RTP Streaming using VLC - Getting huge transmit UDP packet loss on my PC - why?

Question

I am getting strange problems on my Windows 7 PC: I have tried to setup VLC media player to stream audio through RTP, however I am getting huge outbound UDP packet lost.

After some debugging, I observed, my PC can absolutely fine receive the same streamed packets from a different PC (no inbound UDP loss), however when my PC is set-up to transmit the stream, I get over 26% UDP packet loss. Even when PCs are connected using a straight piece of CAT5 cable.

From some Wireshark poking, it seems my PC does not even send those packets, at all. Something is preventing them to be sent. What might be causing it? I have no clue.

Thank you for any hints.

//EDIT: I forgot to add, it doesn't matter if the stream is unicast or multicast. Results are the same.

Answer 1

Finally, I have found a solution: As I was worried from the beginning, the problem was indeed in Windows. For some reason, by default the IP stack uses only 8 kByte buffers. (Hilarious, isn't it?) That is not enough for the few kB UDP bursts that VLC produces.

There is a possibility, to increase the IP stack send/receive buffer sizes, using registry keys. For some reason, my machine was missing these keys completely, so the size was the default one.

To change the send/receive buffer sizes, update (or create) this registry key. In my case setting a 64 kB buffer sizes fixed the problem. No UDP outbound loss now.

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \Afd \Parameters]
DefaultReceiveWindow = 65536
DefaultSendWindow = 65536

Both values are DWORD (32bit), if you need to create those.

(I have also tinkered with the buffer size settings in network controllers, that has however lead nowhere.)