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

At 09:19 AM 12/10/2004, Margaret Wasserman wrote:

>Hi All,
>
>I also have some comments on draft-kashyap-ipoib-connected-mode-02.txt.
>
>I haven't thoroughly reviewed the document, so I may have some further 
>comments later, but in my initial read, one major concern jumped out at me.
>
>If I understand correctly, the reliable connected mode of IPOIB is 
>TCP/IPv6.  Is that correct?

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

>If so, I think that you need to carefully consider the issues associated 
>with tunneling TCP inside of TCP.  In particular, I am concerned about 
>this section:
>
>        The default MTU of the IPoIB-CM interface is 2044 octets i.e.
>         2048 octet IPoIB-link MTU minus the 4 octet encapsulation
>         header.
>
>         The connected modes of InfiniBand allow message sizes up to 2^31
>         octets.  Therefore, IPoIB-CM can use a much larger MTU for
>         unicast communication between any two endpoints. At the same
>         time the maximum and/or optimal payload that can be received or
>         sent over an InfiniBand connection is dependent on the
>         implementation, HCA and the resources configured.
>
>Having a larger MTU at the higher-level TCP layer than at the lower-level 
>TCP layer may lead to situations where the upper and lower layers' 
>retransmit timers will both fire at the same time, causing real problems 
>in the case of congestion.
>
>This may happen because bothTCP layers will use the same algorithm (and 
>maybe the same code) for calculating the retransmission time. When a 
>packet is lost, the lower layer's TCP retransmission timer will fire, 
>causing a retransmission of one lower-layer MTU of data (2048 bytes), AND 
>the upper layer's TCP retransmission timer will also fire, causing a 
>retransmission of one upper-layer MTU of data (up to 2^31 bytes).  The 
>lower layer will not recognize that the upper layer's retransmission is 
>duplicate data, so this will result in the transmission of many 2K packets 
>when a single 2K packet is lost.
>
>We may want to talk to the Transport area to determine if there are other 
>issues associated with tunneling TCP inside of TCP/IP.

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

Mike


>Thoughts?
>
>Margaret
>
>
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