Re: [dhcwg] AD Evaluation: draft-ietf-dhc-dhcpv6-unknown-msg

Qi Sun <> Sun, 02 February 2014 14:53 UTC

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From: Qi Sun <>
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Date: Sun, 2 Feb 2014 22:52:55 +0800
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To: Ted Lemon <>, Brian Haberman <>
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Subject: Re: [dhcwg] AD Evaluation: draft-ietf-dhc-dhcpv6-unknown-msg
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Dear Brian & Ted,

Thank you so much for your evaluation/suggestions! I think the comments/changes are very helpful to make the text clearer. 

I will do the updates asap. Thank you again!

//Sorry for late response because of the Chinese Spring Festival. 

Best Regards,

On 2014-2-2, at 下午9:33, Ted Lemon wrote:

> On Jan 31, 2014, at 10:40 AM, Brian Haberman <> wrote:
>>    I have done my AD Evaluation of draft-ietf-dhc-dhcpv6-unknown-msg
>> (since Ted is listed as a co-author).  I only have a few points of
>> discussion on this document before we move it along in the publication
>> process.  I am by no means a DHCP expert, so let me know if I have gone
>> off in the weeds...
>> 1. I would like to see a little more clarity in the purpose of this
>> document.  It does a good job clarifying what a relay agent should do
>> with existing messages, but I think it would be useful to spell out that
>> is document is also dictating how future documents that define new DHCP
>> messages should specify relay behavior.
> This is a good suggestion (actually, these are all good suggestions!).   I think the right solution to this is twofold.   First, the following change should be made at the end of 4.1 (this also addresses your point 2):
> OLD:
>   In the case that a new type of message is sent by the server to a
>   relay agent but the relay agent does not recognize it, the message is
>   put into a Relay-forward message and sent to the server.
> NEW:
>   New DHCP message types may be defined in future that are intended to
>   convey information from DHCP servers to relay agents.   Relay agents
>   that do not implement these messages will not recognize that such
>   messages are intended for them.   A relay agent that implements this
>   specification will necessarily forward such messages to the DHCP
>   servers to which it is configured to relay client messages.
>   At this time, no messages of this variety have been specified.
>   If such a message is specified in the future, the specification could
>   include text something like the following:
>      DHCP servers that send this new message type MAY, when they
>      receive a relayed message of this type, mark the relay agent to
>      which the message was sent as not implementing messages of this
>      type.   In this case, the DHCP server MAY implement a strategy
>      of probing the relay agent occasionally to see if it has been
>      updated.
>   However, this is not strictly necessary, since DHCP does not provide
>   a signaling message for rejecting unexpected message types, and
>   therefore DHCP servers are not expected to respond to such messages.
>   Documents specifying new message types should use different types for
>   communicating *to* relay agents than are used for communicating *from*
>   relay agents, so that no confusion can occur where a message sent to
>   a relay agent is sent back to the DHCP server, which then tries to
>   process it as if it came from a relay agent.
> In addition to this change, I'd propose the following change to the abstract:
> OLD:
>   DHCPv6 is not specific about handling messages with unknown types.
>   This memo describes the problems and defines how a DHCPv6 server,
>   client or relay agent should behave when receiving unknown DHCPv6
>   messages.  This document updates RFC3315.
> NEW:
>   DHCPv6 is not specific about handling messages with unknown types.
>   This memo describes the problems and defines how a DHCPv6 server,
>   client or relay agent should behave when receiving unknown DHCPv6
>   messages.  This document also provides advice for authors of future
>   documents defining new messages sent from DHCP servers to DHCP relay
>   agents, and should be read by potential authors of such documents.
>   This document updates RFC3315.
>> 2. In section 4.1, the text talks about targeting a message based on its
>> message type.  To me, that suggests that a relay must know about all
>> DHCP message types.  I thought (feel free to correct me) that relay
>> agents were mostly stateless and would forward all messages not
>> addressed to them to the configured DHCP server.  This assumes that all
>> Relay-Reply messages are addressed to the relay directly.
> That is the general effect of 4.1, but I hope the text above makes clearer what the issues are, both to you and to other future readers of the document.
>> 3. After having an exchange with Ralph Droms, I want to ask about
>> messages from servers to clients.  If those messages are *not* in a
>> Relay-Reply message, but are received by a relay, should those messages
>> be dropped?  If a server->client message is forwarded to a server by the
>> relay, should it be dropped by the server?
> If this happens, it's an implementation bug: the DHCP server is not in conformance with RFC3315.   I therefore think that we don't need to address it, although it would certainly break things if an implementation were to erroneously do that.   I think such an implementation would never make it to the field, because it simply wouldn't work.
>> 4. Is there a need to specifically discuss the behavior of single
>> devices that have both a relay agent and a client running at the same time?
> Hm.   We generally treat these as two separate implementations that happen to be running on the same hardware.   Also I think usually it's a client and a server, not a relay agent and a server.   We could add text addressing this, but I don't really see this coming up in practice.   There isn't similar text in RFC3315, for instance, and we haven't seen evidence of implementors getting this wrong.
>> 5. Also from Ralph... Could section 5 simply be written as "A client or
>> server MUST silently discard any received DHCPv6 message with an unknown
>> message type."?
> Yes, I think this is a good edit.   Qi, do you have time to do these edits?
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