PIM Join Attributes: The Protocol Independent Multicast (PIM) Join Attribute Format

Network Working Group                                           A. Boers
Request for Comments: 5384                                   I. Wijnands
Category: Standards Track                                       E. Rosen
                                                     Cisco Systems, Inc.
                                                           November 2008


               The Protocol Independent Multicast (PIM)
                         Join Attribute Format

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2008 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
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Abstract

   A "Protocol Independent Multicast - Sparse Mode" Join message sent by
   a given node identifies one or more multicast distribution trees that
   that node wishes to join.  Each tree is identified by the combination
   of a multicast group address and a source address (where the source
   address is possibly a "wild card").  Under certain conditions it can
   be useful, when joining a tree, to specify additional information
   related to the construction of the tree.  However, there has been no
   way to do so until now.  This document describes a modification of
   the Join message that allows a node to associate attributes with a
   particular tree.  The attributes are encoded in Type-Length-Value
   format.

Table of Contents

   1. Introduction ....................................................2
   2. Specification of Requirements ...................................3
   3. Use of Join Attributes ..........................................3
      3.1. Sending Join Attributes ....................................3
      3.2. The Join Attribute Option in the PIM Hello .................4
      3.3. Receiving Join Attributes ..................................4
           3.3.1. General Considerations ..............................4
           3.3.2. Transitive and Non-Transitive Attributes ............5
           3.3.3. Conflicting Attributes ..............................5
           3.3.4. Attribute Change ....................................6
      3.4. PIM Attribute Packet Format ................................7
           3.4.1. PIM Join Packet Format ..............................7
           3.4.2. PIM Join Attribute Hello Option .....................8
   4. IANA Considerations .............................................8
   5. Security Considerations .........................................9
   6. Acknowledgments .................................................9
   7. Normative References ............................................9
   8. Informative References ..........................................9

1.  Introduction

   In the protocol known as "Protocol Independent Multicast - Sparse
   Mode" [RFC4601] (henceforth referred to as "PIM"), a Join message
   sent by a given node may identify one or more multicast distribution
   trees that that node wishes to join.  Each tree is identified by the
   combination of a multicast group address and a source address (where
   the source address is possibly a "wild card").  Under certain
   conditions it can be useful, when joining a tree, to specify
   additional information related to the construction of the tree.
   However, there has been no way to do so until now.  This document
   describes a modification of the Join message that allows a node to
   associate an attribute, encoded in Type-Length-Value (TLV) format,
   with a particular tree that it wishes to join.  These attributes are
   known as "PIM Join Attributes".

   In the PIM Join message, the Source Address is identified by being
   encoded as an "Encoded-Source Address" ([RFC4601], section 4.9.1).
   Each Encoded-Source Address occurs in the context of a particular
   group address, represented as an "Encoded-Group Address".  Together,
   the Encoded-Source Address and the Encoded-Group Address identify a
   multicast distribution tree.  The Encoded-Source Address contains an
   "encoding type" field.  The only value defined in [RFC4601] is 0.
   This specification is the first to assign another encoding type
   value.

   In order to associate TLVs with a particular tree, this specification
   defines a new encoding type for the Encoded-Source Address: type 1.
   When type 1 is used, the Encoded-Source Address may contain a
   sequence of "Join Attributes", each of which is encoded as a TLV.
   Then the type 1 Encoded-Source Address, in the context of the
   associated Encoded-Group Address, identifies a multicast distribution
   tree and specifies (via the Join Attribute TLVs) the attributes that
   apply to the tree.  Apart from the fact that the type 1 Encoded-
   Source Address may contain Join Attributes, it is otherwise identical
   to the type 0 Encoded-Source Address.

   This document does not contain the specification for any particular
   Join Attribute.  It specifies how Join Attributes are to be encoded
   into the Join messages and it specifies generic procedures that are
   common to all Join Attributes.  The content and purpose of any
   particular Join Attribute is outside the scope of this document.

   The use of Join Attributes in "Protocol Independent Multicast - Dense
   Mode" [RFC3973] is not considered.

2.  Specification of Requirements

   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].

3.  Use of Join Attributes

3.1.  Sending Join Attributes

   Join Attributes are encoded as TLVs into the Encoded-Source Address
   field of a PIM Join message, as specified in section 3.4.1 below.
   Each attribute applies to the same multicast distribution tree that
   is identified by the combination of the Encoded-Source Address and
   the associated Encoded-Group Address.  The multicast distribution
   tree may be either a source-specific tree or a shared tree.

   The encoding of the "source address" field within the Encoded-Source
   Address is exactly the same for a type 1 Encoded-Source Address as
   for a type 0 Encoded-Source Address, specified in [RFC4601].

   A type 1 Encoded-Source Address MUST contain at least one Join
   Attribute.  The way to specify that there are no Join Attributes for
   a particular tree is to use the type 0 Encoded-Source Address.

   Multiple Join Attributes of the same type or of different types may
   occur within a single Encoded-Source Address.  This specification
   does not require all attributes of a given type to occur
   contiguously.  There is no header field that specifies the number of
   attributes; rather the last attribute is specially marked as such.

   Any PIM router that does not understand the type 1 Encoded-Source
   Address will not be able to process a PIM Join message that contains
   it.  Further, if the use of any particular Join Attribute affects the
   construction of the multicast distribution tree, the tree may not be
   formed correctly unless the attribute is understood by all PIM
   routers that receive it.  As a consequence, attributes are only
   useful within a single administrative domain (or perhaps a small set
   of contiguous, cooperating administrative domains) where it can be
   determined a priori that all deployed PIM routers understand the type
   1 Encoded-Source Address, as well as whatever specific attributes are
   in use.

3.2.  The Join Attribute Option in the PIM Hello

   To ensure that a type 1 Encoded-Source Address is not sent to a PIM
   neighbor that does not understand this encoding, a new PIM Hello
   option, the "Join Attribute" option, is defined.  This option MUST be
   included in the PIM Hellos of any PIM router that is willing to
   receive type 1 Encoded-Source Address.  A PIM router MUST NOT send a
   type 1 Encoded-Source Address out any interface on which there is a
   PIM neighbor that has not included this option in its Hellos.  (Even
   a router that is not the upstream neighbor must be able parse the
   packet in order to do Join suppression or overriding.)

   Note that a PIM router that sends the "Join Attribute" Hello option
   does not necessarily understand every possible attribute type.  As
   there is no immediate way to act on a neighbor's inability to process
   certain attribute types, it is not desired to have a Hello option for
   each possible attribute type.

3.3.  Receiving Join Attributes

3.3.1.  General Considerations

   A PIM router that receives a type 1 Encoded-Source Address MUST
   examine all the attributes in the order in which they appear.

   The specification for a given attribute type MUST specify the
   procedure to apply if there are multiple instances of that attribute
   type.

   Processing an attribute may affect the following:

   - the construction of the associated multicast distribution tree,

   - the processing of other attributes of the same type that also occur
     in the type 1 Encoded-Source Address, and

   - the forwarding (or not) of the attribute itself and/or other
     attributes of the same type that also occur in the type 1 Encoded-
     Source Address.

   If the processing of a received attribute has any effect on the
   construction of the multicast distribution tree or on the set of
   attributes that are forwarded up the tree, then state MUST be
   maintained associating the received attribute with the adjacency or
   adjacencies from which it was received.

3.3.2.  Transitive and Non-Transitive Attributes

   If a PIM router understands a particular attribute type, the
   attribute is processed as specified above.

   If a PIM router does not understand the type of a particular
   attribute, the PIM router either forwards that attribute or discards
   it, depending upon the setting of the attribute's F-bit.  If the
   F-bit is set, then the router MUST forward the attribute; if the
   F-bit is clear, then the router MUST discard it.

   If one or more non-transitive attributes are discarded, the rest of
   the Join Attributes (if any) are still forwarded.  If there are no
   Join Attributes left to forward, a Join with a type 0 Encoded-Source
   Address field MUST be forwarded.

3.3.3.  Conflicting Attributes

   It is possible that a router receives conflicting attribute
   information from different downstream routers.  Conflicts only occur
   with attributes of the same type.

       ( Edge A1 )            ( Edge B1 )---- [R1]
      /           \          /
     /             \        /
   [S]              ( Core )
     \             /        \
      \           /          \
       ( Edge A2 )            ( Edge B2 )---- [R2]

                                 Figure 1

   As an example, consider Figure 1 and suppose a Join Attribute is used
   to indicate a choice of exit router.  There are 2 receivers for the
   same group connected to Edge B1 and B2.  Suppose that edge router B1
   prefers A1 and B2 prefers A2 as exit points to reach the source S.
   If both Edge B1 and B2 send a Join including an attribute to prefer
   their exit router in the network and they cross the same core router,
   the core router will get conflicting attribute information for the
   source.  If this happens, we use the attribute from the PIM adjacency
   with the numerically smallest IP address.  In the case of IPv6, the
   link local address will be used.  When two neighbors have the same IP
   address, either for IPv4 or IPv6, the interface index MUST be used as
   a tie breaker.  The attributes from other sending routers MAY be
   remembered; that way, if the adjacency that supplied the selected
   attribute gets pruned or expires, we are able to immediately use the
   attribute that was sent by the adjacency that is next in the order of
   preference.  This enables us to converge quickly without waiting for
   the next periodic update.

   When a particular attribute type is specified, the specification MAY
   include a conflict resolution procedure specific to that type.  If
   so, that conflict resolution procedure MUST be used instead of the
   procedure described in this section.

   It is possible that a router will receive, from two different
   adjacencies, transitive attributes of a given type.  If the router
   does not understand attributes of that type and if the two
   adjacencies have not sent the exact same set of attributes of that
   type, then the conflict resolution procedure described in this
   section MUST be applied to those attributes.  Two adjacencies are
   said to have sent the exact same set of attributes of a given type if
   they have sent the same number of instances of that attribute and if
   corresponding instances are byte-for-byte identical.

3.3.4.  Attribute Change

   A PIM router may decide to change the set of attributes it has
   associated with a given multicast distribution tree.  This can happen
   if one of its downstream neighbors on the tree has changed the set of
   attributes.  It can also happen as a result of processing the
   attributes.  It can also happen for reasons outside the scope of this
   specification (such as a change in configuration).

   If a PIM router needs to change the set of attributes for a given
   tree but does not change its upstream neighbor for that tree, it MUST
   send a new Join for that tree, specifying the new set of attributes.
   If the new set of attributes is the null set, the type 0 Encoded-
   Source format MUST be used.

   If a PIM router needs to change the set of attributes for a given
   tree and as a result changes its upstream neighbor for that tree, it
   sends a Prune to the old upstream neighbor.  The Prune does not need
   to carry any attributes.

   When a PIM router receives a Join for a given tree and the Join does
   not contain exactly the same set of attributes as the prior Join, the
   set of attributes in the new Join becomes the entire new set of
   attributes.  No attribute information from the prior Join is
   retained.  There is no way to advertise incremental changes to the
   set of attributes; any attributes that are no longer present are
   considered to have been withdrawn.  If, as the result of receiving a
   Join, a PIM router determines that the set of attributes has changed,
   it will need to send a new Join upstream that contains the new set of
   attributes.  (Of course, the procedures for resolving attribute
   conflicts may need to be applied first.)

   When a PIM router R1 receives a Prune for a given tree from a given
   downstream neighbor R2, where R2 had previously sent attributes
   applying to that tree, those attributes are considered to have been
   withdrawn.  Depending on the attributes that R1 has received from its
   other downstream neighbors (if any) on the tree, R1 may determine
   that the set of attributes applying to the tree has changed, in which
   case it needs to send a new Join, with the new attribute set, to its
   upstream neighbor on the tree.

3.4.  PIM Attribute Packet Format

3.4.1.  PIM Join Packet Format

   There is no space in the default PIM source encoding to include an
   attribute field.  Therefore we introduce a new source encoding type.
   The attributes are formatted as TLVs.  The new Encoded-Source Address
   looks like this:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Addr Family   | Encoding Type | Rsrvd   |S|W|R|  Mask Len     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Source Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....
   |F|E| Attr_Type | Length        | Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....
   |F|E| Attr_Type | Length        | Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....
        .                    .                     .
        .                    .                     .

   - Encoding Type: 1

   - F-bit, Transitive Attribute.  If this bit is set, the attribute is
     a transitive attribute; otherwise, it is a non-transitive
     attribute.  See section 3.3.2.

   - E-bit, End of Attributes.  If this bit is set, then this is the
     last Join Attribute appearing in this Encoded-Source Address field.

   - "Attr_Type", a 6-bit field identifying the type of the Attribute.

   - Length field, a 1-octet field specifying the length in octets,
     encoded as an unsigned binary integer, of the value field.

   The other fields are the same as described in [RFC4601].

3.4.2.  PIM Join Attribute Hello 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OptionType = 26           |      OptionLength = 0        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   - Option type: 26.

4.  IANA Considerations

   A new IANA registry has been created for "PIM Join Attribute Types".
   These are values of the "Attr_Type" field depicted in section 3.4.1.
   Assignments are to be made according to the policy "IETF Review" as
   defined in [RFC5226].

   IANA has assigned the PIM Hello option value 26 to the "Join
   Attribute" option, with this document as the reference.

   [RFC4601] should have, but did not, create a registry for the
   "Encoding Type" field of the Encoded-Source Address format defined
   therein.  IANA has set up a registry for this, referencing both this
   document and [RFC4601].  Assignments should be made according to the
   policy "IETF Review" as defined in [RFC5226].  Two encoding types are
   defined:

   - The encoding type 0 has been allocated, defined as "native encoding
     for the address family", and [RFC4601] is the reference.

   - The encoding type 1 has been allocated, defined as "native encoding
     for the address family, but with zero or more PIM Join Attributes
     present", and this document is the reference.

5.  Security Considerations

   Security of the Join Attribute is only guaranteed by the security of
   the PIM packet, so the security considerations for PIM Join packets
   as described in [RFC4601] apply here.  Additional security
   considerations may apply to specific attributes; if so, these will
   need to be documented in the specification of those attributes.

   Security considerations from [RFC5015] may apply as well.

6.  Acknowledgments

   The authors would like to thank Stig Venaas, James Lingard, Bharat
   Joshi, Marshall Eubanks, Pekka Savola, Tom Pusateri, and Elwyn Davies
   for their input.

7.  Normative References

   [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.

8.  Informative References

   [RFC3973] Adams, A., Nicholas, J., and W. Siadak, "Protocol
             Independent Multicast - Dense Mode (PIM-DM): Protocol
             Specification (Revised)", RFC 3973, January 2005.

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

   [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 5226, May
             2008.

Authors' Addresses

   Arjen Boers
   Cisco Systems, Inc.
   Avda. Diagnoal, 682
   Barcelona 08034

   EMail: aboers@cisco.com


   IJsbrand Wijnands
   Cisco Systems, Inc.
   De kleetlaan 6a
   Diegem  1831
   Belgium

   EMail: ice@cisco.com


   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA, 01719

   EMail: erosen@cisco.com