From the GDF:

-------------- BROWSE HOST PROTOCOL PROPOSAL --------------
At a high level, the protocol manages the exchange of binary Query
Replies through the HTTP protocol used for Uploads and Downloads.

Here are the details about the proposed standard:

1) Advertising Browse Host through GGEP

'Browse Host' capable clients notify others of this feature via
GGEP. THe new extension ID for Browse Host is 'BH' (subject to
approval) and 'Browse Host' capable clients should add a
corresponding extension header to all Query Replies. There is no
need for any extension data.
In the future, this may be extended to allow more depth in the
description of Browse Host capabilities (via a non-empty extension
data field), ie "text/html" or "binary QR".
For now, simply having a GGEP ext. ID (and no data) is sufficient
because different 'Browse Host' implementations are resolved via the
'Accept' HTTP header flag (see below for more detail).

2) The HTTP-based Browse Host Protocol

A) The Protocol

A Browse Host request will be satisfied through an HTTP GET request.
The request will list media types that are acceptable as a return
value (via the 'Accept' request header field, RFC 1945, pg. 59).
The request lists these media types with a empty-string valued GET.
For example, here is a sample Browse Host HTTP request:

Client (LimeWire)
-------------------------
GET / HTTP/1.1
Host: w.x.y.z
User-Agent: Limewire x.y.z Pro
Accept: text/html, application/x-gnutella-packets
Content-Length: 0
Connection: close

Server (New BearShare)
-------------------------
HTTP/1.1 200 OK
Server: BearShare/x.y
Content-Type: text/html; charset=iso-8859-1

In this example above, LimeWire wants to browse a BearShare and it
accepts responses with media types text/html and
application/x-gnutella-packets.
The BearShare supports text/html only. Therefore it returns a OK
indicating that responses will be sent back as html (the html will
immediately follow the HTTP OK Response). Requesting the root "/"
in the GET request is translated into an Browse Host request.

The Server's IP and port can be retrieved via the standard route, ie a
Query Reply.

As can be seen, servers reply back with the media type that will be
returned - the standard response media type is
"application/x-gnutella-packets" .
This media type implies that a stream of binary Query Replies
describing the shared files of the responding servent will be
returned.

The "text/html" media type is featured in BearShare clients. Bearshare
has had the browse host feature for some time now. The "text/html"
media type simply sends back a HTML page listing the servent's files.
This media type lacks several important features, such as reporting
file size. It is recommended that all implementations use the
"application/x-gnutella-packets" media type as their default but
implement "text/html" reception (to be compatible with older
BearShare's). Minimally, all implementation must support the
"application/x-gnutella-packets" media type for output while being
browsed. The GUID of the query replies sent back for a
"application/x-gnutella-packets" Browse Host request can be
determined arbritrarily by the Server.

Other media types will be followed by different responses, as
necessary.

A server may compress the results of a Browse Host query. First,
the server should confirm that the client accepts the compressed
encoding, ie "deflate". This can be communicated by the client
through use of the HTTP 'Accept-Encoding' header and by the
server's use of the HTTP 'Content-Encoding' header. For example:

Client (LimeWire)
-------------------------
GET / HTTP/1.1
Host: w.x.y.z
User-Agent: Limewire x.y.z Pro
Accept: text/html, application/x-gnutella-packets
Accept-Encoding: deflate
Content-Length: 0
Connection: close

Server (New BearShare)
-------------------------
HTTP/1.1 200 OK
Server: BearShare/x.y
Content-Type: text/html; charset=iso-8859-1
Content-Encoding: deflate

B) Error Cases

If a servent cannot respond to any of the requested media types, an
HTTP error message is returned.
For example:

Client (Old BearShare)
-------------------------
GET / HTTP/1.0
Accept:text/html

Server (LimeWire)
-------------------------
HTTP/1.1 406 Not Acceptable
Content-Type: text/html

Old BearShare's only support text/html and LimeWire does not, so a 406
is returned (This is a example only) .

C) Servent Specific Behavior

A servent may impose whatever limitations on responding to Browse
Host requests that they see fit, such as Upload Slot and Bandwidth
Usage limitations. It is recommended that Upload Slot limitations
are not enforced for the majority of clients though (A client sharing
a large number of files may want to limit 'Browse Host' requests).

3) Browse Host through Pushes

Pushes are also supported. A Push is routed in the normal fashion
(a la Push Requests). The Push Request can technically have any
index.
The GIV message (sent from the server to the client requesting a
browse host) is the same. The Server should connect back to the
client regardless of the value of the index - what is most important
is the <servent-identifier> (the server's guid), which is used by the
client to match up the appropriate state.
For example, in response to the PushRequest, the following transaction
takes place:

Server (just connected to Client)
-------------------------
GIV 4141414141:<servent-identifier>/<lf><lf>

Client (noting the servent-identifier)
-------------------------
GET / HTTP/1.1
Host: w.x.y.z
User-Agent: Limewire x.y.z Pro
Accept: text/html, application/x-gnutella-packets
Content-Length: 0
Connection: close

Server
-------------------------
HTTP/1.1 200 OK
Server: BearShare/x.y
Content-Type: text/html; charset=iso-8859-1


4) Protocol Advantages

This scheme has the following advantages:
1) It works seamlessly with all existing BearShares. This flexibility
also allows clients to create custom media type standards if they
wish,
while still allowing for inter-vendor browsing.
2) No new message format is needed - pre-existing infrastructure is
special cased and the HTTP protocol is leveraged.
3) As a consequence of 2), Browse Host works through Pushes.