Liaison from MPLS & FR Alliance regarding LDP Host

Address FEC

Andrew G. Malis

Host Address FEC vs. Address Prefix FEC

• From RFC 3036:– An Address Prefix FEC element which is a full

address (a /32) has a different effect than a Host Address FEC element which has the same address.

– A packet may match two LSPs, one with a Host Address FEC element and one with an Address Prefix FEC element. In this case, the packet is always assigned to the former.

– A packet which does not match a particular Host Address FEC element may not be sent on the corresponding LSP, even if the Host Address FEC element identifies the packet's egress router.

– This IS allowed on a /32 Address Prefix FEC that matches the egress router.

Ina’s email to the mpls WG list

• Ina proposed removing the Host Address FEC due to non-use in RFC 3036-bis

• She also proposed changing the semantics of Address Prefix FEC with /32 addresses to replicate the original functionality of the Host Address FEC

Liaison Text

A recent message on the MPLS WG mailing list has suggested changes to RFC 3036 ("LDP Specification") and called for comments on the proposed changes. One of the proposed changes is to deprecate the use of the Host Address FEC TLV. Two Implementation Agreements published by the MPLS & Frame Relay Alliance (MFA), "MPLS Proxy Admission Control Definition" and "MPLS Proxy Admission Control Protocol", MFA.6.0.0 and MFA.7.0.0 make use of the Host Address FEC TLV. MPLS Proxy Admission Control provides a bandwidth-based reservation/admission facility for MPLS networks. There are implementations of this protocol in progress.

While the proposed changes make a Prefix FEC TLV with a 32-bit mask equivalent to a Host Address FEC TLV, adopting the Prefix FEC TLV would require changes to approved and published MFA documents, and changes to the implementations. Also, since MPLS Proxy Admission Control only uses host addresses, using the Prefix FEC TLV would necessitate an additional check that the prefix was always 32 bits. Therefore, the MFA kindly requests that the Host Address FEC TLV not be deprecated, and that it continues to be supported in future revisions of LDP.

In RFC 3036, there is a semantic difference between a Host Address and a prefix with length 32. The new version proposes to remove the semantic difference. This is not a problem for the MPLS Proxy Admission Control; we are just requesting that the Host Address codepoint not be deprecated.