[MLB-WIRELESS] Packet scheduling [aka: What do people want to do with the wirele ss connection?]

[Daniel Pittman]

On Tue, 13 Nov 2001, Tyson Clugg wrote:

[...]

> Off the top of my head, isn't there a QOS field in the TCP/IP header
> that is indicitave of the importance of the packet? 

Not really. There is a type-of-service field that let's you ask for low
latency, low cost or high bandwidth. 

> Don't most apps already set this to appropriate values? 

No, never.

> The real question is do routers acutally *use* this information when
> doing packet scheduling, or do most routers simply fire the packets
> out FIFO style? 

FIFO, mostly.

> How can linux or any other OS be told what to do with important
> packets?

The Linux TCP/IP scheduler is rumored to do multi-queue based on the TOS
field by default, but I never saw evidence of it when I wandered around
the code.

Empirical testing on slow links (when I still had them) showed this to
be ineffective in the face of low bandwidth/high latency links, if it
was active at all. Not that this is a surprise...

The alternate set of schedulers allow you to use TOS as a routing key
and, thus, to apply different QOS rules to the various types.


If you want to use it, though, you will need to use netfilter[1] to
mangle packets to carry the right TOS when they hit the border of the
network.

Then, that same machine can have a few buckets into which it tosses the
packets. You will have trouble getting *really* good, fair filtering,
though, in what experience I have.[2]


Note that you can do exactly the same thing by using the firewall mark
as a routing key under netfilter, giving you some thousands of different
types of service, not just three.

        Daniel


Footnotes: 
[1]  ...or your firewall technology of choice.

[2]  I don't do this for a job or anything. :)

-- 
Books won't stay banned. They won't burn. Ideas won't go to jail. In the
long run of history, the censor and the inquisitor have always lost.
        -- A.W. Grisold

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