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    RE: A Transport Protocol Without ACK



    
    
    Dear Mr. Cheng,
    
    I am baffled by your taxonomy. VI and I2O are basically interface (API)
    definitions and
    each of them has a different purpose. VI was ment to be a
    User-Space-to-User-Space
    communication interface and I2O well... a panacea for drivers.
    
    We are trying to build a protocol (to which an API is assumed but not
    mandated).
    
    Could you be a bit more specific in what you criticize or propose?
    
    Julo
    
    "Y P Cheng" <ycheng@advansys.com> on 19/09/2000 03:52:10
    
    Please respond to "Y P Cheng" <ycheng@advansys.com>
    
    To:   "'Ips@Ece. Cmu. Edu'" <ips@ece.cmu.edu>
    cc:    (bcc: Julian Satran/Haifa/IBM)
    Subject:  RE: A Transport Protocol Without ACK
    
    
    
    
    From: randall@stewart.chicago.il.us
    > I see no viable transport protocol here and I don't see this
    > conversation of any use unless you get exact details AND point
    > to a internet draft that defines EXACTLY how it works (or possibly
    > some other standards document).
    
    Both I2O and VI are transport protocols which define the format of a
    request
    to a transport service provider, i.e. an adapter card.  I2O is used but not
    limited to deliver SCSI requests and VI is used for any payload including
    IP
    packets.  VI is mapped into FC with the device headers between the FC
    header
    and data payload.  VI can certainly be used for delivery of SCSI requests
    too.  Both protocols require the service provider to have reliable delivery
    and reception.  VI defines different QoS.
    
    > > I don't claim any credit about this transport layer protocol. Every
    fibre
    > > channel and Infiniband adapter designer knows about this protocol --
    > > although there is no standard.  I am sure the TCP accelerator card is
    doing
    > > the same.  This protocol is a great alternative to the use of TCP/IP
    and
    > > should be incorporated into iSCSI.
    >
    > No it is not. You are not offering an alternative yet..
    
    I did not imply iSCSI should use I2O or VI.  In fact, the purpose iSCSI is
    to map SCSI requests into IP packets as well as to define the delivery .
    It
    seems to me that the working group has set its mind on TCP/IP and is
    believing this is the only solution.  The consensus seems if there is any
    other solutions that address flow control and congestion, it would end up
    like TCP/IP.  I am simply pointing out if we keep an iSCSI request as a
    single atomic transaction without separating it into the
    TCP/IP-stream-oriented Writes and Reads that each deals with a single DU,
    then, the deadlock problem goes away.  While the work group thinks we
    should
    take advantage the flow control and congestion management of TCP/IP, there
    are alternatives known as BB-credit and EE-credit management.  The fibre
    channel adapters make reliable delivery, lost packet detection, and
    retransmission without TCP/IP.
    
    Randall, you are right, I did not spent time to provide the working group a
    draft defining such transaction-oriented protocol.  All I have provided is
    an idea that besides TCP/IP.  The designers for SCSI and fibre channel
    adapters have solved the head-of-queue blocking, the congestion, and
    retransmission problems.  The transaction-oriented WRITE-REQUEST and
    READ-RESPONSE, in my humble opinion, allows us to implement iSCSI simpler
    than that of WRITE and READ stream requests.  The performance cost of
    requiring ACKs on every DU with size greater than MTU on a network with
    long
    latency is very expensive..  By defining a greater ACK granularity is an
    attempt to solve this performance problem.  If we do wish to ACK on every
    DU, then, on a long latency network, we must have a method to stream the
    PDUs to ensure the performance.  The method should not consume a large
    amount of memory space.  One should never ignore the TCP/IP
    memory-to-memory
    copy overhead when the backbone will be running at OC-192 speed in the near
    future.  Finally, please don't ever ask two NIC cards to synchronize with
    each other.  It is really hard to do as those of us in business of
    designing
    NIC cards can testify.
    
    Y.P. Cheng, CTO, ConnectCom Solutions Corp.
    
    
    
    
    


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Last updated: Tue Sep 04 01:07:01 2001
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