Subject: [Sigtran] M2PA I-D version 8, section 4.2.1

From: David Lehmann <[email-masked]>
Date: Wed, 21 May 2003 15:39:32 -0400

In section 4.2.1, it states,
	"For message types other than User Data, the
	Forward Sequence Number is set to the FSN of
	the last User Data message sent."

Why would we do this if, as it states two sentences later,
	"M2PA SHOULD ignore the FSN and BSN contained in a
	Link Status message."

-- 

David Lehmann                          Ulticom, Inc.
AOL/Yahoo IM: davidULCM                1020 Briggs Road
1-856-787-2729                         Mt. Laurel, NJ 08054   USA

_______________________________________________
Sigtran mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/sigtran

---

From: Tom George <[email-masked]>
Date: Wed, 28 May 2003 10:24:41 -0500

Jeff Craig asked me more-or-less the same question a while back. I am
still thinking. I am open to suggestions. 8-(

tomG.

-- 
* Tom George       PB7-24001 (near B-21)      [email-masked]  *
* 972-519-3168   Intranet:  http://www.ssd.usa.alcatel.com/~tgeorge/  *
* Alcatel;  3400 West Plano Pkwy,  MS PB7-SWPRO;  Plano, Texas 75075  *
* I actively disclaim speaking for Alcatel USA, Inc. I do not purport *
* to advertise or represent its position,  operations, or activities. *
_______________________________________________
Sigtran mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/sigtran

---

From: David Lehmann <[email-masked]>
Date: Wed, 28 May 2003 14:37:06 -0400

>>        "M2PA SHOULD ignore the FSN and BSN contained in a

Well, if you are going to ignore the values, you could remove
the "M2PA-specific Header" and add the BSN/FSN to the data
message header for clarity:

2.1.5  Message Length

The Message Length defines the length of the message in octets,
including the Common Header.

2.2 M2PA Messages

The following section defines the messages and parameter contents.  An
M2PA message consists of a Common Message Header followed by the data
appropriate to the message.

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                     Common Message Header                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                         Message Data                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2.1 User Data

The User Data is the data sent from MTP3. The data is an optional
field. It need not be included in an acknowlegement-only message.

The format for the User Data message is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     unused    |                      BSN                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     unused    |                      FSN                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                            Data                               /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2.1.1 Backward Sequence Number (BSN)

This is the FSN of the message last received from the peer.

2.2.1.2 Forward Sequence Number (FSN)

This is the M2PA sequence number of the User Data message being sent.

The FSN and BSN values range from 0 to 16,777,215.

--

---

From: Tom George <[email-masked]>
Date: Wed, 28 May 2003 14:47:58 -0500

Before removing them, it would be a good idea to ask what information
can be gotten from them. What can the receiver validly infer from the
FSN and BSN in a LS or empty User Data message?

FSN: 

The receiver can use the FSN to tell when the message was sent in
relation to non-empty User Data messages. If a LS or empty UD message
has FSN=38, it was sent _after_ the non-empty UD with FSN=38. 

If a LS and empty UD message arrive with the same FSN, there is no way
to tell which was sent first. 

If two LS's arrive with the same FSN, the FSN does not tell you which
was sent first. But since SCTP delivers them in the same order they were
sent, M2PA knows which was sent first.

BSN:

The receiver could use the BSN in an LS just as it is used in non-empty
UD messages. But some care would have to be taken because there could be
some acks received out of order and some messages acked twice. 

The big question is can we use the BSN to resynch the sequence numbers
as in Q.703. It looks possible, but I am not sure yet. But of course if
we do, the statement about ignoring the FSN/BSN in 4.2.1 will have to be
modified. 

tomG. 

> message header for clarity:
  [...]