1. MEGACO SIP State Machine Inter-conversion and Message TranslationBy
Salman Abdul Baset
Zahid Anwar

2. MEGACO/H.248
Megaco is a simple control protocol that enables a network entity (responsible for setting up calls) to control the media streaming devices that perform the actual IP voice streaming.
Megaco is the result of a merger between the simple gateway control protocol (SGCP) from Bellcore, and the IP device control (IPDC) from Level 3 Communications.

3. MEGACO/H.248
Megaco allows software programs known as call agents or media gateway controllers(MGCs) to externally control and manage media streaming devices, or media gateways(MGs), at the edge of multiservice packet networks.
This separation between audio-streaming functions and call-control functions enhances system operation and simplifies operation of the media gateway.

4. Role of SIP in Megaco
Megaco is a Master-Slave protocol used between MGs and MGCs
SIP is a Peer to Peer Protocol used between User Agents to set up calls
What relationship can the two have together?

5. SIP acts as a helper Protocol
The MGC also known as the brain of the popular buzz word “Soft Switch” needs a means to communicate with the outside world
An MGC needs to communicate with other MGCs (inside the IP domain) and with the PSTN domain
SIP comes into play for MGC to MGC communication across different domains

6. Inter-Domain Signaling
Softswitch
Softswitch
SIP-T (IETF) MG MG MG MG MG MG

7. Previous Work
Documentation available for H.323 inter-working with MGCP
SIP-T: Internetworking between SIP and PSTN Networks
No detailed published material on the establishment and tear down of call and handling of different call-procedures using SIP across MGCs that use MEGACO

8. Case 1: Simple Call Flow
We would be discussing three different scenarios of SIP/H.248 communication
How a simple call is made and teared down between two terminations/ endpoints/handsets registered with MGs on different domains. We will see how SIP Capabilities and later tears up the call

9. Initial Megaco Registration

10. Ringing

11. Media Flow

12. Clear Tear Down

13. Case 2: Optimized Call Flow
Eleven SIP Messages exchanged
First INVITE used to detect presence of other MGC
Use OPTIONS instead of first INVITE
Do not enforce reliable delivery of provisional responses

15. Case 3: MGC HandOff MGC can go down for maintenance purposes
Tells its subordinate MGs to register themselves with a new MGC – A HandOff
Established Calls are not disturbed during HandOff
What happens if MG termination was in call with a SIP User Agent
Two(more…) possible approaches

16. How MGs Register with new MGCs
Failing MGC
MG2
MG1
Call Established
Hand Off
New MGC
Register Me
Audit 1 2 3 4

17. What Happens in case of SIP UA?
New MGC
MG2
MG1
Call Established
Failing MGC
SIP UA ? 5 6

18. HandOff MGC Intensive
MGCs subscribe each other for a HandOff Event through a SUBSCRIBE request
MGC going down tells MGC taking over through a NOTIFY request that HandOff occurred
All call states are embedded in XML and sent as NOTIFY Body
Seamless for remote SIP User Agent

19. HandOff MGC Intensive Contd…

20. Handoff Client Intensive
MGC going down sends a re-INVITE with ‘mgcHandOff’ in Contact parameter
SIP Client then sends a re-INVITE to new MGC (specified in earlier re-INVITE) with a ‘mgcHandOff’ parameter in Contact
MGC taking sees ‘mgcHandOff’ and constructs the call state
Not a very elegant and seamless approach
Catch: SIP User Agent must understand ‘mgcHandOff’

21. Handoff Client Intensive Contd…

22. Case 4: MEGACO messages as SIP Message Body
According to Megaco RFC 3015 Section “No recommendation is made on how the MGCs involved in the Handoff maintain state information; this is considered to be out of scope of this recommendation”.
What about in progress transactions with MGs?

23. Case 4: MEGACO Messages as SIP Message Body (Contd…)
SIP “comes to the rescue” of the failing MGC and helps transfer its states
In Progress Megaco Transactions messages embedded as SIP NOTIFY message body and sent to MGC taking over
MGC taking over continues with the Transactions

24. How are Megaco Messages Transferred as SIP Body
Content-Type: application/megaco
Content-Length: 190
MEGACO/1 [ ]:1721 Transaction = 1007{Context = - ServiceChange = ROOTMG1{Services{Method = Restart,Reason = HandOff,ServiceChangeAddress = 1721,Profile = ResGw/0,Version = 1}}}}

25. Questions?