|Subject:||Re: [Linphone-developers] Outgoing call issue in iOS application in Linphne|
|Date:||Tue, 13 Aug 2019 11:21:08 +0200|
Thank you for your interest in Linphone.
However, it is not correct to say that Linphone is missing the a=rtpmap attributes for pcmu and pcma, as these attributes are not mandatory.
Please refer to RFC3264 [https://tools.ietf.org/html/rfc3264] An Offer/Answer Model Session Description Protocol, in section 5 "Generating the answer":
In the case of RTP streams, all media descriptions SHOULD contain "a=rtpmap" mappings from RTP payload types to encodings. If there is no "a=rtpmap", the default payload type mapping, as defined by the current profile in use (for example, RFC 1890 ) is to be used.
RFC1890 has been replaced by RFC3551 but still defines PCMU and PCMA with static payload type numbers 0 and 8.
Equipments that require rtpmap for static payload types are hence non-conformant with RFC3264.
In order to workaround this non-conformance, linphone has a build-time option to request rtpmap to be always used in SDP. Just pass -DENABLE_RTP_MAP_ALWAYS_IN_SDP=ON to the cmake command line (or prepare.py).
For your information, the reason why we decided many years ago not to honor the "SHOULD" for static payload types is simply to save bytes in the INVITE or 200 Ok length by removing redundant information. Indeed, there are still many use case where MTU limit is major problem, for example when interoperating with SIP gateways that only support SIP/UDP transport.
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