<p>Complete direct client-to-client file transfers presents a major problem for clients that are behind a firewall or NAT. Proxy Accept Socket Service (PASS) enables clients to do real-time file transfers via a third party; in addition, it does not limit clients to file transfers but enables any two clients to create a raw TCP socket between them for any purpose, such as VoIP (SIP/RTP), BEEP, or binary game data.</p>
</section1>
<section1topic='Registration/Setup'>
<p>The first step is to communicate with a PASS service to set it up.</p>
<p>At this point, the PASS service is now listening on the given IP and port for incoming connections on behalf of the Jabber Entity. The provided IP and port can now be sent to any other entity as a connection point, for file transfers or any other use.</p>
<p>The default behavior for the PASS service is to listen on the port forever, unless the requesting client specifies an <expire/> value (in seconds). If the service is not configured with an expire value, it will not timeout the connection, and will start listening on the port again after one of the two sides disconnects.</p>
<tablecaption='Error Codes'>
<tr>
<th>Code</th>
<th>Message</th>
<th>Cause</th>
</tr>
<tr>
<td>502</td>
<td>No more ports available. Try again later.</td>
<td>The PASS service is listening on all of its available ports.</td>
</tr>
</table>
</section1>
<section1topic='Incoming Connections'>
<p>When an incoming connection is attempted to that IP and port, the PASS service MUST send an IQ request to the entity on whose behalf it is listening:</p>
<p>The entity SHOULD now immediately connect to the given proxy IP and port, and upon connection all data written to that socket will be copied to the client connection, and vice-versa. Any disconnect on either side MUST cause a disconnect on the other (initiated by the service). If the IQ set to the entity fails or returns an error, the client socket MUST be dropped as well. The client XML element provides the information about the remote end of the incoming socket.</p>
<p>Abuse in bandwidth or resources could become an issue, so PASS implementations SHOULD include strict and detailed rate and usage limits, allowing only limited usage by single entities and rate-limiting bandwidth used if necessary for both single connections or overall usage. These limits are implementation-specific.</p>
</section1>
<section1topic='Controls'>
<p>A Jabber client can send various controls to the PASS service via the set to control how the PASS service behaves, how the server handles a listening port.</p>
<p>This tells the server to shut down the port after a specified number of seconds. After the timeout period, the PASS service MUST send an <iq/> to the JID to tell it that the port has been closed down. It notifies the client with:</p>
<examplecaption='Notification of expiration'><