Re: I-D Action:draft-ietf-tsvwg-sctp-strrst-08.txt

["t.petch" <[e-mail]>]

My previous, somewhat lengthy, worked example was intended to show that yes, it
works, but that care is needed and that perhaps more description is needed in
the I-D.

Quoting from the I-D
"When a peer endpoint requests a Incoming SSN Reset Request it is
   possible that the local endpoint has just sent an Outgoing SSN Reset
   Request on the same stream and has not yet received a response.  In
   such a case the local endpoint SHOULD silently discard the request
   and continue processing any other request found in the Stream Reset
   Chunk."
This will I think fail.  If the local endpoint silently discards the request,
then peer endpoint will never receive a Stream Reset Response Parameter
and the peer will timeout, perhaps taking down the association.  So while it
can work, I do not think that the I-D has it quite right.

Tom Petch

----- Original Message -----
From: "Randy Stewart" <[e-mail]>
To: "t.petch" <[e-mail]>
Cc: "Michael Tuexen" <[e-mail]>; <[e-mail]>
Sent: Wednesday, December 01, 2010 6:09 PM
Subject: Re: I-D Action:draft-ietf-tsvwg-sctp-strrst-08.txt


Tom:

Some comments in-line...

On Dec 1, 2010, at 1:13 AM, t.petch wrote:

> Um; I was expecting perhaps a little more on race conditions.
>
> I had a more complex race condition in mind, which will work, I have
> convinced myself of that, provided everyone expects it to:-)
>
> A appl requests a reset of streams 2 and 3
> A sends Incoming SSN Reset Request Parameter for streams 2 and 3 and starts
> timer
> A receives Outgoing SSN Reset Request Parameter for streams 1 and 2 with zero
> Stream Reset Request Sequence Number
> A sends ACK and  has a think about what to do .....
> - waits for Outgoing SSN Reset Request Parameter for streams 2 and 3?
> - resets streams 1 and 2?
> - raises an event for streams 1 and 2?


Each implementation has to keep a queue of stream reset actions. Basically
consider..

A user does

stream-reset (2,3)
followed by
stream-reset (4,5)

the reset of 2/3 will not be finished when the socketoption (thats how we did it
anyway) is
called on 4-5.. thus the implementation has to keep a queue of requests that
have not been sent.

So its always possible to queue a stream reset.

As to your scenario...

There would be nothing stopping A of responding right away to the Outgoing Reset
request. The
incoming request is outstanding, but it can respond to the outgoing request..
which it would.


>
> Meanwhile,
> B appl requests a reset of streams 1 and 2
> B sends Outgoing SSN Reset Request Parameter for streams 1 and 2 and starts
> timer
> B receives Incoming SSN Reset Request Parameter for streams 2 and 3
> B has timer running so must wait
>

On the other side, B sends his request and then gets the request for the
incoming.
It then adds this to its queue of things to do... and waits its response..

It gets A's reset response.. and then processes its queue sending the
outgoing reset request for 2/3.

> Therefore
> A must reset streams 1 and 2 and send Stream Reset Response Parameter to let B
> proceed

It of course would.. there is nothing stopping it from doing so unless of
course it deny's the request (which is allowed).

> B receives Stream Reset Response Parameter and stops timer
> B can and must now send Outgoing SSN Reset Request Parameter for streams 2 and
3
> with non-zero Stream Reset Request Sequence Number and starts timer
> A receives Outgoing SSN Reset Request Parameter for streams 2 and 3 with
> non-zero Stream Reset Request Sequence Number and stops timer
> A resets streams 2 and 3 and sends Stream Reset Response Parameter
> B receives Stream Reset Response Parameter and stops timer
>
> A appl and B appl both receive events, one for reset of streams 1 and 2, the
> other for streams 2 and 3 (which may puzzle them).
>


Well thats true... but App's are going to have to either coordinate there
reset requests or be able to handle getting a reset on 1/2 followed by
another reset 2/3...


> Of course, a slightly different scenario would be for both A and B to select
the
> same streams, say 1 and 3 (perhaps as a result of an application level event)
in
> which case, the scope for getting it wrong is even greater; when A receives
> Outgoing SSN Reset Request Parameter with zero Stream Reset Request Sequence
> Number, it might send an Error - Bad Sequence Number, which could really
confuse
> B
>

Why would it do that?

In this scenario you have exactly the race condition that I mentioned in the
new version of the document. When B goes to process the "queued" request for 1/3
it
finds they are both already at 1/3... so it could easily send the "success - no
action required" in
response.

The sequence number is unique to each side. I.e. Side A has a number it is
sending and a different
number it is expecting from B..

I.e. (lets use different numbers than 0)

A;

Next_sending_SReset = 1
Next_Expected_SReset_from_peer = 10

B would have:

Next_sending_SReset = 10
Next_Expected_SReset_from_peer = 1

So A does

-----In-Reset(1/3)OSeq=1------->

B does:

<----Reset(1/3)OSeq=10,RSeq=0)-----

Now

A would do reset 1/3 and do

-----Reset-Response(Seq=10)------>

B would now find its queued request and do either (per the draft)

-----Reset-Response(Seq=1)-sucess:nothing to do---->
<or>
-----Reset(1/3)OSeq=11, RSeq=1)----->

Its choice...

R



> Tom Petch
>
> ----- Original Message -----
> From: <[e-mail]>
> To: <[e-mail]>
> Cc: <[e-mail]>
> Sent: Monday, November 29, 2010 6:30 PM
> Subject: I-D Action:draft-ietf-tsvwg-sctp-strrst-08.txt
>