Subject: RE: QoS and adaptive end protocols

From: "Manfredi, Albert E" <[email-masked]>
Date: Wed, 21 Jun 2000 10:21:34 -0400

> -----Original Message-----
> From: demir [mailto:[email-masked]]

> One of the reasons why I asked this question is because there is a
> research on TCP modifications over QoS-network (I'm aware of 
> Diffserv). To
> me it does not make sense to modify TCP ad-hoc (depending on different
> cases) cause adaptation of TCP works over best-effort 
> networks. It makes
> sense to evaulate TCP performance on a QoS-network cause TCP is a
> widely-deployed protocol. May be I should ask my question more
> explicit. If a network is offering a service other than best 
> effort, does
> an end-protocol need to be adaptive? If a network offers a service it
> better satisfies it otherwise the network needs to be re-engineered. I
> can't think of any service (again other than best-effort) will require
> adaptation from users of the service (am I being too restirictive?)

I would ask the question differently to understand why service
differentiation might be desirable. I would ask, if a network offers good
service for adaptive Layer 4 protocols, does it need to do more? Are there
applications running over that same network which don't coexist well with
adaptive Layer 4 protocols? And I think the answer is definitely yes; at
least yes for now.

> To me what to do is not use any adaptive protocol if I am 
> asking a service
> other than best-effort from the network. TCP eventually full 
> its pipe and
> inevitably perform congestion avoidance, results with poorer 
> service than
> the one being asked.

The problem occurs if you are asking for some special type of service _and_
you have adaptive protocols also running on that same network. I agree that
the special service you are asking for would probably not use an adpative
protocol. But the problem of mixed use is still there to be solved.
 
> > So the adaptive protocols like TCP would use services different from
> > non-apaptive protocols like UDP, on the same network.

> Can you name or give an example for such a service?

Two examples.

One is if you are running simultaneously many TCP apps and also many
streaming media apps. If your network is over-provisioned, no problem. But
as your RTP use increases in a network with lots of TCP load, there will be
no way of guaranteeing RTP performance. If people pay for their RTP
services, they will get annoyed at poor performance.

Another example is the usefulness of protocols like TFTP (which runs over
UDP). Since we have FTP, this is not a big item on anyone's agenda. IP
applications evolved around TCP because that is what works best in best
effort networks. But TFTP would be a good option if the lower layers were
themselves adaptive. In the future, as the Internet becomes more general
purpose, allowing more non-adaptive protocols to work well could be a good
thing.

A parallel effort might be to create ever more adaptive protocols, even for
voice and video. But again, people will still demand quality if they are
paying for it. So this is not always the answer.

Bert
[email-masked]

---

From: demir <[email-masked]>
Date: Wed, 21 Jun 2000 14:56:19 -0700 (PDT)

> adaptive Layer 4 protocols? And I think the answer is definitely yes; at
Yes. I agree. However, I am questioning if adaptation is necessary/would
help. The QoS-network can have additional mechanisms to protect adaptive
flows.

> paying for it. So this is not always the answer.
isn't it network's responsibility to provide quality if users are paying
for it?

Alper K. Demir