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IETF RFC 4994
DHCPv6 Relay Agent Echo Request Option
Last modified on Monday, September 17th, 2007
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Network Working Group S. Zeng
Request for Comments: 4994 B. Volz
Category: Standards Track K. Kinnear
Cisco Systems, Inc.
J. Brzozowski
Comcast Cable
September 2007
DHCPv6 Relay Agent Echo Request Option
 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
This memo defines a Relay Agent Echo Request option for the Dynamic
Host Configuration Protocol for IPv6 (DHCPv6). The option allows a
DHCPv6 relay agent to request 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
Zeng, et al. Standards Track PAGE 1 
RFC 4994 Relay Agent ERO September 2007
1. Introduction
DHCPv6 [2] provides a framework for configuring IPv6 clients with
addresses and other network parameters. It includes a relay agent
capability. A relay agent is an intermediary node that delivers DHCP
messages between clients and servers. The relay agent and the server
exchange information using options in relay agent messages. The
relay agent may add relay agent options to the client DHCP message
before forwarding it.
The information that relay agents supply can be used in the server's
decision making about the addresses, delegated prefixes, and
configuration parameters that the client is to receive. Likewise,
the relay may need some of the information to efficiently return
replies to clients.
In DHCPv4, the server generally echoes the relay agent option back
verbatim to the relay agent in server-to-client replies [3].
However, DHCPv6 [2] does not require the server to do so. This could
be problematic, as the relay agent may need to use some relay options
even if the server does not recognize them.
This memo defines a relay agent echo request option that the relay
agent uses to explicitly request 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].
3. The Relay Agent Echo Request Option
The relay agent adds options in the Relay Forward message that the
server uses to guide its decision making with regard to address
assignment, prefix delegation, and configuration parameters. The
relay agent also knows which of these options that it will need to
efficiently return replies to the client. It uses the relay agent
Echo Request option to inform the server of the list of relay agent
options that the server must echo back.
Zeng, et al. Standards Track PAGE 2
RFC 4994 Relay Agent ERO September 2007
The format of the DHCPv6 Relay Agent Echo Request option is shown
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_ERO | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| requested-option-code-1 | requested-option-code-2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 inform the server about options the relay agent wants the
server to echo back to the relay agent. If the relay agent takes
different actions based on whether an option is echoed back or not,
then the relay agent SHOULD NOT include such an option in the Echo
Request option. Note that the relay uses the OPTION_ORO [2] to
request 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 perform ERO processing
after processing 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 continue to process any remaining options
in the ERO.
b. If the option was not in the received Relay-Forward, the server
MUST ignore that option and continue to process 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.
Zeng, et al. Standards Track PAGE 3
RFC 4994 Relay Agent ERO September 2007
6. Security Considerations
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.
Zeng, et al. Standards Track PAGE 4
RFC 4994 Relay Agent ERO September 2007
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
Zeng, et al. Standards Track PAGE 5
RFC 4994 Relay Agent ERO September 2007
Full Copyright Statement
Copyright © The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Zeng, et al. Standards Track PAGE 6
DHCPv6 Relay Agent Echo Request Option
RFC TOTAL SIZE: 10978 bytes
PUBLICATION DATE: Monday, September 17th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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