Subject: RE: [Tsvwg] SCTP query

From: [email-masked]
Date: Mon, 1 Oct 2001 15:45:02 +0530

Hi Armando

Section 4.2.2.17 of RFC 1122 states:

4.2.2.17  Probing Zero Windows: RFC-793 Section 3.7, page 42

            Probing of zero (offered) windows MUST be supported.

            A TCP MAY keep its offered receive window closed
            indefinitely.  As long as the receiving TCP continues to
            send acknowledgments in response to the probe segments, the
            sending TCP MUST allow the connection to stay open.

            DISCUSSION:
                 It is extremely important to remember that ACK
                 (acknowledgment) segments that contain no data are not
                 reliably transmitted by TCP.  If zero window probing is
                 not supported, a connection may hang forever when an
                 ACK segment that re-opens the window is lost.

                 The delay in opening a zero window generally occurs
                 when the receiving application stops taking data from
                 its TCP.  For example, consider a printer daemon
                 application, stopped because the printer ran out of
                 paper.

            The transmitting host SHOULD send the first zero-window
            probe when a zero window has existed for the retransmission
            timeout period (see Section 4.2.2.15), and SHOULD increase
            exponentially the interval between successive probes.

            DISCUSSION:
                 This procedure minimizes delay if the zero-window
                 condition is due to a lost ACK segment containing a
                 window-opening update.  Exponential backoff is
                 recommended, possibly with some maximum interval not
                 specified here.  This procedure is similar to that of
                 the retransmission algorithm, and it may be possible to
                 combine the two procedures in the implementation.

-----------------------------------------------------------------------------------------------------

I interpret from this text and your mail that a separate timer for probing the
receiver
window size should be kept. The Probing timeout should be increased
exponentially. But this should not affect RTO in any case.

Comments?

Vakul

"Armando L. Caro Jr." <[email-masked]> on 09/28/2001 06:24:40 PM

To:   [email-masked]
cc:   Vakul Garg/HSS@HSS, [email-masked]

Subject:  RE: [Tsvwg] SCTP query

On Fri, 28 Sep 2001 [email-masked] wrote:

> > After timeout, TSN = 50 would be retransmitted by SCTP1. As this
> > happens, exponential backoff algoritm is applied to recalculate the
> > value of RTO. Let again SACK with Cummulative TSN = 49 and arwnd = 0
> > arrive. Thus more timeout happen and we end up increasing the RTO
> > to some value RTO.max.
>
>    Yes...

Hmm... I couldn't find any text in the rfc which addressed this scenario,
but I think modifying the RTO due to dropped probes is the wrong thing to
do.

>    Well, there is congestion... Not at the network but at the receiver
> side... Its buffer is full... There are two kinds of congestion, the one
> that cwnd and ssthresh tries to avoid (congestion in the network) and the
> one rwnd tries to avoid (congestion at the receiver). In any of these two
> cases your transmission is not progressing and the datagrams are being
> dropped somewhere, so it makes sense reducing your sending rate as you are
> wasting bandwidth...

Yes... When the receiver's buffer is full, it makes sense to reduce the
sending rate, but NOT increase the RTO.  There is no relationship between
the status of the receiver's buffer and the RTT.

> > Please correct me where I am wrong.
>
>    I don't think you are wrong at all... Maybe somebody else can
> correct us... :-)

I don't think it is wrong to say that this is what would happened if the
rfc is followed. HOWEVER, I do think it is the wrong thing to do.

~armando

-----------------------------------------------------------------------
Armando L. Caro Jr.                                  [email-masked]
University of Delaware                   http://www.cis.udel.edu/~acaro
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From: "Armando L. Caro Jr." <[email-masked]>
Date: Mon, 1 Oct 2001 12:02:07 -0400 (EDT)

On Mon, 1 Oct 2001 [email-masked] wrote:
>
> I interpret from this text and your mail that a separate timer for
> probing the receiver window size should be kept. The Probing timeout
> should be increased exponentially. But this should not affect RTO in
> any case.
>
> Comments?

Yes, I agree.

~armando