TitleICMP Extensions for Multiprotocol Label Switching
AuthorR. Bonica, D. Gan, D. Tappan, C. Pignataro
DateAugust 2007
Format:TXT, HTML

Network Working Group                                          R. Bonica
Request for Comments: 4950                              Juniper Networks
Category: Standards Track                                         D. Gan
                                                               D. Tappan
                                                            C. Pignataro
                                                     Cisco Systems, Inc.
                                                             August 2007

           ICMP Extensions for Multiprotocol Label Switching

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This memo defines an extension object that can be appended to
   selected multi-part ICMP messages.  This extension permits Label
   Switching Routers to append MPLS information to ICMP messages, and
   has already been widely deployed.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Conventions Used in This Document . . . . . . . . . . . . . . . 3
   3.  Application to TRACEROUTE . . . . . . . . . . . . . . . . . . . 3
   4.  Disclaimer  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   5.  MPLS Label Stack Object . . . . . . . . . . . . . . . . . . . . 4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

RFC 4950                       ICMP MPLS                     August 2007

1.  Introduction

   IP routers use the Internet Control Message Protocol, ICMPv4
   [RFC0792] and ICMPv6 [RFC4443], to convey control information to
   source hosts.  Network operators use this information to diagnose
   routing problems.

   When a router receives an undeliverable IP datagram, it can send an
   ICMP message to the host that originated the datagram.  The ICMP
   message indicates why the datagram could not be delivered.  It also
   contains the IP header and leading payload octets of the "original
   datagram" to which the ICMP message is a response.

   MPLS Label Switching Routers (LSR) also use ICMP to convey control
   information to source hosts.  Section 2.3 of [RFC3032] describes the
   interaction between MPLS and ICMP, and Sections 2.4 and 3 of
   [RFC3032] provide applications of that interaction.

   When an LSR receives an undeliverable MPLS-encapsulated datagram, it
   removes the entire MPLS label stack, exposing the previously
   encapsulated IP datagram.  The LSR then submits the IP datagram to an
   error processing module.  Error processing can include ICMP message

   The ICMP message indicates why the original datagram could not be
   delivered.  It also contains the IP header and leading octets of the
   original datagram.

   The ICMP message, however, contains no information regarding the MPLS
   label stack that encapsulated the original datagram when it arrived
   at the LSR.  This omission is significant because the LSR would have
   forwarded the original datagram based upon information contained by
   the MPLS label stack.

   This memo defines an ICMP extension object that permits an LSR to
   append MPLS information to ICMP messages.  Selected ICMP messages
   SHOULD include the MPLS label stack, as it arrived at the router that
   is sending the ICMP message.  The ICMP message MUST also include the
   IP header and leading payload octets of the original datagram.

   The ICMP extensions defined in this document must be preceded by an
   ICMP Extension Structure Header and an ICMP Object Header.  Both are
   defined in [RFC4884].

   The ICMP extension defined in this document is equally applicable to
   ICMPv4 [RFC0792] and ICMPv6 [RFC4443].  Throughout this document,
   unless otherwise specified, the acronym ICMP refers to multi-part
   ICMP messages, encompassing both ICMPv4 and ICMPv6.

RFC 4950                       ICMP MPLS                     August 2007

2.   Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119 [RFC2119].

3.  Application to TRACEROUTE

   The ICMP extension defined in this memo supports enhancements to
   TRACEROUTE.  Enhanced TRACEROUTE applications, like older
   implementations, indicate which nodes the original datagram visited
   en route to its destination.  They differ from older implementations
   in that they also reflect the original datagram's MPLS encapsulation
   status as it arrived at each node.

   Figure 1 contains sample output from an enhanced TRACEROUTE

   > traceroute

     traceroute to (, 30 hops max, 40 byte packets

      1 (  0.661 ms  0.618 ms  0.579 ms

      2 (  0.861 ms  0.718 ms  0.679 ms

        MPLS Label=100048 Exp=0 TTL=1 S=1

      3 (  0.822 ms  0.731 ms  0.708 ms

        MPLS Label=100016 Exp=0 TTL=1 S=1

      4 (  0.961 ms  8.676 ms  0.875 ms

                Figure 1: Enhanced TRACEROUTE Sample Output

4.  Disclaimer

   This memo does not define the general relationship between ICMP and
   MPLS.  Section 2.3 of [RFC3032] defines this relationship.

   The current memo does not define encapsulation-specific TTL (Time to
   Live) manipulation procedures.  It defers to Section 5.4 of RFC 3034
   [RFC3034] and Section 10 of [RFC3035] in this matter.

RFC 4950                       ICMP MPLS                     August 2007

   When encapsulation-specific TTL manipulation procedures defeat the
   basic TRACEROUTE mechanism, they will also defeat enhanced TRACEROUTE

5.  MPLS Label Stack Object

   The MPLS Label Stack Object can be appended to the ICMP Time Exceeded
   and Destination Unreachable messages.  A single instance of the MPLS
   Label Stack Object represents the entire MPLS label stack, formatted
   exactly as it was when it arrived at the LSR that sends the ICMP

   Figure 2 depicts the MPLS Label Stack Object.  It must be preceded by
   an ICMP Extension Structure Header and an ICMP Object Header.  Both
   are defined in [RFC4884].

   In the object payload, octets 0-3 depict the first member of the MPLS
   label stack.  Each remaining member of the MPLS label stack is
   represented by another 4 octets that share the same format.

                   Class-Num = 1, MPLS Label Stack Class
                   C-Type = 1, Incoming MPLS Label Stack
                   Length = 4 + 4 * (number of MPLS LSEs)

              0             1             2            3
      |              Label               |EXP |S|     TTL     |
      |                                                       |
      |       // Remaining MPLS Label Stack Entries //        |
      |                                                       |

                     Figure 2: MPLS Label Stack Object

   Label: 20 bits

   Exp: Experimental Use, 3 bits

   S: Bottom of Stack, 1 bit

   TTL: Time to Live, 8 bits

RFC 4950                       ICMP MPLS                     August 2007

6.  Security Considerations

   This memo does not specify the conditions that trigger the generation
   of ICMP Messages for Labeled IP Packets.  It does not define the
   interaction between MPLS and ICMP.  However, this document defines an
   extension that allows an MPLS router to append MPLS information to
   multi-part ICMP messages, and therefore can provide the user of the
   TRACEROUTE application with additional information.  Consequently, a
   network operator may wish to provide this information selectively
   based on some policy; for example, only include the MPLS extensions
   in ICMP messages destined to addresses within the network management
   blocks with administrative control over the router.  An
   implementation could determine whether to include the MPLS Label
   Stack extensions based upon the destination address of the ICMP
   message, or based on a global configuration option in the router.
   Alternatively, an implementation may determine whether to include
   these MPLS extensions when TTL expires based on the number of label
   stack entries (depth of the label stack) of the incoming packet.
   Finally, an operator can make use of the TTL treatment on MPLS Pipe
   Model LSPs defined in [RFC3443] for a TTL-transparent mode of
   operation that would prevent ICMP Time Exceeded altogether when
   tunneled over the MPLS LSP.

7.  IANA Considerations

   IANA has assigned the following object Class-num in the ICMP
   Extension Object registry:

             Class-Num   Description
                     1   MPLS Label Stack Class

   IANA has established a registry for the corresponding class sub-type
   (C-Type) space, as follows:

             MPLS Label Stack Class Sub-types:

                C-Type  Description
                     0  Reserved
                     1  Incoming MPLS Label Stack
             0x02-0xF6  Available for assignment
             0xF7-0xFF  Reserved for private use

   C-Type values are assignable on a first-come-first-serve (FCFS) basis

RFC 4950                       ICMP MPLS                     August 2007

8.  References

8.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
              "Extended ICMP to Support Multi-Part Messages", RFC 4884,
              April 2007.

8.2.  Informative References

   [RFC3034]  Conta, A., Doolan, P., and A. Malis, "Use of Label
              Switching on Frame Relay Networks Specification",
              RFC 3034, January 2001.

   [RFC3035]  Davie, B., Lawrence, J., McCloghrie, K., Rosen, E.,
              Swallow, G., Rekhter, Y., and P. Doolan, "MPLS using LDP
              and ATM VC Switching", RFC 3035, January 2001.

   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks",
              RFC 3443, January 2003.

RFC 4950                       ICMP MPLS                     August 2007

Authors' Addresses

   Ronald P. Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, VA  20171

   EMail: rbonica@juniper.net

   Der-Hwa Gan

   EMail: derhwagan@yahoo.com

   Daniel C. Tappan

   EMail: Dan.Tappan@gmail.com

   Carlos Pignataro
   Cisco Systems, Inc.
   7025 Kit Creek Road
   Research Triangle Park, NC  27709

   EMail: cpignata@cisco.com

RFC 4950                       ICMP MPLS                     August 2007

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