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Re: B2B proposal

  • From: Binod < >
  • To:
  • Subject: Re: B2B proposal
  • Date: Mon, 17 Jun 2013 09:41:50 +0530

"Likely" was not an accident :-)

It is not specified anywhere. Actually, as you say below in this thread, we should clarify that
and say it is a "must" to make it obvious for the developer.

thanks,
Binod.

On Friday 14 June 2013 06:15 PM, Strickland, Tom wrote:
" type="cite">
Likely to clone the SipServletRequest when the derived session is created? I've probably missed this (it wouldn't be the first time :-) but...
Where in the spec does it say that they should/may/must create the cloned request? Quoting from the spec:

--- Start-quote ---
6.2.3.2 Derived SipSessions
A derived SipSession is essentially a copy of the SipSession associated with the original request. It is constructed at the time the message creating the new dialog is passed to the application. The new SipSession differs only in the values for the tag parameter of the address of the callee (this is the value used for the To header in subsequent outgoing requests) and possibly the route set. These values are derived from the dialog-establishing message as defined by the SIP specification. The set of attributes in the cloned SipSession is the same as that of the original—in particular, the values are not cloned.
New SipSessions corresponding to the second and subsequent 2xx responses (or 1xx responses with To tags) are available through the getSession method on the SipServletResponse. The “original” SipSession of the request continues to be available through the original request object.


12.5 Original Request and Session Cloning
The incoming request that results in creation of a SipSession is termed as the original request, a response to this original request can be created by the application even if the request was committed and application does not have a reference to this request. This is necessary because the B2BUA application may require to send more than one successful response to a request. For example, when a downstream proxy forked and more than one success responses are to be forwarded upstream. This can only be required on initial requests, as only original requests shall need such multiple responses.

SipServletResponse
B2buaHelper.createResponseToOriginalRequest(SipSession session, int
status, String reasonPhrase) throws IllegalStateException;

The response thus generated MUST have a different "To" tag from the other responses generated to the request and must result in a different SipSession. In this (and similar) cases the container clones the original SipSession for the second and subsequent dialogs, as detailed in 6.2.3.2 Derived SipSessions. The cloned session object will contain the same application data but its createRequest method will create requests belonging to that second or subsequent dialog, that is, with a "To" tag specific to that dialog.
--- End-quote ---

Common sense would suggest that it would be useful if the container cloned the request, but I don't read this text as implying that the container may/should/must do this. I would say that it must, but there is nothing that I can lean on in the spec text as an app developer that would allow me to rely on that hope :-)

Tom



On 14 June 2013 12:55, Binod < " target="_blank"> > wrote:

>
> --- SipInviteRequest.getUnacknowledgedProvisionalResponse ---
>
> Why do we need to pass in the SipSession? If we do, it seems like this
> method should be on the SipSession as opposed to the request. Since it
> is on the request, I would assume this method would only return
> responses for this particular transaction, correct? And if so, the
> SipSession is not needed since the request you are operating on
> explicitly is associated with a SipSession, or perhaps I'm missing
> something...
>
> Version 2 of my proposal did have this method on the SipSession. I realised
> a problem with that as explained in the email that accompanied version 3.
>
> The rseq value in a reliable provisional is only unique for that dialog. Due
> to forking there can be several dialogs which return provisional responses.
> In order to locate a specific response at a UAC which is downstream of a
> forking proxy we need to pass the session (or derived session) along with
> the rseq to the method on SipInviteRequest.
>
> If we put the method on the SipSession there is still a problem with the
> uniqueness of the rseq value. PRACK can arrive after the corresponding ACK
> by which time a new INVITE transaction can be in progress which also has
> provisional responses. To retrieve the correct response we would need to
> pass it the requestId of the INVITE transaction as well as the rseq.
>
> It seems best to avoid cluttering the SipSession interface with methods such
> as this one.
Ok, I think you lost me but I guess my original question was also that
the request itself is uniquely associated with a particular sip
session (it cannot have two) so if you ask the request, the sip
session is implied. Right?

A SIP request is uniquely associated with a SIP dialog, which is uniquely associated with a SipSession. This all seems simple enough, until you consider forking. When downstream forking occurs, you will have more than one SipSession, but still only have one SipServletRequest. This is not so bad when you are acting as a UAC, but when you are running a B2BUA and acting as a UAS, what happens when you want to "fork back" to the caller?

    Alice          B2BUA          Proxy          Bob1          Bob2
 1. INV(a1)---------->
 2.                   INV(b1)------->
 3.                                 INV(b1)------->
 4.                                 INV(b2)--------------------->
 5.                                 <-------180(b1)
 6.                   <-------180(b1)
 7. <----------180(a1)
 8.                                 <---------------------180(b2)
 9.                   <-------180(b2)
10. <----------180(a2)

Here we have an example of parallel downstream forking. Assume that the B2BUA is an app running on a SipServlet container. Everything else is a normal SIP actor outside of the container. At step (9), the response from Bob2 leads the container to realise that forking has occurred, and so it creates a derived SipSession from the session used to send the initial INVITE to Bob in step (2). When we need to send the first response to Alice in (7), then the existing SipSession is used, but to create the second response in (10) requires a new SipSession to be created. After all this forking: Alice has two SipSessions, but only one INVITE SipServletRequest - when that SipServletRequest is queried for Alice each has one INVITE. When you call getSession() on the INVITE SipServletRequest, it will return the original SipSession. All quite weird.
Containers are likely to clone the SipServletRequest when the derived session is created.
So, though I see your point, we can not assert that there can be more than one sipsession
for a sipservlet request.

thanks,
Binod.


Anyway, I suck at the prack stuff so I'm going to go with whatever you
want as long as we make sure that the methods are clear, the
documentation is there and we don't run the risk of confusing
container implementors as well as users.
 
As for the PRACK stuff, we are discussing this and Keith is writing up a further proposal that is hopefully a little more clear when it comes to handling PRACK transactions from one INVITE transaction overlapping with a different INVITE transaction (and thus possibly overlapping with its PRACK transactions).

Tom

>>
>> --- SipSession.getActiveInvites ---
>>
>> What about getActiveXXX? i.e., what about all the other methods?
>
> Only INVITE transactions require an ACK, other transactions are complete
> once the final response is sent/received. Similarly only INVITES can have
> reliable provisional responses.

Sure, but until you get a final response the "other" type of requests
are still "active". And if you are after only whether or not you have
received an ACK or not, wouldn't a method following the other
getUnacknowledgedXXXX methods be more appropriate?

--
Tom Strickland, Software Developer, Thrupoint Software. Tel: +44 (0) 2920 005110
 
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--
Tom Strickland, Software Developer, Thrupoint Software. Tel: +44 (0) 2920 005110
 
--------------------
Note: The information contained in this message may be privileged and confidential 
and protected from disclosure. If the reader of this message is not the intended 
recipient, or an employee or agent responsible for delivering this message to the 
intended recipient, you are hereby notified that any dissemination, distribution or 
copying of this communication is strictly prohibited. If you have received this 
communication in error, please notify us immediately by replying to the message and 
deleting it from your computer. Thank you. Thrupoint, Inc.
nXaR2cC3






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