path: root/doc/rfc/rfc8490.txt

Internet Engineering Task Force (IETF)                         R. Bellis
Request for Comments: 8490                                           ISC
Updates: 1035, 7766                                          S. Cheshire
Category: Standards Track                                     Apple Inc.
ISSN: 2070-1721                                             J. Dickinson
                                                            S. Dickinson
                                                                 Sinodun
                                                                T. Lemon
                                                     Nibbhaya Consulting
                                                             T. Pusateri
                                                            Unaffiliated
                                                              March 2019


                        DNS Stateful Operations

Abstract

   This document defines a new DNS OPCODE for DNS Stateful Operations
   (DSO).  DSO messages communicate operations within persistent
   stateful sessions using Type Length Value (TLV) syntax.  Three TLVs
   are defined that manage session timeouts, termination, and encryption
   padding, and a framework is defined for extensions to enable new
   stateful operations.  This document updates RFC 1035 by adding a new
   DNS header OPCODE that has both different message semantics and a new
   result code.  This document updates RFC 7766 by redefining a session,
   providing new guidance on connection reuse, and providing a new
   mechanism for handling session idle timeouts.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8490.









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Copyright Notice

   Copyright (c) 2019 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
   (https://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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.1.1.  Session Management  . . . . . . . . . . . . . . . . .   9
       4.1.2.  Long-Lived Subscriptions  . . . . . . . . . . . . . .   9
     4.2.  Applicable Transports . . . . . . . . . . . . . . . . . .  10
   5.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  DSO Session Establishment . . . . . . . . . . . . . . . .  12
       5.1.1.  DSO Session Establishment Failure . . . . . . . . . .  13
       5.1.2.  DSO Session Establishment Success . . . . . . . . . .  14
     5.2.  Operations after DSO Session Establishment  . . . . . . .  14
     5.3.  DSO Session Termination . . . . . . . . . . . . . . . . .  15
       5.3.1.  Handling Protocol Errors  . . . . . . . . . . . . . .  15
     5.4.  Message Format  . . . . . . . . . . . . . . . . . . . . .  16
       5.4.1.  DNS Header Fields in DSO Messages . . . . . . . . . .  17
       5.4.2.  DSO Data  . . . . . . . . . . . . . . . . . . . . . .  18
       5.4.3.  DSO Unidirectional Messages . . . . . . . . . . . . .  20
       5.4.4.  TLV Syntax  . . . . . . . . . . . . . . . . . . . . .  21
       5.4.5.  Unrecognized TLVs . . . . . . . . . . . . . . . . . .  22
       5.4.6.  EDNS(0) and TSIG  . . . . . . . . . . . . . . . . . .  23
     5.5.  Message Handling  . . . . . . . . . . . . . . . . . . . .  24
       5.5.1.  Delayed Acknowledgement Management  . . . . . . . . .  25
       5.5.2.  MESSAGE ID Namespaces . . . . . . . . . . . . . . . .  26
       5.5.3.  Error Responses . . . . . . . . . . . . . . . . . . .  27
     5.6.  Responder-Initiated Operation Cancellation  . . . . . . .  28
   6.  DSO Session Lifecycle and Timers  . . . . . . . . . . . . . .  29
     6.1.  DSO Session Initiation  . . . . . . . . . . . . . . . . .  29
     6.2.  DSO Session Timeouts  . . . . . . . . . . . . . . . . . .  30
     6.3.  Inactive DSO Sessions . . . . . . . . . . . . . . . . . .  31



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     6.4.  The Inactivity Timeout  . . . . . . . . . . . . . . . . .  32
       6.4.1.  Closing Inactive DSO Sessions . . . . . . . . . . . .  32
       6.4.2.  Values for the Inactivity Timeout . . . . . . . . . .  33
     6.5.  The Keepalive Interval  . . . . . . . . . . . . . . . . .  34
       6.5.1.  Keepalive Interval Expiry . . . . . . . . . . . . . .  34
       6.5.2.  Values for the Keepalive Interval . . . . . . . . . .  34
     6.6.  Server-Initiated DSO Session Termination  . . . . . . . .  36
       6.6.1.  Server-Initiated Retry Delay Message  . . . . . . . .  37
       6.6.2.  Misbehaving Clients . . . . . . . . . . . . . . . . .  38
       6.6.3.  Client Reconnection . . . . . . . . . . . . . . . . .  38
   7.  Base TLVs for DNS Stateful Operations . . . . . . . . . . . .  40
     7.1.  Keepalive TLV . . . . . . . . . . . . . . . . . . . . . .  40
       7.1.1.  Client Handling of Received Session Timeout Values  .  42
       7.1.2.  Relationship to edns-tcp-keepalive EDNS(0) Option . .  43
     7.2.  Retry Delay TLV . . . . . . . . . . . . . . . . . . . . .  44
       7.2.1.  Retry Delay TLV Used as a Primary TLV . . . . . . . .  44
       7.2.2.  Retry Delay TLV Used as a Response Additional TLV . .  46
     7.3.  Encryption Padding TLV  . . . . . . . . . . . . . . . . .  46
   8.  Summary Highlights  . . . . . . . . . . . . . . . . . . . . .  47
     8.1.  QR Bit and MESSAGE ID . . . . . . . . . . . . . . . . . .  47
     8.2.  TLV Usage . . . . . . . . . . . . . . . . . . . . . . . .  48
   9.  Additional Considerations . . . . . . . . . . . . . . . . . .  50
     9.1.  Service Instances . . . . . . . . . . . . . . . . . . . .  50
     9.2.  Anycast Considerations  . . . . . . . . . . . . . . . . .  51
     9.3.  Connection Sharing  . . . . . . . . . . . . . . . . . . .  52
     9.4.  Operational Considerations for Middleboxes  . . . . . . .  53
     9.5.  TCP Delayed Acknowledgement Considerations  . . . . . . .  54
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  57
     10.1.  DSO OPCODE Registration  . . . . . . . . . . . . . . . .  57
     10.2.  DSO RCODE Registration . . . . . . . . . . . . . . . . .  57
     10.3.  DSO Type Code Registry . . . . . . . . . . . . . . . . .  57
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  59
     11.1.  TLS Zero Round-Trip Considerations . . . . . . . . . . .  59
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  60
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  60
     12.2.  Informative References . . . . . . . . . . . . . . . . .  61
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  63
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  63













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1.  Introduction

   This document specifies a mechanism for managing stateful DNS
   connections.  DNS most commonly operates over a UDP transport, but it
   can also operate over streaming transports; the original DNS RFC
   specifies DNS-over-TCP [RFC1035], and a profile for DNS-over-TLS
   [RFC7858] has been specified.  These transports can offer persistent
   long-lived sessions and therefore, when using them for transporting
   DNS messages, it is of benefit to have a mechanism that can establish
   parameters associated with those sessions, such as timeouts.  In such
   situations, it is also advantageous to support server-initiated
   messages (such as DNS Push Notifications [Push]).

   The existing Extension Mechanism for DNS (EDNS(0)) [RFC6891] is
   explicitly defined to only have "per-message" semantics.  While
   EDNS(0) has been used to signal at least one session-related
   parameter (edns-tcp-keepalive EDNS(0) Option [RFC7828]), the result
   is less than optimal due to the restrictions imposed by the EDNS(0)
   semantics and the lack of server-initiated signaling.  For example, a
   server cannot arbitrarily instruct a client to close a connection
   because the server can only send EDNS(0) options in responses to
   queries that contained EDNS(0) options.

   This document defines a new DNS OPCODE for DNS Stateful Operations
   (DSO) with a value of 6.  DSO messages are used to communicate
   operations within persistent stateful sessions, expressed using Type
   Length Value (TLV) syntax.  This document defines an initial set of
   three TLVs used to manage session timeouts, termination, and
   encryption padding.

   All three TLVs defined here are mandatory for all implementations of
   DSO.  Further TLVs may be defined in additional specifications.

   DSO messages may or may not be acknowledged.  Whether a DSO message
   is to be acknowledged (a DSO request message) or is not to be
   acknowledged (a DSO unidirectional message) is specified in the
   definition of that particular DSO message type.  The MESSAGE ID is
   nonzero for DSO request messages, and zero for DSO unidirectional
   messages.  Messages are pipelined and responses may appear out of
   order when multiple requests are being processed concurrently.

   The format for DSO messages (Section 5.4) differs somewhat from the
   traditional DNS message format used for standard queries and
   responses.  The standard twelve-byte header is used, but the four
   count fields (QDCOUNT, ANCOUNT, NSCOUNT, ARCOUNT) are set to zero,
   and accordingly their corresponding sections are not present.





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   The actual data pertaining to DNS Stateful Operations (expressed in
   TLV syntax) is appended to the end of the DNS message header.  Just
   as in traditional DNS-over-TCP [RFC1035] [RFC7766], the stream
   protocol carrying DSO messages (which are just another kind of DNS
   message) frames them by putting a 16-bit message length at the start.
   The length of the DSO message is therefore determined from that
   length rather than from any of the DNS header counts.

   When displayed using packet analyzer tools that have not been updated
   to recognize the DSO format, this will result in the DSO data being
   displayed as unknown extra data after the end of the DNS message.

   This new format has distinct advantages over an RR-based format
   because it is more explicit and more compact.  Each TLV definition is
   specific to its use case and, as a result, contains no redundant or
   overloaded fields.  Importantly, it completely avoids conflating DNS
   Stateful Operations in any way with normal DNS operations or with
   existing EDNS(0)-based functionality.  A goal of this approach is to
   avoid the operational issues that have befallen EDNS(0), particularly
   relating to middlebox behavior (see sections discussing EDNS(0), and
   problems caused by firewalls and load balancers, in the recent work
   describing causes of DNS failures [Fail]).

   With EDNS(0), multiple options may be packed into a single OPT
   pseudo-RR, and there is no generalized mechanism for a client to be
   able to tell whether a server has processed or otherwise acted upon
   each individual option within the combined OPT pseudo-RR.  The
   specifications for each individual option need to define how each
   different option is to be acknowledged, if necessary.

   In contrast to EDNS(0), with DSO there is no compelling motivation to
   pack multiple operations into a single message for efficiency
   reasons, because DSO always operates using a connection-oriented
   transport protocol.  Each DSO operation is communicated in its own
   separate DNS message, and the transport protocol can take care of
   packing several DNS messages into a single IP packet if appropriate.
   For example, TCP can pack multiple small DNS messages into a single
   TCP segment.  This simplification allows for clearer semantics.  Each
   DSO request message communicates just one primary operation, and the
   RCODE in the corresponding response message indicates the success or
   failure of that operation.










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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   DSO:  DNS Stateful Operations.

   connection:  a bidirectional byte (or message) stream, where the
      bytes (or messages) are delivered reliably and in order, such as
      provided by using DNS-over-TCP [RFC1035] [RFC7766] or DNS-over-TLS
      [RFC7858].

   session:  the unqualified term "session" in the context of this
      document refers to a persistent network connection between two
      endpoints that allows for the exchange of DNS messages over a
      connection where either end of the connection can send messages to
      the other end.  (The term has no relationship to the "session
      layer" of the OSI "seven-layer model".)

   DSO Session:  a session established between two endpoints that
      acknowledge persistent DNS state via the exchange of DSO messages
      over the connection.  This is distinct from a DNS-over-TCP session
      as described in the previous specification for DNS-over-TCP
      [RFC7766].

   close gracefully:  a normal session shutdown where the client closes
      the TCP connection to the server using a graceful close such that
      no data is lost (e.g., using TCP FIN; see Section 5.3).

   forcibly abort:  a session shutdown as a result of a fatal error
      where the TCP connection is unilaterally aborted without regard
      for data loss (e.g., using TCP RST; see Section 5.3).

   server:  the software with a listening socket, awaiting incoming
      connection requests, in the usual DNS sense.

   client:  the software that initiates a connection to the server's
      listening socket, in the usual DNS sense.

   initiator:  the software that sends a DSO request message or a DSO
      unidirectional message during a DSO Session.  Either a client or
      server can be an initiator.




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   responder:  the software that receives a DSO request message or a DSO
      unidirectional message during a DSO Session.  Either a client or a
      server can be a responder.

   sender:  the software that is sending a DNS message, a DSO message, a
      DNS response, or a DSO response.

   receiver:  the software that is receiving a DNS message, a DSO
      message, a DNS response, or a DSO response.

   service instance:  a specific instance of server software running on
      a specific host (Section 9.1).

   long-lived operation:  an outstanding operation on a DSO Session
      where either the client or server, acting as initiator, has
      requested that the responder send new information regarding the
      request, as it becomes available.

   early data:  a TLS 1.3 handshake containing data on the first flight
      that begins a DSO Session (Section 2.3 of the TLS 1.3
      specification [RFC8446]).  TCP Fast Open [RFC7413] is only
      permitted when using TLS.

   DNS message:  any DNS message, including DNS queries, responses,
      updates, DSO messages, etc.

   DNS request message:  any DNS message where the QR bit is 0.

   DNS response message:  any DNS message where the QR bit is 1.

   DSO message:  a DSO request message, DSO unidirectional message, or
      DSO response to a DSO request message.  If the QR bit is 1 in a
      DSO message, it is a DSO response message.  If the QR bit is 0 in
      a DSO message, it is a DSO request message or DSO unidirectional
      message, as determined by the specification of its Primary TLV.

   DSO response message:  a response to a DSO request message.

   DSO request message:  a DSO message that requires a response.

   DSO unidirectional message:  a DSO message that does not require and
      cannot induce a response.

   Primary TLV:  the first TLV in a DSO request message or DSO
      unidirectional message; this determines the nature of the
      operation being performed.





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   Additional TLV:  any TLVs that follow the Primary TLV in a DSO
      request message or DSO unidirectional message.

   Response Primary TLV:  in a DSO response, any TLVs with the same DSO-
      TYPE as the Primary TLV from the corresponding DSO request
      message.  If present, any Response Primary TLV(s) MUST appear
      first in the DSO response message, before any Response Additional
      TLVs.

   Response Additional TLV:  any TLVs in a DSO response that follow the
      (optional) Response Primary TLV(s).

   inactivity timer:  the time since the most recent non-keepalive DNS
      message was sent or received (see Section 6.4).

   keepalive timer:  the time since the most recent DNS message was sent
      or received (see Section 6.5).

   session timeouts:  the inactivity timer and the keepalive timer.

   inactivity timeout:  the maximum value that the inactivity timer can
      have before the connection is gracefully closed.

   keepalive interval:  the maximum value that the keepalive timer can
      have before the client is required to send a keepalive (see
      Section 7.1).

   resetting a timer:  setting the timer value to zero and restarting
      the timer.

   clearing a timer:  setting the timer value to zero but not restarting
      the timer.



















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4.  Applicability

   DNS Stateful Operations are applicable to several known use cases and
   are only applicable on transports that are capable of supporting a
   DSO Session.

4.1.  Use Cases

   Several use cases for DNS Stateful Operations are described below.

4.1.1.  Session Management

   In one use case, establishing session parameters such as server-
   defined timeouts is of great use in the general management of
   persistent connections.  For example, using DSO Sessions for stub-to-
   recursive DNS-over-TLS [RFC7858] is more flexible for both the client
   and the server than attempting to manage sessions using just the
   edns-tcp-keepalive EDNS(0) Option [RFC7828].  The simple set of TLVs
   defined in this document is sufficient to greatly enhance connection
   management for this use case.

4.1.2.  Long-Lived Subscriptions

   In another use case, DNS-based Service Discovery (DNS-SD) [RFC6763]
   has evolved into a naturally session-based mechanism where, for
   example, long-lived subscriptions lend themselves to 'push'
   mechanisms as opposed to polling.  Long-lived stateful connections
   and server-initiated messages align with this use case [Push].

   A general use case is that DNS traffic is often bursty, but session
   establishment can be expensive.  One challenge with long-lived
   connections is sustaining sufficient traffic to maintain NAT and
   firewall state.  To mitigate this issue, this document introduces a
   new concept for the DNS -- DSO "keepalive traffic".  This traffic
   carries no DNS data and is not considered 'activity' in the classic
   DNS sense, but it serves to maintain state in middleboxes and to
   assure the client and server that they still have connectivity to
   each other.













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4.2.  Applicable Transports

   DNS Stateful Operations are applicable in cases where it is useful to
   maintain an open session between a DNS client and server, where the
   transport allows such a session to be maintained, and where the
   transport guarantees in-order delivery of messages on which DSO
   depends.  Two specific transports that meet the requirements to
   support DNS Stateful Operations are DNS-over-TCP [RFC1035] [RFC7766]
   and DNS-over-TLS [RFC7858].

   Note that in the case of DNS-over-TLS, there is no mechanism for
   upgrading from DNS-over-TCP to DNS-over-TLS mid-connection (see
   Section 7 of the DNS-over-TLS specification [RFC7858]).  A connection
   is either DNS-over-TCP from the start, or DNS-over-TLS from the
   start.

   DNS Stateful Operations are not applicable for transports that cannot
   support clean session semantics or that do not guarantee in-order
   delivery.  While in principle such a transport could be constructed
   over UDP, the current specification of DNS-over-UDP [RFC1035] does
   not provide in-order delivery or session semantics and hence cannot
   be used.  Similarly, DNS-over-HTTP [RFC8484] cannot be used because
   HTTP has its own mechanism for managing sessions, which is
   incompatible with the mechanism specified here.

   Only DNS-over-TCP and DNS-over-TLS are currently defined for use with
   DNS Stateful Operations.  Other transports may be added in the future
   if they meet the requirements set out in the first paragraph of this
   section.






















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5.  Protocol Details

   The overall flow of DNS Stateful Operations goes through a series of
   phases:

   Connection Establishment:  A client establishes a connection to a
      server (Section 4.2).

   Connected but Sessionless:  A connection exists, but a DSO Session
      has not been established.  DNS messages can be sent from the
      client to server, and DNS responses can be sent from the server to
      the client.  In this state, a client that wishes to use DSO can
      attempt to establish a DSO Session (Section 5.1).  Standard DNS-
      over-TCP inactivity timeout handling is in effect [RFC7766] (see
      Section 7.1.2 of this document).

   DSO Session Establishment in Progress:  A client has sent a DSO
      request within the last 30 seconds, but has not yet received a DSO
      response for that request.  In this phase, the client may send
      more DSO requests and more DNS requests, but MUST NOT send DSO
      unidirectional messages (Section 5.1).

   DSO Session Establishment Timeout:  A client has sent a DSO request,
      and after 30 seconds has still received no DSO response for that
      request.  This means that the server is now in an indeterminate
      state.  The client forcibly aborts the connection.  The client MAY
      reconnect without using DSO, if appropriate.

   DSO Session Establishment Failed:  A client has sent a DSO request,
      and received a corresponding DSO response with a nonzero RCODE.
      This means that the attempt to establish the DSO Session did not
      succeed.  At this point, the client is permitted to continue
      operating without a DSO Session (Connected but Sessionless) but
      does not send further DSO messages (Section 5.1).

   DSO Session Established:  A client has sent a DSO request, and
      received a corresponding DSO response with RCODE set to NOERROR
      (0).  A DSO Session has now been successfully established.  Both
      client and server may send DSO messages and DNS messages; both may
      send replies in response to messages they receive (Section 5.2).
      The inactivity timer (Section 6.4) is active; the keepalive timer
      (Section 6.5) is active.  Standard DNS-over-TCP inactivity timeout
      handling is no longer in effect [RFC7766] (see Section 7.1.2 of
      this document).







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   Server Shutdown:  The server has decided to gracefully terminate the
      session and has sent the client a Retry Delay message
      (Section 6.6.1).  There may still be unprocessed messages from the
      client; the server will ignore these.  The server will not send
      any further messages to the client (Section 6.6.1.1).

   Client Shutdown:  The client has decided to disconnect, either
      because it no longer needs service, the connection is inactive
      (Section 6.4.1), or because the server sent it a Retry Delay
      message (Section 6.6.1).  The client closes the connection
      gracefully (Section 5.3).

   Reconnect:  The client disconnected as a result of a server shutdown.
      The client either waits for the server-specified Retry Delay to
      expire (Section 6.6.3) or else contacts a different server
      instance.  If the client no longer needs service, it does not
      reconnect.

   Forcibly Abort:  The client or server detected a protocol error, and
      further communication would have undefined behavior.  The client
      or server forcibly aborts the connection (Section 5.3).

   Abort Reconnect Wait:  The client has forcibly aborted the connection
      but still needs service.  Or, the server forcibly aborted the
      connection, but the client still needs service.  The client either
      connects to a different service instance (Section 9.1) or waits to
      reconnect (Section 6.6.3.1).

5.1.  DSO Session Establishment

   In order for a session to be established between a client and a
   server, the client must first establish a connection to the server
   using an applicable transport (see Section 4.2).

   In some environments, it may be known in advance by external means
   that both client and server support DSO, and in these cases either
   client or server may initiate DSO messages at any time.  In this
   case, the session is established as soon as the connection is
   established; this is referred to as implicit DSO Session
   establishment.

   However, in the typical case a server will not know in advance
   whether a client supports DSO, so in general, unless it is known in
   advance by other means that a client does support DSO, a server MUST
   NOT initiate DSO request messages or DSO unidirectional messages
   until a DSO Session has been mutually established by at least one
   successful DSO request/response exchange initiated by the client, as




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   described below.  This is referred to as explicit DSO Session
   establishment.

   Until a DSO Session has been implicitly or explicitly established, a
   client MUST NOT initiate DSO unidirectional messages.

   A DSO Session is established over a connection by the client sending
   a DSO request message, such as a DSO Keepalive request message
   (Section 7.1), and receiving a response with a matching MESSAGE ID,
   and RCODE set to NOERROR (0), indicating that the DSO request was
   successful.

   Some DSO messages are permitted as early data (Section 11.1).  Others
   are not.  Unidirectional messages are never permitted as early data,
   unless an implicit DSO Session exists.

   If a server receives a DSO message in early data whose Primary TLV is
   not permitted to appear in early data, the server MUST forcibly abort
   the connection.  If a client receives a DSO message in early data,
   and there is no implicit DSO Session, the client MUST forcibly abort
   the connection.  This can only be enforced on TLS connections;
   therefore, servers MUST NOT enable TCP Fast Open (TFO) when listening
   for a connection that does not require TLS.

5.1.1.  DSO Session Establishment Failure

   If the response RCODE is set to NOTIMP (4), or in practice any value
   other than NOERROR (0) or DSOTYPENI (defined below), then the client
   MUST assume that the server does not implement DSO at all.  In this
   case, the client is permitted to continue sending DNS messages on
   that connection but MUST NOT issue further DSO messages on that
   connection.

   If the RCODE in the response is set to DSOTYPENI ("DSO-TYPE Not
   Implemented"; RCODE 11), this indicates that the server does support
   DSO but does not implement the DSO-TYPE of the Primary TLV in this
   DSO request message.  A server implementing DSO MUST NOT return
   DSOTYPENI for a DSO Keepalive request message because the Keepalive
   TLV is mandatory to implement.  But in the future, if a client
   attempts to establish a DSO Session using a response-requiring DSO
   request message using some newly-defined DSO-TYPE that the server
   does not understand, that would result in a DSOTYPENI response.  If
   the server returns DSOTYPENI, then a DSO Session is not considered
   established.  The client is, however, permitted to continue sending
   DNS messages on the connection, including other DSO messages such as
   the DSO Keepalive, which may result in a successful NOERROR response,
   yielding the establishment of a DSO Session.




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   When a DSO message is received by an existing DNS server that doesn't
   recognize the DSO OPCODE, two other possible outcomes exist: the
   server might send no response to the DSO message, or the server might
   drop the connection.

   If the server sends no response to the DSO message, the client SHOULD
   wait 30 seconds, after which time the server will be assumed not to
   support DSO.  If the server doesn't respond within 30 seconds, it can
   be assumed that it is not going to respond; this leaves it in an
   unspecified state: there is no specification requiring that a
   response be sent to an unknown message, but there is also no
   specification stating what state the server is in if no response is
   sent.  Therefore the client MUST forcibly abort the connection to the
   server.  The client MAY reconnect, but not use DSO, if appropriate
   (Section 6.6.3.1).  By disconnecting and reconnecting, the client
   ensures that the server is in a known state before sending any
   subsequent requests.

   If the server drops the connection the client SHOULD mark that
   service instance as not supporting DSO, and not attempt a DSO
   connection for some period of time (at least an hour) after the
   failed attempt.  The client MAY reconnect but not use DSO, if
   appropriate (Section 6.6.3.2).

5.1.2.  DSO Session Establishment Success

   When the server receives a DSO request message from a client, and
   transmits a successful NOERROR response to that request, the server
   considers the DSO Session established.

   When the client receives the server's NOERROR response to its DSO
   request message, the client considers the DSO Session established.

   Once a DSO Session has been established, either end may unilaterally
   send appropriate DSO messages at any time, and therefore either
   client or server may be the initiator of a message.

5.2.  Operations after DSO Session Establishment

   Once a DSO Session has been established, clients and servers should
   behave as described in this specification with regard to inactivity
   timeouts and session termination, not as previously prescribed in the
   earlier specification for DNS-over-TCP [RFC7766].

   Because a server that supports DNS Stateful Operations MUST return an
   RCODE of "NOERROR" when it receives a Keepalive TLV DSO request
   message, the Keepalive TLV is an ideal candidate for use in
   establishing a DSO Session.  Any other option that can only succeed



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   when sent to a server of the desired kind is also a good candidate
   for use in establishing a DSO Session.  For clients that implement
   only the DSO-TYPEs defined in this base specification, sending a
   Keepalive TLV is the only DSO request message they have available to
   initiate a DSO Session.  Even for clients that do implement other
   future DSO-TYPEs, for simplicity they MAY elect to always send an
   initial DSO Keepalive request message as their way of initiating a
   DSO Session.  A future definition of a new response-requiring DSO-
   TYPE gives implementers the option of using that new DSO-TYPE if they
   wish, but does not change the fact that sending a Keepalive TLV
   remains a valid way of initiating a DSO Session.

5.3.  DSO Session Termination

   A DSO Session is terminated when the underlying connection is closed.
   DSO Sessions are "closed gracefully" as a result of the server
   closing a DSO Session because it is overloaded, because of the client
   closing the DSO Session because it is done, or because of the client
   closing the DSO Session because it is inactive.  DSO Sessions are
   "forcibly aborted" when either the client or server closes the
   connection because of a protocol error.

   o  Where this specification says "close gracefully", it means sending
      a TLS close_notify (if TLS is in use) followed by a TCP FIN, or
      the equivalent for other protocols.  Where this specification
      requires a connection to be closed gracefully, the requirement to
      initiate that graceful close is placed on the client in order to
      place the burden of TCP's TIME-WAIT state on the client rather
      than the server.

   o  Where this specification says "forcibly abort", it means sending a
      TCP RST or the equivalent for other protocols.  In the BSD Sockets
      API, this is achieved by setting the SO_LINGER option to zero
      before closing the socket.

5.3.1.  Handling Protocol Errors

   In protocol implementation, there are generally two kinds of errors
   that software writers have to deal with.  The first is situations
   that arise due to factors in the environment, such as temporary loss
   of connectivity.  While undesirable, these situations do not indicate
   a flaw in the software and are situations that software should
   generally be able to recover from.

   The second is situations that should never happen when communicating
   with a compliant DSO implementation.  If they do happen, they
   indicate a serious flaw in the protocol implementation beyond what is
   reasonable to expect software to recover from.  This document



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   describes this latter form of error condition as a "fatal error" and
   specifies that an implementation encountering a fatal error condition
   "MUST forcibly abort the connection immediately".

5.4.  Message Format

   A DSO message begins with the standard twelve-byte DNS message header
   [RFC1035] with the OPCODE field set to the DSO OPCODE (6).  However,
   unlike standard DNS messages, the question section, answer section,
   authority records section, and additional records sections are not
   present.  The corresponding count fields (QDCOUNT, ANCOUNT, NSCOUNT,
   ARCOUNT) MUST be set to zero on transmission.

   If a DSO message is received where any of the count fields are not
   zero, then a FORMERR MUST be returned.

                                                1   1   1   1   1   1
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                          MESSAGE ID                           |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |QR |  OPCODE (6)   |            Z              |     RCODE     |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                     QDCOUNT (MUST be zero)                    |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                     ANCOUNT (MUST be zero)                    |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                     NSCOUNT (MUST be zero)                    |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                     ARCOUNT (MUST be zero)                    |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                                                               |
      /                           DSO Data                            /
      /                                                               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
















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5.4.1.  DNS Header Fields in DSO Messages

   In a DSO unidirectional message, the MESSAGE ID field MUST be set to
   zero.  In a DSO request message, the MESSAGE ID field MUST be set to
   a unique nonzero value that the initiator is not currently using for
   any other active operation on this connection.  For the purposes
   here, a MESSAGE ID is in use in this DSO Session if the initiator has
   used it in a DSO request message for which it is still awaiting a
   response, or if the client has used it to set up a long-lived
   operation that has not yet been canceled.  For example, a long-lived
   operation could be a Push Notification subscription [Push] or a
   Discovery Relay interface subscription [Relay].

   Whether a message is a DSO request message or a DSO unidirectional
   message is determined only by the specification for the Primary TLV.
   An acknowledgment cannot be requested by including a nonzero MESSAGE
   ID in a message that is required according to its Primary TLV to be
   unidirectional.  Nor can an acknowledgment be prevented by sending a
   MESSAGE ID of zero in a message that is required to be a DSO request
   message according to its Primary TLV.  A responder that receives
   either such malformed message MUST treat it as a fatal error and
   forcibly abort the connection immediately.

   In a DSO request message or DSO unidirectional message, the DNS
   Header Query/Response (QR) bit MUST be zero (QR=0).  If the QR bit is
   not zero, the message is not a DSO request or DSO unidirectional
   message.

   In a DSO response message, the DNS Header QR bit MUST be one (QR=1).
   If the QR bit is not one, the message is not a DSO response message.

   In a DSO response message (QR=1), the MESSAGE ID field MUST NOT be
   zero, and MUST contain a copy of the value of the (nonzero) MESSAGE
   ID field in the DSO request message being responded to.  If a DSO
   response message (QR=1) is received where the MESSAGE ID is zero,
   this is a fatal error and the recipient MUST forcibly abort the
   connection immediately.

   The DNS Header OPCODE field holds the DSO OPCODE value (6).

   The Z bits are currently unused in DSO messages; in both DSO request
   messages and DSO responses, the Z bits MUST be set to zero (0) on
   transmission and MUST be ignored on reception.

   In a DSO request message (QR=0), the RCODE is set according to the
   definition of the request.  For example, in a Retry Delay message
   (Section 6.6.1), the RCODE indicates the reason for termination.
   However, in most DSO request messages (QR=0), except where clearly



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   specified otherwise, the RCODE is set to zero on transmission, and
   silently ignored on reception.

   The RCODE value in a response message (QR=1) may be one of the
   following values:

   +------+-----------+------------------------------------------------+
   | Code | Mnemonic  | Description                                    |
   +------+-----------+------------------------------------------------+
   |    0 | NOERROR   | Operation processed successfully               |
   |      |           |                                                |
   |    1 | FORMERR   | Format error                                   |
   |      |           |                                                |
   |    2 | SERVFAIL  | Server failed to process DSO request message   |
   |      |           | due to a problem with the server               |
   |      |           |                                                |
   |    4 | NOTIMP    | DSO not supported                              |
   |      |           |                                                |
   |    5 | REFUSED   | Operation declined for policy reasons          |
   |      |           |                                                |
   |   11 | DSOTYPENI | Primary TLV's DSO-Type is not implemented      |
   +------+-----------+------------------------------------------------+

   Use of the above RCODEs is likely to be common in DSO but does not
   preclude the definition and use of other codes in future documents
   that make use of DSO.

   If a document defining a new DSO-TYPE makes use of response codes not
   defined here, then that document MUST specify the specific
   interpretation of those RCODE values in the context of that new DSO
   TLV.

   The RCODE field is followed by the four zero-valued count fields,
   followed by the DSO Data.

5.4.2.  DSO Data

   The standard twelve-byte DNS message header with its zero-valued
   count fields is followed by the DSO Data, expressed using TLV syntax,
   as described in Section 5.4.4.

   A DSO request message or DSO unidirectional message MUST contain at
   least one TLV.  The first TLV in a DSO request message or DSO
   unidirectional message is referred to as the "Primary TLV" and
   determines the nature of the operation being performed, including
   whether it is a DSO request or a DSO unidirectional operation.  In
   some cases, it may be appropriate to include other TLVs in a DSO
   request message or DSO unidirectional message, such as the DSO



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   Encryption Padding TLV (Section 7.3).  Additional TLVs follow the
   Primary TLV.  Additional TLVs are not limited to what is defined in
   this document.  New Additional TLVs may be defined in the future.
   Their definitions will describe when their use is appropriate.

   An unrecognized Primary TLV results in a DSOTYPENI error response.
   Unrecognized Additional TLVs are silently ignored, as described in
   Sections 5.4.5 and 8.2.

   A DSO response message may contain no TLVs, or may contain one or
   more TLVs, appropriate to the information being communicated.

   Any TLVs with the same DSO-TYPE as the Primary TLV from the
   corresponding DSO request message are Response Primary TLV(s) and
   MUST appear first in a DSO response message.  A DSO response message
   may contain multiple Response Primary TLVs, or a single Response
   Primary TLV, or in some cases, no Response Primary TLV.  A Response
   Primary TLV is not required; for most DSO operations the MESSAGE ID
   field in the DNS message header is sufficient to identify the DSO
   request message to which a particular response message relates.

   Any other TLVs in a DSO response message are Response Additional
   TLVs, such as the DSO Encryption Padding TLV (Section 7.3).  Response
   Additional TLVs follow the Response Primary TLV(s), if present.
   Response Additional TLVs are not limited to what is defined in this
   document.  New Response Additional TLVs may be defined in the future.
   Their definitions will describe when their use is appropriate.
   Unrecognized Response Additional TLVs are silently ignored, as
   described in Sections 5.4.5 and 8.2.

   The specification for each DSO TLV determines what TLVs are required
   in a response to a DSO request message using that TLV.  If a DSO
   response is received for an operation where the specification
   requires that the response carry a particular TLV or TLVs, and the
   required TLV(s) are not present, then this is a fatal error and the
   recipient of the defective response message MUST forcibly abort the
   connection immediately.  Similarly, if more than the specified number
   of instances of a given TLV are present, this is a fatal error and
   the recipient of the defective response message MUST forcibly abort
   the connection immediately.











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5.4.3.  DSO Unidirectional Messages

   It is anticipated that most DSO operations will be specified to use
   DSO request messages, which generate corresponding DSO responses.  In
   some specialized high-traffic use cases, it may be appropriate to
   specify DSO unidirectional messages.  DSO unidirectional messages can
   be more efficient on the network because they don't generate a stream
   of corresponding reply messages.  Using DSO unidirectional messages
   can also simplify software in some cases by removing the need for an
   initiator to maintain state while it waits to receive replies it
   doesn't care about.  When the specification for a particular TLV used
   as a Primary TLV (i.e., first) in an outgoing DSO request message
   (i.e., QR=0) states that a message is to be unidirectional, the
   MESSAGE ID field MUST be set to zero and the receiver MUST NOT
   generate any response message corresponding to that DSO
   unidirectional message.

   The previous point, that the receiver MUST NOT generate responses to
   DSO unidirectional messages, applies even in the case of errors.

   When a DSO message is received where both the QR bit and the MESSAGE
   ID field are zero, the receiver MUST NOT generate any response.  For
   example, if the DSO-TYPE in the Primary TLV is unrecognized, then a
   DSOTYPENI error MUST NOT be returned; instead, the receiver MUST
   forcibly abort the connection immediately.

   DSO unidirectional messages MUST NOT be used "speculatively" in cases
   where the sender doesn't know if the receiver supports the Primary
   TLV in the message because there is no way to receive any response to
   indicate success or failure.  DSO unidirectional messages are only
   appropriate in cases where the sender already knows that the receiver
   supports and wishes to receive these messages.

   For example, after a client has subscribed for Push Notifications
   [Push], the subsequent event notifications are then sent as DSO
   unidirectional messages.  This is appropriate because the client
   initiated the message stream by virtue of its Push Notification
   subscription, thereby indicating its support of Push Notifications
   and its desire to receive those notifications.

   Similarly, after a Discovery Relay client has subscribed to receive
   inbound multicast DNS (mDNS) [RFC6762] traffic from a Discovery
   Relay, the subsequent stream of received packets is then sent using
   DSO unidirectional messages.  This is appropriate because the client
   initiated the message stream by virtue of its Discovery Relay link
   subscription, thereby indicating its support of Discovery Relay and
   its desire to receive inbound mDNS packets over that DSO Session
   [Relay].



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5.4.4.  TLV Syntax

   All TLVs, whether used as "Primary", "Additional", "Response
   Primary", or "Response Additional", use the same encoding syntax.

   A specification that defines a new TLV must specify whether the DSO-
   TYPE can be used as a Primary TLV, and whether the DSO-TYPE can be
   used as an Additional TLV.  Some DSO-TYPEs are dual-purpose and can
   be used as Primary TLVs in some messages, and in other messages as
   Additional TLVs.  The specification for a DSO-TYPE must also state
   whether, when used as the Primary (i.e., first) TLV in a DSO message
   (i.e., QR=0), that DSO message is unidirectional, or is a DSO request
   message that requires a response.

   If a DSO request message requires a response, the specification must
   also state which TLVs, if any, are to be included in the response and
   how many instances of each of the TLVs are allowed.  The Primary TLV
   may or may not be contained in the response depending on what is
   specified for that TLV.  If multiple instances of the Primary TLV are
   allowed the specification should clearly describe how they should be
   processed.

                                                1   1   1   1   1   1
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                           DSO-TYPE                            |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                          DSO-LENGTH                           |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                                                               |
      /                           DSO-DATA                            /
      /                                                               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   DSO-TYPE:  A 16-bit unsigned integer, in network (big endian) byte
      order, giving the DSO-TYPE of the current DSO TLV per the IANA
      "DSO Type Codes" registry.

   DSO-LENGTH:  A 16-bit unsigned integer, in network (big endian) byte
      order, giving the size in bytes of the DSO-DATA.

   DSO-DATA:  Type-code specific format.  The generic DSO machinery
      treats the DSO-DATA as an opaque "blob" without attempting to
      interpret it.  Interpretation of the meaning of the DSO-DATA for a
      particular DSO-TYPE is the responsibility of the software that
      implements that DSO-TYPE.





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5.4.5.  Unrecognized TLVs

   If a DSO request message is received containing an unrecognized
   Primary TLV, with a nonzero MESSAGE ID (indicating that a response is
   expected), then the receiver MUST send an error response with a
   matching MESSAGE ID, and RCODE DSOTYPENI.  The error response MUST
   NOT contain a copy of the unrecognized Primary TLV.

   If a DSO unidirectional message is received containing both an
   unrecognized Primary TLV and a zero MESSAGE ID (indicating that no
   response is expected), then this is a fatal error and the recipient
   MUST forcibly abort the connection immediately.

   If a DSO request message or DSO unidirectional message is received
   where the Primary TLV is recognized, containing one or more
   unrecognized Additional TLVs, the unrecognized Additional TLVs MUST
   be silently ignored, and the remainder of the message is interpreted
   and handled as if the unrecognized parts were not present.

   Similarly, if a DSO response message is received containing one or
   more unrecognized TLVs, the unrecognized TLVs MUST be silently
   ignored and the remainder of the message is interpreted and handled
   as if the unrecognized parts are not present.




























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5.4.6.  EDNS(0) and TSIG

   Since the ARCOUNT field MUST be zero, a DSO message cannot contain a
   valid EDNS(0) option in the additional records section.  If
   functionality provided by current or future EDNS(0) options is
   desired for DSO messages, one or more new DSO TLVs need to be defined
   to carry the necessary information.

   For example, the EDNS(0) Padding Option [RFC7830] used for security
   purposes is not permitted in a DSO message, so if message padding is
   desired for DSO messages, then the DSO Encryption Padding TLV
   described in Section 7.3 MUST be used.

   A DSO message can't contain a TSIG record because a TSIG record is
   included in the additional section of the message, which would mean
   that ARCOUNT would be greater than zero.  DSO messages are required
   to have an ARCOUNT of zero.  Therefore, if use of signatures with DSO
   messages becomes necessary in the future, a new DSO TLV would have to
   be defined to perform this function.

   Note, however, that while DSO *messages* cannot include EDNS(0) or
   TSIG records, a DSO *session* is typically used to carry a whole
   series of DNS messages of different kinds, including DSO messages and
   other DNS message types like Query [RFC1034] [RFC1035] and Update
   [RFC2136].  These messages can carry EDNS(0) and TSIG records.

   Although messages may contain other EDNS(0) options as appropriate,
   this specification explicitly prohibits use of the edns-tcp-keepalive
   EDNS(0) Option [RFC7828] in *any* messages sent on a DSO Session
   (because it is obsoleted by the functionality provided by the DSO
   Keepalive operation).  If any message sent on a DSO Session contains
   an edns-tcp-keepalive EDNS(0) Option, this is a fatal error and the
   recipient of the defective message MUST forcibly abort the connection
   immediately.

















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5.5.  Message Handling

   As described in Section 5.4.1, whether an outgoing DSO message with
   the QR bit in the DNS header set to zero is a DSO request or a DSO
   unidirectional message is determined by the specification for the
   Primary TLV, which in turn determines whether the MESSAGE ID field in
   that outgoing message will be zero or nonzero.

   Every DSO message with the QR bit in the DNS header set to zero and a
   nonzero MESSAGE ID field is a DSO request message, and MUST elicit a
   corresponding response, with the QR bit in the DNS header set to one
   and the MESSAGE ID field set to the value given in the corresponding
   DSO request message.

   Valid DSO request messages sent by the client with a nonzero MESSAGE
   ID field elicit a response from the server, and valid DSO request
   messages sent by the server with a nonzero MESSAGE ID field elicit a
   response from the client.

   Every DSO message with both the QR bit in the DNS header and the
   MESSAGE ID field set to zero is a DSO unidirectional message and MUST
   NOT elicit a response.





























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5.5.1.  Delayed Acknowledgement Management

   Generally, most good TCP implementations employ a delayed
   acknowledgement timer to provide more efficient use of the network
   and better performance.

   With a bidirectional exchange over TCP, such as with a DSO request
   message, the operating system TCP implementation waits for the
   application-layer client software to generate the corresponding DSO
   response message.  The TCP implementation can then send a single
   combined packet containing the TCP acknowledgement, the TCP window
   update, and the application-generated DSO response message.  This is
   more efficient than sending three separate packets, as would occur if
   the TCP packet containing the DSO request were acknowledged
   immediately.

   With a DSO unidirectional message or DSO response message, there is
   no corresponding application-generated DSO response message, and
   consequently, no hint to the transport protocol about when it should
   send its acknowledgement and window update.

   Some networking APIs provide a mechanism that allows the application-
   layer client software to signal to the transport protocol that no
   response will be forthcoming (in effect it can be thought of as a
   zero-length "empty" write).  Where available in the networking API
   being used, the recipient of a DSO unidirectional message or DSO
   response message, having parsed and interpreted the message, SHOULD
   then use this mechanism provided by the networking API to signal that
   no response for this message will be forthcoming.  The TCP
   implementation can then go ahead and send its acknowledgement and
   window update without further delay.  See Section 9.5 for further
   discussion of why this is important.



















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5.5.2.  MESSAGE ID Namespaces

   The namespaces of 16-bit MESSAGE IDs are independent in each
   direction.  This means it is *not* an error for both client and
   server to send DSO request messages at the same time as each other,
   using the same MESSAGE ID, in different directions.  This
   simplification is necessary in order for the protocol to be
   implementable.  It would be infeasible to require the client and
   server to coordinate with each other regarding allocation of new
   unique MESSAGE IDs.  It is also not necessary to require the client
   and server to coordinate with each other regarding allocation of new
   unique MESSAGE IDs.  The value of the 16-bit MESSAGE ID combined with
   the identity of the initiator (client or server) is sufficient to
   unambiguously identify the operation in question.  This can be
   thought of as a 17-bit message identifier space using message
   identifiers 0x00001-0x0FFFF for client-to-server DSO request
   messages, and 0x10001-0x1FFFF for server-to-client DSO request
   messages.  The least-significant 16 bits are stored explicitly in the
   MESSAGE ID field of the DSO message, and the most-significant bit is
   implicit from the direction of the message.

   As described in Section 5.4.1, an initiator MUST NOT reuse a MESSAGE
   ID that it already has in use for an outstanding DSO request message
   (unless specified otherwise by the relevant specification for the
   DSO-TYPE in question).  At the very least, this means that a MESSAGE
   ID can't be reused in a particular direction on a particular DSO
   Session while the initiator is waiting for a response to a previous
   DSO request message using that MESSAGE ID on that DSO Session (unless
   specified otherwise by the relevant specification for the DSO-TYPE in
   question), and for a long-lived operation, the MESSAGE ID for the
   operation can't be reused while that operation remains active.

   If a client or server receives a response (QR=1) where the MESSAGE ID
   is zero, or is any other value that does not match the MESSAGE ID of
   any of its outstanding operations, this is a fatal error and the
   recipient MUST forcibly abort the connection immediately.

   If a responder receives a DSO request message (QR=0) where the
   MESSAGE ID is not zero, the responder tracks request MESSAGE IDs, and
   the MESSAGE ID matches the MESSAGE ID of a DSO request message it
   received for which a response has not yet been sent, it MUST forcibly
   abort the connection immediately.  This behavior is required to
   prevent a hypothetical attack that takes advantage of undefined
   behavior in this case.  However, if the responder does not track
   MESSAGE IDs in this way, no such risk exists.  Therefore, tracking
   MESSAGE IDs just to implement this sanity check is not required.





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5.5.3.  Error Responses

   When a DSO request message is unsuccessful for some reason, the
   responder returns an error code to the initiator.

   In the case of a server returning an error code to a client in
   response to an unsuccessful DSO request message, the server MAY
   choose to end the DSO Session or MAY choose to allow the DSO Session
   to remain open.  For error conditions that only affect the single
   operation in question, the server SHOULD return an error response to
   the client and leave the DSO Session open for further operations.

   For error conditions that are likely to make all operations
   unsuccessful in the immediate future, the server SHOULD return an
   error response to the client and then end the DSO Session by sending
   a Retry Delay message as described in Section 6.6.1.

   Upon receiving an error response from the server, a client SHOULD NOT
   automatically close the DSO Session.  An error relating to one
   particular operation on a DSO Session does not necessarily imply that
   all other operations on that DSO Session have also failed or that
   future operations will fail.  The client should assume that the
   server will make its own decision about whether or not to end the DSO
   Session based on the server's determination of whether the error
   condition pertains to this particular operation or to any subsequent
   operations.  If the server does not end the DSO Session by sending
   the client a Retry Delay message (Section 6.6.1), then the client
   SHOULD continue to use that DSO Session for subsequent operations.

   When a DSO unidirectional message type is received (MESSAGE ID field
   is zero), the receiver should already be expecting this DSO message
   type.  Section 5.4.5 describes the handling of unknown DSO message
   types.  When a DSO unidirectional message of an unexpected type is
   received, the receiver SHOULD forcibly abort the connection.  Whether
   the connection should be forcibly aborted for other internal errors
   processing the DSO unidirectional message is implementation dependent
   according to the severity of the error.














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5.6.  Responder-Initiated Operation Cancellation

   This document, the base specification for DNS Stateful Operations,
   does not itself define any long-lived operations, but it defines a
   framework for supporting long-lived operations such as Push
   Notification subscriptions [Push] and Discovery Relay interface
   subscriptions [Relay].

   Long-lived operations, if successful, will remain active until the
   initiator terminates the operation.

   However, it is possible that a long-lived operation may be valid at
   the time it was initiated, but then a later change of circumstances
   may render that operation invalid.  For example, a long-lived client
   operation may pertain to a name that the server is authoritative for,
   but then the server configuration is changed such that it is no
   longer authoritative for that name.

   In such cases, instead of terminating the entire session, it may be
   desirable for the responder to be able to cancel selectively only
   those operations that have become invalid.

   The responder performs this selective cancellation by sending a new
   DSO response message with the MESSAGE ID field containing the MESSAGE
   ID of the long-lived operation that is to be terminated (that it had
   previously acknowledged with a NOERROR RCODE) and the RCODE field of
   the new DSO response message giving the reason for cancellation.

   After a DSO response message with nonzero RCODE has been sent, that
   operation has been terminated from the responder's point of view, and
   the responder sends no more messages relating to that operation.

   After a DSO response message with nonzero RCODE has been received by
   the initiator, that operation has been terminated from the
   initiator's point of view, and the canceled operation's MESSAGE ID is
   now free for reuse.















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6.  DSO Session Lifecycle and Timers

6.1.  DSO Session Initiation

   A DSO Session begins as described in Section 5.1.

   Once a DSO Session has been created, client or server may initiate as
   many DNS operations as they wish using the DSO Session.

   When an initiator has multiple messages to send, it SHOULD NOT wait
   for each response before sending the next message.

   A responder MUST act on messages in the order they are received, and
   SHOULD return responses to request messages as they become available.
   A responder SHOULD NOT delay sending responses for the purpose of
   delivering responses in the same order that the corresponding
   requests were received.

   Section 6.2.1.1 of the DNS-over-TCP specification [RFC7766] specifies
   this in more detail.































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6.2.  DSO Session Timeouts

   Two timeout values are associated with a DSO Session: the inactivity
   timeout and the keepalive interval.  Both values are communicated in
   the same TLV, the Keepalive TLV (Section 7.1).

   The first timeout value, the inactivity timeout, is the maximum time
   for which a client may speculatively keep an inactive DSO Session
   open in the expectation that it may have future requests to send to
   that server.

   The second timeout value, the keepalive interval, is the maximum
   permitted interval between messages if the client wishes to keep the
   DSO Session alive.

   The two timeout values are independent.  The inactivity timeout may
   be shorter, the same, or longer than the keepalive interval, though
   in most cases the inactivity timeout is expected to be shorter than
   the keepalive interval.

   A shorter inactivity timeout with a longer keepalive interval signals
   to the client that it should not speculatively keep an inactive DSO
   Session open for very long without reason, but when it does have an
   active reason to keep a DSO Session open, it doesn't need to be
   sending an aggressive level of DSO keepalive traffic to maintain that
   session.  An example of this would be a client that has subscribed to
   DNS Push notifications.  In this case, the client is not sending any
   traffic to the server, but the session is not inactive because there
   is an active request to the server to receive push notifications.

   A longer inactivity timeout with a shorter keepalive interval signals
   to the client that it may speculatively keep an inactive DSO Session
   open for a long time, but to maintain that inactive DSO Session it
   should be sending a lot of DSO keepalive traffic.  This configuration
   is expected to be less common.

   In the usual case where the inactivity timeout is shorter than the
   keepalive interval, it is only when a client has a long-lived, low-
   traffic operation that the keepalive interval comes into play in
   order to ensure that a sufficient residual amount of traffic is
   generated to maintain NAT and firewall state, and to assure the
   client and server that they still have connectivity to each other.

   On a new DSO Session, if no explicit DSO Keepalive message exchange
   has taken place, the default value for both timeouts is 15 seconds.

   For both timeouts, lower values of the timeout result in higher
   network traffic and a higher CPU load on the server.



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6.3.  Inactive DSO Sessions

   At both servers and clients, the generation or reception of any
   complete DNS message (including DNS requests, responses, updates, DSO
   messages, etc.) resets both timers for that DSO Session, with the one
   exception being that a DSO Keepalive message resets only the
   keepalive timer, not the inactivity timeout timer.

   In addition, for as long as the client has an outstanding operation
   in progress, the inactivity timer remains cleared and an inactivity
   timeout cannot occur.

   For short-lived DNS operations like traditional queries and updates,
   an operation is considered "in progress" for the time between request
   and response, typically a period of a few hundred milliseconds at
   most.  At the client, the inactivity timer is cleared upon
   transmission of a request and remains cleared until reception of the
   corresponding response.  At the server, the inactivity timer is
   cleared upon reception of a request and remains cleared until
   transmission of the corresponding response.

   For long-lived DNS Stateful Operations (such as a Push Notification
   subscription [Push] or a Discovery Relay interface subscription
   [Relay]), an operation is considered "in progress" for as long as the
   operation is active, i.e., until it is canceled.  This means that a
   DSO Session can exist with active operations, with no messages
   flowing in either direction, for far longer than the inactivity
   timeout.  This is not an error.  This is why there are two separate
   timers: the inactivity timeout and the keepalive interval.  Just
   because a DSO Session has no traffic for an extended period of time,
   it does not automatically make that DSO Session "inactive", if it has
   an active operation that is awaiting events.



















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6.4.  The Inactivity Timeout

   The purpose of the inactivity timeout is for the server to balance
   the trade-off between the costs of setting up new DSO Sessions and
   the costs of maintaining inactive DSO Sessions.  A server with
   abundant DSO Session capacity can offer a high inactivity timeout to
   permit clients to keep a speculative DSO Session open for a long time
   and to save the cost of establishing a new DSO Session for future
   communications with that server.  A server with scarce memory
   resources can offer a low inactivity timeout to cause clients to
   promptly close DSO Sessions whenever they have no outstanding
   operations with that server and then create a new DSO Session later
   when needed.

6.4.1.  Closing Inactive DSO Sessions

   When a connection's inactivity timeout is reached, the client MUST
   begin closing the idle connection, but a client is not required to
   keep an idle connection open until the inactivity timeout is reached.
   A client MAY close a DSO Session at any time, at the client's
   discretion.  If a client determines that it has no current or
   reasonably anticipated future need for a currently inactive DSO
   Session, then the client SHOULD gracefully close that connection.

   If, at any time during the life of the DSO Session, the inactivity
   timeout value (i.e., 15 seconds by default) elapses without there
   being any operation active on the DSO Session, the client MUST close
   the connection gracefully.

   If, at any time during the life of the DSO Session, too much time
   elapses without there being any operation active on the DSO Session,
   then the server MUST consider the client delinquent and MUST forcibly
   abort the DSO Session.  What is considered "too much time" in this
   context is five seconds or twice the current inactivity timeout
   value, whichever is greater.  If the inactivity timeout has its
   default value of 15 seconds, this means that a client will be
   considered delinquent and disconnected if it has not closed its
   connection after 30 seconds of inactivity.

   In this context, an operation being active on a DSO Session includes
   a query waiting for a response, an update waiting for a response, or
   an active long-lived operation, but not a DSO Keepalive message
   exchange itself.  A DSO Keepalive message exchange resets only the
   keepalive interval timer, not the inactivity timeout timer.

   If the client wishes to keep an inactive DSO Session open for longer
   than the default duration, then it uses the DSO Keepalive message to
   request longer timeout values as described in Section 7.1.



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6.4.2.  Values for the Inactivity Timeout

   For the inactivity timeout value, lower values result in more
   frequent DSO Session teardowns and re-establishments.  Higher values
   result in lower traffic and a lower CPU load on the server, but a
   higher memory burden to maintain state for inactive DSO Sessions.

   A server may dictate any value it chooses for the inactivity timeout
   (either in a response to a client-initiated request or in a server-
   initiated message) including values under one second, or even zero.

   An inactivity timeout of zero informs the client that it should not
   speculatively maintain idle connections at all, and as soon as the
   client has completed the operation or operations relating to this
   server, the client should immediately begin closing this session.

   A server will forcibly abort an idle client session after five
   seconds or twice the inactivity timeout value, whichever is greater.
   In the case of a zero inactivity timeout value, this means that if a
   client fails to close an idle client session, then the server will
   forcibly abort the idle session after five seconds.

   An inactivity timeout of 0xFFFFFFFF represents "infinity" and informs
   the client that it may keep an idle connection open as long as it
   wishes.  Note that after granting an unlimited inactivity timeout in
   this way, at any point the server may revise that inactivity timeout
   by sending a new DSO Keepalive message dictating new Session Timeout
   values to the client.

   The largest *finite* inactivity timeout supported by the current
   Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
   days).



















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6.5.  The Keepalive Interval

   The purpose of the keepalive interval is to manage the generation of
   sufficient messages to maintain state in middleboxes (such at NAT
   gateways or firewalls) and for the client and server to periodically
   verify that they still have connectivity to each other.  This allows
   them to clean up state when connectivity is lost and to establish a
   new session if appropriate.

6.5.1.  Keepalive Interval Expiry

   If, at any time during the life of the DSO Session, the keepalive
   interval value (i.e., 15 seconds by default) elapses without any DNS
   messages being sent or received on a DSO Session, the client MUST
   take action to keep the DSO Session alive by sending a DSO Keepalive
   message (Section 7.1).  A DSO Keepalive message exchange resets only
   the keepalive timer, not the inactivity timer.

   If a client disconnects from the network abruptly, without cleanly
   closing its DSO Session, perhaps leaving a long-lived operation
   uncanceled, the server learns of this after failing to receive the
   required DSO keepalive traffic from that client.  If, at any time
   during the life of the DSO Session, twice the keepalive interval
   value (i.e., 30 seconds by default) elapses without any DNS messages
   being sent or received on a DSO Session, the server SHOULD consider
   the client delinquent and SHOULD forcibly abort the DSO Session.

6.5.2.  Values for the Keepalive Interval

   For the keepalive interval value, lower values result in a higher
   volume of DSO keepalive traffic.  Higher values of the keepalive
   interval reduce traffic and the CPU load, but have minimal effect on
   the memory burden at the server because clients keep a DSO Session
   open for the same length of time (determined by the inactivity
   timeout) regardless of the level of DSO keepalive traffic required.

   It may be appropriate for clients and servers to select different
   keepalive intervals depending on the type of network they are on.

   A corporate DNS server that knows it is serving only clients on the
   internal network, with no intervening NAT gateways or firewalls, can
   impose a longer keepalive interval because frequent DSO keepalive
   traffic is not required.

   A public DNS server that is serving primarily residential consumer
   clients, where it is likely there will be a NAT gateway on the path,
   may impose a shorter keepalive interval to generate more frequent DSO
   keepalive traffic.



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   A smart client may be adaptive to its environment.  A client using a
   private IPv4 address [RFC1918] to communicate with a DNS server at an
   address outside that IPv4 private address block may conclude that
   there is likely to be a NAT gateway on the path, and accordingly
   request a shorter keepalive interval.

   By default, it is RECOMMENDED that clients request, and servers
   grant, a keepalive interval of 60 minutes.  This keepalive interval
   provides for reasonably timely detection if a client abruptly
   disconnects without cleanly closing the session.  Also, it is
   sufficient to maintain state in firewalls and NAT gateways that
   follow the IETF recommended Best Current Practice that the
   "established connection idle-timeout" used by middleboxes be at least
   2 hours and 4 minutes [RFC5382] [RFC7857].

   Note that the shorter the keepalive interval value, the higher the
   load on client and server.  Moreover, for a keepalive value that is
   shorter than the time needed for the transport to retransmit, the
   loss of a single packet would cause a server to overzealously abort
   the connection.  For example, a (hypothetical and unrealistic)
   keepalive interval value of 100 ms would result in a continuous
   stream of ten messages per second or more (if allowed by the current
   congestion control window) in both directions to keep the DSO Session
   alive.  And, in this extreme example, a single retransmission over a
   path with, as an example, 100 ms RTT would introduce a momentary
   pause in the stream of messages long enough to cause the server to
   abort the connection.

   Because of this concern, the server MUST NOT send a DSO Keepalive
   message (either a DSO response to a client-initiated DSO request or a
   server-initiated DSO message) with a keepalive interval value less
   than ten seconds.  If a client receives a DSO Keepalive message
   specifying a keepalive interval value less than ten seconds, this is
   a fatal error and the client MUST forcibly abort the connection
   immediately.

   A keepalive interval value of 0xFFFFFFFF represents "infinity" and
   informs the client that it should generate no DSO keepalive traffic.
   Note that after signaling that the client should generate no DSO
   keepalive traffic in this way, the server may at any point revise
   that DSO keepalive traffic requirement by sending a new DSO Keepalive
   message dictating new Session Timeout values to the client.

   The largest *finite* keepalive interval supported by the current
   Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
   days).





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6.6.  Server-Initiated DSO Session Termination

   In addition to canceling individual long-lived operations selectively
   (Section 5.6), there are also occasions where a server may need to
   terminate one or more entire DSO sessions.  An entire DSO session may
   need to be terminated if the client is defective in some way or
   departs from the network without closing its DSO session.  DSO
   Sessions may also need to be terminated if the server becomes
   overloaded or is reconfigured and lacks the ability to be selective
   about which operations need to be canceled.

   This section discusses various reasons a DSO session may be
   terminated and the mechanisms for doing so.

   In normal operation, closing a DSO Session is the client's
   responsibility.  The client makes the determination of when to close
   a DSO Session based on an evaluation of both its own needs and the
   inactivity timeout value dictated by the server.  A server only
   causes a DSO Session to be ended in the exceptional circumstances
   outlined below.  Some of the exceptional situations in which a server
   may terminate a DSO Session include:

   o  The server application software or underlying operating system is
      shutting down or restarting.

   o  The server application software terminates unexpectedly (perhaps
      due to a bug that makes it crash, causing the underlying operating
      system to send a TCP RST).

   o  The server is undergoing a reconfiguration or maintenance
      procedure that, due to the way the server software is implemented,
      requires clients to be disconnected.  For example, some software
      is implemented such that it reads a configuration file at startup,
      and changing the server's configuration entails modifying the
      configuration file and then killing and restarting the server
      software, which generally entails a loss of network connections.

   o  The client fails to meet its obligation to generate the required
      DSO keepalive traffic or to close an inactive session by the
      prescribed time (five seconds or twice the time interval dictated
      by the server, whichever is greater, as described in Section 6.2).

   o  The client sends a grossly invalid or malformed request that is
      indicative of a seriously defective client implementation.

   o  The server is over capacity and needs to shed some load.





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6.6.1.  Server-Initiated Retry Delay Message

   In the cases described above where a server elects to terminate a DSO
   Session, it could do so simply by forcibly aborting the connection.
   However, if it did this, the likely behavior of the client might be
   simply to treat this as a network failure and reconnect immediately,
   putting more burden on the server.

   Therefore, to avoid this reconnection implosion, a server SHOULD
   instead choose to shed client load by sending a Retry Delay message
   with an appropriate RCODE value informing the client of the reason
   the DSO Session needs to be terminated.  The format of the DSO Retry
   Delay TLV and the interpretations of the various RCODE values are
   described in Section 7.2.  After sending a DSO Retry Delay message,
   the server MUST NOT send any further messages on that DSO Session.

   The server MAY randomize retry delays in situations where many retry
   delays are sent in quick succession so as to avoid all the clients
   attempting to reconnect at once.  In general, implementations should
   avoid using the DSO Retry Delay message in a way that would result in
   many clients reconnecting at the same time if every client attempts
   to reconnect at the exact time specified.

   Upon receipt of a DSO Retry Delay message from the server, the client
   MUST make note of the reconnect delay for this server and then
   immediately close the connection gracefully.

   After sending a DSO Retry Delay message, the server SHOULD allow the
   client five seconds to close the connection, and if the client has
   not closed the connection after five seconds, then the server SHOULD
   forcibly abort the connection.

   A DSO Retry Delay message MUST NOT be initiated by a client.  If a
   server receives a DSO Retry Delay message, this is a fatal error and
   the server MUST forcibly abort the connection immediately.

6.6.1.1.  Outstanding Operations

   At the instant a server chooses to initiate a DSO Retry Delay
   message, there may be DNS requests already in flight from client to
   server on this DSO Session, which will arrive at the server after its
   DSO Retry Delay message has been sent.  The server MUST silently
   ignore such incoming requests and MUST NOT generate any response
   messages for them.  When the DSO Retry Delay message from the server
   arrives at the client, the client will determine that any DNS
   requests it previously sent on this DSO Session that have not yet
   received a response will now certainly not be receiving any response.




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   Such requests should be considered failed and should be retried at a
   later time, as appropriate.

   In the case where some, but not all, of the existing operations on a
   DSO Session have become invalid (perhaps because the server has been
   reconfigured and is no longer authoritative for some of the names),
   but the server is terminating all affected DSO Sessions en masse by
   sending them all a DSO Retry Delay message, the reconnect delay MAY
   be zero, indicating that the clients SHOULD immediately attempt to
   re-establish operations.

   It is likely that some of the attempts will be successful and some
   will not, depending on the nature of the reconfiguration.

   In the case where a server is terminating a large number of DSO
   Sessions at once (e.g., if the system is restarting) and the server
   doesn't want to be inundated with a flood of simultaneous retries, it
   SHOULD send different reconnect delay values to each client.  These
   adjustments MAY be selected randomly, pseudorandomly, or
   deterministically (e.g., incrementing the time value by one tenth of
   a second for each successive client, yielding a post-restart
   reconnection rate of ten clients per second).

6.6.2.  Misbehaving Clients

   A server may determine that a client is not following the protocol
   correctly.  There may be no way for the server to recover the DSO
   session, in which case the server forcibly terminates the connection.
   Since the client doesn't know why the connection dropped, it may
   reconnect immediately.  If the server has determined that a client is
   not following the protocol correctly, it MAY terminate the DSO
   Session as soon as it is established, specifying a long retry-delay
   to prevent the client from immediately reconnecting.

6.6.3.  Client Reconnection

   After a DSO Session is ended by the server (either by sending the
   client a DSO Retry Delay message or by forcibly aborting the
   underlying transport connection), the client SHOULD try to reconnect
   to that service instance or to another suitable service instance if
   more than one is available.  If reconnecting to the same service
   instance, the client MUST respect the indicated delay, if available,
   before attempting to reconnect.  Clients SHOULD NOT attempt to
   randomize the delay; the server will randomly jitter the retry delay
   values it sends to each client if this behavior is desired.






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   If a particular service instance will only be out of service for a
   short maintenance period, it should indicate a retry delay value that
   is a little longer than the expected maintenance window.  It should
   not default to a very large delay value, or clients may not attempt
   to reconnect promptly after it resumes service.

   If a service instance does not want a client to reconnect ever
   (perhaps the service instance is being decommissioned), it SHOULD set
   the retry delay to the maximum value 0xFFFFFFFF (2^32-1 milliseconds,
   approximately 49.7 days).  It is not possible to instruct a client to
   stay away for longer than 49.7 days.  If, after 49.7 days, the DNS or
   other configuration information still indicates that this is the
   valid service instance for a particular service, then clients MAY
   attempt to reconnect.  In reality, if a client is rebooted or
   otherwise loses state, it may well attempt to reconnect before 49.7
   days elapse, for as long as the DNS or other configuration
   information continues to indicate that this is the service instance
   the client should use.

6.6.3.1.  Reconnecting after a Forcible Abort

   If a connection was forcibly aborted by the client due to
   noncompliant behavior by the server, the client SHOULD mark that
   service instance as not supporting DSO.  The client MAY reconnect but
   not attempt to use DSO, or it may connect to a different service
   instance if applicable.

6.6.3.2.  Reconnecting after an Unexplained Connection Drop

   It is also possible for a server to forcibly terminate the
   connection; in this case, the client doesn't know whether the
   termination was the result of a protocol error or a network outage.
   When the client notices that the connection has been dropped, it can
   attempt to reconnect immediately.  However, if the connection is
   dropped again without the client being able to successfully do
   whatever it is trying to do, it should mark the server as not
   supporting DSO.

6.6.3.3.  Probing for Working DSO Support

   Once a server has been marked by the client as not supporting DSO,
   the client SHOULD NOT attempt DSO operations on that server until
   some time has elapsed.  A reasonable minimum would be an hour.  Since
   forcibly aborted connections are the result of a software failure,
   it's not likely that the problem will be solved in the first hour
   after it's first encountered.  However, by restricting the retry
   interval to an hour, the client will be able to notice when the
   problem has been fixed without placing an undue burden on the server.



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7.  Base TLVs for DNS Stateful Operations

   This section describes the three base TLVs for DNS Stateful
   Operations: Keepalive, Retry Delay, and Encryption Padding.

7.1.  Keepalive TLV

   The Keepalive TLV (DSO-TYPE=1) performs two functions.  Primarily, it
   establishes the values for the Session Timeouts.  Incidentally, it
   also resets the keepalive timer for the DSO Session, meaning that it
   can be used as a kind of "no-op" message for the purpose of keeping a
   session alive.  The client will request the desired Session Timeout
   values and the server will acknowledge with the response values that
   it requires the client to use.

   DSO messages with the Keepalive TLV as the Primary TLV may appear in
   early data.

   The DSO-DATA for the Keepalive TLV is as follows:

                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 INACTIVITY TIMEOUT (32 bits)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 KEEPALIVE INTERVAL (32 bits)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   INACTIVITY TIMEOUT:  The inactivity timeout for the current DSO
      Session, specified as a 32-bit unsigned integer, in network (big
      endian) byte order in units of milliseconds.  This is the timeout
      at which the client MUST begin closing an inactive DSO Session.
      The inactivity timeout can be any value of the server's choosing.
      If the client does not gracefully close an inactive DSO Session,
      then after five seconds or twice this interval, whichever is
      greater, the server will forcibly abort the connection.

   KEEPALIVE INTERVAL:  The keepalive interval for the current DSO
      Session, specified as a 32-bit unsigned integer, in network (big
      endian) byte order in units of milliseconds.  This is the interval
      at which a client MUST generate DSO keepalive traffic to maintain
      connection state.  The keepalive interval MUST NOT be less than
      ten seconds.  If the client does not generate the mandated DSO
      keepalive traffic, then after twice this interval the server will
      forcibly abort the connection.  Since the minimum allowed
      keepalive interval is ten seconds, the minimum time at which a
      server will forcibly disconnect a client for failing to generate
      the mandated DSO keepalive traffic is twenty seconds.



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   The transmission or reception of DSO Keepalive messages (i.e.,
   messages where the Keepalive TLV is the first TLV) reset only the
   keepalive timer, not the inactivity timer.  The reason for this is
   that periodic DSO Keepalive messages are sent for the sole purpose of
   keeping a DSO Session alive when that DSO Session has current or
   recent non-maintenance activity that warrants keeping that DSO
   Session alive.  Sending DSO keepalive traffic itself is not
   considered a client activity; it is considered a maintenance activity
   that is performed in service of other client activities.  If DSO
   keepalive traffic itself were to reset the inactivity timer, then
   that would create a circular livelock where keepalive traffic would
   be sent indefinitely to keep a DSO Session alive.  In this scenario,
   the only activity on that DSO Session would be the keepalive traffic
   keeping the DSO Session alive so that further keepalive traffic can
   be sent.  For a DSO Session to be considered active, it must be
   carrying something more than just keepalive traffic.  This is why
   merely sending or receiving a DSO Keepalive message does not reset
   the inactivity timer.

   When sent by a client, the DSO Keepalive request message MUST be sent
   as a DSO request message with a nonzero MESSAGE ID.  If a server
   receives a DSO Keepalive message with a zero MESSAGE ID, then this is
   a fatal error and the server MUST forcibly abort the connection
   immediately.  The DSO Keepalive request message resets a DSO
   Session's keepalive timer and, at the same time, communicates to the
   server the client's requested Session Timeout values.  In a server
   response to a client-initiated DSO Keepalive request message, the
   Session Timeouts contain the server's chosen values from this point
   forward in the DSO Session, which the client MUST respect.  This is
   modeled after the DHCP protocol, where the client requests a certain
   lease lifetime using DHCP option 51 [RFC2132], but the server is the
   ultimate authority for deciding what lease lifetime is actually
   granted.

   When a client is sending its second and subsequent DSO Keepalive
   request messages to the server, the client SHOULD continue to request
   its preferred values each time.  This allows flexibility so that if
   conditions change during the lifetime of a DSO Session, the server
   can adapt its responses to better fit the client's needs.

   Once a DSO Session is in progress (Section 5.1), a DSO Keepalive
   message MAY be initiated by a server.  When sent by a server, the DSO
   Keepalive message MUST be sent as a DSO unidirectional message with
   the MESSAGE ID set to zero.  The client MUST NOT generate a response
   to a server-initiated DSO Keepalive message.  If a client receives a
   DSO Keepalive request message with a nonzero MESSAGE ID, then this is
   a fatal error and the client MUST forcibly abort the connection
   immediately.  The DSO Keepalive unidirectional message from the



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   server resets a DSO Session's keepalive timer and, at the same time,
   unilaterally informs the client of the new Session Timeout values to
   use from this point forward in this DSO Session.  No client DSO
   response to this unilateral declaration is required or allowed.

   In DSO Keepalive response messages, exactly one instance of the
   Keepalive TLV MUST be present and is used only as a Response Primary
   TLV sent as a reply to a DSO Keepalive request message from the
   client.  A Keepalive TLV MUST NOT be added to other responses as a
   Response Additional TLV.  If the server wishes to update a client's
   Session Timeout values other than in response to a DSO Keepalive
   request message from the client, then it does so by sending a DSO
   Keepalive unidirectional message of its own, as described above.

   It is not required that the Keepalive TLV be used in every DSO
   Session.  While many DSO operations will be used in conjunction with
   a long-lived session state, not all DSO operations require a long-
   lived session state, and in some cases the default 15-second value
   for both the inactivity timeout and keepalive interval may be
   perfectly appropriate.  However, note that for clients that implement
   only the DSO-TYPEs defined in this document, a DSO Keepalive request
   message is the only way for a client to initiate a DSO Session.

7.1.1.  Client Handling of Received Session Timeout Values

   When a client receives a response to its client-initiated DSO
   Keepalive request message, or receives a server-initiated DSO
   Keepalive unidirectional message, the client has then received
   Session Timeout values dictated by the server.  The two timeout
   values contained in the Keepalive TLV from the server may each be
   higher, lower, or the same as the respective Session Timeout values
   the client previously had for this DSO Session.

   In the case of the keepalive timer, the handling of the received
   value is straightforward.  The act of receiving the message
   containing the DSO Keepalive TLV itself resets the keepalive timer
   and updates the keepalive interval for the DSO Session.  The new
   keepalive interval indicates the maximum time that may elapse before
   another message must be sent or received on this DSO Session, if the
   DSO Session is to remain alive.

   In the case of the inactivity timeout, the handling of the received
   value is a little more subtle, though the meaning of the inactivity
   timeout remains as specified; it still indicates the maximum
   permissible time allowed without useful activity on a DSO Session.
   The act of receiving the message containing the Keepalive TLV does
   not itself reset the inactivity timer.  The time elapsed since the
   last useful activity on this DSO Session is unaffected by exchange of



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   DSO Keepalive messages.  The new inactivity timeout value in the
   Keepalive TLV in the received message does update the timeout
   associated with the running inactivity timer; that becomes the new
   maximum permissible time without activity on a DSO Session.

   o  If the current inactivity timer value is less than the new
      inactivity timeout, then the DSO Session may remain open for now.
      When the inactivity timer value reaches the new inactivity
      timeout, the client MUST then begin closing the DSO Session as
      described above.

   o  If the current inactivity timer value is equal to the new
      inactivity timeout, then this DSO Session has been inactive for
      exactly as long as the server will permit, and now the client MUST
      immediately begin closing this DSO Session.

   o  If the current inactivity timer value is already greater than the
      new inactivity timeout, then this DSO Session has already been
      inactive for longer than the server permits, and the client MUST
      immediately begin closing this DSO Session.

   o  If the current inactivity timer value is already more than twice
      the new inactivity timeout, then the client is immediately
      considered delinquent (this DSO Session is immediately eligible to
      be forcibly terminated by the server) and the client MUST
      immediately begin closing this DSO Session.  However, if a server
      abruptly reduces the inactivity timeout in this way, then, to give
      the client time to close the connection gracefully before the
      server resorts to forcibly aborting it, the server SHOULD give the
      client an additional grace period of either five seconds or one
      quarter of the new inactivity timeout, whichever is greater.

7.1.2.  Relationship to edns-tcp-keepalive EDNS(0) Option

   The inactivity timeout value in the Keepalive TLV (DSO-TYPE=1) has
   similar intent to the edns-tcp-keepalive EDNS(0) Option [RFC7828].  A
   client/server pair that supports DSO MUST NOT use the edns-tcp-
   keepalive EDNS(0) Option within any message after a DSO Session has
   been established.  A client that has sent a DSO message to establish
   a session MUST NOT send an edns-tcp-keepalive EDNS(0) Option from
   this point on.  Once a DSO Session has been established, if either
   client or server receives a DNS message over the DSO Session that
   contains an edns-tcp-keepalive EDNS(0) Option, this is a fatal error
   and the receiver of the edns-tcp-keepalive EDNS(0) Option MUST
   forcibly abort the connection immediately.






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7.2.  Retry Delay TLV

   The Retry Delay TLV (DSO-TYPE=2) can be used as a Primary TLV
   (unidirectional) in a server-to-client message, or as a Response
   Additional TLV in either direction.  DSO messages with a Relay Delay
   TLV as their Primary TLV are not permitted in early data.

   The DSO-DATA for the Retry Delay TLV is as follows:

                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     RETRY DELAY (32 bits)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   RETRY DELAY:  A time value, specified as a 32-bit unsigned integer in
      network (big endian) byte order, in units of milliseconds, within
      which the initiator MUST NOT retry this operation or retry
      connecting to this server.  Recommendations for the RETRY DELAY
      value are given in Section 6.6.1.

7.2.1.  Retry Delay TLV Used as a Primary TLV

   When used as the Primary TLV in a DSO unidirectional message, the
   Retry Delay TLV is sent from server to client.  It is used by a
   server to instruct a client to close the DSO Session and underlying
   connection, and not to reconnect for the indicated time interval.

   In this case, it applies to the DSO Session as a whole, and the
   client MUST begin closing the DSO Session as described in
   Section 6.6.1.  The RCODE in the message header SHOULD indicate the
   principal reason for the termination:

   o  NOERROR indicates a routine shutdown or restart.

   o  FORMERR indicates that a client DSO request was too badly
      malformed for the session to continue.

   o  SERVFAIL indicates that the server is overloaded due to resource
      exhaustion and needs to shed load.

   o  REFUSED indicates that the server has been reconfigured, and at
      this time it is now unable to perform one or more of the long-
      lived client operations that were previously being performed on
      this DSO Session.






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   o  NOTAUTH indicates that the server has been reconfigured and at
      this time it is now unable to perform one or more of the long-
      lived client operations that were previously being performed on
      this DSO Session because it does not have authority over the names
      in question (for example, a DNS Push Notification server could be
      reconfigured such that it is no longer accepting DNS Push
      Notification requests for one or more of the currently subscribed
      names).

   This document specifies only these RCODE values for the DSO Retry
   Delay message.  Servers sending DSO Retry Delay messages SHOULD use
   one of these values.  However, future circumstances may create
   situations where other RCODE values are appropriate in DSO Retry
   Delay messages, so clients MUST be prepared to accept DSO Retry Delay
   messages with any RCODE value.

   In some cases, when a server sends a DSO Retry Delay unidirectional
   message to a client, there may be more than one reason for the server
   wanting to end the session.  Possibly, the configuration could have
   been changed such that some long-lived client operations can no
   longer be continued due to policy (REFUSED), and other long-lived
   client operations can no longer be performed due to the server no
   longer being authoritative for those names (NOTAUTH).  In such cases,
   the server MAY use any of the applicable RCODE values, or
   RCODE=NOERROR (routine shutdown or restart).

   Note that the selection of RCODE value in a DSO Retry Delay message
   is not critical since the RCODE value is generally used only for
   information purposes such as writing to a log file for future human
   analysis regarding the nature of the disconnection.  Generally,
   clients do not modify their behavior depending on the RCODE value.
   The RETRY DELAY in the message tells the client how long it should
   wait before attempting a new connection to this service instance.

   For clients that do in some way modify their behavior depending on
   the RCODE value, they should treat unknown RCODE values the same as
   RCODE=NOERROR (routine shutdown or restart).

   A DSO Retry Delay message (DSO message where the Primary TLV is Retry
   Delay) from server to client is a DSO unidirectional message; the
   MESSAGE ID MUST be set to zero in the outgoing message and the client
   MUST NOT send a response.

   A client MUST NOT send a DSO Retry Delay message to a server.  If a
   server receives a DSO message where the Primary TLV is the Retry
   Delay TLV, this is a fatal error and the server MUST forcibly abort
   the connection immediately.




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7.2.2.  Retry Delay TLV Used as a Response Additional TLV

   In the case of a DSO request message that results in a nonzero RCODE
   value, the responder MAY append a Retry Delay TLV to the response,
   indicating the time interval during which the initiator SHOULD NOT
   attempt this operation again.

   The indicated time interval during which the initiator SHOULD NOT
   retry applies only to the failed operation, not to the DSO Session as
   a whole.

   Either a client or a server, whichever is acting in the role of the
   responder for a particular DSO request message, MAY append a Retry
   Delay TLV to an error response that it sends.

7.3.  Encryption Padding TLV

   The Encryption Padding TLV (DSO-TYPE=3) can only be used as an
   Additional or Response Additional TLV.  It is only applicable when
   the DSO Transport layer uses encryption such as TLS.

   The DSO-DATA for the Padding TLV is optional and is a variable length
   field containing non-specified values.  A DSO-LENGTH of 0 essentially
   provides for 4 bytes of padding (the minimum amount).

                                                1   1   1   1   1   1
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      /                                                               /
      /              PADDING -- VARIABLE NUMBER OF BYTES              /
      /                                                               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   As specified for the EDNS(0) Padding Option [RFC7830], the PADDING
   bytes SHOULD be set to 0x00.  Other values MAY be used, for example,
   in cases where there is a concern that the padded message could be
   subject to compression before encryption.  PADDING bytes of any value
   MUST be accepted in the messages received.

   The Encryption Padding TLV may be included in either a DSO request
   message, response, or both.  As specified for the EDNS(0) Padding
   Option [RFC7830], if a DSO request message is received with an
   Encryption Padding TLV, then the DSO response MUST also include an
   Encryption Padding TLV.

   The length of padding is intentionally not specified in this document
   and is a function of current best practices with respect to the type
   and length of data in the preceding TLVs [RFC8467].



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8.  Summary Highlights

   This section summarizes some noteworthy highlights about various
   aspects of the DSO protocol.

8.1.  QR Bit and MESSAGE ID

   In DSO request messages, the QR bit is 0 and the MESSAGE ID is
   nonzero.

   In DSO response messages, the QR bit is 1 and the MESSAGE ID is
   nonzero.

   In DSO unidirectional messages, the QR bit is 0 and the MESSAGE ID is
   zero.

   The table below illustrates which combinations are legal and how they
   are interpreted:

               +------------------------------+------------------------+
               |       MESSAGE ID zero        |   MESSAGE ID nonzero   |
      +--------+------------------------------+------------------------+
      |  QR=0  |  DSO unidirectional message  |  DSO request message   |
      +--------+------------------------------+------------------------+
      |  QR=1  |    Invalid - Fatal Error     |  DSO response message  |
      +--------+------------------------------+------------------------+

























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8.2.  TLV Usage

   The table below indicates, for each of the three TLVs defined in this
   document, whether they are valid in each of ten different contexts.

   The first five contexts are DSO requests or DSO unidirectional
   messages from client to server, and the corresponding responses from
   server back to client:

   o  C-P - Primary TLV, sent in DSO request message, from client to
      server, with nonzero MESSAGE ID indicating that this request MUST
      generate response message.

   o  C-U - Primary TLV, sent in DSO unidirectional message, from client
      to server, with zero MESSAGE ID indicating that this request MUST
      NOT generate response message.

   o  C-A - Additional TLV, optionally added to a DSO request message or
      DSO unidirectional message from client to server.

   o  CRP - Response Primary TLV, included in response message sent back
      to the client (in response to a client "C-P" request with nonzero
      MESSAGE ID indicating that a response is required) where the DSO-
      TYPE of the Response TLV matches the DSO-TYPE of the Primary TLV
      in the request.

   o  CRA - Response Additional TLV, included in response message sent
      back to the client (in response to a client "C-P" request with
      nonzero MESSAGE ID indicating that a response is required) where
      the DSO-TYPE of the Response TLV does not match the DSO-TYPE of
      the Primary TLV in the request.

   The second five contexts are their counterparts in the opposite
   direction: DSO requests or DSO unidirectional messages from server to
   client, and the corresponding responses from client back to server.

   o  S-P - Primary TLV, sent in DSO request message, from server to
      client, with nonzero MESSAGE ID indicating that this request MUST
      generate response message.

   o  S-U - Primary TLV, sent in DSO unidirectional message, from server
      to client, with zero MESSAGE ID indicating that this request MUST
      NOT generate response message.

   o  S-A - Additional TLV, optionally added to a DSO request message or
      DSO unidirectional message from server to client.





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   o  SRP - Response Primary TLV, included in response message sent back
      to the server (in response to a server "S-P" request with nonzero
      MESSAGE ID indicating that a response is required) where the DSO-
      TYPE of the Response TLV matches the DSO-TYPE of the Primary TLV
      in the request.

   o  SRA - Response Additional TLV, included in response message sent
      back to the server (in response to a server "S-P" request with
      nonzero MESSAGE ID indicating that a response is required) where
      the DSO-TYPE of the Response TLV does not match the DSO-TYPE of
      the Primary TLV in the request.

                +-------------------------+-------------------------+
                | C-P  C-U  C-A  CRP  CRA | S-P  S-U  S-A  SRP  SRA |
   +------------+-------------------------+-------------------------+
   | KeepAlive  |  X              X       |       X                 |
   +------------+-------------------------+-------------------------+
   | RetryDelay |                      X  |       X              X  |
   +------------+-------------------------+-------------------------+
   | Padding    |            X         X  |            X         X  |
   +------------+-------------------------+-------------------------+

   Note that some of the columns in this table are currently empty.  The
   table provides a template for future TLV definitions to follow.  It
   is recommended that definitions of future TLVs include a similar
   table summarizing the contexts where the new TLV is valid.

























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9.  Additional Considerations

9.1.  Service Instances

   We use the term "service instance" to refer to software running on a
   host that can receive connections on some set of { IP address, port }
   tuples.  What makes the software an instance is that regardless of
   which of these tuples the client uses to connect to it, the client is
   connected to the same software, running on the same logical node (see
   Section 9.2), and will receive the same answers and the same keying
   information.

   Service instances are identified from the perspective of the client.
   If the client is configured with { IP address, port } tuples, it has
   no way to tell if the service offered at one tuple is the same server
   that is listening on a different tuple.  So in this case, the client
   treats each different tuple as if it references a different service
   instance.

   In some cases, a client is configured with a hostname and a port
   number.  The port number may be given explicitly, along with the
   hostname.  The port number may be omitted, and assumed to have some
   default value.  The hostname and a port number may be learned from
   the network, as in the case of DNS SRV records.  In these cases, the
   { hostname, port } tuple uniquely identifies the service instance,
   subject to the usual case-insensitive DNS comparison of names
   [RFC1034].

   It is possible that two hostnames might point to some common IP
   addresses; this is a configuration anomaly that the client is not
   obliged to detect.  The effect of this could be that after being told
   to disconnect, the client might reconnect to the same server because
   it is represented as a different service instance.

   Implementations SHOULD NOT resolve hostnames and then perform the
   process of matching IP address(es) in order to evaluate whether two
   entities should be determined to be the "same service instance".














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9.2.  Anycast Considerations

   When an anycast service is configured on a particular IP address and
   port, it must be the case that although there is more than one
   physical server responding on that IP address, each such server can
   be treated as equivalent.  What we mean by "equivalent" here is that
   both servers can provide the same service and, where appropriate, the
   same authentication information, such as PKI certificates, when
   establishing connections.

   If a change in network topology causes packets in a particular TCP
   connection to be sent to an anycast server instance that does not
   know about the connection, the new server will automatically
   terminate the connection with a TCP reset, since it will have no
   record of the connection, and then the client can reconnect or stop
   using the connection as appropriate.

   If, after the connection is re-established, the client's assumption
   that it is connected to the same instance is violated in some way,
   that would be considered an incorrect behavior in this context.  It
   is, however, out of the possible scope for this specification to make
   specific recommendations in this regard; that would be up to follow-
   on documents that describe specific uses of DNS Stateful Operations.




























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9.3.  Connection Sharing

   As previously specified for DNS-over-TCP [RFC7766]:

      To mitigate the risk of unintentional server overload, DNS
      clients MUST take care to minimize the number of concurrent
      TCP connections made to any individual server.  It is RECOMMENDED
      that for any given client/server interaction there SHOULD be
      no more than one connection for regular queries, one for zone
      transfers, and one for each protocol that is being used on top
      of TCP (for example, if the resolver was using TLS).  However,
      it is noted that certain primary/secondary configurations
      with many busy zones might need to use more than one TCP
      connection for zone transfers for operational reasons (for
      example, to support concurrent transfers of multiple zones).

   A single server may support multiple services, including DNS Updates
   [RFC2136], DNS Push Notifications [Push], and other services, for one
   or more DNS zones.  When a client discovers that the target server
   for several different operations is the same service instance (see
   Section 9.1), the client SHOULD use a single shared DSO Session for
   all those operations.

   This requirement has two benefits.  First, it reduces unnecessary
   connection load on the DNS server.  Second, it avoids the connection
   startup time that would be spent establishing each new additional
   connection to the same DNS server.

   However, server implementers and operators should be aware that
   connection sharing may not be possible in all cases.  A single host
   device may be home to multiple independent client software instances
   that don't coordinate with each other.  Similarly, multiple
   independent client devices behind the same NAT gateway will also
   typically appear to the DNS server as different source ports on the
   same client IP address.  Because of these constraints, a DNS server
   MUST be prepared to accept multiple connections from different source
   ports on the same client IP address.














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9.4.  Operational Considerations for Middleboxes

   Where an application-layer middlebox (e.g., a DNS proxy, forwarder,
   or session multiplexer) is in the path, care must be taken to avoid a
   configuration in which DSO traffic is mishandled.  The simplest way
   to avoid such problems is to avoid using middleboxes.  When this is
   not possible, middleboxes should be evaluated to make sure that they
   behave correctly.

   Correct behavior for middleboxes consists of one of the following:

   o  The middlebox does not forward DSO messages and responds to DSO
      messages with a response code other than NOERROR or DSOTYPENI.

   o  The middlebox acts as a DSO server and follows this specification
      in establishing connections.

   o  There is a 1:1 correspondence between incoming and outgoing
      connections such that when a connection is established to the
      middlebox, it is guaranteed that exactly one corresponding
      connection will be established from the middlebox to some DNS
      resolver, and all incoming messages will be forwarded without
      modification or reordering.  An example of this would be a NAT
      forwarder or TCP connection optimizer (e.g., for a high-latency
      connection such as a geosynchronous satellite link).

   Middleboxes that do not meet one of the above criteria are very
   likely to fail in unexpected and difficult-to-diagnose ways.  For
   example, a DNS load balancer might unbundle DNS messages from the
   incoming TCP stream and forward each message from the stream to a
   different DNS server.  If such a load balancer is in use, and the DNS
   servers it points to implement DSO and are configured to enable DSO,
   DSO Session establishment will succeed, but no coherent session will
   exist between the client and the server.  If such a load balancer is
   pointed at a DNS server that does not implement DSO or is configured
   not to allow DSO, no such problem will exist, but such a
   configuration risks unexpected failure if new server software is
   installed that does implement DSO.

   It is of course possible to implement a middlebox that properly
   supports DSO.  It is even possible to implement one that implements
   DSO with long-lived operations.  This can be done either by
   maintaining a 1:1 correspondence between incoming and outgoing
   connections, as mentioned above, or by terminating incoming sessions
   at the middlebox but maintaining state in the middlebox about any
   long-lived operations that are requested.  Specifying this in detail
   is beyond the scope of this document.




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9.5.  TCP Delayed Acknowledgement Considerations

   Most modern implementations of the Transmission Control Protocol
   (TCP) include a feature called "Delayed Acknowledgement" [RFC1122].

   Without this feature, TCP can be very wasteful on the network.  For
   illustration, consider a simple example like remote login using a
   very simple TCP implementation that lacks delayed acks.  When the
   user types a keystroke, a data packet is sent.  When the data packet
   arrives at the server, the simple TCP implementation sends an
   immediate acknowledgement.  Mere milliseconds later, the server
   process reads the one byte of keystroke data, and consequently the
   simple TCP implementation sends an immediate window update.  Mere
   milliseconds later, the server process generates the character echo
   and sends this data back in reply.  The simple TCP implementation
   then sends this data packet immediately too.  In this case, this
   simple TCP implementation sends a burst of three packets almost
   instantaneously (ack, window update, data).

   Clearly it would be more efficient if the TCP implementation were to
   combine the three separate packets into one, and this is what the
   delayed ack feature enables.

   With delayed ack, the TCP implementation waits after receiving a data
   packet, typically for 200 ms, and then sends its ack if (a) more data
   packet(s) arrive, (b) the receiving process generates some reply
   data, or (c) 200 ms elapse without either of the above occurring.

   With delayed ack, remote login becomes much more efficient,
   generating just one packet instead of three for each character echo.

   The logic of delayed ack is that the 200 ms delay cannot do any
   significant harm.  If something at the other end were waiting for
   something, then the receiving process should generate the reply that
   the thing at the other end is waiting for, and TCP will then
   immediately send that reply (combined with the ack and window
   update).  And if the receiving process does not in fact generate any
   reply for this particular message, then by definition the thing at
   the other end cannot be waiting for anything.  Therefore, the 200 ms
   delay is harmless.

   This assumption may be true unless the sender is using Nagle's
   algorithm, a similar efficiency feature, created to protect the
   network from poorly written client software that performs many rapid
   small writes in succession.  Nagle's algorithm allows these small
   writes to be coalesced into larger, less wasteful packets.





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   Unfortunately, Nagle's algorithm and delayed ack, two valuable
   efficiency features, can interact badly with each other when used
   together [NagleDA].

   DSO request messages elicit responses; DSO unidirectional messages
   and DSO response messages do not.

   For DSO request messages, which do elicit responses, Nagle's
   algorithm and delayed ack work as intended.

   For DSO messages that do not elicit responses, the delayed ack
   mechanism causes the ack to be delayed by 200 ms.  The 200 ms delay
   on the ack can in turn cause Nagle's algorithm to prevent the sender
   from sending any more data for 200 ms until the awaited ack arrives.
   On an enterprise Gigabit Ethernet (GigE) backbone with sub-
   millisecond round-trip times, a 200 ms delay is enormous in
   comparison.

   When this issues is raised, there are two solutions that are often
   offered, neither of them ideal:

   1.  Disable delayed ack.  For DSO messages that elicit no response,
       removing delayed ack avoids the needless 200 ms delay and sends
       back an immediate ack that tells Nagle's algorithm that it should
       immediately grant the sender permission to send its next packet.
       Unfortunately, for DSO messages that *do* elicit a response,
       removing delayed ack removes the efficiency gains of combining
       acks with data, and the responder will now send two or three
       packets instead of one.

   2.  Disable Nagle's algorithm.  When acks are delayed by the delayed
       ack algorithm, removing Nagle's algorithm prevents the sender
       from being blocked from sending its next small packet
       immediately.  Unfortunately, on a network with a higher round-
       trip time, removing Nagle's algorithm removes the efficiency
       gains of combining multiple small packets into fewer larger ones,
       with the goal of limiting the number of small packets in flight
       at any one time.

   The problem here is that with DSO messages that elicit no response,
   the TCP implementation is stuck waiting, unsure if a response is
   about to be generated or whether the TCP implementation should go
   ahead and send an ack and window update.

   The solution is networking APIs that allow the receiver to inform the
   TCP implementation that a received message has been read, processed,
   and no response for this message will be generated.  TCP can then




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   stop waiting for a response that will never come, and immediately go
   ahead and send an ack and window update.

   For implementations of DSO, disabling delayed ack is NOT RECOMMENDED
   because of the harm this can do to the network.

   For implementations of DSO, disabling Nagle's algorithm is NOT
   RECOMMENDED because of the harm this can do to the network.

   At the time that this document is being prepared for publication, it
   is known that at least one TCP implementation provides the ability
   for the recipient of a TCP message to signal that it is not going to
   send a response, and hence the delayed ack mechanism can stop
   waiting.  Implementations on operating systems where this feature is
   available SHOULD make use of it.




































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10.  IANA Considerations

10.1.  DSO OPCODE Registration

   The IANA has assigned the value 6 for DNS Stateful Operations (DSO)
   in the "DNS OpCodes" registry.

10.2.  DSO RCODE Registration

   IANA has assigned the value 11 for the DSOTYPENI error code in the
   "DNS RCODEs" registry.  The DSOTYPENI error code ("DSO-TYPE Not
   Implemented") indicates that the receiver does implement DNS Stateful
   Operations, but does not implement the specific DSO-TYPE of the
   Primary TLV in the DSO request message.

10.3.  DSO Type Code Registry

   The IANA has created the 16-bit "DSO Type Codes" registry, with
   initial (hexadecimal) values as shown below:

   +-----------+-----------------------+-------+-----------+-----------+
   | Type      | Name                  | Early | Status    | Reference |
   |           |                       | Data  |           |           |
   +-----------+-----------------------+-------+-----------+-----------+
   | 0000      | Reserved              | NO    | Standards | RFC 8490  |
   |           |                       |       | Track     |           |
   |           |                       |       |           |           |
   | 0001      | KeepAlive             | OK    | Standards | RFC 8490  |
   |           |                       |       | Track     |           |
   |           |                       |       |           |           |
   | 0002      | RetryDelay            | NO    | Standards | RFC 8490  |
   |           |                       |       | Track     |           |
   |           |                       |       |           |           |
   | 0003      | EncryptionPadding     | NA    | Standards | RFC 8490  |
   |           |                       |       | Track     |           |
   |           |                       |       |           |           |
   | 0004-003F | Unassigned, reserved  | NO    |           |           |
   |           | for DSO session-      |       |           |           |
   |           | management TLVs       |       |           |           |
   |           |                       |       |           |           |
   | 0040-F7FF | Unassigned            | NO    |           |           |
   |           |                       |       |           |           |
   | F800-FBFF | Experimental/local    | NO    |           |           |
   |           | use                   |       |           |           |
   |           |                       |       |           |           |
   | FC00-FFFF | Reserved for future   | NO    |           |           |
   |           | expansion             |       |           |           |
   +-----------+-----------------------+-------+-----------+-----------+



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   The meanings of the fields are as follows:

   Type:  The 16-bit DSO type code.

   Name:  The human-readable name of the TLV.

   Early Data:  If OK, this TLV may be sent as early data in a TLS zero
      round-trip (Section 2.3 of the TLS 1.3 specification [RFC8446])
      initial handshake.  If NA, the TLV may appear as an Additional TLV
      in a DSO message that is sent as early data.

   Status:  RFC status (e.g., "Standards Track") or "External" if not
      documented in an RFC.

   Reference:  A stable reference to the document in which this TLV is
      defined.

   Note: DSO Type Code zero is reserved and is not currently intended
   for allocation.

   Registrations of new DSO Type Codes in the "Reserved for DSO session-
   management" range 0004-003F and the "Reserved for future expansion"
   range FC00-FFFF require publication of an IETF Standards Action
   document [RFC8126].

   Requests to register additional new DSO Type Codes in the
   "Unassigned" range 0040-F7FF are to be recorded by IANA after Expert
   Review [RFC8126].  The expert review should validate that the
   requested type code is specified in a way that conforms to this
   specification, and that the intended use for the code would not be
   addressed with an experimental/local assignment.

   DSO Type Codes in the "experimental/local" range F800-FBFF may be
   used as Experimental Use or Private Use values [RFC8126] and may be
   used freely for development purposes or for other purposes within a
   single site.  No attempt is made to prevent multiple sites from using
   the same value in different (and incompatible) ways.  There is no
   need for IANA to review such assignments (since IANA does not record
   them) and assignments are not generally useful for broad
   interoperability.  It is the responsibility of the sites making use
   of "experimental/local" values to ensure that no conflicts occur
   within the intended scope of use.

   Any document defining a new TLV that lists a value of "OK" in the
   Early Data column must include a threat analysis for the use of the
   TLV in the case of TLS zero round-trip.  See Section 11.1 for
   details.




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11.  Security Considerations

   If this mechanism is to be used with DNS-over-TLS, then these
   messages are subject to the same constraints as any other DNS-over-
   TLS messages and MUST NOT be sent in the clear before the TLS session
   is established.

   The data field of the "Encryption Padding" TLV could be used as a
   covert channel.

   When designing new DSO TLVs, the potential for data in the TLV to be
   used as a tracking identifier should be taken into consideration and
   should be avoided when not required.

   When used without TLS or similar cryptographic protection, a
   malicious entity may be able to inject a malicious unidirectional DSO
   Retry Delay message into the data stream, specifying an unreasonably
   large RETRY DELAY, causing a denial-of-service attack against the
   client.

   The establishment of DSO Sessions has an impact on the number of open
   TCP connections on a DNS server.  Additional resources may be used on
   the server as a result.  However, because the server can limit the
   number of DSO Sessions established and can also close existing DSO
   Sessions as needed, denial of service or resource exhaustion should
   not be a concern.

11.1.  TLS Zero Round-Trip Considerations

   DSO permits zero round-trip operation using TCP Fast Open with
   TLS 1.3 [RFC8446] early data to reduce or eliminate round trips in
   session establishment.  TCP Fast Open is only permitted in
   combination with TLS 1.3 early data.  In the rest of this section, we
   refer to TLS 1.3 early data in a TLS zero round-trip initial
   handshake message, regardless of whether or not it is included in a
   TCP SYN packet with early data using the TCP Fast Open option, as
   "early data."

   A DSO message may or may not be permitted to be sent as early data.
   The definition for each TLV that can be used as a Primary TLV is
   required to state whether or not that TLV is permitted as early data.
   Only response-requiring messages are ever permitted as early data,
   and only clients are permitted to send a DSO message as early data
   unless there is an implicit DSO session (see Section 5.1).







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   For DSO messages that are permitted as early data, a client MAY
   include one or more such messages as early data without having to
   wait for a DSO response to the first DSO request message to confirm
   successful establishment of a DSO Session.

   However, unless there is an implicit DSO session, a client MUST NOT
   send DSO unidirectional messages until after a DSO Session has been
   mutually established.

   Similarly, unless there is an implicit DSO session, a server MUST NOT
   send DSO request messages until it has received a response-requiring
   DSO request message from a client and transmitted a successful
   NOERROR response for that request.

   Caution must be taken to ensure that DSO messages sent as early data
   are idempotent or are otherwise immune to any problems that could
   result from the inadvertent replay that can occur with zero round-
   trip operation.

   It would be possible to add a TLV that requires the server to do some
   significant work and send that to the server as initial data in a TCP
   SYN packet.  A flood of such packets could be used as a DoS attack on
   the server.  None of the TLVs defined here have this property.

   If a new TLV is specified that does have this property, that TLV must
   be specified as not permitted in zero round-trip messages.  This
   prevents work from being done until a round-trip has occurred from
   the server to the client to verify that the source address of the
   packet is reachable.

   Documents that define new TLVs must state whether each new TLV may be
   sent as early data.  Such documents must include a threat analysis in
   the security considerations section for each TLV defined in the
   document that may be sent as early data.  This threat analysis should
   be done based on the advice given in Sections 2.3, 8, and
   Appendix E.5 of the TLS 1.3 specification [RFC8446].

12.  References

12.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.



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   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
              <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
              <https://www.rfc-editor.org/info/rfc7766>.

   [RFC7830]  Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
              DOI 10.17487/RFC7830, May 2016,
              <https://www.rfc-editor.org/info/rfc7830>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2.  Informative References

   [Fail]     Andrews, M. and R. Bellis, "A Common Operational Problem
              in DNS Servers - Failure To Communicate", Work in
              Progress, draft-ietf-dnsop-no-response-issue-13, February
              2019.







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   [NagleDA]  Cheshire, S., "TCP Performance problems caused by
              interaction between Nagle's Algorithm and Delayed ACK",
              May 2005,
              <http://www.stuartcheshire.org/papers/nagledelayedack/>.

   [Push]     Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              Work in Progress, draft-ietf-dnssd-push-18, March 2019.

   [Relay]    Lemon, T. and S. Cheshire, "Multicast DNS Discovery
              Relay", Work in Progress, draft-ietf-dnssd-mdns-relay-02,
              March 2019.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <https://www.rfc-editor.org/info/rfc2132>.

   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, DOI 10.17487/RFC5382, October 2008,
              <https://www.rfc-editor.org/info/rfc5382>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

   [RFC7828]  Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", RFC 7828,
              DOI 10.17487/RFC7828, April 2016,
              <https://www.rfc-editor.org/info/rfc7828>.

   [RFC7857]  Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar,
              S., and K. Naito, "Updates to Network Address Translation
              (NAT) Behavioral Requirements", BCP 127, RFC 7857,
              DOI 10.17487/RFC7857, April 2016,
              <https://www.rfc-editor.org/info/rfc7857>.



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   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8467]  Mayrhofer, A., "Padding Policies for Extension Mechanisms
              for DNS (EDNS(0))", RFC 8467, DOI 10.17487/RFC8467,
              October 2018, <https://www.rfc-editor.org/info/rfc8467>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

Acknowledgements

   Thanks to Stephane Bortzmeyer, Tim Chown, Ralph Droms, Paul Hoffman,
   Jan Komissar, Edward Lewis, Allison Mankin, Rui Paulo, David
   Schinazi, Manju Shankar Rao, Bernie Volz, and Bob Harold for their
   helpful contributions to this document.

Authors' Addresses

   Ray Bellis
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063
   United States of America

   Phone: +1 (650) 423-1200
   Email: ray@isc.org


   Stuart Cheshire
   Apple Inc.
   One Apple Park Way
   Cupertino, CA  95014
   United States of America

   Phone: +1 (408) 996-1010
   Email: cheshire@apple.com







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   John Dickinson
   Sinodun Internet Technologies
   Magadalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   United Kingdom

   Email: jad@sinodun.com


   Sara Dickinson
   Sinodun Internet Technologies
   Magadalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   United Kingdom

   Email: sara@sinodun.com


   Ted Lemon
   Nibbhaya Consulting
   P.O. Box 958
   Brattleboro, VT  05302-0958
   United States of America

   Email: mellon@fugue.com


   Tom Pusateri
   Unaffiliated
   Raleigh, NC  27608
   United States of America

   Phone: +1 (919) 867-1330
   Email: pusateri@bangj.com















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