Re: [rtcweb] Pictures of congestion control on the Internet - which is more realistic?

Dave Taht <> Wed, 23 April 2014 15:19 UTC

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Date: Wed, 23 Apr 2014 08:18:48 -0700
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From: Dave Taht <>
To: Harald Alvestrand <>
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Subject: Re: [rtcweb] Pictures of congestion control on the Internet - which is more realistic?
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On Wed, Apr 23, 2014 at 5:30 AM, Harald Alvestrand <> wrote:
> I quote Karl's piece below, because I want other's opinion of whether it's a
> realistic picture.
> My impression is that it is not - in particular that:
> - Lots of interactive traffic goes over the Internet today without layer 2
> separation
> - All UDP endpoints that use the open Internet have some form of backoff on
> congestion - using delay, packet drop or (most likely) both to detect
> congestion

I'm not sure if we have a shared definition of "congestion".  Mine is
network delays exceeding that of what human factors research notes
as "perceptible" with about 20ms as the outer bound.

Kathie/Van something like a sustained  delay > 1.1 * RTT = bad queue

Bittorrent folk  put it at > 100ms

Others use even bigger numbers.

> I'm writing the transport document based on those assumptions - that we
> should specify something that will work as well as it can over an Internet
> that does not care, and try to allow for better things to come along later.
> Am I wrong to write it like this?
> Harald
> ------------------------------------
> [Karl] I wrote this sometime before, regarding how QoS over the internet
> works in general - may be useful for understanding
> For better understanding of these QoS/QoE problems, and methods for
> improving (not specifically discussing the policies of sharing real-time
> traffic space), I would like to explain:
> Over an IP pipe only used for real-traffic (no TCP data traffic), it is
> sufficient that the pipe is wide enough for good QoS. That is often used and
> implemented by separating IP pipes at a lower level using e.g. Ethernet
> VLAN, MPLS, Ethernet over ATM (std for ADSL modems). TURN servers can
> enforce real-traffic into such pipes and QoS is achieved. Let’s call this
> Level 2 QoS (Network level)
> Over an IP pipe shared between quality requiring real-time traffic and less
> demanding data or streaming (usually TCP) traffic, we have the sharing
> between these two traffic classes to consider. The main method (and the
> mechanism making today’s real-time QoE as good as it often is) is that TCP
> endpoints back-off and share their bandwidth usage. Real-time traffic using
> UDP transport do not back-off, the endpoints using UDP occupy the bandwidth
> needed. When an IP pipe gets filled, it is all endpoint’s TCP bandwidth
> usage that is back-off and shared between them, leaving room for the UDP
> traffic. This is mechanism we experience everyday over the Internet, using
> our “Surf IP pipes”. Let’s call this Level 4 QoS (Transport level)
> However, this Level 4 QoS is based on that at congestion times (which happen
> every time we click – setting up a TCP flow and transferring some amount of
> data as quick as possible) the router handling the most narrow part of the
> pipe (the congestion point) drop packets. It is this packet dropping that
> (i) signals to TCP endpoints to reduce their bandwidth (via TCP’s error
> correction/retransmission mechanism) and (ii) destroys the QoE of real-time
> traffic. (Both TCP and UDP packets are dropped in this process that is
> triggered by flow intensive TCP traffic.)

Um, er, the situation today is one of dramatic overbuffering on everything,
triggering packet drop too infrequently, which messes with QoE far worse
for much interactive traffic  (notably  games  and VOIP) than  drop  does.

"once you have bad latency you're stuck with it"  -

> We need (i) but don’t want (ii) and to improve on this we can e.g. use
> diffserve, DSCP bits in IP packets to instruct routers to always forward the
> real-time traffic before any unmarked TCP traffic (which usually fills most
> of the pipe). Then QoS is then achieved for real-time traffic. This is Level
> 3 QoS (IP level). The method used is “traffic shaping”: backing off data
> traffic, leaving real-time traffic free passage without packet loss.

Well,  there  are several techniques involved in traffic  shaping, and in
the end  you  have  to use most of  them comprehensively to  get
good  QoE.

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Dave Täht