[asterisk-bugs] [JIRA] (ASTERISK-21673) Early media not properly handled on outbound TCP trunk

[Michael L. Young (JIRA)]

    [ https://issues.asterisk.org/jira/browse/ASTERISK-21673?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=205785#comment-205785 ] 

Michael L. Young edited comment on ASTERISK-21673 at 4/24/13 5:20 PM:
----------------------------------------------------------------------

Well, it is hard to answer.  I did a bunch of research before replying and I found comments indicating that Lync, for whatever reason, has decided to do something that most commercial implementations do not do; send 183 without SDP.  I even found a Microsoft forum post where someone was asking about this and the response was basically "is the application Lync certified?  Make sure to only use Lync certified apps."  I am paraphrasing that.

I could not find anything stating that 183 _has_ to contain SDP.  I think that is where the sticky point is.

There are some PBX servers out there that send 183 without SDP kind of like a "keepalive" while it is Trying.  That is why Asterisk currently ignores 183 without SDP due to one of those servers according to the comment in the code.  Everything I could find expects 183 to have SDP and I could not find anything stating how to handle 183 without SDP.  I even found where perhaps some PBXes send 183 with SDP after a 100 Trying and then afterwards send 183 without SDP.  If Lync was doing that, then we could handle that easily.

Here is how understand things based on what I have read and going back to the source, that being RFC 3261.