Subject: Re: question: DiffServ versus RSVP

From: jie yang ee stnt <[email-masked]>
Date: Fri, 15 Sep 2000 11:04:30 -0400 (EDT)

>If two providers choose to use different QoS levels for different
>DiffServ classes, then (hopefully), they should change the DiffServ
>values as packets cross from one administrative domain to another.

>>If they don't, then the QoS values for the classes will change over the
>>path of the packet.

> I am doubting whether diffserv could offer QoS that meets various
> end-to-end requirements.

>>It doesn't.  And it isn't meant to.

In my mind, diffserv was proposed because Interserv+RSVP was based on flow
and is not scalable. In Diffserv since traffic is not based on flow but 
based on class, it is a more scalable traffic engineering mechanism. What else
diffserv is meant to? 

> I was thinking the possibility that at edge router we apply Interserv
> mechanisms which is based on flow and at core router we use diffserv
> which is based on class. I doubt, since diffserv is a kind of coarse
> grain flow management, it is not easy to guarantee per-flow QoS as
> many applications require, if not impossible.

what I envisioned here is, even if in core router we don't have per-flow 
management, by some other mechanisms, such as reservation, CAC, priority
queueing, etc, we can offer same QoS via per-class management as via 
per-flow management when traffic comes out of the domain. it means, the internal
forwarding mechanism is transparent from outside. it seems, with
current track on diffserv, it will not work.

>>If you need end-to-end per-flow reservations, then you need something
>>bigger than DiffServ.

agree. but this reservation should happen only at edge router of a domain
to avoid scalability problem. 

>>MPLS is one possibility.  Another is IP over ATM or Frame Relay. 
>>Another is to use hop-by-hop routers that can make reservations.
MPLS is still based on flow. And, it is becoming more and more complex. 
The power of TCP/IP is its simplicity. That is why it overwhelms ATM.
will MPLS repeat the failure of ATM ?

>>Setting up the QoS levels for such a network can be done through
>>configuration, management, or signalling.
come back to diffserv, SLA+RSVP+bandwidth broker may offer some clue
on how to make it. Do yo have any idea on it?

thanks,
Jie

---

From: David Charlap <[email-masked]>
Date: Fri, 15 Sep 2000 11:22:29 -0400

jie yang ee stnt wrote:
> 
> In my mind, diffserv was proposed because Interserv+RSVP was based on
> flow and is not scalable. In Diffserv since traffic is not based on
> flow but based on class, it is a more scalable traffic engineering
> mechanism. What else diffserv is meant to?

It does not work end-to-end.

A host can not specify Diffserve classes in the packets it generates.

Actually, it can, but as soon as it hits an ISP that is implementing
DiffServ, these values will be discarded and replaced with values that
the ISP is using.

DiffServ is primarily being used to establish priorities within a
network core.  Not to provide a consistent end-to-end quality level.

>> If you need end-to-end per-flow reservations, then you need
>> something bigger than DiffServ.
> 
> agree. but this reservation should happen only at edge router of a
> domain to avoid scalability problem.

You're answering a question that was not asked.  The question I was
responding to specifically asked about end-to-end per-flow reservations.

-- David