local copy of the consumer cursor does not match the
      last consumer cursor value sent to the partner).  This allows the
      sender to perform optional diagnostics for detecting a stalled
      receiver application (data has been sent but not consumed).  It is
      recommended that the Consumer Cursor Update Requested flag only be
      sent for diagnostic procedures, as it may result in non-optimal
      data path performance.

4.5.2.  Small Data Sends

   The SMC-R protocol makes no special provisions for handling small
   data segments sent across a stream socket.  Data is always sent if
   sufficient window space is available.  In contrast to the TCP Nagle
   algorithm, there are no special provisions in SMC-R for coalescing
   small data segments.

   An implementation of SMC-R can be configured to optimize its sending
   processing by coalescing outbound data for a given SMC-R connection
   so that it can reduce the number of RDMA write operations it
   performs, in a fashion similar to Nagle's algorithm.  However, any
   such coalescing would require a timer on the sending host that would
   ensure that data was eventually sent.  Also, the sending host would
   have to opt out of this processing if Nagle's algorithm had been
   disabled (programmatically or via system configuration).










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4.5.3.  TCP Keepalive Processing

   TCP keepalive processing allows applications to direct the local
   TCP/IP host to periodically "test" the viability of an idle TCP
   connection.  Since SMC-R connections have a TCP representation along
   with an SMC-R representation, there are unique keepalive processing
   considerations:

   o  SMC-R-layer keepalive processing: If keepalive is enabled for an
      SMC-R connection, the local host maintains a keepalive timer that
      reflects how long an SMC-R connection has been idle.  The local
      host also maintains a timestamp of last activity for each SMC-R
      link (for any SMC-R connection on that link).  When it is
      determined that an SMC-R connection has been idle longer than the
      keepalive interval, the host checks to see whether or not the
      SMC-R link has been idle for a duration longer than the keepalive
      timeout.  If both conditions are met, the local host then performs
      a TEST LINK LLC command to test the viability of the SMC-R link
      over the RoCE fabric (RC-QPs).  If a TEST LINK LLC command
      response is received within a reasonable amount of time, then the
      link is considered viable, and all connections using this link are
      considered viable as well.  If, however, a response is not
      received in a reasonable amount of time or there's a failure in
      sending the TEST LINK LLC command, then this is considered a
      failure in the SMC-R link, and failover processing to an alternate
      SMC-R link must be triggered.  If no alternate SMC-R link exists
      in the SMC-R link group, then all of the SMC-R connections on this
      link are abnormally terminated by resetting the TCP connections
      represented by these SMC-R connections.  Given that multiple SMC-R
      connections can share the same SMC-R link, implementing an SMC-R
      link-level probe using the TEST LINK LLC command will help reduce
      the amount of unproductive keepalive traffic for SMC-R
      connections; as long as some SMC-R connections on a given SMC-R
      link are active (i.e., have had I/O activity within the keepalive
      interval), then there is no need to perform additional link
      viability testing.















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   o  TCP-layer keepalive processing: Traditional TCP "keepalive"
      packets are not as relevant for SMC-R connections, given that the
      TCP path is not used for these connections once the SMC-R
      Rendezvous processing is completed.  All SMC-R connections by
      default have associated TCP connections that are idle.  Are TCP
      keepalive probes still needed for these connections?  There are
      two main scenarios to consider:

      1. TCP keepalives that are used to determine whether or not the
         peer TCP endpoint is still active.  This is not needed for
         SMC-R connections, as the SMC-R-level keepalives mentioned
         above will determine whether or not the remote endpoint
         connections are still active.

      2. TCP keepalives that are used to ensure that TCP connections
         traversing an intermediate proxy maintain an active state.  For
         example, stateful firewalls typically maintain state
         representing every valid TCP connection that traverses the
         firewall.  These types of firewalls are known to expire idle
         connections by removing their state in the firewall to conserve
         memory.  TCP keepalives are often used in this scenario to
         prevent firewalls from timing out otherwise idle connections.
         When using SMC-R, both endpoints must reside in the same
         Layer 2 network (i.e., the same subnet).  As a result,
         firewalls cannot be injected in the path between two SMC-R
         endpoints.  However, other intermediate proxies, such as
         TCP/IP-layer load balancers, may be injected in the path of two
         SMC-R endpoints.  These types of load balancers also maintain
         connection state so that they can forward TCP connection
         traffic to the appropriate cluster endpoint.  When using SMC-R,
         these TCP connections will appear to be completely idle, making
         them susceptible to potential timeouts at the load-balancing
         proxy.  As a result, for this scenario, TCP keepalives may
         still be relevant.

   The following are the TCP-level keepalive processing requirements for
   SMC-R-enabled hosts:

   o  SMC-R peers should allow TCP keepalives to flow on the TCP path of
      SMC-R connections based on existing TCP keepalive configuration
      and programming options.  However, it is strongly recommended that
      platforms provide the ability to specify very granular keepalive
      timers (for example, single-digit-second timers) and should
      consider providing a configuration option that limits the minimum
      keepalive timer that will be used for TCP-layer keepalives on
      SMC-R connections.  This is important to minimize the amount of
      TCP keepalive packets transmitted in the network for SMC-R
      connections.



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   o  SMC-R peers must always respond to inbound TCP-layer keepalives
      (by sending ACKs for these packets) even if the connection is
      using SMC-R.  Typically, once a TCP connection has completed the
      SMC-R Rendezvous processing and is using SMC-R for data flows, no
      new inbound TCP segments are expected on that TCP connection,
      other than TCP termination segments (FIN, RST, etc.).  TCP
      keepalives are the one exception that must be supported.  Also,
      since TCP keepalive probes do not carry any application-layer
      data, this has no adverse impact on the application's inbound data
      stream.

4.6.  TCP Connection Failover between SMC-R Links

   A peer may change which SMC-R link within a link group it sends its
   writes over in the event of a link failure.  Since each peer
   independently chooses which link to send writes over for a specific
   TCP connection, this process is done independently by each peer.

4.6.1.  Validating Data Integrity

   Even though RoCE is a reliable transport, there is a small subset of
   failure modes that could cause unrecoverable loss of data.  When an
   RNIC acknowledges receipt of an RDMA write to its peer, that creates
   a write completion event to the sending peer, which allows the sender
   to release any buffers it is holding for that write.  In normal
   operation and in most failures, this operation is reliable.

   However, there are failure modes possible in which a receiving RNIC
   has acknowledged an RDMA write but then was not able to place the
   received data into its host memory -- for example, a sudden,
   disorderly failure of the interface between the RNIC and the host.
   While rare, these types of events must be guarded against to ensure
   data integrity.  The process for switching SMC-R links during
   failover, as described in this section, guards against this
   possibility and is mandatory.

   Each peer must track the current state of the CDC sequence numbers
   for a TCP connection.  The sender must keep track of the sequence
   number of the CDC message that described the last write acknowledged
   by the peer RNIC, or Sequence Sent (SS).  In other words, SS
   describes the last write that the sender believes its peer has
   successfully received.  The receiver must keep track of the sequence
   number of the CDC message that described the last write that it has
   successfully received (i.e., the data has been successfully placed
   into an RMBE), or Sequence Received (SR).






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   When an RNIC fails and the sender changes SMC-R links, the sender
   must first send a CDC message with the F-bit (failover validation
   indicator; see Appendix A.4) set over the new SMC-R link.  This is
   the failover data validation message.  The sequence number in this
   CDC message is equal to SS.  The CDC message key, the length, and the
   SMC-R alert token are the only other fields in this CDC message that
   are significant.  No reply is expected from this validation message,
   and once the sender has sent it, the sender may resume sending on the
   new SMC-R link as described in Section 4.6.2.

   Upon receipt of the failover validation message, the receiver must
   verify that its SR value for the TCP connection is equal to or
   greater than the sequence number in the failover validation message.
   If so, no further action is required, and the TCP connection resumes
   on the new SMC-R link.  If SR is less than the sequence number value
   in the validation message, data has been lost, and the receiver must
   immediately reset the TCP connection.

4.6.2.  Resuming the TCP Connection on a New SMC-R Link

   When a connection is moved to a new SMC-R link and the failover
   validation message has been sent, the sender can immediately resume
   normal transmission.  In order to preserve the application message
   stream, the sender must replay any RDMA writes (and their associated
   CDC messages) that were in progress or failed when the previous SMC-R
   link failed, before sending new data on the new SMC-R link.  The
   sender has two options for accomplishing this:

   o  Preserve the sequence numbers "as is": Retry all failed and
      pending operations as they were originally done, including
      reposting all associated RDMA write operations and their
      associated CDC messages without making any changes.  Then resume
      sending new data using new sequence numbers.

   o  Combine pending messages and possibly add new data: Combine failed
      and pending messages into a single new write with a new sequence
      number.  This allows the sender to combine pending messages into
      fewer operations.  As a further optimization, this write can also
      include new data, as long as all failed and pending data are also
      included.  If this approach is taken, the sequence number must be
      increased beyond the last failed or pending sequence number.










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4.7.  RMB Data Flows

   The following sections describe the RDMA wire flows for the SMC-R
   protocol after a TCP connection has switched into SMC-R mode (i.e.,
   SMC-R Rendezvous processing is complete and a pair of RMB elements
   has been assigned and communicated by the SMC-R peers).  The ladder
   diagrams below include the following:

   o  RMBE control information kept by each peer.  Only a subset of the
      information is depicted, specifically only the fields that reflect
      the stream of data written by Host A and read by Host B.

   o  Time line 0-x, which shows the wire flows in a time-relative
      fashion.

   o  Note that RMBE control information is only shown in a time
      interval if its value changed (otherwise, assume that the value is
      unchanged from the previously depicted value).

   o  The local copy of the producer cursors and consumer cursors that
      is maintained by each host is not depicted in these figures.  Note
      that the cursor values in the diagram reflect the necessity of
      skipping over the eye catcher in the RMBE data area.  They start
      and wrap at 4, not 0.

4.7.1.  Scenario 1: Send Flow, Window Size Unconstrained

            SMC Host A                             SMC Host B
           RMBE A Info                            RMBE B Info
       (Consumer Cursors)                      (Producer Cursors)
   Cursor   Wrap Seq# Time               Time Cursor   Wrap Seq#  Flags
   4        0         0                  0    4        0          0
   0        0         1 ---------------> 1    0        0          0
                        RDMA-WR Data
                          (4:1003)
   4        0         2 ...............> 2    1004     0          0
                        CDC Message

        Figure 16: Scenario 1: Send Flow, Window Size Unconstrained

   Scenario assumptions:

   o  Kernel implementation.

   o  New SMC-R connection; no data has been sent on the connection.






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   o  Host A: Application issues send for 1000 bytes to Host B.

   o  Host B: RMBE receive buffer size is 10,000; application has issued
      a recv for 10,000 bytes.

   Flow description:

   1. The application issues a send() for 1000 bytes; the SMC-R layer
      copies data into a kernel send buffer.  It then schedules an RDMA
      write operation to move the data into the peer's RMBE receive
      buffer, at relative position 4-1003 (to skip the 4-byte
      eye catcher in the RMBE data area).  Note that no immediate data
      or alert (i.e., interrupt) is provided to Host B for this RDMA
      operation.

   2. Host A sends a CDC message to update the producer cursor to
      byte 1004.  This CDC message will deliver an interrupt to Host B.
      At this point, the SMC-R layer can return control back to the
      application.  Host B, once notified of the completion of the
      previous RDMA operation, locates the RMBE associated with the RMBE
      alert token that was included in the message and proceeds to
      perform normal receive-side processing, waking up the suspended
      application read thread, copying the data into the application's
      receive buffer, etc.  It will use the producer cursor as an
      indicator of how much data is available to be delivered to the
      local application.  After this processing is complete, the SMC-R
      layer will also update its local consumer cursor to match the
      producer cursor (i.e., indicating that all data has been
      consumed).  Note that a message to the peer updating the consumer
      cursor is not needed at this time, as the window size is
      unconstrained (> 1/2 of the receive buffer size).  The window size
      is calculated by taking the difference between the producer cursor
      and the consumer cursor in the RMBEs (10,000 - 1004 = 8996).


















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4.7.2.  Scenario 2: Send/Receive Flow, Window Size Unconstrained

             SMC Host A                             SMC Host B
            RMBE A Info                            RMBE B Info
        (Consumer Cursors)                      (Producer Cursors)
    Cursor   Wrap Seq# Time               Time Cursor   Wrap Seq#  Flags
    4        0         0                  0    4        0          0
    0        0         1 ---------------> 1    0        0          0
                         RDMA-WR Data
                           (4:1003)
    4        0         2 ...............> 2    1004     0          0
                         CDC Message

    0        0         3 <--------------  3    1004     0          0
                         RDMA-WR Data
                           (4:503)
    1004     0         4 <..............  4    1004     0          0
                          CDC Message

    Figure 17: Scenario 2: Send/Receive Flow, Window Size Unconstrained

   Scenario assumptions:

   o  New SMC-R connection; no data has been sent on the connection.

   o  Host A: Application issues send for 1000 bytes to Host B.

   o  Host B: RMBE receive buffer size is 10,000; application has
      already issued a recv for 10,000 bytes.  Once the receive is
      completed, the application sends a 500-byte response to Host A.

   Flow description:

   1. The application issues a send() for 1000 bytes; the SMC-R layer
      copies data into a kernel send buffer.  It then schedules an RDMA
      write operation to move the data into the peer's RMBE receive
      buffer, at relative position 4-1003.  Note that no immediate data
      or alert (i.e., interrupt) is provided to Host B for this RDMA
      operation.

   2. Host A sends a CDC message to update the producer cursor to
      byte 1004.  This CDC message will deliver an interrupt to Host B.
      At this point, the SMC-R layer can return control back to the
      application.







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   3. Host B, once notified of the receipt of the previous CDC message,
      locates the RMBE associated with the RMBE alert token and proceeds
      to perform normal receive-side processing, waking up the suspended
      application read thread, copying the data into the application's
      receive buffer, etc.  After this processing is complete, the SMC-R
      layer will also update its local consumer cursor to match the
      producer cursor (i.e., indicating that all data has been
      consumed).  Note that an update of the consumer cursor to the peer
      is not needed at this time, as the window size is unconstrained
      (> 1/2 of the receive buffer size).  The application then performs
      a send() for 500 bytes to Host A.  The SMC-R layer will copy the
      data into a kernel buffer and then schedule an RDMA write into the
      partner's RMBE receive buffer.  Note that this RDMA write
      operation includes no immediate data or notification to Host A.

   4. Host B sends a CDC message to update the partner's RMBE control
      information with the latest producer cursor (set to 503 and not
      shown in the diagram above) and to also inform the peer that the
      consumer cursor value is now 1004.  It also updates the local
      current consumer cursor and the last sent consumer cursor to 1004.
      This CDC message includes notification, since we are updating our
      producer cursor; this requires attention by the peer host.

4.7.3.  Scenario 3: Send Flow, Window Size Constrained

             SMC Host A                             SMC Host B
            RMBE A Info                            RMBE B Info
        (Consumer Cursors)                      (Producer Cursors)
    Cursor   Wrap Seq# Time               Time Cursor   Wrap Seq#  Flags
    4        0         0                  0    4        0          0
    4        0         1 ---------------> 1    4        0          0
                         RDMA-WR Data
                           (4:3003)
    4        0         2 ...............> 2    3004     0          0
                         CDC Message
    4        0         3                  3    3004     0          0
    4        0         4 ---------------> 4    3004     0          0
                         RDMA-WR Data
                           (3004:7003)
    4        0         5 ................> 5   7004     0          0
                         CDC Message
    7004     0         6 <................ 6   7004     0          0
                         CDC Message

         Figure 18: Scenario 3: Send Flow, Window Size Constrained






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   Scenario assumptions:

   o  New SMC-R connection; no data has been sent on this connection.

   o  Host A: Application issues send for 3000 bytes to Host B and then
      another send for 4000 bytes.

   o  Host B: RMBE receive buffer size is 10,000.  Application has
      already issued a recv for 10,000 bytes.

   Flow description:

   1. The application issues a send() for 3000 bytes; the SMC-R layer
      copies data into a kernel send buffer.  It then schedules an RDMA
      write operation to move the data into the peer's RMBE receive
      buffer, at relative position 4-3003.  Note that no immediate data
      or alert (i.e., interrupt) is provided to Host B for this RDMA
      operation.

   2. Host A sends a CDC message to update its producer cursor to
      byte 3003.  This CDC message will deliver an interrupt to Host B.
      At this point, the SMC-R layer can return control back to the
      application.

   3. Host B, once notified of the receipt of the previous CDC message,
      locates the RMBE associated with the RMBE alert token and proceeds
      to perform normal receive-side processing, waking up the suspended
      application read thread, copying the data into the application's
      receive buffer, etc.  After this processing is complete, the SMC-R
      layer will also update its local consumer cursor to match the
      producer cursor (i.e., indicating that all data has been
      consumed).  It will not, however, update the partner with this
      information, as the window size is not constrained
      (10,000 - 3000 = 7000 bytes of available space).  The application
      on Host B also issues a new recv() for 10,000 bytes.

   4. On Host A, the application issues a send() for 4000 bytes.  The
      SMC-R layer copies the data into a kernel buffer and schedules an
      async RDMA write into the peer's RMBE receive buffer at relative
      position 3003-7004.  Note that no alert is provided to Host B for
      this flow.

   5. Host A sends a CDC message to update the producer cursor to
      byte 7004.  This CDC message will deliver an interrupt to Host B.
      At this point, the SMC-R layer can return control back to the
      application.





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   6. Host B, once notified of the receipt of the previous CDC message,
      locates the RMBE associated with the RMBE alert token and proceeds
      to perform normal receive-side processing, waking up the suspended
      application read thread, copying the data into the application's
      receive buffer, etc.  After this processing is complete, the SMC-R
      layer will also update its local consumer cursor to match the
      producer cursor (i.e., indicating that all data has been
      consumed).  It will then determine whether or not it needs to
      update the consumer cursor to the peer.  The available window size
      is now 3000 (10,000 - (producer cursor - last sent consumer
      cursor)), which is < 1/2 of the receive buffer size
      (10,000/2 = 5000), and the advance of the window size is > 10% of
      the window size (1000).  Therefore, a CDC message is issued to
      update the consumer cursor to Peer A.

4.7.4.  Scenario 4: Large Send, Flow Control, Full Window Size Writes

             SMC Host A                             SMC Host B
            RMBE A Info                            RMBE B Info
        (Consumer Cursors)                      (Producer Cursors)
    Cursor   Wrap Seq# Time               Time Cursor   Wrap Seq#  Flags
    1004     1         0                  0    1004     1          0
    1004     1         1 ---------------> 1    1004     1          0
                         RDMA-WR Data
                           (1004:9999)
    1004     1         2 ---------------> 2    1004     1          0
                         RDMA-WR Data
                           (4:1003)
    1004     1         3 ...............> 3    1004     2          Wrt
                         CDC Message                               Blk

    1004     2         4 <............... 4    1004     2          Wrt
                         CDC Message                               Blk

    1004     2         5 ---------------> 5    1004     2          Wrt
                         RDMA-WR Data                              Blk
                           (1004:9999)
    1004     2         6 ---------------> 6    1004     2          Wrt
                         RDMA-WR Data                              Blk
                          (4:1003)
    1004     2         7 ...............> 7    1004     3          Wrt
                         CDC Message                               Blk

    1004     3         8 <............... 8    1004     3          Wrt
                         CDC Message                               Blk

             Figure 19: Scenario 4: Large Send, Flow Control,
                          Full Window Size Writes



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   Scenario assumptions:

   o  Kernel implementation.

   o  Existing SMC-R connection, Host B's receive window size is fully
      open (peer consumer cursor = peer producer cursor).

   o  Host A: Application issues send for 20,000 bytes to Host B.

   o  Host B: RMBE receive buffer size is 10,000; application has issued
      a recv for 10,000 bytes.

   Flow description:

   1. The application issues a send() for 20,000 bytes; the SMC-R layer
      copies data into a kernel send buffer (assumes that send buffer
      space of 20,000 is available for this connection).  It then
      schedules an RDMA write operation to move the data into the peer's
      RMBE receive buffer, at relative position 1004-9999.  Note that no
      immediate data or alert (i.e., interrupt) is provided to Host B
      for this RDMA operation.

   2. Host A then schedules an RDMA write operation to fill the
      remaining 1000 bytes of available space in the peer's RMBE receive
      buffer, at relative position 4-1003.  Note that no immediate data
      or alert (i.e., interrupt) is provided to Host B for this RDMA
      operation.  Also note that an implementation of SMC-R may optimize
      this processing by combining steps 1 and 2 into a single
      RDMA write operation (with two different data sources).

   3. Host A sends a CDC message to update the producer cursor to
      byte 1004.  Since the entire receive buffer space is filled, the
      producer writer blocked flag (the "Wrt Blk" indicator (flag) in
      Figure 19) is set and the producer cursor wrap sequence number
      (the producer "Wrap Seq#" in Figure 19) is incremented.  This CDC
      message will deliver an interrupt to Host B.  At this point, the
      SMC-R layer can return control back to the application.

   4. Host B, once notified of the receipt of the previous CDC message,
      locates the RMBE associated with the RMBE alert token and proceeds
      to perform normal receive-side processing, waking up the suspended
      application read thread, copying the data into the application's
      receive buffer, etc.  In this scenario, Host B notices that the
      producer cursor has not been advanced (same value as the consumer
      cursor); however, it notices that the producer cursor wrap
      sequence number is different from its local value (1), indicating
      that a full window of new data is available.  All of the data in
      the receive buffer can be processed, with the first segment



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      (1004-9999) followed by the second segment (4-1003).  Because the
      producer writer blocked indicator was set, Host B schedules a CDC
      message to update its latest information to the peer: consumer
      cursor (1004), consumer cursor wrap sequence number (the current
      value of 2 is used).

   5. Host A, upon receipt of the CDC message, locates the TCP
      connection associated with the alert token and, upon examining the
      control information provided, notices that Host B has consumed all
      of the data (based on the consumer cursor and the consumer cursor
      wrap sequence number) and initiates the next RDMA write to fill
      the receive buffer at offset 1003-9999.

   6. Host A then moves the next 1000 bytes into the beginning of the
      receive buffer (4-1003) by scheduling an RDMA write operation.
      Note that at this point there are still 8 bytes remaining to be
      written.

   7. Host A then sends a CDC message to set the producer writer blocked
      indicator and to increment the producer cursor wrap sequence
      number (3).

   8. Host B, upon notification, completes the same processing as step 4
      above, including sending a CDC message to update the peer to
      indicate that all data has been consumed.  At this point, Host A
      can write the final 8 bytes to Host B's RMBE into
      positions 1004-1011 (not shown).
























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4.7.5.  Scenario 5: Send Flow, Urgent Data, Window Size Unconstrained

             SMC Host A                             SMC Host B
            RMBE A Info                            RMBE B Info
        (Consumer Cursors)                      (Producer Cursors)
    Cursor   Wrap Seq# Time               Time Cursor   Wrap Seq#  Flag
    1000     1         0                  0    1000     1          0
    1000     1         1 ---------------> 1    1000     1          0
                         RDMA-WR Data
                           (1000:1499)
    1000     1         2 ...............> 2    1500     1          UrgP
                         CDC Message                               UrgA

    1500     1         3 <............... 3    1500     1          UrgP
                         CDC Message                               UrgA

    1500     1         4 ---------------> 4    1500     1          UrgP
                         RDMA-WR Data                              UrgA
                           (1500:2499)
    1500     1         5 ...............> 5    2500     1          0
                         CDC Message

      Figure 20: Scenario 5: Send Flow, Urgent Data, Window Size Open

   Scenario assumptions:

   o  Kernel implementation.

   o  Existing SMC-R connection; window size open (unconstrained); all
      data has been consumed by receiver.

   o  Host A: Application issues send for 500 bytes with urgent data
      indicator (out of band) to Host B, then sends 1000 bytes of
      normal data.

   o  Host B: RMBE receive buffer size is 10,000; application has issued
      a recv for 10,000 bytes and is also monitoring the socket for
      urgent data.

   Flow description:

   1. The application issues a send() for 500 bytes of urgent data; the
      SMC-R layer copies data into a kernel send buffer.  It then
      schedules an RDMA write operation to move the data into the peer's
      RMBE receive buffer, at relative position 1000-1499.  Note that no
      immediate data or alert (i.e., interrupt) is provided to Host B
      for this RDMA operation.




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   2. Host A sends a CDC message to update its producer cursor to
      byte 1500 and to turn on the producer Urgent Data Pending (UrgP)
      and Urgent Data Present (UrgA) flags.  This CDC message will
      deliver an interrupt to Host B.  At this point, the SMC-R layer
      can return control back to the application.

   3. Host B, once notified of the receipt of the previous CDC message,
      locates the RMBE associated with the RMBE alert token, notices
      that the Urgent Data Pending flag is on, and proceeds with out-of-
      band socket API notification -- for example, satisfying any
      outstanding select() or poll() requests on the socket by
      indicating that urgent data is pending (i.e., by setting the
      exception bit on).  The urgent data present indicator allows
      Host B to also determine the position of the urgent data (the
      producer cursor points 1 byte beyond the last byte of urgent
      data).  Host B can then perform normal receive-side processing
      (including specific urgent data processing), copying the data into
      the application's receive buffer, etc.  Host B then sends a CDC
      message to update the partner's RMBE control area with its latest
      consumer cursor (1500).  Note that this CDC message must occur,
      regardless of the current local window size that is available.
      The partner host (Host A) cannot initiate any additional RDMA
      writes until it receives acknowledgment that the urgent data has
      been processed (or at least processed/remembered at the SMC-R
      layer).

   4. Upon receipt of the message, Host A wakes up, sees that the peer
      consumed all data up to and including the last byte of urgent
      data, and now resumes sending any pending data.  In this case, the
      application had previously issued a send for 1000 bytes of normal
      data, which would have been copied in the send buffer, and control
      would have been returned to the application.  Host A now initiates
      an RDMA write to move that data to the peer's receive buffer at
      position 1500-2499.

   5. Host A then sends a CDC message to update its producer cursor
      value (2500) and to turn off the Urgent Data Pending and Urgent
      Data Present flags.  Host B wakes up, processes the new data
      (resumes application, copies data into the application receive
      buffer), and then proceeds to update the local current consumer
      cursor (2500).  Given that the window size is unconstrained, there
      is no need for a consumer cursor update in the peer's RMBE.









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4.7.6.  Scenario 6: Send Flow, Urgent Data, Window Size Closed

             SMC Host A                             SMC Host B
            RMBE A Info                            RMBE B Info
        (Consumer Cursors)                      (Producer Cursors)
    Cursor   Wrap Seq# Time               Time Cursor   Wrap Seq#  Flag
    1000     1         0                  0    1000     2          Wrt
                                                                   Blk

    1000     1         1 ...............> 1    1000     2          Wrt
                         CDC Message                               Blk
                                                                   UrgP

    1000     2         2 <............... 2    1000     2          Wrt
                         CDC Message                               Blk
                                                                   UrgP

    1000     2         3 ---------------> 3    1000     2          Wrt
                         RDMA-WR Data                              Blk
                           (1000:1499)                             UrgP

    1000     2         4 ...............> 4    1500     2          UrgP
                         CDC Message                               UrgA

    1500     2         5 <............... 5    1500     2          UrgP
                         CDC Message                               UrgA

    1500     2         6 ---------------> 6    1500     2          UrgP
                         RDMA-WR Data                              UrgA
                           (1500:2499)
    1000     2         7 ...............> 7    2500     2          0
                         CDC Message

     Figure 21: Scenario 6: Send Flow, Urgent Data, Window Size Closed

   Scenario assumptions:

   o  Kernel implementation.

   o  Existing SMC-R connection; window size closed; writer is blocked.

   o  Host A: Application issues send for 500 bytes with urgent data
      indicator (out of band) to Host B, then sends 1000 bytes of
      normal data.

   o  Host B: RMBE receive buffer size is 10,000; application has no
      outstanding recv() (for normal data) and is monitoring the socket
      for urgent data.



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   Flow description:

   1. The application issues a send() for 500 bytes of urgent data; the
      SMC-R layer copies data into a kernel send buffer (if available).
      Since the writer is blocked (window size closed), it cannot send
      the data immediately.  It then sends a CDC message to notify the
      peer of the Urgent Data Pending (UrgP) indicator (the writer
      blocked indicator remains on as well).  This serves as a signal to
      Host B that urgent data is pending in the stream.  Control is also
      returned to the application at this point.

   2. Host B, once notified of the receipt of the previous CDC message,
      locates the RMBE associated with the RMBE alert token, notices
      that the Urgent Data Pending flag is on, and proceeds with out-of-
      band socket API notification -- for example, satisfying any
      outstanding select() or poll() requests on the socket by
      indicating that urgent data is pending (i.e., by setting the
      exception bit on).  At this point, it is expected that the
      application will enter urgent data mode processing, expeditiously
      processing all normal data (by issuing recv API calls) so that it
      can get to the urgent data byte.  Whether the application has this
      urgent mode processing or not, at some point, the application will
      consume some or all of the pending data in the receive buffer.
      When this occurs, Host B will also send a CDC message to update
      its consumer cursor and consumer cursor wrap sequence number to
      the peer.  In the example above, a full window's worth of data was
      consumed.

   3. Host A, once awakened by the message, will notice that the window
      size is now open on this connection (based on the consumer cursor
      and the consumer cursor wrap sequence number, which now matches
      the producer cursor wrap sequence number) and resume sending of
      the urgent data segment by scheduling an RDMA write into relative
      position 1000-1499.

   4. Host A then sends a CDC message to advance its producer cursor
      (1500) and to also notify Host B of the Urgent Data Present (UrgA)
      indicator (and turn off the writer blocked indicator).  This
      signals to Host B that the urgent data is now in the local receive
      buffer and that the producer cursor points to the last byte of
      urgent data.

   5. Host B wakes up, processes the urgent data, and, once the urgent
      data is consumed, sends a CDC message to update its consumer
      cursor (1500).






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   6. Host A wakes up, sees that Host B has consumed the sequence number
      associated with the urgent data, and then initiates the next RDMA
      write operation to move the 1000 bytes associated with the next
      send() of normal data into the peer's receive buffer at
      position 1500-2499.  Note that the send API would have likely
      completed earlier in the process by copying the 1000 bytes into a
      send buffer and returning back to the application, even though we
      could not send any new data until the urgent data was processed
      and acknowledged by Host B.

   7. Host A sends a CDC message to advance its producer cursor to 2500
      and to reset the Urgent Data Pending and Urgent Data Present
      flags.  Host B wakes up and processes the inbound data.

4.8.  Connection Termination

   Just as SMC-R connections are established using a combination of TCP
   connection establishment flows and SMC-R protocol flows, the
   termination of SMC-R connections also uses a similar combination of
   SMC-R protocol termination flows and normal TCP connection
   termination flows.  The following sections describe the SMC-R
   protocol normal and abnormal connection termination flows.

4.8.1.  Normal SMC-R Connection Termination Flows

   Normal SMC-R connection flows are triggered via the normal stream
   socket API semantics, namely by the application issuing a close() or
   shutdown() API.  Most applications, after consuming all incoming data
   and after sending any outbound data, will then issue a close() API to
   indicate that they are done both sending and receiving data.  Some
   applications, typically a small percentage, make use of the
   shutdown() API that allows them to indicate that the application is
   done sending data, receiving data, or both sending and receiving
   data.  The main use of this API is scenarios where a TCP application
   wants to alert its partner endpoint that it is done sending data but
   is still receiving data on its socket (shutdown for write).  Issuing
   shutdown() for both sending and receiving data is really no different
   than issuing a close() and can therefore be treated in a similar
   fashion.  Shutdown for read is typically not a very useful operation
   and in normal circumstances does not trigger any network flows to
   notify the partner TCP endpoint of this operation.

   These same trigger points will be used by the SMC-R layer to initiate
   SMC-R connection termination flows.  The main design point for SMC-R
   normal connection flows is to use the SMC-R protocol to first shut
   down the SMC-R connection and free up any SMC-R RDMA resources, and
   then allow the normal TCP connection termination protocol (i.e., FIN
   processing) to drive cleanup of the TCP connection.  This design



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   point is very important in ensuring that RDMA resources such as
   the RMBEs are only freed and reused when both SMC-R endpoints
   are completely done with their RDMA write operations to the
   partner's RMBE.

                                      1
                            +-----------------+
            |-------------->|     CLOSED      |<-------------|
        3D  |               |                 |              |  4D
            |               +-----------------+              |
            |                       |                        |
            |                     2 |                        |
            |                       V                        |
    +----------------+     +-----------------+     +----------------+
    |AppFinCloseWait |     |     ACTIVE      |     |PeerFinCloseWait|
    |                |     |                 |     |                |
    +----------------+     +-----------------+     +----------------+
            |                   |         |                   |
            |     Active Close  | 3A | 4A |  Passive Close    |
            |                   V    |    V                   |
            |       +--------------+ | +-------------+        |
            |--<----|PeerCloseWait1| | |AppCloseWait1|--->----|
        3C  |       |              | | |             |        |  4C
            |       +--------------+ | +-------------+        |
            |             |          |         |              |
            |             | 3B       |     4B  |              |
            |             V          |         V              |
            |       +--------------+ | +-------------+        |
            |--<----|PeerCloseWait2| | |AppCloseWait2|--->----|
                    |              | | |             |
                    +--------------+ | +-------------+
                                     |
                                     |

                    Figure 22: SMC-R Connection States

   Figure 22 describes the states that an SMC-R connection typically
   goes through.  Note that there are variations to these states that
   can occur when an SMC-R connection is abnormally terminated, similar
   in a way to when a TCP connection is reset.  The following are the
   high-level state transitions for an SMC-R connection:

   1. An SMC-R connection begins in the Closed state.  This state is
      meant to reflect an RMBE that is not currently in use (was
      previously in use but no longer is, or was never allocated).






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   2. An SMC-R connection progresses to the Active state once the SMC-R
      Rendezvous processing has successfully completed, RMB element
      indices have been exchanged, and SMC-R links have been activated.
      In this state, the TCP connection is fully established, rendezvous
      processing has been completed, and SMC-R peers can begin the
      exchange of data via RDMA.

   3. Active close processing (on the SMC-R peer that is initiating the
      connection termination).

      A. When an application on one of the SMC-R connection peers issues
         a close(), a shutdown() for write, or a shutdown() for both
         read and write, the SMC-R layer on that host will initiate
         SMC-R connection termination processing.  First, if a close()
         or shutdown(both) is issued, it will check to see that there's
         no data in the local RMB element that has not been read by the
         application.  If unread data is detected, the SMC-R connection
         must be abnormally reset; for more details on this, refer to
         Section 4.8.2 ("Abnormal SMC-R Connection Termination Flows").
         If no unread data is pending, it then checks to see whether or
         not any outstanding data is waiting to be written to the peer,
         or if any outstanding RDMA writes for this SMC-R connection
         have not yet completed.  If either of these two scenarios is
         true, an indicator that this connection is in a pending close
         state is saved in internal data structures representing this
         SMC-R connection, and control is returned to the application.
         If all data to be written to the partner has completed, this
         peer will send a CDC message to notify the peer of either the
         PeerConnectionClosed indicator (close or shutdown for both was
         issued) or the PeerDoneWriting indicator.  This will provide an
         interrupt to inform that partner SMC-R peer that the connection
         is terminating.  At this point, the local side of the SMC-R
         connection transitions in the PeerCloseWait1 state, and control
         can be returned to the application.  If this process could not
         be completed synchronously (the pending close condition
         mentioned above), it is completed when all RDMA writes for data
         and control cursors have been completed.

      B. At some point, the SMC-R peer application (passive close) will
         consume all incoming data, realize that that partner is done
         sending data on this connection, and proceed to initiate its
         own close of the connection once it has completed sending all
         data from its end.  The partner application can initiate this
         connection termination processing via close() or shutdown()
         APIs.  If the application does so by issuing a shutdown() for
         write, then the partner SMC-R layer will send a CDC message to
         notify the peer (the active close side) of the PeerDoneWriting
         indicator.  When the "active close" SMC-R peer wakes up as a



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         result of the previous CDC message, it will notice that the
         PeerDoneWriting indicator is now on and transition to the
         PeerCloseWait2 state.  This state indicates that the peer is
         done sending data and may still be reading data.  At this
         point, the "active close" peer will also need to ensure that
         any outstanding recv() calls for this socket are woken up and
         remember that no more data is forthcoming on this connection
         (in case the local connection was shutdown() for write only).

      C. This flow is a common transition from 3A or 3B above.  When the
         SMC-R peer (passive close) consumes all data and updates all
         necessary cursors to the peer, and the application closes its
         socket (close or shutdown for both), it will send a CDC message
         to the peer (the active close side) with the
         PeerConnectionClosed indicator set.  At this point, the
         connection can transition back to the Closed state if the local
         application has already closed (or issued shutdown for both)
         the socket.  Once in the Closed state, the RMBE can now be
         safely reused for a new SMC-R connection.  When the
         PeerConnectionClosed indicator is turned on, the SMC-R peer is
         indicating that it is done updating the partner's RMBE.

      D. Conditional state: If the local application has not yet issued
         a close() or shutdown(both), we need to wait until the
         application does so.  Once it does, the local host will send a
         CDC message to notify the peer of the PeerConnectionClosed
         indicator and then transition to the Closed state.

   4. Passive close processing (on the SMC-R peer that receives an
      indication that the partner is closing the connection).

      A. Upon receipt of a CDC message, the SMC-R layer will detect that
         the PeerConnectionClosed indicator or PeerDoneWriting indicator
         is on.  If any outstanding recv() calls are pending, they are
         completed with an indicator that the partner has closed the
         connection (zero-length data presented to the application).  If
         there is any pending data to be written and
         PeerConnectionClosed is on, then an SMC-R connection reset must
         be performed.  The connection then enters the AppCloseWait1
         state on the passive close side waiting for the local
         application to initiate its own close processing.

      B. If the local application issues a shutdown() for writing, then
         the SMC-R layer will send a CDC message to notify the partner
         of the PeerDoneWriting indicator and then transition the local
         side of the SMC-R connection to the AppCloseWait2 state.





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      C. When the application issues a close() or shutdown() for both,
         the local SMC-R peer will send a message informing the peer of
         the PeerConnectionClosed indicator and transition to the Closed
         state if the remote peer has also sent the local peer the
         PeerConnectionClosed indicator.  If the peer has not sent the
         PeerConnectionClosed indicator, we transition into the
         PeerFinCloseWait state.

      D. The local SMC-R connection stays in this state until the peer
         sends the PeerConnectionClosed indicator in a CDC message.
         When the indicator is sent, we transition to the Closed state
         and are then free to reuse this RMBE.

   Note that each SMC-R peer needs to provide some logic that will
   prevent being stranded in a termination state indefinitely.  For
   example, if an Active Close SMC-R peer is in a PeerCloseWait (1 or 2)
   state waiting for the remote SMC-R peer to update its connection
   termination status, it needs to provide a timer that will prevent it
   from waiting in that state indefinitely should the remote SMC-R peer
   not respond to this termination request.  This could occur in error
   scenarios -- for example, if the remote SMC-R peer suffered a failure
   prior to being able to respond to the termination request or the
   remote application is not responding to this connection termination
   request by closing its own socket.  This latter scenario is similar
   to the TCP FINWAIT2 state, which has been known to sometimes cause
   issues when remote TCP/IP hosts lose track of established connections
   and neglect to close them.  Even though the TCP standards do not
   mandate a timeout from the TCP FINWAIT2 state, most TCP/IP
   implementations assign a timeout for this state.  A similar timeout
   will be required for SMC-R connections.  When this timeout occurs,
   the local SMC-R peer performs TCP reset processing for this
   connection.  However, no additional RDMA writes to the partner RMBE
   can occur at this point (we have already indicated that we are done
   updating the peer's RMBE).  After the TCP connection is reset, the
   RMBE can be returned to the free pool for reallocation.  See
   Section 4.4.2 for more details.

   Also note that it is possible to have two SMC-R endpoints initiate an
   Active close concurrently.  In that scenario, the flows above still
   apply; however, both endpoints follow the active close path (path 3).











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4.8.2.  Abnormal SMC-R Connection Termination Flows

   Abnormal SMC-R connection termination can occur for a variety of
   reasons, including the following:

   o  The TCP connection associated with an SMC-R connection is reset.
      In TCP, either endpoint can send a RST segment to abort an
      existing TCP connection when error conditions are detected for the
      connection or the application overtly requests that the connection
      be reset.

   o  Normal SMC-R connection termination processing has unexpectedly
      stalled for a given connection.  When the stall is detected
      (connection termination timeout condition), an abnormal SMC-R
      connection termination flow is initiated.

   In these scenarios, it is very important that resources associated
   with the affected SMC-R connections are properly cleaned up to ensure
   that there are no orphaned resources and that resources can reliably
   be reused for new SMC-R connections.  Given that SMC-R relies heavily
   on the RDMA write processing, special care needs to be taken to
   ensure that an RMBE is no longer being used by an SMC-R peer before
   logically reassigning that RMBE to a new SMC-R connection.

   When an SMC-R peer initiates a TCP connection reset, it also
   initiates an SMC-R abnormal connection flow at the same time.  The
   SMC-R peers explicitly signal their intent to abnormally terminate an
   SMC-R connection and await explicit acknowledgment that the peer has
   received this notification and has also completed abnormal connection
   termination on its end.  Note that TCP connection reset processing
   can occur in parallel to these flows.




















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                            +-----------------+
            |-------------->|     CLOSED      |<-------------|
            |               |                 |              |
            |               +-----------------+              |
            |                                                |
            |                                                |
            |                                                |
            |           +-----------------------+            |
            |           |     Any state         |            |
            |1B         | (before setting       |          2B|
            |           |  PeerConnectionClosed |            |
            |           |  indicator in         |            |
            |           |  peer's RMBE)         |            |
            |           +-----------------------+            |
            |         1A        |         |      2A          |
            |     Active Abort  |         |  Passive Abort   |
            |                   V         V                  |
            |       +--------------+   +--------------+      |
            |-------|PeerAbortWait |   | Process Abort|------|
                    |              |   |              |
                    +--------------+   +--------------+

      Figure 23: SMC-R Abnormal Connection Termination State Diagram

   Figure 23 above shows the SMC-R abnormal connection termination state
   diagram:

   1. Active abort designates the SMC-R peer that is initiating the TCP
      RST processing.  At the time that the TCP RST is sent, the active
      abort side must also do the following:

      A. Send the PeerConnAbort indicator to the partner in a CDC
         message, and then transition to the PeerAbortWait state.
         During this state, it will monitor this SMC-R connection
         waiting for the peer to send its corresponding PeerConnAbort
         indicator but will ignore any other activity in this connection
         (i.e., new incoming data).  It will also generate an
         appropriate error to any socket API calls issued against this
         socket (e.g., ECONNABORTED, ECONNRESET).

      B. Once the peer sends the PeerConnAbort indicator to the local
         host, the local host can transition this SMC-R connection to
         the Closed state and reuse this RMBE.  Note that the SMC-R peer
         that goes into the active abort state must provide some
         protection against staying in that state indefinitely should
         the remote SMC-R peer not respond by sending its own
         PeerConnAbort indicator to the local host.  While this should
         be a rare scenario, it could occur if the remote SMC-R peer



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         (passive abort) suffered a failure right after the local SMC-R
         peer (active abort) sent the PeerConnAbort indicator.  To
         protect against these types of failures, a timer can be set
         after entering the PeerAbortWait state, and if that timer pops
         before the peer has sent its local PeerConnAbort indicator (to
         the active abort side), this RMBE can be returned to the free
         pool for possible reallocation.  See Section 4.4.2 for more
         details.

   2. Passive abort designates the SMC-R peer that is the recipient of
      an SMC-R abort from the peer designated by the PeerConnAbort
      indicator being sent by the peer in a CDC message.  Upon receiving
      this request, the local peer must do the following:

      A. Using the appropriate error codes, indicate to the socket
         application that this connection has been aborted, and then
         purge all in-flight data for this connection that is waiting to
         be read or waiting to be sent.

      B. Send a CDC message to notify the peer of the PeerConnAbort
         indicator and, once that is completed, transition this RMBE to
         the Closed state.

   If an SMC-R peer receives a TCP RST for a given SMC-R connection, it
   also initiates SMC-R abnormal connection termination processing if it
   has not already been notified (via the PeerConnAbort indicator) that
   the partner is severing the connection.  It is possible to have two
   SMC-R endpoints concurrently be in an active abort role for a given
   connection.  In that scenario, the flows above still apply but both
   endpoints take the active abort path (path 1).

4.8.3.  Other SMC-R Connection Termination Conditions

   The following are additional conditions that have implications for
   SMC-R connection termination:

   o  An SMC-R peer being gracefully shut down.  If an SMC-R peer
      supports a graceful shutdown operation, it should attempt to
      terminate all SMC-R connections as part of shutdown processing.
      This could be accomplished via LLC DELETE LINK requests on all
      active SMC-R links.

   o  Abnormal termination of an SMC-R peer.  In this example, there may
      be no opportunity for the host to perform any SMC-R cleanup
      processing.  In this scenario, it is up to the remote peer to
      detect a RoCE communications failure with the failing host.  This





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      could trigger SMC-R link switchover, but that would also generate
      RoCE errors, causing the remote host to eventually terminate all
      existing SMC-R connections to this peer.

   o  Loss of RoCE connectivity between two SMC-R peers.  If two peers
      are no longer reachable across any links in their SMC-R link
      group, then both peers perform a TCP reset for the connections,
      generate an error to the local applications, and free up all QP
      resources associated with the link group.

5.  Security Considerations

5.1.  VLAN Considerations

   The concepts and access control of virtual LANs (VLANs) must be
   extended to also cover the RoCE network traffic flowing across the
   Ethernet.

   The RoCE VLAN configuration and access permissions must mirror the IP
   VLAN configuration and access permissions over the Converged Enhanced
   Ethernet fabric.  This means that hosts, routers, and switches that
   have access to specific VLANs on the IP fabric must also have the
   same VLAN access across the RoCE fabric.  In other words, the SMC-R
   connectivity will follow the same virtual network access permissions
   as normal TCP/IP traffic.

5.2.  Firewall Considerations

   As mentioned above, the RoCE fabric inherits the same VLAN
   topology/access as the IP fabric.  RoCE is a Layer 2 protocol that
   requires both endpoints to reside in the same Layer 2 network (i.e.,
   VLAN).  RoCE traffic cannot traverse multiple VLANs, as there is no
   support for routing RoCE traffic beyond a single VLAN.  As a result,
   SMC-R communications will also be confined to peers that are members
   of the same VLAN.  IP-based firewalls are typically inserted between
   VLANs (or physical LANs) and rely on normal IP routing to insert
   themselves in the data path.  Since RoCE (and by extension SMC-R) is
   not routable beyond the local VLAN, there is no ability to insert a
   firewall in the network path of two SMC-R peers.

5.3.  Host-Based IP Filters

   Because SMC-R maintains the TCP three-way handshake for connection
   setup before switching to RoCE out of band, existing IP filters that
   control connection setup flows remain effective in an SMC-R
   environment.  IP filters that operate on traffic flowing in an active
   TCP connection are not supported, because the connection data does
   not flow over IP.



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5.4.  Intrusion Detection Services

   Similar to IP filters, intrusion detection services that operate on
   TCP connection setups are compatible with SMC-R with no changes
   required.  However, once the TCP connection has switched to RoCE out
   of band, packets are not available for examination.

5.5.  IP Security (IPsec)

   IP security is not compatible with SMC-R, because there are no IP
   packets on which to operate.  TCP connections that require IP
   security must opt out of SMC-R.

5.6.  TLS/SSL

   Transport Layer Security/Secure Socket Layer (TLS/SSL) is preserved
   in an SMC-R environment.  The TLS/SSL layer resides above the SMC-R
   layer, and outgoing connection data is encrypted before being passed
   down to the SMC-R layer for RDMA write.  Similarly, incoming
   connection data goes through the SMC-R layer encrypted and is
   decrypted by the TLS/SSL layer as it is today.

   The TLS/SSL handshake messages flow over the TCP connection after the
   connection has switched to SMC-R, and so they are exchanged using
   RDMA writes by the SMC-R layer, transparently to the TLS/SSL layer.

6.  IANA Considerations

   The scarcity of TCP option codes available for assignment is
   understood, and this architecture uses experimental TCP options
   following the conventions of [RFC6994] ("Shared Use of Experimental
   TCP Options").

   TCP ExID 0xE2D4C3D9 has been registered with IANA as a TCP Experiment
   Identifier.  See Section 3.1.

   If this protocol achieves wide acceptance, a discrete option code may
   be requested by subsequent versions of this protocol.













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7.  Normative References

   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.

   [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options",
              RFC 6994, DOI 10.17487/RFC6994, August 2013,
              <http://www.rfc-editor.org/info/rfc6994>.

   [RoCE]     InfiniBand, "RDMA over Converged Ethernet specification",
              <https://cw.infinibandta.org/wg/Members/documentRevision/
              download/7149>.






































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Appendix A.  Formats

A.1.  TCP Option

   The SMC-R TCP option is formatted in accordance with [RFC6994]
   ("Shared Use of Experimental TCP Options").  The ExID value is
   IBM-1047 (EBCDIC) encoding for "SMCR".

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Kind = 254  | Length = 6    |   x'E2'       |   x'D4'       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    x'C3'      |    x'D9'      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 24: SMC-R TCP Option Format

A.2.  CLC Messages

   The following rules apply to all CLC messages:

   General rules on formats:

   o  Reserved fields must be set to zero and not validated.

   o  Each message has an eye catcher at the start and another
      eye catcher at the end.  These must both be validated by the
      receiver.

   o  SMC version indicator: The only SMC-R version defined in this
      architecture is version 1.  In the future, if peers have a
      mismatch of versions, the lowest common version number is used.


















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A.2.1.  Peer ID Format

   All CLC messages contain a peer ID that uniquely identifies an
   instance of a TCP/IP stack.  This peer ID is required to be
   universally unique across TCP/IP stacks and instances (including
   restarts) of TCP/IP stacks.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Instance ID          |    RoCE MAC (first 2 bytes)   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    RoCE MAC (last 4 bytes)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 25: Peer ID Format

   Instance ID

      A 2-byte instance count that ensures that if the same RNIC MAC is
      later used in the peer ID for a different TCP/IP stack -- for
      example, if an RNIC is redeployed to another stack -- the values
      are unique.  It also ensures that if a TCP/IP stack is restarted,
      the instance ID changes.  The value is implementation defined,
      with one suggestion being 2 bytes of the system clock.

   RoCE MAC

      The RoCE MAC address for one of the peer's RNICs.  Note that in a
      virtualized environment this will be the virtual MAC of one of the
      peer's RNICs.




















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A.2.2.  SMC Proposal CLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   x'E2'       |   x'D4'       |     x'C3'     |     x'D9'     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 1     |           Length              |Version| Rsrvd |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                       Client's Peer ID                      -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                Client's preferred GID                       -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Client's preferred RoCE                                      |
     +- MAC address                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |Offset to mask/prefix area (0) |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                  Area for future growth                       .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         IPv4 Subnet Mask                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | IPv4 Mask Lgth|           Reserved            |Num IPv6 prfx  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                                                               :
     :           Array of IPv6 prefixes (variable length)            :
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   x'E2'       |   x'D4'       |     x'C3'     |     x'D9'     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 26: SMC Proposal CLC Message Format










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   The fields present in the SMC Proposal CLC message are:

   Eye catchers

      Like all CLC messages, the SMC Proposal has beginning and ending
      eye catchers to aid with verification and parsing.  The hex digits
      spell "SMCR" in IBM-1047 (EBCDIC).

   Type

      CLC message Type 1 indicates SMC Proposal.

   Length

      The length of this CLC message.  If this is an IPv4 flow, this
      value is 52.  Otherwise, it is variable, depending upon how many
      prefixes are listed.

   Version

      Version of the SMC-R protocol.  Version 1 is the only currently
      defined value.

   Client's Peer ID

      As described in Appendix A.2.1 above.

   Client's preferred RoCE GID

      The IPv6 address of the client's preferred RNIC on the RoCE
      fabric.

   Client's preferred RoCE MAC address

      The MAC address of the client's preferred RNIC on the RoCE fabric.
      It is required, as some operating systems do not have neighbor
      discovery or ARP support for RoCE RNICs.

   Offset to mask/prefix area

      Provides the number of bytes that must be skipped after this
      field, to access the IPv4 Subnet Mask field and the fields that
      follow it.  Allows for future growth of this signal.  In this
      version of the architecture, this value is always zero.







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   Area for future growth

      In this version of the architecture, this field does not exist.
      This indicates where additional information may be inserted into
      the signal in the future.  The "Offset to mask/prefix area" field
      must be used to skip over this area.

   IPv4 Subnet Mask

      If this message is flowing over an IPv4 TCP connection, the value
      of the subnet mask associated with the interface over which the
      client sent this message.  If this is an IPv6 flow, this field is
      all zeros.

      This field, along with all fields that follow it in this signal,
      must be accessed by skipping the number of bytes listed in the
      "Offset to mask/prefix area" field after the end of that field.

   IPv4 Mask Lgth

      If this message is flowing over an IPv4 TCP connection, the number
      of significant bits in the IPv4 Subnet Mask field.  If this is an
      IPv6 flow, this field is zero.

   Num IPv6 prfx

      If this message is flowing over an IPv6 TCP connection, the number
      of IPv6 prefixes that follow, with a maximum value of 8.  If this
      is an IPv4 flow, this field is zero and is immediately followed by
      the ending eye catcher.





















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   Array of IPv6 prefixes

      For IPv6 TCP connections, a list of the IPv6 prefixes associated
      with the network over which the client sent this message, up to a
      maximum of eight prefixes.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                  IPv6 prefix value                            +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Prefix Length |
     +-+-+-+-+-+-+-+-+

              Figure 27: Format for IPv6 Prefix Array Element






























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A.2.3.  SMC Accept CLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   x'E2'       |   x'D4'       |     x'C3'     |     x'D9'     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 2     |    Length = 68                |Version|F|Rsrvd|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                       Server's Peer ID                      -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                Server's RoCE GID                            -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Server's RoCE                                                |
     +- MAC address                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |     Server QP (bytes 1-2)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---+
     |Srvr QP byte 3 |         Server RMB RKey (bytes 1-3)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Srvr RMB byte 4|Server RMB indx| Srvr RMB alert tkn (bytes 1-2)|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Srvr RMB alert tkn (bytes 3-4)|Bsize  | MTU   |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                     Server's RMB virtual address            -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Reserved      |    Server's initial packet sequence number    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   x'E2'       |   x'D4'       |     x'C3'     |     x'D9'     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 28: SMC Accept CLC Message Format










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   The fields present in the SMC Accept CLC message are:

   Eye catchers

      Like all CLC messages, the SMC Accept has beginning and ending
      eye catchers to aid with verification and parsing.  The hex digits
      spell "SMCR" in IBM-1047 (EBCDIC).

   Type

      CLC message Type 2 indicates SMC Accept.

   Length

      The SMC Accept CLC message is 68 bytes long.

   Version

      Version of the SMC-R protocol.  Version 1 is the only currently
      defined value.

   F-bit

      First contact flag: A 1-bit flag that indicates that the server
      believes this TCP connection is the first SMC-R contact for this
      link group.

   Server's Peer ID

      As described in Appendix A.2.1 above.

   Server's RoCE GID

      The IPv6 address of the RNIC that the server chose for this SMC-R
      link.

   Server's RoCE MAC address

      The MAC address of the server's RNIC for the SMC-R link.  It is
      required, as some operating systems do not have neighbor discovery
      or ARP support for RoCE RNICs.

   Server's QP number

      The number for the reliably connected queue pair that the server
      created for this SMC-R link.





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   Server's RMB RKey

      The RDMA RKey for the RMB that the server created or chose for
      this TCP connection.

   Server's RMB element index

      Indexes which element within the server's RMB will represent this
      TCP connection.

   Server's RMB element alert token

      A platform-defined, architecturally opaque token that identifies
      this TCP connection.  Added by the client as immediate data on
      RDMA writes from the client to the server to inform the server
      that there is data for this connection to retrieve from the
      RMB element.

   Bsize:

      Server's RMB element buffer size in 4-bit compressed notation:
      x = 4 bits.  Actual buffer size value is (2^(x + 4)) * 1K.
      Smallest possible value is 16K.  Largest size supported by this
      architecture is 512K.

   MTU

      An enumerated value indicating this peer's QP MTU size.  The two
      peers exchange their MTU values, and whichever value is smaller
      will be used for the QP.  This field should only be validated in
      the first contact exchange.

      The enumerated MTU values are:

         0:  reserved

         1:  256

         2:  512

         3:  1024

         4:  2048

         5:  4096

         6-15: reserved




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   Server's RMB virtual address

      The virtual address of the server's RMB as assigned by the
      server's RNIC.

   Server's initial packet sequence number

      The starting packet sequence number that this peer will use when
      sending to the other peer, so that the other peer can prepare its
      QP for the sequence number to expect.









































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A.2.4.  SMC Confirm CLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   x'E2'       |   x'D4'       |     x'C3'     |     x'D9'     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 3     |    Length = 68                |Version| Rsrvd |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                       Client's Peer ID                      -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                Client's RoCE GID                            -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Client's RoCE                                                |
     +- MAC address                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |     Client QP (bytes 1-2)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---+
     |Clnt QP byte 3 |         Client RMB RKey (bytes 1-3)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Clnt RMB byte 4|Client RMB indx| Clnt RMB alert tkn (bytes 1-2)|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Clnt RMB alert tkn (bytes 3-4)|Bsize  | MTU   |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                  Client's RMB Virtual Address               -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Reserved      |    Client's initial packet sequence number    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   x'E2'       |   x'D4'       |     x'C3'     |     x'D9'     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 29: SMC Confirm CLC Message Format

   The SMC Confirm CLC message is nearly identical to the SMC Accept,
   except that it contains client information and lacks a first contact
   flag.






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   The fields present in the SMC Confirm CLC message are:

   Eye catchers

      Like all CLC messages, the SMC Confirm has beginning and ending
      eye catchers to aid with verification and parsing.  The hex digits
      spell "SMCR" in IBM-1047 (EBCDIC).

   Type

      CLC message Type 3 indicates SMC Confirm.

   Length

      The SMC Confirm CLC message is 68 bytes long.

   Version

      Version of the SMC-R protocol.  Version 1 is the only currently
      defined value.

   Client's Peer ID

      As described in Appendix A.2.1 above.

   Client's RoCE GID

      The IPv6 address of the RNIC that the client chose for this SMC-R
      link.

   Client's RoCE MAC address

      The MAC address of the client's RNIC for the SMC-R link.  It is
      required, as some operating systems do not have neighbor discovery
      or ARP support for RoCE RNICs.

   Client's QP number

      The number for the reliably connected queue pair that the client
      created for this SMC-R link.

   Client's RMB RKey

      The RDMA RKey for the RMB that the client created or chose for
      this TCP connection.






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   Client's RMB element index

      Indexes which element within the client's RMB will represent this
      TCP connection.

   Client's RMB element alert token

      A platform-defined, architecturally opaque token that identifies
      this TCP connection.  Added by the server as immediate data on
      RDMA writes from the server to the client to inform the client
      that there is data for this connection to retrieve from the
      RMB element.

   Bsize:

      Client's RMB element buffer size in 4-bit compressed notation:
      x = 4 bits.  Actual buffer size value is (2^(x + 4)) * 1K.
      Smallest possible value is 16K.  Largest size supported by this
      architecture is 512K.

   MTU

      An enumerated value indicating this peer's QP MTU size.  The two
      peers exchange their MTU values, and whichever value is smaller
      will be used for the QP.  The values are enumerated in
      Appendix A.2.3.  This value should only be validated in the first
      contact exchange.

   Client's RMB Virtual Address

      The virtual address of the client's RMB as assigned by the
      server's RNIC.

   Client's initial packet sequence number

      The starting packet sequence number that this peer will use when
      sending to the other peer, so that the other peer can prepare its
      QP for the sequence number to expect.













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A.2.5.  SMC Decline CLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   x'E2'       |   x'D4'       |     x'C3'     |     x'D9'     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 4     |    Length = 28                |Version|S|Rsrvd|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                       Sender's Peer ID                      -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Peer Diagnosis Information                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   x'E2'       |   x'D4'       |     x'C3'     |     x'D9'     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 30: SMC Decline CLC Message Format

   The fields present in the SMC Decline CLC message are:

   Eye catchers

      Like all CLC messages, the SMC Decline has beginning and ending
      eye catchers to aid with verification and parsing.  The hex digits
      spell "SMCR" in IBM-1047 (EBCDIC).

   Type

      CLC message Type 4 indicates SMC Decline.

   Length

      The SMC Decline CLC message is 28 bytes long.

   Version

      Version of the SMC-R protocol.  Version 1 is the only currently
      defined value.

   S-bit

      Sync Bit.  Indicates that the link group is out of sync and the
      receiving peer must clean up its representation of the link group.




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   Sender's Peer ID

      As described in Appendix A.2.1 above.

   Peer Diagnosis Information

      4 bytes of diagnosis information provided by the peer.  These
      values are defined by the individual peers, and it is necessary to
      consult the peer's system documentation to interpret the results.

A.3.  LLC Messages

   LLC messages are sent over an existing SMC-R link using RoCE SendMsg
   and are always 44 bytes long so that they fit into the space
   available in a single WQE without requiring the receiver to post
   receive buffers.  If all 44 bytes are not needed, they are padded out
   with zeros.  LLC messages are in a request/response format.  The
   message type is the same for request and response, and a flag
   indicates whether a message is flowing as a request or a response.

   The two high-order bits of an LLC message opcode indicate how it is
   to be handled by a peer that does not support the opcode.

   If the high-order bits of the opcode are b'00', then the peer must
   support the LLC message and indicate a protocol error if it does not.

   If the high-order bits of the opcode are b'10', then the peer must
   silently discard the LLC message if it does not support the opcode.
   This requirement is included to allow for toleration of advanced, but
   optional, functionality.

   High-order bits of b'11' indicate a Connection Data Control (CDC)
   message as described in Appendix A.4.


















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A.3.1.  CONFIRM LINK LLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 1     |  Length = 44  |   Reserved    |R|  Reserved   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Sender's RoCE                                                |
     +-   MAC address                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     +-                                                             -+
     |                 Sender's RoCE GID                             |
     +-                                                             -+
     |                                                               |
     +-                              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |Sender's QP number, bytes 1-2  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sender QP byte3| Link number   |Sender's link userID, bytes 1-2|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sender's link userID, bytes 3-4| Max links     |  Reserved     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                         Reserved                            -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 31: CONFIRM LINK LLC Message Format

   The CONFIRM LINK LLC message is required to be exchanged between the
   server and client over a newly created SMC-R link to complete the
   setup of an SMC-R link.  Its purpose is to confirm that the RoCE path
   is actually usable.

   On first contact, this message flows after the server receives the
   SMC Confirm CLC message from the client over the IP connection.  For
   additional links added to an SMC-R link group, it flows after the
   ADD LINK and ADD LINK CONTINUATION exchange.  This flow provides
   confirmation that the queue pair is in fact usable.  Each peer echoes
   its RoCE information back to the other.










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   The contents of the CONFIRM LINK LLC message are:

   Type

      Type 1 indicates CONFIRM LINK.

   Length

      The CONFIRM LINK LLC message is 44 bytes long.

   R

      Reply flag.  When set, indicates that this is a CONFIRM LINK
      reply.

   Sender's RoCE MAC address

      The MAC address of the sender's RNIC for the SMC-R link.  It is
      required, as some operating systems do not have neighbor discovery
      or ARP support for RoCE RNICs.

   Sender's RoCE GID

      The IPv6 address of the RNIC that the sender is using for this
      SMC-R link.

   Sender's QP number

      The number for the reliably connected queue pair that the sender
      created for this SMC-R link.

   Link number

      An identifier assigned by the server that uniquely identifies the
      link within the link group.  This identifier is ONLY unique within
      a link group.  Provided by the server and echoed back by the
      client.

   Link user ID

      An opaque, implementation-defined identifier assigned by the
      sender and provided to the receiver solely for purposes of
      display, diagnosis, network management, etc.  The link user ID
      should be unique across the sender's entire software space,
      including all other link groups.






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   Max links

      The maximum number of links the sender can support in a link
      group.  The maximum for this link group is the smaller of the
      values provided by the two peers.

A.3.2.  ADD LINK LLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 2     |  Length = 44  | Rsrvd |RsnCode|R|Z| Reserved  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Sender's RoCE                                                |
     +-   MAC address                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     +-                                                             -+
     |                 Sender's RoCE GID                             |
     +-                                                             -+
     |                                                               |
     +-                              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |Sender's QP number, bytes 1-2  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Sender QP byte3| Link number   |Rsrvd  |  MTU  |Initial PSN    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Initial PSN (continued)      |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                              -+
     |                          Reserved                             |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 32: ADD LINK LLC Message Format

   The ADD LINK LLC message is sent over an existing link in the link
   group when a peer wishes to add an SMC-R link to an existing SMC-R
   link group.  It is sent by the server to add a new SMC-R link to the
   group, or by the client to request that the server add a new link --
   for example, when a new RNIC becomes active.  When sent from the
   client to the server, it represents a request that the server
   initiate an ADD LINK exchange.








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   This message is sent immediately after the initial SMC-R link in the
   group completes, as described in Section 3.5.1 ("First Contact").  It
   can also be sent over an existing SMC-R link group at any time as new
   RNICs are added and become available.  Therefore, there can be as few
   as one new RMB RToken to be communicated, or several.  RTokens will
   be communicated using ADD LINK CONTINUATION messages.

   The contents of the ADD LINK LLC message are:

   Type

      Type 2 indicates ADD LINK.

   Length

      The ADD LINK LLC message is 44 bytes long.

   RsnCode

      If the Z (rejection) flag is set, this field provides the reason
      code.  Values can be:

         X'1' - no alternate path available: set when the server
                provides the same MAC/GID as an existing SMC-R link in
                the group, and the client does not have any additional
                RNICs available (i.e., the server is attempting to set
                up an asymmetric link but none is available).

         X'2' - Invalid MTU value specified.

   R

      Reply flag.  When set, indicates that this is an ADD LINK reply.

   Z

      Rejection flag.  When set on reply, indicates that the server's
      ADD LINK was rejected by the client.  When this flag is set, the
      reason code will also be set.

   Sender's RoCE MAC address

      The MAC address of the sender's RNIC for the new SMC-R link.  It
      is required, as some operating systems do not have neighbor
      discovery or ARP support for RoCE RNICs.






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   Sender's RoCE GID

      The IPv6 address of the RNIC that the sender is using for the new
      SMC-R link.

   Sender's QP number

      The number for the reliably connected queue pair that the sender
      created for the new SMC-R link.

   Link number

      An identifier for the new SMC-R link.  This is assigned by the
      server and uniquely identifies the link within the link group.
      This identifier is ONLY unique within a link group.  Provided by
      the server and echoed back by the client.

   MTU

      An enumerated value indicating this peer's QP MTU size.  The two
      peers exchange their MTU values, and whichever value is smaller
      will be used for the QP.  The values are enumerated in
      Appendix A.2.3.

   Initial PSN

      The starting packet sequence number (PSN) that this peer will use
      when sending to the other peer, so that the other peer can prepare
      its QP for the sequence number to expect.






















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A.3.3.  ADD LINK CONTINUATION LLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 3     |  Length = 44  |  Reserved     |R|  Reserved   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Linknum     | NumRTokens    |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                  RKey/RToken pair                           -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                  RKey/RToken pair or zeros                  -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Reserved                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 33: ADD LINK CONTINUATION LLC Message Format

   When a new SMC-R link is added to an SMC-R link group, it is
   necessary to communicate the new link's RTokens for the RMBs that the
   SMC-R link group can access.  This message follows the ADD LINK and
   provides the RTokens.

   The server kicks off this exchange by sending the first ADD LINK
   CONTINUATION LLC message, and the server controls the exchange as
   described below.

   o  If the client and the server require the same number of ADD LINK
      CONTINUATION messages to communicate their RTokens, the server
      starts the exchange by sending the first ADD LINK CONTINUATION
      request to the client with its (the server's) RTokens.  The client
      then responds with an ADD LINK CONTINUATION response with its
      RTokens, and so on until the exchange is completed.






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   o  If the server requires more ADD LINK CONTINUATION messages than
      the client, then after the client has communicated all of its
      RTokens, the server continues to send ADD LINK CONTINUATION
      request messages to the client.  The client continues to respond,
      using empty (number of RTokens to be communicated = 0) ADD LINK
      CONTINUATION response messages.

   o  If the client requires more ADD LINK CONTINUATION messages than
      the server, then after communicating all of its RTokens, the
      server will continue to send empty ADD LINK CONTINUATION messages
      to the client to solicit replies with the client's RTokens, until
      all have been communicated.

   The contents of the ADD LINK CONTINUATION LLC message are:

   Type

      Type 3 indicates ADD LINK CONTINUATION.

   Length

      The ADD LINK CONTINUATION LLC message is 44 bytes long.

   R

      Reply flag.  When set, indicates that this is an ADD LINK
      CONTINUATION reply.

   LinkNum

      The link number of the new link within the SMC-R link group for
      which RKeys are being communicated.

   NumRTokens

      Number of RTokens remaining to be communicated (including the ones
      in this message).  If the value is less than or equal to 2, this
      is the last message.  If it is greater than 2, another
      continuation message will be required, and its value will be the
      value in this message minus 2, and so on until all RKeys are
      communicated.  The maximum value for this field is 255.










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   RKey/RToken pairs (two or less)

      These consist of an RKey for an RMB that is known on the SMC-R
      link over which this message was sent (the reference RKey), paired
      with the same RMB's RToken over the new SMC-R link.  A full RToken
      is not required for the reference, because it is only being used
      to distinguish which RMB it applies to, not address it.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Reference RKey                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            New RKey                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                       New Virtual Address                   -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 34: RKey/RToken Pair Format

   The contents of the RKey/RToken pair are:

   Reference RKey

      The RKey of the RMB as it is already known on the SMC-R link over
      which this message is being sent.  Required so that the peer knows
      with which RMB to associate the new RToken.

   New RKey

      The RKey of this RMB as it is known over the new SMC-R link.

   New Virtual Address

      The virtual address of this RMB as it is known over the new
      SMC-R link.













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A.3.4.  DELETE LINK LLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 4     |  Length = 44  |  Reserved     |R|A|O| Rsrvd   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Linknum     |         reason code (bytes 1-3)               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |RsnCode byte 4 |                                               |
     +-+-+-+-+-+-+-+-+                                              -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                          Reserved                           -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 35: DELETE LINK LLC Message Format

   When the client or server detects that a QP or SMC-R link goes down
   or needs to come down, it sends this message over one of the other
   links in the link group.

   When the DELETE LINK is sent from the client, it only serves as a
   notification, and the client expects the server to respond by sending
   a DELETE LINK request.  To avoid races, only the server will initiate
   the actual DELETE LINK request and response sequence that results
   from notification from the client.

   The server can also initiate the DELETE LINK without notification
   from the client if it detects an error or if orderly link termination
   was initiated.

   The client may also request termination of the entire link group, and
   the server may terminate the entire link group using this message.





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   The contents of the DELETE LINK LLC message are:

   Type

      Type 4 indicates DELETE LINK.

   Length

      The DELETE LINK LLC message is 44 bytes long.

   R

      Reply flag.  When set, indicates that this is a DELETE LINK reply.

   A

      "All" flag.  When set, indicates that all links in the link group
      are to be terminated.  This terminates the link group.

   O

      Orderly flag.  Indicates orderly termination.  Orderly termination
      is generally caused by an operator command rather than an error on
      the link.  When the client requests orderly termination, the
      server may wait to complete other work before terminating.

   LinkNum

      The link number of the link to be terminated.  If the A flag is
      set, this field has no meaning and is set to 0.

   RsnCode

      The termination reason code.  Currently defined reason codes are:

      Request reason codes:

         X'00010000' = Lost path

         X'00020000' = Operator initiated termination

         X'00030000' = Program initiated termination (link inactivity)

         X'00040000' = LLC protocol violation

         X'00050000' = Asymmetric link no longer needed





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      Response reason code:

         X'00100000' = Unknown link ID (no link)

A.3.5.  CONFIRM RKEY LLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 6     |  Length = 44  |   Reserved    |R|0|Z|C|Rsrvd  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   NumTkns     |  New RMB RKey for this link (bytes 1-3)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |ThisLink byte 4|                                               |
     +-+-+-+-+-+-+-+-+                                              -+
     |           New RMB virtual address for this link               |
     +-              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               |                                               |
     +-+-+-+-+-+-+-+-+                                              -+
     |                                                               |
     +-   Other link RMB specification or zeros                     -+
     |                                                               |
     +-                              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                              -+
     |                                                               |
     +-                                                             -+
     |      Other link RMB specification or zeros                    |
     +-                                              +-+-+-+-+-+-+-+-+
     |                                               |  Reserved     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 36: CONFIRM RKEY LLC Message Format

   The CONFIRM RKEY flow can be sent at any time from either the client
   or the server, to inform the peer that an RMB has been created or
   deleted.  The creator of a new RMB must inform its peer of the new
   RMB's RToken for all SMC-R links in the SMC-R link group.

   For RMB creation, the creator sends this message over the SMC-R link
   that the first TCP connection that uses the new RMB is using.  This
   message contains the new RMB RToken for the SMC-R link over which
   the message is sent.  It then lists the sender's SMC-R links in the
   link group paired with the new RToken for the new RMB for that link.
   This message can communicate the new RTokens for three QPs: the QP
   for the link over which this message is sent, and two others.  If
   there are more than three links in the SMC-R link group, a
   CONFIRM RKEY CONTINUATION will be required.



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   The peer responds by simply echoing the message with the response
   flag set.  If the response is a negative response, the sender must
   recalculate the RToken set and start a new CONFIRM RKEY exchange from
   the beginning.  The timing of this retry is controlled by the C flag,
   as described below.

   The contents of the CONFIRM RKEY LLC message are:

   Type

      Type 6 indicates CONFIRM RKEY.

   Length

      The CONFIRM RKEY LLC message is 44 bytes long.

   R

      Reply flag.  When set, indicates that this is a CONFIRM RKEY
      reply.

   0

      Reserved bit.

   Z

      Negative response flag.

   C

      Configuration Retry bit.  If this is a negative response and this
      flag is set, the originator should recalculate the RKey set and
      retry this exchange as soon as the current configuration change is
      completed.  If this flag is not set on a negative response, the
      originator must wait for the next natural stimulus (for example, a
      new TCP connection started that requires a new RMB) before
      retrying.

   NumTkns

      The number of other link/RToken pairs, including those provided in
      this message, to be communicated.  Note that this value does not
      include the RToken for the link on which this message was sent
      (i.e., the maximum value is 2).  If this value is 3 or less, this
      is the only message in the exchange.  If this value is greater
      than 3, a CONFIRM RKEY CONTINUATION message will be required.




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      Note: In this version of the architecture, eight is the maximum
      number of links supported in a link group.

   New RMB RKey for this link

      The new RMB's RKey as assigned on the link over which this message
      is being sent.

   New RMB virtual address for this link

      The new RMB's virtual address as assigned on the link over which
      this message is being sent.

   Other link RMB specification

      The new RMB's specification on the other links in the link group,
      as shown in Figure 37.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Link number   | RMB's RKey for the specified link (bytes 1-3) |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |New RKey byte 4|                                               |
     +-+-+-+-+-+-+-+-+                                              -+
     |           RMB's virtual address for the specified link        |
     +-              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               |
     +-+-+-+-+-+-+-+-+

                Figure 37: Format of Link Number/RKey Pairs

   Link number

      The link number for a link in the link group.

   RMB's RKey for the specified link

      The RKey used to reach the RMB over the link whose number was
      specified in the Link number field.

   RMB's virtual address for the specified link

      The virtual address used to reach the RMB over the link whose
      number was specified in the Link number field.






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A.3.6.  CONFIRM RKEY CONTINUATION LLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 8     |  Length = 44  |   Reserved    |R|0|Z|  Rsrvd  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  NumTknsLeft  |                                               |
     +-+-+-+-+-+-+-+-+                                              -+
     |                                                               |
     +-          Other link RMB specification                       -+
     |                                                               |
     +-              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               |                                               |
     +-+-+-+-+-+-+-+-+                                              -+
     |                                                               |
     +-   Other link RMB specification or zeros                     -+
     |                                                               |
     +-                              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                              -+
     |                                                               |
     +-                                                             -+
     |      Other link RMB specification or zeros                    |
     +-                                              +-+-+-+-+-+-+-+-+
     |                                               |  Reserved     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 38: CONFIRM RKEY CONTINUATION LLC Message Format

   The CONFIRM RKEY CONTINUATION LLC message is used to communicate any
   additional RMB RTokens that did not fit into the CONFIRM RKEY
   message.  Each of these messages can hold up to three RMB RTokens.
   The NumTknsLeft field indicates how many RMB RTokens are to be
   communicated, including the ones in this message.  If the value is 3
   or less, this is the last message of the group.  If the value is 4 or
   higher, additional CONFIRM RKEY CONTINUATION messages will follow,
   and the NumTknsLeft value will be a countdown until all are
   communicated.

   Like the CONFIRM RKEY message, the peer responds by echoing the
   message back with the reply flag set.









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   The contents of the CONFIRM RKEY CONTINUATION LLC message are:

   Type

      Type 8 indicates CONFIRM RKEY CONTINUATION.

   Length

      The CONFIRM RKEY CONTINUATION LLC message is 44 bytes long.

   R

      Reply flag.  When set, indicates that this is a CONFIRM RKEY
      CONTINUATION reply.

   0

      Reserved bit.

   Z

      Negative response flag.

   NumTknsLeft

      The number of link/RToken pairs, including those provided in this
      message, that are remaining to be communicated.  If this value is
      3 or less, this is the last message in the exchange.  If this
      value is greater than 3, another CONFIRM RKEY CONTINUATION message
      will be required.  Note that in this version of the architecture,
      eight is the maximum number of links supported in a link group.

   Other link RMB specification

      The new RMB's specification on other links in the link group, as
      shown in Figure 37.















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A.3.7.  DELETE RKEY LLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 9     |  Length = 44  |   Reserved    |R|0|Z|  Rsrvd  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Count     | Error Mask    |        Reserved               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                First deleted RKey                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Second deleted RKey or zeros                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Third deleted RKey or zeros                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Fourth deleted RKey or zeros                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Fifth deleted RKey or zeros                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Sixth deleted RKey or zeros                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Seventh deleted RKey or zeros                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Eighth deleted RKey or zeros                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Reserved                                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 39: DELETE RKEY LLC Message Format

   The DELETE RKEY flow can be sent at any time from either the client
   or the server, to inform the peer that one or more RMBs have been
   deleted.  Because the peer already knows every RMB's RKey on each
   link in the link group, this message only specifies one RKey for each
   RMB being deleted.  The RKey provided for each deleted RMB will be
   its RKey as known on the SMC-R link over which this message is sent.

   It is not necessary to provide the entire RToken.  The RKey alone is
   sufficient for identifying an existing RMB.

   The peer responds by simply echoing the message with the response
   flag set.  If the peer did not recognize an RKey, a negative response
   flag will be set; however, no aggressive recovery action beyond
   logging the error will be taken.







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   The contents of the DELETE RKEY LLC message are:

   Type

      Type 9 indicates DELETE RKEY.

   Length

      The DELETE RKEY LLC message is 44 bytes long.

   R

      Reply flag.  When set, indicates that this is a DELETE RKEY reply.

   0

      Reserved bit.

   Z

      Negative response flag.

   Count

      Number of RMBs being deleted by this message.  Maximum value is 8.

   Error Mask

      If this is a negative response, indicates which RMBs were not
      successfully deleted.  Each bit corresponds to a listed RMB; for
      example, b'01010000' indicates that the second and fourth RKeys
      weren't successfully deleted.

   Deleted RKeys

      A list of Count RKeys.  Provided on the request flow and echoed
      back on the response flow.  Each RKey is valid on the link over
      which this message is sent and represents a deleted RMB.  Up to
      eight RMBs can be deleted in this message.












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A.3.8.  TEST LINK LLC Message Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 7     |  Length = 44  |   Reserved    |R|  Reserved   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                         User Data                           -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                          Reserved                             |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-                                                             -+
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 40: TEST LINK LLC Message Format

   The TEST LINK request can be sent from either peer to the other on an
   existing SMC-R link at any time to test that the SMC-R link is active
   and healthy at the software level.  A peer that receives a TEST LINK
   LLC message immediately sends back a TEST LINK reply, echoing back
   the user data.  Refer also to Section 4.5.3 ("TCP Keepalive
   Processing").















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   The contents of the TEST LINK LLC message are:

   Type

      Type 7 indicates TEST LINK.

   Length

      The TEST LINK LLC message is 44 bytes long.

   R

      Reply flag.  When set, indicates that this is a TEST LINK reply.

   User Data

      The receiver of this message echoes the sender's data back in a
      TEST LINK response LLC message.

A.4.  Connection Data Control (CDC) Message Format

   The RMBE control data is communicated using Connection Data Control
   (CDC) messages, which use RoCE SendMsg, similar to LLC messages.
   Also, as with LLC messages, CDC messages are 44 bytes long to ensure
   that they can fit into private data areas of receive WQEs without
   requiring the receiver to post receive buffers.

   Unlike LLC messages, this data is integral to the data path, so its
   processing must be prioritized and optimized similarly to other data
   path processing.  While LLC messages may be processed on a slower
   path than data, these messages cannot be.




















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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   0  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Type = x'FE'  | Length = 44   |      Sequence number          |
   4  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       SMC-R alert token                       |
   8  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Reserved              | Producer cursor wrap seqno    |
   12 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Producer Cursor                         |
   16 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Reserved              | Consumer cursor wrap seqno    |
   20 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Consumer Cursor                         |
   24 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |B|P|U|R|F|Rsrvd|D|C|A|             Reserved                    |
   28 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
   32 +-                                                             -+
      |                                                               |
   36 +-                         Reserved                            -+
      |                                                               |
   40 +-                                                             -+
      |                                                               |
   44 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 41: Connection Data Control (CDC) Message Format

   Type = x'FE'

      This type number has the two high-order bits turned on to enable
      processing to quickly distinguish it from an LLC message.

   Length = 44

      The length of inline data that does not require the posting of a
      receive buffer.

   Sequence number

      A 2-byte unsigned integer that represents a wrapping sequence
      number.  The initial value is 1, and this value can wrap to 0.
      Incremented with every control message sent, except for the
      failover data validation message, and used to guard against
      processing an old control message out of sequence.  Also used in
      failover data validation.  In normal usage, if this number is less





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      than the last received value, discard this message.  If greater,
      process this message.  Old control messages can be lost with no
      ill effect but cannot be processed after newer ones.

      If this is a failover validation CDC message (F flag set), then
      the receiver must verify that it has received and fully processed
      the RDMA write that was described by the CDC message with the
      sequence number in this message.  If not, the TCP connection must
      be reset to guard against data loss.  Details of this processing
      are provided in Section 4.6.1.

   SMC-R alert token

      The endpoint-assigned alert token that identifies to which TCP
      connection on the link group this control message refers.

   Producer cursor wrap seqno

      A 2-byte unsigned integer that represents a wrapping counter
      incremented by the producer whenever the data written into this
      RMBE receive buffer causes a wrap (i.e., the producer cursor
      wraps).  This is used by the receiver to determine when new data
      is available even though the cursors appear unchanged, such as
      when a full window size write is completed (producer cursor of
      this RMBE sent by peer = local consumer cursor) or in scenarios
      where the producer cursor sent for this RMBE < local consumer
      cursor.

   Producer Cursor

      A 4-byte unsigned integer that is a wrapping offset into the RMBE
      data area.  Points to the next byte of data to be written by the
      sender.  Can advance up to the receiver's consumer cursor as known
      by the sender.  When the urgent data present indicator is on,
      points 1 byte beyond the last byte of urgent data.  When computing
      this cursor, the presence of the eye catcher in the RMBE data area
      must be accounted for.  The first writable data location in the
      RMBE is at offset 4, so this cursor begins at 4 and wraps to 4.

   Consumer cursor wrap seqno

      A 2-byte unsigned integer that mirrors the value of the producer
      cursor wrap sequence number when the last read from this RMBE
      occurred.  Used as an indicator of how far along the consumer is
      in reading data (i.e., processed last wrap point or not).  The
      producer side can use this indicator to detect whether or not more
      data can be written to the partner in full window write scenarios
      (where the producer cursor = consumer cursor as known on the



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      remote RMBE).  In this scenario, if the consumer sequence number
      equals the local producer sequence number, the producer knows that
      more data can be written.

   Consumer Cursor

      A 4-byte unsigned integer that is a wrapping offset into the
      sender's RMBE data area.  Points to the offset of the next byte of
      data to be consumed by the peer in its own RMBE.  When computing
      this cursor, the presence of the eye catcher in the RMBE data area
      must be accounted for.  The first writable data location in the
      RMBE is at offset 4, so this cursor begins at 4 and wraps to 4.
      The sender cannot write beyond this cursor into the peer's RMBE
      without causing data loss.

   B-bit

      Writer blocked indicator: Sender is blocked for writing.  If this
      bit is set, sender will require explicit notification when receive
      buffer space is available.

   P-bit

      Urgent data pending: Sender has urgent data pending for this
      connection.

   U-bit

      Urgent data present: Indicates that urgent data is present in the
      RMBE data area, and the producer cursor points to 1 byte beyond
      the last byte of urgent data.

   R-bit

      Request for consumer cursor update: Indicates that an immediate
      consumer cursor update is requested, regardless of whether or not
      one is warranted according to the window size optimization
      algorithm described in Section 4.5.1.

   F-bit

      Failover validation indicator: Sent by a peer to guard against
      data loss during failover when the TCP connection is being moved
      to another SMC-R link in the link group.  When this bit is set,
      the only other fields in the CDC message that are significant are
      the Type, Length, SMC-R alert token, and Sequence number fields.
      The receiver must validate that it has fully processed the RDMA
      write described by the previous CDC message bearing the same



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      sequence number as this validation message.  If it has, no further
      action is required.  If it has not, the TCP connection must be
      reset.  This processing is described in detail in Section 4.6.1.

   D-bit

      Sending done indicator: Sent by a peer when it is done writing new
      data into the receiver's RMBE data area.

   C-bit

      PeerConnectionClosed indicator: Sent by a peer when it is
      completely done with this connection and will no longer be making
      any updates to the receiver's RMBE or sending any more control
      messages.

   A-bit

      Abnormal close indicator: Sent by a peer when the connection is
      abnormally terminated (for example, the TCP connection was reset).
      When sent, it indicates that the peer is completely done with this
      connection and will no longer be making any updates to this RMBE
      or sending any more control messages.  It also indicates that the
      RMBE owner must flush any remaining data on this connection and
      generate an error return code to any outstanding socket APIs on
      this connection (same processing as receiving a RST segment on a
      TCP connection).

Appendix B.  Socket API Considerations

   A key design goal for SMC-R is to require no application changes for
   exploitation.  It is confined to socket applications using stream
   (i.e., TCP) sockets over IPv4 or IPv6.  By virtue of the fact that
   the switch to the SMC-R protocol occurs after a TCP connection is
   established, no changes are required in a socket address family or in
   the IP addresses and ports that the socket applications are using.
   Existing socket APIs that allow applications to retrieve local and
   remote socket address structures for an established TCP connection
   (for example, getsockname() and getpeername()) will continue to
   function as they have before.  Existing DNS setup and APIs for
   resolving hostnames to IP addresses and vice versa also continue to
   function without any changes.  In general, all of the usual socket
   APIs that are used for TCP communications (send APIs, recv APIs,
   etc.) will continue to function as they do today, even if SMC-R is
   used as the underlying protocol.






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   Each SMC-R-enabled implementation does, however, need to pay special
   attention to any socket APIs that have a reliance on the underlying
   TCP and IP protocols and also ensure that their behavior in an SMC-R
   environment is reasonable and minimizes impact on the application.
   While the basic socket API set is fairly similar across different
   operating systems, there is more variability when it comes to
   advanced socket API options.  Each implementation needs to perform a
   detailed analysis of its API options, any possible impact that SMC-R
   may have, and any resultant implications.  As part of that step, a
   discussion or review with other implementations supporting SMC-R
   would be useful to ensure consistent implementation.

B.1.  setsockopt() / getsockopt() Considerations

   These APIs allow socket applications to manipulate socket, transport
   (TCP/UDP), and IP-level options associated with a given socket.
   Typically, a platform restricts the number of IP options available to
   stream (TCP) socket applications, given their connection-oriented
   nature.  The general guideline here is to continue processing these
   APIs in a manner that allows for application compatibility.  Some
   options will be relevant to the SMC-R protocol and will require
   special processing "under the covers".  For example, the ability to
   manipulate TCP send and receive buffer sizes is still valid for
   SMC-R.  However, other options may have no meaning for SMC-R.  For
   example, if an application enabled the TCP_NODELAY socket option to
   disable Nagle's algorithm, it should have no real effect on SMC-R
   communications, as there is no notion of Nagle's algorithm with this
   new protocol.  But the implementation must accept the TCP_NODELAY
   option as it does today and save it so that it can be later extracted
   via getsockopt() processing.  Note that any TCP or IP-level options
   will still have an effect on any TCP/IP packets flowing for an SMC-R
   connection (i.e., as part of TCP/IP connection establishment and
   TCP/IP connection termination packet flows).

   Under the covers, manipulation of the TCP options will also include
   the SMC-layer setting, as well as reading the SMC-R experimental
   option before and after completion of the three-way TCP handshake.














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Appendix C.  Rendezvous Error Scenarios

   This section discusses error scenarios for setting up and managing
   SMC-R links.

C.1.  SMC Decline during CLC Negotiation

   A peer to the SMC-R CLC negotiation can send an SMC Decline in lieu
   of any expected CLC message to decline SMC and force the TCP
   connection back to the IP fabric.  There can be several reasons for
   an SMC Decline during the CLC negotiation, including the following:

   o  RNIC went down

   o  SMC-R forbidden by local policy

   o  subnet (IPv4) or prefix (IPv6) doesn't match

   o  lack of resources to perform SMC-R

   In all cases, when an SMC Decline is sent in lieu of an expected CLC
   message, no confirmation is required, and the TCP connection
   immediately falls back to using the IP fabric.

   To prevent ambiguity between CLC messages and application data, an
   SMC Decline cannot "chase" another CLC message.  An SMC Decline can
   only be sent in lieu of an expected CLC message.  For example, if the
   client sends an SMC Proposal and then its RNIC goes down, it must
   wait for the SMC Accept from the server and then reply to the
   SMC Accept with an SMC Decline.

   This "no chase" rule means that if this TCP connection is not a first
   contact between RoCE peers, a server cannot send an SMC Decline after
   sending an SMC Accept -- it can only either break the TCP connection
   or fail over if a problem arises in the RoCE fabric after it has sent
   the SMC Accept.  Similarly, once the client sends an SMC Confirm on a
   TCP connection that isn't a first contact, it is committed to SMC-R
   for this TCP connection and cannot fall back to IP.

C.2.  SMC Decline during LLC Negotiation

   For a TCP connection that represents a first contact between RoCE
   pairs, it is possible for SMC to fall back to IP during the LLC
   negotiation.  This is possible until the first contact SMC-R link is
   confirmed.  For example, see Figure 42.  After a first contact SMC-R
   link is confirmed, fallback to IP is no longer possible.  This
   translates to the following rule: a first contact peer can send an




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   SMC Decline at any time during LLC negotiation until it has
   successfully sent its CONFIRM LINK (request or response) flow.  After
   that point, it cannot fall back to IP.

       Host X -- Server                           Host Y -- Client
    +-------------------+                      +-------------------+
    | Peer ID = PS1     |                      |   Peer ID = PC1   |
    |            +------+                      +------+            |
    |       QP 8 |RNIC 1|    SMC-R Link 1      |RNIC 2|  QP 64     |
    | RKey X |   |MAC MA|<-------------------->|MAC MB|   |        |
    |        |   |GID GA|   attempted setup    |GID GB|   | RKey Y2|
    |       \/   +------+                      +------+  \/        |
    |+--------+         |                      |        +--------+ |
    || RMB    |         |                      |        | RMB    | |
    |+--------+         |                      |        +--------+ |
    |       /\   +------+                      +------+  /\        |
    |        |   |RNIC 3|                      |RNIC 4|   | RKey W2|
    |        |   |MAC MC|                      |MAC MD|   |        |
    |       QP 9 |GID GC|                      |GID GD|  QP 65     |
    |            +------+                      +------+            |
    +-------------------+                      +-------------------+

          SYN / SYN-ACK / ACK TCP three-way handshake with TCP option
         <--------------------------------------------------------->

            SMC Proposal / SMC Accept / SMC Confirm exchange
         <-------------------------------------------------------->

           CONFIRM LINK(request, Link 1)
         .........................................................>

                           CONFIRM LINK(response, Link 1)
                              X...................................
                                :
                                : RoCE write failure
                                :.................................>

           SMC Decline(PC1, reason code)
          <--------------------------------------------------------

              Connection data flows over IP fabric
          <------------------------------------------------------->

                          Legend:
                   ------------   TCP/IP and CLC flows
                   ............   RoCE (LLC) flows

               Figure 42: SMC Decline during LLC Negotiation



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C.3.  The SMC Decline Window

   Because SMC-R does not support fallback to IP for a TCP connection
   that is already using RDMA, there are specific rules on when the
   SMC Decline CLC message, which signals a fallback to IP because of an
   error or problem with the RoCE fabric, can be sent during TCP
   connection setup.  There is a "point of no return" after which a
   connection cannot fall back to IP, and RoCE errors that occur after
   this point require the connection to be broken with a RST flow in the
   IP fabric.

   For a first contact, that point of no return is after the ADD LINK
   LLC message has been successfully sent for the second SMC-R link.
   Specifically, the server cannot fall back to IP after receiving
   either (1) a positive write completion indication for the ADD LINK
   request or (2) the ADD LINK response from the client, whichever comes
   first.  The client cannot fall back to IP after sending a negative
   ADD LINK response, receiving a positive write complete on a positive
   ADD LINK response, or receiving a CONFIRM LINK for the second SMC-R
   link from the server, whichever comes first.

   For a subsequent contact, that point of no return is after the last
   send of the CLC negotiation completes.  This, in combination with the
   rule that error "chasers" are not allowed during CLC negotiation,
   means that the server cannot send an SMC Decline after sending an SMC
   Accept, and the client cannot send an SMC Decline after sending an
   SMC Confirm.

C.4.  Out-of-Sync Conditions during SMC-R Negotiation

   The SMC Accept CLC message contains a first contact flag that
   indicates to the client whether the server believes it is setting up
   a new link group or using an existing link group.  This flag is used
   to detect an out-of-sync condition between the client and the server.
   The scenario for such a condition is as follows: there is a single
   existing SMC-R link between the peers.  After the client sends the
   SMC Proposal CLC message, the existing SMC-R link between the client
   and the server fails.  The client cannot chase the SMC Proposal CLC
   message with an SMC Decline CLC message in this case, because the
   client does not yet know that the server would have wanted to choose
   the SMC-R link that just crashed.  The QP that failed recovers before
   the server returns its SMC Accept CLC message.  This means that there
   is a QP but no SMC-R link.  Since the server had not yet learned of
   the SMC-R link failure when it sent the SMC Accept CLC message, it
   attempts to reuse the SMC-R link that just failed.  This means that
   the server would not set the first contact flag, indicating to the
   client that the server thinks it is reusing an SMC-R link.  However,
   the client does not have an SMC-R link that matches the server's



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   specification.  Because the first contact flag is off, the client
   realizes it is out of sync with the server and sends an SMC Decline
   to cause the connection to fall back to IP.

C.5.  Timeouts during CLC Negotiation

   Because the SMC-R negotiation flows as TCP data, there are built-in
   timeouts and retransmits at the TCP layer for individual messages.
   Implementations also must protect the overall TCP/CLC handshake with
   a timer or timers to prevent connections from hanging indefinitely
   due to SMC-R processing.  This can be done with individual timers for
   individual CLC messages or an overall timer for the entire exchange,
   which may include the TCP handshake and the CLC handshake under one
   timer or separate timers.  This decision is implementation dependent.

   If the TCP and/or CLC handshakes time out, the TCP connection must be
   terminated as it would be in a legacy IP environment when connection
   setup doesn't complete in a timely manner.  Because the CLC flows are
   TCP messages, if they cannot be sent and received in a timely
   fashion, the TCP connection is not healthy and would not work if
   fallback to IP were attempted.

C.6.  Protocol Errors during CLC Negotiation

   Protocol errors occur during CLC negotiation when a message is
   received that is not expected.  For example, a peer that is expecting
   a CLC message but instead receives application data has experienced a
   protocol error; this also indicates a likely software error, as the
   two sides are out of sync.  When application data is expected, this
   data is not parsed to ensure that it's not a CLC message.

   When a peer is expecting a CLC negotiation message, any parsing error
   except a bad enumerated value in that message must be treated as
   application data.  The CLC negotiation messages are designed with
   beginning and ending eye catchers to help verify that a CLC
   negotiation message is actually the expected message.  If other
   parsing errors in an expected CLC message occur, such as incorrect
   length fields or incorrectly formatted fields, the message must be
   treated as application data.

   All protocol errors, with the exception of bad enumerated values,
   must result in termination of the TCP connection.  No fallback to IP
   is allowed in the case of a protocol error, because if the protocols
   are out of sync, mismatched, or corrupted, then data and security
   integrity cannot be ensured.






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   The exception to this rule is enumerated values -- for example, the
   QP MTU values on SMC Accept and SMC Confirm.  If a reserved value is
   received, the proper error response is to send an SMC Decline and
   fall back to IP; this is because the use of a reserved enumerated
   value indicates that the other partner likely has additional support
   that the receiving partner does not have.  This indicated mismatch of
   SMC-R capabilities is not an integrity problem but indicates that
   SMC-R cannot be used for this connection.

C.7.  Timeouts during LLC Negotiation

   Whenever a peer sends an LLC message to which a reply is expected, it
   sets a timer after the send posts to wait for the reply.  An expected
   response may be a reply flavor of the LLC message (for example, a
   CONFIRM LINK reply) or a new LLC message (for example, an ADD LINK
   CONTINUATION expected from the server by the client if there are more
   RKeys to be communicated).

   On LLC flows that are part of a first contact setup of a link group,
   the value of the timer is implementation dependent but should be long
   enough to allow the other peer to have a write complete timeout and
   2-3 retransmits of an SMC Decline on the TCP fabric.  For LLC flows
   that are maintaining the link group and are not part of a first
   contact setup of a link group, the timers may be shorter.  Upon
   receipt of an expected reply, the timer is cancelled.  If a timer
   pops without a reply having been received, the sender must initiate a
   recovery action.

   During first contact processing, failure of an LLC verification timer
   is a "should-not-occur" that indicates a problem with one of the
   endpoints; this is because if there is a "routine" failure in the
   RoCE fabric that causes an LLC verification send to fail, the sender
   will get a write completion failure and will then send an SMC Decline
   to the partner.  The only time an LLC verification timer will expire
   on a first contact is when the sender thinks the send succeeded but
   it actually didn't.  Because of the reliably connected nature of QP
   connections on the RoCE fabric, this indicates a problem with one of
   the peers, not with the RoCE fabric.

   After the reliably connected queue pair for the first SMC-R link in a
   link group is set up on initial contact, the client sets a timer to
   wait for a RoCE verification message from the server that the QP is
   actually connected and usable.  If the server experiences a failure
   sending its QP confirmation message, it will send an SMC Decline,
   which should arrive at the client before the client's verification
   timer expires.  If the client's timer expires without receiving
   either an SMC Decline or a RoCE message confirmation from the server,




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   there is a problem with either the server or the TCP fabric.  In
   either case, the client must break the TCP connection and clean up
   the SMC-R link.

   There are two scenarios in which the client's response to the QP
   verification message fails to reach the server.  The main difference
   is whether or not the client has successfully completed the send of
   the CONFIRM LINK response.

   In the normal case of a problem with the RoCE path, the client will
   learn of the failure by getting a write completion failure, before
   the server's timer expires.  In this case, the client sends an SMC
   Decline CLC message to the server, and the TCP connection falls back
   to IP.

   If the client's send of the confirmation message receives a positive
   return code but for some reason still does not reach the server, or
   the client's SMC Decline CLC message fails to reach the server after
   the client fails to send its RoCE confirmation message, then the
   server's timer will time out and the server must break the TCP
   connection by sending a RST.  This is expected to be a very rare
   case, because if the client cannot send its CONFIRM LINK response LLC
   message, the client should get a negative return code and initiate
   fallback to IP.  A client receiving a positive return code on a send
   that fails to reach the server should also be an extremely rare case.

C.7.1.  Recovery Actions for LLC Timeouts and Failures

   The following list describes recovery actions for LLC timeouts.  A
   write completion failure or other indication of send failure for an
   LLC command is treated the same as a timeout.

   LLC message: CONFIRM LINK from server (first contact, first link in
   the link group)

      Timer waits for: CONFIRM LINK reply from client.

      Recovery action: Break the TCP connection by sending a RST, and
      clean up the link.  The server should have received an SMC Decline
      from the client by now if the client had an LLC send failure.

   LLC message: CONFIRM LINK from server (first contact, second link in
   the link group)

      Timer waits for: CONFIRM LINK reply from client.






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      Recovery action: The second link was not successfully set up.
      Send a DELETE LINK to the client.  Connection data cannot flow in
      the first link in the link group, until the reply to this DELETE
      LINK is received, to prevent the peers from being out of sync on
      the state of the link group.

   LLC message: CONFIRM LINK from server (not first contact)

      Timer waits for: CONFIRM LINK reply from client.

      Recovery action: Clean up the new link, and set a timer to retry.
      Send a DELETE LINK to the client, in case the client has a longer
      timer interval, so the client can stop waiting.

   LLC message: CONFIRM LINK reply from client (first contact)

      Timer waits for: ADD LINK from server.

      Recovery action: Clean up the SMC-R link, and break the TCP
      connection by sending a RST over the IP fabric.  There is a
      problem with the server.  If the server had a send failure, it
      should have sent an SMC Decline by now.

   LLC message: ADD LINK from server (first contact)

      Timer waits for: ADD LINK reply from client.

      Recovery action: Break the TCP connection with a RST, and clean up
      RoCE resources.  The connection is past the point where the server
      can fall back to IP, and if the client had a send problem it
      should have sent an SMC Decline by now.

   LLC message: ADD LINK from server (not first contact)

      Timer waits for: ADD LINK reply from client.

      Recovery action: Clean up resources (QP, RKeys, etc.) for the new
      link, and treat the link over which the ADD LINK was sent as if it
      had failed.  If there is another link available to resend the
      ADD LINK and the link group still needs another link, retry the
      ADD LINK over another link in the link group.

   LLC message: ADD LINK reply from client (and there are more RKeys to
   be communicated)

      Timer waits for: ADD LINK CONTINUATION from server.

      Recovery action: Treat the same as ADD LINK timer failure.



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   LLC message: ADD LINK reply or ADD LINK CONTINUATION reply from
   client (and there are no more RKeys to be communicated, for the
   second link in a first contact scenario)

      Timer waits for: CONFIRM LINK from the server, over the new link.

      Recovery action: The setup of the new link failed.  Send a
      DELETE LINK to the server.  Do not consider the socket opened to
      the client application until receiving confirmation from the
      server in the form of a DELETE LINK request for this link and
      sending the reply (to prevent the partners from being out of sync
      on the state of the link group).

      Set a timer to send another ADD LINK to the server if there is
      still an unused RNIC on the client side.

   LLC message: ADD LINK reply or ADD LINK CONTINUATION reply from
   client (and there are no more RKeys to be communicated)

      Timer waits for: CONFIRM LINK from the server, over the new link.

      Recovery action: Send a DELETE LINK to the server for the new
      link, then clean up any resource allocated for the new link and
      set a timer to send an ADD LINK to the server if there is still an
      unused RNIC on the client side.  The setup of the new link failed,
      but the link over which the ADD LINK exchange occurred is
      unaffected.

   LLC message: ADD LINK CONTINUATION from server

      Timer waits for: ADD LINK CONTINUATION reply from client.

      Recovery action: Treat the same as ADD LINK timer failure.

   LLC message: ADD LINK CONTINUATION reply from client (first contact,
   and RMB count fields indicate that the server owes more ADD LINK
   CONTINUATION messages)

      Timer waits for: ADD LINK CONTINUATION from server.

      Recovery action: Clean up the SMC-R link, and break the TCP
      connection by sending a RST.  There is a problem with the server.

      If the server had a send failure, it should have sent an
      SMC Decline by now.






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   LLC message: ADD LINK CONTINUATION reply from client (not first
   contact, and RMB count fields indicate that the server owes more
   ADD LINK CONTINUATION messages)

      Timer waits for: ADD LINK CONTINUATION from server.

      Recovery action: Treat as if client detected link failure on the
      link that the ADD LINK exchange is using.  Send a DELETE LINK to
      the server over another active link if one exists; otherwise,
      clean up the link group.

   LLC message: DELETE LINK from client

      Timer waits for: DELETE LINK request from server.

      Recovery action: If the scope of the request is to delete a single
      link, the surviving link over which the client sent the
      DELETE LINK is no longer usable either.  If this is the last link
      in the link group, end TCP connections over the link group by
      sending RST packets.  If there are other surviving links in the
      link group, resend over a surviving link.  Also send a DELETE LINK
      over a surviving link for the link over which the client attempted
      to send the initial DELETE LINK message.  If the scope of the
      request is to delete the entire link group, try resending on other
      links in the link group until success is achieved.  If all sends
      fail, tear down the link group and any TCP connections that exist
      on it.

   LLC message: DELETE LINK from server (scope: entire link group)

      Timer waits for: Confirmation from the adapter that the message
      was delivered.

      Recovery action: Tear down the link group and any TCP connections
      that exist on it.

   LLC message: DELETE LINK from server (scope: single link)

      Timer waits for: DELETE LINK reply from client.

      Recovery action: The link over which the server sent the
      DELETE LINK is no longer usable either.  If this is the last link
      in the link group, end TCP connections over the link group by
      sending RST packets.  If there are other surviving links in the
      link group, resend over a surviving link.  Also send a DELETE LINK
      over a surviving link for the link over which the server attempted
      to send the initial DELETE LINK message.  If the scope of the
      request is to delete the entire link group, try resending on other



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      links in the link group until success is achieved.  If all sends
      fail, tear down the link group and any TCP connections that exist
      on it.

   LLC message: CONFIRM RKEY from client

      Timer waits for: CONFIRM RKEY reply from server.

      Recovery action: Perform normal client procedures for detection of
      failed link.  The link over which the message was sent has failed.

   LLC message: CONFIRM RKEY from server

      Timer waits for: CONFIRM RKEY reply from client.

      Recovery action: Perform normal server procedures for detection of
      failed link.  The link over which the message was sent has failed.

   LLC message: TEST LINK from client

      Timer waits for: TEST LINK reply from server.

      Recovery action: Perform normal client procedures for detection of
      failed link.  The link over which the message was sent has failed.

   LLC message: TEST LINK from server

      Timer waits for: TEST LINK reply from client.

      Recovery action: Perform normal server procedures for detection of
      failed link.  The link over which the message was sent has failed.

   The following list describes recovery actions for invalid LLC
   messages.  These could be misformatted or contain out-of-sync data.

   LLC message received: CONFIRM LINK from server

      What it indicates: Incorrect link information.

      Recovery action: Protocol error.  The link must be brought down by
      sending a DELETE LINK for the link over another link in the link
      group if one exists.  If this is a first contact, fall back to IP
      by sending an SMC Decline to the server.








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   LLC message received: ADD LINK

      What it indicates: Undefined enumerated MTU value.

      Recovery action: Send a negative ADD LINK reply with reason
      code x'2'.

   LLC message received: ADD LINK reply from client

      What it indicates: Client-side link information that would result
      in a parallel link being set up.

      Recovery action: Parallel links are not permitted.  Delete the
      link by sending a DELETE LINK to the client over another link in
      the link group.

   LLC message received: Any link group command from the server, except
   DELETE LINK for the entire link group

      What it indicates: Client has sent a DELETE LINK for the link on
      which the message was received.

      Recovery action: Ignore the LLC message.  Worst case: the server
      will time out.  Best case: the DELETE LINK crosses with the
      command from the server, and the server realizes it failed.

   LLC message received: ADD LINK CONTINUATION from server or ADD LINK
   CONTINUATION reply from client

      What it indicates: Number of RMBs provided doesn't match count
      given on initial ADD LINK or ADD LINK reply message.

      Recovery action: Protocol error.  Treat as if detected link
      outage.

   LLC message received: DELETE LINK from client

      What it indicates: Link indicated doesn't exist.

      Recovery action: If the link is in the process of being cleaned
      up, assume timing window and ignore message.  Otherwise, send a
      DELETE LINK reply with reason code 1.

   LLC message received: DELETE LINK from server

      What it indicates: Link indicated doesn't exist.

      Recovery action: Send a DELETE LINK reply with reason code 1.



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   LLC message received: CONFIRM RKEY from either client or server

      What it indicates: No RKey provided for one or more of the links
      in the link group.

      Recovery action: Treat as if detected failure of the link(s) for
      which no RKey was provided.

   LLC message received: DELETE RKEY

      What it indicates: Specified RKey doesn't exist.

      Recovery action: Send a negative DELETE RKEY response.

   LLC message received: TEST LINK reply

      What it indicates: User data doesn't match what was sent in the
      TEST LINK request.

      Recovery action: Treat as if detected that the link has gone down.
      This is a protocol error.

   LLC message received: Unknown LLC type with high-order bits of opcode
   equal to b'10'

      What it indicates: This is an optional LLC message that the
      receiver does not support.

      Recovery action: Ignore (silently discard) the message.

   LLC message received: Any unambiguously incorrect or out-of-sync LLC
   message

      What it indicates: Link is out of sync.

      Recovery action: Treat as if detected that the link has gone down.
      Note that an unsupported or unknown LLC opcode whose two
      high-order bits are b'10' is not an error and must be silently
      discarded.  Any other unknown or unsupported LLC opcode is an
      error.

C.8.  Failure to Add Second SMC-R Link to a Link Group

   When there is any failure in setting up the second SMC-R link in an
   SMC-R link group, including confirmation timer expiration, the SMC-R
   link group is allowed to continue without available failover.
   However, this situation is extremely undesirable, and the server must
   endeavor to correct it as soon as it can.



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   The server peer in the SMC-R link group must set a timer to drive it
   to retry setup of a failed additional SMC-R link.  The server will
   immediately retry the SMC-R link setup when the first of the
   following events occurs:

   o  The retry timer expires.

   o  A new RNIC becomes available to the server, on the same LAN as the
      SMC-R link group.

   o  An ADD LINK LLC request message is received from the client; this
      indicates the availability of a new RNIC on the client side.

Authors' Addresses

   Mike Fox
   IBM
   3039 Cornwallis Rd.
   Research Triangle Park, NC  27709
   United States

   Email: mjfox@us.ibm.com


   Constantinos (Gus) Kassimis
   IBM
   3039 Cornwallis Rd.
   Research Triangle Park, NC  27709
   United States

   Email: kassimis@us.ibm.com


   Jerry Stevens
   IBM
   3039 Cornwallis Rd.
   Research Triangle Park, NC  27709
   United States

   Email: sjerry@us.ibm.com











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