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IETF RFC 5384
The Protocol Independent Multicast (PIM) Join Attribute Format
Last modified on Friday, November 14th, 2008
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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
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.
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.
Boers, et al. Standards Track PAGE 1 
RFC 5384 PIM Join Attribute November 2008
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" [RFC 4601] (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" ([RFC 4601], 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 [RFC 4601] is 0.
This specification is the first to assign another encoding type
value.
Boers, et al. Standards Track PAGE 2
RFC 5384 PIM Join Attribute November 2008
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" [RFC 3973] 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 [RFC 2119].
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 [RFC 4601].
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.
Boers, et al. Standards Track PAGE 3
RFC 5384 PIM Join Attribute November 2008
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.
Boers, et al. Standards Track PAGE 4
RFC 5384 PIM Join Attribute November 2008
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
Boers, et al. Standards Track PAGE 5
RFC 5384 PIM Join Attribute November 2008
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.
Boers, et al. Standards Track PAGE 6
RFC 5384 PIM Join Attribute November 2008
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....
. . .
. . .
Boers, et al. Standards Track PAGE 7
RFC 5384 PIM Join Attribute November 2008
- 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 [RFC 4601].
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 [RFC 5226].
IANA has assigned the PIM Hello option value 26 to the "Join
Attribute" option, with this document as the reference.
[RFC 4601] 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 [RFC 4601]. Assignments should be made according to the
policy "IETF Review" as defined in [RFC 5226]. Two encoding types are
defined:
- The encoding type 0 has been allocated, defined as "native encoding
for the address family", and [RFC 4601] is the reference.
Boers, et al. Standards Track PAGE 8
RFC 5384 PIM Join Attribute November 2008
- 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 [RFC 4601] 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 [RFC 5015] 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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4601] 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
[RFC 3973] Adams, A., Nicholas, J., and W. Siadak, "Protocol
Independent Multicast - Dense Mode (PIM-DM): Protocol
Specification (Revised)", RFC 3973, January 2005.
[RFC 5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-PIM)",
RFC 5015, October 2007.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, May
2008.
Boers, et al. Standards Track PAGE 9
RFC 5384 PIM Join Attribute November 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
Boers, et al. Standards Track PAGE 10
The Protocol Independent Multicast (PIM) Join Attribute Format
RFC TOTAL SIZE: 22330 bytes
PUBLICATION DATE: Friday, November 14th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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