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TRANSCRIPT
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Alcatel 8690 SIGTRAN Description - Page 1All Rights Reserved 2007, Alcatel-Lucent
All rights reserved 2007, Alcatel-Lucent
Alcatel 8690 OSP - Alcatel 8690 SIGTRAN Description
Alcatel 8690 OSPAlcatel 8690 SIGTRAN
Description
TRAINING MANUAL
3FL12936AAAAWBZZAEdition 2
Copyright 2007 by Alcatel-Lucent - All rights reservedPassing on and copying of this document, use and
communication of its contents not permitted without written authorization from Alcatel-Lucent
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2
Legal Notice
Switch to notes view!Safety Warning
Both lethal and dangerous voltages are present within the equipment. Do not wear conductive jewelry
while working on the equipment. Always observe all safety precautions and do not work on the
equipment alone.
Caution
The equipment used during this course is electrostatic sensitive. Please observe correct anti-static
precautions.
Trade Marks
Alcatel and MainStreet are trademarks of Alcatel.
All other trademarks, service marks and logos (Marks) are the property of their respective holders
including Alcatel-Lucent. Users are not permitted to use these Marks without the prior consent of Alcatel
or such third party owning the Mark. The absence of a Mark identifier is not a representation that a
particular product or service name is not a Mark.
Copyright
This document contains information that is proprietary to Alcatel-Lucent and may be used for training
purposes only. No other use or transmission of all or any part of this document is permitted without
Alcatel-Lucents written permission, and must include all copyright and other proprietary notices. No
other use or transmission of all or any part of its contents may be used, copied, disclosed or conveyed to
any party in any manner whatsoever without prior written permission from Alcatel-Lucent.
Use or transmission of all or any part of this document in violation of any applicable Canadian or other
legislation is hereby expressly prohibited.
User obtains no rights in the information or in any product, process, technology or trademark which it
includes or describes, and is expressly prohibited from modifying the information or creating derivative
works without the express written consent of Alcatel-Lucent.
Alcatel-Lucent, The Alcatel-Lucent logo, MainStreet and Newbridge are registered trademarks of Alcatel-
Lucent. All other trademarks are the property of their respective owners. Alcatel-Lucent assumes no
responsibility for the accuracy of the information presented, which is subject to change without notice.
2007 Alcatel-Lucent. All rights reserved.
Disclaimer
In no event will Alcatel-Lucent be liable for any direct, indirect, special, incidental or consequential
damages, including lost profits, lost business or lost data, resulting from the use of or reliance upon the
information, whether or not Alcatel has been advised of the possibility of such damages.
Mention of non-Alcatel-Lucent products or services is for information purposes only and constitutes
neither an endorsement nor a recommendation.
Please refer to technical practices supplied by Alcatel-Lucent for current information concerning Alcatel-
Lucent equipment and its operation.
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Table of Contents
Switch to notes view!1. Sigtran Description
Module 1. Introduction
Module 2. SCTP
Module 3. M3UA
Module 4. Trace
Module 5. OSP 2.4 Sigtran presentation
Module 6. Abbreviations
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Table of Contents [cont.]
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Course Objectives
Switch to notes view!
Welcome to Alcatel 8690 SIGTRAN Description
After successful completion of this course, you should understand:
Describe why SIGTRAN is needed
Describe the SIGTRAN architecture
Describe the main concepts of SCTP layer
Describe SCTP association establishment and shutdown
Describe the main concepts of SCTP data transmission
Describe the main concepts of M3UA layer
Describe the M3UA operation modes and services
Identify the M3UA messages
Describe the association establishment and Traffic failover scenarios
Analyze a SIGTRAN trace
Present the OSP 2.4 SIGTRAN Alcatel-Lucent solution
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Course Objectives [cont.]
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About this Student Guide
Switch to notes view!Conventions used in this guide
Where you can get further information
If you want further information you can refer to the following:
Technical Practices for the specific product
Technical support page on the Alcatel website: http://www.alcatel-lucent.com
Note
Provides you with additional information about the topic being discussed.
Although this information is not required knowledge, you might find it useful
or interesting.
Technical Reference (1) 24.348.98 Points you to the exact section of Alcatel-Lucent Technical
Practices where you can find more information on the topic being discussed.
WarningAlerts you to instances where non-compliance could result in equipment
damage or personal injury.
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About this Student Guide [cont.]
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Self-Assessment of Objectives
At the end of each section you will be asked to fill this questionnaire
Please, return this sheet to the trainer at the end of the training
Switch to notes view!
Instructional objectives Yes (or globally yes)
No (or globally no)
Comments
1 Describe why SIGTRAN is needed
2 Describe the SIGTRAN architecture
3 Describe the main concepts of SCTP layer
4 Describe SCTP association establishment and shutdown
5 Describe the main concepts of SCTP data transmission
6 Describe the main concepts of M3UA layer
7 Describe the M3UA operation modes and services
8 Identify the M3UA messages
9 Describe the association establishment and Traffic failover scenarios
10 Analyze a SIGTRAN trace
11 Present the OSP 2.4 SIGTRAN Alcatel-Lucent solution
Contract number:
Course title: SIGTRAN M3UA Description
Client (Company, Center) :
Language: Dates from: to:
Number of trainees: Location:
Surname, First name:
Did you meet the following objectives?
Tick the corresponding box
Please, return this sheet to the trainer at the end of the training
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Self-Assessment of Objectives [cont.]
Switch to notes view!
Instructional objectives Yes (or Globally yes)
No (or globally no)
Comments
Thank you for your answers to this questionnaire
Other comments
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Sigtran Description
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First editionRosa Maria Izquierdo Kulich
2007-04-2101
RemarksAuthorDateEdition
Document History
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Objectives
At the end of this chapter you will be able to:
Describe why SIGTRAN is needed.
Describe the SIGTRAN architecture.
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Objectives [cont.]
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Table of Contents
Switch to notes view! Page
1 Why SIGTRAN? 71.1 OSP in TDM Network 81.2 SS7 Overview 91.3 OSP in IP Network 101.4 Benefits of signaling over IP 11
2 SIGTRAN Architecture 122.1 SIGTRAN 132.2 SIGTRAN and SS7 152.2.1 SCTP Features 162.2.2 M3UA Features 17
2.3 SIGTRAN (M3UA) and SS7 19
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1 Why SIGTRAN?
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CAP
SSP
1 Why SIGTRAN?
1.1 OSP in TDM Network
MTPMTP
SS7SS7
INAP/CAP/MAP
MSC
SMS-C SGSNHLR
SRP
SP
SP
SP
SP SPSP
SP
INAP
CAP
CAP
MAP
INAP/CAP
FEPSLEE
SMP SDP
SCP
OSP
ISUP
The fundamental principle of CCSS#7 (SS7) is the division of functions into a common Message Transfer
Part (MTP) and several User Parts.
The different user parts use the transport capability provided by MTP.
The complete MTP is capable of sending messages through the network. In addition, error detection and
correction, as well as flow control functionality are provided.
The Signaling System N 7 (SS7) is composed of 3 layers, named Message Transfer Part:
MTP1: Signaling Data Link Functions.
MTP2: Signaling Link Functions.
MTP3: Signaling Network Functions.
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1 Why SIGTRAN?
1.2 SS7 Overview
SCCP
Transmission of bits
Packages the signaling message for error-free transmission
Signaling message handling functions
Connection-oriented protocol
Transfer of information between applications
Intelligent Network and Mobile applications
TCAP
INAP/CAP/MAP
MTP3
MTP2
MTP1 Timeslot E1/T1 board
LinksetLInk
SP, DestinationRoute
GTT GTTRSSN
BeginContinueAbort
IDPPACUI
Signaling Connection Control Part (SCCP)
SCCP is used to increase the functionality of MTP by adding a number of extra features. It corresponds to OSI Layer 3. Routing between two exchanges is now always supported, even when they are located in
different countries. Instead of only using a Point Code (MTP-parameter), which is a unique identification
of a node in a local network, an additional SCCP-parameter called Global Title is used to uniquely
identify a node worldwide.
An important parameter at SCCP level is the Sub System Number (SSN), which identifies the user of
SCCP. By including it in the message, the analysis at the destination will result in the delivery of the
information to the correct SCCP-user. Except when the user of SCCP is the Transaction Capabilities
Application Part (TCAP), in that case the SSN identifies the user of TCAP.
Transaction Capabilities Application Part (TCAP)
TCAP is an example of a user which uses the connectionless services of SCCP. In intelligent networks, the user of TCAP is the Intelligent Network Application Protocol (INAP).
Intelligent Network Application Protocol (INAP)
The user of TCAP in IN is INAP. In this part, the operations and their associated parameters are defined.
This part is related to an application, in this case intelligent networks. For other applications, for
instance mobile telecommunication, another application part is the user of TCAP, namely Mobile Application Part (MAP).
CAMEL Application Part (CAP)
CAP is the implementation of INAP in the mobile world.
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1 Why SIGTRAN?
1.3 OSP in IP Network
CAP
SSP
M3UAM3UA//SCTPSCTP
IPIP
INAP/CAP/MAP
MSC
SMS-C SGSNHLR
SRP
SP
SP
SP
SP SPSP
SP
INAP
CAP
CAP
MAP
INAP/CAP
FEPSLEE
SMP SDP
AS
OSP
ISUP
The proliferation of packet-based protocols throughout the telephony industry has generated a need for
the transmission of signaling information through an IP-based network. Much of the development work on
methods to implement such information transport is still in its infancy. However, a number of standards
are emerging. One of the more notable standards is the work by the Internet Engineering Task Force,
IETF, SIGTRAN group.
The IETF have specified a number of signaling transport protocols and interworking layers that enable
SS7 like information to be conveyed through IP networks. IP is a transport mechanism, whereas SS7 is a
transport mechanism and network structure that provides user services. The IETF specifications provide a
migration path that combines the structure of existing networks with the advantages of IP transport.
M3UA: Message Transfer Part level 3 User Adaptation (one of SIGTRAN components)
SCTP: Stream Control Transmission Protocol (one of SIGTRAN components)
IP: Internet Protocol
SCCP: Signaling Connection Control Part
CAP: Camel Application Part
MAP: Mobile Application Part
INAP: Intelligent Network Application Protocol
SDP: Service Data Point
FEP: Front End Process
SLEE: Service Login Execution Environment
OSP: Open Service Platform
AS: Application Server
SP: Signaling Point
SMP: Service Management Point
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1 Why SIGTRAN?
1.4 Benefits of signaling over IP
No need for specialized SS7 hardware.
Very large bandwidth
Fault resilient
Incremental approach to leveraging IP flexibility:
Flexible network maintenance and expansion: costs are lowered.
It allows migration toward a full NGN network architecture.
The IP network suppresses expensive long-distance TDM leased lines and replaces them by cost-effective IP links, reduces network infrastructure cost by removing Signaling Transfer Points (STPs) when possible and provides on-demand bandwidth opposed to fixed reserved bandwidth when using the TDM technology.
Very large bandwidth: at least 100 Mbits per IP link versus 64 Kbits or 2 Mbits per link with SS7.
Fault resilient: Session failure detection and Redundant Stream Control Transmission Protocol (SCTP).
Incremental approach to leveraging IP flexibility: It maintains existing network functions.
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2 SIGTRAN Architecture
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2 SIGTRAN Architecture
2.1 SIGTRAN
The SIGnaling TRANsport working group (SIGTRAN) of the IETF has defined User Adaptation Protocols
SIGTRAN
Architecture
Adaptation Protocol(xUA xPA)
Common signaling Transport(SCTP)
Standard Internet Protocol
(IP)
RFC 2960
RFC 4666
For M3UA
User Adaptation Protocols to provide the architectural model of signaling transport over IP networks.
The SIGTRAN protocol stack consists of 3 components:
A standard IP stack.
A common signaling transport protocol, Stream Control Transmission Protocol (SCTP): SCTP provides
connection-oriented reliable transfer of user messages between peer SCTP users (Adaptation layer
protocols). Note: The SCTP layer replaces the Transmission Control Protocol (TCP) layer.
Adaptation layer: The protocols defined for this layer are:
MTP2 Peer-to-peer Adaptation (M2PA),
MTP2 User Adaptation (M2UA),
MTP3 User Adaptation (M3UA),
SCCP User Adaptation (SUA).
SIGTRAN defines many IP-based protocols that replace almost all SS7 layers:
M2UA, M2PA for MTP2.
M3UA for MTP3.
ISDN User Adaptation (IUA).
SUA for SCCP.
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2 SIGTRAN Architecture
2.1 SIGTRAN [cont.]
M3UA
SCTP
IP
SUAM2PA
SS7 ApplicationPart
SS7 link SS7 User part
M3UA: MTP3 User Adaptation enables SS7 user parts (SCCP, ISUP) to run over IP.
SCTP: Stream Control Transmission Protocol is a reliable transport
protocol operating on top of IP.
Stream Control Transmission Protocol (SCTP) provides very reliable transport of messages.
MTP2 User Adaptation (M2UA) is ideal for backhauling signaling links at a centralized location.
MTP2 Peer-to-peer Adaptation (M2PA) enables SS7 links to run over IP.
MTP3 User Adaptation (M3UA) enables SS7 User Parts (ISUP, SCCP) to run over IP.
SCCP User Adaptation (SUA) enables SS7 Application Parts (e.g. TCAP) to run over IP.
Why M3UA has been chosen?
M2UA is not suitable:
designed for MTP2 backhauling (invocation of remote MTP2 layer).
no full IP peer-to-peer mode, only IP-SS7 legacy interworking (for example, relay of legacy SS7
associated signaling terminated at an MGW toward an MGC through IP).
SUA is not suitable: unable to transport ISUP & BICC signaling.
M2PA could have been suitable:
only peer-to-peer mode: a legacy SS7 link is replaced by an "SS7 IP link" (an SCTP association).
MTP3 management is kept unchanged.
M2PA enabled STP can be needed in networks.
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2 SIGTRAN Architecture
2.2 SIGTRAN and SS7
Data
User
MTP3
MTP
L1, L2
SS7 Stack
MTP3
IP
M3UAM3UA
SCTP
SUA
M2UAM2PA
TCAP
MAP/CAP/INAP
ISUP/BICC/TUP
IUA
Q931Q931
SIGTRAN Stack
SCCP
Stream Control Transmission Protocol (SCTP) provides very reliable transport required by SCN signaling.
MTP2 User Adaptation (M2UA) is ideal for backhauling signaling links at a centralized location.
MTP2 Peer-to-peer Adaptation (M2PA) enables SS7 links to run over IP.
MTP3 User Adaptation (M3UA) enables SS7 User Parts (ISUP, SCCP) to run over IP.
SCCP User Adaptation (SUA) enables SS7 Application Parts (e.g. TCAP) to run over IP.
The ISDN User Part (ISUP) defines the protocol and procedures used to set up, manage and release trunk circuits that carry voice and data calls over the Public Switched Telephone Network (PSTN). ISUP is used for both Integrated Services Digital Network (ISDN) and non-ISDN calls. Calls that originate and terminate at the same switch do not use ISUP signaling.
Bearer Independent Call Control (BICC) is an implementation of SS7 defined in ITU-T Recommendation Q.BICC and ANSI T1.BICC. BICC provides call control of telephone calls over the ISDN where the bearer is non-traditional. BICC can use the services of the Message Transfer Part.
The Telephone User Part (TUP) provides conventional PSTN telephony services across the SS7 network. TUP was the first layer 4 protocol defined by the standards bodies and as such did not provision for ISDN services. Prior to the introduction of ISUP, national variants of TUP have evolved which provide varying degrees of support for ISDN.
Q.931 (also called Q931) is a signaling protocol for ISDN communications that is used in Voice over IP (VoIP). The Q.931 protocol is involved in the setup and termination of connections.
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2.2 SIGTRAN and SS7
2.2.1 SCTP Features
Ensures the error-free transmission of user messages.
Ensures in-sequence delivery of user messages.
Enables fast delivery.
Avoids head-of-line blocking.
Is a more suitable protocol than the TCP and the UDP.
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2.2 SIGTRAN and SS7
2.2.2 M3UA Features
Provides an interface between SCTP and those applications that typically use the services of MTP3.
SCTP enables seamless peer-to-peer communication between MTP3 user applications in the IP network and identical applications in SS7 networks.
Provides services to the applications in the SS7 network and offers equivalent services to applications in the IP network.
Has a registered port: 2905.
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2.2 SIGTRAN and SS7
2.2.2 M3UA Features [cont.]
M3UA
SCTP
IP
NIF
MTP3
MTP
L1, L2
MTP3 User
M3UA
SCTP
IP
MGC
MTP3 user
MTP3
MTP2
Level 1
SEP or STPSG
IP SS7
Nodal Interworking Function (NIF)
Consider a Media Gateway Controller (MGC) that needs to run an application as ISUP. The MGC can
implement M3UA over SCTP. The point is to identify where the MTP3 function really resides.
The real MTP3 resides at the Signaling Gateway (SG). M3UA simply enables the ISUP application at the
MGC to remotely access the MTP3 function at the SG, without the ISUP application realizing that the
MTP3 function is not local.
The MGC must have its own Signaling Point, separate from the Signaling point of the SG. In that case, the
SG functions like an STP and appears as an STP to the outside of the network. The SS7 network views the
MGC as a typical SS7 signaling endpoint to which access is achieved via one or more SG STPs.
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2 SIGTRAN Architecture
2.3 SIGTRAN (M3UA) and SS7
MTP2 Link management Stream Control Transmission Protocol (SCTP)
MTP3 Configuration and provisioning
M3UA configuration and provisioning
SCCP and TCAP layers remain unchanged
Fully replaces
Fully replace
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Answer the Questions
Why SIGTRAN is needed?
Mention the functions of SCTP.
Mention the functions of M3UA.
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Summary
In this chapter, we have seen:
why SIGTRAN is needed.
the SIGTRAN architecture.
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Self-Assessment on the Objectives
Please be reminded to fill in the formSelf-Assessment on the Objectivesfor this module
The form can be found in the first partof this course documentation
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End of ModuleIntroduction
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RemarksAuthorDateEdition
Document History
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Objectives
At the end of this chapter you will be able to:
Describe the main concepts of SCTP layer.
Describe SCTP association establishment and shutdown.
Describe the main concepts of SCTP data transmission.
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Objectives [cont.]
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Table of Contents
Switch to notes view! Page
1 SCTP Presentation 71.1 SCTP Features 81.2 SCTP, UDP and TCP 91.3 SCTP Main Concepts 101.4 SCTP Overview 111.5 SCTP Packet Presentation 121.6 SCTP Packet and Chunks 131.7 Chunks 14
2 Association 172.1 Association Establishment 182.2 Graceful Termination of an Association 192.3 Aborting the Association 20
3 Data Transmission 223.1 SCTP Data Transmission 233.1.1 Reliable Transfer 243.1.1.1 Checksum 253.1.1.2 Transmission Sequence Number 263.1.1.3 SACK Mechanism 27
3.1.2 Flexible Delivery 293.1.3 Multi-Homing 313.1.3.1 Primary Path 323.1.3.2 Transport address states 333.1.3.3 Rules to Transmit to the Destination Address 343.1.3.4 Path failure detection 353.1.3.5 End point failure detection 363.1.3.6 Network resilience example 373.1.3.7 HEARTBEAT 40
3.2 SCTP Robustness 413.3 SL, Association and Stream Relationship Example 42
4 Annexes: SCTP Messages Format 464.1 INIT and INIT Ack 474.2 Cookie Echo and Cookie Echo ACK 484.3 Shutdown 494.4 Abort Association 504.5 Payload Data 514.6 SACK 534.7 HEARTBEAT 544.8 Operation Error 55
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Table of Contents [cont.]
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1 SCTP Presentation
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1 SCTP Presentation
1.1 SCTP Features
Reliable transport protocol operating on top of an IP, alternative to UDP, TCP.
SCTP offers acknowledged error-free non-duplicated transfer ofdatagrams (messages).
A selective retransmission mechanism is applied to correct loss or corruption of data.
SCTP multi-homing: nodes reached under several IP addresses.
SCTP multi-streaming: separates and transmits user data on multiple SCTP streams.
SCTP multi-homing: Support of multi-homed nodes feature is an essential property of SCTP; i.e. nodes, which can be, reached under several IP addresses.The multi-homing feature enables SCTP endpoints to
support multiple IP addresses. Multi-homing protects an association from potential network failures by
steering traffic to alternate IP addresses. During the initiation of an association, SCTP endpoints
exchange lists of IP addresses. Therefore, each endpoint can send and receive messages from any of the
IP addresses listed at the remote endpoint. For example, one of the listed IP addresses will be
designated as the primary address during the initiation. If the primary address repeatedly drops chunks,
however, all chunks will be transmitted to an alternate address until a connection to the primary address
can be reestablished.
SCTP multi-streaming: A stream is a unidirectional logical channel that permits exchange of messages between SCTP peer entities. When an association is set up, the number of available streams per
direction is exchanged between the peer entities.The multi-streaming feature separates and transmits
user data on multiple SCTP streams. These streams are capable of independent, sequenced delivery.
Message loss in a particular stream will only hinder delivery within that stream. Therefore, other streams
within an association are not affected. Through multi-streaming, SCTP eliminates unnecessary blocking
that often occurs in TCP transmissions. Since SCTP streams are independent, retransmitted and high-
priority messages can bypass less significant messages.
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1 SCTP Presentation
1.2 SCTP, UDP and TCP
SCTPTCPUDP
Framing
Reliable
Ordering
PMTU fragmentation
Bundling
Multi-homing failover
Flow control congestion control
X X
XX
X
Optional &
multi-streamX
XX
X
XX
Multi-homing failover and Bundling are two new features in SCTP.
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1 SCTP Presentation
1.3 SCTP Main Concepts
IP Network service
SCTP Endpoint
A
IP Network service
SCTP Endpoint
B
SCTPM3UA
IP@ IP@ IP@ Transport address
Primary path
Association
IP Address + Port Number
IP@ IP@ IP@
SCTPM3UA
HeaderVerification tag
CRC-32c
HeaderVerification tag
CRC-32cData headerData headerCtrl chunkCtrl chunk
Outbound stream
Inbound stream
Data chunkTSN
Data chunkTSN
Data chunk User message
Data chunkTSN
Data chunkTSN Data header
Data header Ctrl chunkCtrl chunkHeader
Verification tagCRC-32c
HeaderVerification tag
CRC-32c
SCTP Endpoint: The logical sender/receiver of SCTP packets. On a multi-homed host, an SCTP endpoint is represented to its peers as a combination of a set of eligible destination transport addresses to which SCTP packets can be sent and a set of eligible source transport addresses from which SCTP packets can be received. All transport addresses used by an SCTP endpoint must use the same port number, but can use multiple IP addresses. A transport address is unique to an SCTP endpoint.
Association: Relationship between SCTP endpoints that allows their communication. An association is a protocol relationship between SCTP endpoints, composed of the two SCTP endpoints and protocol state information including Verification Tags and the currently active set of Transmission Sequence Numbers (TSNs), etc. An association can be uniquely identified by the transport addresses used by the endpoints in the association. Two SCTP endpoints must not have more than one SCTP association between them at any given time.
Transport Address: Traditionally defined by Network Layer address, Transport Layer protocol and Transport Layer port number. In the case of SCTP running over IP, a transport address is defined by the combination of an IP address and an SCTP port number (where SCTP is the Transport protocol).
SCTP packet: The unit of data delivery across the interface between SCTP and the connectionless packet network (e.g., IP). An SCTP packet includes the common SCTP header, possible SCTP control chunks, and user data encapsulated within SCTP DATA chunks (Protocol Data Unit (PDU) of SCTP that forms the payload of an IP packet).
Stream: A unidirectional logical channel established from one to another associated SCTP endpoint, within which all user messages are delivered in sequence except for those submitted to the unordered delivery service.
Chunk: A unit of information within an SCTP packet, consisting of a chunk header and chunk specific content.
Transmission Sequence Number (TSN): A 32-bit sequence number used internally by SCTP. One TSN is attached to each chunk containing user data to permit the receiving SCTP endpoint to acknowledge its receipt and detect duplicate deliveries.
Bundling: Multiple Data and control chunks may be bundled by the sender into a single SCTP packet for transmission, as long as the final size of the packet does not exceed the current path MTU. Each user message occupies its own data chuck. The receiver will unbundled the packet back into the original chunks. Control chunks must come before Data chunks in the packet. The fragmentation and bundling mechanisms are optional. The chunk bundling function of SCTP is responsible for assembly of the complete SCTP packet and its disassembly at the receiving end.
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1 SCTP Presentation
1.4 SCTP Overview
M3UA layer
Association
Sequenced delivery within streams
User Data Fragmentation
Acknowlegement and congestion
avoidance
Chunk Bundling
Packet Validation
Path management
Upper layers
IP layer
Request from M3UA user
Nb of streamers?
Conforms to PathMTU
TSN to each message; Rx end
Header and Chunks
Tag and 32-bit checksum fields
Destination transport @
The SCTP transport service can be fragmented into several functionalities:
Association Startup and Teardown: An association is initiated by a request from the SCTP user. A cookie mechanism is employed during the initialization to provide protection against security attacks.
Sequenced Delivery within Streams: The SCTP user can specify at association startup time the number of streams to be supported by the association.
User Data Fragmentation: SCTP supports fragmentation and reassembly of user messages to ensure that the SCTP packet passed to the lower layer conforms to the path Multiple-Tenant Unit (MTU).
Acknowledgement and Congestion Avoidance: SCTP assigns a Transmission Sequence Number (TSN) to each user data message (fragmented or unfragmented). The receiving end acknowledges all TSNs
received, even if there are gaps in the sequence.
Chunk Bundling: The SCTP packet delivered to the lower layer consists of a common header followed by one or more chunks.
Packet Validation: A mandatory verification tag field and a 32-bit checksum field are included in the SCTP common header.
Path Management: The SCTP path-management function chooses the destination transport address for each outgoing SCTP packet based on the SCTP user's instructions and the currently perceived
reach ability status of the eligible destination set.
Multiple chunks may be multiplexed into one packet up to the Path-MTU size.
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1 SCTP Presentation
1.5 SCTP Packet Presentation
SCTP packet: Protocol Data Unit (PDU) of SCTP that forms the payload of an IP packet.
Composed of Common Header and Chunks
Chunks: A unit information within an STCP packet that can contain control information or user data
SCTP Common Header Chunk #1 Chunk #N
Source port Dest. port
Verification tag
Checksum
Type Lenght
User data
FlagsProvides a key to verify if the packet in the correct association
Common Header
The common header consists of 12 bytes. For the identification of an association, SCTP uses the same
port concept as TCP and UDP. For the detection of transmission errors, each SCTP packet is protected by
a 32-bit checksum (CRC-32c algorithm), which is more robust than the 16-bit checksum of TCP and UDP.
SCTP packets with an invalid checksum are silently discarded. The common header also contains a 32-bit
value called verification tag. The verification tag is association specific, and exchanged between the endpoints at association startup. So there are two tag values used in one association.
Chunk
Each chunk begins with a chunk type field, which is used to distinguish data chunks and different types
of control chunks, followed by chunk specific flags and a chunk length field needed because chunks have
a variable length. The value field contains the actual payload of the chunk. An SCTP receiver MUST be
able to receive a minimum of 1500 bytes in one SCTP packet. This means that an SCTP endpoint MUST
NOT indicate less than 1500 bytes in its Initial a_rwnd sent in the INIT or INIT ACK. For transmission
efficiency, SCTP defines mechanisms for bundling of small user messages and fragmentation of large user
messages.
The Verification Tag value: Provides a key that allows a receiver to verify that an SCTP packet belongs to the current association; is chosen by each end of the association during association startup. Packets
received without the expected Verification Tag value are discarded. The Verification tag is randomly
generated.
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1 SCTP Presentation
1.6 SCTP Packet and Chunks
SCTP Common Header
Chunk Id (8)
0x00 DATA 0x01 INIT **
0x02 INIT ACK **
0x03 SELECTIVE ACK
0x04 HEARTBEAT
0x05 HEARTBEAT ACK 0x06 ABORT **
0x07 SHUTDOWN
0x08 SHUTDOWN ACK **
0x09 ERROR
0x0A COOKIE 0x0B COOKIE ACK
0x0C ECNE reserved
0x0D CWR reserved
VENDOR SPECIFIC
** Cant be bundled with DATA
Chunk #1 Chunk #N
Chunk Flags (8)
Chunk Dependent
Chunk Length (16) Chunk Value
Parameter Type (16) Parameter Length (16)
Source Port 16 bitsDestination Port 16 bits
Verification Tag 32 bits
CRC-32c Checksum 32 bits
Control Chunks precede Data
Chunks if both are present in
same SCTP PDU
Fixed Parameters
Chunk Value
Parameter Value
Optional / Variable Parameter
VP #1 VP #N
Length (octets) =
len Chunk Id +
len Chunk Flags +
len Chunk Length +
len Chunk Value
Chunk
Common Header
The common header consists of 12 bytes. For the identification of an association, SCTP uses the same
port concept as TCP and UDP. For the detection of transmission errors, each SCTP packet is protected by
a 32-bit checksum (CRC-32c algorithm), which is more robust than the 16-bit checksum of TCP and UDP.
SCTP packets with an invalid checksum are silently discarded. The common header also contains a 32-bit
value called verification tag. The verification tag is association specific, and exchanged between the endpoints at association startup. So there are two tag values used in one association.
Chunk
Each chunk begins with a chunk type field, which is used to distinguish data chunks and different types
of control chunks, followed by chunk specific flags and a chunk length field needed because chunks have
a variable length. The value field contains the actual payload of the chunk.
Chunk Length (16 bits) - (unsigned integer): This value represents the size of the chunk in bytes including
the Chunk Type, Chunk Flags, Chunk Length and Chunk Value fields. Therefore, if the Chunk Value field
is zero-length, the Length field will be set to 4. The Chunk Length field does not count any padding.
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1 SCTP Presentation
1.7 Chunks
Initiation (INIT)
Initiation Acknowledgement (INIT ACK)
Cookie Echo (COOKIE ECHO)
Cookie Acknowledgement (COOKIE ACK)
Shutdown Association (SHUTDOWN)
Shutdown Acknowledgement (SHUTDOWN ACK)
Shutdown Complete (SHUTDOWN COMPLETE)
Abort Association (ABORT)
Payload Data
Selective Acknowledgement (SACK)
Heartbeat Request (HEARBEAT)
Heartbeat Acknowledgement (HEARTBEAT ACK)
Operation error (ERROR)
To establish an association
To gracefully shutdown or terminate an association
Data transfer and selective acknowledgement
To test reachability of a particular destination transport address defined in the association
The Initiation chunk (INIT) is used to initiate an SCTP association between two endpoints.
The Initiation Acknowledgement chunk (INIT ACK) is used to acknowledge the initiation of an SCTP association.
The parameter part of INIT ACK is formatted similarly to the INIT chunk. It uses two extra variable
parameters:
the State Cookie
the Unrecognized Parameter.
The Cookie Echo chunk (COOKIE ECHO) is used only during the initialization of an association. It is sent by the initiator of an association to its peer to complete the initialization process. This chunk MUST
precede any DATA chunk sent within the association, but MAY be bundled with one or more DATA chunks
in the same packet.
The Cookie Acknowledgement chunk (COOKIE ACK) is used only during the initialization of an association. It is used to acknowledge the receipt of a COOKIE ECHO chunk. This chunk MUST precede
any DATA or SACK chunk sent within the association, but MAY be bundled with one or more DATA chunks
or SACK chunk in the same SCTP packet.
The Shutdown Association chunk (SHUTDOWN) MUST be used by an endpoint in an association to initiate a graceful close of the association with its peer.
The Shutdown Acknowledgement chunk (SHUTDOWN ACK) MUST be used to acknowledge the receipt of the SHUTDOWN chunk at the completion of the shutdown process. The SHUTDOWN ACK chunk has no
parameters.
The Shutdown Complete chunk (SHUTDOWN COMPLETE) MUST be used to acknowledge the receipt of the SHUTDOWN ACK chunk at the completion of the shutdown process. The SHUTDOWN COMPLETE chunk
has no parameters.
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The Abort Association chunk (ABORT) is sent to the peer of an association to close the association. The ABORT chunk may contain Cause Parameters to inform the receiver of the reason for the abort. DATA
chunks MUST NOT be bundled with ABORT. Control chunks (except for INIT, INIT ACK and SHUTDOWN
COMPLETE) MAY be bundled with an ABORT but they MUST be placed before the ABORT in the SCTP
packet, or they will be ignored by the receiver.
If an endpoint receives an ABORT with a format error or for an association that doesn't exist, it MUST
silently discard it. Moreover, under any circumstances, an endpoint that receives an ABORT MUST NOT
respond to that ABORT by sending an ABORT of its own.
The Payload Data chunk is used to send user messages.
The Selective Acknowledgement chunk (SACK) is sent to the peer endpoint to acknowledge received DATA chunks and to inform the peer endpoint of gaps in the received subsequences of DATA chunks as
represented by their TSNs. The SACK MUST contain the Cumulative TSN Ack and Advertised Receiver
Window Credit (a_rwnd) parameters.
The Heartbeat Request chunk (HEARTBEAT) is sent by an endpoint to its peer endpoint to probe the reachability of a particular destination transport address defined in the present association.
The parameter field contains the Heartbeat Information which is a variable length opaque data structure
understood only by the sender.
The Heartbeat Acknowledgement chunk (HEARTBEAT ACK) is sent by an endpoint to its peer endpoint as a response to a HEARTBEAT chunk. A HEARTBEAT ACK is always sent to the source IP address of the IP
datagram containing the HEARTBEAT chunk to which this ack is responding.
The parameter field contains a variable length opaque data structure.
The Operation error chunk (ERROR) is sent by an endpoint to its peer endpoint to notify it of certain error conditions. It contains one or more error causes. An Operation Error is not considered fatal in and
of itself, but may be used with an ABORT chunk to report a fatal condition.
Each error cause may carry its own set of parameters. The error causes that have been defined are:
Cause Code Value Cause Code
1 Invalid Stream Identifier
2 Missing Mandatory Parameter
3 Stale Cookie Error
4 Out of Resource
5 Unresolvable Address
6 Unrecognized Chunk Type
7 Invalid Mandatory Parameter
8 Unrecognized Parameters
9 No User Data
10 Cookie Received While Shutting Down
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Answer the Questions
What is an association?
What is a chunk?
What is a transport address?
What is the composition of an SCTP packet?
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2 Association
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2 Association
2.1 Association Establishment
SCTP Endpoint A SCTP Endpoint B
INIT
INIT ACK
COOKIE ECHO
COOKIE ACK
Association is established
Closed
Established
Cookie wait
CookieEchoed
Established
Cookie with values, IP@, secret keyCookie with values,
IP@ secret key
Unpacks data, verifysecret key
Ready to send and receive data through a destination transport address
The server receives an association setup request (an INIT chunk) usually in the CLOSED state and analyzes
the data contained in that chunk. From that, it generates all the values needed on its side to enter an
established association, and generates a secure hash of these values and a secret key. The values are
then put into the so-called COOKIE, along with the derived Message Authentication Code (MAC). This
COOKIE is returned to the sender of the INIT chunk in an INIT ACK chunk. The server remains in the
CLOSED state and forgets all about the received INIT chunk.
Upon reception of a COOKIE ECHO chunk (which contains a COOKIE data structure as parameter), the
server unpacks the data contained in this COOKIE and uses again the MAC contained therein to verify
whether it was the originator of this COOKIE. If the MAC computes okay, it is a valid COOKIE that this
server had created before, and the data values contained in the COOKIE are used to initialize the SCTP
instance. The server will send a COOKIE-ACK to the client (optionally bundling a data chunk with this
COOKIE-ACK chunk) and enter the ESTABLISHED state. It is then ready to accept data or send data chunks
itself.
The INIT chunk can contain one or more IPv4 (4 bytes) or IPv6 (16 bytes) addresses, or it can contain a
host name that can be resolved to one or more IP addresses.
Message Authentication Code (MAC): An integrity check mechanism based on cryptographic hash
functions using a secret key. Typically, message authentication codes are used between two parties that
share a secret key in order to validate information transmitted between these parties. In SCTP, it is used
by an endpoint to validate the State Cookie information that is returned from the peer in the COOKIE
ECHO chunk.
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2 Association
2.2 Graceful Termination of an Association
SCTP Endpoint A SCTP Endpoint B
SHUTDOWN
SHUTDOWN ACK
SHUTDOWN COMPLETE
Closed
Closed
Data acknowledgement
Shutdown primitive
Stops accepting data, outstanding data acknowledgement
Removes all data from this association
Remove all data from this association
An association begins with an "initiation" and is maintained until all data has been successfully
transmitted and received. Once all data is successfully received, the association is gracefully terminated
through a "shutdown."
Upon receiving the SHUTDOWN primitive from its upper layer user process, an SCTP instance should stop
accepting data from this process, and start sending a SHUTDOWN chunk, as soon as all of its outstanding
data has been acknowledged. This process is secured by a timer, that repeats this process, should the
SHUTDOWN be lost.
The peer will, at one point, receive the SHUTDOWN, and reply by sending a SHUTDOWN ACK chunk, as
soon as all of its data has been acknowledged (also secured by a timer!).When the first peer (that started the shutdown procedure) receives the SHUTDOWN ACK, it will stop the
timer, send a SHUTDOWN COMPLETE and remove all data still belonging to that association, and enter
the CLOSED state.
The peer that receives this SHUTDOWN COMPLETE chunk may then also remove all record of this
association, and enter the CLOSED state. Should the last SHUTDOWN COMPLETE message be lost, the
peer will repeat sending SHUTDOWN ACK chunks, until an error counter has been exceeded, which
reports the other peer unreachable.
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2 Association
2.3 Aborting the Association
ABORT (no data chuck)
ClosedClosed
Validates the chunk Removes the association Reports termination to its upper layer process
The peers Verification Tag in the outbound packet must be filled
SCTP SCTP
An endpoint may also decide to abort an existing association, taking into account that data still in flight
may not be acknowledged, by sending an ABORT chunk to its peer endpoint. The sender MUST fill in the
peer's Verification Tag in the outbound packet and MUST NOT bundle any DATA chunk with the ABORT.
The receiver of the ABORT does not reply, but validates the chunk and removes the association, if the
ABORT contains the correct tag value. If so, it also reports termination to its upper layer process.
Should the ABORT be lost, and the endpoint sending it terminates directly after sending it, it will take a
rather long time to determine that the peer has gone (i.e., after the Peer Error Counter has been
exceeded).
The ABORT chunk is sent by an endpoint to end an association abruptly. It may contain cause information
regarding the reason for aborting the association. The ABORT chuck may be multiplexed with other
control chucks into one packet. In this case, the ABORT should be the last chuck in the packet. If it is not
the case, then subsequent chucks in the packet are ignored. DATA chuck should not be included in the
same packet as an ABORT chuck.
Special cases:
Peer restart case, where the peer uses a new tag value.
Cross initialization, where both peers send an INIT chunk at the same time.
Excessive delay of COOKIE chunk.
One peer trying to re-establish an association, while the other one tries to terminate it.
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Answer the Questions
Mention the parameters that are negotiated during the association establishment.
What is the difference between "shutdown" and "abort" an association?
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3 Data Transmission
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3 Data Transmission
3.1 SCTP Data Transmission
Reliable transfer
Flexible delivery
Multi-homing
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3.1 SCTP Data Transmission
3.1.1 Reliable Transfer
Checksum
Transmission Sequence Number (TSN)
Selective retransmission mechanism
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3.1.1 Reliable Transfer
3.1.1.1 Checksum
SCTP Endpoint
ASender
HeaderVerification tag: Chosen during
Association startup
CRC-32c calculated by sender
HeaderVerification tag: Chosen during
Association startup
CRC-32c calculated by sender
DataheaderDataheader
CtrlchunkCtrlchunk
Data chunkTSN
Data chunkTSN
CRC-32c calculated by receiverCRC-32c calculated by sender
PACKET OK
CRC-32c calculated by receiverCRC-32c calculated by sender
INVALID PACKET
Checksum = 0
Checksum = Value
SCTP Endpoint
AReceiver
1
2
3
Checksum = Value
Checksum = 0 and calculates a new checksum
1
3
2
PACKET RECEIVED WITHOUT THE EXPECTED VERIFICATION TAG ARE DISCARDED!
The CRC-32c checksum should be set by the sender of each SCTP packet to provide additional protection
against data corruption in the network. The receiver of an SCTP packet with an invalid CRC-32c
checksum silently discards the packet.
After the packet is constructed (containing the SCTP common header and one or more control or DATA
chunks), the transmitter shall:
1. Fill in the proper Verification Tag in the SCTP common header and initialize the checksum field to 0's.
2. Calculate the CRC-32c checksum of the whole packet, including the SCTP common header and all the
chunks.
3. Put the resultant value into the checksum field in the common header and leave the rest of the bits
unchanged.
The receiver must (numbers in green):
1. Store the received CRC-32c checksum value aside,
2. Replace the 32 bits of the checksum field in the received SCTP packet with all '0's and calculate an
CRC-32c checksum value of the whole received packet.
3. Verify that the calculated CRC-32c checksum is the same as the received CRC-32c checksum. If not,
the receiver must treat the packet as an invalid SCTP packet.
Details about SCTP checksum calculation can be found in the RFC 3309, that updates RFC 2960
concerning Cheksum.
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3.1.1 Reliable Transfer
3.1.1.2 Transmission Sequence Number
SCTP Endpoint A SCTP Endpoint B
PAYLOAD DATA (TSN)
SACK Lost and Duplicated TSNs
TSN: Gaps and Duplication detection
Detection of loss and duplication of data chunk is enabled by numbering
all data chunks in the sender with a TSN.
The acknowledgements sent from the receiver to the sender are based on TSN numbers.
Retransmissions are time controlled. The timer duration is derived from continuous measurements of the round trip delay. Whenever such a transmission timer expires (and congestion control allows transmissions), all non-acknowledged data chunks are retransmitted and the timer is started again doubling its initial duration (like TCP).
When the receiver detects one or more gaps in the sequence of data chunks, each received SCTP packet is acknowledged by sending a SACK which reports all gaps. The SCAK is contained in a specific control chunk. Whenever the sender receives 4 consecutive SACKs reporting the same data chunk missing, this data chunk is immediately retransmitted (fast retransmit).
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3.1.1 Reliable Transfer
3.1.1.3 SACK Mechanism
SCTP Sender SCTP receiver
CHDH CtrlDataTSN 1
DataTSN 2
DataTSN 3
DH DHDHDataTSN 4
CHCHSACK
TSN 8, a_rwnd = 4600SACK
TSN 8, a_rwnd = 4600
One SACK:
is sent for at least every second data packet received
acknowledges multiple data chunks
CHDH CtrlDataTSN 9
DataTSN 10
DataTSN 11
DH DHDHDataTSN 12
cwnd
Payload data packet 1
CHDH CtrlDataTSN 5
DataTSN 6
DataTSN 7
DH DHDHDataTSN 8
Payload data packet 2
An SCTP endpoint must always acknowledge the reception of each valid data chunk.
CH: Common Header
DH: Data Header
The acknowledgements sent from the receiver to the sender are based on TSN numbers.
Retransmissions are time controlled. The timer duration is derived from continuous measurements of the round trip delay. Whenever such a transmission timer expires (and congestion control allows transmissions), all non-acknowledged data chunks are retransmitted and the timer is started again doubling its initial duration (like TCP).
When the receiver detects one or more gaps in the sequence of data chunks, each received SCTP packet is acknowledged by sending a SACK which reports all gaps. The SCAK is contained in a specific control chunk. Whenever the sender receives four consecutive SACKs reporting the same data chunk missing, this data chunk is immediately retransmitted (fast retransmit).
ACK generated within 200ms of the arrival of any unknowledgeable data chunk
If a_rwnd = 0 the sender must not transmit new data.
The sender must not transmit new data to a given transport address if it has cwnd or more bytes of data outstanding to that transport address.
When the time comes for the sender to transmit, before sending new DATA chunks, the sender MUST first transmit any outstanding DATA chunks which are marked for retransmission (limited by the currentcwnd).
The current congestion window (Cwnd) is an SCTP variable that limits the data, in number of bytes, a sender can send to a particular destination transport address before receiving an acknowledgement.
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3.1.1 Reliable Transfer
3.1.1.3 SACK Mechanism [cont.]
11
10
9
8
7
6
5
4
3
2
1
11
10
8
7
4
3
2
1
11
Gap
Cumulative TSN ACK = 4
Gap
Nb of Gap Ack
Blocks N = 2
Nb of Duplicate
TSNs X = 1
Tx Rx
7 4 = 3
8 4 = 4
10 4 = 6
11 4 = 7
Gap Ack Block # 1 and 2
Start and End
calculation
The reliable transfer of user data is achieved by the use of two SCTP chunks:
Payload Data Chunk.
Selective Acknowledgement (SACK).
Detection of loss and duplication of data chunks is enabled by numbering all data chunks in the sender
with the so-called Transmission Sequence Number (TSN). The acknowledgements sent from the receiver
to the sender are based on these sequence numbers.
Retransmissions are timer-controlled.
When the receiver detects one or more gaps in the sequence of data chunks, each received SCTP packet
is acknowledged by sending a SACK which reports all gaps. The SACK is contained in a specific control
chunk. Whenever the sender receives four consecutive SACKs reporting the same data chunk missing, this
data chunk is immediately retransmitted (fast retransmit).
Let's assume that an endpoint has transmitted data chunk 1 through 11. Let's also assume that chunks
with TSNs 1 through 4 and those with TSNs 7, 8, 10 and 11 have been received. Hence, chunk 5, 6, and 9
are missing. Lets also assume that chunks with TSN 8 and 11 have been received twice.
The Cumulative TSN ACK parameter contains the highest TSN value received without any gaps.
The number of Gap Ack Blocks (N) indicates the number of fragments received after the unbroken sequence. So, N=2 (from 7 to 8 and from 10 to 11).
The number of Duplicate TSNs (X) indicates the number of TSNs that have been received more than once.
The Gap Ack Block number 1 start field indicates the offset of the first segment from the unbroken sequence. This is the difference between the TSN value in the Cumulative TSN ACK field and the lowest
TSN value of the first segment.
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3.1 SCTP Data Transmission
3.1.2 Flexible Delivery
User application
SCTP layer
IP layer
User messages
2B 2A 1
Fragmentation
Bundling
2B 2A 1
DATA chunk queue
Control chunks
Message 1 Message 2
Data chunk headerSCTP control chunks
SCTP common header
IP header
Fragmentation
Flexible delivery is based on the notion of several independent streams of datagrams within an
association.
Chunks belonging to one or several streams may be bundled and transmitted in one SCTP packet.
Fragmentation
When needed, SCTP fragments user messages to ensure that the SCTP packet passed to the lower layer
conforms to the path MTU. On receipt, fragments are reassembled into complete messages before being
passed to the SCTP user.
Bundling
Multiple Data and control chunks may be bundled by the sender into a single SCTP packet for
transmission, as long as the final size of the packet does not exceed the current path MTU. Each user
message occupies its own data chuck. The receiver will unbundled the packet back into the original
chunks. Control chunks must come before Data chunks in the packet. The fragmentation and bundling
mechanisms are optional. The chunk bundling function of SCTP is responsible for assembly of the
complete SCTP packet and its disassembly at the receiving end.
An endpoint bundles chunks by simply including multiple chunks in one outbound SCTP packet. The total
size of the resultant IP datagram, including the SCTP packet and IP headers, MUST be less or equal to the
current Path MTU. If its peer endpoint is multi-homed, the sending endpoint shall choose a size not
larger than the latest MTU of the current primary path.
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3.1 SCTP Data Transmission
3.1.2 Flexible Delivery [cont.]
SCTP layer
Stream reordering queues
DATA chunks
Fragmented DATA chunks
Message 1 Message 2
IP layer
Control chunk
UnbundlingReassembly
12B 2AUser messages
User application
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3.1 SCTP Data Transmission
3.1.3 Multi-Homing
Primary path
Rules to transmit to the destination address
Heartbeat
Error
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3.1.3 Multi-Homing
3.1.3.1 Primary Path
Is there any IP IP addressaddress(es) or host name present in
INIT or INIT ACK?
Is there any host host namenameparameter present in INIT or INIT ACK?
Endpoint derives the transport transport addressaddress(es)
Endpoint takes the source IP source IP addressaddresswhich the chunk arrives and derives the transport transport addressaddress
After all the transport addresses are derived, the Endpoint shall select one of the transport addresses as initial Primary Path
Yes No
Yes No
Endpoint resolves the list of IP address(es) and derives the transport transport addressaddress(es)
An SCTP endpoint is considered multi-homed if there are more than one transport address that can be
used as a destination address to reach that endpoint.
Endpoint derives the transport address combining IP address(es) and SCTP source port (from common
header).
The primary path is the destination and source address that will be put into a packet outbound to the
peer endpoint by default. The definition includes the source address since an implementation may wish
to specify both destination and source address to better control the return path taken by reply chunks
and on which interface the packet is transmitted when the data sender is multi-homed.
In case that the receiver of an INIT ACK fails to resolve the address parameter due to an unsupported
type, it can abort the initiation process and then attempt a re-initiation by using a 'Supported Address
Types' parameter in the new INIT to indicate what types of address it prefers.
The receiver of the INIT or INIT ACK MUST NOT send user data (piggy-backed or stand-alone) to its peer
until the host name is successfully resolved.
If the name resolution is not successful, the endpoint MUST immediately send an ABORT with
"Unresolvable Address" error cause to its peer. The ABORT shall be sent to the source IP address from
which the last peer packet was received.
An endpoint should transmit reply chunks (e.g., SACK, HEARTBEAT ACK, etc.) to the same destination
transport address from which it received the DATA or control chunk to which it is replying. This rule
should also be followed if the endpoint is bundling DATA chunks together with the reply chunk. However,
when acknowledging multiple DATA chunks received in packets from different source addresses in a
single SACK, the SACK chunk may be transmitted to one of the destination transport addresses from
which the DATA or control chunks being acknowledged were received.
When a receiver of a duplicate DATA chunk sends a SACK to a multi-homed endpoint, it MAY be
beneficial to vary the destination address and not use the source address of the DATA chunk. The reason
being that receiving a duplicate from a multi-homed endpoint might indicate that the return path (as
specified in the source address of the DATA chunk) for the SACK is broken.
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3.1.3 Multi-Homing
3.1.3.2 Transport address states
Active destination transport address:
A transport address on a peer endpoint which a transmitting endpoint considers available for receiving user messages.
Idle destination transport address:
An address that has not had user messages sent to it within some length of time, normally the HEARTBEAT interval or greater.
Inactive destination transport address:
An address which is considered inactive due to errors and unavailable to transport user messages.
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3.1.3 Multi-Homing
3.1.3.3 Rules to Transmit to the Destination Address
A. By default, an endpoint should always transmit to the primary path.
B. An endpoint should transmit reply chunks (e.g. SACK) to the same destination transport address from which it received the DATA or control chunk to which it is replying.
C. When a receiver of a duplicate DATA chunk sends a SACK to a multi-homed endpoint, it may be beneficial to vary the destination addressand not use the source address of the DATA chunk.
D. When the peer is multi-homed, an endpoint should try to retransmit a chunk to an active destination transport address that is different from the last destination address to which the DATA chunk was sent.
A. Unless the SCTP user explicitly specifies the destination transport address (and possibly source transport address) to use.
B. This rule should also be followed if the endpoint is bundling DATA chunks together with the reply chunk.
C. However, when acknowledging multiple DATA chunks received in packets from different source addresses in a single SACK, the SACK chunk may be transmitted to one of the destination transport addresses from which the DATA or control chunks being acknowledged were received. The reason being that receiving a duplicate from a multi-homed endpoint might indicate that the return path (as specified in the source address of the DATA chunk) for the SACK is broken.
D. Retransmissions do not affect the total outstanding data count. However, if the DATA chunk is retransmitted onto a different destination address, both the outstanding data counts on the new destination address and the old destination address to which the data chunk was last sent shall be adjusted accordingly.
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3.1.3 Multi-Homing
3.1.3.4 Path failure detection
End point 1 End point 2
10.102.1.1 10.102.1.17
10.102.1.1810.102.1.2
DATA TSN = 1
Path.max.Retrans = Y
Error Counter Destination Transport @ 1 = X
T3-rtx
T3-rtx expires
T3-rtx
T3-rtx cancel
X > Y Destination Transport Address is marked as InactiveInactive, Notification is sent to the Upper Layer
DATA TSN = 1
Destination @ 1
Destination @ 2
SACK TSN = 1
Destination @ 1 is unavailable
When its peer endpoint is multi-homed, an endpoint should keep a error counter for each of the
destination transport addresses of the peer endpoint.
Each time the T3-rtx timer expires on any address, or when a HEARTBEAT sent to an idle address is not
acknowledged within a RTO, the error counter of that destination address will be incremented.
When the value in the error counter exceeds the protocol parameter 'Path.Max.Retrans' of that
destination address, the endpoint should mark the destination transport address as inactive, and a
notification SHOULD be sent to the upper layer.
When the primary path is marked inactive (due to excessive retransmissions, for instance), the sender
MAY automatically transmit new packets to an alternate destination address if one exists and is active.
If more than one alternate address is active when the primary path is marked inactive only ONE
transport address SHOULD be chosen and used as the new destination transport address.
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3.1.3 Multi-Homing
3.1.3.5 End point failure detection
End point 1 End point 2
10.102.1.1 10.102.1.17
10.102.1.1810.102.1.2
DATA TSN = 1
DATA TSN = 1
Association.max.Retrans = M
DATA TSN = 1DATA
TSN = 1
Counter = N
SACK TSN = 1Counter = 0
If N > MN > M End point 2 is unreachable!End point 2 is unreachable! Stop transmission to it, associations will pass to CLOSE state; SCTP reports to upper layer
T3-rtx
An endpoint shall keep a counter on the total number of consecutive retransmissions to its peer
(including retransmissions to all the destination transport addresses of the peer if it is multi-homed).
If the value of this counter exceeds the limit indicated in the protocol parameter
'Association.Max.Retrans', the endpoint shall consider the peer endpoint unreachable and shall stop
transmitting any more data to it (and thus the association enters the CLOSED state).
In addition, the endpoint shall report the failure to the upper layer, and optionally report back all
outstanding user data remaining in its outbound queue.
The counter shall be reset each time a DATA chunk sent to that peer endpoint is acknowledged (by the
reception of a SACK), or a HEARTBEAT-ACK is received from the peer endpoint.
When configuring the SCTP endpoint, the user should avoid having the value of 'Association.Max.Retrans'
larger than the summation of the 'Path.Max.Retrans' of all the destination addresses for the remote
endpoint. Otherwise, all the destination addresses may become inactive while the endpoint still
considers the peer endpoint reachable. When this condition occurs, how the SCTP chooses to function
is implementation specific.
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3.1.3 Multi-Homing
3.1.3.6 Network resilience example
SGP 1SGP 1
SCTP endpoint A
IP5 IP6
IP1 IP2
IP
network
ASP 1ASP 1
SCTP associations
Endpoint A -> Endpoint B
IP1IP3
IP1- IP4
IP2-IP3
IP2-IP4
primary path = IP1 IP3
Endpoint A -> Endpoint C
IP1-IP5
IP1-IP6
IP2-IP5
IP2-IP6
primary path = IP2 IP6
SCTP associations
Endpoint A -> Endpoint B
IP1IP3
IP1- IP4
IP2-IP3
IP2-IP4
primary path = IP1 IP3
Endpoint A -> Endpoint C
IP1-IP5
IP1-IP6
IP2-IP5
IP2-IP6
primary path = IP2 IP6SGP 1SGP 1
IP3 IP4
SCTP endpoint B SCTP endpoint C
Network resilience provided through SCTP is :
independent of the transport technology and its redundancy mechanisms
provide end-to-end protection (network, access routers, platform switches, NICs ..)
provides fast recovery (~500 ms)
But
leads to IP routing configuration complexity in core network and NEs
need to dominate source IP address assignment and SCTP primary paths assignments at local and
remote nodes
leads to high CPU processing for heartbeat messages processing (or dedicated offload hardware)
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3.1.3 Multi-Homing
3.1.3.6 Network resilience example [cont.]
SCTP active path:
primary path
SCTP inactive path
IPnetworks
SCTP endpoint A
IP1 IP2
ASP 1ASP 1
SGP 1SGP 1
IP5IP6
SGP 1SGP 1
IP3 IP4
Endpoint B Endpoint C
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3.1.3 Multi-Homing
3.1.3.6 Network resilience example [cont.]
IPnetworks
SCTP inactive path
SCTP Active path:
primary path
SCTP endpoint A
IP1 IP2
ASP 1ASP 1
SGP 1SGP 1
IP5IP6
Endpoint CSGP 1SGP 1
IP3 IP4
Endpoint B
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3.1.3 Multi-Homing
3.1.3.7 HEARTBEAT
SCTP Endpoint A SCTP Endpoint B
HEARBEAT (source IP@)
HEARBEAT ACK
IP@
Is an association Is idle
The HEARTBEAT chunk is used to query the reachability of a particular endpoint. Let's assume that no chunks need to be sent from endpoint A to endpoint B during a particular period of time. In that case, endpoint A will sent periodic HEARTBEAT messages to endpoint B, just to make sure that endpoint B is still alive. The HEARTBEAT contains sender-specific information and the receiver should respond with a HEARTBEAT ACK chunk containing heartbeat information copied form the receiver HEARBEAT chunk.
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3 Data Transmission
3.2 SCTP Robustness
SCTP implements congestion control mechanisms:
a_rwnd
Path MTU
Multi-homed:
INIT
INIT ACK
SCTP ensures that an endpoint is aware of the reachability of another endpoint:
SACK
HEARTBEAT
Robustness means the network should implement procedures whereby failures or undesired occurrences are minimized. It is also the capability to handle a certain amount of failure in the network without a
significant reduction of quality. Furthermore, the network should provide a graceful rather than a drastic
degradation in the event of failure or overload.
Congestion control mechanisms
SCTP implements congestion control mechanisms to ensure that one endpoint does not flood another
with messages. SCTP incorporates Path MTU discovery so that messages are not send if they are too long
to be handled by the intervening transport network.
Multi-homed
Common header contains source and destination port numbers. INIT and INIT ACK chunks may contain
one or more IP addresses or a host name that can be resolved to one or more addresses. The inclusion of
theses parameters enables a given endpoint to be multi-homed (to have multiple addresses). If