Thread: out-of-order packets

Or use SCTP, which preserves both message boundaries (which TCP does not) and message order (which UDP does not).

Originally Posted by Bryan Fok

How to ensure my udp sockets receive all packets in order? Assuming the packets are guarantee arrive in order at the receiver machine, and no packet drop.

Even with those assumptions in place, UDP packets logically have no relation to each other, so the kernel doesn't have to make any special effort to ensure they are seen by the application in the same order they are seen by the network interface. You'd think that would be the case, but there are all sorts of reasons why it might not be. For instance, the packets might be queued in a LIFO fashion instead of a FIFO, simply because that was easier to implement and permitted to do.

You're relying on undefined behavior by relying on the order of UDP packets.

If they must be in order then attach a value that indicates the number of the packet in the sequence and re-order them yourself when they're received. Also, instead of assuming that packets won't be dropped, decide whether you can afford to drop them, and if you can't, send something back indicating that the other side successfully received the packet, and keep sending the same data until that is received. You can safely fit around 512 bytes into a UDP datagram, so keep that in mind when you're designing your protocol.