RFC 4994

DHCPv6 Relay Agent Echo Request Option

📅 published September 2007, from ietf

RFC 4994 🎀 DHCPv6 Relay Agent Echo Request Option

Network Working Group ✨ Category: Standards Track 💅 (Track.. it's an Internet standards track protocol bestie) Date: September 2007 🌸 Authors: Shengyou Zeng, Bernard Volz, and Kim Kinnear (all Cisco Systems, Inc.), plus John Jason Brzozowski (Comcast Cable) 🫶 Obsoletes: none listed 🙈 Updates: none listed 🙈

Status of This Memo 📜

okay so this doc is an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 💜. For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🦋.

Abstract ✨

this memo defines a Relay Agent Echo Request option for the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) 🧚. It lets a DHCPv6 relay agent ask for a list of relay agent options that the server echoes back to the relay agent 🔁.

Table of Contents 🍓

1. Introduction ....................................................2
2. Requirements Terminology ........................................2
3. The Relay Agent Echo Request Option .............................2
4. DHCPv6 Relay Agent Behavior .....................................3
5. DHCPv6 Server Behavior ..........................................3
6. Security Considerations .........................................4
7. IANA Considerations .............................................4
8. Acknowledgements ................................................4
9. References ......................................................4
   9.1. Normative References .......................................4
   9.2. Informative References .....................................4

1. Introduction 🌈

okay so DHCPv6 [2] is the framework that configures IPv6 clients with addresses and other network parameters, and it has a relay agent capability 💖. A relay agent is an intermediary node that carries DHCP messages between clients and servers 🚚. The relay agent and the server swap info using options in relay agent messages. The relay agent may add relay agent options to the client DHCP message before forwarding it 🎀.

The info relay agents supply can help the server decide about the addresses, delegated prefixes, and configuration parameters the client gets 👑. And honestly, the relay may need some of that same info to send replies back to clients efficiently 💅.

In DHCPv4, the server generally echoes the relay agent option back verbatim to the relay agent in server-to-client replies [3]. But DHCPv6 [2] does not require the server to do that 😭. This could be a problem, because the relay agent may need some relay options even when the server doesn't recognize them 💀.

So this memo defines a relay agent echo request option. The relay agent uses it to explicitly ask for a list of options that the server echoes back to the relay agent ✨🔥.

2. Requirements 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 [1] 💜. These are the BCP 14 key words, so read them exactly as written 🫶.

3. The Relay Agent Echo Request Option 🦋

the relay agent adds options in the Relay Forward message, and the server uses them to guide its decisions about address assignment, prefix delegation, and configuration parameters 🎯. The relay agent also knows which of these options it will need to return replies to the client efficiently 🙌. So it uses the relay agent Echo Request option to tell the server the list of relay agent options the server must echo back. (Yes, "must" is doing the work there, babe 💅.)

The format of the DHCPv6 Relay Agent Echo Request option looks like this 👀:

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           OPTION_ERO          |           option-len          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    requested-option-code-1    |    requested-option-code-2    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              ...                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

the fields, explained:

option-code              OPTION_ERO (43).
option-len               2 * number of requested options.
requested-option-code-n  The option code for an option requested by
                         the relay agent.

4. DHCPv6 Relay Agent Behavior 🌸

a relay agent MAY include an Echo Request option in a Relay Forward message to tell the server which options the relay agent wants echoed back to it ✨. If the relay agent does different things depending on whether an option gets echoed back or not, then the relay agent SHOULD NOT put that option in the Echo Request option 🙈. Also, the relay uses the OPTION_ORO [2] to ask the server to return options (e.g., [4]) other than relay agent options in the Relay Forward message 💖.

5. DHCPv6 Server Behavior 👑

when a server creates a Relay-Reply, it SHOULD do ERO processing after the ORO and other options processing 🧚. For each option in the ERO:

a. If the option is already in the Relay-Reply, the server MUST ignore that option and keep processing any remaining options in the ERO 🙅‍♀️.
b. If the option was not in the received Relay-Forward, the server MUST ignore that option and keep processing any remaining options in the ERO 🙅‍♀️.
c. Otherwise, the server MUST copy the option, verbatim, from the received Relay-Forward to the Relay-Reply, even if the server does not otherwise recognize that option 🫶.

6. Security Considerations 🔐

⚠️ Okay bestie, this part is SERIOUS. Don't skip it.

As the Echo Request option is only exchanged between relay agents and DHCPv6 servers, section 21.1 of [2] provides details on securing DHCPv6 messages sent between servers and relay agents 🛡️. And section 23 of [2] provides general DHCPv6 security considerations 💀.

7. IANA Considerations 🌷

IANA has assigned a DHCPv6 option code for the OPTION_ERO (Relay Agent Echo Request) Option (43) 🎀.

8. Acknowledgements 🫶

Thanks to Ralph Droms, Josh Littlefield, Richard Johnson, and Hemant Singh for their consistent input, ideas, and review during the production of this document 💜🌈.

9. References 📚

9.1. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
[3] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001.

9.2. Informative References

[4] Droms, R., Volz, B., and O. Troan, "DHCPv6 Relay Agent Assignment Notification (RAAN) Option", Work in Progress, November 2006.

Authors' Addresses 📮

Shengyou Zeng
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA  01719
USA

Phone: +1 978 936 0000
EMail: szeng@cisco.com


Bernard Volz
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA  01719
USA

Phone: +1 978 936 0000
EMail: volz@cisco.com


Kim Kinnear
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA  01719
USA

Phone: +1 978 936 0000
EMail: kkinnear@cisco.com


John Jason Brzozowski
Comcast Cable
1800 Bishops Gate Boulevard
Mt. Laurel, NJ  08054
USA

Phone: +1 856 324 2671
EMail: john_brzozowski@cable.comcast.com

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