UDP Packet size and fragements

Question

Let's say I am trying to send data using udp socket. If the data is big then I think the data is going to be divided into several packets and sent to the destination.

At the destination, if there is more than one incoming packets then how to I combined those separated packets into the original packet? Do I need to have a data structure that save all the incoming udp based on the sender ? Thanks in advance..

Answer 1

If you are simply sending the data in one datagram, using a single send() call, then the fragmentation and reassembly will be done for you, by the transport layer. All you need to do is supply a large enough buffer to recv(), and if all the fragments have arrived, then they will be reassembled and presented to you as a single datagram.

Basically, this is the service that UDP provides you (where a "datagram" is a single block of data sent by a single send() call):

• The datagram may not arrive at all;
• The datagram may arrive out-of-order with respect to other datagrams;
• The datagram may arrive more than once;
• If the datagram does arrive, it will be complete and correct¹.

However, if you are performing the division of the data into several UDP datagrams yourself, at the application layer, then you will of course be responsible for reassembling it too.

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^{1. Correct with the probability implied by the UDP checksum, anyway.}

Answer 2

You should use TCP for this. TCP is for structured data that needs to arrive in a certain order without being dropped.

On the other hand, UDP is used when the packet becomes irrelevant after ~500 ms. This is used in games, telephony, and so on.

Answer 3

If your problem requires UDP, then you need to handle any lost, duplicate, or out-of-order packets yourself, or at least write code that is resilient to that possibility.

http://en.wikipedia.org/wiki/User_Datagram_Protocol

If you can't afford lost packets, then TCP is probably a better option than UDP, since it provides that guarantee out of the box.