Internet Telephony: Architecture and Protocols an IETF Perspective

1. Internet Telephony: Architecture and Protocols an IETF Perspective Authors:Henning Schulzrinne, Jonathan Rosenberg. Presenter: Sambhrama Mundkur

2. Contents • Introduction • Features of IPTelephony • Protocols – RTP, RTCP, SIP, WASRV, RTSP • Protocol Integration • How does IPTel differ from GSTN • Conclusion

3. Introduction • IPTel – Real time delivery of voice between two or more parties, across the network using IP and the exchange of the information required to control this delivery. • Global multimedia communication system. • Differs from multimedia streaming protocols. • Uses SIP as a signaling protocol and RTP to transmit the packets.

4. Internet protocol stack

5. Features of IPTelephony • Adjustable quality • Security • User Identification • User Interface • Computer-Telephony integration • Feature Ubiquity • Multimedia • Silence Suppression and Compression • Shared Facilities and advanced services • Separation of voice and control flow

6. IPTel Requirements • Sequencing • Intra-media synchronization • Inter-media synchronization • Payload identification • Frame indication • Multicast-friendly • Media-independent • Mixers and Translators • QoS Feedback • Loose Session control • Encryption

7. Real Time Protocol

8. Real-Time Control Protocol • Companion control protocol for RTP. • Sender Report, Receiver Report, Source Description. • Participants send RTCP packets periodically. • Ensures bandwidth used for RTCP reports is fixes independent of the group size. • RTCP provides the following services: Q0S monitoring and congestion control, source identification, inter-media synchronization, • RTCP can be used for resource reservation also.

9. Session Initiation Protocol- SIP • SIP is the IPTelephony signaling protocol • Name Translation and User Location • Feature Negotiation • Call Participant Negotiation • Feature Changes

10. Overview of SIP • Client-server text-based protocol; used over UDP or TCP • User Agent Server – SIP enabled end-system • Proxy Servers • Redirect Servers • SIP Entities – Logical connection source, Logical connection destination, Media destination, Media capabilities • SIP methods – INVITE, BYE, OPTIONS, STATUS, ACK, CANCEL, REGISTER

11. SIP Proxy Server Operation

12. SIP Redirect Server Operation

13. Telephony services using SIP • SIP doesn’t explicitly describe the implementation of a particular service. • Uses headers (Ex. Also, Replaces) and methods (Ex. STATUS) to construct the services . • Example of Call Forwarding Service

14. Multi-party protocols • Via network-level multicast • Via one or more bridges (multipoint control units) • Mesh of unicast connections

15. Additional protocols • IP host communicates with a GSTN endpoint using an Internet Telephone Gateway (ITG) • WASRV – Wide Area Service Location server • Real Time Streaming Protocol (RTSP) – establishes and controls streams of continuous audio and video between the media servers and the clients.

16. Protocol Integration

17. IPTel versus GSTN • IPTel - End to end paradigm • IPTel - Separates call setup from resource reservation • IPTel - Addresses for end-point identification and basic service indication • IPTel - Better scaling for services (Authenticated caller ID etc.) • IPTel - No UNI/NNI distinction both at signaling and transport • IPTel - Components can be provided by different vendors.

18. Conclusion • Paper presents the protocols that support IP telephony – RTP for transport, SIP for signaling, RTSP for stored media retrieval. • IPTel has to overcome unpredictable QoS in the wide area, the lack of reliable and cheap end systems, Internet and the lack of billing infrastructure.