Average UDP packet loss and packet re-ordering

Question

I'd like to garner fellow SO'ers experience with regards to the issue of UDP packet loss (or drop-out).

Initially my understanding is that given direct point to point connections where the NICs are connected via a crossover cable and ample buffer on the NICs and timely processing of said buffers, that there 'should' be no packet loss or packet ordering issues. I believe this is also the case given one good/high-end switch in between the points.

1. Excluding the above scenario, what is the expected average UDP packet loss over a LAN

2. What scenarios cause UDP packet ordering issues?

Answer 1

No idea on the UDP packetloss on average LANs. I assume reasonably low on modern switched networks, otherwise your LAN or endpoints are too highly loaded. :)

The re-ordering is probably easiest to achieve when routes are brought up and down; say, one of the switches in your organization is under enough load that re-organizing the tree makes sense and traffic is sent through different switches. More likely is your ISP's peers coming and going, or reaching traffic limits, and the priority of packets through them changes -- old packets were in flight on the heavy-loaded network, new packets are in flight on the lighter-loaded network, and they arrive out of order.

Answer 2

I too am looking for an expected average. I found that from a direct link (PC to PC) packet loss occurs very rarely, although it definitely occurs. Availability was something like 99.9% at 1 kB packets @ 50 Hz.

I have seen reordering just by sending and receiving on the same network interface. I concluded that this occurs because each packet is handled asynchronously so that there is a chance of a newly arrived packet being processed before packets received prior to the newly received one.