HELSINKI UNIVERSITY OF TECHNOLOGY NETWORKING LABORATORY. Assignment 2: sipspy Jegadish.D 1

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1 Assignment 2: sipspy 1

2 The Tasks of Assignment-2 The second assignment (sipspy)would build upon the tcpbridge that you had prepared. Here the tcpbridge would be used as SIP Proxy. Then the messages that are exchanged via your SIP proxy need to be modified so as to take control of the media traffic And Finally, sipspy will accept a RTSP control connection from a media player and forward the media packets to the media player Now Lets look at each step individually 2

3 Step 1- Using tcpbridge as a SIP proxy Download a SIP soft phone (a free phone is available at the URI or ) A SIP server is already up and running at nmps.netlab.hut.fi Create a SIP identity as your_name(at)nmps.netlab.hut.fi The SIP server does not require authentication. Configure the SIP phone to use the sipspy as Outbound proxy. Now, when the SIP phone is restarted, it would exchange SIP messages with nmps.netlab.hut.fi through the sipspy program. Make a call to the URI a2-test-1(at)nmps.netlab.hut.fi The SIP server has been configured to send an audio stream, once the above call is established successfully.. Your SIP phone would now play the audio stream from the SIP server 3

4 Step 1- Overview 4

5 Step-2: sipspy takes control of media packets The SIP message exchanged contains information about connection (address) related values Modify the required values of the messages, so that the media packets reach sipspy instead of SIP phone The message from both the SIP phone and SIP server need to be modified, so that you could successfully establish yourself as a man in the middle. The media packets received by the sipspy should then be forwarded to the SIP phone. Extend the tcpbridge to accept and forward the UDP packets (RTP/RTCP carried over UDP) 5

6 Step 2- Overview 6

7 Step-2 Hints SIP facilitates in describing the media stream and transport addresses for the session SIP carries SDP description Sample SDP v=0 o=username 0 0 IN IP s=announcement c=in IP t=0 0 m=audio RTP/AVP a=rtpmap:0 PCMU/8000 a=rtpmap:3 GSM/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:97 ilbc/8000 7

8 Step-3: Hearing the audio data (eavesdropping) Accepts RTSP control connection from a media player and forward media packets to the media player. Choose one media player (for example: vlc, mplayer) Make sure to check that media player has support for RTSP based streaming Machines at Maarintalo Computing Centre has vlc and mplayer installed with the required streaming libraraies. Provide support for simple RTSP messages like OPTIONS, DESCRIBE, SETUP, PLAY, PAUSE and TEARDOWN Note: Construct simple responses that the media player can understand. With regard to the message construction, a little trial and error approach is needed as different media players may behave differently 8

9 Finally we spied on the announcement 9

10 Step-3 Hints RTSP facilitates in describing the media stream and transport addresses for the session RTSP carries SDP description RTP-Info: header(s) in required in PLAY response Include most recent RTP sequence number and timestamp Collect from incoming media streams, by parsing the RTP packets A sample that gives idea on the construction of RTSP reply messages Response to SETUP RTSP/ OK\r\n CSeq: %d\r\n Session: %d\r\n Transport: RTP/AVP;unicast; \ client_port=p1-p2; \ server_port= \r\n \r\n 10

11 sipspy Listen for an incoming TCP connection from a SIP Phone When a SIP phone makes a SIP based control connection request, the connection is accepted and another connection is made by the sipspy to the SIP server address (specified in the command line) So, Now a connection between the SIP Phone and the SIP server gets established through the sipspy. The messages from either side need to be transfered across. The message from the SIP phone and SIP server need to be modified, so that the RTP media packets gets forwarded to the sipspy instead of SIP Phone. The SIP Phone would be receiving media packets through sipspy. sipspy is also listening on an another port number specified on the command line. When a Media Player (like vlc, mplayer) makes a RTSP based control connection to the specified listening port, the connection is accepted and a session is established. 11

12 sipspy contd... The media packets received from the SIP announcement server, would be forwarded to both the media player and SIP Phone. Terminating the program with Ctrl-C (SIGINT) will cause it to dump a summary on the number of RTP packets received so far. The announcement from the SIP server gets played on both the SIP Phone as well as the Media Player. So, when the security of SIP proxy is compromised, it would be possible to eavesdrop on a conversation that was established using SIP signaling. 12

13 sipspy -h <SIP-Server-addr> -i <if-addr> -r <rtsp-addr> -S <sip-addr> -h: SIP server name to which the connection need to be forwarded <<IPv4 address>:<port No.>> <<Server Name>:<Port No.>> -i: Address of the local interface to use for listening for control connections <IPv4 address> -r: Transport address to accept RTSP control connection request <Port Number> -s: Transport address to accept SIP connection request <Port Number> (Optionally you could provide support for IPv6) Examples: sipspy -h nmps.netlab.hut.fi:5060 -i xx.yy -r s 5060 sipspy -h i:5060 -i xx.yy -r s

14 sipspy ports The below picture gives an overview of the port usage of sipspy In the command line, we have not specified details on the port usage for RTP and RTCP packets. You are free to chose the port numbers that are available for use. 14

15 Notes: read() from TCP sockets Some players send multiple requests and do not wait for the response Need to parse what you read from TCP Also: you may not get all of a (large) request in a single read() system call There MAY be multiple requests received in a single read() system call Example: SET_PARAMETER rtsp:// :554/trailer RTSP/1.0 CSeq: 4 Subscribe: stream=0;rule=0,stream=0;rule=1 Session: PLAY rtsp:// :554/trailer RTSP/1.0 CSeq: 5 User-Agent: RealMedia Player (HelixDNAClient)/ (win32) Session: Range: npt=0-15

16 Assignment-2 Documentation Submit a document along with the code of the second assignment The document should contain details about The design decisions made during the implementation of sipspy Modules of your sipspy code Special features if any The setup for which your implementation works (details like the type of media player and sip phone used) Issues faced during the implementation. (the feedback would help us, in providing better course environment) The above information would also help us in understanding and testing your implementations Keep the documentation short and simple. 16

17 Slide Access SIP and Assignment-2 slides will only be accessible from HUT network ( /16) 17