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rfc:rfc6559

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

Farinacci, et al. Experimental [Page 1] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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

Farinacci, et al. Experimental [Page 2] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 3] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 4] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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

Farinacci, et al. Experimental [Page 5] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 6] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 7] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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].

Farinacci, et al. Experimental [Page 8] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 9] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 10] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 11] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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

Farinacci, et al. Experimental [Page 12] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 13] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 14] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 15] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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

Farinacci, et al. Experimental [Page 16] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 17] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 18] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 19] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 20] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 21] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 22] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 23] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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.

Farinacci, et al. Experimental [Page 24] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 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

Farinacci, et al. Experimental [Page 25] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 26] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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.

Farinacci, et al. Experimental [Page 27] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

 [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.

Farinacci, et al. Experimental [Page 28] RFC 6559 A Reliable Transport Mechanism for PIM March 2012

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

Farinacci, et al. Experimental [Page 29]

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