Re: [9fans] datakit

[geoff@collyer.net]

George commented:
> you can't take the checksums out of the IP/above layers because one
> sub-net claims to be reliable, so you wind up doing checksums in all
> layers (this wouldn't have only been true for IP btw.  links are not
> homogenous, so absent AT&T owning everywhere, if you want to talk
> everywhere you wind up having to do your own anyway)

Just to clarify, I'm unaware of any datakit/ip gateways (other than
multi-homed hosts, such as a Plan 9 cpu server (or two?) used to be).
So traffic within the world of datakit presumably could have lived
without upper-level checksums.  Looking at the second edition
port/stasync.c, it appears that CRCs were used in some cases.

To address the larger issue of trusting the network, that Ron raised,
I don't know how much intelligence was in a Datakit host interface,
but one could imagine end-to-end checksums or CRCs being handled by
the host interfaces (and probably between intermediate nodes too).
The better gigabit Ethernet cards offer to compute UDP and TCP
checksums incoming and outgoing for you, thus effectively pushing the
checksums and their verification very nearly into `the network' (your
network interface driver still has to check a bit that says `yup, the
checksums are okay'), at least within a single gigabit Ethernet.  Can
you trust these cards and your PCI buses to deliver your data
reliably?  Would you compute and verify the checksums in the IP stack
instead?  What about at 10Gb/s?