LSP ping Router Alert: Deprecating the Use of Router Alert in LSP Ping

Internet Engineering Task Force (IETF)                       K. Kompella
Request for Comments: 9570                                     R. Bonica
Updates: 8029                                           Juniper Networks
Category: Standards Track                                 G. Mirsky, Ed.
ISSN: 2070-1721                                                 Ericsson
                                                                May 2024


            Deprecating the Use of Router Alert in LSP Ping

Abstract

   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), are encapsulated in IP
   packets with headers that include a Router Alert Option (RAO).  In
   actual deployments, the RAO was neither required nor used.
   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.

   Also, this document recommends the use of an IPv6 loopback address
   (::1/128) as the IPv6 destination address for an MPLS echo request
   message.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9570.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Router Alert for LSP Ping (RFC 8029)
     2.1.  MPLS Echo Request
     2.2.  MPLS Echo Reply
   3.  Reclassification of RFC 7506 as Historic
   4.  Update to RFC 8029
   5.  Backwards Compatibility
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   "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."  This document
   explains why an RAO was not needed in both cases.  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.

1.1.  Requirements Language

   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.

2.  Router Alert for LSP Ping (RFC 8029)

2.1.  MPLS Echo Request

   While the MPLS echo request message must traverse every node in the
   LSP under test, it must not traverse any other nodes.  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]:

   1.  When the MPLS echo request message is encapsulated in IPv4, the
       IPv4 destination address must be chosen from the subnet 127/8.
       When the MPLS echo request message is encapsulated in IPv6, the
       IPv6 destination address must be chosen from the subnet
       0:0:0:0:0:FFFF:7F00:0/104.

   2.  When the MPLS echo request message is encapsulated in IPv4, the
       IPv4 TTL must be equal to 1.  When the MPLS echo request message
       is encapsulated in IPv6, the IPv6 Hop Limit must be equal to 1.
       For further information on the encoding of the TTL / Hop Limit in
       an MPLS echo request message, see Section 4.3 of [RFC8029].

   3.  When the MPLS echo request message is encapsulated in IPv4, the
       IPv4 header must include an RAO with the option value set to
       "Router shall examine packet" [RFC2113].  When the MPLS echo
       request message is encapsulated in IPv6, the IPv6 header chain
       must include a hop-by-hop extension header and the hop-by-hop
       extension header must include an RAO with the option value set to
       MPLS OAM [RFC7506].

   Currently, all of these are required.  However, any one is sufficient
   to prevent forwarding the packet beyond the egress LSR.

   Therefore, this document updates 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 has been reported to the MPLS
   Working Group.

2.2.  MPLS Echo Reply

   An LSP ping replies to the MPLS echo request message with an MPLS
   echo reply message.  Four reply modes are defined in [RFC8029]:

   1.  Do not reply

   2.  Reply via an IPv4/IPv6 UDP packet

   3.  Reply via an IPv4/IPv6 UDP packet with Router Alert

   4.  Reply via application-level control channel

   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 updates RFC 8029 [RFC8029] in that
   mode 3 is removed.

   No implementations of mode 3 have been reported to the MPLS Working
   Group.


3.  Reclassification of RFC 7506 as Historic

   RFC 7506 [RFC7506] defines the IPv6 Router Alert Option for MPLS
   Operations, Administration, and Maintenance.  This document explains
   why RFC 7506 [RFC7506] has been reclassified as Historic.

4.  Update to RFC 8029

   [RFC8029] requires that the IPv6 Destination Address used in IP/UDP
   encapsulation of an MPLS echo request packet be 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.
   |
   |  RFC 1812 [RFC1812] states:
   |
   |     A router SHOULD NOT forward, except over a loopback interface,
   |     any packet that has a destination address on network 127.  A
   |     router MAY have a switch that allows the network manager to
   |     disable these checks.  If such a switch is provided, it MUST
   |     default to performing the checks.
   |
   |  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.
   |
   |  RFC 1812 [RFC1812] states:
   |
   |     A router SHOULD NOT forward, except over a loopback interface,
   |     any packet that has a destination address on network 127.  A
   |     router MAY have a switch that allows the network manager to
   |     disable these checks.  If such a switch is provided, it MUST
   |     default to performing the checks.
   |
   |  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:
   |
   |  *  The IPv6 loopback address ::1/128 SHOULD be used.
   |
   |  *  The sender of an MPLS echo request MAY select the IPv6
   |     destination address from the 0:0:0:0:0:FFFF:7F00/104 range.
   |
   |  *  To exercise all paths in an ECMP environment, the source of
   |     entropy other than the IP destination address SHOULD be used.
   |     For example, the MPLS Entropy Label [RFC6790] or IPv6 Flow
   |     Label [RFC6438] can be used as the source of entropy.

   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.

5.  Backwards Compatibility

   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.

6.  IANA Considerations

   IANA has marked the IPv6 RAO value of MPLS OAM (69) in
   [IANA-IPV6-RAO] as "DEPRECATED".

   IANA has marked 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".

7.  Security Considerations

   The recommendations this document makes do not compromise security.
   Using the IPv6 loopback address ::1/128 strengthens security for LSP
   ping because it is standardized and has well-defined behavior.

8.  References

8.1.  Normative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <https://www.rfc-editor.org/info/rfc1812>.

   [RFC2113]  Katz, D., "IP Router Alert Option", RFC 2113,
              DOI 10.17487/RFC2113, February 1997,
              <https://www.rfc-editor.org/info/rfc2113>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC6398]  Le Faucheur, F., Ed., "IP Router Alert Considerations and
              Usage", BCP 168, RFC 6398, DOI 10.17487/RFC6398, October
              2011, <https://www.rfc-editor.org/info/rfc6398>.

   [RFC7506]  Raza, K., Akiya, N., and C. Pignataro, "IPv6 Router Alert
              Option for MPLS Operations, Administration, and
              Maintenance (OAM)", RFC 7506, DOI 10.17487/RFC7506, April
              2015, <https://www.rfc-editor.org/info/rfc7506>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

   [IANA-IPV6-RAO]
              IANA, "IPv6 Router Alert Option Values",
              <https://www.iana.org/assignments/ipv6-routeralert-
              values>.

   [IANA-LSP-PING]
              IANA, "Multiprotocol Label Switching (MPLS) Label Switched
              Paths (LSPs) Ping Parameters",
              <https://www.iana.org/assignments/mpls-lsp-ping-
              parameters>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <https://www.rfc-editor.org/info/rfc6438>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

Acknowledgments

   The authors express their appreciation to Adrian Farrel and Gyan
   Mishra for their suggestions that improved the readability of the
   document.

Authors' Addresses

   Kireeti Kompella
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, CA 94089
   United States of America
   Email: kireeti.ietf@gmail.com


   Ron Bonica
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, CA 94089
   United States of America
   Email: rbonica@juniper.net


   Greg Mirsky (editor)
   Ericsson
   Email: gregimirsky@gmail.com