[openib-general] IB Address Translation service

[David M. Brean]

Greg Lindahl wrote:

>On Fri, Mar 04, 2005 at 11:58:33AM -0800, Tom Duffy wrote:
>
>  
>
>>In any event, I think being able to plop an IB network in an Ethernet
>>world will require things like RARP to work.  If there is no spec now,
>>it should be written.
>>    
>>
>
>Much more important is understanding the role of RARP in the ethernet
>world.
>
>It is *not* something you do to find _someone else's_ IP addr from
>their MAC addr. It's what you do to find your _own_ IP addr because
>you're booting. Ethernet protocols such as IP include enough IP
>information to talk back to someone who sent you a packet. So you
>don't need to find out an IP addr from a MAC for remote nodes on a
>regular basis. Instead, you find out a MAC addr from an IP address,
>which is ARP.
>
>  
>

Right, RARP won't satisfy the reverse lookup requirement being put 
forward, so I don't think it's relevant to this address resolution 
discussion.
 

>RARP is little used now that DHCP is popular.
>
>Now it would be nice for ethernet broadcast packets to just work(tm)
>with IPoIB. "ping -b" is an example of a user-level program that
>generates a broadcast packet.  DHCP clients also generate such
>packets, and DHCP servers listen for them. Getting a RARP client and
>server to work ought to be the same as a DHCP client and server.
>
>  
>

There is an I-D for DHCP on IB.  IPoIB defines a "broadcast" address and 
DHCP (and ARP) on IB use it.  Could make RARP work using this mechanism, 
but as someone else pointed out, the IB hardware address contains a 
QPN.  The I-D for IPoIB says something like:

    The link-layer address for IPoIB includes the QPN which might not be
    constant across reboots or even across network interface resets.
    Cached QPN entries, such as in static ARP entries or in RARP servers
    will only work if the implementation(s) using these options ensure
    that the QPN associated with an interface is invariant across
    reboots/network resets.

So, there are requirements on the IPoIB implementation to make RARP 
work.  Folks in the IPoIB work group decided not to go much further than 
these statements for RARP support since most folks felt that DHCP is (de 
facto) replacement.

-David


>-- greg
>
>
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