[Sigtran] M2PA questions on synchronization and message handling

HI,

here are three questions for M2PA RFC 4165:

1.	Synchronization after PO during Initial Alignment
the question is: is the synchronization in Initial Alignment identical to
synchronization in stable PO state in terms of exchanged messages and
procedures?
(at least – during Initial Alignment , there was no User Data messages sent,
so what is the purpose of such sequence?)

2.	when FLUSH primitive is received from MTP level 3 in M2PA, should M2PA:
a.	flush outgoing buffers
b.	flush both outgoing and incoming buffers?

Perhaps it is not clear are both points a) and b) listed in RFC section
4.1.4.  to be executed?

3.	Unexpected FSN and/or invalid BSN
The RFC states that 
„If M2PA receives a User Data message with an FSN that is out of order, M2PA
shall discard the message“. (RFC4165, section 4.2.1)

In addition to that, Q.703 states
„In the case that a message signal unit or fill-in signal unit is received
having a backward sequence number value which is not the same as the
previous one or one of the forward sequence number values of the signal
units available for transmission, the signal unit is discarded. The
following message signal unit or fill-in signal unit is discarded.
If any two backward sequence number values in three consequtively 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 retransmission buffer at the time that they are received, then
they are received, then level 3 is infomed that the link is faulty“
(section 5.3.1)

If M2PA is to follow all these statements that it would mean that reception
of message with invalid sequence number would cause transition to Out Of
Service state:

For example:
-	User Data message with either unexpected FSN or invalid BSN is received:
discard it
-	User Data with correct FSN (FSN=x) is received, it must be discarded due
to Q.703 statement
-	Nex User Data with FSN=x+1 is received. This is now unexpected FSN since
previously received message is discarded. Etc.

Best regards, Dario

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