M2UA AND M2PA

1
M2UA AND M2PA

• Submitted by,
• Srinivas Kommineni,
• Gayathri Sarivisetti,
• Vivek Nemarugommula.

2
Agenda

• Introduction of SS7
• M2UA
• M2PA
• Differences between M2UA and M2PA
• Conclusion
• References

3
SS7 Protocols

• Common Channel Signaling System No. 7 (i.e., SS7
  or C7) is a global standard for
  telecommunications defined by the International
  Telecommunication Union (ITU) Telecommunication
  Standardization Sector (ITU-T).
• The standard defines the procedures and protocol
  by which network elements in the public switched
  telephone network (PSTN) exchange information
  over a digital signaling network to effect
  wireless (cellular) and wireline call setup,
  routing and control.

4
Standard SS7 Layer Summary
5
Message Transfer Part

• The lowest level, MTP Level 1, is equivalent to
  the OSI Physical Layer. MTP Level 1 defines the
  physical, electrical, and functional
  characteristics of the digital signaling link.
• MTP Level 2 ensures accurate end-to-end
  transmission of a message across a signaling
  link. Level 2 implements flow control, message
  sequence validation, and error checking.
• MTP Level 3 provides message routing between
  signaling points in the SS7 network. MTP Level 3
  re-routes traffic away from failed links and
  signaling points and controls traffic when
  congestion occurs. MTP Level 3 is equivalent to
  the OSI Network Layer.

6
SCCP TCAP

• Signaling Connection Control Part (SCCP)SCCP is
  used as the transport layer for TCAP-based
  services. SCCP provides global title translation
  (GTT) capabilities above MTP Level 3.
• TCAP supports the exchange of non-circuit related
  data between applications across the SS7 network
  using the SCCP connectionless service.

7
SS7 Classic

• The term SS7 classic differentiates between SS7
  over IP and narrowband 64-kilobit SS7.
• SS7 classic is signaling for call delivery that
  follows a separate physical path from the bearer
  channel to set up calls.

8
Evolution to SS7 over IP

• A Signaling Transport (sigtran) working group is
  focusing on how the existing SS7 protocol might
  run over IP.
• The first step is converting elementssuch as
  simple control transport protocol (SCTP) to run
  directly over IP, thus replacing transmission
  control protocol (TCP) and user datagram protocol
  (UDP) to provide a reliable transport for
  signaling in the telephony networks.

9
Uses of SS7 Network

• The SS7 network and protocol are used for
• basic call setup, management, and tear down
• wireless services such as personal communications
  services (PCS), wireless roaming, and mobile
  subscriber authentication
• local number portability (LNP)
• toll-free (800/888) and toll (900) wireline
  services
• efficient and secure worldwide telecommunications

10
Introduction (M2UA)

• M2UA is a protocol for transporting SS7 MTP2-User
  signaling e.g., MTP3 messages over IP using the
  services of the Stream Control Transmission
  Protocol (SCTP).
• The M2UA protocol is the layer between SCTP and
  MTP3 that separates the physical SS7 termination
  from the actual signaling point within the
  network.

11
M2UA Overview

• M2UA deployments consist of 2 entities, the
  client and the server.
• The server provides physical SS7 termination and
  communicates with the client over an SCTP
  association using IP.
• The client houses the MTP3 and thus is the point
  code addressable element within the SS7 network.

12
M2UA in the SG to MGC Application
13
Architecture of M2UA
14
Common Message Header
15
M2UA Message Header
16
Applications

• M2UA serves several purposes.
• The first purpose is to provide a mechanism for
  the transport of SS7 MTP2 user signaling (e.g.,
  MTP3 messages) over IP using SCTP.
• The second purpose is to allow remote placement
  of SS7 link terminations and back haul SS7
  traffic to a centralized point in the network.

17
Services Provided by the M2UA Adaptation Layer

• The SS7 MTP3/MTP2(MTP2-User) interface is
  retained at the termination point in the IP
  network, so that the M2UA protocol layer is
  required to provide the equivalent set of
  services to its users as provided by the MTP
  Level 2 to MTP Level 3.
• Support for MTP Level 2 / MTP Level 3 interface
  boundary
• Support for communication between Layer
  Management modules on SG and MGC
• Support for management of active associations
  between SG and MGC

18
Functions Provided by the M2UA Layer

• Mapping
• Flow Control / Congestion
• SCTP Stream Management
• Seamless SS7 Network Management Interworking
• Active Association Control

19
Security

• M2UA is designed to carry signaling messages for
  telephony services. As such, M2UA MUST involve
  the security needs of several parties the end
  users of the services the network providers and
  the applications involved.
• As a transport protocol, M2UA has the following
  security objectives
• Availability of reliable and timely user
  data transport.
• Integrity of user data transport.
• Confidentiality of user data.

20
Threats

• Blind Denial of Service Attacks
• Flooding
• Masquerade
• Improper Monopolization of Services
• When the network in which M2UA runs in involves
  more than one party, it MAY NOT be reasonable to
  expect that all parties have implemented security
  in a sufficient manner. In such a case, it is
  recommended that IPSEC is used to ensure
  confidentiality of user payload.

21
M2PA-Message Transport protocol peer-to-peer
adaptation layer

• M2PA is the peer-to-peer equivalent of M2UA.
• M2PA allows communication between SS7 systems
  over IP rather than T-1 or E-1 TDM links.
• An M2PA link may be used in place of an MTP2
  link, removing the need for dedicated and
  expensive SS7 hardware.
• The M2PA protocol is the layer between SCTP and
  MTP Level 3.
• M2PA provides a means for peer MTP3 layers in SGs
  to communicate directly, it extends the reach of
  SS7 over the IP network.

22
Role of M2PA in Evolution to SS7 over IP

• M2PA allows the classical SS7 link to be replaced
  by SS7 over IP while maintaining the SS7 link
  topology.

23
Purpose of M2PA

• Provides a mechanism for the transport of SS7
  MTP2 user signaling (e.g., MTP3 messages) over IP
  using SCTP.
• Enables seamless operation between MTP2 user
  peers in the SS7 and IP space.

24
M2PA Symmetrical Peer-to-Peer Architecture
25
M2PA Symmetrical Peer-to-Peer Architecture

• MTP3 is adapted to the SCTP layer using M2PA.
• All primitives between MTP3 and MTP2 are
  supported by M2PA.

26
Architecture of M2PA in a Signaling Gateway
27
M2PA in IP Signaling Gateway
28
Architecture of M2PA in a Signaling Gateway

• SG is an IPSP that is equipped with both
  traditional SS7 and IP network connections.
• Architecture is applicable for an SG to SG
  connection, used to bridge SS7 network islands.
• SG and the IPSP communicate through an IP link
  using the M2PA protocol. Messages sent from the
  SEP to the IPSP (and vice versa) are routed by
  the SG.
• MTP3 is present on each SG to provide routing and
  management of the MTP2/M2PA links. Because of the
  presence of MTP3, each SG would require its own
  SS7 point code.
• M2PA has no knowledge of the upper SS7 layer.

29
M2PA in IP Signaling Gateway

• The IPSP's MTP3 uses its underlying M2PA as a
  replacement for MTP2.
• Communication between the two layers MTP3/M2PA
  is defined by the same primitives as in SS7
  MTP3/MTP2.
• M2PA uses the SCTP association as an SS7 link.
  The M2PA/SCTP/IP stack can be used in place of an
  MTP2/MTP1 stack.

30
Functions Provided by M2PA

• MTP2 Functionality M2PA provides MTP2
  functionality that is not provided by SCTP thus,
  together M2PA and SCTP provide functionality
  similar to that of MTP2.
• SCTP provides reliable, sequenced delivery of
  messages.
• M2PA functionality includes
• Data retrieval to support the MTP3 changeover
  procedure.
• Reporting of link status changes to MTP3.
• Processor outage procedure.
• Link alignment procedure.

31
SCTP Association Management

• SCTP allows a user-specified number of streams to
  be opened during initialization.
• Responsibility of M2PA to ensure proper
  management of the streams.
• M2PA uses two streams in each direction for each
  association.
• - Stream 0 is designated for Link Status
  messages.
• - Stream 1 is designated for User Data
  messages, as well as Link Status messages that
  must remain in sequence with the User Data
  messages.
• Separating results in M2PA to prioritize the
  messages in a manner similar to MTP2.

32
M2PA Association State Transition Diagram
33
Description of M2PA Association states

• IDLE State of the association during power up
  initialization
• ASSOCIATING M2PA is attempting to establish an
  SCTP association.
• ESTABLISHED SCTP association is established.

34
M2PA Link State Control

• M2PA link moves from one state to another in
  response to various events. The events that may
  result in a change of state include
• - MTP3 primitive requests
• Receipt of messages from the peer M2PA
• Expiration of timers
• SCTP notifications

35
M2PA Applications
36
M2PA Applications

• M2PA used in SS7 offloading applicationsCommunica
  tion between node SEP1 and SEP2 is done via two
  SGs. Both SEP1 and SEP2 are connected to two
  different Signaling Gateways via SS7 interface.
  These Signaling Gateways are connected to each
  other via SIGTRAN (M2Pa SCTP) and acts as STP
  Nodes. Signaling messages from SEP1 and SEP2 are
  passed via these two Signaling Gateways. This
  application can be termed as SS7 offload.
• M2PA used in IP based signaling pointsIn this
  case Signaling Points are connected to each other
  using IP network. These IP based signaling points
  (IPSP) uses M2PA links instead of MTP2 links.
  These IP bases signaling points can also connect
  to signaling points in SS7 network, via M2PA
  based Signaling Gateway.

37
Services provided by M2PA

• M2PA receives the primitives sent from MTP3 to
  its lower layer.
• M2PA processes these primitives or maps them to
  appropriate primitives at the M2PA/SCTP
  interface.
• Also M2PA sends primitives to MTP3 similar to
  those used in the MTP3/MTP2 interface.

38
Types of messages

• Message Signal Units (MSUs)
• Link Status Signal Units (LSSUs)
• Fill-In Signal Units (FISUs)

39
Types of messages (contd..)

• MSUs originate at a higher level than MTP2, and
  are destined for a peer at another node. M2PA
  passes these messages from MTP3 to SCTP as data
  for transport across a link. These are called
  User Data messages in M2PA.
• LSSUs allow peer MTP2 layers to exchange status
  information. Analogous messages are needed for
  M2PA. The Link Status message serves this
  purpose.
• FISUs are transmitted continuously when no other
  signal units are waiting to be sent. FISUs also
  carry acknowledgement of messages. Since an IP
  network is a shared resource, it would be
  undesirable to have a message type that is sent
  continuously as is the case with FISUs.
  Furthermore, SCTP does not require its upper
  layer to continuously transmit messages.
  Therefore, M2PA does not provide a protocol data
  unit like the FISU. The M2PA User Data message is
  used to carry acknowledgement of messages. If
  M2PA needs to acknowledge a message, and it has
  no MTP3 message of its own to send, an empty User
  Data message can be sent.

40
M2PA Procedures

• Messages passed between MTP3 and M2PA are the
  same as those passed between MTP3 and MTP2.
• M2PA interprets messages from MTP3 and sends the
  appropriate message to SCTP. Likewise, messages
  from SCTP are used to generate a meaningful
  message to MTP3.
• LINK Initialization Alignment
• An example of the message flow used to bring an
  SS7 link in service is shown
• The purposes of the alignment procedure
  are
• (1) To provide a handshaking procedure so that
  both endpoints are prepared to send SS7 traffic,
  and to prevent traffic from being sent before the
  other end is ready.
• (2) To verify that the SCTP association is
  suitable for use as an SS7 link.

41
Link Initialization - Alignment
42
Link Initialization

• If SCTP fails to establish the association, and
  M2PA has received a Start Request from its MTP3,
  then M2PA SHALL report to MTP3 that the link is
  out of service.
• The Link Status Out of Service message replaces
  the SIOS message of MTP2
• After the association is established, M2PA SHALL
  send a Link Status Out of Service message to its
  peer. Prior to the beginning of alignment, M2PA
  MAY send additional Link Status Out of Service
  messages.
• M2PA MAY send additional Link Status Alignment
  until it receives Link Status Alignment, Link
  Status Proving Normal, or Link Status Proving
  Emergency from the peer.
• If proving is performed, then during the proving
  period (i.e., after M2PA starts the proving
  period timer T4), M2PA SHALL send Link Status
  Proving messages to its peer at an interval
  defined by the protocol parameter
  Proving_Interval
• The Link Status Ready message is used to verify
  that both ends have completed proving. When M2PA
  starts timer T1, it SHALL send a Link Status
  Ready message to its peer in the case where MTP2
  would send a FISU after proving is complete.

43
Link Initialization - Proving
44
Message Transmission and Reception

• Link Initialization In Service
• Messages are transmitted using the Data Request
  primitive from MTP3 to M2PA.
• The message is passed from MTP3 of the source to
  MTP3 of the destination.

45
Link Status Indication

• If SCTP sends a Communication Lost primitive to
  M2PA, M2PA notifies MTP3 that the link is out of
  service. MTP3 responds in its usual way.

46
Processor Outage

• The Link Status Processor Outage message replaces
  the SIPO message of MTP2.
• M2PA SHALL send a Link Status Processor Outage
  message to its peer at the beginning of a
  processor outage condition where MTP2 would send
  SIPO. M2PA MAY send additional Link Status
  Processor Outage messages as long as that
  condition persists.
• M2PA sends a Link Status message to its peer. The
  peer M2PA notifies MTP3 of the outage. MTP3 can
  then follow the processor outage procedures.
• When the local processor outage condition ends,
  M2PA SHALL send a Link Status Processor Recovered
  message to its peer on the User Data stream. This
  message is used to signal the end of the
  processor outage condition, instead of an MSU or
  FISU, as is used in MTP2.
• Upon receiving the Link Status Processor
  Recovered message, the M2PA in RPO SHALL respond
  with a Link Status Ready message on the User Data
  stream.
• When M2PA experiences a local processor outage,
  it MAY put the link out of service by sending a
  Link Status Out of Service message, if this is
  allowed by the applicable MTP2 standard

47
Processor Outage
48
Flow control

• M2PA SHALL send a Link Status Busy message to its
  peer at the beginning of a receive congestion
  condition.
• M2PA MAY send additional Link Status Busy
  messages as long as that condition persists. When
  the condition ends, M2PA SHALL send a Link Status
  Busy Ended message to its peer
• When the peer M2PA receives the first Link Status
  Busy message, it SHALL start the Remote
  Congestion timer T6 if there are messages in the
  retransmission buffer awaiting acknowledgement
  (i.e., T7 is running). M2PA SHALL stop the T7
  timer if it is running. Additional Link Status
  Busy messages received while T6 is running do not
  cause T6 to be reset and do not cause T7 to be
  started. While T6 is running, T7 SHALL NOT be
  started.
• When the peer M2PA receives the Link Status Busy
  Ended message and T6 has not expired, it SHALL
  stop T6 (if T6 is running) and start T7 (if there
  are messages awaiting acknowledgement in the
  retransmission buffer).
• The peer M2PA SHOULD continue receiving and
  acknowledging messages while the other end is
  busy, but MUST NOT send User Data messages after
  receiving Link Status Busy and before receiving
  Link Status Busy Ended.

49
Flow Control

• Level 2 Flow Control- Congestion Ceases

50
Level 2 Flow Control-Timer T6 Expires
51
MTP3 Signaling Link Congestion

• M2PA SHALL detect transmit congestion in its
  buffers according to the requirements for
  signaling link transmit congestion in MTP3
• M2PA notifies MTP3 of congestion onset and
  abatement. The notification includes the
  congestion level, if there are levels of
  congestion defined.

52
Link Deactivation

• MTP3 can request that a link be taken out of
  service.
• M2PA SHALL send a Link Status Out of Service
  message to its peer at the beginning of a
  condition where MTP2 would send SIOS. M2PA MAY
  send additional Link Status Out of Service
  messages as long as that condition persists.

53
Link Changeover

• The objective of the changeover is to ensure that
  signaling traffic carried by the unavailable
  signaling link is diverted to the alternative
  signaling links as quickly as possible while
  avoiding message loss, duplication, or
  mis-sequencing.
• MTP3 performs a changeover because the link went
  out of service. MTP3 selects a different link to
  retransmit the unacknowledged and unsent
  messages.
• MTP2's Forward and Backward Sequence Numbers are
  only seven bits long. Hence, it is necessary for
  MTP3 to accommodate the larger sequence numbers.
  This is done through the use of the Extended
  Changeover Order (XCO) and Extended Changeover
  Acknowledgement (XCA) messages instead of the
  Changeover Order (COO) and Changeover
  Acknowledgement (COA) messages.
• If M2PA receives a Retrieval Request and FSNC
  request from MTP3, M2PA SHALL retrieve from its
  buffers and deliver to MTP3
• BSNT - Backward Sequence Number to be Transmitted
  
• FSNC - Forward Sequence Number of last message
  accepted by remote level 2
• For emergency changeover, MTP3 retrieves only the
  unsent messages for transmission on the alternate
  links. If M2PA receives a Retrieval Request and
  FSNC request with no FSNC value, or with an
  invalid FSNC, then M2PA SHALL retrieve from its
  buffers and deliver to MTP3.

54
Link Changeover
55
Security Issues

• M2PA is designed to carry signaling messages for
  telephony services. As such, M2PA MUST involve
  the security needs of several parties
• - the end users of the services
• - the network providers
• - the applications involved

56
M2PA Protocol Extensions

• This protocol may be extended through IANA
  (Internet Assigned Numbers Authority) in three
  ways
• - through definition of additional message
  classes,
• - through definition of additional message types,
  and
• - through definition of additional message
  parameters.

57
Differences between M2PA and M2UA

• M2PA IPSP processes MTP3/MTP2 primitives.
• M2UA MGC transports MTP3/MTP2 primitives
  between the SG's MTP2 and the MGC's MTP3 (via the
  NIF) for processing.
• M2PA SG-IPSP connection is an SS7 link.
• M2UA SG-MGC connection is not an SS7 link. It
  is an extension of MTP to a remote entity.
• M2PA SG is an SS7 node with a point code.
• M2UA SG is not an SS7 node and has no point
  code.
• M2PA SG can have upper SS7 layers, e.g., SCCP.
• M2UA SG does not have upper SS7 layers since it
  has no MTP3.
• M2PA relies on MTP3 for management procedures.
• M2UA uses M2UA management procedures.

58
M2PA and M2UA similarities

• Both transport MTP3 messages.
• Both present an MTP2 upper interface to MTP3.

59
Specification Issues

• In M2PA/SCTP, there is no mechanism for
  immediately stopping acknowledgement of incoming
  messages.
• No Link Status Out of Service message
• - If M2PA keeps an association up when the
  link is out of service, there should be a Link
  Status Out of Service message. M2PA could then
  inform its peer that it is in the Out of Service
  state.
• M2PA draft does not give clear advice on when to
  abort an association because of poor association
  performance.

60
Benefits of SS7oIP

• reduced infrastructure costs.
• enhanced efficiency.
• new opportunities to deploy revenue-generating
  applications and services.

61
Conclusions

• The goal of SS7 is to provide a signaling
  network, and performance characteristics to
  facilitate communication between carrier grade
  network elements in circuit switched and mobile
  networks.
• The purpose is to provide a mechanism for the
  transport of SS7 MTP2 user signaling (e.g., MTP3
  messages) over IP using SCTP.
• M2PA provides MTP2 functionality that is not
  provided by SCTP thus, together M2PA and SCTP
  provide functionality similar to that of MTP2.
• M2PA interprets messages from MTP3 and sends the
  appropriate message to SCTP. Likewise, messages
  from SCTP are used to generate a meaningful
  message to MTP3.

62
Questions

• 1. Differences between M2UA and M2PA?
• - Refer slide 57.
• 2. What functions does M2PA support?
• a.) seamless operation of MTP3 protocol peers
  over an IP network connection.
• b.) The MTP2/MTP3 interface boundary,
  management of SCTP transport associations, and
  traffic instead of MTP2 Links.
• c.) Asynchronous reporting of status changes to
  management.
• 3. Services Provided by the M2UA Adaptation Layer
  ?
• - Refer slide 17.

63
References

• http//www.hssworld.com/voip/stacks/sigtran/Sigtra
  n_M2PA/overview.htmm2pa
• http//www.protocols.com/pbook/sigtran.htm
• http//www.commsdesign.com/design_corner/showArtic
  le.jhtml?articleID16502464
• http//www.analogzone.com/nett0105.pdf
• http//www.zytrax.com/tech/ss7/sigtran_intro.html
• http//www.faqs.org/ftp/rfc/pdf/rfc4165.txt.pdf
• http//quimby.gnus.org/internet-drafts/draft-georg
  e-sigtran-m2pa-interop-00.txt
• http//community.roxen.com/developers/idocs/rfc/rf
  c4165.html
• http//www.pt.com/tutorials/iptelephony/tutorial_v
  oip_mtp.html
• http//www.ietf.org/rfc/rfc3331.txt
• http//www.ulticom.com/html/products/sigtran/m2ua.
  asp