ICMP extension to include underlay information
draft-jags-intarea-icmp-ext-underlay-info-03
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draft-jags-intarea-icmp-ext-underlay-info-03
Internet Area Working Group J. Rajamanickam, Ed.
Internet-Draft D. Dukes
Intended status: Standards Track M. Sankaranarayanan
Expires: 19 February 2026 Cisco Systems, Inc.
14 October 2025
ICMP extension to include underlay information
draft-jags-intarea-icmp-ext-underlay-info-03
Abstract
Network operators managing overlay networks require visibility into
underlay network hops during traceroute operations from overlay
endpoints. This document defines an ICMP extension object, the
Underlay Information Object (UIO), which allows underlay head-end
nodes to encapsulate underlay error information within ICMP error
messages. This mechanism provides overlay operators with crucial
visibility into underlay network paths for troubleshooting.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 19 February 2026.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Underlay Information Object . . . . . . . . . . . . . . . . . 4
3.1. UIO Object Format . . . . . . . . . . . . . . . . . . . . 4
3.2. Underlay Information Object Encoding Process . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4.1. Information Disclosure . . . . . . . . . . . . . . . . . 7
4.2. Privacy Considerations . . . . . . . . . . . . . . . . . 7
4.3. Message Size and Amplification . . . . . . . . . . . . . 7
4.4. Spoofing and Forgery . . . . . . . . . . . . . . . . . . 7
4.5. Intended Use . . . . . . . . . . . . . . . . . . . . . . 8
4.6. Rate Limiting . . . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. ICMP Extension Object Class . . . . . . . . . . . . . . . 8
5.2. C-Type Values . . . . . . . . . . . . . . . . . . . . . . 9
6. Operational Considerations . . . . . . . . . . . . . . . . . 9
6.1. Configuration . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Troubleshooting Workflow . . . . . . . . . . . . . . . . 10
6.3. Multi-Vendor Interoperability . . . . . . . . . . . . . . 10
7. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. UIO ICMP Extension Message Examples . . . . . . . . . . . 10
7.1.1. UIO carrying IPv6 information to the IPv4 source . . 10
7.1.2. UIO carrying IPv4 information to the IPv6 source . . 11
8. Normative References . . . . . . . . . . . . . . . . . . . . 12
9. Informative References . . . . . . . . . . . . . . . . . . . 13
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 13
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The mechanism for ICMP messages to carry additional information is
defined in [RFC4884]. ICMP message extensions that enable ICMP
messages to carry additional information about the system where an
error occurred are defined in [RFC5837], [RFC8335], and [RFC8883].
These extensions transmit enhanced diagnostic information to the
source node.
Network operators who manage both overlay and underlay networks,
such as those operating VPN segments connected through an SRv6 core
network, require the ability to trace paths through the underlay
infrastructure. Currently, when performing traceroute operations
from an overlay endpoint, operators lack visibility into the
underlay path and cannot identify the specific underlay node where
a failure occurred. For instance, imagine a VPN service (overlay)
running over an SRv6 network (underlay). If a packet gets dropped
within the SRv6 network, the VPN operator currently has no direct
way to pinpoint the exact underlay node causing the issue.
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The Underlay Information Object (UIO) defined in this document
addresses this operational requirement by enabling underlay head-end
nodes to include underlay-specific diagnostic information in ICMP
error messages sent to overlay endpoints, thereby providing crucial
visibility for troubleshooting.
2. Conventions and Definitions
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.
This document uses the following terms:
Overlay Network: A virtual network built on top of an existing
underlying network infrastructure, often providing services like
VPNs or tunnels.
Underlay Network: The physical or logical network infrastructure
over which an overlay network operates, responsible for forwarding
packets between overlay endpoints.
Overlay Endpoint: A device or system that terminates an overlay
network segment and originates or receives traffic for the
overlay.
Underlay Head-End Node: The node in the underlay network responsible
for encapsulating overlay traffic and often the first point of
contact for an overlay packet entering the underlay.
3. Underlay Information Object
This section defines a new ICMP extension object called Underlay
Information Object (UIO) that is encoded as part of ICMP extension
message. A new Class-Num value TBA (To Be Assigned) is assigned to
identify the UIO. As per [RFC4884], this object MAY be appended to
one of the following ICMP messages:
ICMPv4 Time Exceeded
ICMPv4 Destination Unreachable
ICMPv4 Parameter Problem
ICMPv6 Time Exceeded
ICMPv6 Destination Unreachable
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3.1. UIO Object Format
The UIO ICMP extension object has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Class-Num=TBA | C-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Object-Payload (Other ICMP Extension Objects...) ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Underlay Information Object Format
Length (16 bits):
The length of this object, measured in octets, including the
object header and object payload. The length MUST be a multiple
of 4 octets and MUST be at least 8 octets.
Class-Num (8 bits):
The ICMP extension object class number that identifies this as
a UIO object. IANA is requested to assign a value from the "ICMP
Extension Object Classes and Class Sub-types" registry (see
Section 5).
C-Type (8 bits):
The object sub-type. This document defines C-Type value 0.
Additional C-Type values may be defined in future documents.
Implementations MUST set this field to 0 and SHOULD ignore the
value upon receipt.
Object-Payload (variable length):
Contains one or more ICMP Extension Objects that provide
information about underlay nodes. The payload MUST contain at
least one ICMP extension object. Each encapsulated ICMP extension
object MUST be formatted according to [RFC4884] and the
specifications for that particular object class.
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This ICMP extension object acts as an envelope to carry other ICMP
extension objects related to the underlay. Primarily, the UIO ICMP
extension object is encoded in the ICMP extension message by the
underlay head-end when it receives an ICMP error message from one of
its intermediate nodes.
This UIO ICMP extension object can encapsulate one or more relevant
ICMP extension objects that are related to the underlay node. When
the underlay head-end encodes its ICMP extension object, the first
object MUST contain the ICMP extension object that carries IP address
or the hostname of the node where the initial ICMP error was
generated. The ICMP extension objects encoded within the UIO ICMP
extension objects can belong to any address family, irrespective of
the address family of the source node that decapsulates the UIO ICMP
extension objects, as opposed to what is stated in [RFC5837]
Section 4.2.
If the node decoding the ICMP extension header does not recognize the
UIO ICMP extension object, it SHOULD ignore this object and
continue processing the other objects.
3.2. Underlay Information Object Encoding Process
When an underlay head-end node receives an ICMP error message from
an underlay node and needs to forward information about this error
to an overlay endpoint, it follows this process:
1. The underlay head-end node constructs an ICMP error message
destined for the overlay endpoint.
2. The node appends a UIO ICMP extension object to this ICMP error
message according to the procedures defined in [RFC4884].
3. Within the UIO object payload, the node includes one or more
ICMP extension objects that carry information about the underlay
node where the original error occurred.
4. The first ICMP extension object within the UIO payload MUST
contain addressing information (e.g., using the Interface
Information Object defined in [RFC5837]) that identifies the
underlay node that generated the original error. This ensures
that the most critical diagnostic information for pinpointing
the failure source is immediately available.
5. Additional ICMP extension objects MAY be included to provide
supplementary diagnostic information about the underlay path.
6. The encapsulated ICMP extension objects within the UIO may
belong to any address family, regardless of the address family
used between the underlay head-end and the overlay endpoint.
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7. The total length of the ICMP message, including all extensions,
MUST NOT exceed 576 octets for IPv4 or 1280 octets for IPv6
(the minimum reassembly buffer sizes defined in [RFC791] and
[RFC8200], respectively).
Implementations SHOULD provide configuration options to control
which underlay information is included in UIO objects, considering
security and privacy implications discussed in Section 4.
4. Security Considerations
The UIO extension introduces several security considerations that
implementations and operators must address:
4.1. Information Disclosure
The UIO extension reveals information about the underlay network
topology and addressing to overlay endpoints. In many deployments,
the overlay and underlay networks are operated by different
administrative entities, and underlay topology information may be
considered sensitive.
Implementations MUST provide configuration options to control the
generation of UIO extensions. The default configuration MUST
disable UIO generation. Operators SHOULD enable UIO only for
authenticated and authorized overlay endpoints or networks. The
specific mechanisms for such authentication and authorization are
outside the scope of this document but are crucial for secure
deployment.
4.2. Privacy Considerations
Underlay information may reveal details about network architecture,
capacity, and routing that could be exploited for reconnaissance
or targeted attacks. Operators SHOULD carefully consider which
underlay information to expose through UIO extensions.
4.3. Message Size and Amplification
Including UIO extensions increases ICMP message size.
Implementations MUST enforce the message size limits specified in
Section 3.2 to prevent fragmentation issues and potential
amplification attacks.
4.4. Spoofing and Forgery
As with all ICMP messages, UIO extensions are subject to spoofing
attacks. The authenticity and integrity of UIO information cannot
be guaranteed without additional security mechanisms.
Implementations and operators SHOULD NOT use UIO information for
security-critical decisions.
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4.5. Intended Use
The extensions defined in this document are intended exclusively
for administrative debugging and troubleshooting purposes. They
provide diagnostic information in ICMP responses and are not
designed for use in production protocols, automation systems, or
non-debugging applications.
4.6. Rate Limiting
Implementations SHOULD apply rate limiting to the generation of
ICMP messages containing UIO extensions to prevent resource
exhaustion and potential denial-of-service conditions.
5. IANA Considerations
5.1. ICMP Extension Object Class
IANA is requested to assign a new value from the "ICMP Extension
Object Classes and Class Sub-types" registry
(https://www.iana.org/assignments/icmp-parameters/) for the
Underlay Information Object (UIO) as follows:
Class Value: TBA (suggested value: TBD by IANA)
Class Name: Underlay Information Object
Reference: [This RFC]
5.2. C-Type Values
IANA is requested to establish a new sub-registry titled "Underlay
Information Object C-Types" under the "ICMP Extension Object
Classes and Class Sub-types" registry.
Initial values for this registry are as follows:
C-Type Value | Description | Reference
-------------|-----------------------|------------
0 | Reserved/Unspecified | [This RFC]
1-246 | Unassigned |
247-255 | Reserved for Private | [This RFC]
| or Experimental Use |
The registration procedure for values 1-246 is Standards Action or
IESG Approval as defined in [RFC8126].
6. Operational Considerations
6.1. Configuration
Operators SHOULD carefully configure which overlay endpoints or
networks are authorized to receive UIO information. To effectively
manage the security and operational aspects of UIO,
implementations SHOULD provide configuration options, including
but not limited to:
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- Enable/disable UIO generation (default: disabled)
- Whitelist of authorized overlay prefixes
- Maximum UIO object payload size
- Rate limiting parameters
6.2. Troubleshooting Workflow
The intended use case for UIO is as follows:
1. An overlay operator performs traceroute from an overlay endpoint
2. The traceroute reveals a failure point in the path
3. ICMP error messages include UIO extensions with underlay details
4. The overlay operator uses this information to coordinate with
the underlay operator for problem resolution
6.3. Multi-Vendor Interoperability
Implementations SHOULD be tested for interoperability, particularly
when overlay and underlay equipment are from different vendors.
7. Appendix
7.1. UIO ICMP Extension Message Examples
This section lists examples of UIO encoding.
7.1.1. UIO carrying IPv6 information to the IPv4 source
In this example, a host receives an IPv4 ICMPv4 Time Exceeded error
message in response to an ICMP Echo Request as part of the traceroute
application. It also contains an UIO ICMP extension object with IPv6
interface address information as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ IPv4 Header ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=11 | Code=0 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | Length=32 | Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Part of Original Datagram (128 bytes) ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Ver=2 | Unused | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length=28 | Class-Num=TBA | C-Type=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length=24 | Class-Num=2 | C-Type=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AFI=2 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ IPv6 Address (Original Error Device) ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ICMPv4 packet carrying UIO ICMP extension
The traceroute application displays the IPv6 Address in the UIO to
allow an administrator to trace the underlay path of the route being
traced.
7.1.2. UIO carrying IPv4 information to the IPv6 source
In this example, a host receives an IPv6 ICMPv6 Time Exceeded error
message in response to an ICMP Echo Request as part of the traceroute
application. It contains a UIO ICMP extension object with IPv4
interface address information as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ IPv6 Header ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=3 | Code=0 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length=32 | Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Part of Original Datagram (128 bytes) ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Ver=2 | Unused | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length=16 | Class-Num=TBA | C-Type=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length=12 | Class-Num=2 | C-Type=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AFI=1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address (Original Error Device) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: UIO carrying IPv4 information to the IPv6 source
The traceroute application displays the IPv4 Address in the UIO to
allow an administrator to trace the underlay path of the route being
traced.
8. Normative References
[This RFC] This document.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/rfc/rfc791>.
[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/rfc/rfc2119>.
[RFC4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
"Extended ICMP to Support Multi-Part Messages", RFC 4884,
DOI 10.17487/RFC4884, April 2007,
<https://www.rfc-editor.org/rfc/rfc4884>.
[RFC5837] Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
N., and JR. Rivers, "Extending ICMP for Interface and
Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
April 2010, <https://www.rfc-editor.org/rfc/rfc5837>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing RFCs with Nits", BCP 14, RFC 8126,
DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/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/rfc/rfc8174>.
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[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/rfc/rfc8200>.
[RFC8335] Bonica, R., Thomas, R., Linkova, J., Lenart, C., and M.
Boucadair, "PROBE: A Utility for Probing Interfaces",
RFC 8335, DOI 10.17487/RFC8335, February 2018,
<https://www.rfc-editor.org/rfc/rfc8335>.
[RFC8883] Herbert, T., "ICMPv6 Errors for Discarding Packets Due to
Processing Limits", RFC 8883, DOI 10.17487/RFC8883,
September 2020, <https://www.rfc-editor.org/rfc/rfc8883>.
9. Informative References
[IANA.address-family-numbers]
IANA, "Address Family Numbers",
<http://www.iana.org/assignments/address-family-numbers>.
Acknowledgments
The authors thank the contributors listed below for their substantial
input and review.
Contributors
Tamilselvan Murugan
Cisco Systems, Inc.
Email: tammurug@cisco.com
Dhilip Sekar
Email: dhilipsekar1998@gmail.com
Authors' Addresses
Jaganbabu Rajamanickam (editor)
Cisco Systems, Inc.
Canada
Email: jrajaman@cisco.com
Darren Dukes
Cisco Systems, Inc.
Canada
Email: ddukes@cisco.com
Madhan Sankaranarayanan (editor)
Cisco Systems, Inc.
India
Email: madsanka@cisco.com
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