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Simon Millard Professional Services Manager Aculab – booth 402

The State of SIP. Simon Millard Professional Services Manager Aculab – booth 402. The state of SIP. Agenda SIP concepts Media SIP signalling NAT traversal Security. SIP concepts. SIP is the Session Initiation Protocol

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Simon Millard Professional Services Manager Aculab – booth 402

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  1. The State of SIP Simon Millard Professional Services Manager Aculab – booth 402

  2. The state of SIP • Agenda • SIP concepts • Media • SIP signalling • NAT traversal • Security

  3. 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

  4. 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

  5. Media For IP telephony we are concerned with RTP RTCP RTP CODECs UDP IP Ethernet, optical, radio, …

  6. ETH IP UDP RTP AUDIO CHK Media • More data is sent than in a TDM call • Silence elimination • CNG • VAD

  7. 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

  8. 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 • ++

  9. 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

  10. SIP signalling • Media for the session is described by the SDP (session description protocol)

  11. sip:simon@192.168.0.100 response Signalling – UAs • SIP based on UAs (User Agents) • UAC initiates requests • UAS responds to requests UAC UAS

  12. sip:simon@work sip:simon@aculab.com sip:simon@home Signalling – Proxies • Route signalling • Do not initiate requests or responses • Pass through unknown messages unchanged • Stateless or stateful Aculab Proxy

  13. Signalling – Registrars • Allow a SIP device to dynamically register a location • This allows them to be contactable when mobile Aculab Registrar REGISTER sip:simon@aculab.com Location database 192.168.0.102

  14. request for sip:simon@aculab.com moved to sip:simon@xx.xx.xx.xx request for sip:simon@xx.xx.xx.xx sip:simon@aculab.com registered from xx.xx.xx.xx Signalling – Redirect Servers • Respond to a request by redirecting it to another device Aculab Redirect Server 192.168.0.102

  15. Signalling – B2BUA • A back-to-back User Agent is somewhat similar to a Proxy, but terminates and initiates SIP signalling B2BUA UA UA

  16. location server DNS server simon? simon@192.168.0.100:5060 SIP SRV b.com proxy.b.com proxy.b.com INVITE INVITE INVITE BYE Putting it all together proxy.a.com RTP

  17. 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

  18. 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

  19. 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

  20. 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

  21. NAT traversal • Port restricted • External host can reach internal port only if traffic has already been sent to it from that port

  22. 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

  23. 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

  24. 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

  25. Security • SIP signalling • Digest authentication, based on knowledge of a shared secret

  26. 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

  27. 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)

  28. Summary • Session Initiation Protocol leverages Internet technologies • Signalling and media paths • Other devices • NAT traversal issues • Security

  29. Thank you simon.millard@aculab.comVisit Aculab on booth 402

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