| Hi Arun,        I didnt get one thing.........i have written 
      that thing below in the capital letters...   Regards, Vijay        Hi all, In the 
      draft-ietf-tsvwg-sctpimpguide-03.txt section 2.15 "Window probes in
 SCTP"....    we felt some contradictions....
 --------- 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.
     When the receiver's 
      advertised window is 0, the receiver MUST drop all new incoming DATA chunk and 
      immediately send back a SACK with
 the current receive window showing only 
      DATA chunks received and
 accepted so far.
         The above point 
      specifies that when we receive any Data chunks and the Rwnd is zero 
      then we have to drop those packets and  those chunks' TSN will
 not be included in the SACK sent........ Consider the following 
      example...
 * Sender sends a probing data chunk 
      of  "Tsn x" to Receiver when the Receiver
 has advertised the the 
      ARwnd as zero....
 * Receiver upon receiving this 
      chunk will check its Rwnd, Since the Rwnd is zero
 it will drop the chunk and sends 
      a SACK with CUM_TSN as " x-1" (not
 including "TSN x"....
 * Sender upon receiving this SACK cannot send 
      another "Zero Window Probe"
 because there is an OutStanding Data chunk ( this condition is from the 
      same
 section.. "Note that zero window probe SHOULD only be sent when 
      all
 outstanding DATA 
      chunks have been cumulatively acknowledged "...)  but its
 RTX timer will expire and 
      sends the  same Data chunk "TSN x" one more
 time...( note, that zero 
      window probe is no more used...) ...
 * Receiver 
      again drops the packet and sends a SACK as above......
  
        
       I THINK WHENEVER WE RECEIVE THE SACK WE SET THE ASSOC RETRANS COUNT AND 
      PATH RETRANS COUNT TO ZERO....MAY BE I M WRONG 
        
        
                   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 )
 ********************* 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...)
 *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). 
      )
     *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......
 
 
 Are the issues making sense... Atleast for me... 
      its totally confusing.....  The Probing ideas given in the draft is condradictory in the sense, the 
      Send side Alg  follows the
 TCP way, 
      but not the Receive side.... ( I guess in TCP,  when the ARwnd is 
      zero,
 it will still accept the data chunks 
      from the Sender.... anyway in TCP case the
 zero window probing will be done with "one byte", but in 
      SCTP a whole CHUNK
 is sent....)
 
 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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