[altq 704] Re: ALTQ inbound processing

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

Lars Eggert wrote:
> > ALTQ doesn't support queueing disciplines on the input path but
> > diffserv-style traffic conditioning is supported.
> 
> You mean dropping packets that fall outside certain specified service level
> limits? (Sorry, I'm not too familiar with diffserv terminology.)

You can also mark packets for differentiated treatment by the
queueing disciplines along the path.

> If so, that's unfortunately not fine-grained enough for what I'm looking
> into. Is there anything fundamental that would prohibit ALTQ from working on
> the inbound queue? I've looked into is a little bit, and it seems to be a
> straightforward extension (basically, ALTQify ipintrq). If that's the case,
> I'll start working on it soon...

The problem is that input queues are empty most of the time so that
there isn't much to do with queueing.

Quoted from an ALTQ paper:
    Input queues and output queues on a router normally have asymmetric
    loads.  When the speed of a link is slower than the forwarding speed
    of a router, packets are stored in the output queue most of the time.
    On the other hand, when the speed of the link is faster, packets need to 
    wait in the input queue.

    The former case, the link is a bottleneck, is predominant in the
    Internet and we can use various software techniques to manage packets
    in the output queue.
    Thus, traffic management and QoS related techniques are usually
    applied only to output queueing. 

    The latter case occurs when the router is a bottleneck.
    For example, a lookup operation for the next hop can be expensive when
    a routing table becomes large.  It also happens with a router with
    high-speed ports or with a large number of ports; arriving packets can
    exceed the internal switching capacity of a router.
    Whereas input queueing is important to build high-speed routers, 
    special hardware is required to solve performance problems,
    and the situation is quite different from output queueing.
    Thus, we do not discuss input queueing in this paper, and 
    our focus is on output queueing in order to manage traffic.

-Kenjiro