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


Chronological Thread 
  • From: "Brian F. G. Bidulock" <bidulock AT openss7.org>
  • 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 07:02:00 2002
  • Dsn-notification-to: <bidulock AT openss7.org>
  • List-id: SCTP Discussions <discussion.sctp.de>
  • Organization: http://www.openss7.org/
Arun,

On Fri, 15 Mar 2002, Arun Prasad wrote:

>
> Hi Brian,
> Comments inline......
>
> "Brian F. G. Bidulock" wrote:
>>
>> Arun,
>>
>> The association does not time out because when doing a zero
>> window probe there is nothing to use to calculate RTT, so
>> a Heartbeat must be sent to the destination within the hearbeat
>> interval. When the Hearbeat Ack is succesfully returned, the
>> retransmission counts are cleared. If you use the recommended
>> values for heartbeat interval, RTO min, and path max retrans
>> you will not lose the association on a zero window probe of
>> indefinite length.
>>
> Ha, I got the point.....The thing which I missed is the
> HeartBeatAck and clearing of the Rtx counter..... I think the the first
> issue is cleared ... Now I could derive the solution for it... See if
> that is ok...
>
> * There is no need of maintaining a seperate timer for the Zero
> Window probing, as the T3 timer will automatically do this job....
> * There is no special processing required for the zero window
> probing except the following check, the rest can follow the procedures
> Rfc 2960...
>
> Check:
> *While trying to send a Data chunk (either new or Retransmitted
> one..) and if the SND.WND=0 (Effective SendWindow) then the Data
> chunk is sent to the peer only if there is no Outstanding
> chunks......... And by continuiuing this, will lead to the Rtx
> counter exceeding the Max.Retrans threshold.... then the path will
> become inactive and a HB is sent.... and when an HB-ACK is
> received.... the Path will again go to the ACTIVE state... and
> continue Retransmitting that Data chunk till the ARwnd is
> Non-zero....

Yup. However, the imp guide also allows the implementation to not
timeout the destination in the first place if the a_rwnd = 0. This is
necessary because heartbeats can be suppressed on a destination basis.

>
>> In your example with a receive buffer of 500 and an incoming
>> data chunk of 1500, it is typical to not discard this chunk,
>> but to process it (let the rwnd slop by 1000 bytes) and then
>> start advertizing a zero receive window. This way there is no
>> problem.
>>
> But the above point contradicts the implementation guide...
> If I have understood the below lines properly..... "The SCTP endpoint
> MUST always acknowledge the reception of each valid DATA chunk when the
> DATA chunk received is inside its receive window."

This does not say that the endpoint must discard anything. Nor does it
say that the endpoint is not permitted to ack chunks and slop the receive
window.

> But even the Receiver discards the data chunk and sends a SACK
> of ARwnd = non_zero_value..... the sender will still have the SND.WND as
> zero and will be doing the zero window probing....... The below is that
> scenario....
>
> * Sender sends a data chunk of size 1500 when its SND.WND=500
> (because one data chunk is allowed to be sent when the Rwnd is not
> sufficient...)
> * Receiver receiving this will discard the chunk as it has the
> Rwnd= 500..... And Sends the Sack as ARwnd = 500...

Don't discard the chunk. Accept it, SACK it, and advertize a_rwnd = 0.
IMO an implementation should never discard messages due to insufficient
receive buffer space without advertising a_rwnd = 0.

--brian

> * Sender upon receiving the SACK, will update its SND.WND as
> SND.WND = (500 - 1500) ( ARwnd - number of bytes in flight)
> SND.WND = 0
> * Then when the T3 timer expires, the Sender will have no
> OutStanding Data bytes and its SND.WND is zero...... So it will again
> resend this Data chunk....
>
> This continues as the first case (explained above in the first
> comment...), as usual
> T3 expires for N times, the path goes INACTIVE... and after
> sending and
> receiving the HB-ACK, again transmission continues......
>
> I think the above illustrations seems to solve the
> querries... If my assumption is not valid or if I have missed some point,
> please point out....
>
> Thanks
> -arun
>
>
>
> Both these procedures are provided for by RFC 2960 and the
> implementation guide. We just wanted to clarify some things
> that was not so clear in RFC 2960.
>
> --brian
>
> On Fri, 15 Mar 2002, 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......
> >
> >
> >
> >
> >
> > * 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
> >
> ****************************************************************
>
> --
> Brian F. G. Bidulock ¦ The reasonable man adapts himself to the
> ¦
>
> bidulock AT openss7.org
> ¦ world; the unreasonable one persists in
> ¦
> [1]http://www.openss7.org/ ¦ trying to adapt the world to
> himself. ¦
> ¦ Therefore all progress depends on the
> ¦
> ¦ unreasonable man. -- George Bernard Shaw
> ¦
>
> _______________________________________________
> tsvwg mailing list
>
> tsvwg AT ietf.org
> [2]https://www1.ietf.org/mailman/listinfo/tsvwg
>
> --
> ****************************************************************
> V.Arun Prasad
> HCL Technologies Ltd.
> 51, Jawaharlal Nehru Road,
> Ekkattuthangal,
> Guindy Industrial Estate,
> Chennai - 600097.
>
> Contact # : 9144 - 2334174
> 9144 - 2334181
> extn: 233
> ****************************************************************
>
> References
>
> 1. http://www.openss7.org/
> 2. https://www1.ietf.org/mailman/listinfo/tsvwg

--
Brian F. G. Bidulock ¦ The reasonable man adapts himself to the ¦
bidulock AT openss7.org
¦ world; the unreasonable one persists in ¦
http://www.openss7.org/ ¦ trying to adapt the world to himself. ¦
¦ Therefore all progress depends on the ¦
¦ unreasonable man. -- George Bernard Shaw ¦


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