[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]

The following issue has been UPDATED. 
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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:                     new
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 11:36 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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 (0125897) ebroad (manager) - 2010-08-12 11:36
 https://issues.asterisk.org/view.php?id=17849#c125897 
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"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"

That is correct, see the last 20 lines or so of add_sdp() in chan_sip. 

"This allows for streams to be matched up based on their order." does
imply that the order must match, though the wording of "The answer MUST
contain exactly the same number of "m=" lines as the offer." could have
been less vague if it had has been worded as "The answer MUST contain
exactly the same number and order of "m=" lines as the offer."

my .02

elazar   

Issue History 
Date Modified    Username       Field                    Change               
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2010-08-12 11:36 ebroad         Asterisk Version         1.6.2.11 => SVN     
2010-08-12 11:36 ebroad         Note Added: 0125897                          
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