The "Connect Back" Vendor-specific Messages
                              Version 1

                          Raphael Manfredi
                    <Raphael_Manfredi@pobox.com>
                          January 3rd, 2003
                      Revised January 27th, 2003
                     Revised September 20th, 2004
                

[These specifications are a summary of messages exchanged on the GDF]

INTRODUCTION

Please read the Vendor-Messages specifications if you have not done so
already.

This document specified the "Connect Back" vendor-specific messages.
There are two of them: one for TCP, one for UDP.

SPECIFICATIONS

* TCP Connect Back

    Name: TCP Connect Back
    Vendor: BEAR
    ID: 7
    Version: 1
    TTL: 1
    Payload:
        unsigned short: port number (little-endian)

The "Connect Back" message is meant to be sent by servents that still
think they are firewalled, right after a connection to a remote servent
has been established (but after the initial handshaking ping).

This message carries a single 16-bit unsigned integer representing the
port to which the receiver of the message should try to connect to via TCP.
The target IP address for the connection is based on the IP address
determined from the connection endpoint from which the message is
received.

When the connection is made, the connecting servent emits the 2-byte
string "\n\n" and closes the connection.

If the remote servent is able to receive the connection on the port it
specified (usually its listening Gnutella port) AND read back that string,
it will know it is not firewalled.

* UDP Connect Back

    Name: UDP Connect Back
    Vendor: GTKG
    ID: 7
    Version: 2
    TTL: 1
    Payload:
        unsigned short: port number (little-endian)

This message is similar to "BEAR/7v1", but handles UDP connections
instead of TCP ones.

The "UDP Connect Back" message is meant to be sent by servents that still
think they are firewalled, UDP-wise, right after a connection to a remote
servent has been established (but after the initial handshaking ping).

This message carries a 16-bit unsigned integer representing the
port to which the receiver of the message should try to send a Gnutella Ping
to via UDP. The target IP address for the connection is based on the IP address
determined from the connection endpoint from which the message is
received.  Moreover, the GUID of the GTKG/7v2 message should be used as the
GUID of the ping sent via UDP.

The connection is made by sending a Gnutella Ping with the specified
guid to the specified port. If the remote servent is able to receive that
Gnutella ping on the port it specified (its listening UDP Gnutella port) with
the guid it specified, it will know it is not UDP firewalled.

An older version of this message, GTKG/7v1, was defined as:

    Name: UDP Connect Back
    Vendor: GTKG
    ID: 7
    Version: 1
    TTL: 1
    Payload:
        unsigned short: port number (little-endian)
        16 byte guid: should be used as the GUID of the ping sent

The difference with GTKG/7v2 being that the GUID of GTKG/7v1 was ignored and
the desired GUID for the Ping was enclosed in the payload, thereby making
the message size larger than that of GTKG/7v2.

To know which version of the "UDP Connect Back" message is supported by
a given peer, look at its "Messages Supported" null/0v0 message, which
must be sent unsolicited right after the handshaking ping.