HTTP1.0 / HTTP1.1

Squid is a HTTP/1.0 proxy. But all HTTP/1.1 clients and servers MUST interoperate with HTTP/1.0 to be compliant.

The main difference this brings to the protocol (apart from version number) is that aplications or servers can't use transfer-encoding. The rest of HTTP/1.1 is supported fine over HTTP/1.0.

Transfer-Encoding tells the receiver what encoding has been performed on the message in order for it to be safely transported.
All transfer-encoding values are case-insensitive. HTTP/1.1 uses transfer-encoding values in the TE header field and in the Transfer-Encoding header field. The latest HTTP specification defines only one transfer encoding, chunked encoding.