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Unread 10-30-2007, 06:35 PM   #1
telenerd
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Default Another misrouted ACK

I believe I have found another variation on the misrouted ACK theme. It involves a call from a SIP UA through a PSTN gateway and the chosen carrier has optimize=yes. The familiar symptom is the call being dropped after 30 seconds.

The call goes astray during the re-INVITE. I think the UA at the gateway end is not in compliance with the following section from RFC 3261 ----

12.2 Requests within a Dialog
Once a dialog has been established between two UAs, either of them MAY initiate new transactions as needed within the dialog. The UA sending the request will take the UAC role for the transaction. The UA receiving the request will take the UAS role. Note that these may be different roles than the UAs held during the transaction that established the dialog.
Requests within a dialog MAY contain Record-Route and Contact header fields. However, these requests do not cause the dialog's route set to be modified, although they may modify the remote target URI. Specifically, requests that are not target refresh requests do not modify the dialog's remote target URI, and requests that are target refresh requests do. For dialogs that have been established with an INVITE, the only target refresh request defined is re-INVITE (see Section 14). Other extensions may define different target refresh requests for dialogs established in other ways.
Note that an ACK is NOT a target refresh request.
Target refresh requests only update the dialog's remote target URI, and not the route set formed from the Record-Route. Updating the latter would introduce severe backwards compatibility problems with RFC 2543-compliant systems.

Another way of saying that is once a dialogue is established the signalling path used by subsequent requests within the dialogue remains fixed. The route set was determined by the Record-Route's that the UAS collected from the original INVITE and forwarded to the UAC in the 200 OK. This applies only to establishing a dialogue and further Record-Route's within the dialogue should be ignored. Section 12.1.1 of the RFC describes establishing a dialogue.

How does this relate to my problem?

UA's can behave differently with regard to the 200 OK that they send to the UAC in response to a re-INVITE. Some forward the list of Record-Route's to the UAC just the same as if a dialogue were being established and others do not. The RFC to my mind is not clear on this point but it is clear that a re_INVITE should not alter the route set.

I believe the UA associated with the gateway is using the Record-Route list or the absence of one to update its route set during a re-INVITE. If there were no Record-Route's it is creating an empty route set. If there is a proxy in the signalling path, the ACK will not reach the UAS and the call will be dropped. If there is a RR list and it has not changed then no harm is done even if it is used to update the route set.

I am attaching a trace of a call that fails as described above.

It should be noted the incoming calls from the gateway to this particular UA do not fail.
I do not know if it is significant but when the call is incoming the gateway acts as UAC for the initial INVITE and the re-INVITE. On an outgoing call the gateway acts first as an UAS and then switches to UAC mode for the re-INVITE.
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Unread 10-30-2007, 07:09 PM   #2
telenerd
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Default

My file was not attached. Perhaps to big.
Attached Files
File Type: txt re-invite.txt (16.3 KB, 9 views)
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Unread 11-08-2007, 05:39 PM   #3
telenerd
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Hi Martin, are you back from VON?
How is it going with our effort to make Voxalot/Sipbroker the best voip service in the world?
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Unread 01-31-2008, 07:51 PM   #4
telenerd
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Hi martin
There is now a fix in asterisk that will prevent this trouble.
See bugs.digium.com bug #11545
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