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

OK - a couple of misconceptions

IB Layer 3 is NOT IPv6.  It has this thing that looks ALMOST like an
IPv6 header - but it does routing completely differently.  Infact it
isn't until you do IB Routing that you would even put a GRH (Global
Route Header) that is what ends up looking like a IPv6 header.  If you
are staying on the local IB subnet - you are only required to put a LRH
(Local Route Header) that looks nothing like IPv6.

The reliability mechanism is not TCP therefore.  I will agree with
Michael on this one.  That said - the same problems that you discuss can
happen between the layer 4 IB and the Layer 4 TCP/IP.

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 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

Bill

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Margaret Wasserman
Sent: Saturday, December 11, 2004 4:47 AM
To: [email protected]
Subject: Re: [Ipoverib] comments on
draft-kashyap-ipoib-connected-mode-02.txt

Michael Krause wrote:
>No.  It uses InfiniBand RC or UC to communicate IP datagrams (v4 / 
>v6) between connected endnodes.

And what is InfiniBand RC, under the covers?

>Rest is based on a misconception as there is only IB below IP and no 
>tunneling of TCP over TCP occurs.

Well, I know that there is another IP(v6) in there, as IB is very 
closely based on IPv6.  Is RC based on TCP? SCTP?  Or did IB define a 
different reliable, connection-oriented IP protocol?  If the latter, 
what type of retransmission and congestion control algorithms are 
used by IB RC?  And, has there been any study regarding how they 
would interact with the retransmission and congestion control 
algorithms of TCP or other reliable upper-layer transports?

Even if this isn't TCP, per se, there may still be issues, but it 
will take more careful research to determine what they are.

Margaret



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