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
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   This Internet-Draft will expire on 19 February 2026.

Copyright Notice

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   document authors.  All rights reserved.

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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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