2. January 23-26, 2007 Ft. Lauderdale, Florida An introduction to SIP Simon Millard Professional Services Manager Aculab

3. January 23-26, 2007 Ft. Lauderdale, Florida An introduction to SIP Agenda –SIP concepts –Media –SIP signalling –NAT traversal –Security

4. January 23-26, 2007 Ft. Lauderdale, Florida SIP concepts SIP is the Session Initiation Protocol –Its job is to set up a session (maybe a phone call) between two or more users

5. January 23-26, 2007 Ft. Lauderdale, Florida SIP concepts SIP’s view of the network is the same as the Internet’s –Intelligence at the edge –Re-use of proven devices and concepts There is the ability to negotiate supported features –Can set up any type of media SIP separates media from signalling

6. January 23-26, 2007 Ft. Lauderdale, Florida Media For IP telephony we are concerned with RTP Ethernet, optical, radio, … IP UDP RTP CODECs RTCP

7. January 23-26, 2007 Ft. Lauderdale, Florida Media More data is sent than in a TDM call CHKETHIPUDPRTPAUDIO Silence elimination –CNG –VAD

8. January 23-26, 2007 Ft. Lauderdale, Florida Media compression The rain in Spain falls mainly on the plain –Lossless $ r# in Sp# falls m#ly on $ pl# –$ = the #=ain –Lossy Th rn n Spn flls mnly n th pln

9. January 23-26, 2007 Ft. Lauderdale, Florida SIP signalling Coded in ASCII Verbs (methods) and responses –INVITE initiate a session –ACK confirm session established –BYE terminate a session –CANCEL cancel a pending INVITE –REGISTER bind an address to a location –++

10. January 23-26, 2007 Ft. Lauderdale, Florida SIP signalling Responses – as per HTTP 1xx information –100 trying, 180 ringing 2xx success –200 OK 3xx redirection –300 multiple choices 4xx client error –404 not found 5xx server failure 6xx global failure

11. January 23-26, 2007 Ft. Lauderdale, Florida SIP signalling Media for the session is described by the SDP (session description protocol)

12. January 23-26, 2007 Ft. Lauderdale, Florida Signalling – UAs SIP based on UAs (User Agents) –UAC initiates requests –UAS responds to requests sip:simon@192.168.0.100 response UACUAS

13. January 23-26, 2007 Ft. Lauderdale, Florida Signalling – Proxies Route signalling –Do not initiate requests or responses –Pass through unknown messages unchanged –Stateless or stateful sip:simon@aculab.com Aculab Proxy sip:simon@work sip:simon@home

14. January 23-26, 2007 Ft. Lauderdale, Florida Signalling – Registrars Allow a SIP device to dynamically register a location –This allows them to be contactable when mobile Aculab Registrar 192.168.0.102 REGISTER sip:simon@aculab.com Location database

15. January 23-26, 2007 Ft. Lauderdale, Florida Signalling – Redirect Servers Respond to a request by redirecting it to another device Aculab Redirect Server 192.168.0.102 request for sip:simon@aculab.com moved to sip:simon@xx.xx.xx.xx sip:simon@aculab.com registered from xx.xx.xx.xx request for sip:simon@xx.xx.xx.xx

16. January 23-26, 2007 Ft. Lauderdale, Florida Signalling – B2BUA A back-to-back User Agent is somewhat similar to a Proxy, but terminates and initiates SIP signalling B2BUA UA

17. January 23-26, 2007 Ft. Lauderdale, Florida Putting it all together proxy.a.com INVITE DNS server SIP SRV b.com proxy.b.com INVITE location server simon? simon@192.168.0.100:5060 INVITE RTP BYE

18. January 23-26, 2007 Ft. Lauderdale, Florida NAT traversal Network Address Translation –IP-Masquerading Source and/or destination addresses re-written Most widely used to allow multiple hosts on a private network to access the Internet from a single public IP address Solved the IP address shortage of IPv4

19. January 23-26, 2007 Ft. Lauderdale, Florida NAT traversal NAT binding is created by the NAT to map a private to a public address Binding lifetime –Period of time for which the binding remains open –Binding will be closed if there is no traffic for a period of time

20. January 23-26, 2007 Ft. Lauderdale, Florida NAT traversal Full cone Internal IP address and port mapped one-to-one to external IP address and port External host can reach internal by sending to IP:port

21. January 23-26, 2007 Ft. Lauderdale, Florida NAT traversal Restricted cone Internal IP:port mapped one-to-one to external IP:port External host can reach internal client only if traffic has already been sent to it

22. January 23-26, 2007 Ft. Lauderdale, Florida NAT traversal Port restricted External host can reach internal port only if traffic has already been sent to it from that port

23. January 23-26, 2007 Ft. Lauderdale, Florida NAT traversal Symmetric Requests from an internal IP:port are mapped to a unique external IP:port Only a host which receives a packet can send packets back

24. January 23-26, 2007 Ft. Lauderdale, Florida NAT traversal STUN STUN is a client/server protocol Client sends request to STUN server which responds with the IP address of the NAT and the port which was opened for the request

25. January 23-26, 2007 Ft. Lauderdale, Florida NAT traversal STUN works with full cone, restricted cone and port restricted NATs Will not work with symmetric NAT –IP address of the STUN server is different to that of the destination endpoint Peers communicate discovered IP:port information –In a full cone, any endpoint can initiate the session

26. January 23-26, 2007 Ft. Lauderdale, Florida Security SIP signalling –Digest authentication, based on knowledge of a shared secret

27. January 23-26, 2007 Ft. Lauderdale, Florida Security SIP signalling –TLS – Transport Layer Security –Based on public key cryptography Client requests TLS session Server responds with public certificate Client verifies certificate Mutual exchange of session keys Send/receive application data using keys –Can be used hop-by-hop –SIPS requires TLS used end-to-end

28. January 23-26, 2007 Ft. Lauderdale, Florida Security Media –Uses SRTP (secure RTP) –AES encryption typically using 128 bit keys –Assumes secure key exchange prior to the session running Most commonly used are Mikey and SDES (SDES within SDP so need to secure the SIP session)

29. January 23-26, 2007 Ft. Lauderdale, Florida Summary Session Initiation Protocol leverages Internet technologies Signalling and media paths Other devices NAT traversal issues Security

30. January 23-26, 2007 Ft. Lauderdale, Florida Thank you simon.millard@aculab.com Visit Aculab on booth 1217