[asterisk-dev] [Code Review] Fix issue of unrecognized inbound ACK when Asterisk responds to an INVITE with a 481

[Mark Michelson]

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Review request for Asterisk Developers.


Summary
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When Asterisk receives what it believes to be a dialog-starting INVITE with a to-tag, Asterisk will reply with a 481. There was an issue where Asterisk would not recognize the incoming ACK of the 481 because Asterisk's internally generated to-tag did not match the actual to-tag used in the transaction. My initial fix for this was to make Asterisk send its generated to-tag in the 481 to essentially "change" the to-tag of the transaction. This way, the UA would respond with an ACK with a matching to-tag and all would be happy.

Unfortunately, it was pointed out to me that just because Asterisk thinks that an INVITE is out-of-dialog doesn't make it so. For instance, Asterisk could crash with a SIP dialog up, be restarted, and then receive a reinvite for that dialog. The new Asterisk instance would have no knowledge of the dialog, so it would think the INVITE was out-of-dialog with an illegitimate to-tag. In this case, the sender of the reinvite would not recognize the 481 as being in-dialog and would thus continue retransmitting the reinvite until the transaction timed out.

The new approach here is to update Asterisk's local tag to be the to-tag present in the INVITE in the case that Asterisk will be transmitting a 481 response. Taking this approach meant having to change the local tag to be a string field instead of a static buffer and rewriting the make_our_tag() method.


This addresses bug ASTERISK-19892.
    https://issues.asterisk.org/jira/browse/ASTERISK-19892


Diffs
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  /branches/1.8/channels/chan_sip.c 368625 
  /branches/1.8/channels/sip/include/sip.h 368624 

Diff: https://reviewboard.asterisk.org/r/1977/diff


Testing
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I have tested using Walter Doekes's 481 test. The 481 that Asterisk sends has the same to-tag as the INVITE, and Asterisk properly recognizes the inbound ACK.


Thanks,

Mark

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