linux - internet - Difference between UNIX domain STREAM and DATAGRAM sockets?




unix socket vs pipe (3)

This question is NOT for the difference between STREAM type and DATAGRAM type INTERNET sockets. I know that STREAM sockets use TCP, Datagram sockets use UDP and all the TCP,UDP stuff, packets arriving in order, ACK, NACK etc. I understand the importance of these over internet.

Q1) When I create a UNIX domain socket which is a local socket, how would it matter if the socket is STREAM socket or DATAGRAM socket. This type of socket would write the data to the socket file, would the protocol matter in this case since I am not transmitting data over a network? Is there any chance of data loss in this case if I use UNIX-based DATAGRAM sockets?

Q2) Does UNIX DATAGRAM sockets provide better performance than UNIX STREAM sockets?

Q3) How to decide for a STREAM/DATAGRAM UNIX based socket in my application?


Thanks


  1. One likely difference are message boundaries. Datagrams will be delivered as a whole with the datagrams being the natural message boundaries. With stream sockets you can read N bytes and the socket will block until N bytes are ready. But this means no obvious message boundaries.
  2. Maybe. A stream socket with TCP at least needs the initial three way handshake to establish the connection. A UDP socket does not.
  3. All things being equal, if speed is a concern, instrument and measure. (I assume you already know that only a TCP stream socket provides built-in reliable in-order transport, and only datagram sockets can be used to send to multiple receivers).

If the clients and servers will always be on the same machine and the goal is to have minimal latency and maximum bandwidth, use shared memory.


The main difference is that one is connection based (STREAM) and the other is connection-less (DGRAM) - the difference between stream and packet oriented communication is usually much less important.

With SOCK_STREAM you still get all the connection handling, i.e. listen/accept and you can tell if a connection is closed by the other side.

Note that there is also a SEQPACKET socket type that's still connection oriented, but preserves message boundaries (which might save you from implementing a message-oriented layer on top of a STREAM socket).

I would expect data transfer performance to be similar for all of these types, the main difference is just what semantics you want.





udp