[asterisk-bugs] [JIRA] (ASTERISK-28686) chan_sip strictrtp=yes fails when media source is changed: no audio

[Michael L. Young (JIRA)]

    [ https://issues.asterisk.org/jira/browse/ASTERISK-28686?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=249342#comment-249342 ] 

Michael L. Young commented on ASTERISK-28686:
---------------------------------------------

Sorry, I misspoke about the purpose of "strictrtp" and what to expect from it.  It is in the title and sort of mislead me from the actual issue at hand.  I retract that statement.

Not sure if chan_sip is worth that much effort on my part anymore since I don't really use it.

The description of the issue is talking about an active session (audio) and changing the media source.  That is all that my comments were mainly focused on.  You are quoting from the SIP RFC 3261 which talks about sessions and modifying sessions.  What I am referring to is the SDP RFC 3264 which is very specific about modifying the SDP characteristics in an active session.

My point was that if we want to go down the path of history, there are several issues on this issue tracker where we have already had discussions around endpoints trying to change the media source on an active session and they do so by changing the "o=" line.  In those discussions the conclusion was that trying to change the "o=" line more than just incrementing the version number with an active session is not proper behavior.  Those conclusions are based on RFC3264 . We have directed people to use "ignoresdpversion=yes" if they so desire.

This just change just doesn't feel right but I am not going to declare that I am an expert or authority on this.

> chan_sip strictrtp=yes fails when media source is changed: no audio
> -------------------------------------------------------------------
>
>                 Key: ASTERISK-28686
>                 URL: https://issues.asterisk.org/jira/browse/ASTERISK-28686
>             Project: Asterisk
>          Issue Type: Bug
>      Security Level: None
>          Components: Channels/chan_sip/Interoperability
>    Affects Versions: 13.30.0
>            Reporter: Walter Doekes
>            Assignee: Walter Doekes
>
> Hi!
> When the SDP origin changes, the SDP session version from the new media is not necessarily higher than the existing session version.
> chan_sip contains this bit:
> {code}
>         if (ast_test_flag(&p->flags[1], SIP_PAGE2_IGNORESDPVERSION) ||
>             (p->sessionversion_remote < 0) ||
>             (p->sessionversion_remote != rua_version)) {
>                 p->sessionversion_remote = rua_version;
>         } else {
> ...
>                         ast_debug(2, "Call %s responded to our reinvite without changing SDP version; ignoring SDP.\n", p->callid);
>                         return FALSE;
> {code}
> That means that the updated SDP will be ignored for a change like this:
> - Initial o= line (for example in 183):
> {noformat}
> o=root 1845921041 1845921041 IN IP4 193.x.x.215
> {noformat}
> - Updated o= line (for example in 200):
> {noformat}
> o=- 1126144851 1126144853 IN IP4 87.x.x.235
> {noformat}
> In this example, the new SDP sess_version 1126144851 is lower than 1845921041, causing the SDP to be ignored. And because the SDP is not processed, {{ast_rtp_instance_set_remote_address}} is never called, and the new RTP source is discarded.
> The fix: do not only check sess_version, but also check the "unique parts" from the SDP. And if they change, then also update the SDP.
> Basically this:
> {noformat}
>         if (ast_test_flag(&p->flags[1], SIP_PAGE2_IGNORESDPVERSION) ||
> -           (p->sessionversion_remote < 0) ||
> -           (p->sessionversion_remote < rua_version)) {
> -               p->sessionversion_remote = rua_version;
> +                       sess_version > p->sessionversion_remote ||
> +                       strcmp(unique, S_OR(p->sessionunique_remote, ""))) {
> +               p->sessionversion_remote = sess_version;
> +               ast_string_field_set(p, sessionunique_remote, unique);
>         } else {
> {noformat}
> Along with a marginally bigger patch (to be put on gerrit), we then get these:
> {noformat}
> [2020-01-13 10:31:20] VERBOSE[10720][C-00000001] chan_sip.c:
>   Got SDP version 203027438 and unique parts [- 203027438 IN IP4 10.x.x.161]
> [2020-01-13 10:31:20] VERBOSE[10720][C-00000001] chan_sip.c:
>   Got SDP version 1845921041 and unique parts [root 1845921041 IN IP4 193.x.x.215]
> [2020-01-13 10:31:31] VERBOSE[10720][C-00000001] chan_sip.c: 
>   Comparing SDP version 1845921041 -> 1126144853 and unique parts [root 1845921041 IN IP4 193.x.x.215] -> [- 1126144851 IN IP4 87.x.x.235]
> {noformat}
> And then everything works as intended.



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