Subject: [ipoverib] reasons for not using LID ?

From: Yaron Haviv <[email-masked]>
Date: Fri, 18 Jan 2002 02:46:47 +0200

By now it is obvious that there are the ones that wants LID's in the ARP
packets

Could the ones that don't give their reasons for why not to
Not just things like “you don't need it”, “we want to save 16-bit”, etc'
But, reasons for what problems could arouse from having it in the packet
I haven't heard any real argument that would make me consider removing it
until now
we then will review them and see if it is possible to deal with those cases,
if not I personally would drop my support in LID's

If people want it for their reasons, the ones that don't also need to do
some explaining

Yaron

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From: Libor Michalek <[email-masked]>
Date: Thu, 17 Jan 2002 19:52:39 -0800

On Fri, Jan 18, 2002 at 02:46:47AM +0200, Yaron Haviv wrote:
> Not just things like 'you don't need it', 'we want to save 16-bit', etc

  If a single IP subnet spans two IB subnets, (the IP subnet uses them as 
a single layer 2 domain) the LID in an ARP which crosses the IB router
will be incorrect, because the router is an IB router, and does not
necessarily understand IP traffic.

  For example:

  A single IP subnet spans two IB subnets, (subnets X and Y) and there is 
an IB router between the two subnets. Hosts A and B are on the same IP
subnets, but host A is on IB subnet X and host B is on IB subnet Y. Host
A sends an ARP request for host B's address, and host B responds. Host A
will have a LID for host B which is incorrect, because the LID is for a
different IB subnet. What host A needs is the LID of the IB router.

  In this scenario the IB router needs to understand IP ARP packets and
be able to look into each packet to determine if it is an ARP, and
modify the LID. This requires all IB routers to implement proxy ARP. 
This seems to be an unnecessary constraint on IB routers that want to
carry IP traffic.

  The suggestion has been made to include the LID and people can chose
to use it or not. The problem there is if a host driver does use the
LID to communicate on topology described above, it will fail unless the
IB router understands IP. Unless the recipient looks to see if the LID
in the ARP and IB packet SLID match, but this opens problems for legitamate
cases where you want an entity answering ARPs for some other entity.

  Requiring IB routers to understand IP breaks one of the 
mandates/assumptions under which we've been working, and that is to use
all of IB as a strictly layer 2 transport. I favor that we stay with that
course.

-Libor
--
Libor Michalek
[email-masked]
Topspin Communications