[asterisk-bugs] [Asterisk 0017849]: Asterisk does not properly align SDP "m=" lines when answering an SDP offer (provoking a T.38 negociation issue)

[Asterisk Bug Tracker]

A NOTE has been added to this issue. 
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https://issues.asterisk.org/view.php?id=17849 
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Reported By:                frawd
Assigned To:                
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Project:                    Asterisk
Issue ID:                   17849
Category:                   Channels/chan_sip/General
Reproducibility:            always
Severity:                   major
Priority:                   normal
Status:                     acknowledged
Asterisk Version:           SVN 
JIRA:                        
Regression:                 No 
Reviewboard Link:            
SVN Branch (only for SVN checkouts, not tarball releases): N/A 
SVN Revision (number only!):  
Request Review:              
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Date Submitted:             2010-08-12 10:23 CDT
Last Modified:              2010-08-12 11:56 CDT
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Summary:                    Asterisk does not properly align SDP "m=" lines when
answering an SDP offer (provoking a T.38 negociation issue)
Description: 
It appears that Asterisk generates SDP replies with "m=" lines always
aligned in the same order:
1. m=audio
2. m=video
3. m=text
4. m=image

When it should reply with the same order that was in the initial offer per
RFC-3264 (page 8):

[QUOTE]For each "m=" line in the offer, there MUST be a corresponding "m="
line in the answer. The answer MUST contain exactly the same number of "m="
lines as the offer. This allows for streams to be matched up based on their
order. This implies that if the offer contained zero "m=" lines, the answer
MUST contain zero "m=" lines.[/QUOTE]

This could potentially provoke many interoperability issues, one of which
is presented below.
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 (0125898) frawd (reporter) - 2010-08-12 11:56
 https://issues.asterisk.org/view.php?id=17849#c125898 
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Thanks for giving me arguments to put responsabilities to Huawei, but come
on...

The older SIP/SDP RFC (RFC-2543) says:
"The caller and callee align their media descriptions so that the nth
media stream ("m=" line) in the caller's session description corresponds to
the nth media stream in the callee's description."


The new SDP RFC (RFC-3264) implies it quite strongly:
page 8:
- "[...] there MUST be a corresponding "m=" line in the answer."
- "This allows for streams to be matched up based on their order."

See section 6 "Generating the Answer" and 8 "Modifying the Session" from
the same RFC, always referring to the "corresponding" m= line.


Other RFCs, for example RFC-5888 refer to 3264 saying:
"[RFC3264] describes the usage of SDP in text of SIP. The offerer and the
answerer align their media description so that the nth media stream ("m="
line) in the offerer's session description corresponds to the nth media
stream in the answerer's description."

Also get a look in the "SIP implementors" mailing list, a few threads talk
about this.

I mean, come on! :-D 

Issue History 
Date Modified    Username       Field                    Change               
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2010-08-12 11:56 frawd          Note Added: 0125898                          
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