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Internet Telephony: Architecture and Protocols

an IETF Perspective

Authors:Henning Schulzrinne, Jonathan Rosenberg.

Presenter: Sambhrama Mundkur

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

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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.

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Internet protocol stack

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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

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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

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Real Time Protocol

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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.

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

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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

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SIP Proxy Server Operation

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SIP Redirect Server Operation

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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

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Multi-party protocols Via network-level multicast Via one or more bridges (multipoint control

units) Mesh of unicast connections

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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.

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Protocol Integration

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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.

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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.