Retiring TLVs from the Associated Channel Header of the MPLS Generic Associated Channel
updates
- rfc-5586 — MPLS Generic Associated Channel
Extracted elements (11)
ACH TLVs are retired because no ACH Channel Type defined at the time of writing uses them, and handling variable-length TLVs in hardware introduces significant problems to the fast path. Since G-ACh messages are intended to be processed substantially in hardware, ACH TLVs are considered undesirable.
Of the 18 ACH Channel Types defined at the time of writing, none allows the use of ACH TLVs, and there were no unexpired Internet-Drafts utilizing ACH TLVs. This absence of adoption confirms the feature is not useful.
The document deliberately excludes from scope any proposals to use TLVs within ACH messages or as an appendage to ACH messages; only the ACH TLV construct placed between the fixed ACH header fields and the G-ACh message is retired.
Packet sniffers already implemented to look for ACH TLVs will not be negatively impacted by the deletion of the construct, since no ACH Types currently allow TLV use and the absence of TLVs is already the operational norm.
A G-ACh message MUST NOT be preceded by an ACH TLV. This directly deprecates the ACH TLV construct defined in RFC 5586 Section 3.
RFC 5586 Section 3, which defined ACH TLV constructs, is deleted in its entirety by this document. References to ACH TLVs in RFC 5586 Section 4 (which used phrases like 'ACH TLV(s), if present') should also be disregarded.
The MPLS Generic Associated Channel (G-ACh) provides a common encapsulation header for control channel messages associated with MPLS Sections, LSPs, and PWs, generalized from the Pseudowire Associated Channel Header (ACH) defined in RFC 4385.
The 'Associated Channel Header TLV Registry' subregistry within 'Pseudowire Name Spaces (PWE3)' is entirely deleted by IANA. A tombstone record remains in the top-level registry list reading 'Associated Channel Header TLV Registry (DELETED)' referencing RFC 5586 and RFC 7026.
The 'Pseudowire Associated Channel Types' registry previously included a column marked 'TLV Follows'. IANA has entirely deleted this column, leaving no record, as part of retiring ACH TLVs.
Deleting the ACH TLV has a marginal positive security effect because it removes a feature that could have been used as an attack vector to carry false information or to bloat G-ACh messages.
It was suggested that ACH TLVs could have carried security parameters to secure G-ACh messages generically, but no such mechanisms were proposed. Security requirements are considered the responsibility of each individual G-ACh message specification.