| Internet-Draft | RAO-less Label Switched Path (LSP) Ping | February 2024 |
| Kompella, et al. | Expires 16 August 2024 | [Page] |
The MPLS echo request and MPLS echo response messages, defined in RFC 8029 "Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures" (usually referred to as LSP ping messages), are encapsulated in IP headers that include a Router Alert Option (RAO). The rationale for using an RAO as the exception mechanism is questionable. Furthermore, RFC 6398 identifies security vulnerabilities associated with the RAO in non-controlled environments, e.g., the case of using the MPLS echo request/reply as inter-area Operations, Administration, and Maintenance (OAM), and recommends against its use outside of controlled environments.¶
Therefore, this document retires the RAO for MPLS OAM and updates RFC 8029 to remove the RAO from LSP ping message encapsulations. Furthermore, this document explains why RFC 7506 has been reclassified as Historic.¶
This document also recommends the use of an IPv6 loopback address (::1/128) and not the use of an IPv4 loopback address mapped to IPv6.¶
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RFC 8029 - "Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures" (usually referred to as LSP Ping) [RFC8029] detects data-plane failures in MPLS Label Switched Paths (LSPs). It can operate in "ping mode" or "traceroute mode". When operating in ping mode, it checks LSP connectivity. When operating in traceroute mode, it can trace an LSP and localize failures to a particular node along an LSP.¶
The reader is assumed be familiar with [RFC8029] and its terminology.¶
LSP ping defines a probe message called the "MPLS echo request". It also defines a response message called the "MPLS echo reply". Both messages are encapsulated in UDP and IP. The MPLS echo request message is further encapsulated in an MPLS label stack, except when all of the Forwarding Equivalency Classes in the stack correspond to Implicit Null labels.¶
When operating in ping mode, LSP ping sends a single MPLS echo request message, with the MPLS TTL set to 255. This message is intended to reach the egress Label Switching Router (LSR). When operating in traceroute mode, MPLS ping sends multiple MPLS echo request messages as defined in Section 4.3 of [RFC8029]. It manipulates the MPLS TTL so that the first message expires on the first LSR along the path and subsequent messages expire on subsequent LSRs.¶
According to [RFC8029], the IP header that encapsulates an MPLS echo request message must include a Router Alert Option (RAO). Furthermore, [RFC8029] also says that the IP header that encapsulates an MPLS echo reply message must include an RAO if the value of the Reply Mode in the corresponding MPLS echo request message is "Reply via an IPv4/IPv6 UDP packet with Router Alert". In both cases, the rationale for including an RAO is questionable. Furthermore, [RFC6398] identifies security vulnerabilities associated with the RAO in non-controlled environments, e.g., the case of using the MPLS echo request/reply as inter-domain OAM over the public Internet, and recommends against its use outside of controlled environments, e.g., outside a single administrative domain.¶
Therefore, this document updates RFC 8029 [RFC8029] to retire the RAO from both LSP ping message encapsulations and explains why RFC 7506 [RFC7506] has been reclassified as Historic.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
While the MPLS echo request message must traverse every node in the LSP under test, it must not traverse any other node. Specifically, the message must not be forwarded beyond the egress Label Switching Router (LSR). To achieve this, a set of the mechanisms that are used concurrently to prevent leaking MPLS echo request messages has been defined in [RFC8029]:¶
Currently, all of these are required. However, any one is sufficient to prevent forwarding the packet beyond the egress LSR.¶
Therefore, this document changes RFC 8029 [RFC8029] in that Requirement 3 is removed.¶
No implementation that relies on the RAO to prevent packets from being forwarded beyond the egress LSR have been reported to the MPLS working group.¶
An LSP ping replies to the MPLS echo request message with an MPLS echo reply message. Four reply modes are defined in [RFC8029]:¶
The rationale for mode 3 is questionable, if not wholly misguided. According to RFC 8029 [RFC8029], "If the normal IP return path is deemed unreliable, one may use 3 (Reply via an IPv4/IPv6 UDP packet with Router Alert)."¶
However, it is not clear that the use of the RAO increases the reliability of the return path. In fact, one can argue it decreases the reliability in many instances, due to the additional burden of processing the RAO. This document changes RFC 8029 [RFC8029] in that mode 3 is removed.¶
No implementations of mode 3 have been reported to the MPLS working group.¶
RFC 7506 [RFC7506] defines the IPv6 Router Alert Option for MPLS Operations, Administration, and Management. This document explains why RFC 7506 [RFC7506] has been reclassified as Historic.¶
[RFC8029] requires that the IPv6 Destination Address used in IP/UDP encapsulation of an MPLS echo request packet is selected from the IPv4 loopback address range mapped to IPv6. Such packets do not have the same behavior as prescribed in [RFC1122] for an IPv4 loopback addressed packet.¶
[RFC4291] defines ::1/128 as the single IPv6 loopback address. Considering that, this specification updates Section 2.1 of [RFC8029] regarding the selection of an IPv6 destination address for an MPLS echo request message as follows:¶
OLD¶
The 127/8 range for IPv4 and that same range embedded in an IPv4-mapped IPv6 address for IPv6 was chosen for a number of reasons.¶
RFC 1122 allocates the 127/8 as the "Internal host loopback address" and states: "Addresses of this form MUST NOT appear outside a host." Thus, the default behavior of hosts is to discard such packets. This helps to ensure that if a diagnostic packet is misdirected to a host, it will be silently discarded.¶
This helps to ensure that diagnostic packets are never IP forwarded.¶
The 127/8 address range provides 16M addresses allowing wide flexibility in varying addresses to exercise ECMP paths. Finally, as an implementation optimization, the 127/8 range provides an easy means of identifying possible LSP packets.¶
NEW¶
The 127/8 range for IPv4 was chosen for a number of reasons.¶
RFC 1122 [RFC1122] allocates the 127/8 as the "Internal host loopback address" and states: "Addresses of this form MUST NOT appear outside a host." Thus, the default behavior of hosts is to discard such packets. This helps to ensure that if a diagnostic packet is misdirected to a host, it will be silently discarded.¶
This helps to ensure that diagnostic packets are never IP forwarded.¶
The 127/8 address range provides 16M addresses allowing wide flexibility in varying addresses to exercise ECMP paths. Finally, as an implementation optimization, the 127/8 range provides an easy means of identifying possible LSP packets.¶
The IPv6 destination address for an MPLS echo request message is selected as follows:¶
END¶
Additionally, this specification updates Section 2.2 of [RFC8029] to replace the whole of the section with the following text:¶
LSP Ping implementations SHOULD ignore RAO options when they arrive on incoming MPLS echo request and MPLS echo reply messages.¶
Resulting from the removal of the Reply mode 3 "Reply via an IPv4/IPv6 UDP packet with Router Alert" (see Section 2.2), this specification updates Section 4.5 of [RFC8029] by removing the following text:¶
If the Reply Mode in the echo request is "Reply via an IPv4 UDP packet with Router Alert", then the IP header MUST contain the Router Alert IP Option of value 0x0 [RFC2113] for IPv4 or 69 [RFC7506] for IPv6. If the reply is sent over an LSP, the topmost label MUST in this case be the Router Alert label (1) (see [RFC3032]).¶
Furthermore, this specification updates Section 4.3 of [RFC8029] as follows:¶
OLD:¶
The Router Alert IP Option of value 0x0 [RFC2113] for IPv4 or value 69 [RFC7506] for IPv6 MUST be set in the IP header.¶
NEW:¶
The Router Alert IP Option of value 0x0 [RFC2113] for IPv4 or value 69 [RFC7506] for IPv6 MUST NOT be set in the IP header.¶
END¶
LSP Ping implementations that conform to this specification SHOULD ignore RAO options when they arrive on incoming MPLS echo request and MPLS echo reply messages. However, this will not harm backwards compatibility because other mechanisms will also be in use by all legacy implementations in the messages they send and receive.¶
Section 6 of this document deprecates the IPv6 RAO value for MPLS OAM (69) in [IANA-IPV6-RAO] and the Reply Mode 3 ("Reply via an IPv4/IPv6 UDP packet with Router Alert") in [IANA-LSP-PING].¶
[RFC8126] offers a formal description of the word "Deprecated". In this context, "Deprecated" means that the deprecated values SHOULD NOT be used in new implementations, and that deployed implementations that already use these values continue to work seamlessly.¶
IANA is requested to mark the IPv6 RAO value of MPLS OAM (69) in [IANA-IPV6-RAO] as "Deprecated".¶
IANA is also requested to mark Reply Mode 3 ("Reply via an IPv4/IPv6 UDP packet with Router Alert") in "Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"[IANA-LSP-PING] as "Deprecated".¶
The recommendations this document makes do not compromise security. In case of using IPv6 loopback address ::1/128 strengthens security for LSP Ping by using the standardized loopback address with well-defined behavior.¶
The authors express their appreciation to Adrian Farrel and Gyan Mishra for for their suggestions that improved the readability of the document.¶