[c-nsp] 6500 service-policy

[John Kristoff]

On Fri, 23 Sep 2005 08:37:27 -0600
Clinton Work <clinton at scripty.com> wrote:

> I have seen tech notes by Cisco that if you want the policer to be
> friendly  to TCP traffic you should allow up to 1 second of burst at
> the committed rate.

I had just recently reviewed some doc related to policers and their
interaction with TCP so I'll regurgitate what I had recently come
across.  Here is the relevant doc if you haven't already seen it:

  QoS Policing on the Catalyst 6500/6000 Series Switches
  http://www.cisco.com/warp/public/473/102.html

I think the section your most interested is the one entitled
"Calculate Parameters".  They cannot give a hard an fast rule, but
do say this:

  "When you configure the burst rate, take into account that some
  protocols (like TCP) implement a flow-control mechanism that reacts
  to packet loss. For example, TCP reduces windowing by half for each
  lost packet. Consequently, when policed to a certain rate, the
  effective link utilization is lower than the configured rate. You
  can increase the burst to achieve better utilization. A good start
  for such traffic is to double the burst size. (In this example, the
  burst size is increased from 13 kbps to 26 kbps). Then, monitor
  performance and make further adjustments if needed."

John