[erlang-questions] gen_tcp:send(Sock, SmallPacket) issue

Fredrik Thulin <>
Thu Jun 21 12:24:24 CEST 2007


Matthias Lang wrote:
> Fredrik Thulin writes:
> 
>  > The complexity comes from having to implement retransmissions
>  > yourself, and even though SIP has done just that, it hasn't solved
>  > all issues with flow control and messages larger than your typical
>  > MTU.
> 
> I'm aware of two MTU issues. One is that a fragmented UDP packet
> stands an uncomfortably high chance of being dropped (if any one of
> the fragments gets dropped, you lose the whole UDP packet). The other
> is that some networks discard fragmented packets. I never really
> understood why that is (paranoid firewalls?). Is there more?

Those are the ones I was thinking of.

In SIP, it is basically solved by mandating that you use TCP when a 
request approaches the MTU in size. Since requests grow in size for 
every node they pass, this might happen somewhere in the middle of the 
requests path from client to server.

This adds more complexity than one might think, because the responses 
are symmetrically routed back from server to client, so the node 
switching from TCP to UDP will have to do the reverse for responses 
coming back from the server. Might seem simple, but then there are 
different timeout values for TCP and UDP, since a single timeout 
wouldn't work well for both reliable and unreliable transports.

This part of SIP is way into the land of dragons and madness.

/Fredrik



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