Basics of Protocols SIP H 323 MGCP Presented

Basics of Protocols SIP / H. 323 / MGCP Presented by Jonathan Cumming jonathan. cumming@dataconnection. com

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

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 o Call Establishment Media Negotiation • What sort of session – voice, video, messaging? o Media Transport

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

Session Initiation Protocol • IETF-based • Developed from work on multi-party conferences • Releases o o RFC 2543 RFC 3261 1999 2002 - Better scalability and resilience • The protocol chosen for next generation mobile and fixed networks (3 GPP and IMS) • Huge amount of work extending the protocol

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

SIP Architecture • SIP is used for o o o Registration and Call Routing Call Admission Control (performed by proxy) 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

SIP Terminology • Endpoints are SIP User Agents (UA) o o o User Agent Clients (UAC) send requests User Agent Servers (UAS) process requests and send responses 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

Structure of a SIP message • Request o o o Request URI Headers Body sip: user@host To: …, From: …, etc. SDP offer • Response o o o Status Line Headers Body 180 Ringing To: …, From: …, etc. SDP answer

SIP Extensibility • Anyone can define SIP extensions o o New Message types 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

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 o Offer contains list of supported streams and codecs Answer contains list of accepted streams and codecs • Supports wide range of media: RTP, ATM, video

H. 323 • Published by ITU-T • Developed from H. 320 - Conferencing over ISDN o Adapted for unreliable packet-based networks. • Widely used for conferencing and IP telephony • Releases o o H. 323 v 1 v 2 v 3 v 4 1996 1998 - Useable Vo. IP support 1999 - Improved scalability 2000 - Improved web support, inc URLs

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

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

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

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

H. 245 • Provides o o o Terminal capability exchange, e. g. Codecs supported Channel signaling to open the media sessions 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

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

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

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

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 (Packet. Cable standards) • Megaco/H. 248 jointly developed by IETF and ITU as a replacement to MGCP o o Cleaner and more powerful architecture => simpler flows Most MGCP messages have direct equivalents in Megaco • Mandated for next generation networks (inc. IMS), but not yet as widely deployed

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

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

MGCP/Megaco Terminology • MCGP • Megaco o Endpoint A media source or sink. A media gateway is considered as a collection of endpoints, e. g. DS 0, Analog line, etc. o Termination A media source or sink. This could be either a physical device, e. g. DS 0, or an ephemeral termination such as an RTP stream. o Connection A connection is an association between two endpoints, which may be on the same or different MGs. o Context A connection is created by placing terminations into the same context

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

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

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 o SIP is easier to extend and develop with than H. 323 Megaco has a more powerful architecture that MGCP • But H. 323 and MGCP will still be here in 10 years