Re: VOIP Peering Questions

[Mark R. Lindsey <lindsey@e-c-group.com>]

On May 25, 2004, at 9:55 PM, Jere Retzer wrote:
> - Does anyone have any technical suggestions on implementing VOIP 
> peering
> across a layer 2 Internet exchange?

RTP flows are unresponsive to congestion; i.e., if they have to compete 
with TCP (*), you can expect two effects:
	1> unfairly-depressed TCP throughput
	2> loss in your RTP streams

What's typically done is QoS using class-based queueing in both the IP 
routers and in the L2 equipment. You'll need to convince your L2 gear 
to classify RTP as better-than-best-effort -- i.e., 802.1p 
classification, probably with 802.1q VLAN tagging on the frames from 
your exchange customers.

There's another issue that's trickier for an exchange: admission 
control of the RTP flows. Normal Internet traffic handles congestion 
gracefully (thanks VJ for the CWND), but RTP doesn't.

Assuming all traffic flows through the same gear, the RTP traffic needs 
to get a premium service so that it doesn't have to compete. 
Corresponding somewhat to the two points above,
	3> every RTP stream slows *all* non-RTP throughput, for a given output 
queue
	4> RTP doesn't degrade gracefully in the presence of RTP congestion

This last point could be a real issue, particularly so if you have 
rate-limited the RTP class or if you just have more RTP flows going 
through a link than the link can handle. At some point, you have to 
determine the number of RTP flows you intend to handle through each 
link, and somehow enforce that limit. If peer A can send as many as 150 
RTP streams to peer B, then you have to effectively "reserve" that 
bandwidth for those flows.


(*) TCP and things made to work well in a TCP-friendly environment are 
responsive to congestion; e.g., streaming audio even sent over UDP can 
reduce its bit-rate in the presence of congestion. By contrast, RTP 
flows are unresponsive -- then they drop packets, then it just sounds 
bad, but generally they don't attempt to slow down.

---
Mark R. Lindsey
ECG, Inc. http://www.e-c-group.com/
Office: 229-244-2099x2207; Mobile: 229-630-5553


--
To unsubscribe send a message to voip-peering-request@psg.com with
the word 'unsubscribe' in a single line as the message text body.
An archive is at <http://psg.com/lists/voip-peering/>.