[OpenSER-Users] TM-module and ACK to 2xx (doesnt terminate txn?)

[Taisto Qvist]

Oups, I seemed to have my maillist-subscription disabled so I never
got the answers....

I seems like we dont agree on the interpretation of the txn handling
for invite. Sure, there is a timer for handling retransmitted responses,
but thats in earlier states than Terminated.
Sure, there is nothing that says that the inner workings of your 
transactions
must exactly behave as the statemachine diagrams in the rfc as long as the
external behavior stays the same, but still, I think that the ACK for 2xx
should not match the INVITE txn.
(It probably boils down to how rfc-isch you want to be.)

 >The end-to-end ACK establish a separate transaction (RFC 3261) and these
 >ACKs are not match as part of the INVITE transactions, but correlated
 >with them.

[TQ]
Where does it say that ACKs establish a separate transaction?
There is no defined ACK transaction, only INVITE/nonInvite, server and 
client.
The ACK is either a part of the INVITE txn, or its a separate request, 
but I would
never call it a standalone transaction.
The real purpose of txn's (in my view), is to enable/simplify forking, 
and to
handle retransmissions. Retransmissions of ACK and 2xx, are done by the 
UAC/UAS,
so there is no need for a ack-txn.

 >but even describe the wait timer. So, there is no contradiction.
[TQ]
I'd say there is. Where does it describe that this wait-timer should be
used for all responses, or for matching ALL acks?

My two main reasons for saying that ACKs for 2xx should not be matched,
come from the following two texts. (17.1.1.2, and 17.2.1)

17.1.1.2 (client invite txn)
   The client transaction MUST be destroyed the instant it enters the
 >  "Terminated" state.  This is actually necessary to guarantee correct
   operation.  The reason is that 2xx responses to an INVITE are treated
   differently; each one is forwarded by proxies, and the ACK handling
   in a UAC is different.  Thus, each 2xx needs to be passed to a proxy
   core (so that it can be forwarded) and to a UAC core (so it can be
   acknowledged).  No transaction layer processing takes place.
   Whenever a response is received by the transport, if the transport
   layer finds no matching client transaction (using the rules of
   Section 17.1.3), the response is passed directly to the core.  Since
   the matching client transaction is destroyed by the first 2xx,
 >  subsequent 2xx will find no match and therefore be passed to the
   core.

[TQ]
It even says "actually nessesary to guarantee...."
Since the txn's are there to (among other things) absorb retransmissions,
the receiving the 2xx MUST destroy the txn, so that when the next 
retransmission
of 2xx is received, it is not consumed by the txn layer.
This is what your txn-layer does for 3++ right? Any additional 3++ 
received on
a txn while waiting for Timer D to expire, will just be consumed, and 
the proxy
core will never have to process it.


17.2.1. (server invite txn)
The purpose of the "Confirmed" state is to absorb any additional ACK
messages that arrive, triggered from retransmissions of the final
response.
..snip...
Once the transaction is in the "Terminated" state, it MUST be destroyed
immediately.

[TQ]
The Confirmed state is there to absorb retransmissions, not the terminated.
When receiving 2xx you go directly to terminated bypassing confirmed.

Also, in 18.2.1 the text explicitly says:

   .. Note that when a UAS core sends a 2xx response to INVITE,
   the server transaction is destroyed.  This means that when the ACK 
arrives,
   there will be no matching server transaction, and based on this rule,
   the ACK is passed to the UAS core, where it is processed.

Thats faily clear right? :-)
The think that puzzles my a little bit though, is that you must have added
"extra" code for this matching...?
I mean, you're using the ;branch=z9xxxx for transaction matching right, if
its there? And the ACK for 2xx doesnt have the same branch, indicating a 
"new txn".

So matching ALL ACKs would imply that for any ACK that doesnt match a txn,
your really checking up Call-Id's, tags and stuff, just to be able to match
this ACK to the 2xx?

Or are you checking callid's, cseq's, tags, and stuff, for ALL requests 
in your
txn matching?

Now, I been working on a sip-stack the last few years myself, and I 
naturally
know that there is often a need to match the ACK, and the stack I've 
build has
similar functions, it just doesnt do it on the txn layer, where we try 
to be as
RFC-isch as possible.

What I would like, is simply have some function-parameters to the 
t_check_trans(<param>),
or even better in my little mind, a "modparam("tm", "ack_2xx_matching", 1)".

I am looking forward to hearing your reply, and in the mean time,
thanks to all of you developers for an extraordinary software :-)

Regards
Taisto Qvist

 > Hi guys,
 >
 > Just to bring some clarifications on the TM module.
 >
 > once a transaction is completed (negative or 2xx reply), it is put on
 > wait (wait timer - see RFC3261) for catching any potential
 > retransmissions of replies.
 > So, the transaction is completed, but not removed from memory - RFC does
 > not say that you need to trash immediately all the transaction
 > information, but even describe the wait timer. So, there is no
 > contradiction.
 >
 > The ACK (for 2xx)catching is done based on the completed INVITE
 > transaction (from wait timer) - nothing else.
 >
 >
 > Inaki, could you please detail what you mean by:
 >
 > <quote>
 > In my opinion OpenSer does a special treatment for ACK in tm mode, 
even if
 > they are for failed transaction (hop-by-hop ACK's) or succesfull INVITE
 > (end-to-end ACK's).
 >
 > </quote>
 >
 > Maybe I can explain you more if I understand you question....
 >
 > Regards,
 > Bogdan

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