Stream Control Transmission Protocol (SCTP): A Reference Guide

"This book...offers inside insights (by two of the SCTP designers) into the design of the protocol and discusses why the protocol operates as it does. This insight takes much of the 'black magic' out of SCTP and makes the protocol more accessible."
--From the Foreword by Mark Allman

Named one of the Top 10 Hottest Technologies of 2001 by Telecommunications Magazine, Stream Control Transmission Protocol (SCTP) is considered by many to be the TCP of the future. Originally designed for Internet telephony, SCTP has also proven itself a superb general purpose transport protocol for IP network data communications. More robust and secure than TCP, SCTP is the technology of choice for building next generation commercial grade infrastructures for telecommunications and e-commerce. Anyone involved with the technical aspects of Internet communications should have a working knowledge of this new transmission protocol, which is rapidly being adopted by standardization bodies and networking organizations.

Written by the creators of SCTP, Stream Control Transmission Protocol (SCTP): A Reference Guide is a comprehensive, practical, guide to SCTP technology. Examining the protocol in depth, the book provides useful perspective on design rationale and highlights SCTP's improvements over TCP in the areas of security and reliability. The book also demonstrates how to take full advantage of SCTP to make IP-based communications more reliable and robust.

Topics covered include:

• An item-by-item comparison between the functions of SCTP and TCP/UDP
• SCTP packet formats
• Setup and closing of an association
• User data transfer
• Congestion control and avoidance, covering the SCTP Congestion Control Algorithm
• Failure detection and recovery, covering the interaction with multihoming and route arrangements
• "Out-of-the-blue" SCTP packets and verification tag rules
• IANA and future extension of SCTP
• A preview of the proposed SCTP Socket API extension
• Use of Streams in SCTP, featuring file transfer, call control, and Web browser applications

Numerous examples and diagrams throughout the book clarify important concepts and techniques. The final chapter presents a user space implementation of SCTP, and the accompanying CD-ROM contains the open source code for this implementation.

An important complement to the official IETF standards specification, this book is required reading for all Internet programmers, developers, managers, and implementors of SCTP.

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Index

Numerics

3GPP (Third Generation Partnership Project) 329

A

a_rwnd (advertised receive window credit) 145-146

calculating 145

for INIT messages 46

ABORT chunk 33

description of 40

fields of 72-73

receiving 249-250

in SCTP packets 233-234

sending 248-249

using 248

abortive shutdown 248-250

accept() function 259

acknowledgment

cumulative 138

delayed 138-139

generation of 28

revoking 146-148

to SCTP sender 131

additive increase multiplicative decrease (AIMD) 180

ADDRESS_ADDED state 274

ADDRESS_AVAILABLE state 274

ADDRESS_MADE_PRIM state 274

ADDRESS_REMOVED state 274

ADDRESS_UNREACHABLE state 274

addresses 2-3

ambiguity in detecting the reachability of 208-208

broadcast 22

heartbeat period of 205

hostname 48, 48

inactive 160

multicast 22

source 161

unicast 22

unreachability detection algorithm for 205-206

unreachable 201-204

unresolvable 69

use in association set-up 97-101

address-unreachability detection algorithm 205-206

Adler-32 checksum 37

generation by data senders 171

validation by data receivers 172

advertised receive window credit (a_rwnd) 46

calculating 145

AF_INET 259

AIMD (additive increase multiplicative decrease) 180

ancillary data 268

API (application programming interface) 24, 297

application programming interface. See API

ASSOCIATE primitive 30

association 21, 24-25

asymmetric multi-homing configuration of 220-221

between single-homed and multi-homed endpoints 220-221

chunk rejections during setup of 101-102

CLOSED state 237-238

definition of 329

dormant state of 216-218

duplication of 307-308

four-way initiation sequence of 13

functional view of 26-27

graceful shutdown of 238-239

operation states of 30-33

setup of 298

shutdown of 237-238

       peer endpoint request 322

       upper-layer request 321-322

tie-tags 104-105

use of addresses when starting 97-101

See also chunks

See also endpoints

association PMTU 166-166

changes in 167

definition of 329

asymmetric multi-homing 220-222

AUTOCLOSE option 265

B

B bit 56

bandwidth 281

Big Endian. See network byte order

bind() system call 24

Birds of a Feather (BOF) 329

BOF (Birds of a Feather) 329

broadcast addresses 22

buffer space 139

receive 145-146

bundling 13

of data 28

DATA chunk with COOKIE-ACK chunk 96-97

data with COOKIE-ECHO chunk 91

definition of 329

disabling 128

SACK chunk with COOKIE-ACK chunk 96-97

of user messages 169-170

C

calculateRTO() function 317

call processing centers 15

CANT_START_ASSOC notification 273

cause code

of cookie received while shutting down error 72

of invalid mandatory parameter error 70

of invalid stream identifier error 67

of missing mandatory parameter error 67

of no user data error 71

of out of resource error 68

of stake cookie error 68

of unrecognized chunk type error 69

of unrecognized parameter error 71

of unresolvable address error 69

cause length

of cookie received while shutting down error 72

of invalid mandaratory parameter error 70

of invalid stream identifier error 67

of missing mandatory parameter error 67

of no user data error 71

of out of resource error 68

of stale cookie error 68

of unrecognized chunk type error 69

of unrecognized parameter error 71

of unresolvable address error 69

chunk flags

for ABORT chunk 72

for COOKIE-ACK chunk 55

for COOKIE-ECHO chunk 54

for DATA chunk 55

for HEARTBEAT chunk 61

for HEARTBEAT-ACK chunk 61

for INIT-ACK chunk 51

for OPERATIONAL ERROR chunk 65

for SACK chunk 59

for SHUTDOWN chunk 63

for SHUTDOWN-ACK chunk 64

for SHUTDOWN-COMPLETE chunk 64

chunk header 330

chunk length 38

of ABORT chunk 73

of COOKIE-ACK chunk 55

of COOKIE-ECHO chunk 54

of DATA chunk 57

of HEARTBEAT chunk 61

of HEARTBEAT-ACK chunk 61

of INIT messages 46

of INIT-ACK chunk 51

of OPERATIONAL ERROR chunk 65

of SACK chunk 59

of SHUTDOWN chunk 63

of SHUTDOWN-ACK chunk 64

of SHUTDOWN-COMPLETE chunk 64

chunk parameters 42-44

data 43

definition of 330

length 43

optional 42

permanent 42, 332

type 43

chunk type 38

bit patterns 43

by number 38-41

extensibility of 254-255

for ABORT chunk 72

for COOKIE-ACK chunk 55

for COOKIE-ECHO chunk 54

for DATA chunk 55

for HEARTBEAT chunk 61

for HEARTBEAT-ACK chunk 61

for INIT messages 46

for INIT-ACK chunk 51

for OPERATIONAL ERROR chunk 65

for SACK chunk 59

for SHUTDOWN chunk 63

for SHUTDOWN-ACK chunk 64

for SHUTDOWN-COMPLETE chunk 64

reserved values for 254-255

selecting the values of 42

chunks 35

ABORT 72-73

control 35, 41

COOKIE-ACK 55

COOKIE-ECHO 54-55, 89-90

DATA 55-58

data 35, 41

definition of 330

elements in 37-38

ERROR 65

flags 38

forming SCTP packets from 127-128

general rules in dealing with 41-42

header 330

HEARTBEAT 60-61

INIT 44-47

INIT-ACK 50-53

length 38

OPERATIONAL ERROR 65, 65

out-of-the-blue 173

parameter length 43

parameter type 43

parameters 42-44

parameters data 43

rejections during association setup 101-102

SACK 58-60

SHUTDOWN 62-63

SHUTDOWN-ACK 63-64

SHUTDOWN-COMPLETE 64-65

types 38

       by number 38-41

unrecognized 69

See also association

See also endpoints

client-side applications 259-261

CLOSE primitive 33

close() function 260

CLOSED state 30, 32

of association 237-238

procedures for entering 247

CMSG_HDR structure 265

command channel 288

commercial telephone services 9

common header 36-37

communication nodes 4

multi-homed 6

protocols 5

communication software 3

COMMUNICATION_LOST notification 273

COMMUNICATION_UP notification 273

communications 1

complex cookies 306

processing of 308-309

congestion 10

avoidance of 28

control of 13

congestion avoidance algorithm 184

congestion avoidance mechanism 132

congestion collapse 177-178

congestion control 280, 315-316

algorithms 178-180, 186-193

basic concepts of 178-180

for SCTP (Stream Control Transmission Protocol) 180-186

congestion window (cwnd) 132, 180

decreasing 185

increasing 183

congestion window reduce (CWR) 197

connect() function 259-260

connection-oriented communications 1

control chunks 35, 41

cookie preservative parameter 48, 82

description of 48

cookie received while shutting down error 71-72

COOKIE_ECHOED state 30, 308

handlin

Preface

This book describes, from the two primary designers' view, the design and operation of the Stream Control Transmission Protocol (SCTP)--a new general-purpose IP transport protocol recently standardized by the Internet Engineering Task Force (IETF).

For those who are familiar with TCP/IP networking, SCTP can be simply viewed as a "super-TCP," that is, a TCP-like, general-purpose reliable data transport protocol with an enhanced set of capabilities.

SCTP is a key piece of the puzzle for making IP a truly viable technology choice in building the next-generation commercial grade infrastructure for telecommunications and e-commerce. For instance, SCTP has been required by the 3rd Generation Partnership Project (3GPP) to carry call signaling traffic in the third-generation cellular systems, and recommended by the IETF to carry the mission-critical Authentication, Authorization, and Accounting (AAA) messages in any future IP service networks.

Telecommunications Magazine selected SCTP as one of the "10 Hottest Technologies of Year 2001" in a cover story in its May 2001 issue.

Why Do We Need SCTP?

The basic design of TCP/IP has worked remarkably well and remained largely unchanged. All the while the Internet has gone through its exponential growth, from a network of only a few nodes connecting a few university campuses, to a truly global Internet with tens of millions of hosts.

The Internet's explosive growth, witnessed by the nineties, continues into the new millennium. At the same time, IP technology has been steadily transforming itself into a cornerstone of the new information-era economy. One of the most visible results of this transformation is that IP technology is beginning to see more and more commercial uses. This is in sharp contrast to the seventies and eighties, when almost the only mention of TCP/IP was heard in academic and research communities.

It is inevitable that this commercialization process has brought with it new requirements to the technology. The original design of TCP, which was conceived and incubated in a mostly academic and research environment, has started to show its limitations in meeting some of these new commercial requirements.

It is this commercialization challenge on IP technology that has brought on the birth of SCTP.

When we first brought the original idea of SCTP (then called MDTP) to the IETF, we were trying to create a protocol to solve a very particular and practical problem; that is, how to transport telephony signaling messages using IP technology in commercial-grade systems. But, at the end of the process, the success of SCTP has grown far beyond our widest expectations.

Readers

This book is written with two different groups of readers in mind. The first group consists of those who want to understand SCTP. These readers may include programmers who need to work with SCTP, managers who want to make informed decisions about SCTP, students who want to learn the basics of SCTP, etcetera. For them, this book can serve as an easy-to-read alternative to the IETF standards specification.

In this book they will find not only detailed explanations of all the basic parts and operations of the protocol, often accompanied by a good deal of examples and diagrams, but also a thorough item-by-item comparison between functions of SCTP and TCP/UDP. Some of the chapters also give this group of readers guidance on which parts of the chapter they can safely skip without hindering their understanding of the general protocol operation.

The second group of readers we have in mind consists of the implementers of SCTP. For them, this book can be used as an annotation to the IETF standards specification. It is our hope that they will find some helpful clarifications of certain complex internal mechanisms of the protocol through the examples and illustrations we put in the book. Moreover, the recounts we have in the book on many important IETF debates and consensus-building processes may give them some insight into the protocol design. This in turn may improve their understanding over the rationale behind those same key design decisions.

Prerequisites

We assume that the readers have preliminary knowledge about TCP/IP and IP networking. (An excellent book for this is W. Richard Stevens' and Gary R. Wright's The TCP/IP Illustrated, Volume 1: The Protocols, published by Addison-Wesley.) Moreover, some experience with C programming and Unix systems will be very helpful, though it is not an absolute requirement.

For those who would like to refresh their knowledge of IP networking, we have a short review of the IP networking basics at the beginning of Chapter 1.

Coverage

This book covers all aspects of SCTP standards as defined in the IETF's Request for Comments 2960 (RFC2960) (Stewart et al. 2000). We also include a preview of the proposed SCTP sockets API (application programming interface) extension, which is a work in progress in the IETF.

A CD-ROM containing the open-source user-space SCTP reference implementation is included in this book. Readers who want to read the reference implementation code can find a chapter at the end of the book explaining the overall design and internal structures of the code. This code has been compiled and tested with FreeBSD 4.2, Linux 2.2.17 and 2.4.1, and Solaris 2.8.1 operating systems. Updates and new releases of this open-source SCTP reference implementation can be found at http://www.sctp.org.

For those interested in doing experiments with the SCTP reference implementation, a good tool to use to capture and analyze SCTP traffic is the Ethereal Network Analyzer, which is freely downloadable from http://www.ethereal.com.

0201721864P10152001

Table of Contents

Preface.


1. Introduction and Overview.

IP Networking Basics.

How Messages are Delivered in an IP Network.

IP Protocols.

IP Multi-homing.

What Is SCTP?

Where Does SCTP Fit in the IP Architecture?

Motivation for Developing SCTP.

TCP Limitations.

UDP Limitations.

SCTP Enhancements Over TCP and UDP.

A Short History of SCTP Development.

Early Works Before IETF and MDTP.

IETF Refinements.

Major General SCTP Issues Debated in IETF.

Do We Really Need a New Transport Protocol?

Over UDP vs. Over IP.

Organization of This Book.

Summary.

Questions.

2. SCTP Endpoint and Association.

SCTP Transport Address.

SCTP Endpoint.

Multi-homed SCTP Endpoint.

SCTP Association.

Operation of an SCTP Association.

Functional View of an Association.

Functional Responsibilities of an SCTP Endpoint.

Operation States of an Association.

IETF Debate And Issues.

Summary.

Questions.

3. Format of SCTP Messages.

Basic Layout—the Bits on the Wire.

The SCTP Common Header.

Elements in a Chunk.

Defined Chunk Types and Extensibility.

General Rules When Dealing with Chunks.

Chunk Parameters.

Other Considerations.

Basic Chunk Descriptions.

The INIT Chunk.

The INIT-ACK Chunk.

The COOKIE-ECHO Chunk.

The COOKIE-ACK Chunk.

The DATA Chunk.

The SACK Chunk.

The HEARTBEAT Chunk 98.

The HEARTBEAT-ACK Chunk.

The SHUTDOWN chunk.

The SHUTDOWN-ACK Chunk.

The SHUTDOWN-COMPLETE Chunk.

The ERROR Chunk.

The Error Cause Parameters.

The ABORT Chunk.

IETF Debate And Issue.

Summary.

Questions.

4. Setup of an Association.

The INIT Chunk.

Formulating the Chunk.

INIT Parameters.

Timer and Retransmission For INIT.

The INIT-ACK Response.

Formulating the Response.

INIT-ACK Parameters.

Preventing Resource Attacks.

The COOKIE-ECHO Chunk.

Formulating the Chunk.

Bundling Data with the COOKIE-ECHO.

Timer and Retransmission For COOKIE-ECHO.

The COOKIE-ACK Chunk.

Validating and Unpacking the Cookie.

Formulating The COOKIE-ACK.

Bundling DATA and SACK with COOKIE-ACK.

Address Usage When Starting An Association.

Chunk Rejections During an Association Setup.

When an INIT Is Unacceptable?

When is a Cookie Not Worth Eating?

Handle Unexpected Initialization Chunks.

Association Tie-Tags and their values.

Handle Overlapping Initialization Chunks.

Case Study #1: Initialization Collision.

Case Study #2: Peer Restart.

A lost COOKIE-ACK.

IETF Debate and Issues.

Summary.

Questions.

5. User Data Transfer.

User Message Transfer Overview.

How User Messages Are Sent.

How User Messages Are Received.

Summary on Data Transfer Overview.

Obey The Transmission Rules.

Congestion Window And Receiver Window.

Rules for Data Transmission.

Acknowledgment Rules.

Where to Send a SACK?

When to Send a SACK?

What to Report in a SACK?

Revoke an Acknowledgment by Receiver.

Process Received SACK by Data Sender.

Management of Retransmission Timer.

Determine Retransmission Time-out (RTO).

Retransmission Timer Rules.

Handle Retransmission Timer Expiration.

Multi-homed Data Sender and Receiver.

Selection of Primary Transport Address.

Transfer Data Between Multi-homed Endpoints.

Sending SACK on Duplicate Data.

Stream Identifier and Stream Sequence Number.

Ordered and Un-ordered Message Delivery.

Passing Payload Protocol Identifier.

Fragmentation and Bundling Of User Messages.

When to Fragment?

How to Fragment a User Message?

Reassembling Fragmented Message at Data Receiver.

User Message Bundling.

Adler-32 Checksum.

Generation of Adler-32 Checksum by Data Sender.

Validate Adler-32 Checksum by Data Receiver.

Error Handling.

Handle Data Chunk with Invalid Stream Identifier.

Handle Empty Data Chunk.

Handle Out-of-state Data.

IETF Debate and Issues.

Summary.

Questions.

6. Congestion Control and Avoidance.

The Cause of Network Congestion Collapse.

Basic Concept of Congestion Control.

SCTP Congestion Control Algorithm.

How Does Your cwnd Grow?

How Does Your cwnd Shrink?

Interesting Details of the Algorithms.

Path MTU Discovery.

A Refresher on How PMTU Works.

PMTU Discoveryin SCTP.

Explicit Congestion Notification.

IETF Debate and Issues.

Summary.

Questions.

7. Failure Detection and Recovery.

What Types of Failure SCTP Detects.

Destination Address Unreachable.

Peer Endpoint Unreachable.

How To Detect an Unreachable Address.

Use HEARTBEAT to Monitor Reachability.

Address Unreachability Detection Algorithm.

More on Using Heartbeat.

Handle Unreachable Primary Destination Address.

How To Detect an Unreachable Peer.

Association Entering Dormant State.

Fault Resilience Communication and Routing Configuration.

Maximize Path Diversity.

Asymmetric Multi-homing Configuration.

Effects of Source Address Selection.

IETF Debate and Issues.

Summary.

Questions.

8. Auxiliary SCTP Packet Handling Functions.

Handle Out-of-the-blue SCTP Packets.

SCTP Packet Verification Tag Rules.

Basic Verification Tag Rules.

Special Verification Tag Rules.

IETF Debate and Discussion.

Summary.

Questions.

9. Close of an Association.

The Graceful Shutdown.

Sending SHUTDOWN.

Receiving SHUTDOWN.

Sending SHUTDOWN-COMPLETE.

Race Condition And Lost Message.

The Abortive Shutdown.

Abort Usage.

Sending ABORT.

Receiving ABORT.

IETF Debate and Issues.

Summary.

Questions.

10. IANA And Future Extension Of SCTP.

SCTP Port Assignment.

IETF Chunk Extensions.

IETF Parameter Extensions.

IETF Extensions to the Error Causes.

Payload Protocol Identifier.

Summary.

11. A Socket API For SCTP.

A Quick Review of the TCP SOCKET API.

The Application Server Side.

The Application Client Side.

Applying the Typical TCP API to SCTP.

The Un-typical SCTP API.

A View of a Client.

The Receiving Side View.

Common API Mechanisms for Both Models.

Using the sendmsg and recvmsg Calls with SCTP.

Notification Information.

Summary.

Questions.

12. Comparing SCTP to TCP.

The Similarities Between SCTP and TCP.

Start-up.

Reliability and Ordering.

Congestion Control.

Closing Down.

The Differences between SCTP and TCP.

Differences at Start-up.

Head of Line Blocking.

Message Oriented Boundaries.

Un-ordered Delivery.

Selective Acknowledgment.

Multi-homing.

A Closing Difference.

Summary.

Questions.

13. Using Streams In SCTP.

A File Transfer Application.

A Call Control Application.

A Web Browser.

Summary.

Questions.

14. A User Space Implementation Of SCTP.

The Big Picture.

Data Structures.

Events and the API.

Association Setup.

When an Upper Layer Issues an Associate Primitive.

When a Peer Sends the SCTP Endpoint an INIT.

Data Transfer.

SACK Processing.

Inbound DATA Processing

Finishing Off By Sending.

Congestion Control, PMTU, And RTO.

Congestion Control.

PMTU Discovery.

RTO Calculation.

Fault Management.

Tag and Out-of-the-blue Handling.

Association Close-down.

At the Upper Layers Request.

Reception of a SHUTDOWN.

Summary.

Glossary and Abbreviations.


References.


Index. 0201721864T10022001