Re: [Serusers] what makes a transaction?

At 05:20 PM 2/27/2003, Greg Fausak wrote:

I'm trying to understand stateful vs. stateless transactions.

A stateless 'transaction' simply lasts as long as it takes to process the current sip request. It is then forgotten.

yes

However, a stateful transaction I'm having a little problem grasping.

it holds the original requests and for a while the received replies too. That's good when you wish to correlate these, for example for purpose of accounting, serial forking to somewhere else or parallel forking. none of these things works with "forward", you need "t_relay". Stateful proxy also carries out the retransmission job on behalf of the sender. See also http://www.iptel.org/ser/doc/seruser-html/x740.html#AEN839

For instance, if I t_relay() an INVITE, logically the transaction is started....so when the response comes it isn't delivered to the main route {} block, but instead it is delivered to the awaiting stateful t_relay(), probably keyed by callid. So, for instance:

Roughly like that. t_relay establishes a transaction, but it does not block. Replies are keyed through via/branch parameter.

INVITE <- 401 Unauthorized ->

Is this entire sequence a transaction? Right after this the UA sends an ACK, this is a transaction unto itself? ACK <-

If ser challenges, it does so statelessly to scale better. It would have to remember all unathenticated requests for some time (not good for your memory). When the 401 reply is lost and INVITE is retransmitted, ser challenges newly from scratch -- that costs more effort but less memory. If it was stateful, it would just resend the cached reply.

INV--> compute and forget 407<-- ACK-->

The stateful case occurs when ser acts as proxy server and relays the replies coming from downstream for requests previously processed using the t_relay family. ACK for negative replies is part of transaction (unlike ACK for 200). Its purpose is tranport confirmation of negative replies, which are retranmitted until ACK is received. They are local -- proxy confirms on behalf of client.

UAC SER/proxy UAS INV--> t_relay --> <--407 ACK--> (generated by ser to stop 407 retransmissions from downstream) <--407 ACK--> (generated by UAC to stop 407 retranmissions from ser in the middle)

The UA then sends :

INVITE <- 100 Trying -> 180 Ringing -> 200 OK ->

All of these messages are a 'transaction' handled by t_relay()?

yes

The next transaction is:

ACK <-

Yes. ACK for 200 is an interesting thing -- it does not belong to a transaction per specification. It is a request which is always processed statelessly.

Finally, the last transaction would be (as an example):

BYE <- 200 OK ->

So the ser route{} block would need to recognize and handle

   INVITE
   ACK
   BYE

in this context. Is that correct? If I was running 'statelessly' I would need to handle and send every single message listed?

You need to handle all of these messages if you plan to deploy VoIP services, and you should not forget CANCEL either. It does not matter whether statelessly, or statefuly unless you need some of the things above (accounting, forking, ...)

-Jiri