path: root/doc/rfc/rfc6559.txt

Internet Engineering Task Force (IETF)                      D. Farinacci
Request for Comments: 6559                                  IJ. Wijnands
Category: Experimental                                         S. Venaas
ISSN: 2070-1721                                            Cisco Systems
                                                            M. Napierala
                                                               AT&T Labs
                                                              March 2012


                 A Reliable Transport Mechanism for PIM

Abstract

   This document defines a reliable transport mechanism for the PIM
   protocol for transmission of Join/Prune messages.  This eliminates
   the need for periodic Join/Prune message transmission and processing.
   The reliable transport mechanism can use either TCP or SCTP as the
   transport protocol.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6559.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  4
     1.2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  5
   3.  PIM Hello Options  . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  PIM over the TCP Transport Protocol  . . . . . . . . . . .  6
     3.2.  PIM over the SCTP Transport Protocol . . . . . . . . . . .  7
     3.3.  Interface ID . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Establishing Transport Connections . . . . . . . . . . . . . .  9
     4.1.  Connection Security  . . . . . . . . . . . . . . . . . . . 11
     4.2.  Connection Maintenance . . . . . . . . . . . . . . . . . . 11
     4.3.  Actions When a Connection Goes Down  . . . . . . . . . . . 13
     4.4.  Moving from PORT to Datagram Mode  . . . . . . . . . . . . 14
     4.5.  On-Demand versus Pre-Configured Connections  . . . . . . . 14
     4.6.  Possible Hello Suppression Considerations  . . . . . . . . 15
     4.7.  Avoiding a Pair of TCP Connections between Neighbors . . . 15
   5.  PORT Message Definitions . . . . . . . . . . . . . . . . . . . 16
     5.1.  PORT Join/Prune Message  . . . . . . . . . . . . . . . . . 18
     5.2.  PORT Keep-Alive Message  . . . . . . . . . . . . . . . . . 19
     5.3.  PORT Options . . . . . . . . . . . . . . . . . . . . . . . 20
       5.3.1.  PIM IPv4 Join/Prune Option . . . . . . . . . . . . . . 21
       5.3.2.  PIM IPv6 Join/Prune Option . . . . . . . . . . . . . . 21
   6.  Explicit Tracking  . . . . . . . . . . . . . . . . . . . . . . 22
   7.  Support of Multiple Address Families . . . . . . . . . . . . . 23
   8.  Miscellany . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   9.  Transport Considerations . . . . . . . . . . . . . . . . . . . 23
   10. Manageability Considerations . . . . . . . . . . . . . . . . . 24
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
     12.1. PORT Port Number . . . . . . . . . . . . . . . . . . . . . 25
     12.2. PORT Hello Options . . . . . . . . . . . . . . . . . . . . 25
     12.3. PORT Message Type Registry . . . . . . . . . . . . . . . . 26
     12.4. PORT Option Type Registry  . . . . . . . . . . . . . . . . 26
   13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 26
   14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 27
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 27
     15.2. Informative References . . . . . . . . . . . . . . . . . . 28







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1.  Introduction

   The goals of this specification are:

   o  To create a simple incremental mechanism to provide reliable PIM
      Join/Prune message delivery in PIM version 2 for use with PIM
      Sparse-Mode (PIM-SM) [RFC4601], including PIM Source-Specific
      Multicast (PIM-SSM), and Bidirectional PIM [RFC5015].

   o  When a router supports this specification, it need not use the
      reliable transport mechanism with every neighbor.  It can be
      negotiated on a per-neighbor basis.

   The explicit non-goals of this specification are:

   o  Making changes to the PIM message formats as defined in [RFC4601].

   o  Providing support for automatic switching between the reliable
      transport mechanism and the regular PIM mechanism defined in
      [RFC4601].  Two routers that are PIM neighbors on a link will
      always use the reliable transport mechanism if and only if both
      have enabled support for the reliable transport mechanism.

   This document will specify how periodic Join/Prune message
   transmission can be eliminated by using TCP [RFC0793] or SCTP
   [RFC4960] as the reliable transport mechanism for Join/Prune
   messages.  The destination port number is 8471 for both TCP and SCTP.

   This specification enables greater scalability in terms of control-
   traffic overhead.  However, for routers connected to multi-access
   links, scalability comes at the price of increased PIM state and the
   overhead required to maintain this state.

   In many existing and emerging networks, particularly wireless and
   mobile satellite systems, link degradation due to weather,
   interference, and other impairments can result in temporary spikes in
   the packet loss rate.  In these environments, periodic PIM joining
   can cause join latency when messages are lost, causing a
   retransmission only 60 seconds later.  By applying a reliable
   transport, a lost Join is retransmitted rapidly.  Furthermore, when
   the last user leaves a multicast group, any lost Prune is similarly
   repaired, and the multicast stream is quickly removed from the
   wireless/satellite link.  Without a reliable transport, the multicast
   transmission could otherwise continue until it timed out, roughly 3
   minutes later.  As network resources are at a premium in many of
   these environments, rapid termination of the multicast stream is
   critical for maintaining efficient use of bandwidth.




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   This is an experimental extension to PIM.  It makes some fundamental
   changes to how PIM works in that Join/Prune state does not require
   periodic updates, and it partly turns PIM into a hard-state protocol.
   Also, using reliable delivery for PIM messages is a new concept, and
   it is likely that experiences from early implementations and
   deployments will lead to at least minor changes in the protocol.
   Once there is some deployment experience, making this a Standards
   Track protocol should be considered.  Experiments using this protocol
   only require support by pairs of PIM neighbors, and need not be
   constrained to isolated networks.

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.2.  Definitions

   PORT:   Stands for PIM Over Reliable Transport, which is the short
      form for describing the mechanism in this specification where PIM
      can use the TCP or SCTP transport protocol.

   Periodic Join/Prune message:   A Join/Prune message sent periodically
      to refresh state.

   Incremental Join/Prune message:   A Join/Prune message sent as a
      result of state creation or deletion events.  Also known as a
      triggered message.

   Native Join/Prune message:   A Join/Prune message that is carried
      with an IP protocol type of PIM.

   PORT Join/Prune message:   A Join/Prune message using TCP or SCTP for
      transport.

   Datagram Mode:   The procedures whereby PIM encapsulates triggered or
      periodic Join/Prune messages in IP packets.

   PORT Mode:   The procedures used by PIM and defined in this
      specification for sending Join/Prune messages over the TCP or SCTP
      transport layer.









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2.  Protocol Overview

   PIM Over Reliable Transport (PORT) is a simple extension to PIMv2 for
   refresh reduction of PIM Join/Prune messages.  It involves sending
   incremental rather than periodic Join/Prune messages over a TCP/SCTP
   connection between PIM neighbors.

   PORT only applies to PIM Sparse-Mode [RFC4601] and Bidirectional PIM
   [RFC5015] Join/Prune messages.

   This document does not restrict PORT to any specific link types.
   However, the use of PORT on, e.g., multi-access LANs with many PIM
   neighbors should be carefully evaluated.  This is due to the facts
   that there may be a full mesh of PORT connections and that explicit
   tracking of all PIM neighbors is required.

   PORT can be incrementally used on a link between PORT-capable
   neighbors.  Routers that are not PORT-capable can continue to use PIM
   in Datagram mode.  PORT capability is detected using new PORT-Capable
   PIM Hello Options.

   Once PORT is enabled on an interface and a PIM neighbor also
   announces that it is PORT enabled, only PORT Join/Prune messages will
   be used.  That is, only PORT Join/Prune messages are accepted from,
   and sent to, that particular neighbor.  Native Join/Prune messages
   are still used for PIM neighbors that are not PORT enabled.

   PORT Join/Prune messages are sent using a TCP/SCTP connection.  When
   two PIM neighbors are PORT enabled, both for TCP or both for SCTP,
   they will immediately, or on demand, establish a connection.  If the
   connection goes down, they will again immediately, or on demand, try
   to reestablish the connection.  No Join/Prune messages (neither
   Native nor PORT) are sent while there is no connection.  Also, any
   received native Join/Prune messages from that neighbor are discarded,
   even when the connection is down.

   When PORT is used, only incremental Join/Prune messages are sent from
   downstream routers to upstream routers.  As such, downstream routers
   do not generate periodic Join/Prune messages for state for which the
   Reverse Path Forwarding (RPF) neighbor is PORT-capable.

   For Joins and Prunes that are received over a TCP/SCTP connection,
   the upstream router does not start or maintain timers on the outgoing
   interface entry.  Instead, it keeps track of which downstream routers
   have expressed interest.  An interface is deleted from the outgoing
   interface list only when all downstream routers on the interface no
   longer wish to receive traffic.  If there also are native Joins/
   Prunes from a non-PORT neighbor, then a router can maintain timers on



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   the outgoing interface entry as usual, while at the same time keep
   track of each of the downstream PORT Joins/Prunes.

   This document does not update the PIM Join/Prune packet format.  In
   the procedures described in this document, each PIM Join/Prune
   message is included in the payload of a PORT message carried over
   TCP/SCTP.  See Section 5 for details on the PORT message.

3.  PIM Hello Options

3.1.  PIM over the TCP Transport Protocol

   Option Type: PIM-over-TCP-Capable

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Type = 27           |         Length = 4 + X        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     TCP Connection ID AFI     |        Reserved       |  Exp  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       TCP Connection ID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Assigned Hello Type values can be found in [HELLO-OPT].

   When a router is configured to use PIM over TCP on a given interface,
   it MUST include the PIM-over-TCP-Capable Hello Option in its Hello
   messages for that interface.  If a router is explicitly disabled from
   using PIM over TCP, it MUST NOT include the PIM-over-TCP-Capable
   Hello Option in its Hello messages.

   All Hello messages containing the PIM-over-TCP-Capable Hello Option
   MUST also contain the Interface ID Hello Option, see Section 3.3.

   Implementations MAY provide a configuration option to enable or
   disable PORT functionality.  It is RECOMMENDED that this capability
   be disabled by default.

   Length:   Length in bytes for the value part of the Type/Length/Value
      encoding, where X is the number of bytes that make up the
      Connection ID field.  X is 4 when AFI of value 1 (IPv4) [AFI] is
      used, 16 when AFI of value 2 (IPv6) [AFI] is used, and 0 when AFI
      of value 0 is used.







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   TCP Connection ID AFI:   The AFI value to describe the address family
      of the address of the TCP Connection ID field.  Note that this
      value does not need to match the address family of the PIM Hello
      message that carries it.  When this field is 0, a mechanism
      outside the scope of this document is used to obtain the addresses
      used to establish the TCP connection.

   Reserved:   Set to zero on transmission and ignored on receipt.

   Exp:   For experimental use [RFC3692].  One expected use of these
      bits would be to signal experimental capabilities.  For example,
      if a router supports an experimental feature, it may set a bit to
      indicate this.  The default behavior, unless a router supports a
      particular experiment, is to ignore the bits on receipt.

   TCP Connection ID:   An IPv4 or IPv6 address used to establish the
      TCP connection.  This field is omitted (length 0) for the
      Connection ID AFI 0.

3.2.  PIM over the SCTP Transport Protocol

   Option Type: PIM-over-SCTP-Capable

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Type = 28           |         Length = 4 + X        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     SCTP Connection ID AFI    |        Reserved       |  Exp  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       SCTP Connection ID                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Assigned Hello Type values can be found in [HELLO-OPT].

   When a router is configured to use PIM over SCTP on a given
   interface, it MUST include the PIM-over-SCTP-Capable Hello Option in
   its Hello messages for that interface.  If a router is explicitly
   disabled from using PIM over SCTP, it MUST NOT include the PIM-over-
   SCTP-Capable Hello Option in its Hello messages.

   All Hello messages containing the PIM-over-SCTP-Capable Hello Option
   MUST also contain the Interface ID Hello Option; see Section 3.3.

   Implementations MAY provide a configuration option to enable or
   disable PORT functionality.  It is RECOMMENDED that this capability
   be disabled by default.




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   Length:   Length in bytes for the value part of the Type/Length/Value
      encoding, where X is the number of bytes that make up the
      Connection ID field.  X is 4 when AFI of value 1 (IPv4) [AFI] is
      used, 16 when AFI of value 2 (IPv6) [AFI] is used, and 0 when AFI
      of value 0 is used.

   SCTP Connection ID AFI:   The AFI value to describe the address
      family of the address of the SCTP Connection ID field.  Note that
      this value does not need to match the address family of the PIM
      Hello message that carries it.  When this field is 0, a mechanism
      outside the scope of this document is used to obtain the addresses
      used to establish the SCTP connection.

   Reserved:   Set to zero on transmission and ignored on receipt.

   Exp:   For experimental use [RFC3692].  One expected use of these
      bits would be to signal experimental capabilities.  For example,
      if a router supports an experimental feature, it may set a bit to
      indicate this.  The default behavior, unless a router supports a
      particular experiment, is to ignore the bits on receipt.

   SCTP Connection ID:   An IPv4 or IPv6 address used to establish the
      SCTP connection.  This field is omitted (length 0) for the
      Connection ID AFI 0.

3.3.  Interface ID

   All Hello messages containing PIM-over-TCP-Capable or PIM-over-SCTP-
   Capable Hello Options MUST also contain the Interface ID Hello Option
   [RFC6395].

   The Interface ID is used to associate a PORT Join/Prune message with
   the PIM neighbor from which it is coming.  When unnumbered interfaces
   are used or when a single transport connection is used for sending
   and receiving Join/Prune messages over multiple interfaces, the
   Interface ID is used to convey the interface from Join/Prune message
   sender to Join/Prune message receiver.  The value of the Interface ID
   Hello Option in Hellos sent on an interface MUST be the same as the
   Interface ID value in all PORT Join/Prune messages sent to a PIM
   neighbor on that interface.

   The Interface ID need only uniquely identify an interface of a
   router; it does not need to identify to which router the interface
   belongs.  This means that the Router ID part of the Interface ID MAY
   be 0.  For details on the Router ID and the value 0, see [RFC6395].






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4.  Establishing Transport Connections

   While a router interface is PORT enabled, a PIM-over-TCP-Capable or a
   PIM-over-SCTP-Capable Option MUST be included in the PIM Hello
   messages sent on that interface.  When a router on a PORT-enabled
   interface receives a Hello message containing a PIM-over-TCP-Capable/
   PIM-over-SCTP-Capable Option from a new neighbor, or an existing
   neighbor that did not previously include the option, it switches to
   PORT mode for that particular neighbor.

   When a router switches to PORT mode for a neighbor, it stops sending
   and accepting Native Join/Prune messages for that neighbor.  Any
   state from previous Native Join/Prune messages is left to expire as
   normal.  It will also attempt to establish a transport connection
   (TCP or SCTP) with the neighbor.  If both the router and its neighbor
   have announced both PIM-over-TCP-Capable and PIM-over-SCTP-Capable
   Options, SCTP MUST be used.  This resolves the issue where two
   transports are both offered.  The method prefers SCTP over TCP,
   because SCTP has benefits such as handling of call collisions and
   support for multiple streams, as discussed later in this document.

   When the router is using TCP, it will compare the TCP Connection ID
   it announced in the PIM-over-TCP-Capable Option with the TCP
   Connection ID in the Hello received from the neighbor.  Unless
   connections are opened on demand (see below), the router with the
   lower Connection ID MUST do an active transport open to the neighbor
   Connection ID.  The router with the higher Connection ID MUST do a
   passive transport open.  An implementation MAY open connections only
   on demand; in that case, it may be that the neighbor with the higher
   Connection ID does the active open (see Section 4.5).  If the router
   with the lower Connection ID chooses to only do an active open on
   demand, it MUST do a passive open, allowing for the neighbor to
   initiate the connection.  Note that the source address of the active
   open MUST be the announced Connection ID.

   When the router is using SCTP, the IP address comparison need not be
   done since the SCTP protocol can handle call collision.

   The decisions whether to use PORT, which transport to use, and which
   Connection IDs to use are made independently for IPv4 and IPv6.
   Thus, if PORT is used both for IPv4 and IPv6, both IPv4 and IPv6 PIM
   Hello messages MUST be sent, both containing PORT Hello Options.  If
   two neighbors announce the same transport (TCP or SCTP) and the same
   Connection IDs in the IPv4 and IPv6 Hello messages, then only one
   connection is established and is shared.  Otherwise, two connections
   are established and are used separately.





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   The PIM router that performs the active open initiates the connection
   with a locally generated source transport port number and a well-
   known destination transport port number.  The PIM router that
   performs the passive open listens on the well-known local transport
   port number and does not qualify the remote transport port number.
   See Section 5 for the well-known port number assignment for PORT.

   When a transport connection is established (or reestablished), the
   two routers MUST both send a full set of Join/Prune messages for
   state for which the other router is the upstream neighbor.  This is
   needed to ensure that the upstream neighbor has the correct state.
   When moving from Datagram mode, or when the connection has gone down,
   the router cannot be sure that all the previous Join/Prune state was
   received by the neighbor.  Any state that was created before the
   connection was established (or reestablished) and that is not
   refreshed MUST be left to expire and be deleted.  When the non-
   refreshed state has expired and been deleted, the two neighbors will
   be in sync.

   When not running PORT, a full update is only needed when a router
   restarts; with PORT, it must be done every time a connection is
   established.  This can be costly, although it is expected that a PORT
   connection will go up and down rarely.  There may be a need for
   extensions to better handle this.

   It is possible that a router starts sending Hello messages with a new
   Connection ID, e.g., due to configuration changes.  A router MUST
   always use the last announced and last seen Connection IDs.  A
   connection is identified by the local Connection ID (the one we are
   announcing on a particular interface), and the remote Connection ID
   (the one we are receiving from a neighbor on the same interface).
   When either the local or remote ID changes, the Connection ID pair we
   need a connection for changes.  There may be an existing connection
   with the same pair, in which case the router will share that
   connection.  Or, a new connection may need to be established.  Note
   that for link-local addresses, the interface should be regarded as
   part of the ID, so that connection sharing is not attempted when the
   same link-local addresses are seen on different interfaces.

   When a Connection ID changes, if the previously used connection is
   not needed (i.e., there are no other PIM neighborships using the same
   Connection ID pair), both peers MUST attempt to reset the transport
   connection.  Next (even if the old connection is still needed), they
   MUST, unless a connection already exists with the new Connection ID
   pair, immediately or on demand attempt to establish a new connection
   with the new Connection ID pair.





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   Normally, the Interface ID would not change while a connection is up.
   However, if it does, the change does not affect the connection.  It
   just means that when subsequent PORT Join/Prune messages are
   received, they should be matched against the last seen Interface ID.

   Note that a Join sent over a transport connection will only be seen
   by the upstream router; thus, it will not cause non-PORT routers on
   the link with the upstream router to delay the refresh of Join state
   for the same state.  Similarly, a Prune sent over a transport
   connection will only be seen by the upstream router; thus, it will
   never cause non-PORT routers on the link with the upstream router to
   send a Join to override this Prune.

   Note also that a datagram PIM Join/Prune message for a said (S,G) or
   (*,G) sent by some router on a link will not cause routers on the
   same link that use a transport connection with the upstream router
   for that state to suppress the refresh of that state to the upstream
   router (because they don't need to periodically refresh this state)
   or to send a Join to override a Prune.  The latter will not occur
   because the upstream router will only stop forwarding the traffic
   when all joined routers that use a transport connection have
   explicitly sent a Prune for this state, as explained in Section 6.

4.1.  Connection Security

   TCP/SCTP packets used for PORT MUST be sent with a TTL/Hop Limit of
   255 to facilitate the enabling of the Generalized TTL Security
   Mechanism (GTSM) [RFC5082].  Implementations SHOULD provide a
   configuration option to enable the GTSM check at the receiver.  This
   means checking that inbound packets from directly connected neighbors
   have a TTL/Hop Limit of 255, but implementations MAY also allow for a
   different TTL/Hop Limit threshold to check that the sender is within
   a certain number of router hops.  The GTSM check SHOULD be disabled
   by default.

   Implementations SHOULD support the TCP Authentication Option (TCP-AO)
   [RFC5925] and SCTP Authenticated Chunks [RFC4895].

4.2.  Connection Maintenance

   TCP is designed to keep connections up indefinitely during a period
   of network disconnection.  If a PIM-over-TCP router fails, the TCP
   connection may stay up until the neighbor actually reboots, and even
   then it may continue to stay up until PORT tries to send the neighbor
   some information.  This is particularly relevant to PIM since the
   flow of Join/Prune messages might be in only one direction and the
   downstream neighbor might never get any indication via TCP that the
   other end of the connection is not really there.



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   SCTP has a heartbeat mechanism that can be used to detect that a
   connection is not working, even when no data is sent.  Many TCP
   implementations also support sending keep-alives for this purpose.
   Implementations MAY make use of TCP keep-alives, but the PORT keep-
   alive mechanism defined below allows for more control and
   flexibility.

   One can detect that a PORT connection is not working by regularly
   sending PORT messages.  This applies to both TCP and SCTP.  For
   example, in the case of TCP, the connection will be reset if no TCP
   ACKs are received after several retries.  PORT in itself does not
   require any periodic signaling.  PORT Join/Prune messages are only
   sent when there is a state change.  If the state changes are not
   frequent enough, a PORT Keep-Alive message (defined in Section 5.2)
   can be sent instead.  For example, if an implementation wants to send
   a PORT message, to check that the connection is working, at least
   every 60 seconds, then whenever 60 seconds have passed since the
   previous message, a Keep-Alive message could be sent.  If there were
   less than 60 seconds between each Join/Prune, no Keep-Alive messages
   would be needed.  Implementations SHOULD support the use of PORT
   Keep-Alive messages.  It is RECOMMENDED that a configuration option
   be available to network administrators to enable it when needed.
   Note that Keep-Alives can be used by a peer, independently of whether
   the other peer supports it.

   An implementation that supports Keep-Alive messages acts as follows
   when processing a received PORT message.  When processing a Keep-
   Alive message with a non-zero Holdtime value, it MUST set a timer to
   the value.  We call this timer Connection Expiry Timer (CET).  If the
   CET is already running, it MUST be reset to the new value.  When
   processing a Keep-Alive message with a zero Holdtime value, the CET
   (if running) MUST be stopped.  When processing a PORT message other
   than a Keep-Alive, the CET MUST be reset to the last received
   Holdtime value if running.  If the CET is not running, no action is
   taken.  If the CET expires, the connection SHOULD be shut down.  This
   specification does not mandate a specific default Holdtime value.
   However, the dynamic congestion and flow control in TCP and SCTP can
   result in variable transit delay between the endpoints.  When
   capacity varies, there may be loss in the network or variable link
   performance.  Consistent behavior therefore requires a sufficiently
   large Holdtime value, e.g., 60 seconds to prevent premature
   termination.

   It is possible that a router receives Join/Prune messages for an
   interface/link that is down.  As long as the neighbor has not
   expired, it is RECOMMENDED to process those messages as usual.  If
   they are ignored, then the router SHOULD ensure it gets a full update




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   for that interface when it comes back up.  This can be done by
   changing the GenID (Generation Identifier; see [RFC4601]) or by
   terminating and reestablishing the connection.

   If a PORT neighbor changes its GenID and a connection is established
   or is in the process of being established, the local side should
   generally tear down the connection and do as described in
   Section 4.3.  However, if the connection is shared by multiple
   interfaces and the GenID changes for only one of them, the local side
   SHOULD simply send a full update, similar to other cases when a GenID
   changes for an upstream neighbor.

4.3.  Actions When a Connection Goes Down

   A connection may go down for a variety of reasons.  It may be due to
   an error condition or a configuration change.  A connection SHOULD be
   shut down as soon as there are no more PIM neighbors using it.  That
   is, for the connection in question (and its associated local and
   remote Connection IDs), when there is no PIM neighbor with that
   particular remote Connection ID on any interface where we announce
   the local Connection ID, the connection SHOULD be shut down.  This
   may happen when a new Connection ID is configured, PORT is disabled,
   or a PIM neighbor expires.

   If a PIM neighbor expires, one should free connection state and
   downstream outgoing interface list (oif-list) state for that
   neighbor.  A downstream router, when an upstream neighboring router
   has expired, will simply update the RPF neighbor for the
   corresponding state to a new neighbor where it would trigger Join/
   Prune messages.  This behavior is according to [RFC4601], which
   defines the term "RPF neighbor".  It is required of a PIM router to
   clear its neighbor table for a neighbor who has timed out due to
   neighbor Holdtime expiration.

   When a connection is no longer available between two PORT-enabled PIM
   neighbors, they MUST immediately, or on demand, try to reestablish
   the connection following the normal rules for connection
   establishment.  The neighbors MUST also start expiry timers so that
   all oif-list state for the neighbor using the connection gets expired
   after J/P_Holdtime, unless it later gets refreshed by receiving new
   Join/Prunes.

   The value of J/P_Holdtime is 210 seconds.  This value is based on
   Section 4.11 of [RFC4601], which says that J/P_HoldTime should be 3.5
   * t_periodic where the default for t_periodic is 60 seconds.






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4.4.  Moving from PORT to Datagram Mode

   There may be situations where an administrator decides to stop using
   PORT.  If PORT is disabled on a router interface, or a previously
   PORT-enabled neighbor no longer announces any of the PORT Hello
   Options, the router follows the rules in Section 4.3 for taking down
   connections and starting timers.  Next, the router SHOULD trigger a
   full state update similar to what would be done if the GenID changed
   in Datagram mode.  The router SHOULD send Join/Prune messages for any
   state where the router switched from PORT to Datagram mode for the
   upstream neighbor.

4.5.  On-Demand versus Pre-Configured Connections

   Transport connections could be established when they are needed or
   when a router interface to other PIM neighbors has come up.  The
   advantage of on-demand transport connection establishment is the
   reduction of router resources, especially in the case where there is
   no need for a full mesh of connections on a network interface.  The
   disadvantage is additional delay and queueing when a Join/Prune
   message needs to be sent and a transport connection is not
   established yet.

   If a router interface has become operational and PIM neighbors are
   learned from Hello messages, at that time, transport connections may
   be established.  The advantage is that a connection is ready to
   transport data by the time a Join/Prune message needs to be sent.
   The disadvantage is there can be more connections established than
   needed.  This can occur when there is a small set of RPF neighbors
   for the active distribution trees compared to the total number of
   neighbors.  Even when transport connections are pre-established
   before they are needed, a connection can go down and an
   implementation will have to deal with an on-demand situation.

   Note that for TCP, it is the router with the lower Connection ID that
   decides whether to open a connection immediately or on demand.  The
   router with the higher Connection ID SHOULD only initiate a
   connection on demand, that is, if it needs to send a Join/Prune
   message and there is no currently established connection.

   Therefore, this specification RECOMMENDS but does not mandate the use
   of on-demand transport connection establishment.









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4.6.  Possible Hello Suppression Considerations

   Based on this specification, a transport connection cannot be
   established until a Hello message is received.  Reasons for this are
   to determine if the PIM neighbor supports this specification and to
   determine the remote address to use for establishing the transport
   connection.

   There are cases where it is desirable to suppress entirely the
   transmission of Hello messages.  In this case, how to determine if
   the PIM neighbor supports this specification and how to determine
   out-of-band (i.e., outside of the PIM protocol) the remote address
   for establishing the transport connection are outside the scope of
   this document.  In this case, the following is outside the scope of
   this document: how to determine if the PIM neighbor supports this
   specification as well as an out-of-band (outside of the PIM protocol)
   method to determine the remote address to establish the transport
   connection.

4.7.  Avoiding a Pair of TCP Connections between Neighbors

   To ensure that there is only one TCP connection between a pair of PIM
   neighbors, the following set of rules MUST be followed.  Note that
   this section applies only to TCP; for SCTP, this is not an issue.
   Let nodes A and B be two PIM neighbors where A's Connection ID is
   numerically smaller than B's Connection ID, and each is known to the
   other as having a potential PIM adjacency relationship.

   At node A:

   o  If there is already an established TCP connection to B, on the
      PIM-over-TCP port, then A MUST NOT attempt to establish a new
      connection to B.  Rather, it uses the established connection to
      send Join/Prune messages to B.  (This is independent of which node
      initiated the connection.)

   o  If A has initiated a connection to B, but the connection is still
      in the process of being established, then A MUST refuse any
      connection on the PIM-over-TCP port from B.

   o  At any time when A does not have a connection to B (which is
      either established or in the process of being established), A MUST
      accept connections from B.








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   At node B:

   o  If there is already an established TCP connection to A on the PIM-
      over-TCP port, then B MUST NOT attempt to establish a new
      connection to A.  Rather, it uses the established connection to
      send Join/Prune messages to A.  (This is independent of which node
      initiated the connection.)

   o  If B has initiated a connection to A, but the connection is still
      in the process of being established, then if A initiates a
      connection too, B MUST accept the connection initiated by A and
      release the connection that it (B) initiated.

5.  PORT Message Definitions

   For scaling purposes, it may be desirable to allow Join/Prune
   messages from different PIM protocol families to be sent over the
   same transport connection.  Also, it may be desirable to have a set
   of Join/Prune messages for one address family sent over a transport
   connection that is established over a different address-family
   network layer.

   To be able to do this, we need a common PORT message format.  This
   will provide both record boundary and demux points when sending over
   a stream protocol like TCP/SCTP.

   A PORT message may contain PORT Options; see Section 5.3.  We will
   define two PORT Options for carrying PIM Join/Prune messages -- one
   for IPv4 and one for IPv6.  For each PIM Join/Prune message to be
   sent over the transport connection, we send a PORT Join/Prune message
   containing exactly one such option.

   Each PORT message will have the Type/Length/Value format.  Multiple
   different TLV types can be sent over the same transport connection.

   To make sure PIM Join/Prune messages are delivered as soon as the TCP
   transport layer receives the Join/Prune buffer, the TCP Push flag
   will be set in all outgoing Join/Prune messages sent over a TCP
   transport connection.

   PORT messages will be sent using destination TCP port number 8471.
   When using SCTP as the reliable transport, destination port number
   8471 will be used.  See Section 12 for IANA considerations.

   PORT messages are error checked.  This includes unknown/illegal type
   fields or a truncated message.  If the PORT message contains a PIM
   Join/Prune Message, then that is subject to the normal PIM error




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   checks, including checksum verification.  If any parsing errors occur
   in a PORT message, it is skipped, and we proceed to any following
   PORT messages.

   When an unknown type field is encountered, that message MUST be
   ignored.  As specified above, one then proceeds as usual when
   processing further PORT messages.  This is important in order to
   allow new message types to be specified in the future, without
   breaking existing implementations.  However, if only unknown or
   invalid messages are received for a longer period of time, an
   implementation MAY alert the operator.  For example, if a message is
   sent with a wrong length, the receiver is likely to see only unknown/
   invalid messages thereafter.

   The checksum of the PIM Join/Prune message MUST be calculated exactly
   as specified in Section 4.9 of [RFC4601].  For IPv6, [RFC4601]
   specifies the use of a pseudo-header.  For PORT, the exact same
   pseudo-header MUST be used, but its source and destination address
   fields MUST be set to 0 when calculating the checksum.

   The TLV type field is 16 bits.  The range 65532 - 65535 is for
   experimental use [RFC3692].

   This document defines two message types.



























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5.1.  PORT Join/Prune Message

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Type = 1             |        Message Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            Reserved                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Interface                           |
       |                               ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    PORT Option Type           |      Option Value Length      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             Value                             |
       |                               .                               |
       |                               .                               |
       |                               .                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       \                               .                               \
       /                               .                               /
       \                               .                               \
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    PORT Option Type           |      Option Value Length      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             Value                             |
       |                               .                               |
       |                               .                               |
       |                               .                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          PORT Join/Prune Message

   The PORT Join/Prune Message is used for sending a PIM Join/Prune.

   Message Length:   Length in bytes for the value part of the Type/
      Length/Value encoding.  If no PORT Options are included, the
      length is 12.  If n PORT Options with Option Value lengths L1, L2,
      ..., Ln are included, the message length is 12 + 4*n + L1 + L2 +
      ... + Ln.

   Reserved:   Set to zero on transmission and ignored on receipt.

   Interface ID:   This MUST be the Interface ID of the Interface ID
      Hello Option contained in the PIM Hello messages that the PIM
      router is sending to the PIM neighbor.  It indicates to the PIM
      neighbor what interface to associate the Join/Prune with.  The
      Interface ID allows us to do connection sharing.



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   PORT Options:   The message MUST contain exactly one PIM Join/Prune
      PORT Option, either one PIM IPv4 Join/Prune or one PIM IPv6 Join/
      Prune.  It MUST NOT contain both.  It MAY contain additional
      options not defined in this document.  The behavior when receiving
      a message containing unknown options depends on the option type.
      See Section 5.3 for option definitions.

5.2.  PORT Keep-Alive Message

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Type = 2             |        Message Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            Reserved                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Holdtime            |       PORT Option Type        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Option Value Length      |            Value              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+              .                +
       |                                              .                |
       |                                              .                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       \                               .                               \
       /                               .                               /
       \                               .                               \
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    PORT Option Type           |      Option Value Length      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             Value                             |
       |                               .                               |
       |                               .                               |
       |                               .                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          PORT Keep-Alive Message

   The PORT Keep-alive Message is used to regularly send PORT messages
   to verify that a connection is alive.  They are used when other PORT
   messages are not sent at the desired frequency.

   Message Length:   Length in bytes for the value part of the Type/
      Length/Value encoding.  If no PORT Options are included, the
      length is 6.  If n PORT Options with Option Value lengths L1, L2,
      ..., Ln are included, the message length is 6 + 4*n + L1 + L2 +
      ... + Ln.

   Reserved:   Set to zero on transmission and ignored on receipt.



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   Holdtime:   This specifies a Holdtime in seconds for the connection.
      A non-zero value means that the connection SHOULD be gracefully
      shut down if no further PORT messages are received within the
      specified time.  This is measured on the receiving side by
      measuring the time from when one PORT message has been processed
      until the next has been processed.  Note that this MUST be done
      for any PORT message, not just keep-alive messages.  A Holdtime of
      0 disables the keep-alive mechanism.

   PORT Options:   A keep-alive message MUST NOT contain any of the
      options defined in this document.  It MAY contain other options
      not defined in this document.  The behavior when receiving a
      message containing unknown options depends on the option type.
      See Section 5.3 for option definitions.

5.3.  PORT Options

   Each PORT Option is a TLV.  The type is 16 bits.  The PORT Option
   type space is split in two ranges.  The types in the range 0 - 32767
   (the most significant bit is not set) are for Critical Options.  The
   types in the range 32768 - 65535 (the most significant bit is set)
   are for Non-Critical Options.

   The behavior of a router receiving a message with an unknown PORT
   Option is determined by whether the option is a Critical Option.  If
   the message contains an unknown Critical Option, the entire message
   must be ignored.  If the option is Non-Critical, only that particular
   option is ignored, and the message is processed as if the option was
   not present.

   PORT Option types are assigned by IANA, except the ranges 32764 -
   32767 and 65532 - 65535, which are for experimental use [RFC3692].
   The length specifies the length of the value in bytes.  Below are the
   two options defined in this document.

















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5.3.1.  PIM IPv4 Join/Prune Option

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      PORT Option Type = 1     |      Option Value Length      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   PIMv2 Join/Prune Message                    |
       |                               .                               |
       |                               .                               |
       |                               .                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     PIM IPv4 Join/Prune Option Format

   The IPv4 Join/Prune Option is used to carry a PIMv2 Join/Prune
   message that has all IPv4-encoded addresses in the PIM payload.

   Option Value Length:   The number of bytes that make up the PIMv2
      Join/Prune message.

   PIMv2 Join/Prune Message:   PIMv2 Join/Prune message and payload with
      no IP header in front of it.

5.3.2.  PIM IPv6 Join/Prune Option

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      PORT Option Type = 2     |      Option Value Length      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   PIMv2 Join/Prune Message                    |
       |                               .                               |
       |                               .                               |
       |                               .                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     PIM IPv6 Join/Prune Option Format

   The IPv6 Join/Prune Option is used to carry a PIMv2 Join/Prune
   message that has all IPv6-encoded addresses in the PIM payload.

   Option Value Length:   The number of bytes that make up the PIMv2
      Join/Prune message.

   PIMv2 Join/Prune Message:   PIMv2 Join/Prune message and payload with
      no IP header in front of it.




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6.  Explicit Tracking

   When explicit tracking is used, a router keeps track of Join state
   for individual downstream neighbors on a given interface.  This MUST
   be done for all PORT Joins and Prunes.  Note that it may also be done
   for native Join/Prune messages, if all neighbors on the LAN have set
   the T bit of the LAN Prune Delay Option (see definition in Section
   4.9.2 of [RFC4601]).  The discussion below covers ET (explicit
   tracking) neighbors and non-ET neighbors.  The set of ET neighbors
   MUST include the PORT neighbors.  The set of non-ET neighbors
   consists of all the non-PORT neighbors, unless all neighbors have set
   the LAN Prune Delay T bit -- in which case, the ET neighbors set
   contains all neighbors.

   For some link-types, e.g., point-to-point, tracking neighbors is no
   different than tracking interfaces.  It may also be possible for an
   implementation to treat different downstream neighbors as being on
   different logical interfaces, even if they are on the same physical
   link.  Exactly how this is implemented, and for which link types, is
   left to the implementer.

   For (*,G) and (S,G) state, the router starts forwarding traffic on an
   interface when a Join is received from a neighbor on such an
   interface.  When a non-ET neighbor sends a Prune, as specified in
   [RFC4601], if no Join is sent to override this Prune before the
   expiration of the Override Timer, the upstream router concludes that
   no non-ET neighbor is interested.  If no ET neighbors are interested,
   the interface can be removed from the oif-list.  When an ET neighbor
   sends a Prune, one removes the Join state for that neighbor.  If no
   other ET or non-ET neighbors are interested, the interface can be
   removed from the oif-list.  When a PORT neighbor sends a Prune, there
   can be no Prune Override, since the Prune is not visible to other
   neighbors.

   For (S,G,rpt) state, the router needs to track Prune state on the
   shared tree.  It needs to know which ET neighbors have sent Prunes,
   and whether any non-ET neighbors have sent Prunes.  Normally, one
   would forward a packet from a source S to a group G out on an
   interface if a (*,G) Join is received, but no (S,G,rpt) Prune.  With
   ET, one needs to do this check per ET neighbor.  That is, the packet
   should be forwarded except in two cases: all ET neighbors that have
   sent (*,G) Joins have also sent (S,G,rpt) Prunes, and if a non-ET
   neighbor has sent a (*,G) Join, whether there also is non-ET
   (S,G,rpt) Prune state.







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7.  Support of Multiple Address Families

   To allow for efficient use of router resources, one can mux Join/
   Prune messages of different address families on the same transport
   connection.  There are two ways this can be accomplished -- using a
   common message format over a TCP connection or using multiple streams
   over a single SCTP connection.

   Using the common message format described in this specification, and
   using different PORT Options, both IPv4- and IPv6-based Join/Prune
   messages can be encoded within the same transport connection.

   When using SCTP multi-streaming, the common message format is still
   used to convey address-family information, but an SCTP association is
   used, on a per-family basis, to send data concurrently for multiple
   families.  When data is sent concurrently, head-of-line blocking
   (which can occur when using TCP) is avoided.

8.  Miscellany

   There are no changes to processing of other PIM messages like PIM
   Asserts, Grafts, Graft-Acks, Registers, and Register-Stops.  This
   goes for Bootstrap Router (BSR) and Auto-RP type messages as well.

   This extension is applicable only to PIM-SM, PIM-SSM, and
   Bidirectional PIM.  It does not take requirements for PIM Dense Mode
   (PIM-DM) into consideration.

9.  Transport Considerations

   As noted in the introduction, this is an experimental extension to
   PIM, and using reliable delivery for PIM messages is a new concept.
   There are several potential transport-related concerns.  Hopefully,
   experiences from early implementations and deployments will reveal
   what concerns are relevant and how to resolve them.

   One consideration is keep-alive mechanisms.  We have defined an
   optional keep-alive mechanism for PORT; see Section 4.2.  Also, SCTP
   and many TCP implementations provide keep-alive mechanisms that could
   be used.  When to use keep-alive messages and which mechanism to use
   are unclear; however, we believe the PORT Keep-alive allows for
   better application control.  It is unclear what Holdtimes are
   preferred for the PORT Keep-alives.  For now, it is RECOMMENDED that
   administrators be able to configure whether to use keep-alives, what
   Holdtimes to use, etc.






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   In a stable state, it is expected that only occasional small messages
   are sent over a PORT connection.  This depends on how often PIM Join/
   Prune state changes.  Thus, over a long period of time, there may be
   only small messages that never use the entire TCP congestion window,
   and the window may become very large.  This would then be an issue if
   there is a state change that makes PORT send a very large message.
   It may be good if the TCP stack provides some rate-limiting or burst-
   limiting.  The congestion control mechanism defined in [RFC3465] may
   be of help.

   With PORT, it is possible that only occasional small messages are
   sent (as discussed in the previous paragraph).  This may cause
   problems for the TCP retransmit mechanism.  In particular, the TCP
   Fast Retransmit algorithm may never get triggered.  For further
   discussion of this and a possible solution, see [RFC3042].

   There may be SCTP issues similar to the TCP issues discussed in the
   above two paragraphs.

10.  Manageability Considerations

   This document defines using TCP or SCTP transports between pairs of
   PIM neighbors.  It is recommended that this mechanism be disabled by
   default.  An administrator can then enable PORT TCP and/or SCTP on
   PIM-enabled interfaces.  If two neighbors both have PORT SCTP (or
   both have PORT TCP), they will only use SCTP (or alternatively, TCP)
   for PIM Join/Prune messages.  This is the case even when the
   connection is down (there is no fallback to native Join/Prune
   messages).

   When PORT support is enabled, a router sends PIM Hello messages
   announcing support for TCP and/or SCTP and also Connection IDs.  It
   should be possible to configure a local Connection ID, and also to
   see what PORT capabilities and Connection IDs PIM neighbors are
   announcing.  Based on these advertisements, pairs of PIM neighbors
   will decide whether to try to establish a PORT connection.  There
   should be a way for an operator to check the current connection
   state.  Statistics on the number of PORT messages sent and received
   (including number of invalid messages) may also be helpful.

   For connection security (see Section 4.1), it should be possible to
   enable a GTSM check to only accept connections (TCP/SCTP packets)
   when the sender is within a certain number of router hops.  Also, one
   should be able to configure the use of TCP-AO.







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   For connection maintenance (see Section 4.2), it is recommended to
   support Keep-Alive messages.  It should be configurable whether to
   send Keep-Alives -- and if doing so, whether to use a Holdtime and
   what Holdtime to use.

   There should be some way to alert an operator when PORT connections
   are going down or when there is a failure in establishing a PORT
   connection.  Also, information like the number of connection
   failures, and how long the connection has been up or down, is useful.

11.  Security Considerations

   There are several security issues related to the use of TCP or SCTP
   transports.  By sending packets with a spoofed source address, off-
   path attackers might establish a connection or inject packets into an
   existing connection.  This might allow an attacker to send spoofed
   Join/Prune messages and/or reset a connection.  Mechanisms that help
   protect against this are discussed in Section 4.1.

   For authentication, TCP-AO [RFC5925] may be used with TCP,
   Authenticated Chunks [RFC4895] may be used with SCTP.  Also, GTSM
   [RFC5082] can be used to help prevent spoofing.

12.  IANA Considerations

   This specification makes use of a TCP port number and an SCTP port
   number for the use of the pim-port service that has been assigned by
   IANA.  It also makes use of IANA PIM Hello Options assignments that
   have been made permanent.

12.1.  PORT Port Number

   IANA previously had assigned a port number that is used as a
   destination port for pim-port TCP and SCTP transports.  The assigned
   number is 8471.  References to this document have been added to the
   Service Name and Transport Protocol Port Number Registry for pim-
   port.

12.2.  PORT Hello Options

   In the "PIM-Hello Options" registry, the following options have been
   added for PORT.

    Value    Length      Name                    Reference
   -------  ----------  -----------------------  ---------------
    27       Variable    PIM-over-TCP-Capable     this document
    28       Variable    PIM-over-SCTP-Capable    this document




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12.3.  PORT Message Type Registry

   A registry for PORT message types has been created.  The message type
   is a 16-bit integer, with values from 0 to 65535.  An RFC is required
   for assignments in the range 0 - 65531.  This document defines two
   PORT message types: Type 1 (Join/Prune) and Type 2 (Keep-alive).  The
   type range 65532 - 65535 is for experimental use [RFC3692].

   The initial content of the registry is as follows:

    Type           Name                             Reference
   -------------  -------------------------------  ---------------
    0              Reserved                         this document
    1              Join/Prune                       this document
    2              Keep-alive                       this document
    3-65531        Unassigned
    65532-65535    Experimental                     this document

12.4.  PORT Option Type Registry

   A registry for PORT Option types.  The option type is a 16-bit
   integer, with values from 0 to 65535.  The type space is split in two
   ranges, 0 - 32767 for Critical Options and 32768 - 65535 for Non-
   Critical Options.  Option types are assigned by IANA, except the
   ranges 32764 - 32767 and 65532 - 65535 that are for experimental use
   [RFC3692].  An RFC is required for the IANA assignments.  An RFC
   defining a new option type must specify whether the option is
   Critical or Non-Critical in order for IANA to assign a type.  This
   document defines two Critical PORT Option types: Type 1 (PIM IPv4
   Join/Prune) and Type 2 (PIM IPv6 Join/Prune).

   The initial content of the registry is as follows:

    Type           Name                               Reference
   -------------  ----------------------------------  ---------------
    0              Reserved                            this document
    1              PIM IPv4 Join/Prune                 this document
    2              PIM IPv6 Join/Prune                 this document
    3-32763        Unassigned Critical Options
    32764-32767    Experimental                        this document
    32768-65531    Unassigned Non-Critical Options
    65532-65535    Experimental                        this document

13.  Contributors

   In addition to the persons listed as authors, significant
   contributions were provided by Apoorva Karan and Arjen Boers.




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14.  Acknowledgments

   The authors would like to give a special thank you and appreciation
   to Nidhi Bhaskar for her initial design and early prototype of this
   idea.

   Appreciation goes to Randall Stewart for his authoritative review and
   recommendation for using SCTP.

   Thanks also goes to the following for their ideas and review of this
   specification: Mike McBride, Toerless Eckert, Yiqun Cai, Albert Tian,
   Suresh Boddapati, Nataraj Batchu, Daniel Voce, John Zwiebel, Yakov
   Rekhter, Lenny Giuliano, Gorry Fairhurst, Sameer Gulrajani, Thomas
   Morin, Dimitri Papadimitriou, Bharat Joshi, Rishabh Parekh, Manav
   Bhatia, Pekka Savola, Tom Petch, and Joe Touch.

   A special thank you goes to Eric Rosen for his very detailed review
   and commentary.  Many of his comments are reflected as text in this
   specification.

15.  References

15.1.  Normative References

   [RFC0793]    Postel, J., "Transmission Control Protocol", STD 7,
                RFC 793, September 1981.

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4601]    Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
                "Protocol Independent Multicast - Sparse Mode (PIM-SM):
                Protocol Specification (Revised)", RFC 4601,
                August 2006.

   [RFC4895]    Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
                "Authenticated Chunks for the Stream Control
                Transmission Protocol (SCTP)", RFC 4895, August 2007.

   [RFC4960]    Stewart, R., "Stream Control Transmission Protocol",
                RFC 4960, September 2007.

   [RFC5015]    Handley, M., Kouvelas, I., Speakman, T., and L.
                Vicisano, "Bidirectional Protocol Independent Multicast
                (BIDIR-PIM)", RFC 5015, October 2007.






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   [RFC5082]    Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
                Pignataro, "The Generalized TTL Security Mechanism
                (GTSM)", RFC 5082, October 2007.

   [RFC5925]    Touch, J., Mankin, A., and R. Bonica, "The TCP
                Authentication Option", RFC 5925, June 2010.

   [RFC6395]    Gulrajani, S. and S. Venaas, "An Interface Identifier
                (ID) Hello Option for PIM", RFC 6395, October 2011.

15.2.  Informative References

   [AFI]        IANA, "Address Family Numbers",
                <http://www.iana.org/assignments/
                address-family-numbers>.

   [HELLO-OPT]  IANA, "PIM-Hello Options",
                <http://www.iana.org/assignments/pim-parameters>.

   [RFC3042]    Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
                TCP's Loss Recovery Using Limited Transmit", RFC 3042,
                January 2001.

   [RFC3465]    Allman, M., "TCP Congestion Control with Appropriate
                Byte Counting (ABC)", RFC 3465, February 2003.

   [RFC3692]    Narten, T., "Assigning Experimental and Testing Numbers
                Considered Useful", BCP 82, RFC 3692, January 2004.























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Authors' Addresses

   Dino Farinacci
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: dino@cisco.com


   IJsbrand Wijnands
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: ice@cisco.com


   Stig Venaas
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: stig@cisco.com


   Maria Napierala
   AT&T Labs
   200 Laurel Drive
   Middletown, New Jersey  07748
   USA

   EMail: mnapierala@att.com















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