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IETF RFC 4950
ICMP Extensions for Multiprotocol Label Switching
Last modified on Thursday, August 2nd, 2007
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Network Working Group R. Bonica
Request for Comments: 4950 Juniper Networks
Category: Standards Track D. Gan
D. Tappan
Consultant
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 © The IETF Trust (2007).
Abstract
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
Bonica, et al. Standards Track PAGE 1 
RFC 4950 ICMP MPLS August 2007
1. Introduction
IP routers use the Internet Control Message Protocol, ICMPv4
[RFC 792] and ICMPv6 [RFC 4443], 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 [RFC 3032] describes the
interaction between MPLS and ICMP, and Sections 2.4 and 3 of
[RFC 3032] 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
generation.
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 [RFC 4884].
The ICMP extension defined in this document is equally applicable to
ICMPv4 [RFC 792] and ICMPv6 [RFC 4443]. Throughout this document,
unless otherwise specified, the acronym ICMP refers to multi-part
ICMP messages, encompassing both ICMPv4 and ICMPv6.
Bonica, et al. Standards Track PAGE 2
RFC 4950 ICMP MPLS August 2007
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
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
implementation.
> traceroute 192.0.2.1
traceroute to 192.0.2.1 (192.0.2.1), 30 hops max, 40 byte packets
1 192.0.2.13 (192.0.2.13) 0.661 ms 0.618 ms 0.579 ms
2 192.0.2.9 (192.0.2.9) 0.861 ms 0.718 ms 0.679 ms
MPLS Label=100048 Exp=0 TTL=1 S=1
3 192.0.2.5 (192.0.2.5) 0.822 ms 0.731 ms 0.708 ms
MPLS Label=100016 Exp=0 TTL=1 S=1
4 192.0.2.1 (192.0.2.1) 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 [RFC 3032] 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
[RFC 3034] and Section 10 of [RFC 3035] in this matter.
Bonica, et al. Standards Track PAGE 3
RFC 4950 ICMP MPLS August 2007
When encapsulation-specific TTL manipulation procedures defeat the
basic TRACEROUTE mechanism, they will also defeat enhanced TRACEROUTE
implementations.
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
message.
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 [RFC 4884].
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
Bonica, et al. Standards Track PAGE 4
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 [RFC 3443] 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 2434].
Bonica, et al. Standards Track PAGE 5
RFC 4950 ICMP MPLS August 2007
8. References
8.1. Normative References
[RFC 792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC 3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC 4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC 4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
"Extended ICMP to Support Multi-Part Messages", RFC 4884,
April 2007.
8.2. Informative References
[RFC 3034] Conta, A., Doolan, P., and A. Malis, "Use of Label
Switching on Frame Relay Networks Specification",
RFC 3034, January 2001.
[RFC 3035] 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.
[RFC 3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks",
RFC 3443, January 2003.
Bonica, et al. Standards Track PAGE 6
RFC 4950 ICMP MPLS August 2007
Authors' Addresses
Ronald P. Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, VA 20171
US
EMail: rbonica@juniper.net
Der-Hwa Gan
Consultant
EMail: derhwagan@yahoo.com
Daniel C. Tappan
Consultant
EMail: Dan.Tappan@gmail.com
Carlos Pignataro
Cisco Systems, Inc.
7025 Kit Creek Road
Research Triangle Park, NC 27709
US
EMail: cpignata@cisco.com
Bonica, et al. Standards Track PAGE 7
RFC 4950 ICMP MPLS August 2007
Full Copyright Statement
Copyright © The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Bonica, et al. Standards Track PAGE 8
ICMP Extensions for Multiprotocol Label Switching
RFC TOTAL SIZE: 15091 bytes
PUBLICATION DATE: Thursday, August 2nd, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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