usrsctp send buffer does not free itself

Question

We're working with a C++ webrtc data channels library and in our test application, upon sending a few small packets that would totally amount to about 256kB, the usrsctp_sendv() call returns -1 (with errno as EWOULDBLOCK/EAGAIN which means "Resource is temporarily unavailable"). We believe this is because we're hitting the usrsctp's send buffer limit, which is 256 kB by default. We've tried adding several sleep delays in between each send call hoping it clears that buffer, but nothing works.

The receiving side, (a JS web page) does indeed receive all the bytes that we've sent up until it errors out. It's also worth noting that this only happens when we try to send data from the C++ application to the JS and not the other way around. We tried looking around mozilla's datachannels implementation, but can't seem to draw any conclusions on what the issue could be about.

Answer 1

It is hard to answer such question straight away. I would start looking into wireshark traces in order to see if your remote side (JS page) actually acknowledges data you send (e.i. if SACK chunks are sent back) and what is the value of received buffer (a_rwnd) reported in these SACKs. It might be possible that it is not an issue on your side, but you are getting EWOULDBLOCKS just because sending side SCTP cannot flush the data from buffers because it is still awaiting for delivery confirmation from remote end. Please provide more details about your case, also if this is possible provide sample code for your JS page.