Internet-Draft Stub Router RA Flag July 2022
Hui Expires 8 January 2023 [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 to distinguish configuration information sent by stub routers from infrastructure routers. For example, the Stub Router flag allows stub routers to easily identify when an infrastructure router is advertising a usable IPv6 prefix, triggering the stub router to not advertise its own routable prefix.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 8 January 2023.

Table of Contents

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. Routers on the infrastructure network may configure routable IPv6 prefixes using Router Advertisements (RAs) containing Prefix Information Options (PIOs) [RFC4861].

In cases where the infrastructure network does not provide routable IPv6 address configuration, the stub router is responsible for configuring routable IPv6 addresses on the infrastructure network. If a stub router determines that routable IPv6 addresses are not available on the infrastructure link, it will locally generate a Unique Local Address (ULA) Prefix [RFC4193] and begin advertising that via a PIO contained in RA messages.

Both infrastructure routers and stub routers may advertise routable IPv6 prefixes in the same way (via RAs containing PIOs). To distinguish between an infrastructure router and a stub router, this document defines a new Stub Router flag in the RA message. The Stub Router flag allows stub routers to easily identify when an infrastructure router is advertising a routable IPv6 prefix, triggering the stub router to stop advertising its ULA prefix.

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 8 in the Router Advertisement flags field. Setting the Stub Router flag requires use of the RA Flags Extension Option [RFC5175] to encode the flag.

 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      |    Length     |S|       Bit fields available ..
... for assignment                                              |
Figure 1: TCP Header Format

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 includes PIOs in outgoing RAs is specified in [I-D.lemon-stub-networks].

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
8 S - Stub Router Flag This Document

Note that RA Option Bit 8 requires use of the RA Flags Extension Option [RFC5175] to encode the flag.

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., "Connecting Stub Networks to Existing Infrastructure", Work in Progress, Internet-Draft, draft-lemon-stub-networks-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