[Lab] Instant Messaging

1. Start Wireshark, and capture with the filter "udp port 5060 and udp[4:2] > 10".
   • PJSUA keeps sending a keep-alive message. This is for NAT traversal. (We shall talk more about this topic in the following weeks. If you are curious, you may read RFC 3489 to get a basic understanding.)
   • The keep-alive message has a payload of 2 bytes ("\r\n"), so the length (including UDP header) is 10.
   • To filter these packets out, we specify that we only want UDP packet with length greater than 10 bytes.
2. Start PJSUA as "pjsua.exe --no-tcp --id=sip:111321___@sip.ncnu.net --registrar=sip:sip.ncnu.net".
3. You will see a command menu of PJSUA.

   +=============================================================================+
   |       Call Commands:         |   Buddy, IM & Presence:  |     Account:      |
   |                              |                          |                   |
   |  m  Make new call            | +b  Add new buddy        | +a  Add new accnt.|
   |  M  Make multiple calls      | -b  Delete buddy         | -a  Delete accnt. |
   |  a  Answer call              |  i  Send IM              | !a  Modify accnt. |
   |  h  Hangup call  (ha=all)    |  s  Subscribe presence   | rr  (Re-)register |
   |  H  Hold call                |  u  Unsubscribe presence | ru  Unregister    |
   |  v  re-inVite (release hold) |  t  Toggle online status |  >  Cycle next ac.|
   |  U  send UPDATE              |  T  Set online status    |  <  Cycle prev ac.|
   | ],[ Select next/prev call    +--------------------------+-------------------+
   |  x  Xfer call                |      Media Commands:     |  Status & Config: |
   |  X  Xfer with Replaces       |                          |                   |
   |  #  Send RFC 2833 DTMF       | cl  List ports           |  d  Dump status   |
   |  *  Send DTMF with INFO      | cc  Connect port         | dd  Dump detailed |
   | dq  Dump curr. call quality  | cd  Disconnect port      | dc  Dump config   |
   |                              |  V  Adjust audio Volume  |  f  Save config   |
   |  S  Send arbitrary REQUEST   | Cp  Codec priorities     |                   |
   +-----------------------------------------------------------------------------+
   |  q  QUIT      L  ReLoad       I  IP change     n  detect NAT type           |
   |  sleep MS     echo [0|1|txt]                                                |
   +=============================================================================+ 

4. Type the command "i" and press ENTER.
5. It will ask you to input the SIP URI of your partner (e.g., "sip:111321xxx@sip.ncnu.net".
6. Your partner will now receive the "Message Composing Indication", among some debugging messages of PJSUA.
   • In the future, you may wish to re-compile PJSUA to hide these debugging messages.
   • The receiver will see a highlighted message like

     pjsua_app.c  .IM indication: <sip:111321xxx@sip.ncnu.net> is typing..

   • In Wireshark you will observe a SIP message sent to the receiver:
     1. The method shown in the request line is "MESSAGE".
     2. The Content-Type header field has the value "application/im-iscomosing+xml".
     3. The contents has a <isComposing> tag, in which the <state> tage is "active".
7. Type the message you want to send to your partner and press ENTER. Now he/she will see the 1-line message.
8. Observe the MESSAGE requests in Wireshark.
   1. On the receiver side, there are two "Via" header fields because this message is routed via a SIP proxy server. The value of "Max-Forwards" header fields is decremented from 70 to 69.
   2. The "Content-Type" header field is "text/plain".
   3. The message which you typed is in the message body.