The SNAC Router Flag in ICMPv6 Router Advertisement Messages

Internet Engineering Task Force (IETF)                            J. Hui
Request for Comments: 10075                                   Google LLC
Category: Standards Track                                   October 2026
ISSN: 2070-1721


      The SNAC Router Flag in ICMPv6 Router Advertisement Messages

Abstract

   This document defines a new flag, the SNAC Router flag, in the Router
   Advertisement message that can be used to distinguish configuration
   information sent by Stub Network Auto-Configuring (SNAC) routers from
   information sent by transit routers.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc10075.

Copyright Notice

   Copyright (c) 2026 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 and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  SNAC Router Flag
   3.  IANA Considerations
   4.  Operational Considerations
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Author's Address

1.  Introduction

   Per Section 2.1 of [RFC2328], networks can either be transit or stub
   networks.  Transit networks are those capable of carrying data
   traffic that is neither locally originated nor locally destined.

   A Stub Network Auto-Configuring (SNAC) router is an autonomously
   configuring router that provides IP connectivity between one or more
   stub networks and one or more transit networks.  A common example of
   a SNAC router is a device that attaches a 6LoWPAN-based network
   [RFC4919] to a home network, automatically providing IPv6 forwarding
   between the two networks without explicit operator configuration.
   This document defines a new IPv6 Neighbor Discovery Router
   Advertisement (RA) flag, the SNAC Router flag, which SNAC routers use
   to identify RAs sent by other SNAC routers.

   Readers can refer to [AUTO-STUB] for an overview of a set of
   practices for automatically connecting IPv6 stub networks to transit
   networks.

2.  SNAC Router Flag

   The SNAC Router flag is bit 6 in the RA Flags Extension Option
   [RFC5175].

   The SNAC Router flag bit with a value of '1' is reserved for use by
   SNAC routers.  Receipt of an RA with that bit set to '1' indicates
   that the sending router is a SNAC router.  Describing the exact
   triggers when a SNAC router sets this flag to '1' is out of scope for
   this document.  An example of such considerations is documented in
   [AUTO-STUB].

   Consistent with Section 4.2 of [RFC4861], devices that do not operate
   as SNAC routers will not set the SNAC Router flag bit to '1', and
   devices that do not understand the SNAC Router flag will silently
   ignore it.  This means that setting the flag bit to '1' or '0' should
   not change the behavior of such devices in any way (other than that
   it is permissible to log and cache the value of the flag bit as part
   of normal router advertisement processing, where applicable).

3.  IANA Considerations

   IANA has allocated a flag from the "IPv6 ND Router Advertisement
   Flags" registry in the "Internet Control Message Protocol version 6
   (ICMPv6) Parameters" registry group [IANA-RA-FLAGS] as specified
   below:

         +===============+======================+===============+
         | RA Option Bit | Description          | Reference     |
         +===============+======================+===============+
         | 6             | S - SNAC Router Flag | This Document |
         +---------------+----------------------+---------------+

                                 Table 1

4.  Operational Considerations

   In environments that implement RA-Guard [RFC7113] in a way that
   filters RAs sent by SNAC routers, devices on the transit network will
   never receive an RA with the SNAC Router flag bit set to '1'.

5.  Security Considerations

   How SNAC routers process RAs is dependent on the SNAC Router flag
   value.

   The security considerations of IPv6 Neighbor Discovery are documented
   in Section 11 (Security Considerations) of [RFC4861].  The addition
   of the SNAC Router flag does not change these considerations.

6.  References

6.1.  Normative References

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC5175]  Haberman, B., Ed. and R. Hinden, "IPv6 Router
              Advertisement Flags Option", RFC 5175,
              DOI 10.17487/RFC5175, March 2008,
              <https://www.rfc-editor.org/info/rfc5175>.

6.2.  Informative References

   [AUTO-STUB]
              Lemon, T., Hui, J., and E. Dijk, "Automatically Connecting
              Stub Networks to Unmanaged Infrastructure", Work in
              Progress, Internet-Draft, draft-ietf-snac-simple-13, 21
              October 2026, <https://datatracker.ietf.org/doc/html/
              draft-ietf-snac-simple-13>.

   [IANA-RA-FLAGS]
              IANA, "IPv6 ND Router Advertisement Flags",
              <https://www.iana.org/assignments/icmpv6-parameters>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, DOI 10.17487/RFC4919, August 2007,
              <https://www.rfc-editor.org/info/rfc4919>.

   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", RFC 7113,
              DOI 10.17487/RFC7113, February 2014,
              <https://www.rfc-editor.org/info/rfc7113>.

Author's Address

   Jonathan Hui
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043
   United States of America
   Email: jonhui@google.com