Saturday, August 6, 2011

SOCK_DGRAM Socket Type

There are some situations in which it is not absolutely required that data must arrive at the remote end in sequence. Additionally, it might not even be required that the data delivery be reliable. The following lists the characteristics of a SOCK_DGRAM socket type:
  • Packets are delivered, possibly out of order at the receiving end.
  • Packets might be lost. No attempt is made at recovering from this type of error. Nor is it necessarily known at the receiving end that a packet was lost.
  • Datagram packets have practical size limits. Exceeding these limits will make them undeliverable through certain routers and nodes.
  • Packets are sent to remote processes in an unconnected manner. This permits a program to address its message to a different remote process, with each message written to the same socket.
Reliability is not a concern when noncritical logging information is transmitted. This information is transmitted on a "best efforts" basis. When a noncritical log packet is lost, it is considered an acceptable loss.

Unlike a streamed connected socket, a datagram socket simply passes data by individual packets. Remember that for protocols such as IP, individual packets can be routed different ways. This frequently causes packets to arrive at the destination in a different sequence from which they were sent. The socket type SOCK_DGRAM implies that receiving these messages out of order is acceptable to the application. Sending a datagram packet is unreliable. If a packet is transmitted and not received correctly by an intervening router or the receiving host, then the packet is simply lost. No record of its existence is kept, and no attempt to recover from the transmission error is made.

Packets can also be lost if they are unsuitably large. Routers in the path between the sending host and the receiving host will drop a packet if it is too large or lacks the buffer space to pass it. Again, there is no error recovery implied in a SOCK_DGRAM socket when this happens.

The last characteristic that is of interest to you is the fact that the SOCK_DGRAM type does not imply a connection. Each time you send a message with your socket, it can be destined for another recipient. This property of the SOCK_DGRAM type makes it attractive and efficient.

A connection-oriented protocol, on one hand, requires that a connection establishment procedure be carried out. This requires a certain number of packets to be sent and received in order to establish the connection. The SOCK_DGRAM type, on the other hand, is efficient because no connection is established.
Before choosing to use SOCK_DGRAM, however, you must carefully weigh the following:

• Need for reliability
• Need for sequenced data
• Data size requirements

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