Re: Packet ordering

[Greg Troxel <gdt%lexort.com@localhost>]

Jan Danielsson <jan.m.danielsson%gmail.com@localhost> writes:

>    Where, technically, do packet orders get mixed up?
>
>    I know UDP packet ordering is not guaranteed.  I have seen UDP
> packets arrive in the wrong order when sent over the Internet.  However,
> I have never seen UDP packets arrive in the wrong order when sent
> between two hosts on our LAN, and it made me wonder where they actually
> happen.  I obviously understand that it could simply be that I haven't
> observed it, not that it doesn't happen -- but it seems pretty easy to
> trigger them over the Internet.
>
>    I understand that it's conceptually wrong to assume packet ordering
> regardless -- but I'm curious if it can happen anywhere in the NetBSD
> network stack.

Generally the notion in a single network device is that it's best that
packets be kept in order.  While it's not an internet standards
violation to reorder them, I think most view it happening more than once
in a great while to be a defective implementation.

So two hosts with one Ethernet switch probably have packets remain in
order.

Packets can get reordered from:

  queueing disciplines that deviate from FIFO to do priority or
  something like that (although typically they try to not reorder within
  a flow)

  equal-cost multipath in routing, or link trunking, or things like that

  routing changes, where a high-latency path is switched to a lower one
  and the later packets arrive first


I would expect quite a lot more of that sort of thing in the
ISP/big-router portion of the world than in a house LAN.