First login?
Lost password?
 
 
sctp-discussion AT lists.franken.de 
SCTP Discussions

Text archives Help


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


Chronological Thread 
  • From: Randall Stewart <randall AT stewart.chicago.il.us>
  • To: Arun Prasad <arun AT netlab.hcltech.com>
  • Cc: discussion AT sctp.de, 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 13:47:41 2002
  • List-id: SCTP Discussions <discussion.sctp.de>
Arun:

Some early morning comments.. coffee firmly in hand :>

Arun Prasad wrote:
>
> 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......
>
>


Any time you send a chunk with the rwnd set to 0 this IS a
window probe. We probably need some additional clearification in
the implementors guide here.. When doing a zero window probe
then you should NOT count against the error thresholds.


>
>
> >
> >
> > * 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......
>

Actually I think that is quoted from the RFC.. and just reproduced
in the draft... i.e. it does not change.

Whenever you have a data chunk to send that exceeds the rwnd
(i.e. rwnd=500 and say my only data chunk is 1000bytes) and
no other data in-flight then you WOULD need to treat this send
as a window probe. Now the big difference with this one is that
the receiver IS allowed to accept it.

Also note the receiver and the sender SHOULD be doing SWS avoidance
so that hopefully this situation would be less likely.

Note because of this MUST discard when the rwnd is 0 provision we
do have an interesting scenario. Since you may want the next
chunk and if the rwnd is 0 you are required to drop the chunk.

What I did (at Michael Tuexen's nice suggestion) is that when
I kick in SWS avoidance I set my rwnd to 1 NOT 0. This then
allows me to accept the data (if it is the next TSN I want).
And of course my sender side SWS will consider 1 to be a rwnd
of 0... so this works very nicely :>

R


>
> >
> > *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
> ****************************************************************
>

--
Randall R. Stewart
randall AT stewart.chicago.il.us
815-342-5222 (cell phone)


Archive powered by MHonArc 2.6.19+.

Top of page