[SAGE] Free Networking Textbook?

[Jason Antman <jason@xxxxxxxxxxxxxxx>]

I'm a student (junior, IT major) at Rutgers University.

I was in a networking class this semester - pretty basic stuff. 
Unfortunately, a *lot* of information on error detection, data 
transmission and encoding, etc - a lot of stuff that I'd expect in a 
lower-level engineering class - and very little information on protocols 
(SNMP was mentioned, nothing more, no real information given on *any* 
common protocols). My instructor gave me the option of an alternate 
final exam - instead of doing a case study, he gave me the option of 
finding a new textbook.

When I was a CS major (the switch is a long story), both of our intro 
Java books were available online (at no cost) as PDFs.

I was wondering if anyone knows of any good general networking books 
that are available as PDFs, for no cost? I could go crazy with 
suggesting a whole bookshelf, but I'd really like to emphasize student 
cost when selecting a book.

Thanks in advance for any suggestions!

-Jason Antman