Re: IP QoS issues

[Karl Auerbach <karl at CaveBear.com>]

> Is QoS on TCP/IP reliable enough for applications that are
> bit-rate-sensitive (like real-time video and toll quality voice)?

The empirical answer is yes.  I (I work at Cisco Systems via acquisition
of Precept Software) find IP a perfectly adequate vehicle to move
near-broadcast quality audio/video streams over IP networks, even networks
with a significant number of routers and links of diverse types. 

Last May I put together a multi-vendor QoS/RSVP test in which we ran
RTP/RTCP based audio/video and IP telephony over a network with four
brands of routers connected by links of such diverse types as T-1, ADSL,
fast ethernet, and standard ethernet.  We used a number of host platforms,
including Win 98, which, as shipped, includes RSVP.

Interoperability of RSVP/QoS was quite good, indeed better than I had
expected. It was about as non-proprietary as one can get.

Of course, we still have a very long way to go before we achieve QoS
across broad areas of the Internet.  Aggregation techniques, such as
diff-serve, are needed to keep router state within reasonable bounds.  And
we need to get policy controls in place so that we can prevent everyone
from simply asking for, and receiving, preferential QoS values. 

And instability of routes is going to make it difficult to maintain QoS
once a path is established. 

As one who has developed QoS enabled applications -- I suggest that one
should not build an application that depends entirely on the network for
QoS, for smoothing packets into isochronous bit streams.

Rather, applications need to take an active part in working with the net
and should be able to gracefully deal with failures to obtain the desired
QoS.

And applications need to avoid doing silly things (like forming IP
fragments)  or carving their media representations across packet
boundaries so that a single lost packet causes the invalidation of data in
adjacent parts of the media stream.

QoS networking requires that one think in terms of the network as a whole,
as *all* the layers working together.

		--karl--