Subject: [dhcwg] Relay behavior in dhcpv4 and dhcpv6

From: "Templin, Fred L" <[email-masked]>
Date: Tue, 7 Feb 2006 15:21:12 -0800

I believe there is a difference in the way relays work
between dhcpv4 and dhcpv6. In dhcpv4, it seems that the
server unicasts the reply to the first-hop relay that is
connected to the client's link, whereas with dhcpv6 it
seems that the chain of relays is revisited in reverse
order when the server issues the reply. Can someone
confirm whether I have read this correctly? And, what
is the reason for the different behavior between dhcpv4
and dhcpv6?

Fred
[email-masked]

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From: Ralph Droms <[email-masked]>
Date: Tue, 07 Feb 2006 18:45:05 -0500

Fred - you've read the specs correctly.  The DHCPv6 behavior is an explicit
fix to the DHCPv4 behavior, to guarantee that in case there is more than one
relay agent, any relay agent option in a server->client message can be
processed and removed by the relay agents, in order.

- Ralph

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From: Ted Lemon <[email-masked]>
Date: Tue, 7 Feb 2006 17:10:20 -0700

On Tuesday 07 February 2006 16:21, Templin, Fred L wrote:
> And, what

The different behavior in DHCPv6 ensures that if there is some setup work that 
needs to be done by relay agents along the way, it can be done.   It also 
means that if there is some sort of weird routing in place that would 
ordinarily prevent the tail-end relay agent from communicating with the DHCP 
server, but that does not prevent the relay chain from working, that the DHCP 
packet still goes through.

You may also notice that in DHCPv6, the client _always_ multicasts unless the 
server tells it it doesn't have to, even when it has an address and is 
attempting to extend its valid lifetime.   This again ensures that every time 
a packet comes from a client, it follows the same chain of relay agents to 
the server, or if the client has moved, meaning that it's connecting through 
a different set of relay agents, then the server will be made aware of that 
fact.