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, . [RFC5175] Haberman, B., Ed. and R. Hinden, "IPv6 Router Advertisement Flags Option", RFC 5175, DOI 10.17487/RFC5175, March 2008, . 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, . [IANA-RA-FLAGS] IANA, "IPv6 ND Router Advertisement Flags", . [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998, . [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, . [RFC7113] Gont, F., "Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)", RFC 7113, DOI 10.17487/RFC7113, February 2014, . Author's Address Jonathan Hui Google LLC 1600 Amphitheatre Parkway Mountain View, California 94043 United States of America Email: jonhui@google.com