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.
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.
DHCPv6  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 . However, DHCPv6  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.
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:
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  to request the server to return options (e.g., ) other than relay agent options in the Relay Forward message.
As the Echo Request option is only exchanged between relay agents and DHCPv6 servers, section 21.1 of  provides details on securing DHCPv6 messages sent between servers and relay agents. And, section 23 of  provides general DHCPv6 security considerations.
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