GoogleTalk

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Introduction

Google Talk is an instant messaging service that provides both text and voice communication. The instant messaging service is colloquially known as "gtalk" or "gchat" to its users. Because the Google Talk servers communicate with clients using an open protocol, Extensible Messaging and Presence Protocol XMPP, the service can also be accessed using any other client that supports XMPP. Such clients are available for a number of operating systems not supported by the Google Talk client.

Google Talk makes use of libjingle, which is Google's implementation of Jingle. Clients which implement support for Jingle are incompatible with libjingle as they are different enough.

Features

Google Talk supports the following features :

  • File transfer
  • Instant messaging
  • Voice chat
  • Webcam support

File Transfer

The scenario is as follows :

<romeo@montague.lit> (sender) wants to send a file somefile.tar.gz of size (in bytes) 24410321 to <juliet@capulet.lit> (receiver).

The procedure is as follows:

1. Sender sends an "initiate" message to the receiver for initiating a session.

<iq from="romeo@montague.lit" to="juliet@capulet.lit" type="set" id="purple937c6e2e">
  <session xmlns="http://www.google.com/session" 
           type="initiate" 
           id="purple937c6e2d" 
           initiator="romeo@montague.lit">
     <description xmlns="http://www.google.com/session/share">
        <manifest>
           <file size="24410321">
              <name>somefile.tar.gz</name>
           </file>
        </manifest>
        <protocol>
           <http>
              <url name="source-path">/temporary/x9w0a15d8gll334681a9tps7j838f104/</url>
              <url name="preview-path">/temporary/p4e2a64s2ugd204688x7prk8s601m238/</url>
           </http>
        </protocol>
     </description>
     <transport xmlns="http://www.google.com/transport/p2p" />
  </session>
</iq>

The receipt of the "initiate" message is acknowledged by the sender by a result IQ.

<iq to="romeo@montague.lit" id="purple937c6e2e" type="result" from="juliet@capulet.lit"/>

2. The receiver replies with a "transport-accept" message if the "initiate" message contains the required(and correct!) transport.

<iq to="romeo@montague.lit" type="set" id="58" from="juliet@capulet.lit">
  <session xmlns="http://www.google.com/session" 
           type="transport-accept" 
           id="purple937c6e2d" 
           initiator="romeo@montague.lit">
     <transport xmlns="http://www.google.com/transport/p2p" />
  </session>
</iq>

The receipt of the "initiate" message is acknowledged by a result IQ.

<iq type='result' to='juliet@capulet.lit' id='58' from="romeo@montague.lit"/>

3. When the receiver accepts the file transfer, an "accept" message is sent to the sender.

<iq to="romeo@montague.lit" type="set" id="59" from="juliet@capulet.lit">
  <session xmlns="http://www.google.com/session" 
           type="accept" 
           id="purple937c6e2d" 
           initiator="romeo@montague.lit">
     <description xmlns="http://www.google.com/session/share">
        <manifest />
        <protocol>
           <http />
        </protocol>
     </description>
  </session>
</iq>

The sender acknowledges the "accept" message with:

<iq type='result' to='juliet@capulet.lit' id='59' from="romeo@montague.lit"/>

4. The receiver now sends an "info" message containing information about the channel which will consequently allow the file bytes to flow.

<iq to="romeo@montague.lit" type="set" id="60" from="juliet@capulet.lit">
   <session xmlns="http://www.google.com/session" 
            type="info" 
            id="purple937c6e2d"
            initiator="romeo@montague.lit">
      <channel xmlns="http://www.google.com/session/share" name="private-1" />
   </session>
</iq>

Sender sends an IQ of type "result" for acknowledgement:

<iq type='result' to='juliet@capulet.lit' id='60' from"romeo@montague.lit"/>

5. Now, candidates are exchanged, which will serve as the candidates for the actual file transfer.

Each "transport-info" message contains the information about a single candidate.

5a. The sender sends it's candidates:

<iq type="set" id="purple937c6e2f" to="juliet@capulet.lit" from="romeo@montague.lit">
  <session xmlns="http://www.google.com/session" 
           id="purple937c6e2d" 
           initiator="romeo@montague.lit" 
           type="transport-info">
     <transport xmlns="http://www.google.com/transport/p2p">
        <candidate address="192.168.160.101" 
                   port="55433" 
                   name="private-1" 
                   username="ccgg8psCDi8ALHPN" 
                   password="" 
                   preference="1" 
                   protocol="udp" 
                   type="host" 
                   generation="0" 
                   network="0" />
     </transport>
  </session>
</iq>

The receiver acknowledges this with an IQ:

<iq type='result' from='juliet@capulet.lit' id='purple937c6e2f' to="romeo@montague.lit"/>


5b. The receiver sends it's candidates:

<iq to="romeo@montague.lit" type="set" id="61" from="juliet@capulet.lit">
  <session xmlns="http://www.google.com/session" 
           type="transport-info"
           id="purple937c6e2d"
           initiator="romeo@montague.lit">
     <transport xmlns="http://www.google.com/transport/p2p">
        <candidate name="private-1" 
                   address="192.168.160.50" 
                   port="55455" 
                   preference="1" 
                   username="sVfmPJvuPBdh+ail" 
                   protocol="udp" 
                   generation="0" 
                   password="+9SLmzhJo6nqxYgE"  
                   type="local" 
                   network="0" />
     </transport>
  </session>
</iq>

The sender acknowledges the "transport-info" with an IQ:

<iq type='result' to='juliet@capulet.lit' id='61' from="romeo@montague.lit"/>

6. Finalizing a candidate pair for the file transfer, the receiver sends an HTTP GET request in the following format:

GET /temporary/x9w0a15d8gll334681a9tps7j838f104/somefile.tar.gz HTTP/1.1
Connection: Keep-Alive
Content-Length: 0
Host: romeo@montaglue.lit:0
User-Agent: Google Talk

The URL following "GET" in the above request is the same as the "source-path" URL that was sent as a part of the "initiate" message (Step #1).

7. The sender replies with an HTTP response, in this format :

HTTP/1.1 200
Connection: Keep-Alive
Content-Length: 24410321
Content-Type: application/octet-stream

The Content-Length field is changed from 0 to the size of the file under question.

8. The actual transfer of file bytes is underway.

The sender is writing bytes to the open stream while the receiver reads all the bytes. As soon as the receiver has received the number of bytes mentioned in the HTTP response, it considers the file transfer to be complete and sends an "info" message containing "complete".

<iq to="romeo@montague.lit" type="set" id="68" from="juliet@capulet.lit">
  <session xmlns="http://www.google.com/session" 
           type="info" 
           id="purple937c6e2d" 
           initiator="romeo@montague.lit">
     <complete xmlns="http://www.google.com/session/share" />
  </session>
</iq>

Sender acknowledges "info" message with IQ :

<iq type='result' to='juliet@capulet.lit' id='68' from="romeo@montague.lit"/>

9. The file transfer is complete.

The receiver now sends a "terminate" message to the sender, showing it's intention of tearing down the session.

<iq to="romeo@montague.lit" type="set" id="69" from="juliet@capulet.lit">
  <session xmlns="http://www.google.com/session" 
           type="terminate" 
           id="purple937c6e2d" 
           initiator="romeo@montague.lit" />
</iq>

The sender acknowledges this with an IQ:

<iq type='result' to='juliet@capulet.lit' id='69' from="romeo@montague.lit"/>


This completes the whole process of file transfer.