RE: comments on draft-kashyap-ipoib-connected-mode-02.txt

Hi Bill,

At 6:53 AM -0800 12/11/04, Bill Strahm wrote:
>OK - a couple of misconceptions

I am sorry for my limited understanding of Infiniband...  Hopefully 
we all have enough information between us to reach the right answers 
here...

>I do not believe there will be a problem however because the timers are
>on such a different scale.  Let me try and give an example... There are
>retries in Ethernet (I am talking about collision detection and back
>off) - What if we decided to worry about TCP retransmitting because the
>Layer 2 packet hasn't been able to get on the wire and the TCP timers
>went off and started retransmitting ?  People don't worry because the
>timers are SO different - this will be even more pronounced in the IB
>world.

I think I understand your point about timers, but I don't really 
understand how this works...  Does IB RC actually retransmit lost 
packets?  Or does it just have some type of collision-detection & 
back-off mechanism, like Ethernet?  If it retransmits, how can it do 
that without waiting for at least one round-trip-time to know that 
data has been lost?  And, if it doesn't retranmit, how can it 
guarantee reliability if data is lost?

Ethernet does not actually retransmit packets... an Ethernet device 
will detect a collision during transmission of the packet preamble 
and wait a very short period before trying again.  This time is much 
shorter than the Ethernet round-trip-time, so it is guaranteed (if I 
understand this correctly) to be shorter than the smallest possible 
TCP retransmission timer (which will continuously decrease if things 
are going well until it gets quite close to the round trip time for 
the TCP connection).

The fact that Ethernet does not actually retransmit is also 
important, as you don't need to worry about retransmissions at the 
Ethernet layer exacerbating a congestion condition.

>I don't think we will be able to move the MTU above 64K - just because I
>believe the IP packet size is (was ??? Has the size parameter gone up ?)
>64K so don't worry about someone trying to stick a 2G IP packet onto the
>wire.  At the same time if it is possible to specify a single IP packet
>that is 2G, maybe we need some text staying the MTU should be limited to
>something sane

IPv6 includes a Jumbo Payload hop-by-hop option which allows the 
transmission of IPv6 packets with payloads of >64K bytes.  RFC 2147 
(TCP and UDP over IPv6 Jumbograms) defines how to use TCP and UDP 
with IPv6 Jumbograms.  I don't now if either of these mechanisms is 
widely implemented, though.

Margaret