Label Switched Path (LSP) Ping and Traceroute Reply Mode Simplification
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- rfc-7110 — Return Path Specified Label Switched Path (LSP) Ping
Extracted elements (23)
Different implementations chose different default return-path encodings for different LSP types, causing MPLS echo replies to fail in many cases because responder LSRs drop echo requests with unavailable return paths. The Reply Mode Order TLV eliminates guessing by letting the responder pick the first available path from a preference list.
Ordering Reply Mode values from most to least preferred in the TLV allows a responder that cannot honor the top choice to fall back automatically, avoiding the timeout-then-retry cycle that operators would otherwise experience when a preferred return path is unavailable.
Relaxing the RFC 7110 requirement that 'Reply via Specified Path' (value=5) must always carry a Reply Path TLV creates a simple shorthand: value=5 alone means 'use the reverse LSP', avoiding the overhead of constructing a Reply Path TLV with the B bit set for the common bidirectional-LSP case.
Reply Mode value 1 (Do not reply) is explicitly prohibited in the Reply Mode Order TLV because including it would defeat the purpose of the TLV: the initiator always needs a reply to determine which return path was chosen.
If an initiator that sent an echo request with 'Reply via Specified Path' (value=5) and no Reply Path TLV receives back a 'Malformed echo request received' return code, it SHOULD assume the responder does not support the relaxed semantics defined in this document.
The Reply Mode Order TLV type 32770 is assigned from the range 32768-49161 in the MPLS LSP Ping Parameters registry. Values in this range designate optional TLVs that MUST be silently dropped if not recognized by the responder.
When multiple consecutive 'Reply via Specified Path' (value=5) entries appear in the Reply Mode Order TLV but fewer corresponding Reply Path TLVs exist, the earlier value=5 entries consume the available TLVs and the remaining ones imply the reverse LSP.
For proxy LSP Ping, it is RECOMMENDED that Reply Mode 2 (Reply via an IPv4/IPv6 UDP packet) be used in the Reply Mode field of the MPLS proxy ping request message.
If a responder LSR understands the Reply Mode Order TLV but the TLV is invalid (too few values, repeated non-5 modes, value 1 present, or missing Reply Path TLVs for repeated value 5), the responder MUST ignore the entire TLV and use only the Reply Mode field.
Reply Mode value 5 MAY appear more than once in the Reply Mode Order TLV, but each instance MUST have a corresponding Reply Path TLV. Three instances of value 5 require three Reply Path TLVs.
The Reply Mode field of an MPLS echo request MUST be set to a valid value even when the Reply Mode Order TLV is included. The initiator SHOULD set it to the value most likely to be available, in case the responder does not understand the TLV.
The Reply Mode Order TLV MUST contain at least one Reply Mode value. Reply Mode value 1 (Do not reply) MUST NOT appear in the TLV. Any Reply Mode value other than 5 MUST NOT be repeated in the TLV.
The Reply Mode Order TLV MUST NOT be included in any MPLS echo reply. If a Reply Mode Order TLV is received in an echo reply, the initiator MUST ignore it entirely and use only the Reply Mode field of the received echo reply.
The 'Reply via Specified Path' Reply Mode (value=5) MAY be used without including a Reply Path TLV. This usage implies the reverse LSP as the return path, equivalent to including a Reply Path TLV with the B bit set.
The responder LSR SHOULD select the first available return path listed in the Reply Mode Order TLV. The Reply Mode value of the selected path MUST be set in the Reply Mode field of the MPLS echo reply to inform the initiator which return path was chosen.
When a proxy LSR forwards an MPLS echo request, it MUST copy the Reply Mode field, the Reply Mode Order TLV, and all Reply Path TLV(s) from the proxy ping request. If more than one Reply Path TLV is present, their ordering MUST be preserved.
When a proxy LSR sends an MPLS proxy ping reply (as opposed to an echo request), it is RECOMMENDED to ignore the Reply Mode Order TLV and use the Reply Mode field from the original proxy ping request message.
When sending an echo request, the Reply Mode MUST be set to 'Reply via Specified Path'; a Reply Path TLV SHOULD be carried correspondingly. If the Reply Path TLV is absent, it indicates the reverse LSP as the reply path. This updates RFC 7110 Section 5.1.
The Reply Mode Order TLV (type 32770) is a variable-length optional TLV carried in MPLS echo requests. It contains an ordered list of Reply Mode values; the first entry is most preferred and the last is least preferred.
IANA assigned Reply Mode Order TLV type value 32770 in the 'TLVs' sub-registry of the 'Multiprotocol Label Switching Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters' registry.
MPLS echo request and reply messages are not encrypted, so their contents—including return-path information—may be exposed within the MPLS domain. Encryption mechanisms may be employed if exposure is a concern.
The Reply Mode Order TLV provides a new vector for unauthorized sources to discover return-path topology. Implementations SHOULD check source addresses of MPLS echo request messages against an access list before accepting them, as also specified in RFC 4379.
The Reply Mode Order TLV has a 16-bit Type field (value 32770), a 2-octet Length field defining the byte length of the Reply Mode list, followed by variable-length Reply Mode values each encoded as 1 octet with no padding.