[asterisk-users] RTP/SIP traffic prioritization and Linux issues

[John A. Sullivan III]

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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