Re: Router with bandwidth management

[Brian Candler <B.Candler at pobox.com>]

On Sat, May 17, 2003 at 01:44:35PM +0300, Mark Tinka wrote:
>    As you can see above, the access list that matches your wireless
>    client's IP address is placed onto the Ethernet interface that is
>    connected to your wireless segment. In the rate-limit statement on the
>    Ethernet interface, 32000 is the bits per second allowed, 8000 is the
>    normal burst bytes and 16000 is the maximum burst bytes. As the last
>    few lines say, if the user's IP address conforms to this setting,
>    transmit his packets through. If the assigned capacity is exceeded,
>    begin to apply congestion control - basically, drop packets exceeding
>    allocated capacity.

That's the problem with Cisco's solution: it doesn't shape your traffic, it
just polices it, which means throwing away packets which exceed the
bandwidth limit. This does not give a graceful control, but relies on TCP's
backoff algorithm sensing the packet loss to ultimately reduce the
throughput, but it will never settle properly at the level you want.

For true traffic shaping, the incoming data would be stuffed straight into a
queue, and the output interface would pull packets out of this queue at your
predetermined rate (say 64kbps):

               Any  +--------------+ 64kbps
Incoming ---------> |    Queue     | ---------> Out
                    +--------------+

This is how the FreeBSD ipfw dummynet shaper works (see 'man 4 ipfw' and
scan down to 'TRAFFIC SHAPER CONFIGURATION')

This approach gives no packet loss, unless of course the queue fills up. But
each TCP session will only have at most one window's worth of data "in
transit" (typically 32K bytes) waiting for an acknowledgement.

The only way I know to do this with a Cisco is to use an ATM interface,
where you can specify the output PCR/SCR as the speed you want packets to
exit.

Regards,

Brian.

-----
This is the afnog mailing list, managed by Majordomo 1.94.5

To send a message to this list, e-mail afnog at afnog.org
To send a request to majordomo, e-mail majordomo at afnog.org and put
your request in the body of the message (i.e use "help" for help)

This list is maintained by owner-afnog at afnog.org