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Re: [SCTP-Discussion] RE: [Tsvwg] contradiction in draft-ietf-tsvwg-sctpimpguide-03.txt


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  • From: Arun Prasad <arun AT netlab.hcltech.com>
  • To: discussion AT sctp.de
  • Cc: tsvwg AT ietf.org, sctpimp AT netlab.hcltech.com
  • Subject: Re: [SCTP-Discussion] RE: [Tsvwg] contradiction in draft-ietf-tsvwg-sctpimpguide-03.txt
  • Date: Fri Mar 15 04:52:00 2002
  • List-id: SCTP Discussions <discussion.sctp.de>
Hi Ivan,
    Thanks for the response....  Please see the comments inline...
 
[Arias-Rodriguez Ivan (NRC/Helsinki)] :-/

    What is then a "zero window probe"? I guess that the concept applies to any DATA chunk sent when rwnd is zero... So, it seems to me that you are doing a zero window probe here...

    Maybe the mistake comes from the fact that once the T3 timer expires, the DATA chunk is marked for retransmission and thus, not considered outstanding any more...

    Since it is not outstanding, you immediately make a zero window probe... I hope I'm not missing anything...
 
 

        I accept with you, that when T3 timer expires,  there will be no Outstanding
data, and the same data chunk will be retransmitted... as a zero window probe...
But when the procedure continues, the T3 timer will expire for say the Max.Retrans
threshold value and then the Path goes down (if its the only path Assoc goes down..)
The error counter will not be clear when SACK is received due to the following
condition....
Sec 8.2 of 2960:
    When an outstanding TSN is acknowledged or a HEARTBEAT sent to that
   address is acknowledged with a HEARTBEAT ACK, the endpoint shall
   clear the error counter of the destination transport address to which
   the DATA chunk was last sent (or HEARTBEAT was sent).
 

So I think it should be made clear....... My suggestiong are ...

    * When a Data Chunk is to be sent and (SND.WND = 0) then  the transmission
        should be considered "Zero Window Probe"  and T3 timer should not be
        started.....  Instead some timer say like "Persist Timer" should be started.
        When the Persist timer expires, the same data chunk should be again used
        for "Zero Wnd Probe".... this procedure should follow till the Peer advertises
        a non-zero window....
                            OR
    * When a T3 timer expires and the (ARwnd = 0) then the Rtx Counter should
        not be increamented  ( this ensures that the counter will not reach the threshold
        and make the Path INACTIVE)...    This ensures that during "Zero Window
        probe" the T3 timer will act as the "Persist timer"... and this will come to
        normal when the  the endpoint receives a SACK with non-zero ARwnd.....

            I think if we dont follow any of the schemes as above, we will end up in
mixing the Retransmission procedure with the Zero Windo Probe.. and Aborting the
Association... which should not be done......
 
 
 

 

     * Receiver again drops the packet and sends a SACK as above......

            This procedure continues till the Maximum Retransmission Counter goes
    off and the Association closes.... (According to section 2.15.2 "When a sender is
doing zero window probing, it should not time out the association if it continues to
receive new packets from the receiver", this again contradicts )

[Arias-Rodriguez Ivan (NRC/Helsinki)] I guess this question is answered by my previous comment...
 
 

 *********************
    Next issue is from the lines
"---------
   New text: (Section 6.2)
   ---------

   The SCTP endpoint MUST always acknowledge the reception of each
   valid DATA chunk when the DATA chunk received is inside its receive
   window."

       Considere the below scenario:
    * Sender sending a data chunk of size 1000 when SND.WND = 1500
    * Receiver receives the data chunk and sends a SACK wth  ARwnd = 500
        (It cant apply SWS avoidance here by sending a (ARwnd = 0), because that
           would violate SCTP principle that a window cannot shrink by moving the
            right edge...)

[Arias-Rodriguez Ivan (NRC/Helsinki)]
 

    *Sender receiving the SACK, will send another Data chunk of size 1500 when
      SND.WND = 500 ( this is allowed according to the RFC, "However,
      regardless of the value of rwnd (including if it  is 0), the data sender can always
      have one DATA chunk in flight to    the receiver if allowed by cwnd (see rule B
      below). )

[Arias-Rodriguez Ivan (NRC/Helsinki)] Yes, but this means that you are not supposed to surpass rwnd. You can always have one DATA chunk outstanding, but I think that the only special case when you can surpass the rwnd is when the received a_rwnd is 0 and you don't have any TSN outstanding...
 

        I think ur statement above contradicts the the statement from the draft.....
"However,   regardless of the value of rwnd (including if it  is 0), the data sender
can always have one DATA chunk in flight to    the receiver if allowed by cwnd"
    It says, "Including if it is 0".... which I think means... any ARwnd value even it is
zero.....  Have I misunderstood something here......
 
 
     *Receiver on receivin this data chunk will do the check as specified above
        ( data chunk's size is less than Rwnd), since this check fails, it will drop the
        packet and sends a SACK of ARwnd = 500....

            Here again we came to the same issue as the first one.... Here we never
go into "Zero Window Probing" as we never get ARwnd = 0......
 

[Arias-Rodriguez Ivan (NRC/Helsinki)] Yes, you are right... The point is that you shouldn't send a DATA chunk carrying more user data than the rwnd allows... unless it is completely impossible not to surpass this value, i.e. you make a 0 window probe...

        Again Iam to the same confusion.....  and couldnt get the solutions for my
doubt of "when the Receiver drops the Data chunk from the Sender because it
doesnt contain enough Rwnd.... and it contiues to advertise the small ARwnd (non
zero) it has....  Which will not trigger the "Zero window probe procedure" in the
Sender side...
        Not sure where Iam missing, but Iam missing something.....
 
 
Thanks
-arun

****************************************************************
V.Arun Prasad
HCL Technologies Ltd.
51, Jawaharlal Nehru Road,
Ekkattuthangal,
Guindy Industrial Estate,
Chennai - 600097.

Contact # : 9144 - 2334174
            9144 - 2334181
            extn : 233
****************************************************************
 


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