Hi,
I am trying to stream RTP over IPv6 link-local multicast, but despite
everything I am doing I cannot get ffmpeg to receive it. I write here
because I've ran out of options.
To transmit the stream I use the following, with the interface in the
local address:
ffmpeg -f alsa -channels 1 -sample_rate 8000 -i default -c:a pcm_mulaw
-f rtp
"rtp://[ff12::acab:cafe:beef:caca]:10000?localaddr=fe80::d65d:64ff:fe81:7095%enp4s0"
And to receive it, I had to patch libavformat/udp.c to join the
multicast group in the correct interface provided by addr.sin6_scope_id
(PR#21439). The following was used, with the interface specified in the
rtp address:
ffplay -protocol_whitelist file,udp,rtp
rtp://[ff12::acab:cafe:beef:caca%enp4s0]:10000
I could confirm packets were being emitted in the enp4s0 interface with
the correct destination. And I could see ffplay bind() to the right
address and joined the right multicast group as confirmed by ss and ip:
# ss -6a
udp UNCONN 0 0
[ff12::acab:cafe:beef:caca]%enp4s0:10000
[::]:*
udp UNCONN 0 0
[ff12::acab:cafe:beef:caca]%enp4s0:10001
[::]:*
# ip -6 maddress
2: enp4s0
inet6 ff12::acab:cafe:beef:caca users 2
I was also able to receive the RTP packets if I replicated this bind()
and join behaviour in a minimal C example program:
$ ./ipv6_multicast_recv ff12::acab:cafe:beef:caca%enp4s0 10000
�{��g��%{vtup��������������������������������������������~�������������������������������������������������������������������������������|��g��%���������������������������������|z{|zz||xyvrrsponmnmlnnnpostuvyy{�~{������~���~�������������������������������}|~}z{zy|yy{y{|{{�}��hE�%��}�������������������������������������~~~}|xwxwtpoooqomlklljjjikjjkklmmmnmorpnqqrtuy|z|�}�������~}zzzxwwwrrpnnppprsqpppqprpopo�~��hy�%pqprpprtvwxxy|}|~
[...]
But whatever I did, I could not get ffplay (or ffmpeg) to receive the
multicast packets. I have tried -loglevel trace but nothing of use appears.
I have confirmed that global multicast works, with routable addresses,
such as this (note the ff1e:: scope prefix):
ffmpeg -f alsa -channels 1 -sample_rate 8000 -i default -c:a pcm_mulaw
-f rtp "rtp://[ff1e::acab:cafe:beef:caca]:10000?localaddr=fd7c:3f3e:c7e2::1"
ffplay -protocol_whitelist file,udp,rtp
rtp://[ff1e::acab:cafe:beef:caca]:10000
So clearly IPv6 multicast works. The problem is somewhere in the
handling of link-local multicast group addresses. But I am not familiar
enough with the libav source to debug this, I have ran out of options here.
Also for some reason I cannot observe any bind() call in ffplay with
strace, strange.
Thank you for reading, any help appreciated!
Angel
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