[asterisk-bugs] [JIRA] (ASTERISK-26221) chan_sip: iLBC does not include correct mode

[Aaron Meriwether (JIRA)]

    [ https://issues.asterisk.org/jira/browse/ASTERISK-26221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=231525#comment-231525 ] 

Aaron Meriwether commented on ASTERISK-26221:
---------------------------------------------

I need to test further, but I think it should work fine in conjunction with your 20ms patch.  I also suspect your 20ms patch alone may not solve our issue in all cases, since behavior will still be technically undefined if Asterisk does not advertise any particular framing. (e.g., if client advertises 20ms and Asterisk advertises nothing, some clients might expect Asterisk to transmit 30ms and others might expect 20ms).

> chan_sip: iLBC does not include correct mode
> --------------------------------------------
>
>                 Key: ASTERISK-26221
>                 URL: https://issues.asterisk.org/jira/browse/ASTERISK-26221
>             Project: Asterisk
>          Issue Type: Bug
>      Security Level: None
>          Components: Channels/chan_sip/CodecHandling
>    Affects Versions: 13.9.1
>            Reporter: Aaron Meriwether
>            Assignee: Aaron Meriwether
>            Severity: Minor
>         Attachments: asterisk-13-ilbc-sdp.patch
>
>
> As of Asterisk-13, the format/channel handling was overhauled.  Bug ASTERISK-25309 was introduced as a result and eventually closed, but the fix for that bug was not complete.
> The old (pre-Asterisk-13) behavior was that whenever iLBC would be listed in an SDP message, it would be qualified with "mode=30" because Asterisk supports only the 30ms framing of iLBC.  As of Asterisk-13, the new format/channel handling overhaul caused Asterisk to offer "mode=20" in some cases, resulting in some connections negotiating an unsupported framing.  ASTERISK-25309 addressed this by removing the mode value entirely, which resolved the issue for some SIP clients, but because the mode is omitted entirely from the Asterisk side, if the client offers mode=20 and Asterisk does not override that offer with a mode=30 offer, some clients take that as an invitation to begin transmitting iLBC-20 data.
> ASTERISK-25309 cites RFC 3952 section 5 as justification for removing the "mode=" offer, however if that specification is read carefully, it can be seen that it only defines the "always use iLBC-30" behavior when at least one side offers mode=30.  If one side offers mode=20 and the other makes no framing offer, behavior is undefined.
> The attached patch causes iLBC negotiation to return to its pre-Asterisk-13 behavior of always offering mode=30 regardless of the other side's offer.  It does this by reinstating the "mode=" output, and taking the "framing" value from "ast_format_get_default_ms" rather than "ast_format_cap_get_format_framing".  This also causes "ptime" to be output correctly, and does not preclude using a patched iLBC-20 format module (as hard-coding the value in chan_sip.c would have).



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