asterisk users - Jun 2009 - RTP/SIP traffic prioritization and Linux issues

Hello, all.  I've stumbled across what seems to be a traffic
prioritization issue in a Linux environment and wonder if anyone else
has encountered or addressed this issue.

We had planned to use expedited forwarding for our RTP and perhaps our
SIP packets.  Our plan was to set DSCP to 101110 (by the way, I think
document http://www.voip-info.org/wiki/view/snom+360 is in error as I'm
almost certain the expedited forwarding bits are 101110 and not 100010).
However, we realized that when these passed through Linux based routers
or firewalls using the default pfifo_fast packet scheduler, it would
look at bits 3-7 for placement in band 0, 1, or 2.  Using the standard
expedited forwarding DSCP means pfifo_fast will see 1100 and place the
packets in band 1 - the default band for all traffic.  Thus, they will
receive no prioritization.

We are planning to thus change the DSCP to 101100 (b0 instead of b8 for
Asterisk, 176 instead of 184 for our Snom phones) and map 101100 to
802.1p priority 7 on our switches.

I am imagining this or is it a real issue when using Linux based
firewalls and routers with default packet schedulers and expedited
forwarding? Thanks - John
-- 
John A. Sullivan III
Open Source Development Corporation

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John A. Sullivan III wrote:
> Hello, all.  I've stumbled across what seems to be a traffic
> prioritization issue in a Linux environment and wonder if anyone else
> has encountered or addressed this issue.
> 
> We had planned to use expedited forwarding for our RTP and perhaps our
> SIP packets.  Our plan was to set DSCP to 101110 (by the way, I think
> document http://www.voip-info.org/wiki/view/snom+360 is in error as I'm
> almost certain the expedited forwarding bits are 101110 and not 100010).
> However, we realized that when these passed through Linux based routers
> or firewalls using the default pfifo_fast packet scheduler, it would
> look at bits 3-7 for placement in band 0, 1, or 2.  Using the standard
> expedited forwarding DSCP means pfifo_fast will see 1100 and place the
> packets in band 1 - the default band for all traffic.  Thus, they will
> receive no prioritization.
> 
> We are planning to thus change the DSCP to 101100 (b0 instead of b8 for
> Asterisk, 176 instead of 184 for our Snom phones) and map 101100 to
> 802.1p priority 7 on our switches.
> 
> I am imagining this or is it a real issue when using Linux based
> firewalls and routers with default packet schedulers and expedited
> forwarding? Thanks - John
You are correct, EF is 101110.

I recently started using dscp on my network and ran into similar issues 
as you. I have cisco routers (not on smartnet) in my environment and 
some (v 12.x) understood dscp and some (<=v 11.x) did not. For those 
that did not I had to match on the precedence bits instead and 
everything thus far is working like it is supposed to.

As for linux, I couldn't find anything online that actually implemented 
diffserv-style traffic management. I ended up writing a script that 
would generate a set of queues and used the dscp to drop packets into 
the appropriate queues and another script to set the dscp for programs 
that could not on their own.

It's still a bit of a work in process and I'm sure there are 
improvements to be made, but if you'd like to look at it I can send it 
to you off-list.

-Dave