Registry and Extensions for P-Multicast Service Interface Tunnel Attribute Flags
updates
- rfc-6514 — BGP Encodings and Procedures for Multicast in MPLS/BGP IP VPNs
Extracted elements (16)
The original 'PMSI Tunnel' attribute Flags field defined in RFC 6514 is only one octet, allowing only eight flags, which is considered insufficient for all future applications. This document creates a new Extended Community to carry additional flags beyond the eight-bit limit.
Whether all members of a particular set of BGP speakers support a given flag bit is outside the scope of this document. Applications that require all speakers in a set to support a particular flag must define their own mechanism for determining support.
A BGP speaker MUST NOT attach more than one 'Additional PMSI Tunnel Attribute Flags' Extended Community to a given BGP UPDATE. If one is already present, no additional such Extended Communities may be attached.
A BGP speaker that does not recognize a particular flag (in either the 'PMSI Tunnel' Flags field or the 'Additional PMSI Tunnel Attribute Flags' Extended Community) MUST simply ignore that flag and SHOULD leave its setting unchanged when propagating the attribute or Extended Community.
If a BGP speaker modifies or replaces the 'PMSI Tunnel' attribute such that the Extension flag is no longer set, any 'Additional PMSI Tunnel Attribute Flags' Extended Communities MUST be removed before propagating the UPDATE. If the 'PMSI Tunnel' attribute is removed entirely, those Extended Communities MUST also be removed.
If a BGP speaker receives a BGP UPDATE with more than one 'Additional PMSI Tunnel Attribute Flags' Extended Community, only the flag settings in the first occurrence are significant; settings in subsequent occurrences MUST be ignored. When propagating the UPDATE, all instances other than the first SHOULD be removed.
If a BGP speaker receives an UPDATE containing a 'PMSI Tunnel' attribute with the Extension flag set but no 'Additional PMSI Tunnel Attribute Flags' Extended Community, the UPDATE is considered malformed and the 'treat-as-withdraw' procedure of RFC 7606 MUST be applied.
If a BGP speaker receives an UPDATE with one or more 'Additional PMSI Tunnel Attribute Flags' Extended Communities but either no 'PMSI Tunnel' attribute is present or the Extension flag is not set, the Extended Community(ies) SHOULD be removed and SHOULD NOT be redistributed; the UPDATE MUST be processed as if the Extended Community(ies) had not been present.
The 'Additional PMSI Tunnel Attribute Flags' Extended Community MUST NOT be carried by a BGP UPDATE unless that UPDATE also carries a 'PMSI Tunnel' attribute and the 'Extension' flag of that attribute's Flags field is set.
A new bit flag called the 'Extension' flag is defined at bit position 1 (left to right) in the one-octet 'Flags' field of the 'PMSI Tunnel' attribute. When set, it signals that an 'Additional PMSI Tunnel Attribute Flags' Extended Community is also present in the same BGP UPDATE.
The 'Additional PMSI Tunnel Attribute Flags' Extended Community is a new Transitive Opaque Extended Community (per RFC 4360 and RFC 7153). Its six-octet value field is treated as a string of 48 one-bit flags, numbered 0 (MSB of most significant octet) through 47 (LSB of least significant octet).
IANA assigned codepoint 0x07 from the 'First Come, First Served' range of the 'Transitive Opaque Extended Community Sub-Types' registry for the 'Additional PMSI Tunnel Attribute Flags' Extended Community.
IANA created the 'P-Multicast Service Interface (PMSI) Tunnel Attribute Flags' registry under 'Border Gateway Protocol (BGP) Parameters'. Bit 7 (Leaf Information Required, L) is assigned per RFC 6514; bit 1 (Extension) is assigned by this document; bits 0 and 2–6 are unassigned. Registration procedure is Standards Action.
IANA established a registry for bit flags carried in the 'Additional PMSI Tunnel Attribute Flags' Extended Community. Bits are numbered 0–47 (0 = most significant). The initial state has all 48 bits unassigned. Registration policy is Standards Action.
Incorrectly setting the 'Extension' flag in a 'PMSI Tunnel' attribute without including the corresponding 'Additional PMSI Tunnel Attribute Flags' Extended Community causes the receiving BGP speaker to treat the UPDATE as a withdrawal via the RFC 7606 treat-as-withdraw procedure. This can be exploited to cause spurious route withdrawals.
The value field of the 'Additional PMSI Tunnel Attribute Flags' Extended Community is six octets wide, encoding 48 individual flag bits. Bit 0 is the leftmost (most significant) bit and bit 47 is the rightmost (least significant) bit of the six-octet field.