Gsm protl

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Gsm protl

  1. 1. GSM PROTOCOL skgochhayat
  2. 2. Functions Of Protocols• Session establishment and termination between users.• Orderly exchange of Data messages.• Coding of the information.• Routing and Sequencing• Flow control and Congestion control.• Error checking and recovery.• Efficient network resource utilization
  3. 3. Example Of Some Functions• Code conversion to facilitate understanding of the meaning.• Routing of the messages through Network.• Error control to counteract effect of disturbances.• Transmission of Electrical Signals.
  4. 4. OSI Layers• The OSI model is built of seven ordered layers: – Layer-7: Application – Layer-6: Presentation Application – Layer-5: Session Presentation – Layer-4: Transport Session Transport – Layer-3: Network Network – Layer-2: Data Link Data Link Physical – Layer-1: Physical
  5. 5. OSI Layers• The seven layers can be thought of as belonging to three sub groups – Network Support Layers (Layers 1-3) • Deal with the physical aspects of moving data from one device to another – User Support Layers (Layers 5-7) • Allow interoperability among unrelated software systems – Layer-4 ensures end to end reliable data transmission
  6. 6. OSI Layers End System Application User Support Presentation Layers Session Network Support Layers TransportNetwork Network Network NetworkData Link Data Link Data Link Data LinkPhysical Physical Physical PhysicalPSPDN PSTN Dedicated CSPDN
  7. 7. Hierarchical communication.• Within a single machine, each layer calls upon the services of the layer just below it• The passing of data and network information is made possible by an interface between each pair of adjacent layers• The messages exchanged between the adjacent layers, to obtain the required services, are called Interface Control Information (ICI)
  8. 8. Peer-to-Peer communication• Between machines, layer-n on one machine communicates with layer-n on another machine• This communication is governed by an agreed-upon series of rules and conventions called protocols• The processes on each machine that communicates at a given layer are called peer-to-peer processes• At the physical layer, communication is direct• At higher layers, communication moves down through the layers on the transmitting machine and back up through the layers at receiving machine
  9. 9. Hierarchical & Peer-to-Peer CommunicationsHierarchicalN+1-Layer PCI N+1-Layer Interface Interface N-Layer Peer-to-Peer N-Layer ICI InterfaceControl Information (ICI) Protocol Control Information (PCI)
  10. 10. Data Units in the OSI Model (N+1)-PDU (N+1)-PCI (N+1)-SDU (N)-ICI(N+1)-Layer (N)-IDU (N+1)-PCI (N+1)-SDU (N)-ICI(N)-Layer (N)-ICI (N)-SDU (N+1)-PCI (N+1)-SDU (N)-PCI (N)-PDU (N)-PCI (N+1)-PCI (N+1)-SDU
  11. 11. Data Units• Protocol Control Information (PCI) – (N)-PCI is the protocol control information exchanged between the (N)-entities to coordinate their functions• Service Data Units (SDU) – (N)-SDU is the data unit transferred between the ends of a (N)-connection whose identity is preserved during the transfer• Protocol Data Unit (PDU) – (N)-PDU is the combination of (N)-PCI and (N)-SDU
  12. 12. Data Units• Interface Control Information (ICI) – (N)-ICI is the information exchanged between (N+1)-entity and (N)-entity to coordinate their functions• Interface Data Unit (IDU) – (N)-IDU is the total data unit transferred across the SAP between (N+1)-entity and (N)-entity
  13. 13. OSI Layers7-Application 7-Application Link Intermediate Link 7-6 Interface 7-6 Interface Node6-Presentation 6-Presentation 6-5 Interface 6-5 Interface 5-Session 5-Session 5-4 Interface 5-4 Interface 4-Transport 4-Transport 4-3 Interface 4-3 Interface 3-Network 3-Network 3-Network 3-2 Interface 3-2 Interface 3-2 Interface 2-Data Link 2-Data Link 2-Data Link 2-1 Interface 2-1 Interface 2-1 Interface 1-Physical 1-Physical 1-Physical
  14. 14. Summary of OSI Layers Functions Allow access to network Application resources 7-6 Interface Translate, encrypt and Presentation compress data 6-5 Interface Establish, manage and Session terminate sessions 5-4 InterfaceReliable end to end delivery Transport & error recovery 4-3 Interface Movement of packets; Network Provide internetworking 3-2 InterfaceOrganise bits into streams; Data Link Node to node delivery 2-1 Interface Transmit bits; Mechanical Physical and electrical specifications
  15. 15. Summary of OSI Layers Functions Allow access to network Application resources 7-6 Interface Translate, encrypt and Presentation compress data 6-5 Interface Establish, manage and Session terminate sessions 5-4 InterfaceReliable end to end delivery Transport & error recovery 4-3 Interface Movement of packets; Network Provide internetworking 3-2 InterfaceOrganise bits into streams; Data Link Node to node delivery 2-1 Interface Transmit bits; Mechanical Physical and electrical specifications
  16. 16. GSM System Architecture BSS NSS 16Kb/s GSTN R TRAU MSC 64Kb/s A BSC ISDN D VLR BTS SS7 I O HLR NSSMS BSS : Base Station Sub-system BSC : Base Station Controller NSS : Network and Switching Sub-system BTS : Base Transceiver Station MSC: Mobile service Switching Center TRAU : Transcoder / Rate Adapter Unit HLR : Home Location Register VLR : Visitors Location Register
  17. 17. GSM protocol layers for signaling Um Abis A MS BTS BSC MSC CM CMMM MM BSSAP BSSAP RR RR’ RR’ BTSM BTSM SS7 SS7LAPDm LAPDm LAPD LAPDradio radio PCM PCM PCM PCM 16/64 kbit/s 64 kbit/s / 2.048 Mbit/s
  18. 18. ISDN Protocol• Two types of ISDN Interfaces : Basic Rate Interface (BRI), and Primary rate interface (PRI), provide multiple digital bearer channels over which temporary connections can be made and data can be sent.• The result is digital dial access to multiple site concurrently. Type of Interface Number of Bearer Number of Signaling Channels (B Channels (D channels) Channels) BRI 2 1 (16 Kbps) PRI(T/1) 23 1 (64 Kbps) PRI(E/1) 30 1 (64 Kbps)
  19. 19. ISDN Channels• B Channels : Bearer channels (B channels) are used to transport data. B Channels are called bearer channels because they bear the burden of transporting the data. B channels operate up to 64 Kbps, although the speed might be lower depending on the service provider.• D Channels are used for signaling. LAPD is used to deliver signaling message to the ISDN switch
  20. 20. LAPD and PPP on D and B Channels BRI BRI B0 SS7 B0 B1 Call Setup B1 D LAPD Flows D LAPD Call Setup Flows Call Setup Flows ISDN Network BRI PPP BRI B0 B0 B1 B1 D LAPD D LAPD ISDN Network
  21. 21. LAPD and PPP on D and B Channels• The call is established through the service provider network; PPP is used as the data link protocol on the B channel from end to end. LAPD is used between the router and the ISDN switch at each local central office (CO) and remains up so that new signaling messages can be sent and received. Because the signals are sent outside the channel used for data, this is called out-of-band signaling.• The BRI encodes bits at 192 kbps, out of which 144 Kbps is used by B and D Channels rest is used for framing.
  22. 22. Use of ISDN as WAN Protocol• Dial on Demand Routing – Logic is configured in the routers to trigger the dial when that traffic needs to get to another site is sent by user.• Telecommuting Environment• Backup to leased lines – When leased line fails, an ISDN call is established between two routers.
  23. 23. ISDN as WAN LinkDial onDemandRouting ISDN Network Telecommuting NT1 ISDN Network Computer with ISDN Inerface Leased Line Backup Leased Line BRI BRI ISDN Network
  24. 24. Layer 2 LAPDM Protocol• Establishment and release of signalling layer 2 connections.• Multiplexing and de multiplexing of several signalling layer 2 connection on a dedicated control channel and discrimination between them by including different Service Access Point Identifiers (SAPI).• Mapping of signalling layer 2 service data units on protocol data unit (in case of acknowledged operation service data units may be segmented and reassembled at destination).• Detection and recovery of errors due to loss, duplication, and disorder.• Flow control.
  25. 25. LAPDM Protocol• The establishment and release of layer 2 connection coincides with the allocation release, and change of dedication radio channels. Signaling layer 2 connections are frequently established and released, and thus an average lifetime of a connection is short. Multiplexing and demultiplexing deals with arranging different user (eight channels per frame) in a frame format.• LAPDm uses on the two modes of operation for the transmission of layer 3 message; unacknowledged operation of multiple frame operation.
  26. 26. LAPDM Protocol• On the DCCH both unacknowledged and acknowledged operations are used, DCCHs (SDCCH, SACCH and FACCH)• whereas on the CCCHs only unacknowledged operation is applied. Thus, both modes are applicable for transmission over on of the in contrast top information transfer over CCCHs (BCCH, PCH and AGCH)
  27. 27. LAPDM Protocol• For an unacknowledged information transfer, the use of layer 3 service implies that the information transfer is not acknowledged by the data link layer, and thus error check facilities are not provided. The transmission and reception of messages here use data link service primitives, that is, DL-DATA-REQUEST and DL-DATA-INDICATION.
  28. 28. LAPD and LAPDm• The main distinction between LAPD and LAPDm is the absence of address and control fields. Thus, the protocol is only used for the unacknowledged mode of operation, which applies to BCCHs and CCCHs only. Both FCCH and SCH under BCCH do not require unacknowledged. Similarity, no acknowledgement is needed for PCH and AGCH.• The LAPD frame is used internal to BSS, namely, between BTS and BSC.
  29. 29. MTP3, SCCP, and TCAP Protocols.
  30. 30. The Telephone Network [1/2]SS7 Signaling Service Service ISUP Messages + Control Data INAP/TCAP Messages Point Point Signal Transfer Control Layer Point Intelligent Transport Layer Peripheral Class 4 Class 5 Tandem Switch End Office Switch Circuit Switched Network
  31. 31. The Telephone Network [2/2]• 5 Basic Components in Intelligent Networks – SSP/Service Switching Point • switching, service invocation – STP/Service Transfer Point • signal routing SCP SDP – SCP/Service Control Point TCAP messages • service logic execution IP – SDP/Service Data Point STP STP • subscriber data storage, access – IP/Intelligent Peripheral SSP ISUP messages SSP • resources such as customized voice announcement, voice recognition, Voice DTMF digit collection
  32. 32. Signalling example BBSR Exch CUTTACKUser A Exch User B(calling Exch (called user) user) KOLKOTA DatabaseA typical scenario:User A calls mobile user B. The call is routed to a specificgateway exchange (GMSC) that must contact a database(HLR) to find out under which exchange (MSC) the mobileuser is located. The call is then routed to this exchange.
  33. 33. SS7 Protocol Suite OSI Layers IN Application Application INAP MAP Part Transaction Mobile Capabilities Presentation Application Applications Session TCAP ISUP Part Part ISDN UserSignaling Transport SCCP PartConnectionControl Part Network MTP Level 3 Data Link MTP Level 2 Physical MTP Level 1
  34. 34. MTP Levels 1 & 2• Message Transfer Part• Level 1 – Handling the issues related to the signals on the physical links between one signaling node and another – Closely to layer 1 of the OSI stack• Level 2 – Dealing with the transfer of messages on a given link from one node to another – Providing error detection/correction and sequenced delivery of the SS7 messages – signalling network supervision and maintenance functions
  35. 35. MTP Level 3• Signaling message handling – Providing message routing between signaling points in the SS7 network – May pass a number of intermediate nodes (STP, Signal Transfer Point) – MTP level 3 ”users” are ISUP and SCCP• Signaling network management – Rerouting traffic to other SS7 signaling links in the case of link failure, congestion or node failure – Load-sharing
  36. 36. Services– Provides a number of services to the protocol layer above it • The transfer of messages • Indicating availability of resources • MTP-Transfer request, MTP-Transfer indication, MTP_Pause indication, MTP-Resume indication, and MTP-Status indication
  37. 37. ISUP• ISDN User Part• Used as the protocol for setting up and tearing down phone calls between switches• Initial Address Message (IAM) – To initiate a call between two switches• Answer Message (ANM) – To indicate that a call has been accepted by the called party• Release Message (REL) – To initiate call disconnection
  38. 38. Connection-Oriented Protocol– A connection-oriented protocol • Related to the establishment of connections between users • The path of messages and the path of the bearer might be different
  39. 39. SCCP• Signaling Connection Control Part• Used as the transport layer for TCAP-based services – Free phone (800/888), calling card, wireless roaming• Both connection-oriented and connectionless – Mostly connectionless signaling• Global title translation (GTT) capabilities – The destination signaling point and subsystem number is determined from the global title
  40. 40. TCAP, MAP and INAP• TCAP (Transaction Capabilities Applications Part) – Supporting the exchange of non-circuit related information between signaling points – Queries and responses sent between SSPs and SCPs are carried in TCAP messages• Provides services to – INAP (IN Application Part) – MAP (Mobile Application Part)
  41. 41. SS7 Network Architecture• Figure 7-4 depicts a typical SS7 network arrangement.• This configuration serves several purposes. – No direct signaling links – A fully meshed signaling network is not required. – The quad arrangement ensures great robustness.
  42. 42. Signaling Point (SP)• Each node in an SS7 network is an SP.• The signaling address of the SP is known as a signaling point code (SPC).• Linkset – Group of signaling links directly connecting two SPCs – For capability and security reasons• Service Switching Point (SSP)
  43. 43. Signal Transfer Point (STP)• To transfer messages from one SPC to another
  44. 44. Service Control Point (SCP)• A network entity that contains additional logic and that can be used to offer advanced services• The switch sends a message to the SCP asking for instructions. – The SCP, based upon data and service logic that is available, will tell the switch which actions need to be taken.• An good example – toll-free 800 number
  45. 45. – An example • A subscriber dials a toll-free 800 number • The SSP knows that it needs to query the SCP • The SCP contains the translation information • The SCP responds to the SSP with a routable number • The SSP routes the call • Connectionless signaling • The application use the services of TCAP, which in turn uses the services of SCCP
  46. 46. Message Signal Units (MSUs) – The messages sent in the SS7 network•Backward Sequence Number•BSN Indicator Bit•Forward Sequence Number•Length Indicator
  47. 47. Message Signal Units (MSUs)• The messages sent in the SS7 network• The format of an MSU – SIO – Service Information Octet • Indicate the upper-level protocol (e.g., SCCP or ISUP) • A sub-service field indicating the signaling numbering plan – SIF – Signaling Information Field • The actual user information • The ANSI version and the ITU-T version • The routing label – The Destination Point Code (DPC) – The Originating Point Code (OPC)
  48. 48. • Signaling Link Selection (SLS) – The particular signaling link to be used
  49. 49. • SS7 addressing – The ANSI version, 24 bits • Member, cluster, network codes • An operator has a network code – The ITU-T version, 14 bits• International Signaling Gateway – Use sub-service field • National, Nation Spare, International, International Spare • An international gateway has one national point code and one international code
  50. 50. • International Signaling
  51. 51. Same SPCs can be reused at different network levelsInternational SPC = 277National SPC = 277SPC = 277 means different signalling points (network elements)at different network levels.The Service Information Octet (SIO) indicates whether the DPCand OPC are international or national signalling point codes. F CK SIF SIO LI Control F
  52. 52. ISDN User Part (ISUP)ISUP is a signalling application protocol that is used for establishingand releasing circuit-switched connections (calls). • Only for signalling between exchanges (ISUP can never be used between an exchange and a stand-alone database) • Not only for ISDN (=> ISUP is generally used in the PSTN)Structure of ISUP message: SIO (one octet) Routing label (four octets) CIC (two octets) Must always be included in ISUP message Message type (one octet) E.g., IAM message Mandatory fixed part Mandatory variable part E.g., contains called (user B) number in Optional part IAM message
  53. 53. The ISDN User Part (ISUP)• ISUP – The most-used SS7 application – The establishment and release of telephone calls – IAM • Called number, calling number, transmission requirement, type of caller, … – ACM • The call is through-connected to the destination • A one-way-audio path is opened for ring-back tone • Optional – If not returned, no ring-back tone at all
  54. 54. – CPG, Call Progress • Optional; provide information to the calling switch – ANM, Answer Message • Open the transmission path in both directions • Instigate charging for the call – REL, Release – RLC, Release Complete• CIC, circuit identification Code – Indicates the specific trunk between two switches – OPC, DPC, and CIC
  55. 55. Difference between SLS and CICThe four-bit signalling link selection (SLS) field in the routinglabel defines the signalling link which is used for transfer of thesignalling information.The 16-bit circuit identification code (CIC) contained in theISUP message defines the TDM time slot or circuit with whichthe ISUP message is associated. Signalling link STP Exchange Exchange Circuit
  56. 56. ISUP Call Establishment and• A given circuit between two Releaseswitches is identified by OPC,DPC and CIC.
  57. 57. Signalling using IAM message STP STP SL 4 SL 7 SPC = 82 SPC = 22 Circuit SPC = 60 20 Circuit 14 Exchange Exchange ExchangeOutgoing message: Processing in (transit) exchange(s):OPC = 82 CIC = 14 Received IAM message contains B-number.DPC = 22 SLS = 4 Exchange performs number analysis (not part of ISUP) and selects new DPC (60) and CIC (20).
  58. 58. Setup of a call using ISUPUser A LE A Transit exchange LE B User B Setup IAM IAM Setup DSS1signalling Number analysisassumed Alert ACM ACM Alert Connect ANM ANM Connect Charging of call starts now
  59. 59. ISUP message format
  60. 60. Signalling Connection Control Part (SCCP)SCCP is required when signalling information is carried betweenexchanges and databases in the network.An important task of SCCP is global title translation (GTT): Exchange STP Database STP with GTT capability1. Exchange knows the global title (e.g. 0800 number or IMSI number in a mobile network) but does not know the DPC of the database related to this global title.2. SCCP performs global title translation in the STP (0800 or IMSI number => DPC) and the SCCP message can now be routed to the database.
  61. 61. Example: SCCP usage in mobile callMobile switching center (MSC) needs to contact the home locationregister (HLR) of a mobile user identified by his/her InternationalMobile Subscriber Identity (IMSI) number. SCCP/GTT functionality STP SCCP SPC = 32 SCCP MSC located in Espoo HLR located in Oslo SPC = 82 SPC = 99Outgoing message: Processing in STP:OPC = 82 DPC = 32 Received message is given to SCCP for GTT.SCCP: IMSI global title SCCP finds the DPC of the HLR: DPC = 99
  62. 62. To sum it up with an example…Part B, Section 3.3 in ”UnderstandingTelecommunications 2” PSTN Typical operation of Transmission a local exchange (PDH, SDH) Databases in Subscriber signalling the network Network- (HLR) (analog or ISDN=DSS1) internal signalling (SS7)
  63. 63. Basic local exchange (LE) architectureModern trend: Switching and control functions are separated intodifferent network elements (separation of user and control plane). Subscriber stage Switching system TDM links LIC Time ETC to other Group switch Tone network switch LIC Rx ETC elements Tone generator Sign. ExchangeLine terminalinterface circuit • Switch controlcircuit • E.164 number analysis SS7 Signalling • Charging equipment • User databases • O&M functions Control system
  64. 64. Setup of a call (1)Phase 1. User A lifts handset and receives dial tone. Local exchange of user A 4. Tone Rx is connected Switching system1. Off hook LIC Time ETC Group switch Tone switch LIC Rx ETC5. Dial tone Tone generator Sign.is sent(indicating“network is 2. Check user database. For instance, isalive”) user A barred for outgoing calls? 3. Reserve memory for user B number Control system
  65. 65. Setup of a call (2)Phase 2. Exchange receives and analyzes user B number. Local exchange of user A Switching system LIC Time ETC Group switch Tone switch LIC Rx ETC1. User A 2. Number (DTMF signal) received Sign.dials user Bnumber 3. Number analysis 4. IN triggering actions? Should an external database (e.g. SCP, HLR) be contacted? Control system
  66. 66. Setup of a call (3)Phase 3. Outgoing circuit is reserved. ISUP Initial address message(IAM) is sent to next exchange. Local exchange of user A Switching system E.g., CIC = 24 LIC Time ETC Group switch Tone switch LIC Rx ETC 1. Tone receiver IAM is disconnected Sign. (contains information CIC = 24) 2. Outgoing circuit is reserved 3. Outgoing signalling message (ISUP IAM) contains user B number Control system
  67. 67. Setup of a call (4)Phase 4. ACM received => ringback or busy tone generated. ANMreceived => charging starts. Local exchange of user A Switching system LIC Time ETC Group switch switch LIC ETC ACM,2. Ringback Tone generator Sign. ANMor busy toneis locallygenerated 1. ISUP ACM message indicates free or busy user B 3. Charging starts when ISUP ANM message4. Call is receivedcontinues… Control system
  68. 68. Performance Requirements for SS7• Bellcore spec. GR-246-Core – MTP • A given route set should not be out of service for more than 10 minutes per year • < 1*10-7 messages should be lost • < 1*10-10 messages should be delivered out of sequence – ISUP • Numerous timing requirements• A VoIP network that uses SS7 – Must meet the stringent requirements – Signaling Transport (Sigtran) group of the IETF
  69. 69. Performance Requirements for SS7• Long-distance VoIP network• A given route set should not be out of service for more than 10 minutes per year.• No more than 1x10-7 messages should be lost.• No more than 1x10-10 messages should be delivered out of sequence.• In ISUP, numerous timing requirements must be met.• How to make sure that VoIP networks can emulate the signaling performance of SS7.• SIGTRAN (Signaling Transport) group of IETF
  70. 70. Softswitch ArchitectureSS7 Network Internet Signaling SCP (SS7) MGCP/ Gateway SIGTRAN Call MEGACO STP Agent MGCP/ MEGACO Trunking Residential Trunking Gateway Residential Gateway CO Trunking RTP Residential Gateway Gateway Switch Gateway Gateway
  71. 71. Signaling Transport (SIGTRAN)• Addressing the issues regarding the transport of signaling within IP networks – The issues related to signaling performance within IP networks and the interworking with PSTN• SIP/MEGACO/ISUP Interworking – Translating the MTP-based SS7 message (e.g., IAM) to IP-based message (e.g., IP IAM) – Just a simple translation from point code to IP address ???
  72. 72. SIGTRAN• Issues discussed in SIGTRAN – Address translation – How can we deploy an SS7 application (e.g., ISUP) that expects certain services from lower layers such as MTP when lower layers do not exist in the IP network? – For transport layer, the ISUP message must be carried in the IP network with the same speed and reliability as in the SS7. • UDP x • TCP x• RFC 2719, “Framework Architecture for Signaling Transport”
  73. 73. SIGTRAN Architecture• Signaling over standard IP uses a common transport protocol that ensures reliable signaling delivery. – Error-free and in-sequence – Stream Control Transmission Protocol (SCTP)• An adaptation layer is used to support specific primitives as required by a particular signaling application. – The standard SS7 applications (e.g., ISUP) do not realize that the underlying transport is IP.
  74. 74. ISUP Transport to MGC• NIF (Nodal Interworking Function) is responsible for interworking between the SS7 and IP networks
  75. 75. SIGTRAN Protocol Stack SCTP: fast delivery of messages (error-free, in sequence delivery), network-level fault tolerance
  76. 76. • Adaptation Layer Layer) M2UA (MTP-2 User Adaptation [1/3]
  77. 77. • Adaptation Layer [2/3] M2PA (MTP-2 Peer-to-Peer Adaptation Layer) – An SG that utilizes M2PA is a signaling node for the MGC. • It is effectively an IP-based STP. – SG can processing higher-layer signaling functions, such as SCCP GTT.
  78. 78. Adaptation Layer [3/3]• M3UA (MTP3-User Adaptation Layer)• SUA (SCCP-User Adaptation Layer) – Applications such as TCAP use the services of SUA.• IUA (ISDN Q.921-User Adaptation Layer)• V5UA (V5.2-User Adaptation Layer)
  79. 79. SCTP• To offer the fast transmission and reliability required for signaling carrying.• SCTP provides a number of functions that are critical for telephony signaling transport. – It can potentially benefit other applications needing transport with additional performance and reliability.• SCTP must meet the Functional Requirements of SIGTRAN.
  80. 80. Why not use TCP?• TCP provides both reliable data transfer and strict order-of-transmission, but SS7 may not need ordering. – TCP will cause delay for supporting order-of- transmission.• The limited scope of TCP sockets complicates the task of data transmission using multi- homed hosts.• TCP is relatively vulnerable to DoS attack, such as SYN attacks.
  81. 81. What Supported By Using SCTP?• To ensure reliable, error-free, in-sequence delivery of user messages (optional).• To support fast delivery of messages and avoid head-of-line blocking.• To support network-level fault tolerance that is critical for carrier-grade network performance by using multi-home hosts.• To provide protection against DoS attack by using 4-way handshake and cookie.
  82. 82. SCTP Endpoint & Association• Endpoint – The logical sender/receiver of SCTP packets. – Transport address = IP address + SCTP port number – An endpoint may have multiple transport addresses (for multi-homed host, all transport addresses must use the same port number.)• Association – A protocol relationship between SCTP endpoints. – Two SCTP endpoints MUST NOT have more than one SCTP association.
  83. 83. Multi-Homed Host Host A Host B SCTP User SCTP User SCTP SCTPOne IP address One IP address One IP address One SCTP association with multi-homed redundant
  84. 84. SCTP Streams• A stream is a one-way logical channel between SCTP endpoints. – The number of streams supported in an association is specified during the establishment of the association.• To avoid head-of-line blocking and to ensure in-sequence delivery – In-sequence delivery is ensured within a single stream.
  85. 85. SCTP Functional View SCTP User Application Sequenced deliveryAssociation within streams startup and User Data Fragmentation takedown Acknowledgement and Congestion Avoidance Chunk Bundling Packet Validation Path Management
  86. 86. SCTP Packets & Chunks• A SCTP packet can comprise several chunks.• Chunk – Data or control 0 . . . 15 16 . . . 31 Source Port Number Destination Port Number Common Header Verification Tag Checksum Chunk Type Chunk Flags Chunk Length Chunk 1 Chunk Value Chunk N . . .
  87. 87. Chunk Type– ID Value Chunk Type– -------- ---------------– 0 - Payload Data (DATA)– 1 - Initiation (INIT)– 2 - Initiation Acknowledgement (INIT ACK)– 3 - Selective Acknowledgement (SACK)– 4 - Heartbeat Request (HEARTBEAT)– 5 - Heartbeat Acknowledgement (HEARTBEAT ACK)– 6 - Abort (ABORT)– 7 - Shutdown (SHUTDOWN)– 8 - Shutdown Acknowledgement (SHUTDOWN ACK)– 9 - Operation Error (ERROR)– 10 - State Cookie (COOKIE ECHO)– 11 - Cookie Acknowledgement (COOKIE ACK)– 12 - Reserved for Explicit Congestion Notification Echo (ECNE)– 13 - Reserved for Congestion Window Reduced (CWR)– 14 - Shutdown Complete (SHUTDOWN COMPLETE)– … - Reserved for IETF
  88. 88. SCTP control chunks• INIT chunk – Initiate an SCTP association between two endpoints – Cannot share an SCTP packet with any other chunk• INIT ACK – Acknowledge the initiation – Must not share a packet with any other chunk• SACK – Acknowledge the receipt of Data chunks – Inform the sender of any gaps • Only the gaps need to be resent
  89. 89. • HEARTBEAT – When no chunks need to be sent – Send periodic HEARTBEAT messages – Contain sender-specific information• HEARTBEAT ACK – Containing heartbeat information copied form HEARTBEAT• ABORT – End an association abruptly – Cause information – Can be multiplexed with other SCTP control chunks • Should be the last chunk, or …
  90. 90. • SHUTDOWN – A graceful termination of an association – Stop sending any new data – Wait until all data sent has been acknowledged – Send a SHUTDOWN to the far end • Indicate the chunk received – Upon receipt of a SHUTDOWN • Retransmit data that are not acknowledged • Send a SHUTDOWN ACK• SHUTDOWN ACK• SHUTDOWN COMPLETE
  91. 91. • ERROR – Some error condition detected • E.g., a chunk for a non-existent stream• COOKIE ECHO – Used only during the initiation of an association – An INIT ACK includes a cookie parameter – Information specific to the endpoint, a timestamp, a cookie lifetime – Upon receipt of an INIT ACK • Return the cookie information in COOKIE ECHO • Can be multiplexed; must be the first chunk• COOKIE ACK – Can be multiplexed; must be the first chunk
  92. 92. INIT Chunk0 . . . 15 16 . . . 31 Type = 1 Chunk Flags Chunk Length Initial Tag Advertised Receiver Window Credit (a_rwnd)Number of Outbound Streams Number of Inbound Streams Initial TSN (Transmission Sequence Number) Optional / Variable-Length Parameter . . .
  93. 93. Association Establishment INIT [I-Tag=Tag_A] INIT ACK [V-Tag=Tag_A, I-Tag=Tag_Z, Cookie_Z]A COOKIE [Cookie_Z] Z COOKIE ACK allocating resources
  94. 94. User Data Transfer User MessagesSCTP user SCTP DATA Chunks SCTP Control Chunks SCTP packetsSCTP Connectionless Packet Transfer Service (e.g. IP)
  95. 95. DATA Chunk0 . . . 15 16 . . . 31 Type = 0 Reserved UB E Chunk Length TSN Stream ID = S Stream Sequence Number = n Payload Protocol ID User Data (Sequence n of Stream S) . . . U : unordered B : begin E : end
  96. 96. • Payload data chunk – Carry information to and from the ULP – U: unordered bit • The information should be passed to the ULP without regard to sequencing – B and E: beginning and end bits • Segment a given user message – TSN: Transmission Sequence Number (32-bit) • Independent of any streams • Assigned by SCTP • An INIT has the same TSN as the first DATA chunk • TSN ++ for each new DATA chunk
  97. 97. – S: Stream Identifier (16-bit)– n: stream sequence number (16-bit) • Begins at zero • Increments for each new message– Payload protocol identifier • For the users to pass further information about the chunk but is not examined by the SCTP
  98. 98. SACK Chunk0 . . . 15 16 . . . 31 Type = 3 Chunk Flags Chunk Length Cumulative TSN Ack Advertised Receiver Window Credit (a_rwnd) Number of Gap Ack Blocks = n Number of Duplicate TSNs = x Gap Ack Block #1 Start Gap Ack Block #1 End . . . Duplicate TSN #1 . . .
  99. 99. • Transferring data – Reliable transfer – SACK chunk • Cumulative TSN – The highest TSN value received without any gaps –4 • The number of Gap Ack Blocks – The number of fragments received after the unbroken sequence –2 • The number of duplicate TSNs –2
  100. 100. • Gap Ack Block number 1 start – The offset of the first segment from the unbroken sequence – 3 (7-4)• Gap Ack Block number 1 end – The offset of the first segment from the unbroken sequence – 8 (8-4)• a_rwnd – The updated buffer space of the sender
  101. 101. SCTP Robustness• Robustness is a key characteristic of any carrier- grade network. – To handle a certain amount of failure in the network without a significant reduction in quality• INIT and INIT ACK chunks may optionally include one or more IP addresses (a primary address + several secondary addresses). – Multi-homes hosts• SCTP ensures that endpoint is aware of the reachability of another endpoint through the following mechanisms. – SACK chunks if DATA chunk have been sent – HEARTBEAT chunks if an association is idle
  102. 102. M3UA Operation• M3UA over SCTP• Application Server – A logical entity handling signaling for a scope – A CA handles ISUP signaling for a SS7 DPC/OPC/CIC- range – An AS contains a set of Application Server Processes (ASPs)• ASP – A process instance of an AS – Can be spread across multiple IP addresses – Active ASPs and standby ASPs
  103. 103. • Routing Key – A set of SS7 parameters that identifies the signaling for a given AS – OPC/DPC/CIC-range• Network Appearance – A mechanism for separating signaling traffic between an SG and an ASP – E.g., international signaling gateway
  104. 104. Signaling Network Architecture• No single point of failure – SGs should be set up at least in pairs – ASPs • A redundant or load-sharing configuration • Spread over different hosts• Point code – All ASPs and the connected SG share the same PC • A single SS7 signaling endpoint – All ASPs share a PC != that of the SG • ASPs: a signaling endpoint; SG: an STP – A group of ASPs share a PC
  105. 105. • Robust Signaling Architecture
  106. 106. Services Provided by M3UA• Offer the same primitives as offered by MTP3 – MTP-Transfer request – MTP-Transfer indication – MTP-Pause indication • Signaling to a particular destination should be suspended – MTP-Resume indication • Signaling to a particular destination can resume – MTP-Status indication • Some change in the SS7 network • E.g., network congestion or a destination user part becoming unavailable
  107. 107. • Transferring application message – A CA sends an ISUP message – MTP-Transfer request – A SCTP DATA chunk – Transmitted to a SG – M3UA – MTP3 – To the SS7 network
  108. 108. • M3UA Messages – Messages between peer M3UA entities – A header + the M3UA message content – The entities can communicate information regarding the SS7 network • If a remote destination becomes unavailable • The SG becomes aware of this through SS7 signaling- network management messages • The SG pass M3UA messages to the CA • The ISUP application at the CA is made aware – MTP-Pause indication
  109. 109. Signaling Network Management MSGs• S7ISO – SS7 Network Isolation – When all links to the SS7 network have been lost• DUNA – Destination Unavailable – Sent from the SG to all connected ASPs – Destination(s) within the SS7 network is not available • Allocate 24 bits for each DPC – DUNA is generated at the SG • It determines from MTP3 network management message – The M3UA of the ASP • Create MTP-Pause indication
  110. 110. • DAVA - Destination Available – Sent from SG to all concerned ASPs – Mapped to the MTP-Resume indication• DAUD – Destination State Audit – Sent from an ASP to an SG – To query the status of one or more destination – The SG responds with DAVA, DUNA, or SCON• SCON – SS7 Network Congestion – Sent from the SG to ASPs – The route to an SS7 destination is congested – Mapped to the MTP-Status indication
  111. 111. • DUPU – Destination User Part Unavailable – Sent from the SG to ASPs – A given user part at a destination is not available – The DPC and the user part in question – Mapped to MTP-Status indication – Cause codes• DRST – Destination Restricted – Sent from the SG to ASPs – One or more SS7 destinations are restricted from – The M3UA may use a different SG
  112. 112. ASP management• ASPUP – ASP Up – Used between M3UA peers – The adaptation layer is ready to receive traffic or maintenance messages• ASPDN – ASP Down – An ASP is not ready• UP ACK – ASP Up Ack• DOWN ACK – ASP Down Ack
  113. 113. • ASPAC – ASP Active – Sent by an ASP – Indicate that it is ready to be used – To receive all messages or in a load-sharing mode – Routing context • Indicate the scope is applicable to the ASP – DPC/OPC/CIC-range• ASPIA – ASP Inactive• ACTIVE ACK – ASP Active Ack• INACTIVE ACK – ASP Inactive Ack
  114. 114. • BEAT – Heartbeat – Between M3UA peers – Still available to each other – When M3UA use the services of SCTP • The BEAT message is not required at the M3UA level – SCTP includes functions for reachability information• ERR – Error message – A received message with invalid contents• NFTY – Notify – Between M3UA peers – To communicate the occurrence of certain events
  115. 115. Routing Key Management Messages• Registration Request (REG REQ) – An ASP = a DPC/OPC/CIC range• Registration Response• Deregistration Request• Deregistration Response
  116. 116. M2UA Operation• MTP3/M2UA/SCTP• The CA has more visibility of the SS7 network – More tightly coupled to the SG• MTP3 – Routing and distribution capabilities• M2UA uses similar concepts to those used by M3UA – ASPUP, ASPDN, ASPAC, ASPIA and ERR – Exactly the same functions – In M2UA, the ASP is an instance of MTP3
  117. 117. • M2UA-specific messages – DATA • Carry an MTP2-user Protocol Data Unit – ESTABLISH REQUEST • To establish a link to the SG – ESTABLISH CONFIRMATION – RELEASE REQUEST • Request the SG to take a particular signaling link out of service – RELEASE CONFIRM – RELEASE INDICATION • The SG autonomously take a link out of service

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