Proxy Specifications

I am looking for some documentation on www proxy servers.   I am looking

to build a *custom* proxy server, thus, I am looking for a specification

( or rfc) that details the required features, optional features and
forbidden features of a www proxy server.

I would appreciate it if you could point me to some documentation.
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fn:             Kwang-Rim Ahn
n:              Ahn;Kwang-Rim
org:            Inha Univ.
email;internet: g9711307@inhavisioin.inha.ac.kr
x-mozilla-cpt:  ;0
x-mozilla-html: FALSE
version:        2.1
end:            vcard