[Standards] Coping with low bandwidth channels in Jingle

Dave Cridland dave at cridland.net
Wed Feb 13 14:08:37 UTC 2008

On Wed Feb 13 13:13:26 2008, Lauri Kaila wrote:
> If I understood you, a client should know its network capacity. Then
> it tells that infromation to the other end so they can agree on the
> best media fromat that doesn't exceed the limit. But isn't it almost
> impossible to know actual end-to-end bandwidth beforehand unless  
> some
> kind of QoS is present?

Sometimes the user knows the bandwidth of the immediate upstream  
link. Often, though, they don't - people usually know the advertized  
maximum downlink bandwidth, but usually don't know either the  
upstream, nor the actual bandwidth of either direction.

The actual device may know it's immediate link rate. Sometimes, at  
least - often actual link rates are deeply buried inside weird APIs,  
and finding overhead, latency, and other somewhat related link  
characteristics isn't usually possible.

But there is indeed no practical way to know the end to end bandwidth  
short of measuring it, and that measurement is quite tricky. There's  
a very interesting paper on the subject of intermediate link  
measurement by van Jacobson, actually. People interested in this sort  
of thing can probably find it with a Google for pathchar, or download  
pchar (an open source implementation). You'll note it takes several  
minutes for each hop, and uses quite a bit of traffic to do so.

Dave Cridland - mailto:dave at cridland.net - xmpp:dwd at jabber.org
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