[altq 696] Re: How to configure altq as a RED router

[Kenjiro Cho <kjc@csl.sony.co.jp>]

Tatsuhiko Terai wrote:
> If I simply think that ALTQ works for only 'packet dropping'
> used some probability (such as RED), is the below network topology
> more suitable for the purpose?
> 
>  +-----------------+           +---------+
>  +Server           +-----------+Client   + 
>  +(installed ALTQ) + 1000Base  +         +
>  +--------+--------+           +---------+
>           |      (ti0)
>           +------------------(some interface, 1000Base or 100Base)
> 
> I understood that ALTQ works at the outgoing device (ti0),
> so I think the server also works as a RED router with proper configuration,
> and I don't need to gateway overhead with 2 NICs.
> Is it right??

RED works in the same way but there is a big difference for TCP.
When a packet is dropped on a local interface queue, an error
(ENOBUFS) is returned to tcp_output() so that TCP immediately reacts
to the drop.  On the other hand, when a packet is dropped at an
intermediate router, TCP learns the drop only after 3 dup acks or
timeout.

-Kenjiro