RFC 5075

IPv6 Router Advertisement Flags Option

📅 published November 2007, from ietf

RFC 5075: IPv6 Router Advertisement Flags Option 🎀

Okay bestie, this is RFC 5075 and it's a Standards Track doc, so it's a real protocol spec for the whole Internet community ✨. It came out in November 2007. The authors are Brian Haberman, Ed. from JHU APL and Robert Hinden from Nokia. The title block doesn't list anything it obsoletes or updates, so it's standing on its own 💅.

Status of This Memo 📜

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Abstract 💭

So the IPv6 Neighbor Discovery Router Advertisement message has an 8-bit field that's reserved for single-bit flags 🚩. Several protocols already reserved flags in there, and others are getting ready to reserve enough to use up the whole field 😭. This document defines an option for the Router Advertisement message that expands the number of flag bits available. It's giving more room 🫶.

Table of Contents 🗂️

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3.  Current Router Advertisement Flags  . . . . . . . . . . . . . . 2
4.  Flags Expansion Option  . . . . . . . . . . . . . . . . . . . . 3
5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
  7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
  7.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

1. Introduction ✨

The IPv6 Neighbor Discovery Protocol (NDP) [RFC4861] has a Router Advertisement message with an 8-bit field reserved for single-bit flags. Several protocols have grabbed flags from that field, and others are about to grab enough to run it out 🍓.

This document defines an option for the Router Advertisement message that expands the number of flag bits by adding 48 more flag bits to NDP messages 🧚.

2. Terminology 📖

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Current Router Advertisement Flags 🚩

Right now, the NDP Router Advertisement message has these one-bit flags defined in published RFCs 👑:

 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|M|O|H|Prf|P|R|R|
+-+-+-+-+-+-+-+-+

                Figure 1: Router Advertisement Flags
  • M - Managed Address Configuration Flag [RFC4861]
  • O - Other Configuration Flag [RFC4861]
  • H - Mobile IPv6 Home Agent Flag [RFC3775]
  • Prf - Router Selection Preferences [RFC4191]
  • P - Neighbor Discovery Proxy Flag [RFC4389]
  • R - Reserved

With other protocols in the works (for example, Detecting Network Attachment) that want to use flags in NDP messages, we need a way to expand the flags so new features can fit 🌈.

4. Flags Expansion Option 🧩

The Neighbor Discovery specification [RFC4861] already lets you define NDP options 🙌. So the Expanded Flags Option (EFO) for NDP Router Advertisement messages is defined in Figure 2 below.

 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     |         Bit fields available ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... for assignment                                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 2: Router Advertisement Expanded Flags Option

Here's the breakdown of the fields 🔍:

  • Type - TBD (to be assigned by IANA)
  • Length - The length MUST be checked when processing the option, so that future expansion of this option is possible. An implementation of this specification MUST set the Length to 1, MUST ignore any unrecognized data, and MUST be able to recognize the specific length so it can skip over unrecognized bits.
  • Bits - allocated by IANA

The definition and usage of these bits can be found in the document that requests their allocation 📄.

When this option is being built and sent, it:

  • MUST only occur in Router Advertisement messages.
  • MUST occur prior to any additional options associated with any flags set in this option.
  • MUST only occur once in the Router Advertisement message.
  • MUST NOT be added to a Router Advertisement message if no flags in the option are set.
  • MUST set all unused flags to zero.

When a receiver gets an NDP message containing this option, it:

  • MUST ignore the option if it occurs in a message other than a Router Advertisement.
  • MUST ignore all instances of the option except the first one encountered in the Router Advertisement message.
  • MUST ignore the option if the Length is less than 1.
  • MUST ignore any unknown flag bits.
The bit fields in the option are numbered from left to right, from 8 to 55 (starting at bit offset 16 in the option). They follow the numbering of the flag bits in the RA option described in Figure 1. Flag bits 0 to 7 live in the Router Advertisement message header defined in [RFC4861] 🧠.

5. IANA Considerations 🏛️

IANA has defined a new IPv6 Neighbor Discovery option for the option defined in this document, like so 💌:

          +------+---------------------------+-----------+
          | Type | Description               | Reference |
          +------+---------------------------+-----------+
          | 26   | RA Flags Extension Option | [RFC5075] |
          +------+---------------------------+-----------+

The registry for these options is at:

http://www.iana.org/assignments/icmpv6-parameters

IANA has also created a new registry for IPv6 ND Router Advertisement flags. It should include the current flags in the RA option and in the extension option defined in this document. The new registry has been added to icmpv6-parameters as shown above. The format for the registry is:

+---------------+---------------------------------------+-----------+
| RA Option Bit | Description                           | Reference |
+---------------+---------------------------------------+-----------+
| 0             | M - Managed Address Configuration     | [RFC4861] |
|               | Flag                                  |           |
| 1             | O - Other Configuration Flag          | [RFC4861] |
| 2             | H - Mobile IPv6 Home Agent Flag       | [RFC3775] |
| 3             | Prf - Router Selection Preferences    | [RFC4191] |
| 4             | Prf - Router Selection Preferences    | [RFC4191] |
| 5             | P - Neighbor Discovery Proxy Flag     | [RFC4389] |
| 6-53          | R - Reserved; Available for           |           |
|               | assignment                            |           |
| 54-55         | Private Experimentation               |           |
+---------------+---------------------------------------+-----------+

Assigning new RA flags in the RA option header, and the bits defined in the RA extension option from this document, requires standards action or IESG approval [RFC2434] 🔐.

6. Security Considerations 🔐

This is serious business, so please read this section carefully 🚨.

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

Also, adding more optional bit fields creates a potential covert channel that's useful for passing information. That's a real risk, so keep it in mind 👀💀.

7. References 📚

7.1. Normative References

  • [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
  • [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
  • [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007.

7.2. Informative References

  • [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in IPv6", RFC 3775, June 2004.
  • [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, November 2005.
  • [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery Proxies (ND Proxy)", RFC 4389, April 2006.

Authors' Addresses 📮

Brian Haberman (editor), Johns Hopkins University Applied Physics Lab, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA. Phone: +1 443 778 1319. EMail: brian@innovationslab.net

Robert Hinden, Nokia, 313 Fairchild Drive, Mountain View, CA 94043, USA. Phone: +1 650 625 2004. EMail: bob.hinden@nokia.com

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