RTSP over UDP missing

[hanjo]

Which version are you using?

1.11.3

Which operating system are you using?

Linux amd64 Docker

Describe how to replicate the issue

Hi,
I'm using mediamtx as proxy for RTSP streams. I noticed that when I access such a proxied stream, VLC takes quite a long time for some streams to start the playback. Upon investigation, I noticed that VLC always connects through TCP to mediamtx, which it shouldn't, as the default settings for VLC are to use UDP and I did not ask the app to use TCP.
So I checked on the server side, and I noticed that mediamtx is not exposing the configued RTSP port through UDP.

Consider the following (standard) config:

###############################################
# Global settings -> RTSP server

# Enable publishing and reading streams with the RTSP protocol.
rtsp: yes
# List of enabled RTSP transport protocols.
# UDP is the most performant, but doesn't work when there's a NAT/firewall between
# server and clients, and doesn't support encryption.
# UDP-multicast allows to save bandwidth when clients are all in the same LAN.
# TCP is the most versatile, and does support encryption.
# The handshake is always performed with TCP.
rtspTransports: [udp, multicast, tcp]
# Encrypt handshakes and TCP streams with TLS (RTSPS).
# Available values are "no", "strict", "optional".
rtspEncryption: "no"
# Address of the TCP/RTSP listener. This is needed only when encryption is "no" or "optional".
rtspAddress: :8554
# Address of the TCP/TLS/RTSPS listener. This is needed only when encryption is "strict" or "optional".
rtspsAddress: :8322
# Address of the UDP/RTP listener. This is needed only when "udp" is in rtspTransports.
rtpAddress: :8000
# Address of the UDP/RTCP listener. This is needed only when "udp" is in rtspTransports.
rtcpAddress: :8001
# IP range of all UDP-multicast listeners. This is needed only when "multicast" is in rtspTransports.
multicastIPRange: 224.1.0.0/16
# Port of all UDP-multicast/RTP listeners. This is needed only when "multicast" is in rtspTransports.
multicastRTPPort: 8002
# Port of all UDP-multicast/RTCP listeners. This is needed only when "multicast" is in rtspTransports.
multicastRTCPPort: 8003
# Path to the server key. This is needed only when encryption is "strict" or "optional".
# This can be generated with:
# openssl genrsa -out server.key 2048
# openssl req -new -x509 -sha256 -key server.key -out server.crt -days 3650
rtspServerKey: server.key
# Path to the server certificate. This is needed only when encryption is "strict" or "optional".
rtspServerCert: server.crt
# Authentication methods. Available are "basic" and "digest".
# "digest" doesn't provide any additional security and is available for compatibility only.
rtspAuthMethods: [basic]

This will result in the following listeners:

# netstat -lnp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 :::8554                 :::*                    LISTEN      1/mediamtx
udp        0      0 :::8000                 :::*                                1/mediamtx
udp        0      0 :::8001                 :::*                                1/mediamtx
Active UNIX domain sockets (only servers)
Proto RefCnt Flags       Type       State         I-Node PID/Program name    Path

As you can see, ports 8000 (RTP) and 8001 (RTCP) are on UDP, as expected, but 8554 is only on TCP, where it should be on both TCP and UDP.
The description above for the rtspAddress setting does say it is for the Address of the TCP/RTSP listener, but how to configure the address for UDP/RTSP then?

Thanks for looking into this!

Server logs

No response

Network dump

No response

[jjbarbosa7]

The rtsp transports specify what mechanism(s) are supported in delivering the video, but the 'primary' connection (port 8554 in your case), which is required for rtsp signaling, always only uses tcp.

[hanjo]

So what you are saying is that the tcp port is used for signaling in any case, but the actual video payload could be send either over tcp or udp, correct?
I tried some configuration combinations and checked on the network traffic with tcpdump and wireshark, but I always only saw tcp traffic, both on the receiving end of mediamtx as well as the sending end of mediamtx.

[hanjo]

@aler9 I see you closed this as not planned - which is fair - but I still don't understand if mediamtx is generally not doing RTSP over UDP or if it is just my configuration that is causing this. Could you please provide your point of view?

[aler9]

@hanjo the RTSP protocol is not invented from scratch but follows specifications defined in RFC2326 and implemented in all major players. Handshake always happens with TCP, while transport happens with a protocol of choice. The same architecture is used by WebRTC. There is no RTSP-over-UDP.

If you have further questions about the protocol (since this is not a bug report) you can take a look at the specification and at the README, which describes available transports and how to use them.

[github-actions]

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