Unreliable Unix Domain DGRAM socket?

[Robert Watson]

On Fri, 7 Apr 2006, Qiao Yang wrote:

> Does anyone know what possible scenarios may cause the Unix Domain DGRAM 
> socket to lose packet? We know, in theory, it is unreliable, but in reality, 
> I rarely see packet loss happens to this type of socket. Unfortunately, I 
> believe it happened a few times in the last couple of months in a FreeBSD 
> 5.3 box and I wasn't able to track down what the cause was.
>
> One related question. Considering the following setup, client (socket_A) 
> send a packet to server (socket_B). If the sending buffer of socket_A is ok 
> but the receiving buffer of socket_B is full, will sendto() return error?

Yes -- there are several situations where packets can be lost.  The most 
typical one is that there is insufficient room in the receive buffer. 
uipc_usrreq.c:uipc_send() calls sbappendaddr_locked() to append to the remote 
receive socket:

         if (space > sbspace(sb))
                 return (0);

uipc_send() converts this to ENOBUFS.  Other loss scenarios include EMSGSIZE 
if the message is too large.  If sendto() is called with the flag 
MSG_DONTWAIT, you can also get ENOBUFS due to mbuf allocator exhaustion.

Robert N M Watson