when is IPPROTO_UDP required?
When is IPPROTO_UDP required?
Is there ever a case where UDP is not the default protocol for SOCK_DGRAM? (real cases, not hypothetical "it might be", please")
i.e., what are the situations where the following two lines would not produce identical behavior?
if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
if ((s=socket(AF_INET, SOCK_DGRAM, 0))==-1)
Answer
Some operating systems (eg. Linux kernel after 2.6.20) support a second protocol for SOCK_DGRAM, called UDP-Lite. If supported by your system, it would be enabled by providing IPPROTO_UDPLITE as the third argument to the socket() call.
It is differentiated from normal UDP by allowing checksumming to be applied to only a portion of the datagram. (Normally, UDP checksumming is an all-or-nothing effort.) That way, the protocol can be more resistant to checksum failures due to fragmented transmission, in the event that some fragments outside the checksummed area may have been lost in transit. As long as the fragments covering the checksummed portion were successfully received, as much of the datagram as possible will still be delivered to the application.
For backwards compatibility with existing code, I suspect (but I cannot guarantee) that the call socket(AF_INET,SOCK_DGRAM,0) will continue to default to normal UDP, even in systems that additionally support UDP-Lite.