[asterisk-bugs] [JIRA] (ASTERISK-25537) [patch] format-attribute module: RFC or internal defaults?

[Alexander Traud (JIRA)]

Alexander Traud created ASTERISK-25537:
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             Summary: [patch] format-attribute module: RFC or internal defaults?
                 Key: ASTERISK-25537
                 URL: https://issues.asterisk.org/jira/browse/ASTERISK-25537
             Project: Asterisk
          Issue Type: Bug
      Security Level: None
          Components: Formats/General
    Affects Versions: 13.6.0, 11.20.0
            Reporter: Alexander Traud
            Severity: Minor


For example, the format Opus has an attribute module registered thanks to {{res/res_format_attr_opus.c}}. When Opus is negotiated in SIP/SDP, {{ast_format_parse_sdp_fmtp}} calls this module to parse the line {{fmtp}}. However, {{fmtp}} is optional for many formats including Opus. Therefore, the attribute module might not be called in all SIP/SDP negotiations correctly.

Without a {{fmtp}}, default values are specified. These defaults describe the RTP payload which is brought to Asterisk for example by a VoIP/SIP client. Or stated differently: A VoIP client specifies its defaults to Asterisk. Asterisk negotiates between its internal defaults and the external {{fmtp}}. When a VoIP client does not send {{fmtp}} at all, the last sentence changes into: Asterisk negotiates between it’s internal defaults and the defaults of the RFC.

However, the defaults of the RFC might not be the internal defaults of Asterisk. For example, some RFCs are backward compatible:
* iLBC 20: Asterisk {{mode=20}} [RFC 3952|https://tools.ietf.org/html/rfc3952#section-4.2] {{=30}}
* AMR: Asterisk {{mode-change-capability=2}} [RFC 4867|https://tools.ietf.org/html/rfc4867#section-8] {{=1}}
* Opus: Asterisk {{useinbandfec=1}} [RFC 7587|https://tools.ietf.org/html/rfc7587#section-6.1] {{=0}}

Currently, I am not aware of any trick to determine whether a format was created because of external means (for example a VoIP client). Consequently, I am not able to apply defaults while doing {{format_get_joint}} or {{format_cmp}}, in case {{format_parse_sdp_fmtp}} was not called. Therefore, I am not able to negotiate correctly within SDP.



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