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Basics of Protocols SIP / H.323 / MGCP Presented by Jonathan Cumming

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1 Basics of Protocols SIP / H.323 / MGCP Presented by Jonathan Cumming

2 Basics of Protocols - SIP/H.323/MGCP Introduction o Terminology o Concepts Call vs. Media Gateway Control Protocols Call Control Protocols o SIP o H.323 o How SIP and H.323 compare Media Gateway Control Protocols o MGCP and Megaco/H.248 Summary

3 Concepts Multimedia requires the following features o Endpoint Registration and Call Routing Where am I and how do I find someone else? o Call Admission Control Am I allowed to make/receive a call? o Call Establishment o Media Negotiation What sort of session – voice, video, messaging? o Media Transport

4 Call vs. Media Gateway Control Protocols Call Control – SIP and H.323 o Peer-to-Peer protocol o Call Routing by proxy / gatekeeper Media Gateway Control – MGCP, Megaco/H.248 o Master-Slave protocol o Media Gateway (MG) is controlled by the Media Gateway Controller (MGC)

5 Session Initiation Protocol IETF-based Developed from work on multi-party conferences Releases o RFC o RFC Better scalability and resilience The protocol chosen for next generation mobile and fixed networks (3GPP and IMS) Huge amount of work extending the protocol

6 SIP Standards RFC 3261 defines the core SIP protocol Many extensions defined in additional RFCs o RFC 3262Reliable Provisional Responses o RFC 3263Server Location o RFC 3265SIP Events o RFC 3264Offer / Answer model Organizations, including ETSI, have defined higher- level features and conformance feature sets.

7 SIP Architecture SIP is used for o Registration and Call Routing o Call Admission Control (performed by proxy) o Call Establishment SDP (attached to SIP messages) is used to negotiate the media for the call RTP/RTCP carries the media directly between the endpoints

8 SIP Terminology Endpoints are SIP User Agents (UA) o User Agent Clients (UAC) send requests o User Agent Servers (UAS) process requests and send responses o Most endpoints are both UAC and UAS Proxies forward requests and responses o They cannot generate new requests Registrars are UAS that record the location of clients o A Registrar is normally colocated with a proxy

9 Structure of a SIP message Request o Request o HeadersTo: …, From: …, etc. o BodySDP offer Response o Status Line180 Ringing o HeadersTo:…, From: …, etc. o BodySDP answer

10 SIP Extensibility Anyone can define SIP extensions o New Message types o New Headers Unrecognized message types and headers ignored If a UAC requires a UAS to support an extension it can mandate it using Require: UAS and UAC advertise supported extensions using Supported: and Allow: headers For example: Presence and Instant Messaging have been added without changes to the core protocol

11 Session Description Protocol (SDP) Used to negotiate media channels and codecs Text-based protocol defined by the IETF Also used by MGCP and Megaco Offer/Answer handshake o Offer contains list of supported streams and codecs o Answer contains list of accepted streams and codecs Supports wide range of media: RTP, ATM, video

12 H.323 Published by ITU-T Developed from H Conferencing over ISDN o Adapted for unreliable packet-based networks. Widely used for conferencing and IP telephony Releases o H.323 v11996 o H.323 v Useable VoIP support o H.323 v Improved scalability o H.323 v Improved web support, inc URLs

13 H.323 Standards H.323 is a collection of standards o H.225Signaling: RAS, Call Signaling and Annex G o H.245Multimedia Control Protocol Related standards o H.235Security within H.245-based systems o H.245Interworking with the PSTN o H.450Supplementary Services

14 H.323 Architecture H.225 RAS is used with a Gatekeeper for o Registration o Call Routing H.225 Call signaling is used for Call Establishment H.245 is used to control the established multi- media session RTP/RTCP is normally used to carry the media

15 H.225 RAS Registration, Admission and Status Used between endpoint and gatekeeper to o allow the gatekeeper to manage the endpoint o allow the endpoint to request admission for a call o allow the endpoint to resolve addresses RAS messages o Gatekeeper Discovery (Gxx), and Registration (Rxx) o Admission (Axx), Location (Lxx), and Bandwidth (Bxx) o Disengage (Dxx), Information (Ixx), and various others

16 H.225 Call Signaling Used to establish calls between entities Derived from Q.931 Example messages o Setup o Call Proceeding o Alerting

17 H.245 Provides o Terminal capability exchange, e.g. Codecs supported o Channel signaling to open the media sessions o Conference control Optimization options o H.245 can be tunneled in H.225 Call Signaling channel Normally transmitted in its own TCP connection o Fast connect does not establish an H.245 channel Relevant H.245 fields passed in H.225 Setup message

18 Comparing SIP and H.323 Similarities o Use RTP and RTCP for media transport o Support call routing through proxies/gatekeepers using username, phone numbers or URLs o Similar flows Differences o Encoding (Text vs. ASN.1) o Standardized Feature sets Conference control Attended and blind transfer Caller Preferences H.323 Advantages o More compact messages o More mature (in some areas) SIP Advantages o Easier to prototype (text) o More flexible extensibility o More scalable loop detection o Same messages used throughout network o Cleaner separation of layers o Use of SDP is compatible with MGCP and Megaco

19 Example 1: Endpoint Registration SIP o Discovery and Registration REGISTER -> Registrar 200 OK on success >= 300 on error o DHCP can also be used for discovery H.323 o Discovery GRQ -> Gatekeeper GCF returned on success GRJ on error o Registration RRQ -> Gatekeeper Returns RCF or RQJ

20 Example 2: Call Setup SIP o Admission control and routing Provided by routing INVITE message through proxies o Call Setup INVITE 180 ringing 200 OK ACK H.323 with FastConnect o Admission control and routing ARQ -> Gatekeeper ACF returned on success o Call Setup Setup -> remote terminal Proceeding returned Connect returned Ack -> remote terminal (UDP)

21 MGCP and Megaco Originally designed to control PSTN access by IP terminals Developed by Cisco, Telcordia and Level 3 Published by IETF as RFC 2705, Oct 1999 Widely used in cable networks (PacketCable standards) Megaco/H.248 jointly developed by IETF and ITU as a replacement to MGCP o Cleaner and more powerful architecture => simpler flows o Most MGCP messages have direct equivalents in Megaco Mandated for next generation networks (inc. IMS), but not yet as widely deployed

22 MGCP/Megaco Architecture Text protocol o Binary coding available for Megaco, but hardly used Uses SDP to describe the media Uses RTP and RTCP as the media transport Packages define extensions o Many defined, e.g. Basic Line, ATM, DTMF

23 MGCP/Megaco Operation MG contacts its MGC when initialized o MGC can AUDIT the MG to discover available resources MGC tells MG o Media streams to establish o Tones to play and events to monitor o Digit maps against which to map received digits MG notifies MGC when a monitored event is detected

24 MGCP/Megaco Terminology MCGP o Endpoint A media source or sink. A media gateway is considered as a collection of endpoints, e.g. DS0, Analog line, etc. o Connection A connection is an association between two endpoints, which may be on the same or different MGs. Megaco o Termination A media source or sink. This could be either a physical device, e.g. DS0, or an ephemeral termination such as an RTP stream. o Context A connection is created by placing terminations into the same context

25 MGCP/Megaco Features Media bridging o MGCP – By connecting endpoints together o Megaco– By placing two terminations in the same context Conferencing o MCGP– By connection multiple endpoints to a conference bridge o Megaco- By placing multiple terminations in the same context Media transcoding o By bridging endpoints using different codecs

26 MGCP Example: Call Setup MG notifies MGC that the phone has gone off hook MGC tells MG to play dialtone and collect digits matching a given digit map MG notifies MGC when digits are received MGC tells MG to create a new connection and play ringing tone MGC tells MG the remote RTP info for the connection

27 Summary SIP and H.323 are equivalent MCGP and Megaco are equivalent o SIP and H.323 are complementary to MGCP and Megaco SIP and Megaco are the protocols of the future o SIP is easier to extend and develop with than H.323 o Megaco has a more powerful architecture that MGCP But H.323 and MGCP will still be here in 10 years

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