Subject: RE: [Sigtran] M2PA : LS messages on stream 0

From: [email-masked]
Date: Mon, 22 Jul 2002 14:59:02 +0100

Tolga,

"Tolga Asveren" <[email-masked]> on 22/07/2002 14:42:55

To:   [email-masked]
cc:    (bcc: Amish CHANA/EN/HTLUK)

Subject:  RE: [Sigtran] M2PA : LS messages on stream 0

> Can you provide a good example where missequencing between Stream
> 0 and Stream 1
> makes the delivery of MSUs less reliable.
[TOLGA] And can you give an example, where sending LS from stream0 increases
reliability of sending MSUs?

[Amish] As I mentioned numerous times already - if a message is corrupted, lost
or delayed on stream 1 the SCTP receiver will not withhold delivery of Link
Status messages. As yet I have not seen a "technical reason" from you as to why
missequencing between stream 0 and stream 1 is detrimental.

A.

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---

From: "Tolga Asveren" <[email-masked]>
Date: Mon, 22 Jul 2002 10:22:33 -0400

Amish,

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of
> [email-masked]
> Sent: Monday, July 22, 2002 9:59 AM
> To: Tolga Asveren
> Cc: [email-masked]
[TOLGA] And I replied several times, that SCTP is a perfect wire and it is
impossible that something SCTP delivers to M2PA is corrupted. You are mixing
what is happening at which layer and responsibilities of layers. Retransmits
on SCTP are transparent to M2PA. I think it is enough "technical reason",
that this approach is against the layered protocol model, at least in my
opinion. I hope somebody can also give the reasons, why it was decided to
use stream0 for LS messages at the beginning, maybe there were other reaons,
which I cannot think of for now. As I said earlier, justifying this approach
with 11.2 in Q.703 is not the correct interpretation of Q.703 in my opinion.
There, it is about when MTP2 sends/processes messages, the same is
applicable for M2PA, while it is sending and processing received messages. I
don't see this as a reason to use stream0 for LS messages.

BTW for FISU, well now I think after some time people might ask for it, but
for another purpose than in MTP2 case. In the draft it is stated that LSP
might be used through the slow start period. What will happen, after a link
didn't carry MSU for a while? During that time cwnd again will shrink.

---

From: "Tolga Asveren" <[email-masked]>
Date: Mon, 22 Jul 2002 11:28:00 -0400

And here an example where using stream0 for LS messages is not a good idea:
(although in my opinion, to have a model where responsibilities of layers
are not mixed is a more important argument)

A was sending Data to B. Then it decides to send LS OOS, and it is delivered
to the receiving M2PA before some Data messages, which were sent before LS
OO. So, those messages, which have already reached receiving SCTP
successfully won't be delivered to M2PA. So, after changeover they need to
be retransmitted. If sequence was preserved -like in the case with a
physicall wire-, there wouldn't be a need to retransmit those, because
during changeover, B would inform A that it received those messages.

(There is Retrieval state in M2PA, but I think this is just for M2PA buffers
and is not related with messages in SCTP)

    Tolga

> Tolga Asveren
> Sent: Monday, July 22, 2002 10:23 AM
> To: [email-masked]

---

From: Al Varney <[email-masked]>
Date: Mon, 22 Jul 2002 21:14:16 -0500

>are not mixed is a more important argument)

    I don't believe there is a "right" answer to this particular issue -- 
either one works well, considering the inadequacy of STCP to exactly 
emulate a "wire".  But since you are the one arguing "layer purity", I 
think you need reconsider your argument ... see below.

>during changeover, B would inform A that it received those messages.

    But the exact same thing can happen with SS7 MTP2 -- if you respect the 
protocol layering.  If A (MTP3 layer) sends "Data" to B, MTP2 will 
typically place "Data" in the transmit buffer of the specified link.  If A 
(MTP3 layer) then indicates "send LS OOS" for the same link, the MPT2 
priority rules state that LS OOS will be sent IN FRONT OF the "Data" 
message.  And thus "Data" will not be delivered to the far-end MTP3 -- nor 
will those messages in the re-transmit buffer (or SCTP equivalent).  (This 
presumes "Data" isn't in the process of being sent when the LS OOS 
indication arrives at MTP2.)

    So if you look at it from the MTP3 layer rather than the wire itself 
(Layer 1), you have the same potential out-of-order processing possible 
with both SS7 and with M2PA using Stream 0 for LS messages.  SS7 does NOT 
promise to deliver LSSUs and MSUs in the same order indicated by the 
sending MTP3.

    I don't claim there aren't other arguments for a single-stream for all 
M2PA data (simplicity of reading 1 stream vs. 2, for example).  I'm not 
sure they are sufficient to remove the ability to "expedite" LS messages 
using Stream 0.  (Presuming SCTP will send a new Stream 0 message before a 
whole buffer full of Stream 1 messages, that is.)

Al Varney

---

From: "Tolga Asveren" <[email-masked]>
Date: Tue, 23 Jul 2002 09:01:15 -0400

Al,

> Sent: Monday, July 22, 2002 10:14 PM
> To: [email-masked]
[TOLGA] The example I gave was not to show how the model is against the
layer purity. It was an example to show how a problem with sream0/stream1
approach might arise, which is non-existent with single-stream approach.
What I would call "layer purity" is:
1- Clear distinction of responsibilities between layers
Personally I would go with the idea expecting services provided by a
physical wire from SCTP. IMO, layer2 services should be provided either by
M2PA or by SCTP, but I wouldn't try to mix them. Now one could argue, what
about retransmissions of SCTP? But if SCTP is abstracted as a perfect
physicall wire with no message loss, I would consider them some procedure
belonging below layer2, which should be done by it, to behave as a wire with
no message loss. I would agree that SCTP is not a wire, but in that model,
the only services I would expect from SCTP as a "wire" are, to provide
sequenced delivery of messages and no message loss. IMO, SCTP does this
perfectly.
2- To mimic exactly MTP2 behavior
If messageA arrives after messageB with MTP2 to the remote MTP2, IMO the
same should happen with M2PA. IMO, we shouldn't go case by case and
investigate, what advantage/disadvantage changing order of messages in
certain scenarios will bring. Here, one would argue, what about
retransmission? It is not needed, because in that model, we have a perfect
wire with no message loss. If there was retransmit mechanism, it wouldn't be
used at all. Unfortuantely MTP2 does not have a non-error wire, so should
have retransmit mechanisms.

This is how I would model it (in fact it is already quite close to this,
except using two streams).

---

From: Al Varney <[email-masked]>
Date: Tue, 23 Jul 2002 11:14:11 -0500

At 09:01 AM 7/23/02 -0400, Tolga Asveren wrote:
> > From: Al Varney
> > >are not mixed is a more important argument.

>about retransmissions of SCTP?

    So SCTP is a perfect, but potentially very LONG wire -- M2PA's inputs 
to it arrive at the peer M2PA after a long, variable delay.  OK -- in that 
sense, M2PA should put LS messages on the same stream as MSUs.  And SCTP 
should have a buffer only big enough for one message, so LS messages can be 
inserted in front of the MSUs being buffered by M2PA.  Now THAT would mimic 
MTP2 pretty closely, assuming LS messages could be delivered to the 
receiving M2PA regardless of the correct receipt of a preceding MSU.

>same should happen with M2PA.

    My reply was to demonstrate that you were arguing the need to mimic 
MTP1-to-MTP2 behavior (not MTP2-to-MTP2), without recognizing that MTP3 
does NOT expect order preservation of MSUs and LS indications from 
MTP2.  That is, the sending MTP2 itself can scramble the order of the 
messages.  An argument that dual streams could do so is, in my opinion, not 
relevant.

    I think you could make the opposite "MTP2 behavior" argument:  MTP3 
sending an LS indication followed by an MSU will always result in the 
remote MTP3 seeing the LS indication first.  With dual streams, M2PA cannot 
assure that result -- SCTP might deliver the MSU, or several of them, 
before the LS indication.  (This doesn't apply to LS OOS, since it would be 
stupid for MTP3 to send an MSU to a link it had just put OOS.  But would 
apply to LS Busy, for example.)

Al Varney

---

From: "Tolga Asveren" <[email-masked]>
Date: Tue, 23 Jul 2002 13:26:57 -0400

Al,

> Sent: Tuesday, July 23, 2002 12:14 PM
> To: [email-masked]
[TOLGA] I disagree that SCTP should have a buffer of only one message, to be
able to assume that it behaves as a perfect wire. IMO, what is expected from
wire in that context is, when you send a message through it, the remote
user at the other end of the wire recives the message, without error and
with no missequencing.
> messages.  An argument that dual streams could do so is, in my
[TOLGA] I think you are right that it is not about MTP2-to-MTP2 behavior but
also not about MTP1-to-MTP2 behavior. IMO, it is about MTP1-to-MTP1
behavior.

---

From: "Tolga Asveren" <[email-masked]>
Date: Tue, 23 Jul 2002 13:35:46 -0400

Al,

One more comment below.

> receiving M2PA regardless of the correct receipt of a preceding MSU.
[TOLGA] I wouldn't consider retransmission on SCTP visible to M2PA. SCTP
provides the "perfect wire", when it delivers something to M2PA, there is no
chance that there is missequencing or error. Only when a message is ready at
SCTP to be delivered to M2PA, it can be considered as reached the other end
of the wire. Lets's say msg1 and afterwards msg2 is sent. Receiving SCTP
receives first msg2, this does not mean that msg2 has reached "the other end
of the wire", only after msg1 is received by SCTP, it can be considered like
this.