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    Re: [rddp] Re: iSCSI/iWARP drafts and flow control

    On Thursday, July 31, 2003, at 07:43 PM, Mallikarjun C. wrote:
    >> No new wire protocol is required.
    > Please explain to me how credit can be replenished
    > in the following example - without new positive flow
    > control protocol.
    I thought of at least one algorithm overnight. This is
    not presented as a definitive proposal, merely to show
    a strategy that can allow iSER to provide flow control
    of *all* untagged messages.
    iSER untagged messages are divided into a predictable
    portion regulated by CmdSNs in a very direct fashion
    and asynchronous messages. The latter category can be
    characterized as having a low sustained rate compared
    to the CmdSN-related untagged messages, but that the
    traffic is very bursty. That is, the peak rate can be
    That suggests adapting a classic "leaky bucket" credit
    system of the type typically used to regulate burst
    traffic over rate controlled networks. The key
    difference is that the "clock" is the Max CmdSN.
    What is required is the following:
    The sender maintains a asynch-credit counter. It is
    initialized to a known value. This value could be
    negotiated if it is believed that there is enough
    variation to warrant negotiation, otherwise it would
    be fixed. That caveat applies to all other "constants"
    cited in this algorithm.
    When the ULP desires to send an untagged message using
    one of these credits:
         The credit count is brought up to date: The
         current Max CmdSN is compared with the value when
         the prior "fringe" send was performed. The delta
         is used to grant new credits, however there is a
         maximum number of credits that may be accumulated.
         If there are enough credits: the credit count is
         reduced and the untagged message may be sent.
         If there are not enough credits: the untagged
         message must be delayed until the Max CmdSN is
         advanced. After a configurable delay, iSER
         SHOULD send some form of NOP command to cause
         Max CmdSN to advance.
    Note that there are NO iSER defined messages for
    the purpose of flow control.  Untagged messages
    may be delayed, but that could happen over TCP
    just as easily. The only change is that *only*
    untagged messages will be delayed, never tagged
    messages, and *where* they are delayed (iSER
    versus TCP).


Last updated: Tue Aug 05 12:46:07 2003
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