MEGACO%20SIP%20State%20Machine%20Inter-conversion%20and%20Message%20Translation

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MEGACO SIP State Machine Inter-conversion and
Message Translation

• By
• Salman Abdul Baset
• Zahid Anwar

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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.

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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.

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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?

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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

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

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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

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Initial Megaco Registration
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Ringing
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Media Flow
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Clear Tear Down
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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

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(No Transcript)
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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

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How MGs Register with new MGCs
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What Happens in case of SIP UA?
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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

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HandOff MGC Intensive Contd
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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

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Handoff Client Intensive Contd
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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?

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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

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How are Megaco Messages Transferred as SIP Body

• Content-Type application/megaco
• Content-Length 190
•  
• MEGACO/1 172.16.15.11721 Transaction
  1007Context - ServiceChange
  ROOTMG1ServicesMethod Restart,Reason
  HandOff,ServiceChangeAddress 1721,Profile
  ResGw/0,Version 1

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Questions?