Re: [9fans] Inducing artificial latency

[erik quanstrom <quanstro@quanstro.net>]

> You'd need some plumbing in devether to demultiplex incoming
> packets addressed to this device (assuming it has its own MAC
> address).

i don't think you would.  if you're using the latency device as your
ethernet device, no muxing is required.

> Something like a tap device would allow you to simulate high
> latency link, a level-2 bridge or whatever in usercode.

this is difficult to do in user space for two reasons
1.  the difficulty of getting precision timing.
2.  copies into/out of the kernel.

> [BTW, is there a strong reason why devether.c is in 9/pc/ and
>  not 9/port/?  vlan code can be factored out from various
>  ether*.c to a similar portable file (if not devether.c)]

actually, most of the code has been factored out, but one step
better than you imagine.  most of the code is in ../port/netif.c
which should be a good start on any type of network interface.

what remains is code littered with system dependencies.  it's
worthwhile comparing pc/devether.c with kw/devether.c

by the way, there is no vlan code, as far as i know, which
is absolutely fine by me.

> > ip is not the only protocol!  that's what loopback(3) does, but without
> > the real network.  it would be good to plug loopback or similar into a real
> > ethernet.  it's also worth looking at loopback's implementation strategy,
>
> Or bridge(3).

not sure that is too informative.

> > which allows for µs delays.  sleep is just too course-grained for my
> > testing.
> >
> > dialing up random reordering would seem to me to be a feature at
> > this level.
>
> A user level simulator will make it easy to add random drops,
> reordering, filtering, NAT, etc.

what do you mean by mac-level natting?  something like
hp vc?  seems way too extravagant for a simple packet muncher.

by the way,  if you keep the packets in the order they should
be released from the delay queue, reordering is automatic.

i suppose the delay queue gives new meaning to the term
"storage network".

- erik