Internet-Draft Stub Router RA Flag February 2024
Hui Expires 30 August 2024 [Page]
Internet Engineering Task Force
Intended Status:
Standards Track
J. Hui
Google LLC

Stub Router Flag in ICMPv6 Router Advertisement Messages


This document defines a new Stub Router flag in the Router Advertisement message that can be used to distinguish configuration information sent by stub routers from information sent by infrastructure routers.

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 working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on 30 August 2024.

1. Introduction

A stub router provides IP connectivity between a stub network and an infrastructure network. A common stub router example is a device that attaches a 6LoWPAN-based network to a home network.

To support IPv6 reachability between infrastructure network devices and stub network devices, routable IPv6 addresses must be configured on both the infrastructure and stub networks. Stub routers decide whether or not to advertise their ULA prefixes [RFC4193] on the infrastructure network to which they are connected by tracking the presence of infrastructure-provided IPv6 service. When a stub router sees a new prefix advertised on infrastructure, this prefix may be coming from an infrastructure router, or from another stub router. In the second case, the two stub routers could wind up in a cycle of publishing and deprecating their prefixes as they see prefixes from the other stub router show up.

The stub router document [I-D.ietf-snac-simple] explains how two stub routers decide which one has precedence in the event of a conflict. However, the infrastructure prefix always has precedence over a prefix provided by any stub router. In order to differentiate between prefixes advertised by infrastructure and those advertised by stub routers, it is necessary to be able to mark RAs sent by stub routers.

Additionally, the RA header includes M and O flags that indicate whether DHCPv6 is available on the link. Section 6.3.4 of [RFC4861] specifies that hosts consider the most recently received information as authoritative. As a result, stub routers must mirror the M and O values in RAs received from infrastructure routers. The Stub Router flag allows stub routers to easily identify which RAs are sent by infrastructure routers.

2. Terminology

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.

3. Stub Router Flag

This document defines a new "Stub Router" flag as flag bit 6 in the Router Advertisement flags field.

 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
|     Type      |     Code      |          Checksum             |
| Cur Hop Limit |M|O|H|Prf|P|S|R|       Router Lifetime         |
|                         Reachable Time                        |
|                          Retrans Timer                        |
|   Options ...
Figure 1: outer Advertisement Message Format

The S flag is the "Stub Router" flag.

The M, O, H, Prf, P, and R flags are as defined in [RFC5175].

4. Router Advertisement Transmission

A stub router that is not explicitly configured as part of the infrastructure network MUST set the Stub Router flag in outgoing RA messages.

How and when a stub router sets the M and O flags in outgoing RAs is specified in [I-D.ietf-snac-simple].

5. IANA Considerations

IANA is requested to allocate a flag from the "IPv6 ND Router Advertisement flags" registry of [RFC5175], as specified below:

Table 1
RA Option Bit Description Reference
6 S - Stub Router Flag This Document

6. Security Considerations

This protocol shares the security issues of NDP that are documented in the "Security Considerations" section of [RFC4861].

7. Normative References

Lemon, T. and J. Hui, "Automatically Connecting Stub Networks to Unmanaged Infrastructure", Work in Progress, Internet-Draft, draft-ietf-snac-simple-03, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, DOI 10.17487/RFC4193, , <>.
Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, , <>.
Haberman, B., Ed. and R. Hinden, "IPv6 Router Advertisement Flags Option", RFC 5175, DOI 10.17487/RFC5175, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

Author's Address

Jonathan Hui
Google LLC
1600 Amphitheatre Parkway
Mountain View, California 940432
United States of America