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UDP Protocol, no sequencing of packets

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by harvardd

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IF UDP protocol does not provide sequencing numbers on each packet, then how does the receiving node know how to re-assemble the packets in the correct sequence ?

I know that TCP does sequence packets. But in a broadcast, how do the packets get reassembled in the correct order.

7 replies

by beermonster

I'm a bit confused about
a) why you would post this question in a forum about windows 2000 and
b) why you didn't just look this up in a reference book.
UDP relies on the sending and receiving applications to resequence the data. TCP does order packets thats true, but all that actually means is that it 'numbers' the packets as it sends them, theres no guarantee that they will arrive in sequence. That is why the receiving tcp client has the task of reordering the packets should that be required. It does this (depending on implementation) by buffering out of sequence packets until the missing portions arive. Broadcasting is no different, as it is simply a sequence of packets addressed to anyone that is listening. May I suggest a site toyou that I have found invaluable for quick reference - www.whatis.com - Hope this helps...

by ebob

basically, UDP does not sequence packets (as you say), but that does not mean that packets can NOT be sequenced. It simply means that UDP (a Transport Layer/Layer 4 protocol) does not do the job. If sequencing is important, and if UDP is chosen as the L4 protocol, then it is up to higher levels (i.e., the "application") to do the job of managing sequencing.

TCP, on the other hand, uses sequence numbers in the TCP header to manage sequencing. In this case, the "application" does not need to bother with doing any packet sequence management. It simply hands the data off to TCP knowing that it will be managed there.

When somebody is writing a network application, they have to (among many other things) determine the balance between havinga fast L4 protocol with the application managing aspects of data integrity (e.g., UDP) vs. a more robust, but slightly slower (due to more overhead) L4 protocol, and less issues in the application (e.g., TCP).

by ntoz

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