Subject: [Sigtran] M2PA BSN dis-ordering

From: "Bibhuti Bhusan Tosh" <[email-masked]>
Date: Fri, 26 May 2006 11:09:32 +0530

Hi All

Kindly consider the following call flow and suggest me for the valid FSN and BSN values.

  A    B 
---------------------------- ----------------------------

MTP3  M2PA  SCTP                   SCTP  M2PA  MTP3 
----  ----  ----                   ----  ----  ---- 

User Data FSN=1 BSN=10 . .
     ------------------------------------> . . 
User Data FSN=2 BSN=10 . .
     ------------------------------------> . . 
User Data FSN=3 BSN=10 . . . . 
     ------------------------------------> . .
                              User Data FSN=11 BSN=3 . .
     <------------------------------------ . . . . 
                              User Data FSN=12 BSN=3 . .
     <------------------------------------ . . . . 
                              User Data FSN=13 BSN=3 . .
     <------------------------------------ . . . . 
User Data FSN=4 BSN=12 . . . . 
     ------------------------------------> . . . . 
                              User Data FSN=? BSN=? . .
     <------------------------------------ . . . . 
User Data FSN=5 BSN=11 . . . . 
     ------------------------------------> . . . . 
                              User Data FSN=? BSN=? . . 
     <------------------------------------ . . . . 

    Is there any impact of T7 timer expiry?

Regards,
Bibhuti

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---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Fri, 26 May 2006 02:20:02 -0600

Bibhuti,

If you are following Q.703, the handling is per that standard.  One
isolated abnormal BSN is ignored, but two consecutive abnormal BSNs
cause the link to be put out of service.  So at the end of your sequence
B sends FSN=14 BSN=4 and FSN=15 BSN=5, however, if another abnormal BSN
follows, OUT-OF-SERVICE is sent from B.

You can see this in test cases 3.8.10 and 3.8.11 in
draft-bidulock-sigtran-m2pa-test-06.txt and in Q.781/Test 8.10 and
Q.781/Test 8.11.  T7 is not impacted by the abnormal BSN per the Q.703
SDLs because the abnormal BSN does not negatively or positively
acknowledge any message.

But this is defined by SS7 MTP2 and not altered by M2PA: therefore, it
is of some advantage to learn how a traditional SS7 link works before
working with M2PA.

--brian

Bibhuti Bhusan Tosh wrote:             (Fri, 26 May 2006 11:09:32)

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

---

From: "Bibhuti Bhusan Tosh" <[email-masked]>
Date: Fri, 26 May 2006 14:30:09 +0530

Thanks Brian

    For the details. But on my point of view the first DATA with FSN = 4 and 
BSN =12 is valid one and endpoint B flushes all the packets upto FSN = 12 at 
its side. So when it again receives a packet with FSN=5(correct) and 
BSN=11(incorrect i.e. already flushed) then this packet will be ignored.

    As per you both the packets are invalid? Or only last packet is invalid. 
kindly clarify this.

Regards,
Bibhuti Bhusan Tosh

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Fri, 26 May 2006 04:00:01 -0600

Bibhuti,

Bibhuti Bhusan Tosh wrote:                                         (Fri, 26 May 2006 14:30:09)
> BSN=11(incorrect i.e. already flushed) then this packet will be ignored.

It needs to be processed because M2PA cannot negatively acknowledge the valid
FSN as would Q.703.

>     As per you both the packets are invalid? Or only last packet is invalid. 

Both packets are valid (they have a monotonically increasing FSN per RFC 4165
section 4.2.1), one simply contains an abnormal BSN.  See test case 3.8.10
and 3.8.11 in draft-bidulock-m2pa-test-06.txt.

--brian

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