ICMP MPLS Extensions: ICMP Extensions for Multiprotocol Label Switching
How a router can tell you, inside an ICMP error, which MPLS labels your packet carried when it arrived.
Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "ICMP Extensions for Multiprotocol Label Switching", RFC 4950, DOI 10.17487/RFC4950, August 2007, <https://www.rfc-editor.org/info/rfc4950>.
@misc{rfc4950,
series = {Request for Comments},
number = 4950,
howpublished = {RFC 4950},
publisher = {RFC Editor},
doi = {10.17487/RFC4950},
url = {https://www.rfc-editor.org/info/rfc4950},
author = {R. Bonica and D. Gan and D. Tappan and C. Pignataro},
title = {{ICMP Extensions for Multiprotocol Label Switching}},
pagetotal = 8,
year = 2007,
month = aug,
} RFC text
Contents
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 (C) 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
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
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 [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.
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 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
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 [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.
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 [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
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
[RFC2434].
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.
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
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AI ELI5
On this page
§1The Missing MPLS Information
Routers use ICMP (ICMPv4 and ICMPv6) to send control information back to the host that originated a datagram. When a router cannot deliver a datagram, the ICMP message says why, and includes the IP header and leading payload octets of the original datagram.
A Label Switching Router (LSR) that receives an undeliverable MPLS-encapsulated datagram removes the entire label stack, exposing the IP datagram inside, and hands that datagram to error processing. Any ICMP message that results contains no information about the label stack. That is a gap, because the LSR forwarded the packet based on that stack.
The general relationship between ICMP and MPLS is not defined here. It comes from Section 2.3 of RFC 3032.
The router used a secret code on the package to decide where to send it, but when it reports a problem it forgets to mention the code. This RFC adds the code to the report.
§2The Extension Approach
The memo defines an ICMP extension object that lets an LSR append MPLS information to ICMP messages. Selected ICMP messages SHOULD include the MPLS label stack as it arrived at the router sending the ICMP message. The message MUST also still include the IP header and leading payload octets of the original datagram.
The object must be preceded by an ICMP Extension Structure Header and an ICMP Object Header, both defined in RFC 4884. The extension applies equally to ICMPv4 (RFC 792) and ICMPv6 (RFC 4443). Here, ICMP means multi-part ICMP messages.
The report keeps its usual details and gets an extra attachment with the labels. Both the old notes and the new attachment travel together.
§3The MPLS Label Stack Object
The object can be appended to the ICMP Time Exceeded and Destination Unreachable messages. A single instance represents the entire label stack, formatted exactly as it was when it arrived at the LSR that sends the ICMP message.
The header values are Class-Num = 1 (MPLS Label Stack Class) and C-Type = 1 (Incoming MPLS Label Stack). The length is 4 + 4 * (number of MPLS label stack entries). Octets 0-3 of the payload hold the first stack entry, and each remaining entry takes another 4 octets in the same format.
Each entry has a 20-bit Label, a 3-bit Exp field (experimental use), a 1-bit S flag (bottom of stack) and an 8-bit TTL.
Each label is a small four-part sticker. The object is just all the stickers the package had, in order, stuck onto the report.
§4Enhanced Traceroute
The extension supports enhancements to TRACEROUTE. Enhanced implementations, like older ones, show which nodes the datagram visited on the way to its destination. They also show the datagram's MPLS encapsulation status as it arrived at each node.
The RFC's sample output (traceroute to 192.0.2.1, 30 hops max, 40 byte packets) lists four hops. Hops 2 and 3 each carry an extra line such as MPLS Label=100048 Exp=0 TTL=1 S=1.
The memo does not define encapsulation-specific TTL handling and defers to Section 5.4 of RFC 3034 and Section 10 of RFC 3035. When such procedures defeat basic traceroute, they also defeat enhanced traceroute.
A traceroute is like checking each stop on a bus route. Now it can also tell you which special lane you were riding in at each stop.
§5Security Considerations
The extension can give the user of traceroute additional information about the network, so an operator may want to share it selectively. The RFC suggests, for example, including the extension only in ICMP messages destined to addresses within the network management blocks that have administrative control over the router.
An implementation could decide based on the destination address of the ICMP message, on a global configuration option, or on the depth of the incoming label stack. An operator can also use the TTL treatment for MPLS Pipe Model LSPs from RFC 3443 for a TTL-transparent mode that prevents ICMP Time Exceeded altogether when tunneled over the LSP.
Telling everyone how your lanes work may be more than you want to share, so a network owner can choose who gets to see the labels.
§6IANA Registrations
IANA assigned Class-Num 1, the MPLS Label Stack Class, in the ICMP Extension Object registry. IANA also established a registry for the class sub-types (C-Types) of this class.
In that registry C-Type 0 is Reserved and C-Type 1 is Incoming MPLS Label Stack. The range 0x02-0xF6 is available for assignment and 0xF7-0xFF is reserved for private use. Values are assigned first-come-first-served under RFC 2434.
A librarian keeps the official list of attachment types, and this RFC got the first slot for label stacks.