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

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according rfc 4566 https://tools.ietf.org/html/rfc4566#page-22, m= line in sdp contains port:

m=audio 49170 rtp/avp 0

aren't ports determined ice candidates already? these contain ports well:

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

as rfc 5245 https://tools.ietf.org/html/rfc5245#page-9 states,

"ice extension offer/answer model, , works including multiplicity of ip addresses , ports in sdp offers , answers, tested connectivity peer-to-peer connectivity checks."

would mean port specified in m= line obsolete?

when using libjingle create new sdp offer, shows ip address of 0.0.0.0, port 9 (which "discard" port). not see port in m= line 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 know?

ip , candidates in m= line relict pre-candidate eras, fixed ips , ports used communication.

at least webrtc, specified in https://tools.ietf.org/html/draft-ietf-rtcweb-jsep-15 on page 33,

"the port value set port of default ice candidate m= section, given no candidates have yet been gathered, "dummy" port value of 9 (discard) must used, indicated in [i-d.ietf-ice-trickle], section 5.1.", and

"the m= line must followed "c=" line, specified in [rfc4566], section 5.7. again, no candidates have yet been gathered, "c=" line must contain "dummy" value "in ip4 0.0.0.0", defined in [i-d.ietf-ice-trickle], section 5.1.",

it in simple words specified, m= line contains dummy ip , port, long no candidates have been gathered , no media flow desired, , ip , port of default candidate thereafter.


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