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

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

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

4. Call vs. Media Gateway Control Protocols
Call Control – SIP and H.323
Peer-to-Peer protocol
Call Routing by proxy / gatekeeper
Media Gateway Control – MGCP, Megaco/H.248
Master-Slave protocol
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
RFC
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
RFC 3262 Reliable Provisional Responses
RFC 3263 Server Location
RFC 3265 SIP Events
RFC 3264 Offer / Answer model
Organizations, including ETSI, have defined higher-level features and conformance feature sets.

7. SIP Architecture SIP is used for
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

8. SIP Terminology Endpoints are SIP User Agents (UA)
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
They cannot generate new requests
Registrars are UAS that record the location of clients
A Registrar is normally colocated with a proxy

9. Structure of a SIP message
Request
Request URI
Headers To: …, From: …, etc.
Body SDP offer
Response
Status Line 180 Ringing
Headers To:…, From: …, etc.
Body SDP answer

10. SIP Extensibility Anyone can define SIP extensions
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

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
Offer contains list of supported streams and codecs
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.320 - Conferencing over ISDN
Adapted for unreliable packet-based networks.
Widely used for conferencing and IP telephony
Releases
H.323 v1 1996
H.323 v Useable VoIP support
H.323 v Improved scalability
H.323 v Improved web support, inc URLs

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

14. H.323 Architecture H.225 RAS is used with a Gatekeeper for
Registration
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
allow the gatekeeper to manage the endpoint
allow the endpoint to request admission for a call
allow the endpoint to resolve addresses
RAS messages
Gatekeeper Discovery (Gxx), and Registration (Rxx)
Admission (Axx), Location (Lxx), and Bandwidth (Bxx)
Disengage (Dxx), Information (Ixx), and various others

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

17. H.245 Provides Optimization options
Terminal capability exchange, e.g. Codecs supported
Channel signaling to open the media sessions
Conference control
Optimization options
H.245 can be tunneled in H.225 Call Signaling channel
Normally transmitted in its own TCP connection
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 Differences H.323 Advantages
Use RTP and RTCP for media transport
Support call routing through proxies/gatekeepers using username, phone numbers or URLs
Similar flows
Differences
Encoding (Text vs. ASN.1)
Standardized Feature sets
Conference control
Attended and blind transfer
Caller Preferences
H.323 Advantages
More compact messages
More mature (in some areas)
SIP Advantages
Easier to prototype (text)
More flexible extensibility
More scalable
loop detection
Same messages used throughout network
Cleaner separation of layers
Use of SDP is compatible with MGCP and Megaco

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

20. Example 2: Call Setup SIP H.323 with FastConnect
Admission control and routing
Provided by routing INVITE message through proxies
Call Setup
INVITE
180 ringing
200 OK
ACK
H.323 with FastConnect
Admission control and routing
ARQ -> Gatekeeper
ACF returned on success
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
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

22. MGCP/Megaco Architecture
Text protocol
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
Many defined, e.g. Basic Line, ATM, DTMF

23. MGCP/Megaco Operation
MG contacts its MGC when initialized
MGC can AUDIT the MG to discover available resources
MGC tells MG
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

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

25. MGCP/Megaco Features Media bridging Conferencing Media transcoding
MGCP – By connecting endpoints together
Megaco – By placing two terminations in the same context
Conferencing
MCGP – By connection multiple endpoints to a conference bridge
Megaco - By placing multiple terminations in the same context
Media transcoding
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
SIP and H.323 are complementary to MGCP and Megaco
SIP and Megaco are the protocols of the future
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