ensuring packet order in UDP

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+2 Q:

ensuring packet order in UDP

hey. im using 2 computers with an application to send and recieve udp datagrams.
there is no flow control (ICMP disabled)
frequently when i send a file as udp datagrams via the application, i get 2 packets changing their order and therefor - packetloss.

is there a way to make sure winsock and send() will send the packets the same way they got there?
or is the OS doing that? ive disabled and kind of firewall and there is no hardware switch connected between the computers (they are directly wired). network device configuration needed?
thanks

+5 A:

UDP is a lightweight protocol that by design doesn't handle things like packet sequencing. TCP is a better choice if you want robust packet delivery and sequencing.

UDP is generally designed for applications where packet loss is acceptable or preferable to the delay which TCP incurs when it has to re-request packets. UDP is therefore commonly used for media streaming.

If you're limited to using UDP you would have to develop a method of identifying the out of sequence packets and resequencing them.

PaulG 2010-09-19 09:22:14

+4 A:

UDP does not guarantee that your packets will arrive in order. (It does not even guarantee that your packets will arrive at all.) If you need that level of robustness you are better off with TCP. Alternatively you could add sequence markers to your datagrams and rearrange them at the other end, but why reinvent the wheel?

crazyscot 2010-09-19 09:48:13

from a praticular reason i cant describe im limited to udp only. a rare packetloss once and then is accaptable. it just that 1 of 3 files gets a loss. i just need to find a way for the sender to send the packets in the right order, the rest doesnt matter for me. thanks

Davidallencoe 2010-09-19 11:09:32

It's not so much a question of the sender sending the packets in the right order as for the receiver to reassemble them into the correct order. Like I said, add sequence markers at some appropriate level.

crazyscot 2010-09-19 11:40:48

UDP does guarantee that a packet will arrive intact or not at all (i.e., it has a checksum) and it also adds port numbers to raw IP. It doesn't guarantee delivery or sequencing; those are what TCP adds (by basically shouting out a packet until the other end says that it has arrived). Guaranteed correct in-order delivery is also enough that you can pretend you've got a stream of data (hence TCP is a streaming socket, since that's pretty commonly desired).

Donal Fellows 2010-10-03 19:52:47

+1 A:

is there a way to make sure winsock and send() will send the packets the same way they got there?

It's called TCP.

Alternatively try a reliable UDP protocol such as UDT. I'm guessing you might be on a small embedded platform so you want a more compact protocol like Bell Lab's RUDP.

Steve-o 2010-09-20 13:33:02

Try enet: http://enet.bespin.org/

ENet's purpose is to provide a relatively thin, simple and robust network communication layer on top of UDP (User Datagram Protocol). The primary feature it provides is optional reliable, in-order delivery of packets.

bertelmonster2k 2010-10-03 19:46:02

there is no flow control (ICMP disabled)

You can implement your own flow control using UDP:

Send one or more UDP packets
Wait for acknowledgement (sent as another UDP packets from receiver to sender)
Repeat as above

See Sliding window protocol for further details.

[This would be in addition to having a sequence number in the packets which you send.]

ChrisW 2010-10-03 19:53:56

ansaurus

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ensuring packet order in UDP

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