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Internet Engineering Task Force                                 A. Lange
INTERNET DRAFT                                                   Alcatel
                                                              March 2004
                                                  Expires September 2004


                        Flexible BGP Communities
             <draft-lange-flexible-bgp-communities-02.txt>


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   This document defines a new attribute for BGP called the Flexible
   Community attribute.  Flexible Communities build on the experience
   and utility of the standard BGP community, and the extended BGP
   community attributes.  This attribute allows operators to associate
   structured information with a route or set of routes.  This
   information can be then be used to execute routing policy.  An
   enhanced version of communities is necessary to accommodate IPv6,
   4-byte ASN's, and introduce a more extensible and flexible policy
   expression.  This document also introduces the concept of Neighbor
   Classes.  A Neighbor Class is applied to a group of BGP neighbors who
   share certain attributes.  For example, the PEER Neighbor Class could
   be applied to BGP sessions between ASN X and other networks with
   which ASN X has a non-transit peering relationship.





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Table of Contents
   Status of this Memo.........................................   1
   Abstract....................................................   1
   1.  Introduction............................................   2
   2.  The BGP Flexible Community Attribute....................   3
   2.1  Transitivity Field.....................................   4
   2.2  Structure Field........................................   4
   2.3  Type Field.............................................   5
   2.4  Originating ASN Field..................................   5
   2.5  Length Field...........................................   6
   3.  Locally Defined Structures and Types....................   6
   4.  Neighbor Classes........................................   6
   4.1  Locally Defined Neighbor Classes.......................   7
   4.2  Defined Neighbor Classes...............................   8
   5.  Defined Flexible BGP Community Structures...............   9
   6.  Base BGP Flexible Community Type (Opaque Type)..........  10
   7.  Defined Flexible BGP Community Types....................  10
   7.1  NO_EXPORT..............................................  10
   7.2  ONLY_EXPORT............................................  11
   7.3  ANNOUNCE_WITH..........................................  12
   7.4  PREPEND................................................  13
   7.5  The BGP VPN Communities................................  13
   8.  New Capability Code for Flexible Communities............  14
   9.  Aggregation.............................................  14
   10. Operations..............................................  15
   11. Security Considerations.................................  15
   12. IANA Considerations.....................................  16
   13. References..............................................  16
   14. Acknowledgements........................................  17
   15. Author's Address........................................  17

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 [RFC2119].

1. Introduction

   This attribute represents the third generation of the BGP community
   attribute.  The first generation is documented in RFC1997 [RFC1997].
   The second generation, the extended community, is documented in EX-
   COMM [EX-COMM].

   The Flexible Community Attribute provides a number of important
   enhancements over the existing BGP Community Attribute and BGP
   Extended Community Attribute.  These enhancements are:




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     - Support for IPv6.
     - More efficient encoding of lists of data, for example, a list of
     ASN's.
     - Clean support for a broad range of future data field structures
     and interpretations.
     - Support for locally defined community structures, and
     interpretations.
     - Easy extensibility for a range of future applications.

   The continuation and expansion of the structure introduced with
   Extended Communities allows for policy based on the application where
   the community is being used.  The separation of structure and
   interpretation allows for easier machine and human parsing of
   community types which do the same thing with slightly different
   input.

   This attribute continues the use of the Transitivity community bit,
   first introduced in the Extended Community Attribute.

   We also define a set of well-known values for this attribute which
   can be used to replicate and extend the functionality of the existing
   well-known community values.

   The concept of Neighbor Classes is introduced.  A Neighbor Class is
   defined on a BGP peering session.  It allows neighbors that share a
   set of administrative attributes to be easily grouped together.
   Policy can then be defined through Flexible Communities in reference
   to these groupings.

2. The BGP Flexible Community Attribute

   The Flexible Community Attribute is a transitive optional BGP
   attribute with a Type Code of <TBD>.  The attribute consists of a
   list of "flexible communities."

   Each Flexible Community is encoded as a variable length quantity.
   The encoding scheme is:

     - Transitivity Field: 1 bit
     - Structure Field: 7 bits
     - Type Field: 2 octets
     - Length Field: 1 octet
     - Value Field: 0-255 octets

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |T|   Struct.   |             Type              |  Originating  |



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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      ASN                      |    Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Value Field  (0-255 octets)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.1. Transitivity Field:

   The Transitivity Field is one bit.  It can take the following values:

   Value 0: The community is transitive across ASes

   Value 1: The community is non-transitive across ASes

   It is important to note that the transitivity defined by this field
   is different from the general transitivity of a BGP attribute.  A
   single Flexible Community Attribute, can contain multiple Flexible
   Communities, each of which may or may not be transitive.  If a route
   originates in an AS with the transitivity bit set, indicating that
   the community is non-transitive, then that AS MUST NOT propagate that
   community to its peers.  However, if a community with the transitive
   bit set is applied on an outbound policy expression (e.g., a route-
   map), the community will be conveyed to the immediately adjacent
   peer.  That peer, in turn, will NOT propagate the community to its
   peers.  The one exception to this is as-confederations.  For the
   purposes of this attribute, confederation boundaries should be
   treated the same as IBGP.     In other words, non-transitive flexible
   communities should be propagated to other members of the as-
   confederation, unless overridden by local policy.

   If the community is transitive, then the Value Field MUST contain the
   originating ASN.  This ASN is encoded as a 4-octet value, occupying
   the first 4 octets of the Value Field.  Two-octet ASN numbers are
   padded out to 4 octets.  Any additional information in the Value
   Field comes after this origin ASN data.

2.2. Structure Field:

   The Structure Field's contents modify the Type Field.  For example, a
   Flexible Community which specifies SPECIFIC_NO_EXPORT in its Type
   Field, can be modified by the contents of the Structure Field to let
   the receiver know if the list of data on which it must act is a list
   of 2 octet or 4 octet ASNs.  A set of commonly used Structure values
   is defined later in this document.

   The Structure field is the latter 7 bits of the first octet.  It is
   split into two sub-fields.




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        0
        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |T|L|  Struct.  |
       +-+-+-+-+-+-+-+-+

   L - Local bit

   The Local bit can take two values:

     Value 0: The Structure is Locally Defined.

     Value 1: The Structure is Well Known.

2.3. Type Field:

   The Type Field is two octets.  This contents of this field are used
   to define an action for the recipient to take on the route, or to
   define and attribute that is related to that route.  An example of
   the former would be a Type which requests that a route be
   ONLY_EXPORTed to a specific set of peers.  An example of the latter
   would be a Type that defines the LINK_BANDWIDTH associated with a
   certain NLRI.

   Like the Structure Field the Type Field is split into two Sub-Fields:

            1                   2
        8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |L|            Type             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The Local bit can take two values:

       Value 0: The Type is Locally Defined.

       Value 1: The Type is Well Known.

   An implementation MUST allow an operator to filter out entire Types
   of Flexible Communities from their peering sessions if they so
   choose.

2.4 Originating ASN Field:

   This field contains the ASN that added the flexible community to the
   route.  It is 4 octets long.  In the case of a 2-byte ASN, the first
   2 octets are set to zero, as padding.




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2.5. Length Field:

   The Length Field specifies the length, in octets, of the Value Field.

3. Locally Defined Structures and Types

   The Local bit allows the operator of the network to define Structures
   and Types that are relevant only within that ASN's boundaries.  The
   definition the term "local" used throughout this document is: "A
   value used by ad hoc agreement or convention outside the scope of
   standardization, which has meaning only between the parties using the
   Flexible Community in question."  This typically means that the Local
   value only has meaning within an AS or set of ASes controlled by a
   single entity.

   A Locally Defined Structure or Type will have a syntax for
   interpretation defined on the routers that need to interpret it.  If
   a router receives a community with a Locally Defined Structure or
   Type that it does not recognize, then it should ignore the contents
   and process the route based on the information in the route that it
   does understand.  This includes obeying the transitivity bit, in the
   Flexible Community.  If the community is set to non-transitive, even
   if the router does not understand the rest of the Structure or Type
   of the community, that community should not be forwarded outside the
   AS.

   In order to prevent collisions with other operators' Locally Defined
   values, Flexible Communities containing Locally Defined Structures or
   Types MUST be non-transitive (have their Transitivity Field set to
   1).

4. Neighbor Classes

   A Neighbor Class is a value which represents a certain class, or
   group, of BGP neighbors.  Each BGP peering session can be configured
   with zero or more Neighbor Classes.  This value will allow a general
   classification of what sort of relationship the BGP session
   represents.  With the sort of session defined, it becomes easier to
   apply policy to only that class of neighbors.  Neighbor Classes make
   the expression of policy through flexible communities much easier.
   There are a number of examples in the sections on defined values.

   Neighbor Classes can be used in two main ways.  First they can be
   used in a passive manner, where the configuration acts as a policy
   expression for communities matching it.  An example of this would be
   configuring a neighbor with the PEER neighbor class, and having a
   community that, say is set to NO_EXPORT for the NEIGHBOR CLASS PEER.
   In this configuration, all the neighbors that are configured as PEERs



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   would match and filter out the route carrying the NO_EXPORT, NEIGHBOR
   CLASS PEER community.  This is the standard way of using neighbor
   classes.

   A second, optional, way to use Neighbor Classes would be to allow
   inbound community tagging.  In this usage, routes traversing the
   session would be automatically tagged with a flexible community with
   the appropriate neighbor class value.  This eases configuration.  To
   extend the example above, our neighbor designated PEER would add a
   community with NEIGHBOR CLASS PEER to routes traversing the BGP
   session.  This community could then be used further down the line to
   just announce PEER routes to a particular customer.  This usage is
   configurable on a per-neighbor basis.

   A Neighbor Class is encoded as a 2-octet value with 2 parts:

        0                   1
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |L|       Neighbor Class        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The Local bit can take two values:

       Value 0: The Neighbor Class is Locally Defined.

       Value 1: The Neighbor Class is Well Known.

4.1. Locally Defined Neighbor Classes

   If a router receives a Flexible Community containing a Neighbor Class
   that it does not recognize, then it should ignore the contents and
   process the route based on the information in the route that it does
   understand.  If the Transitivity Field of the Flexible Community with
   the Locally Defined Structure or Type is set to 1 (the community is
   non-transitive) then the router MUST NOT forward the Flexible
   Community.  Similarly, if the Transitivity Field is set to 0 (the
   community is transitive) the router MUST forward the community along
   with the NLRI.

   Using Locally Defined Neighbor Classes an operator could easily
   define a set that is locally useful.  This set could be used in a
   flat name space, for example, one could say that "31" would be "Asian
   Public Peers", and "34" would be European Private Peers.

   Also, a given BGP Neighbor can be part of multiple Neighbor Classes.
   This would allow for a hierarchical or additive name space. For
   example, a neighbor could be part of both "PEER", and locally defined



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   "ASIAN" and "PUBLIC PEER" Classes.  The logical matching
   functionality available is left implementation-dependent.  However,
   the default in such as case is logical OR functionality for matching
   this neighbor class.  In the case where routes are being tagged
   inbound, then by default a single community with all configured
   neighbor classes in a list is added.  Implementation dependent knobs
   are suggested to allow more fine grained control.

4.2. Defined Neighbor Classes

   This document defines Neighbor Class values for common BGP neighbor
   groupings:

   ALL NEIGHBORS

     This class is the default Neighbor Class for all BGP peers.

     This class is represented by a value of 0 (0x8000).

   PEER

     This class is typically applied to sessions where a transit-free
     relationship exists between the two providers.

     This class is represented by a value of 1 (0x8001).

   CUSTOMER

     This class is typically applied to sessions where the remote end of
     the session is operated by a customer.

     This class is represented by a value of 2 (0x8002).

   UPSTREAM

     This class is typically applied to sessions where the remote end of
     the session is operated by a network from which you receive transit
     routes.

     This class is represented by a value of 3 (0x8003).

   CONFEDERATION PEER

     This class is typically applied to sessions where the remote end of
     the session is part of a confederation.

     This class is represented by a value of 4 (0x8004).




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5. Defined Flexible BGP Community Structures

   This section defines a number of Structure values which different
   Type values can inherit.

   Summary of the Defined Values:

    - opaque/variable (0x40 or 0xC0)
    - list of ASN's (0x41 or 0xC1)
    - list of IPv4 addresses (0x42 or 0xC2)
    - list of IPv6 addresses (0x43 or OxC3)
    - list of neighbor_classes (0x44 or 0xC4)

   Defined Structure can be transitive or non-transitive, they are well
   known.

   Opaque/Variable Structure

     This sort of structure defers interpretation of the community and
     value field to the Type value.  Typically this structure value will
     be used when the Type value does not have a lot of variations, but
     rather one structure for the Value Field.

     This structure is represented by the value 0x00.

   List of ASN's

     This structure value means that the Type Field's action is
     qualified by a list of ASN's, contained in the Value Field.  In the
     case of 2 byte ASN's the value is padded to 4 bytes by inserting 2
     octets worth of zeros in the leftmost portion of the value.

     This structure is represented by the value 0x01.

   List of IPv4 Addresses

     This structure value means that the Type Field's action is
     qualified by a list of IPv4 addresses, contained in the Value
     Field.

     This structure is represented by the value 0x02.

   List of IPv6 Addresses

     This structure value means that the Type Field's action is
     qualified by a list of IPv6 addresses, contained in the Value
     Field.




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     This structure is represented by the value 0x03.

   List of Neighbor Classes

     This structure value means that the Type Field's action is
     qualified by a list of neighbor classes, contained in the Value
     Field.

     This structure is represented by the value 0x04.

6. Base BGP Flexible Community Type (Opaque Type)

   This section defines the base BGP community specification.  Since the
   root value of communities is the ability to tag a route with
   arbitrary information, and then create a system to give that
   information meaning, the base flexible community type (type 0), is
   very simple.  This base type could also be described as the "opaque
   type."  All actions can be replicated with a well-thought out,
   provider dependent, implementation of a scheme using this opaque
   type.

   Like all flexible communities, this type can be transitive or non-
   transitive.  This is a well known type.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   0x00,0xC0   |             0x0000            |  Originating  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      ASN                      |    Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Value Field  (0-255 octets)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


7. Defined Flexible BGP Community Types

   The Type Field specifies the subgroup that a set of communities
   belongs to.  Typically this subgroup represents an action to be taken
   on the data.  A variety of well-known Type Values follow.

7.1. NO_EXPORT

   This grouping of well-known communities specify a list of ASNs, Peer
   IPs, or Neighbor Classes NOT to announce a route to.

   Name: NO_EXPORT
   Type Code: 0x0001



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   Can Take Structures:
     Ox01 (ASN), 0x02 (IPv4), 0x03 (IPv6), 0x04 (neighbor-class)
   Transitive: Non-Transitive
   Min Length of Value Field: 2 octets
   Max Length of Value Field: 254 octets
   Behavior:
     All routes received with this community MUST NOT be advertised to
     the list of ASNs, Peer IPs, or Neighbor Classes contained in the
     Value Field.
   Notes:
     GLOBAL_NO_EXPORT is accomplished by sending a NO_EXPORT Flexible
     Community with the Neighbor Class of 0x00 (ALL NEIGHBORS).

     GLOBAL_NO_EXPORT's NO_EXPORT behavior is defined as:
       All routes received with this community MUST NOT be advertised
       outside a BGP confederation boundary (a stand-alone autonomous
       system that is not part of a confederation should be considered a
       confederation itself.)    [RFC1997]

     This is analogous to the NO_EXPORT community defined in [RFC1997].

7.2. ONLY_EXPORT

   This grouping of well-known communities specify a list of ASNs, Peer
   IPs, or Neighbor Classes to announce a route to.

   Name: ONLY_EXPORT
   Type Code: 0x0002
   Can Take Structures:
     Ox01 (ASN), 0x02 (IPv4), 0x03 (IPv6), 0x04 (neighbor-class)
   Transitive: Non-Transitive
   Min Length of Value Field: 0 octets
   Max Length of Value Field: 254 octets
   Behavior:
     The Value Field contains a list of ASNs, neighbor IP addresses, or
     Neighbor Classes to which the route should be advertised.

     The default behavior of a route carrying this community is the same
     as the GLOBAL_NO_EXPORT behavior, except for the ASNs, IPs, or
     Neighbor Classes listed in the Value Field.
   Notes:
     This community can be used to replicate the NO_ADVERTISE
     functionality from [RFC1997].  To do so, simply announce
     ONLY_EXPORT with a Structure of 0x03 or 0x04 (one of the IP address
     Structures), but with no IP address in the list.  This will tell
     the receiving router that you wish to ONLY_EXPORT this route to NO
     peer IPs.




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     This community can also be use to replicate the NO_EXPORT_SUBCONFED
     functionality from [RFC1997].  To do so, simply announce
     ONLY_EXPORT with a Neighbor Class of CONFEDERATION PEER (4,
     0x8004).  This will tell the receiving router that you wish to
     ONLY_EXPORT this route to Confederation Peers.

7.3. ANNOUNCE_WITH

   This group of well-known communities allows a network to announce a
   community to an ASN beyond those that it directly peers with,
   assuming its direct peers allow it to transit the community value.
   This community group has a lot of flexibility, and could be used to
   nest another ANNOUNCE_WITH community to gain reach greater than 2
   ASN-hops away.  If this is a good idea or not is unknown and is left
   to further study. The only theoretical restriction to the amount of
   nesting is that the community cannot exceed the maximum size for the
   Value Field.

   Since true transitivity can be obtained by simply setting a bit, this
   community is mainly useful for propagating NO_EXPORT or ONLY_EXPORT
   (which are non-transitive) to your neighbor's neighbors.

   In effect, this community, if allowed by the BGP neighbors in the
   chain, can be used for an originating network to very specifically
   control the distribution of its routes.  This community type does
   contain a LOT of rope, and should be used with care.  In the end,
   though, a mistake should only effect the person originating the
   route.

   Name: ANNOUNCE_WITH
   Type Code: 0x0003
   Can Take Structures:
     Ox01 (ASN), 0x02 (IPv4), 0x03 (IPv6), 0x04 (neighbor-class)
   Transitive: Non-Transitive
   Min Length of Value Field: 8 octets
   Max Length of Value Field: 254 octets
   Behavior:
     This community's Value Field is split into two sections.
     - The first section is a variable length field that contains the
     full community value that you wish to announce.
     - The second section is a variable length field that contains the
     list of, ASN's, IP addresses, or neighbor_classes you wish to
     propagate this community to.  If you wish to propagate to all
     peers, use the ALL NEIGHBORS neighbor class.

     When a router receives this community value, it should strip the
     ANNOUNCE_WITH community and announce the underlying community value
     to its neighbor.



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7.4. PREPEND

   This community can be used to ask a BGP peer to prepend its own ASN
   to its peers.

   Name: PREPEND
   Type Code: 0x0004
   Can Take Structures:
     Ox01 (ASN), 0x02 (IPv4), 0x03 (IPv6), 0x04 (neighbor-class)
   Transitive: Non-Transitive
   Min Length of Value Field: 3 octets
   Max Length of Value Field: 254 octets
   Behavior:
     This community has 2 sections:
     The first section:
       Is a one-octet value which specifies the number of times that the
       ASN should prepend its ASN.  It is recommended that operators
       constrain this value to no more than 3.  Implementations MUST
       offer the ability for an operator to set a maximum bound for this
       field.  The suggested default is also 3.
     The second section:
       Contains a list of ASNs, peer IPs, or Neighbor Classes to which
       the originator of this community wishes its peer to prepend its
       ASN.

7.5. The BGP VPN Communities

   These communities are used mostly for BGP MPLS VPN's.  Please see
   RFC2547 [RFC2547] for more detail on how these VPNs are constructed.

   Name: ROUTE_TARGET
   Type Code: 0x0005
   Can Take Structures:
     0x02 (IPv4), 0x03 (IPv6)
   Transitive: Transitive or Non-Transitive
   Min Length of Value Field: 4 octets
   Max Length of Value Field: 254 octets
   Behavior:
     The Value Field of this community represents a list of the IP
     addresses where this route is to be announced.

   Name: ROUTE_ORIGIN
   Type Code: 0x0006
   Can Take Structures:
     0x02 (IPv4), 0x03 (IPv6)
   Transitive: Transitive or Non-Transitive
   Min Length of Value Field: 4 octets
   Max Length of Value Field: 16 octets



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   Behavior:
     The Value Field of this community represents a list of the IP
     address where this route is originated.  This community can only
     contain one IP address.

   Name: LINK_BANDWIDTH
   Type Code: 0x0007
   Can Take Structures:
     0x00 (opaque), Ox01 (ASN)
   Transitive: Transitive or Non-Transitive
   Min Length of Value Field: 4 octets
   Max Length of Value Field: 6 octets
   Behavior:
     This community consists of two parts:
     The first part represents the bandwidth of the link in bits-per-
     second, encoded in IEEE floating point format.     This part is 4
     octets long.

     The second part consists of a list of ASNs of the peer whose link
     bandwidth you wish to propagate.

8. New Capability Code for Flexible Communities

To ensure compatibility between implementations that may or may not
implement this protocol extension, this document defines a new
capability.

     Capability Code: TBD
     Capability Length: 1
     Capability Value: 0x00 for unsupported
                       0x01 for supported

   Capability negotiation is especially important for this attribute
   because we are creating a transitivity action within an optional,
   transitive attribute.  If an implementation sends a flexible
   community with the non-transitive bit set within the community to a
   router that does not support flexible communities, that router will
   send the community on to its peers when it should not do so.

9. Aggregation

   Aggregation behaves the same as with other community types.

   By default if a range of routes is to be aggregated and the resultant
   aggregates path attributes do not carry the ATOMIC_AGGREGATE
   attribute, then the resulting aggregate should have an Flexible
   Communities path attribute which contains the set union of all the
   Flexible Communities from all of the aggregated routes. The default



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   behavior could be overridden via local configuration, in which case
   handling the Flexible Communities attribute in the presence of route
   aggregation becomes a matter of the local policy of the BGP speaker
   that performs the aggregation.

10. Operations

   Flexible Communities are handled operationally in a manner very
   similar to other community values.

   A BGP speaker may use the Flexible Communities attribute to control
   which routing information it accepts or distributes to its peers.

   The Flexible Community attribute MUST NOT be used to modify the BGP
   best path selection algorithm in a way that leads to forwarding
   loops.

   A BGP speaker receiving a route that doesn't have the Flexible Commu-
   nities attribute MAY append this attribute to the route when
   propagating it to its peers.

   A BGP speaker receiving a route with the Flexible Communities
   attribute MAY modify this attribute according to the local policy.
   If a route has a non-transitivity flexible community, then before
   advertising the route across the Autonomous system boundary the
   community SHOULD be removed from the route. However, the community
   SHOULD NOT be removed when advertising the route across the BGP
   Confederation boundary.

   A route may carry the BGP Communities attribute as defined in
   [RFC1997]), the Extended BGP Communities attribute as defined in [EX-
   COMM], and the Flexible Communities attribute. In this case the BGP
   Communities attribute is handled as specified in [RFC1997], the
   Extended BGP Communities attribute is handled as specified in [EX-
   COMM], and the Flexible Communities attribute is handled as specified
   in this document.

   If older community types are present in addition to the flexible
   community there is the possibility that the information contained may
   be redundant.  Although effort should be made to avoid this
   situation, if it does occur the BGP Speaker shall prefer the flexible
   community first, then the extended community second and then the base
   community.

11. Security Considerations

   This extension to BGP does not change the underlying security issues.




Lange                                                          [Page 15]


INTERNET DRAFT                                             December 2002


12. IANA Considerations

   The values for the Transitivity Field (1 or 0) are completely defined
   in this document.

   The assignment policy for the Structure Field is:
     o The "L" bit's usage is completely defined in this document.
     o Values of the Structure Field where the "L" bit is "0" are to be
     assigned in accordance with the Private Use policy outlined in
     RFC2434 [RFC2434].
     o Values of the Structure Field where the "L" bit is "1" defined in
     this document are: 0-4 (0x40-0x44 and 0xC1-0xC4).  Remaining values
     in this range are to be assigned using the IETF Consensus policy
     outlined in RFC2434 [RFC2434].

   The assignment policy for the Type Field is:
     o The "L" bit's usage is completely defined in this document.
     o Values of the Type Field where the "L" bit is "0" are to be
     assigned in accordance with the Private Use policy outlined in
     RFC2434 [RFC2434].
     o Values of the Type Field where the "L" but is "1" defined in this
     document are: 0-7 (0x0000-0x0007).  Remaining values in this range
     are to be assigned using the IETF Consensus policy outlined in
     RFC2434 [RFC2434].

   The assignment policy for Neighbor Classes is:
     o The "L" bit's usage is completely defined in this document.
     o Values of the Type Field where the "L" bit is "0" are to be
     assigned in accordance with the Private Use policy outlined in
     RFC2434 [RFC2434].
     o Values of the Type Field where the "L" but is "1" defined in this
     document are: 0-4 (0x8000-0x8004).  Remaining values in this range
     are to be assigned using the IETF Consensus policy outlined in
     RFC2434 [RFC2434].

13. References

   [RFC1771] Rekhter, Y., Li, T., "A Border Gateway Protocol 4 (BGP-4)",
   RFC 1771, March 1995.

   [RFC1997] Chandra, R., Traina, P., Li, T., "BGP Communities
   Attribute", RFC 1997, August 1996.

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

   [RFC2434] Narten, T., Alvestrand, H., "Guidelines for Writing an IANA
   Considerations Section in RFCs", RFC 2434, October 1998



Lange                                                          [Page 16]


INTERNET DRAFT                                             December 2002


   [RFC2547] Rosen, E., Rekhter, Y., "BGP/MPLS VPNs", RFC 2547, March
   1999.

   [RFC2842] Chandra, R., Scudder, J., "Capabilities Advertisement with
   BGP-4", RFC 2842, May 2000.

   [EX-COMM] Sangli, S., Tappan, D., Rekhter, Y., "BGP Extended
   Communities Attribute", draft-ietf-idr-bgp-ext-communities-05.txt,
   work-in-progress, expires November, 2002.

   [NOPEER] Huston, G., "NOPEER community for BGP route scope control",
   draft-ietf-ptomaine-nopeer-00.txt, work-in-progress, expires October,
   2002.

   [REDIST] Bonaventure, O., et. al., "Controlling the redistribution of
   BGP routes", draft-ietf-ptomaine-redistribution-01.txt, work-in-
   progress, expires February, 2003.

14. Acknowledgements
   I would like to thank Tom Barron, Dave Ward and especially Hal
   Peterson for their valuable comments and feedback.

15. Author's  Address:

   Andrew Lange
   Alcatel
   274 Ferguson Drive
   Mountain View, CA  94043
   andrew.lange@alcatel.com






















Lange                                                          [Page 17]


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