Firewalls at lower levels?

[smith135 @ mc . duke . edu]

I was at a seminar presented by Stuart Holoman, Holocon Inc. 
yesterday, and he said firewalls are not effective/implementable 
below the session layer:

layer 7 - App support
layer 6 - Presentation
layer 5 - Session
layer 4 - Transport
layer 3 - Network
layer 2 - Data link
layer 1 - Physical

Any comments?
I don't know if he was speaking in abstract terms (e.g., not many 
people know how to make them effective). 

Michael Smith                                    Voice: 919-613-7633 
Duke University and Medical Center               Fax:   919-613-7631 
E-mail: smith135 @
 mc .
 duke .
 edu