Why does the m= line in sdp contain a port? Isn't this redundant with candidates?

Question

According to RFC 4566 https://www.rfc-editor.org/rfc/rfc4566#page-22, the m= line in an sdp contains a port:

m=audio 49170 RTP/AVP 0

Aren't ports determined by ICE candidates already? These contain ports as well:

a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host

As RFC 5245 https://www.rfc-editor.org/rfc/rfc5245#page-9 states,

"ICE is an extension to the offer/answer model, and works by including a multiplicity of IP addresses and ports in SDP offers and answers, which are then tested for connectivity by peer-to-peer connectivity checks."

Would that mean the port specified in the m= line is obsolete?

When using libjingle to create a new sdp offer, it shows an IP address of 0.0.0.0, together with port 9 (which is the "discard" port). So it obviously does not see the port in the m= line as relevant:

m=audio 9 UDP/TLS/RTP/SAVPF 111 103 9 102 0 8 106 105 13 127 126
c=IN IP4 0.0.0.0
a=rtcp:9 IN IP4 0.0.0.0

Does anybody know?

Answer 1

IP and candidates in the m= line is a relict from pre-candidate eras, where fixed IPs and ports were used for communication.

At least with WebRTC, as specified in https://datatracker.ietf.org/doc/html/draft-ietf-rtcweb-jsep-15 on page 33,

"The port value is set to the port of the default ICE candidate for this m= section, but given that no candidates have yet been gathered, the "dummy" port value of 9 (Discard) MUST be used, as indicated in [I-D.ietf-ice-trickle], Section 5.1.", and

"The m= line MUST be followed immediately by a "c=" line, as specified in [RFC4566], Section 5.7. Again, as no candidates have yet been gathered, the "c=" line must contain the "dummy" value "IN IP4 0.0.0.0", as defined in [I-D.ietf-ice-trickle], Section 5.1.",

it is in simple words specified, that the m= line contains a dummy IP and port, as long as no candidates have been gathered and no media flow is desired, and the IP and port of the default candidate thereafter.