Subject: RE: Do me a favor, please.

From: Yuxi Li <[email-masked]>
Date: Thu, 9 Dec 1999 23:07:01 -0700 (MST)

Mary,
Thank you!

The original statement is
" to compare and contrast flow-based switching with VLANs:
  compare the two by identifying their points of similiarities
  contrast the two by discussing how they differ
... stree any aspects related to the quantitative performance 
of the two techniques. ......"
I think they are solving different problems,
although both of them are switching technologies,
so, it does not make sense to identify the similarities.
And, for layer 3 switching such as MPLS,
there are few quantitative results avalable (except IP switching)
so, how to treat quantitative performance?
That is what made me confused.
Hope to hear from you. (It is due next Tuesday noon :(   )

Regards,
Li

---

From: Fred Baker <[email-masked]>
Date: Fri, 10 Dec 1999 09:43:44 -0800

>although both of them are switching technologies,

yes, they are indeed solving different problems. 

A Virtual LAN is, by definition, an ATM network that carries data encoded
as Ethernet frames, and externalizing Ethernet interfaces. The devices
around it use straight Ethernet encoding (perhaps augmented by 802.1Q tags,
which allow you to prioritize traffic in a simple manner). One can overlay
the ATM domain with many virtual LANs, which are enumerated; you route
traffic between them in the same way that you would route between separate
physical LANs. It's quality of service is based on a combination of
over-provisioning and simple prioritization.

The problem that a Virtual LAN addresses is, in a nutshell, the problem of
making a network design comparable to an organizational chart, simplifying
IP Address management by moving IP addresses with people when they change
locations rather than assignment of new IP Addresses upon a move. Moves,
and therefore drops and adds, can be quite frequent and a bit of overhead.

An MPLS network is addressing more or less the opposite problem. It
presumes that there are no people involved per se, but rather it is
interconnecting routing for a set of connected networks across a transit
domain. If you have N connectivity points (routers, interfaces of routers,
that sort of thing) interconnected, you need at most N(N-1) interconnecting
tunnels corresponding to the IP Routes in question. The interesting thing
is that IP routing may be seen as identifying the ingress/egress pair for a
particular flow of traffic, but a separate traffic engineering function
seen as providing the path from ingress to egress. This allows us to
maximize the use of each of the trunks and better use the bandwidth investment.

This is not to say that one cannot do traffic engineering using the ATM
underlying the VLAN; one certainly can. But one generally doesn't, as the
objective is very different and the traffic patterns are very different.
Local Area Networks, physical or virtual, are generally deigned for high
availability; bandwidth is thrown at the problem in order to make specific
routing unimportant, and this is done in configurations where bandwidth is
essentially free. MPLS could be used in a local area, but is really
targeted at wide area transit networks which have relatively consistent
traffic densities on a point to point basis and cost optimization is a very
relevant factor.

Quantitative performance could mean a lot of things - the contrast between
Ethernet+ATM overheads for IP traffic to MPLS+Packet on SONET overheads,
for example, is quite large, and the engineering differences can be large.
But worrying at that level, IMHO, concentrates on the trees and very much
misses the forest. The forest is "why are you using this kind of network,
what assumptions therefore apply, and what are you trying to optimize." If
you're not trying to optimize the quantified numbers, but the management
processes of drops and adds, a quantitative analysis is not very interesting.