RTSP handshaking delay

[cctvcam]

I am using VLC to access an RTSP stream over the network. When I connect from a client via a firewall that has all ports open from client -> server I experience a startup delay of 12 sec. If I try the same from a client on the same subnet then the delay is only 1sec.

Monitoring the firewall I can see VLC is accessing 5 times on UDP port 4893 and getting a "Close - ICMP Unreach" log entries.

What is vlc trying to do on this port and would this cause delays through networks + firewalls?

The ping and trace routes are reporting <1ms response times, and only 1 hop. The network guys say its something at the application level.

[Rémi Denis-Courmont]

Because RTSP transmits data over UDP by default.

[cctvcam]

thanks for the reply, but why udp from vlc to server cause cause a delay through an open firewall/network rather than on a local subnet?

[Rémi Denis-Courmont]

Because network address translation and/or firewalling is blocking the UDP stream.