Firewall Sizing?

[Paul Hutchings <PAUL () SPAMCOP NET>]

How do you go about sizing a firewall?

I ask both generally and specifically.  Right now I need to replace  
an existing ISA server, and top of the list is a Secure Computing  
Sidewinder (those Palo Alto boxes look nice but they're just too much  
$$$ to go beyond looking at the features on the website :-)).

Anyway, as with most vendors there's a number of models and a number  
of specs that vary as you move up the range - throughput, max  
sessions, recommended users etc.

In our case I suspect we're a bit of an oddity, as we have a fat  
internet pipe and a few hundred users, but not all have full internet  
access and there's very little in the way of concurrent access (I  
think the most concurrent sessions I've ever seen was around 3000 and  
that depends on the vendors idea of a session).

Because of this, with most vendors I'm thinking of our situation and  
on paper 9/10 times the low end units appear suitable, the vendors  
seem to simply hear "few hundred users" and "fat internet pipe" and  
try and persuade me I need the higher end models.

What puts the most load on a modern firewall such as a Sidewinder, is  
it sheer throughput, is it keeping track of X sessions to/from Y  
clients and so on?

I'd appreciate any thoughts/input on how you go about sizing/speccing  
these things if you don't have the budget to simply buy a the mid to  
top range unit.

cheers,
Paul
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