Does UDP allow repacketization?

Question

I know that for TCP you can have for example Nagle\'s Algorithm enabled. However, can you have something similar for UDP?

Practical Question(assume UDP socke

Answer 1

No. UDP datagrams are delivered intact exactly as sent, or not at all.

Answer 2

Not according to the RFC (RFC 768). Above IP facilities themselves, UDP really only provides, as extras, port-based routing and a little bit of extra detection for corruption or misrouting.

That means there's no facility to combine datagrams. In fact, since it's meant to be transaction oriented, I would say that combining two transactions into one may well be a bad idea in terms of keeping these transaction disparate.

Otherwise, you would need a layer above UDP which could figure out how to extract these transactions from a datagram. At the moment, that's not necessary since the datagram is the transaction.

As added support (though not, of course, definitive) for this contention, see the UDP wikipedia page:

Datagrams – Packets are sent individually and are checked for integrity only if they arrive. Packets have definite boundaries which are honored upon receipt, meaning a read operation at the receiver socket will yield an entire message as it was originally sent.

However, the best support for it comes from one of its clients. UDP was specially engineered for TFTP (among other things) and that protocol breaks down if you cannot distinguish a transaction.

Specifically, one of the TFTP transaction types is the data transaction which consists of an opcode, block number and up to 512 bytes of data. Without a length indication at the start or a sentinel value at the end, there is no way to work out where the next transaction would start unless there is a one-to-one mapping between transaction and datagram.

As an aside, the other four TFTP transaction types have either a fixed length or end-of-string sentinel values but the data transaction is the decider here.