Subject: Re: [Sigtran] M2PA:Questions on sequence number

From: Tom George <[email-masked]>
Date: Mon, 26 Aug 2002 16:13:47 -0500

Since M2PA sends acknowledgements on both the Status and Data streams,
it seems possible that an acks could contain a BSNR that is out of
order, or that contains a sequence number for a message that has been
removed from the transmit buffer because of a previous ack (on a
different stream).

Should we abandon this BSNR test for M2PA, or restrict it to the BSNRs
received on a single stream? 

tomG.

On Fri, 24 May 2002 Lisa Gorgone wrote:    
> > Hi all,
> > We have some questions about M2PA:
> >
> > 1)About BSNR, Q703 states:
> >
> > "If any two backward sequence number values in three consecutively received
> > message signal units or fill-in signal units are not the same as the
> > previous one or any of the forward sequence number values of the signal
> > units in the retrasmission buffer at the time that they are received,
> > then level 3 is informed that the link is faulty."
> >
> > Does this apply also to M2PA or BSNR is used just to acknowledge the MSU (ie
> > the link is never failed when the BSNR is wrong)?
  [...] 

"Brian F. G. Bidulock" replied:
> I take the same approach as abnormal BSNR in Q.703.  I believe it is the
> rational approach and should be included in the draft.

-- 
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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Mon, 26 Aug 2002 15:53:58 -0600

Tom,

I never considered acks on the status stream: is there any reason
to allow acks on status stream at all?  IMO acks SHOULD only be
sent on the data stream.

--brian

Tom George wrote:                                                                      (Mon, 26 Aug 2002 16:13:47)

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/

---

From: Tom George <[email-masked]>
Date: Mon, 26 Aug 2002 17:32:28 -0500

What if M2PA has to ack a message, but has not User Data to send? I had
dealt with that by having M2PA send a Link Status In Service message.

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Mon, 26 Aug 2002 17:05:50 -0600

Tom,

A separate ACK message (or message with no data payload) would
solve that.

--brian

Tom George wrote:                                                                      (Mon, 26 Aug 2002 17:32:28)

---

From: "Tolga Asveren" <[email-masked]>
Date: Tue, 27 Aug 2002 09:08:02 -0400

Tom,

I also think such an ACK should be sent from data stream. Otherwise, some
other LS message might get stuck behind those LS In Service messages, in
case of retransmission (isn't this the whole idea behind multiple streams
afterall, although personally I still think it is violation of a clean,
layered architecture?)

    Tolga

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of
> Brian F. G. Bidulock
> Sent: Monday, August 26, 2002 7:06 PM
> To: Tom George
> Cc: [email-masked]
> Subject: Re: [Sigtran] M2PA:Questions on sequence number
>
>
> Tom,
>

---

From: Tom George <[email-masked]>
Date: Tue, 27 Aug 2002 13:58:12 -0500

Brian and others,

I like the Data message with no data in it. It would just have the M2PA
common header and the M2PA specific header. It would be easy to
recognize because of the message length.

Anshoo suggested removing the M2PA specific header (FSN/BSN) from the
Link Status message altogether. I could go either way. If it is ignored
all the time, it seems like a good idea remove it. Unless it helps for
the Data and Status messages to have the same headers.

tomG.

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Tue, 27 Aug 2002 13:11:46 -0600

Tom,

I agree with using a data message with no data in it (it is
strikingly similar to a FISU anyway).  However, leaving the
M2PA specific header in the Link Status messages would allow
Tolga to resynchronize LS against the data stream if that
should be really necessary to an implementation.

--brian

Tom George wrote:                              (Tue, 27 Aug 2002 13:58:12)

---

From: Tom George <[email-masked]>
Date: Tue, 27 Aug 2002 14:52:16 -0500

OK, I will leave the FSN/BSN header in the Link Status message. 

I can get rid of the hapless Link Status In Service message, since its
only purpose was to carry an ack.

tomG.

---

From: Tom George <[email-masked]>
Date: Tue, 27 Aug 2002 15:11:26 -0500

Now that all acks are going over the Data stream, let's get back to the
question that started this discussion. Now BSNR values should not get
out of order. 

What is the purpose of the "2 out of 3 consecutive messages" criterion?
It would seem that if one bad BSNR is received, that should be enough to
identify a problem.

tomG.

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Tue, 27 Aug 2002 14:20:56 -0600

Tom,

I can still see a purpose for the procedure: if one is directly bridging
an MTP2 link to an M2PA link.  The BSNR values from the MTP2 link are not
necessarily always in order regardless of SCTP's transport abilities.

--brian

Tom George wrote:                                                                      (Tue, 27 Aug 2002 15:11:26)

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Tue, 27 Aug 2002 14:23:13 -0600

Tom,

It can go.  It might be worthwhile re-labeling LS "Ready" back to
"In-Service" after it is removed.

--brian

Tom George wrote:                                                                      (Tue, 27 Aug 2002 14:52:16)

---

From: Tom George <[email-masked]>
Date: Tue, 27 Aug 2002 15:36:12 -0500

I want to keep the name for Link Status Ready. When M2PA sends that
message, it is not necessarily true that the link is "in service" at
that point. Some other things have to happen. 

And it prevents someone confusing it with the Link In Service primitive
from M2PA to MTP3. 

tomG.

  [...]

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Tue, 27 Aug 2002 15:05:57 -0600

Tom,

That's ok, it was just a thought.  It used to be called In Service a
few drafts ago.

--brian

Tom George wrote:                             (Tue, 27 Aug 2002 15:36:12)

---

From: Tom George <[email-masked]>
Date: Tue, 27 Aug 2002 17:22:22 -0500

Brian,

Good point. I will go with the 2-out-of-3 criterion.

tomG.