MEGACO SIP State Machine Inter-conversion and Message Translation By Salman Abdul Baset Zahid Anwar

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.

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.

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?

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

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

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

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

Initial Megaco Registration

Ringing

Media Flow

Clear Tear Down

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

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

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

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

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

HandOff MGC Intensive Contd…

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’

Handoff Client Intensive Contd…

Case 4: MEGACO messages as SIP Message Body According to Megaco RFC 3015 Section 11.5 “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?

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

How are Megaco Messages Transferred as SIP Body Content-Type: application/megaco Content-Length: 190   MEGACO/1 [172.16.15.1]:1721 Transaction = 1007{Context = - ServiceChange = ROOTMG1{Services{Method = Restart,Reason = HandOff,ServiceChangeAddress = 1721,Profile = ResGw/0,Version = 1}}}}

Questions?