DHCP support for Infiniband

David W. Hankins David_Hankins at isc.org
Thu Mar 16 16:34:07 UTC 2006


On Thu, Mar 16, 2006 at 05:27:16PM +0200, Eli Cohen wrote:
> In the past I have raised here the issue of supporting InfiniBand by the
> 
> DHCP server. I was concerned then by the fact that the server did not
> transmit back the client identifier sent by the client. I actually was
> seeking a way to filter out responses from the server since they are
> broadcast (the client sets the broadcast flag). Recently I learned that
> the xid field of the bootp protocol serves just for this purpose. So
> that means that the the server is not required to send back the client
> identifier. In fact RFC 2131 says the server MUST NOT send the client
> identifier. However, the xid field is chosen by each client is random
> value and it is possible that two clients will choose the same value
> which could cause a problem. 
> 
> I would appreciate comments on this.

http://tools.ietf.org/html/draft-ietf-ipoib-dhcp-over-infiniband-10.txt

I'm not seeing anything in there that relies on the server to echo the
client id back.

You're right that it's possible that the XID may collide.  It's only
a 16 bit field, there are only 65536 possible values, and worse, some
implementations may fill xid with pseudo-static information (ISC DHCP
dhclient fills 'xid' with a hash of the address of a memory structure
the client is using at the time) - but in its defense, it is also in
the universe where chaddr is always verifiably itself, we don't support
infiniband, USB, or Firewire yet.

2 hits in a field of 65536 where you probably have no more than a few
hundred clients on the same broadcast domain is a pretty hard thing to
reach in the general case.


If you have any thoughts on the subject, I encourage you to take it
up with the DHC or IPOIB workgroups.

I'm sorry I don't know a lot about Infiniband (or even the related
problems on Firewire and USB).

-- 
David W. Hankins		"If you don't do it right the first time,
Software Engineer			you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins


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