Deprecating the Use of Router Alert in LSP Ping
updates
- rfc-8029 — Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures
Extracted elements (16)
Reply Mode 3 ('Reply via an IPv4/IPv6 UDP packet with Router Alert') was removed because the RAO does not clearly increase return-path reliability and may actually decrease it due to the additional processing burden it imposes. No implementations of Mode 3 were reported.
RFC 7506, which defined the IPv6 Router Alert Option value for MPLS OAM, was reclassified as Historic because this document removes the requirement for the RAO in LSP ping, making that option value obsolete.
The 127/8 IPv4 range was chosen as the MPLS echo request destination because RFC 1122 mandates hosts discard such packets, RFC 1812 directs routers not to forward them, and the range provides 16M addresses enabling ECMP path variation. These properties ensure diagnostic packets are contained.
The IPv6 destination address for MPLS echo requests was changed from the IPv4-mapped range (0:0:0:0:0:FFFF:7F00:0/104) to ::1/128 because IPv4-mapped addresses do not exhibit the same loopback behavior prescribed in RFC 1122, whereas ::1/128 is the standardized IPv6 loopback address per RFC 4291 with well-defined discard behavior.
The RAO was not needed for MPLS echo request containment because the loopback destination address and TTL=1 mechanisms are each independently sufficient to prevent forwarding beyond the egress LSR. No implementation relying solely on the RAO for this purpose was reported.
Backwards compatibility is preserved because legacy implementations also use the loopback destination address and TTL=1 mechanisms. Ignoring the RAO on receipt (as now specified) does not harm interoperability since those other mechanisms remain in place.
Deprecated values (IPv6 RAO value 69 and Reply Mode 3) SHOULD NOT be used in new implementations; deployed implementations using these values may continue to work seamlessly.
For IPv6-encapsulated MPLS echo request messages, the IPv6 loopback address ::1/128 SHOULD be used as the destination address. The sender MAY alternatively select from the 0:0:0:0:0:FFFF:7F00/104 range.
LSP Ping implementations SHOULD ignore RAO options when they arrive on incoming MPLS echo request and MPLS echo reply messages.
The Router Alert IP Option MUST NOT be set in the IP header of an MPLS echo request message. This reverses the prior requirement in RFC 8029 Section 4.3 that mandated setting the RAO.
To exercise all paths in an ECMP environment, a source of entropy other than the IP destination address SHOULD be used, such as the MPLS Entropy Label (RFC 6790) or IPv6 Flow Label (RFC 6438).
MPLS echo request messages use three concurrent mechanisms to prevent leaking beyond the egress LSR: (1) a loopback destination address (127/8 for IPv4, ::1/128 or mapped range for IPv6), (2) IP TTL/Hop Limit of 1, and (3) previously an RAO (now removed). Any one of the first two is sufficient.
RFC 8029 defined four LSP ping reply modes: (1) Do not reply, (2) Reply via IPv4/IPv6 UDP packet, (3) Reply via IPv4/IPv6 UDP packet with Router Alert (now removed/deprecated), and (4) Reply via application-level control channel.
IANA has marked Reply Mode 3 ('Reply via an IPv4/IPv6 UDP packet with Router Alert') in the MPLS LSP Ping Parameters registry as DEPRECATED.
IANA has marked the IPv6 Router Alert Option value for MPLS OAM (value 69) in the 'IPv6 Router Alert Option Values' registry as DEPRECATED.
RFC 6398 identifies security vulnerabilities associated with the RAO in non-controlled environments (e.g., inter-domain OAM over the public Internet). Removing the RAO from LSP ping eliminates these risks; using ::1/128 as the IPv6 destination strengthens security due to its standardized and well-defined behavior.