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.


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

More information about the dhcp-users mailing list