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The Internet and Its Protocols
A Comparative Approach

This page intentionally left blank

The Internet and Its Protocols
ADRIAN FARREL

For Eleanor and Elliot in the hope that they
need never read it.

vii
Contents
Preface
Preface
Preface
Preface xix
xix
xix
xix
About the Author
About the Author
About the Author
About the Author xxix
xxix
xxix
xxix
Chapter 1
Chapter 1
Chapter 1
Chapter 1 Overview of Essentials 1
1.1 Physical Connectivity 1
1.2 Protocols and Addressing 2
1.3 The OSI Seven-Layer Model 4
1.4 An Architecture for the Network 7
1.5 Packaging Data 9
1.6 Data-link Protocols 10
1.6.1 Ethernet 10
1.6.2 Token Ring 12
1.6.3 Asynchronous Transfer Mode 14
1.6.4 Packet over SONET 16
1.6.5 Dial-Up Networking 18
1.6.6 802.2 and Logical Link Control 18
1.7 The Protocols at a Glance 19
1.8 Further Reading 20
Chapter 2
Chapter 2
Chapter 2
Chapter 2 The Internet Protocol 23
2.1 Choosing to Use IP 24
2.1.1 Connecting across Network Types 24
2.2 IPv4 25
2.2.1 IP Datagram Formats 26
2.2.2 Data and Fragmentation 30
2.2.3 Choosing to Detect Errors 34
2.3 IPv4 Addressing 37
2.3.1 Address Spaces and Formats 37
2.3.2 Broadcast Addresses 39
2.3.3 Address Masks, Prefixes, and Subnetworks 39
2.3.4 Network Address Translation (NAT) 41
2.4 IP in Use 42
2.4.1 Bridging Function 42
2.4.2 IP Switching and Routing 44
2.4.3 Local Delivery and Loopbacks 46
2.4.4 Type of Service 47

viii Contents
2.4.5 Address Resolution Protocol 49
2.4.6 Dynamic Address Assignment 53
2.5 IP Options and Advanced Functions 59
2.5.1 Route Control and Recording 61
2.6 Internet Control Message Protocol (ICMP) 64
2.6.1 Messages and Formats 64
2.6.2 Error Reporting and Diagnosis 65
2.6.3 Flow Control 70
2.6.4 Ping and Traceroute 70
2.6.5 Discovering Routers 74
2.6.6 Path MTU Discovery 75
2.6.7 Security Implications 76
Chapter 3
Chapter 3
Chapter 3
Chapter 3 Multicast 79
3.1 Choosing Unicast or Multicast 79
3.1.1 Applications That Use Multicast 83
3.2 Multicast Addressing and Forwarding 84
3.3 Internet Group Management Protocol (IGMP) 87
3.3.1 What are Groups? 87
3.3.2 IGMP Message Formats and Exchanges 88
Chapter 4
Chapter 4
Chapter 4
Chapter 4 IP Version Six 93
4.1 IPv6 Addresses 94
4.1.1 IPv6 Address Formats 95
4.1.2 Subnets and Prefixes 99
4.1.3 Anycast 99
4.1.4 Addresses with Special Meaning 100
4.1.5 Picking IPv6 Addresses 101
4.2 Packet Formats 102
4.3 Options 103
4.4 Choosing IPv4 or IPv6 110
4.4.1 Carrying IPv4 Addresses in IPv6 110
4.4.2 Interoperation between IPv4 and IPv6 111
4.4.3 Checksums 111
4.4.4 Effect on Other Protocols 112
4.4.5 Making the Choice 113
Chapter 5
Chapter 5
Chapter 5
Chapter 5 Routing 115
5.1 Routing and Forwarding 116
5.1.1 Classless Inter-Domain Routing (CIDR) 116
5.1.2 Autonomous Systems 118
5.1.3 Building and Using a Routing Table 119
5.1.4 Router IDs, Numbered Links, and Unnumbered Links 122

Contents ix
5.2 Distributing Routing Information 124
5.2.1 Distance Vectors 125
5.2.2 Link State Routing 131
5.2.3 Path Vectors and Policies 137
5.2.4 Distributing Additional Information 141
5.2.5 Choosing a Routing Model 141
5.3 Computing Paths 142
5.3.1 Open Shortest Path First (OSPF) 143
5.3.2 Constrained Shortest Path First (CSPF) 145
5.3.3 Equal Cost Multi-Path (ECMP) 146
5.3.4 Traffic Engineering 146
5.3.5 Choosing How to Compute Paths 147
5.4 Routing Information Protocol (RIP) 147
5.4.1 Messages and Formats 148
5.4.2 Overloading the Route Entry 150
5.4.3 Protocol Exchanges 151
5.4.4 Backwards Compatibility with RIPv1 153
5.4.5 Choosing to Use RIP 154
5.5 Open Shortest Path First (OSPF) 155
5.5.1 Basic Messages and Formats 155
5.5.2 Neighbor Discovery 157
5.5.3 Synchronizing Database State 159
5.5.4 Advertising Link State 161
5.5.5 Multi-Access Networks and Designated
Routers 167
5.5.6 OSPF Areas 170
5.5.7 Stub Areas 172
5.5.8 Not So Stubby Areas (NSSAs) 172
5.5.9 Virtual Links 174
5.5.10 Choosing to Use Areas 175
5.5.11 Other Autonomous Systems 177
5.5.12 Opaque LSAs 178
5.6 Intermediate-System to Intermediate-System (IS-IS) 179
5.6.1 Data Encapsulation and Addressing 180
5.6.2 Fletcher’s Checksum 181
5.6.3 Areas 181
5.6.4 IS-IS Protocol Data Units 184
5.6.5 Neighbor Discovery and Adjacency Maintenance 185
5.6.6 Distributing Link State Information 190
5.6.7 Synchronizing Databases 195
5.7 Choosing between IS-IS and OSPF 196
5.8 Border Gateway Protocol 4 (BGP-4) 199
5.8.1 Exterior Routing and Autonomous Systems 199
5.8.2 Basic Messages and Formats 200
5.8.3 Advanced Function 214
5.8.4 Example Message 217
5.8.5 Interior BGP 218
5.8.6 Choosing to Use BGP 222

x Contents
5.9 Multicast Routing 223
5.9.1 Multicast Routing Trees 224
5.9.2 Dense-Mode Protocols 225
5.9.3 Sparse-Mode Protocols 227
5.9.4 Protocol Independent Multicast Sparse-Mode
(PIM-SM) 227
5.9.5 Multicast OSPF (MOSPF) 231
5.9.6 Distance Vector Multicast Routing Protocol
(DVMRP) 232
5.9.7 The MBONE 234
5.9.8 A New Multicast Architecture 236
5.9.9 Choosing a Multicast Routing Protocol 239
5.10 Other Routing Protocols 241
5.10.1 Inter-Gateway Routing Protocol (IGRP) and
Enhanced Inter-Gateway Routing Protocol (EIGRP) 242
5.10.2 ES-IS 242
5.10.3 Inter-Domain Routing Protocol (IDRP) 243
5.10.4 Internet Route Access Protocol 243
5.10.5 Hot Standby Router Protocol (HSRP) and Virtual Router
Redundancy Protocol (VRRP) 243
5.10.6 Historic Protocols 245
Chapter 6
Chapter 6
Chapter 6
Chapter 6 IP Service Management 249
6.1 Choosing How to Manage Services 251
6.2 Differentiated Services 253
6.2.1 Coloring Packets in DiffServ 253
6.2.2 DiffServ Functional Model 255
6.2.3 Choosing to Use DiffServ 257
6.3 Integrated Services 257
6.3.1 Describing Traffic Flows 258
6.3.2 Controlled Load 260
6.3.3 Guaranteed Service 260
6.3.4 Reporting Capabilities 262
6.3.5 Choosing to Use IntServ 264
6.3.6 Choosing a Service Type 265
6.3.7 Choosing between IntServ and DiffServ 266
6.4 Reserving Resources Using RSVP 266
6.4.1 Choosing to Reserve Resources 267
6.4.2 RSVP Message Flows for Resource Reservation 267
6.4.3 Sessions and Flows 270
6.4.4 Requesting, Discovering, and Reserving 271
6.4.5 Error Handling 272
6.4.6 Adapting to Changes in the Network 274
6.4.7 Merging Flows 277
6.4.8 Multicast Resource Sharing 280

Contents xi
6.4.9 RSVP Messages and Formats 281
6.4.10 RSVP Objects and Formats 286
6.4.11 Choosing a Transport Protocol 296
6.4.12 RSVP Refresh Reduction 297
6.4.13 Choosing to Use Refresh Reduction 303
6.4.14 Aggregation of RSVP Flows 304
Chapter 7
Chapter 7
Chapter 7
Chapter 7 Transport Over IP 307
7.1 What Is a Transport Protocol? 307
7.1.1 Choosing to Use a Transport Protocol 308
7.1.2 Ports and Addresses 309
7.1.3 Reliable Delivery 311
7.1.4 Connection-Oriented Transport 312
7.1.5 Datagrams 312
7.2 User Datagram Protocol (UDP) 313
7.2.1 UDP Message Format 313
7.2.2 Choosing to Use the UDP Checksum 314
7.2.3 Choosing between Raw IP and UDP 316
7.2.4 Protocols That Use UDP 316
7.2.5 UDP Lite 317
7.3 Transmission Control Protocol (TCP) 318
7.3.1 Making IP Connection Oriented 318
7.3.2 TCP Messages 318
7.3.3 Connection Establishment 319
7.3.4 Data Transfer 322
7.3.5 Acknowledgements and Flow Control 324
7.3.6 Urgent Data 329
7.3.7 Closing the Connection 330
7.3.8 Implementing TCP 331
7.3.9 TCP Options 334
7.3.10 Choosing between UDP and TCP 336
7.3.11 Protocols That Use TCP 337
7.4 Stream Control Transmission Protocol (SCTP) 337
7.4.1 SCTP Message Formats 339
7.4.2 Association Establishment and Management 341
7.4.3 Data Transfer 348
7.4.4 SCTP Implementation 352
7.4.5 Choosing between TCP and SCTP 353
7.4.6 Protocols That Use SCTP 353
7.5 The Real-time Transport Protocol (RTP) 354
7.5.1 Managing Data 354
7.5.2 Control Considerations 358
7.5.3 Choosing a Transport for RTP 363
7.5.4 Choosing to Use RTP 363

xii Contents
Chapter 8
Chapter 8
Chapter 8
Chapter 8 Traffic Engineering 367
8.1 What Is IP Traffic Engineering? 367
8.2 Equal Cost Multipath (ECMP) 369
8.3 Modifying Path Costs 369
8.4 Routing IP Flows 371
8.5 Service-Based Routing 372
8.6 Choosing Offline or Dynamic Traffic Engineering 373
8.7 Discovering Network Utilization 374
8.7.1 Explicit Congestion Notification 375
8.8 Routing Extensions for Traffic Engineering 376
8.8.1 OSPF-TE 377
8.8.2 IS-IS-TE 379
8.9 Choosing to Use Traffic Engineering 381
8.9.1 Limitations of IP Traffic Engineering 382
8.9.2 Future Developments in Traffic Engineering 382
Chapter 9
Chapter 9
Chapter 9
Chapter 9 Multiprotocol Label Switching (MPLS) 385
9.1 Label Switching 386
9.1.1 Choosing between Routing and Switching 387
9.2 MPLS Fundamentals 388
9.2.1 Labeling Packets 388
9.2.2 Label Swapping and the Label Switched Path (LSP) 389
9.2.3 Inferred Labels in Switching Networks 390
9.2.4 Mapping Data to an LSP 391
9.2.5 Hierarchies and Tunnels 393
9.2.6 Choosing MPLS Over Other Switching Technologies 396
9.3 Signaling Protocols 397
9.3.1 What Does a Signaling Protocol Do? 397
9.3.2 Choosing an IP-Based Control Plane 397
9.3.3 Routing-Based Label Distribution 398
9.3.4 On-Demand Label Distribution 399
9.3.5 Traffic Engineering 399
9.3.6 Choosing to Use a Signaling Protocol 400
9.4 Label Distribution Protocol (LDP) 401
9.4.1 Peers, Entities, and Sessions 403
9.4.2 Address Advertisement and Use 409
9.4.3 Distributing Labels 411
9.4.4 Choosing a Label Distribution Mode 417
9.4.5 Choosing a Label Retention Mode 418
9.4.6 Stopping Use of Labels 419
9.4.7 Error Cases and Event Notification 423
9.4.8 Further Message Flow Examples 426
9.4.9 Choosing Transport Protocols for LDP 429
9.4.10 Surviving Network Outages 429
9.4.11 LDP Extensions 430

Contents xiii
9.5 Traffic Engineering in MPLS 431
9.5.1 Explicit Routes 433
9.5.2 Reserving Resources and Constraint-Based Routing 436
9.5.3 Grooming Traffic 437
9.5.4 Managing the Network 437
9.5.5 Recovery Procedures 438
9.5.6 Choosing to Use a Constraint-Based Signaling Protocol 438
9.6 Constraint-Based LSP Setup Using LDP (CR-LDP) 439
9.6.1 Adding Constraints to LDP 439
9.6.2 New TLVs 440
9.6.3 New Status Codes 451
9.6.4 CR-LDP Messages 452
9.7 Extensions to RSVP for LSP Tunnels (RSVP-TE) 456
9.7.1 Re-use of RSVP Function 457
9.7.2 Distributing Labels 458
9.7.3 Identifying LSPs 458
9.7.4 Managing Routes 459
9.7.5 Resource Requests and Reservation 464
9.7.6 Priorities, Preemption, and Other Attributes 465
9.7.7 Coloring the LSP 466
9.7.8 Detecting Errors and Maintaining Connectivity 466
9.7.9 Summary of Messages and Objects 468
9.7.10 Choosing a Transport Protocol 470
9.7.11 Security, Admission Control, and Policy Considerations 471
9.7.12 New Error Codes and Values 471
9.7.13 Message Flows 472
9.7.14 Sample Messages 476
9.8 Choosing Between CR-LDP and RSVP-TE 479
9.8.1 Why Are There Two Protocols? 479
9.8.2 Applicability and Adoption 479
9.8.3 Comparison of Functionality 480
9.9 Prioritizing Traffic in MPLS 481
9.9.1 Inferring Priority from Labels 482
9.9.2 Inferring Priority from Experimental Bits 483
9.9.3 New Error Codes 484
9.9.4 Choosing between L-LSPs and E-LSPs 485
9.10 BGP-4 and MPLS 486
9.10.1 Distributing Labels for BGP Routes 486
9.10.2 New and Changed Message Objects 488
9.10.3 Constructing MPLS VPNs 489
Chapter 10
Chapter 10
Chapter 10
Chapter 10 Generalized MPLS (GMPLS) 491
10.1 A Hierarchy of Media 492
10.1.1 Layer Two Switching 492
10.1.2 Packet Switching 492
10.1.3 Time Division Multiplexing 492

xiv Contents
10.1.4 Lambda Switching 493
10.1.5 Waveband Switching 493
10.1.6 Fiber and Port Switching 493
10.1.7 Choosing Your Switching Type 493
10.1.8 What is a Label? 494
10.2 Generic Signaling Extensions for GMPLS 494
10.2.1 Generic Labels 494
10.2.2 Requesting Labels 496
10.2.3 Negotiating Labels 497
10.2.4 Bidirectional Services 502
10.2.5 Protection Services 503
10.2.6 Managing Connections and Alarms 503
10.2.7 Out of Band Signaling 505
10.2.8 Choosing to Use GMPLS Signaling 507
10.3 Choosing RSVP-TE or CR-LDP in GMPLS 508
10.4 Generalized RSVP-TE 509
10.4.1 Enhanced Route Control 509
10.4.2 Reducing Protocol Overheads 511
10.4.3 Notification Requests and Messages 512
10.4.4 Graceful Restart 513
10.4.5 New and Changed Message Objects 514
10.4.6 Message Formats 516
10.4.7 Message Exchanges 516
10.5 Generalized CR-LDP 520
10.5.1 New TLVs 521
10.5.2 Message Formats 521
10.6 Hierarchies and Bundles 521
10.7 OSPF and IS-IS in GMPLS 523
10.7.1 A New Meaning for Bandwidth 524
10.7.2 Switching and Protection Capabilities 524
10.7.3 Shared Risk Link Groups 525
10.7.4 OSPF Message Objects 526
10.7.5 IS-IS Message Objects 528
10.7.6 Choosing between OSPF and IS-IS
in GMPLS 529
10.8 Optical VPNs 530
10.9 Link Management Protocol (LMP) 531
10.9.1 Links, Control Channels, and Data
Channels 533
10.9.2 Discovering and Verifying Links 537
10.9.3 Exchanging Link Capabilities 542
10.9.4 Isolating Faults 544
10.9.5 Authentication 545
10.9.6 Choosing to Use LMP 546

Contents xv
Chapter 11
Chapter 11
Chapter 11
Chapter 11 Switches and Components 549
11.1 General Switch Management Protocol 549
11.1.1 Distributed Switches 550
11.1.2 Overview of GSMP 551
11.1.3 Common Formats 551
11.1.4 Establishing Adjacency 554
11.1.5 Switch Configuration 556
11.1.6 Port Management 560
11.1.7 Connection Management 561
11.1.8 Pre-reservation of Resources 562
11.1.9 Events, State and Statistics 563
11.1.10 Choosing to Use GSMP 565
11.2 Separating IP Control and Forwarding 566
11.2.1 The ForCES Working Group and Netlink 566
11.3 LMP-WDM 569
11.3.1 Distributed WDM Architectures 569
11.3.2 Control Channel Management 569
11.3.3 Link Management 569
11.3.4 Fault Management 571
Chapter 12
Chapter 12
Chapter 12
Chapter 12 Application Protocols 575
12.1 What Is an Application? 576
12.1.1 Clients and Servers 576
12.1.2 Ports 576
12.2 Choosing a Transport 578
12.2.1 Choosing to Use Sockets 579
12.3 Domain Name System 579
12.3.1 Host Names 579
12.3.2 The DNS Protocol 582
12.3.3 Distribution of DNS Databases 582
12.3.4 DNS Message Formats 584
12.3.5 Extensions to DNS 588
12.4 Telnet 588
12.4.1 Choosing between Character and Graphic Access 590
12.4.2 Network Virtual Terminal 590
12.4.3 How Does Telnet Work? 591
12.4.4 Telnet Authentication 595
12.4.5 Telnet Applications 597
12.5 File Transfer Protocol (FTP) 598
12.5.1 A Simple Application Protocol 598
12.5.2 Connectivity Model 600
12.5.3 FTP Message Format 601
12.5.4 Managing an FTP Session 602
12.5.5 Data Connection Control 603

xvi Contents
12.5.6 Moving Files in FTP 607
12.5.7 FTP Replies 608
12.5.8 Could It Be Simpler? Trivial FTP 611
12.5.9 Choosing a File Transfer Protocol 614
12.6 Hypertext Transfer Protocol (HTTP) 615
12.6.1 What Is Hypertext 616
12.6.2 Universal Resource Locators (URLs) 617
12.6.3 What Does HTTP Do? 618
12.6.4 Multipurpose Internet Message Extensions (MIME) 621
12.6.5 HTTP Message Formats 622
12.6.6 Example Messages and Transactions 626
12.6.7 Securing HTTP Transactions 630
12.7 Choosing an Application Protocol 630
Chapter 13
Chapter 13
Chapter 13
Chapter 13 Network Management 635
13.1 Choosing to Manage Your Network 635
13.2 Choosing a Configuration Method 637
13.2.1 Command Line Interfaces 637
13.2.2 Graphical User Interfaces 638
13.2.3 Standardized Data Representations and Access 639
13.2.4 Making the Choice 641
13.3 The Management Information Base (MIB) 641
13.3.1 Representing Managed Objects 644
13.4 The Simple Network Management Protocol (SNMP) 646
13.4.1 Requests, Responses, and Notifications 646
13.4.2 SNMP Versions and Security 647
13.4.3 Choosing an SNMP Version 648
13.5 Extensible Markup Language (XML) 648
13.5.1 Extensibility and Domains of Applicability 649
13.5.2 XML Remote Procedure Calls 650
13.5.3 Simple Object Access Protocol (SOAP) 652
13.5.4 XML Applicability to Network Management 652
13.6 Common Object Request Broker Architecture
(CORBA) 652
13.6.1 Interface Definition Language (IDL) 652
13.6.2 The Architecture 653
13.6.3 CORBA Communications 656
13.7 Choosing a Configuration Protocol 660
13.8 Choosing to Collect Statistics 660
13.9 Common Open Policy Service Protocol (COPS) 663
13.9.1 Choosing to Apply Policy 663
13.9.2 The COPS Protocol 666
13.9.3 COPS Message Formats 668
13.9.4 The Policy Information Base 672

Contents xvii
Chapter 14
Chapter 14
Chapter 14
Chapter 14 Concepts in IP Security 677
14.1 The Need for Security 678
14.1.1 Choosing to Use Security 679
14.2 Choosing Where to Apply Security 681
14.2.1 Physical Security 681
14.2.2 Protecting Routing and Signaling Protocols 682
14.2.3 Application-Level Security 682
14.2.4 Protection at the Transport Layer 684
14.2.5 Network-Level Security 684
14.3 Components of Security Models 684
14.3.1 Access Control 685
14.3.3 Encryption 688
14.4 IPsec 689
14.4.1 Choosing between End-to-End and
Proxy Security 689
14.4.3 Authentication and Encryption 692
14.5 Transport-Layer Security 695
14.5.1 The Handshake Protocol 697
14.5.2 Alert Messages 701
14.6 Securing the Hyper-Text Transfer Protocol 701
14.7 Hashing and Encryption: Algorithms and Keys 703
14.7.1 Message Digest Five (MD5) 704
14.7.2 Data Encryption Standard (DES) 714
14.8 Exchanging Keys 714
14.8.1 Internet Key Exchange 715
Chapter 15
Chapter 15
Chapter 15
Chapter 15 Advanced Applications 723
15.1 IP Encapsulation 723
15.1.1 Tunneling through IP Networks 724
15.1.2 Generic Routing Encapsulation 725
15.1.3 IP in IP Encapsulation 726
15.1.4 Minimal IP Encapsulation 728
15.1.5 Using MPLS Tunnels 729
15.1.6 Choosing a Tunneling Mechanism 730
15.2 Virtual Private Networks (VPN) 730
15.2.1 What Is a VPN 731
15.2.2 Tunneling and Private Address Spaces 732
15.2.3 Solutions Using Routing Protocols 732
15.2.4 Security Solutions 735
15.2.5 MPLS VPNs 735
15.2.6 Optical VPNs 737
15.2.7 Choosing a VPN Technology 737

xviii Contents
15.3 Mobile IP 738
15.3.1 The Requirements of Mobile IP 739
15.3.2 Extending the Protocols 740
15.3.3 Reverse Tunneling 745
15.3.4 Security Concerns 745
15.4 Header Compression 746
15.4.1 Choosing to Compress Headers 746
15.4.2 IP Header Compression 747
15.4.3 MPLS and Header Compression 751
15.5 Voice Over IP 752
15.5.1 Voice Over MPLS 753
15.6 IP Telephony 753
15.6.1 The Protocols in Brief 754
15.7 IP and ATM 756
15.7.1 IP Over ATM 756
15.7.2 Multi-Protocol Over ATM 757
15.7.3 LAN Emulation 759
15.7.4 MPLS Over ATM 760
15.8 IP Over Dial-Up Links 760
15.8.1 Serial Line Internet Protocol 760
15.8.2 Point-to-Point Protocol 762
15.8.3 Choosing a Dial-Up Protocol 763
15.8.4 Proxy ARP 763
Concluding Remarks
Concluding Remarks
Concluding Remarks
Concluding Remarks 767
767
767
767
Index
Index
Index
Index 773
773
773
773

xix
Preface
The Internet is now such a well-known concept that it no longer needs intro-
duction. Yet only a relatively small proportion of people who make regular use
of email or the World Wide Web have a clear understanding of the computers
and telecommunications networks that bring them together across the World.
Even within this group that understands, for example, that a router is a special
computer that forwards data from one place to another, there is often only
a sketchy understanding of what makes the routers tick, how they decide where
to send data, and how the data is packaged to be passed from one computer to
another.
The Internet is a mesh of computer networks that spans the World.
Computers that connect to the Internet or form part of its infrastructure use
a common set of languages to communicate with each other. These are the Internet
protocols. These languages cover all aspects of communication, from how data
is presented on the link between two computers so that they can both have the
same understanding of the message, to rules that allow routers to exchange and
negotiate capabilities and responsibilities so that the network becomes a fully
connected organism. Internet protocols are used to establish conversations
between remote computers. These conversations, or logical connections, may
span thousands of miles and utilize many intervening routers. They may make
use of all sorts of physical connections, including satellite links, fiber optic
cables, or the familiar twisted-pair telephone wire. The conversations may be
manipulated through Internet protocols to allow data traffic to be placed within
the Internet to optimize the use of resources, to avoid network congestion, and
to help network operators guarantee quality of service to the users. In short, the
Internet without protocols would be a very expensive and largely useless collec-
tion of computers and wires.
The protocols used in the Internet are, therefore, of special interest to
everyone concerned with the function of the Internet. Software developers and
vendors making Web browsers, email systems, electronic commerce packages,
or even multi-user domain games must utilize the protocols to run smoothly
over the Internet and to ensure that their products communicate successfully
with those from other vendors. Equipment manufacturers need to implement the
protocols to provide function and value to their customers and to offer solutions
that interoperate with hardware bought from other suppliers. Network operators
and managers need to be especially aware of how the protocols function so that
they can tune their networks and keep them functioning, even through dramatic
changes in traffic demand and resource availability.

xx Preface
There are already a large number of books devoted to descriptions of the
protocols that run the Internet. Some describe a cluster of protocols with a view
to showing how a particular service (for example, Virtual Private Networks)
can be provided across and within the Internet. Others take a field of operation
(such as routing) and discuss the specific protocols relevant to that area. Still
more books give a highly detailed anatomy of an individual protocol, describing
all of its features and foibles.
The aim of this book is to give a broader picture, showing all of the common
Internet protocols and how they fit together. This lofty aim is, of course, not
easily achieved without some compromises. In the first instance, it is necessary
to include only those protocols that receive widespread and public use—there
are over one thousand protocols listed by the Internet Assigned Numbers
Authority (IANA) and clearly these could not all be covered in a single work.
Second, some details of each individual protocol must be left out in order to fit
everything between the covers. Despite these constraints, this book gives more
than an overview of the established protocols. It examines the purpose and
function of each and provides details of the messages used by the protocols,
including byte-by-byte descriptions and message flow examples.
The Internet is a rapidly evolving entity. As the amount of traffic increases
and advances in hardware technology are made, new demands are placed on the
inventors of Internet protocols—the Internet Engineering Task Force (IETF)—
leading to the development of new concepts and protocols. Some of these recent
inventions, such as Multiprotocol Label Switching (MPLS), are already seeing
significant deployment within the Internet. Others, such as Generalized MPLS
(GMPLS), are poised to establish themselves as fundamental protocols within
the Internet’s transport core. This book recognizes the importance of these new
technologies and gives them their appropriate share of attention.
Underlying the whole of this book is a comparative thread. Deployment of
Internet protocols is fraught with decisions: How should I construct my network?
Which protocol should I use? Which options within a protocol should I use?
How can I make my network perform better? How can I provide new services to
my customers? At each step this book aims to address these questions by giving
guidance on choices and offering comparative analysis.
It would not have been possible to write this book without reference to
many of the existing texts that provide detailed descriptions of individual proto-
cols. At the end of each chapter some suggestions for further reading are made
to point the reader to sources of additional information.
Audience
This book is intended to be useful to professionals and students with an interest
in one or more of the protocols used in the Internet. No knowledge of the Internet is

Preface xxi
assumed, but the reader will find it helpful to have a general understanding of the
concepts of communication protocols.
Readers will probably have varying degrees of familiarity with some of the
protocols described in this book. This book can be used to learn about unfamiliar
protocols, as a refresher for rusty areas, or as a reference for well-known protocols.
Software and hardware developers, together with system testers, will find
this book useful to broaden their understanding and to give them a solid
grounding in new protocols before they move into new areas or start new
projects. It will help them understand how protocols relate to each other and
how they differ while providing similar function.
Network operators are often required to adopt new technologies as new
equipment is installed, and must rapidly come up to speed on the new and
different protocols. New developments such as MPLS are making a strong
impression in the Internet, and technologies like GMPLS are bringing IP-based
control protocols into core transport networks. This book should appeal to the
many core network operators who suddenly discover that IP is invading their world.
A third category of readers consists of decision-makers and managers
tasked with designing and deploying networks. Such people can be expected
already to have a good understanding of the use and purpose of many protocols,
but they will find the comparison of similar protocols useful and will be able to
update their knowledge from the description of the new protocols.
Organization of This Book
Network protocols are often considered with respect to a layered model.
Applications form the top layer and talk application level protocols to each
other. In doing so, they utilize lower layer protocols to establish connections,
encapsulate data, and route the data through the network.
This book is organized by layer from the bottom up so that network layer
protocols precede transport protocols, and application protocols come last. Like
all good generalizations, the statement that protocols fit within layers is badly
flawed, and many protocols do not fit easily into that model. MPLS, for
example, has often been described as a “layer two-and-a-half” protocol. With
these difficult cases, the protocols are described in chapters ordered according
to where the functional responsibility fits within a data network.
Chapter 1 provides an overview of essentials designed to consolidate termi-
nology within the rest of the book and to bring readers who are unfamiliar with
communication protocols up to speed. It introduces the OSI seven-layer model,
describes some common data link protocols, and presents a picture of how the
Internet protocols described in this book all fit together.
Chapter 2, The Internet Protocol (IP), introduces the essential data transfer
protocol on which all other Internet protocols are built. It discusses addressing
and describes the most popular form of the Internet Protocol, IPv4. This chapter

xxii Preface
also includes information about the Internet Control Message Protocol (ICMP)
which is fundamental to the operation of IP networks.
Chapter 3 provides a short overview of multicast. Techniques for mass
distribution of IP messages are covered together with the Internet Group Man-
agement Protocol (IGMP). The topic of multicast routing is deferred to Chapter 5.
Chapter 4 outlines the next generation of the Internet Protocol, IPv6, and
looks at the problems it sets out to solve.
Chapter 5 introduces routing as a concept and describes some of the important
routing protocols in use within the Internet. This is the largest chapter in the
book and covers a crucial topic. It details the four most deployed unicast routing
protocols: the Routing Information Protocol (RIP), the Open Shortest Path First
protocol (OSPF), the Intermediate System to Intermediate System protocol (IS-IS),
and the Border Gateway Protocol (BGP). Chapter 5 also includes an introduction
to some of the concepts in multicast routing and gives an overview of some of
the multicast routing protocols.
Chapter 6 is devoted to IP service management and describes how services
and features are built on top of IP using Differentiated Services (DiffServ), Inte-
grated Services (IntServ), and the Resource Reservation Protocol (RSVP).
Chapter 7 addresses the important area of transport over IP. Transport
protocols are responsible for delivering end-to-end data across the Internet, and
they provide different grades of service to the applications that use them. This
chapter describes the User Datagram Protocol (UDP), the Transmission Control
Protocol (TCP), the Streams Control Transmission Protocol (SCTP), and the
Real-time Transport Protocol (RTP).
Chapter 8 is a digression into the field of traffic engineering. It describes
some of the important concepts in optimal placement of traffic within a network
and outlines the extensions to routing protocols to provide some of the informa-
tion that a traffic engineering application needs to do its job. This chapter also
sets out the extensions to the OSPF and IS-IS routing protocols in support of
traffic engineering.
Chapters 9 and 10 describe Multiprotocol Label Switching (MPLS) and
Generalized MPLS (GMPLS). These important new technologies utilize IP to
establish data paths through networks to carry traffic that may or may not itself
be IP. Chapter 9 explains the fundamentals of MPLS before giving details of
three MPLS signaling protocols: the Label Distribution Protocol (LDP), Constraint-
Based LSP Setup Using LDP (CR-LDP), and traffic engineering extensions to the
Resource Reservation Protocol (RSVP-TE). Chapter 10 explains how the MPLS
protocols have been extended to use an IP infrastructure to manage network
hardware that might switch optical data rather than IP packets. Chapter 10 also
includes a description of the Link Management Protocol (LMP).
Chapter 11 is devoted to managing switches and components. Although
switches and components are at the lowest level in the layered protocol model,
their management is an application-level issue and the protocols used utilize
IP and many of the other features already described. The General Switch

Preface xxiii
Management Protocol (GSMP) and extensions to LMP for managing optical
components (LMP-WDM) are described, and there is a brief introduction to the
work of the IETF’s Forwarding and Element Control Separation (ForCES) Work-
ing Group.
Chapter 12 brings us at last to application protocols. Applications are what
it is all about; there is no point in any of the other protocols without applications
that need to exchange data between different sites. This chapter describes a few
of the very many protocols that applications use to talk amongst themselves
across the Internet. The Domain Name System protocol (DNS), Telnet, the File
Transfer Protocol (FTP), the Trivial File Transfer Protocol (TFTP), and the
Hyper-Text Transfer Protocol (HTTP) are used as examples.
Chapter 13 develops the previous two chapters to discuss network manage-
ment. The control protocols used to gather information about the network and
to control the resources are increasingly important in today’s complex networks.
This chapter includes an overview of the Management Information Base (MIB)
that acts as a distributed database of information on all elements of a network.
There is also a description of three important techniques for distributing man-
agement information: the Simple Network Management Protocol (SNMP), the
Extensible Markup Language (XML), and the Common Object Request Broker
Architecture (CORBA). The chapter concludes with some comments on managing
policy within a network, and with a description of the Common Open Policy
Service protocol (COPS).
Chapter 14 looks at the important subject of IP Security and how messages
can be authenticated and protected when they are sent through the Internet.
Special attention is given to the ways in which security can be applied at the
network layer (IPsec), at the transport layer using the Transport Layer Security
protocol (TLS) and the Secure Sockets Layer (SSL), and at the application layer,
with security techniques for HTTP providing an example.
Chapter 15 briefly dips into some advanced applications such as IP Encap-
sulation, Virtual Private Networks, Mobile IP, and Voice over IP. Some of these
topics are new uses of IP that are requiring the development of new protocols
and extensions to existing protocols. Others are almost as old as IP itself and are
well-established techniques.
Finally, the closing remarks look toward future developments and attempt
to predict the next steps in the development and standardization of Internet
protocols.
Each chapter begins with a brief introduction that lists the topics that will
be covered and explains why the material is important. The chapters all end
with suggestions for further reading, pointing the reader to books and other
material that cover the topics in greater detail.
Throughout the book, comparisons are made between protocols, and
between implementation/deployment options, in the form of sections with titles
such as Choosing Between TCP and SCTP, or Choosing Between CR-LDP and
RSVP-TE.

xxiv Preface
Conventions Used in This Book
A byte is an eight-bit quantity, sometimes known as an octet. Bits are numbered
within a byte in the order that they would arrive in a transmission. The first bit
is numbered 0 (zero) and is the most significant bit.
Where integers are transmitted as part of a protocol, they are sent in ‘line
format’—that is, with the most significant bit first. This can most easily be seen
by converting the number into binary representation with the right number of
bits (that is, padding with zeros on the left) and numbering the bits from left to
right starting with zero. Thus, the number 26025 (which is 0x65A9 in hexadecimal)
is represented as a 16-bit binary number as 0110010110101001. Bit zero has
value zero and bit 15 has value one.
Diagrammatic representation of messages is achieved by showing bits run-
ning from left to right across the page with bit zero of byte zero in the top left
corner. Thirty-two bits (four bytes) are shown in a row. For example, Figure 0.1
shows the Protocol Data Unit (PDU) header used to prefix all messages in the
Label Distribution Protocol (LDP). The header is ten bytes long and comprises
four fields: the Version, the PDU Length, an LSR Identifier and a Label Space
Identifier. The Version field is 16 bits (two bytes) long and is transmitted (and
received!) first.
Sample networks are shown in figures using some of the symbols shown in
Figure 0.2. A distinction is made between IP routers and Multiprotocol Label
Switching (MPLS) Label Switching Routers (LSRs). Multi-access networks are
typically represented as Ethernets, and more general IP networks are shown as
“clouds.” Users’ computers and workstations (hosts) attached to the networks
are usually shown as personal computers with monitors. Larger computers that
may act as application servers are represented as tower systems.
Protocol exchanges are shown diagrammatically using vertical lines to
represent network nodes and horizontal lines to represent messages with the
message name written immediately above them. Time flows down the diagram;
in Figure 0.3, which illustrates the events and exchange of messages between
two RSVP-TE LSRs, the first events are Path messages that are passed from one
LSR to the next. Dotted vertical lines are used to illustrate the passing of time,
such as when waiting for a timer to expire or waiting for application instructions.
0 1 9
8
7
6
5
4
3
2 0 1 9
8
7
6
5
4
3
2 0 1 9
8
7
6
5
4
3
2 0 1
0 1 2 3
Version
LSR Identifier
PDU Length
Label Space Identifier
Figure 0.1 LDP PDU header.

Preface xxv
The Backus Naur Form (BNF) is sometimes used to describe message formats
when the messages are built from component parts. Each component is identified
by angle brackets <as here> and optional components are placed in square
brackets [<like this>]. The other symbol used is the pipe ‘|’, a vertical bar that
A host or workstation A host or application server
A label switching router (LSR)
An IP router
An Ethernet segment with four hosts and one router
connected to an IP network containing three routers
Figure 0.2 Some of the symbols used in the figures in this book.
LSR A LSR D
LSR C
LSR B
Path
Path
Path
Resv
1
2
PathTear
PathTear
3
5
6
Resv
Resv
PathTear
4
7
8
Figure 0.3 Normal LSP setup and teardown in an RSVP-TE network.

xxvi Preface
indicates an exclusive or, so that <either this component is present> | <or
this one>. Figure 0.4 shows the COPS Decision message that is built from two
mandatory components (the common header and the client handle), a choice
between the Decisions component and the Error component (exactly one of
which must be present), and an optional Integrity component.
About the IETF
The Internet Engineering Task Force (IETF) is the principle standards-making
body documenting standards for use in the Internet and in relation to the Internet
Protocol, IP. The body is a loose affiliation of individuals who supposedly
eschew their corporate affiliations and work together to produce the best technical
solutions in a timely manner. Membership doesn’t exist as such, and everyone
is free to participate in the discussions of new standards and problems with
existing ones.
Most of the work of the IETF is carried out within Working Groups, each
chartered to address a reasonably small set of problems. At the time of writing
there are 133 active Working Groups in operation. Each Working Group maintains
an email list that is used for discussions and holds a meeting once every four
months when the IETF meets up “in the flesh.”
Standards are developed through a process of drafting. Internet Drafts may
be the work of groups of individuals or of a Working Group, and are published
and republished until they are acceptable or until everyone loses interest and
they are dropped. Acceptable drafts are put to last call within the Working Group
and then again across the whole IETF to allow everyone to express any last-minute
objections. If all is well and the draft is approved by the Internet Engineering
Steering Group (IESG) it is published as a Request For Comment (RFC).
An RFC is not automatically a standard. It must go through a process of
implementation, deployment, and assessment before it is given that mark of
approval. There are over 3500 RFCs published to date, but only 62 of those
have been certified as standards. For the sake of clarity, RFCs and Standards
are referred to only through their RFC number within this book.
Most of the protocols described in this book are the subject of more than
one RFC. The further reading sections at the end of each chapter list the relevant
RFCs, which can be found through the IETF’s Web site.
Figure 0.4 The COPS protocol decision message represented in BNF.
<Decision Message>:: = <Common Header>
<Client Handle>
<Decisions> | <Error>
[<Integrity>]

Preface xxvii
Two other important groups contribute to the IETF’s success. The RFC
editor is responsible for formatting, checking, and publishing RFCs. The Internet
Assigned Numbers Authority (IANA) maintains a repository of all allocated
protocol numbers and values so that there is no risk of accidental double usage
of the same value.
The IETF maintains a Web site at http://www.ietf.org from where links
exist to each of the Working Groups, to IANA, to a list of all of the published
RFCs, and to a search engine to search the repository of Internet Drafts. The
IETF publishes a useful document, RFC 3160—The Tao of the IETF, that serves
as an introduction to the aims and philosophy of the IETF; it can be found at
http://www.ietf.org/rfc/rfc3160.txt.
A Note on Gender
Within this book it is occasionally necessary to refer to an individual (for
example, a network operator or the implementer of a software component)
using a third-person pronoun. The word ‘he’ is used without prejudice and is not
intended to imply that a disproportionate number of techno-nerds are male nor
that women are too clever to waste their time in such jobs.
Acknowledgments
I wrote this book while working for a startup company making high-tech optical
switches for the Internet during a severe downturn in the telecoms market, and
living in a country that was new and strange to me during a time of heightened
security and stress caused by terrorism and war. There was never a dull
moment, and very few that were dedicated to sleep.
Most of my writing time was squeezed into spare moments in evenings or
weekends that should have been spent being a house-husband or a tourist. My
thanks go, therefore, to my wife, Catherine, and dog, Bracken, for putting up with
my turned back as I sat typing, and for amusing each other without my input.
I am grateful, too, to my reviewers who took such pains to wade through
the manuscript, making helpful suggestions and factual changes. Loa Andersson,
Paul Turcotte, Judith M. Myerson, and Y. Reina Wang all contributed significantly
to the form of the book and to my comfort level as I wrote it. Thanks also to
Phillip Matthews for stepping in to provide prompt, substantial, and detailed
feedback on Chapter 5. My gratitude goes to the team at Morgan Kaufmann for
all their hard work: especially Kanyn Johnson and Marcy Barnes-Henrie.
Finally, special thanks to Philip Yim for providing encouragement at a difficult
time.

xxix
About the Author
Adrian Farrel has almost 20 years of experience designing and developing port-
able communications software ranging from various aspects of SNA and OSI
through ATM and into IP. At Data Connection Ltd., he was MPLS Architect and
Development Manager, leading a team that produced a carrier-class MPLS
implementation for customers in the router space, while their GMPLS imple-
mentation pioneered the protocols working closely with optical companies that
were developing the standards. As Director of Protocol Development for Movaz
Networks Inc., Adrian had the opportunity to build a cutting-edge system integrat-
ing many IP-based protocols to control and manage optical switches offering
wavelength services.
Adrian is very active within the IETF, where he is co-chair of the Common
Control and Management Protocols (CCAMP) Working Group that is responsible
for the GMPLS family of protocols. He has co-authored and contributed to
numerous Internet Drafts and RFCs on MPLS, GMPLS, and related technologies.
He was a founding board member of the MPLS Forum, frequently speaks at
conferences, and is the author of several white papers on GMPLS.
He lives in North Wales, from where he runs an Internet Protocols consultancy,
Old Dog Consulting, and lives the good life with his wife Catherine and dog
Bracken.

1
Chapter 1
Overview of Essentials
This first chapter provides an overview of some of the essentials for discussion
of the Internet and its protocols. It may safely be skipped by anyone with a
good background in computers and networking, or skimmed by those who
want to check that they have the right level of information to tackle the
remainder of the book. The chapter examines aspects of physical connectivity
before looking at the fundamentals of communications protocols. The Open
Systems Interconnection (OSI) seven-layer architectural model for communication
protocols is introduced and used to reference some of the protocols described
in this book. There follows a brief examination of how data is packaged and
exchanged, and a description of some of the common link level protocols that
are hardware dependent and provide essential support for the Internet. The
chapter concludes with an overview of network layer addressing, a chart
showing how the protocols discussed in this book fit together, and some sug-
gestions for further reading.
1.1 Physical Connectivity
What is the point of connecting two computers together? Why do we go to such
lengths to invent languages to allow computers to communicate? The answer is
simply to enable the distribution of data of all forms both between computers
and between users. It has been suggested (by Robert Metcalfe, former chief of
3Com) that the value of a network increases as the square of the number of
computers in the network. If that is true, linking two computers together doubles
their value, and linking one hundred computers in an office achieves a 10,000
percent increase in value.
But we should recall that linking computers together has only recently
become a simple concept. These days nearly every office computer ships with
an Ethernet card built in and most home or portable computers include
modems—it is relatively simple to achieve physical connectivity by plugging in
the right cable and performing simple configuration. Other local area network
(LAN) technologies do still have their footholds and many offices use Token
Ring or FDDI in place of Ethernet. Similarly, there are other wide area networking

2 Chapter 1 Overview of Essentials
technologies that may be used to connect computers at remote sites in place of
dial-up links—these include ISDN, SDLC, and X.25.
The immediate connection between a computer and its network is only the
first step in connecting a computer to a remote partner. There may be many
computers on the path from data source to destination and these computers may
be linked together using a variety of technologies, some of which are designed
specifically for bulk data transfer and for building links between computers that
run in the core of the network. Increasingly, such technologies utilize fiber
optics and rely on special encodings (ATM, SONET, SDH, etc.) to carry data. Of
course, as wireless networking grows in popularity there is no obvious physical
linkage between computers, but they are still linked and exchanging data on
point-to-point connections made across the airwaves.
So the physical links traversed by data exchanged between computers may
vary widely. Each link between a pair of directly connected computers is of just
one type, but there may be multiple parallel links of different (or the same)
types between computers. The physical connection is responsible for delivering
bits and bytes from one end of the link to another and for reassembling them
in the same order as they were presented for dispatch. There are no further
rules that can be applied universally, and each medium has a different way of ensur-
ing that the signal can be clearly and unambiguously converted from and to
data (for example, consider how data bits are converted to line voltages using
NRZI, or how photons are used to represent electrical signals).
In order to manage the way that data is run across these links, computers
employ data-link level protocols. These are specific communication languages
designed to address the requirements of individual physical networking con-
straints, and are largely concerned with packaging of data so that it can be
recognized and delivered to the correct user at the other end of the link. For
the data to be delivered to the correct user, it is necessary to have some form
of addressing that identifies computers and users within the network.
1.2 Protocols and Addressing
Computer protocols can be said to serve four purposes. Chiefly, they exist to
encode and transfer data from one point to another. To enable this primary
function they may need to control how the data is distributed by designating
paths that the data must follow, and in order to achieve this they may need to
exchange network state information. Finally, the protocols may be needed
to manage network resources (computers and links) in order to control their
behavior.
Data transfer protocols may be the most important from the user’s
perspective since all they want to do is send their data, but these protocols are
relatively simple and also form only a small percentage of the protocols actually
needed to build and maintain the infrastructure of a network. The information

1.2 Protocols and Addressing 3
distribution, control, and management protocols that serve the other three
purposes described in the preceding paragraph are often far more complex and
sophisticated.
For any of these protocols to operate, they must have a way to identify the
source and destination of messages. Just as the sender of a letter must write
the recipient’s address on the front of the envelope, the sender of a protocol
message must provide the address for the remote computer that is the desired destin-
ation. Similarly, if a letter writer wishes to receive a response, he is well advised
to supply his name and return address, and so should the sender of a protocol
message. It should be clear that computers need names and addresses to identify
themselves.
At a physical level, computers and devices are usually identified by unique
numbers burned in to ROM. These numbers often identify the equipment man-
ufacturer, and the product type and version, and have a component that is
unique to each individual item to come off the production line. An increasingly
common format for identifiers is the Media Access Control (MAC) address
shown in Figure 1.1, and these are used by several data-link layer protocols to
directly identify the computer or interface card that is communicating. Other
data-link layer protocols, however, have different addressing schemes ranging
from simple 16-bit integers to complex 40-byte structures, and these rely on
careful configuration policies to ensure that everyone has a unique address (just
as two people with the same name living on the same street will lead to fun and
games in an Italian farce, so two computers with the same address in the same
network will result in chaos and confusion).
A protocol message, then, has three components: addressing information to
define the source and destination of the data, control information to regulate
the flow and manner of distribution of the data, and payload data. The payload
data is, from the user’s perspective, the important part—the information being
transferred—although, if the message is being exchanged between control
40-bitManufacturer'sExtensionID
0 × FFFE
24-bit Manufacturer's Extension
ID
24-bit Manufacturer's Extension
ID
24-bit Company ID
24-bit Company ID
24-bit Company ID
Figure 1.1 MAC addresses may be encoded as 60 bits or 48 bits. The 48-bit variety can be mapped
into a 64-bit address by inserting 0× FFFE.

programs on the two communicating computers, the payload data may be
control state information such as instructions to regulate the flow of user
data, to exchange addresses, or to establish connections over which to exchange
user data.
Protocol messages are usually constructed as a header followed by data.
The header contains the addressing and control information and is, itself, some-
times broken into two parts: a standard header that has a well-known format for
all messages within a protocol, and header extensions that vary per message.
Sometimes messages will also include a trailer that comes after the payload
data. Usually the standard message header includes a length field that tells the
protocol how many bytes of message are present. This structure is represented
in Figure 1.2.
1.3 The OSI Seven-Layer Model
It is apparently impossible to write a book about networking protocols without
reference to the seven-layer architectural model devised by the International
Standards Organization (ISO) and used to classify and structure their protocol
suite, the Open Systems Interconnection (OSI) protocols. The seven-layer model
includes many useful concepts, although it is not as applicable to the entire
Internet Protocol suite as it might once have been, with many protocols sitting
uncomfortably on the architectural fence. Figure 1.3 shows the seven layers and
how they are used in communication between two computers across a network
of devices. The lowest layer, the physical layer, provides connectivity between
devices. The next layer up, the data-link layer, is responsible for presenting
data to the physical layer and for managing data exchanges across the physical
media. Data-link exchanges are point-to-point between computers that terminate
physical links, although the concept of bridging (see Chapter 2) offers limited
forwarding capabilities within the data-link layer. The network layer is responsible
for achieving end-to-end delivery of data (that is, from source to destination),
but achieves it in a hop-by-hop manner (that is, by passing it like a hot potato
from one node to the next). Examples of network layer protocols include X.25,
CLNP, and IP (the Internet Protocol). An important fact about the network layer
Payload Data Trailer
ProtocolMessage
Message
Header
Message-
Specific
Header
Message Header
Figure 1.2 A protocol message may be comprised of a header, payload data, and a trailer.

1.3 The OSI Seven-Layer Model 5
is that it aims to be independent of the underlying data-link technology—this
has been achieved with varying degrees of success, but the designers of IP are
proud of the fact that it can be run over any data-link type from the most
sophisticated free-space optics to the less-than-reliable tin cans and string.
Above the network layer comes the transport layer. Transport protocols,
described in Chapter 7, manage data in a strictly end-to-end manner and are
responsible for providing predictable levels of data-delivery across the network.
Examples from the IP world include TCP, UDP, and SCTP. Next up the stack
comes the session layer, which manages associations (or sessions) between
applications on remote computers using the transport layer to deliver data from
site to site. The presentation layer contains features such as national language
support, character buffering, and display features. It is responsible for converting
data into the right format to be transmitted across the network and for receiving
the data and making it available to applications which make up the top layer
of the model, the application layer.
As shown in Figure 1.3, protocol message exchanges are between entities at
the same level within the protocol stack. That is, application layer protocols are
used to communicate between applications on different computers, and they
send their messages as if they were colocated (along the dotted lines in Figure 1.3).
In fact, however, they achieve this communication by passing the messages to
the next lower layer in the stack. So with each layer in the stack, the protocol
code communicates directly with its peer, but does so by passing the message
down to the next layer, and it is only when the data reaches the physical layer
that it is actually encoded and put on the “wire” to reach the next node.
As described earlier in this section, physical communications are hop-by-hop
and are terminated at each node, but at each node the protocols are terminated
only if they are relevant to the type of node and the layer in the protocol stack.
Presentation
Physical
Data Link
Network
Transport
Session
Physical Physical
Data Link
Physical
Data Link
Network
Physical
Data Link
Network
Application
Presentation
Data Link
Network
Transport
Session
Repeater Router
Switch or Bridge
Host Router Host
Application
Physical
Figure 1.3 Connectivity within the seven-layer model allows neighboring entities at the same level of the stack to
consider themselves adjacent regardless of the number of intervening hops between lower layer entities.
End-to-end connectivity is, in fact, achieved by passing the data down the stack.

So, as shown by the gray line in Figure 1.3, at some nodes the data may rise as
far as the network layer while at others it only reaches the data-link layer.
The IP protocols do not sit particularly well in the seven-layer model,
although the concepts illustrated in the diagram are very useful. The lower layers
(one through four) are well matched, with IP itself fitting squarely in the net-
work layer and the transport protocols situated in the transport layer. Many of
the protocols that support applications (such as HTTP, the Hypertext Transfer
Protocol) encompass the session and presentation layers and also stray into the
application layer to provide services for the applications they support.
Matters get more fuzzy when we consider the routing protocols. Some of
these operate directly over data-link layer protocols, some use IP, and others
utilize transport protocols. Functionally, many routing protocols maintain
sessions between adjacent or remote computers, making matters still more
confusing. Operationally, however, the routing protocols are network layer
commodities.
The world is really turned on its head by the Multiprotocol Label Switching
(MPLS) protocols described in Chapter 9. These are often referred to as “layer
two-and-a-half protocols” because they exist to transport network protocol data
over the data-link layer connections, and MPLS relays data in a hop-by-hop way
and delivers it end-to-end. However, the MPLS protocols themselves are
responsible for installing the forwarding rules within the network, and they
operate more at the level of routing protocols running over IP or making use of
the transport protocols and establishing sessions between neighbors.
Figure 1.4 shows some of the IP protocols in the context of the OSI seven
layers. Note that there is no implied relationship between the protocols in the
figure—they are simply placed in the diagram according to their position in
Application
Presentation
Physical
Data Link
Network
Transport
Session
IP
OSPF
IS-IS
X.25
ARP
Ethernet
Frame
Relay
ATM PPP
V.34
UDP TCP SCTP
RIP
Sockets
HTML
ASN.1
HTTP Telnet
FTP SNMP
Token
Ring
Figure 1.4 Some of the Internet protocols as they fit within the OSI seven-layer model.

1.4 An Architecture for the Network 7
the seven-layer model. Refer to Figure 1.17 for a more comprehensive view of how
the protocols described in this book fit together.
Some people, it should be pointed out, don’t see much point in the seven-
layer model. In some cases a five-layer IP model is used that merges the top
three OSI layers into a single application layer, but others choose to discard the
model entirely after introducing it as a concept to explain that features and
functions are provided by protocols in a layered manner. This book takes a middle
road and only uses the architectural model loosely to draw distinctions between
the data-link protocols that are responsible for transporting IP data, the IP protocol
itself as a network protocol, and the transport protocols that provide distinctive
services to application programs.
1.4 An Architecture for the Network
It is sometimes convenient to consider network computers as split into distinct
components, each with a different responsibility. One component might handle
management of the router, another could have responsibility for forwarding
data, and yet another might be given the task of dealing with control protocol
interactions with other computers in the network.
When a network is viewed as a collection of computers partitioned in this
way, it can be seen that messages and information move around the network
between components with the same responsibility. For example, one computer
might process some data using its dedicated data-processing component. The
first computer sends the data on to another computer where it is also processed
by the dedicated data-processing component, and so on across the network.
This view builds up to the concept of processing planes in which networked
computers communicate for different purposes. Communications between com-
puters do not cross from one plane to another, so that, for example, the
management component on one computer does not talk to the control protocol
component on another computer. However, within a single computer there is
free communication between the planes.
Figure 1.5 displays how this model works. Four planes are generally
described. The Data Plane is responsible for the data traffic that passes across
the network. The Management Plane handles all management interactions
such as configuration requests, statistics gathering, and so forth. The Control
Plane is where the signaling and control protocols operate to dynamically
interact between network computers. The Routing Plane is usually considered
as distinct from the Control Plane simply because the routing protocols that
dynamically distribute connectivity and reachability information within the
network are usually implemented as separate components within network
computers.
Some people like to add a fifth plane, the Application Plane. However,
application transactions tend to be end-to-end and do not require any interaction

from other computers in the network, so there is not much benefit in defining
a separate plane in the model.
Of course, the key interaction at each computer is that every other plane
uses the Data Plane to transfer data between computers. Other interactions
might include the Routing Plane telling the Data Plane in which direction to
send data toward its destination, the Data Plane reporting to the Management
Plane how much data is being transmitted, and the Management Plane instructing
the Control Plane to provision some resources across the network.
In Figure 1.5, the vertical lines represent each network computer’s presence
in all of the planes. The dotted lines within each plane indicate the communi-
cation paths between the computers. In the Data Plane, the communication
paths map to the physical connections of the network, but in the other planes
the communications use logical connections and the underlying Data Plane to
form arbitrary associations between the computers. The connectivity can be dif-
ferent in each plane.
The Transport Plane is sometimes shown as separate from the Data Plane.
This allows a distinction between the physical transport network which may
include fiber rings, repeaters, and so forth, and the components such as the
Internet Protocol and data-link layer software that manage the data transfer
between computers.
Data Plane
Routing Plane
Control Plane
Management
Plane
Network computers have a presence in each of the planes
Figure 1.5 The network may be viewed as a set of planes passing through all of the computers within the network.

1.5 Packaging Data 9
1.5 Packaging Data
In a full protocol stack the effect of all the protocols is quite significant. An
application generates a stream of data to be sent to a remote application (for
example, the contents of a file being sent across FTP) and hands it to the
presentation layer for buffering, translation, and encoding into a common
format. This “network-ready” stream of data is passed to the session layer for
transmission. There is then a pause while the session layer sets up an end-to-end
connection.
The session layer passes its connection requests and the application’s data
(usually prepended by a session protocol message header) to the transport layer
as buffers or byte streams. The transport layer chops this data up into manageable
pieces for transmission and prepends a header to give coordinates to the remote
transport component, and then passes the data to the network layer. The network
layer chops up the data again according to the capabilities of the underlying
data link, making it ready for transmission, and adds its own header to give
Translated Data
Application Data
Data
Transport
Header
Data
Network
Header
Session
Header
Data Link
Header
Data Link
Header
Data Link
Header
Transport
Header
Network
Header
Transport
Header
Data
Network
Header
Session
Header
Transport
Header
Network
Header
Network
Header
Data
Network
Header
Data
Data
Transport
Header
Data
Session
Header
Transport
Header
Data
Data
Session
Header
Application
Header
Application
Header
Figure 1.6 The imposition of message headers at each layer in the protocol stack can create a large protocol overhead
relative to the amount of application data actually transferred.

hop-by-hop and end-to-end information before passing it to the data-link layer.
The data-link layer prepends its own header and may also chop the data up further,
if necessary. The data-link layer presents the data to the physical layer, which
encodes it for transmission as a bit stream according to the physical medium.
The effect of this is that a considerable amount of protocol overhead may
be needed to transmit some data end to end, as shown in Figure 1.6.
At the data-link layer, protocol and data messages are known as frames.
At the network and transport layers they are called packets. At higher layers
they are known simply as messages. The term Protocol Data Unit (PDU) can be
applied at any level of the protocol stack, is synonymous with message, and
may carry control information and/or data. One last term, Maximum Transmis-
sion Unit (MTU), is also applicable: it is usually applied only at the network and
data-link layers, and refers to the largest packet or frame that can be supported
by a link, network, or path through a network. An MTU at the network layer,
therefore, describes the largest network layer packet that can be encapsulated
into a data-link layer frame. The MTU at the data-link layer describes the largest
frame that can be supported by the physical layer.
1.6 Data-Link Protocols
This book is about Internet protocols, and these can loosely be defined as those
protocols that utilize IP, make IP possible, or are IP. This means that the oper-
ational details of the data-link layer protocols are beyond the scope of the book.
However, the following short sections give an overview of some of the import-
ant data-link technologies and provide useful background to understanding
some of the reasons behind the nature of IP and its related protocols. It is
important to understand how IP is encapsulated as a payload of data-link protocols
and also how data-link technologies are used to construct networks of differing
topologies. This can help when decoding packet traces and can explain why
IP packets are a particular size, why the Internet protocols have their specific
behaviors, and how IP networks are constructed from a collection of networks
built from different data-link technologies.
There is a very large number of data-link layer protocols. The five (Ethernet,
Token Ring, Asynchronous Transfer Mode, Packet over SONET, and dial-up
networking) introduced in the following sections constitute some of the most
common for specific uses, but this does not invalidate other protocols such as
Frame Relay, FDDI, X.25 and so on.
1.6.1 Ethernet
Ethernet is the most popular office or home networking system. The specifications
include the physical and data-link layer, with the IEEE’s 802.3 standard being
the most common and most familiar. Normal data speeds are either 10 or 100

1.6 Data-Link Protocols 11
megabits per second and are run over copper wires; more recent developments
have led to gigabit and 10-gigabit Ethernet run over fiber.
Ethernet is a point-to-point or multi-access technology. A pair of nodes may
be connected by a single cable, or multiple nodes may participate in a network.
In the latter case, the network is typically drawn as on the left-hand side of
Figure 1.7, with each of the nodes attached to a common cable. In practice,
however, connectivity is provided through hubs, which allow multiple nodes to
connect in. A hub is not much more than a cable splitter: each junction in the
network on the left of Figure 1.7 could be a hub, but a more likely configuration
is shown on the right side of the figure.
Ethernet messages, as shown in Figure 1.8, carry source and destination
addresses. These are 6-byte (48-bit) MAC addresses that uniquely identify the
sender and intended recipient. When a node wants to send an Ethernet message
it simply formats it as shown and starts to send. This can cause a problem
(called a collision) if more than one node sends at once. Collisions result in lost
frames because the signal from the two sending nodes gets garbled. This is
often given as a reason not to use Ethernet, but a node that wants to send can
perform a simple test to see if anyone else is currently sending to considerably
reduce the chance of a collision. This can be combined with a random delay if
someone is sending so that the node comes back and tries again when there is
silence on the wire. The risk of collisions can be further reduced by the use of
Ethernet switches that replace hubs in Figure 1.7 and are configured to terminate
one network and only forward frames into another network if the destination is
not in the source network.
As can be seen in Figure 1.8, an Ethernet frame begins with seven bytes of
preamble and a start delimiter byte containing the value 0 ×AB. These fields
together allow the receiver to synchronize and know that a data frame is coming.
The first proper fields of the frame are the destination and source addresses.
In the 802.3 standard, the next field gives the length of the payload in bytes.
The minimum frame length (not counting preamble and start delimiter) is
64 bytes, so the minimum payload length is 46 bytes. If fewer bytes need to be
sent, the data is padded up to the full 46 bytes. The maximum payload length is
Figure 1.7 An Ethernet network showing logical connectivity and usual notations on the left, and actual physical connectivity
using two hubs on the right.

1,500 bytes. The 802.3 standard specifies that the payload data is encoded
according to the 802.2 standard, so that the receiving node can determine the
application to which the data should be delivered (see Section 1.6.6).
Ethernet differs from 802.3 in that 802.2 is not used to wrap the data.
Instead, the length field is reinterpreted as a payload type indicator. Values
greater than 1,500 (that is values that could not be misinterpreted as lengths) are
used to indicate the type of the payload (for example, IP) so that the receiver
can deliver the data to the right application. In this format, the payload length is
still constrained to be between 46 and 1,500 bytes. The last 4 bytes of the message
carry a cyclic redundancy check (CRC). The CRC is a simple checksum computed
on the whole frame to protect against accidental corruption.
It is worth noting that the simplicity, stability, and relative cheapness of
Ethernet lead not only to its popularity as a networking protocol but also to its
use as a communications infrastructure in compound devices, allowing line
cards and central processors to communicate across a bus or backplane.
1.6.2 Token Ring
Another popular local area networking protocol is Token Ring, for many
years the principal local area networking technology promoted by IBM and
documented by the IEEE as the 802.5 standard. As its name suggests, the
0 1 9
8
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2 0 1 9
8
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2 0 1 9
8
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2 0 1
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Preamble
Source Address (continued)
Payload Data
Start Delimiter
(0×AB)
Preamble (continued)
Source Address
Destination Address
Destination Address (continued)
Length/Type
Pad
CRC
Figure 1.8 An Ethernet frame.

computers attached to a Token Ring are arranged in a ring, as shown on the
left of Figure 1.9. A token passes around the ring from node to node, and when
a node wishes to transmit it must wait until it has the token. This prevents the
data collisions seen in Ethernet, but increases the amount of time that a node
must wait before it can send data.
As with Ethernet, Token Ring is a multi-access network, meaning that any
node on the ring can send to any other node on the ring without assistance
from a third party. It also means that each node sees data for which it is not the
intended recipient. In Ethernet, each node discards any frames that it receives
for which it is not the destination, but in Token Ring the node must pass the
frame further around the ring, and it is the responsibility of the source node to
intercept frames that it sent to stop them from looping around the ring forever.
Of course, a major disadvantage of a ring is that it is easily broken by the
failure of one node. To manage this, Token Rings are actually cabled as shown
on the right-hand side of Figure 1.9. Each computer is on a twin cable spur from
a Multiple Access Unit (MAU), making the network look like a hub-and-spoke
configuration. The MAU is responsible for taking a frame and sending it to a
node; the node examines the frame and passes it on along the ring by sending it
back to the MAU on its second cable; the MAU then sends the frame to the next
node on the ring. MAUs contain relays, and can detect when any node on the
ring is down and can “heal” the break in the ring. MAUs may also be chained
together (as shown in Figure 1.9) to increase the size of the ring.
The twin cables and the sophistication of MAUs make Token Rings notably
more expensive to deploy than Ethernet.
Token Ring frames are not substantially different from Ethernet frames
because they have to do the same things: identify source and destination, carry
data, and check for corruption. There are three fields that comprise the token
(shown in gray in Figure 1.10) when there is no data flowing; the token still
circulates on the ring as a simple 3-byte frame (start delimiter, access control,
Figure 1.9 A Token Ring network showing logical connectivity and usual notation on the left, and actual
physical connectivity using two MAUs on the right.

and end delimiter) so that any node that wishes to transmit can receive the token
and start to send.
1.6.3 Asynchronous Transfer Mode
The Asynchronous Transfer Mode (ATM) is an end-to-end data transfer proto-
col. It is connection-oriented, meaning that data between two end points flows
down the same path through transit nodes in a regulated way. The connections
may be switched virtual circuits (SVCs), which are established using a control
protocol such as Private Network to Node Interface (PNNI) or Multiprotocol
Label Switching (MPLS, see Chapter 9). Alternatively, the connections may be
preestablished through management or configuration actions, in which case
they are known as permanent virtual circuits (PVCs).
The links in an ATM network are point-to-point, with each ATM switch
responsible for terminating a link and either switching the ATM frames (called
cells) on to the next link or delivering the data to the local application. ATM
nodes are often shown connected together in a ring topology. This has nothing
to do with the data-link or physical layer technologies but much to do with the
economics and the applications that can be built. A full mesh of point-to-point
links connecting each pair of nodes in a network would be very expensive since
it requires a lot of fiber, as shown in the network on the left of Figure 1.11. Full
internode connectivity can be achieved through a much more simple network
since ATM can switch cells along different paths to reach the right destination.
However, as shown in the network in the center of Figure 1.11, a simply
0 1 9
8
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6
5
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2 0 1 9
8
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2 0 1 9
8
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2 0 1
0 1 2 3
Payload Data
Start Delimiter
Source Address
Dest Address
(continued)
Access Control
CRC
Frame Control
Destination
Address
End Delimiter Frame Status
Figure 1.10 A Token Ring frame.

connected network is vulnerable to a single point of failure. The network on the
right-hand side of Figure 1.11 demonstrates how a ring topology provides an
alternative route between all nodes, making it possible to survive single failures
without requiring substantial additional fiber.
ATM cells are all always exactly 53 bytes long. The standard data-bearing
cell, as shown in Figure 1.12, has 5 bytes of header information, leaving
48 bytes to carry data. This is a relatively high protocol overhead (15 percent) and
is known by ATM’s detractors as the cell tax. The header information indicates
whether flow control is used on the connection (Generic Flow Control field),
the destination address of the connection (Virtual Path Indicator and Virtual
Channel Indicator), how the cell should be handled in case of congestion (the
Cell Loss Priority field), and the Header Error Control field.
The last remaining field (the Payload Type field) indicates how the data is
wrapped. For packet data, the payload type normally indicates ATM Adaptation
Layer 5 (AAL5), meaning that no further wrapping of data is performed. Note
that since the cells are always 53 bytes long, the data portion may need to be
Preferred
Route
Alternate
Route
Figure 1.11 Full mesh topologies require a large amount of fiber, but simply-connected networks are
vulnerable to single failures. ATM networks are often fibered as rings, providing cheap
resilience.
0 1 9
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5
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3
2 0 1 9
8
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6
5
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3
2 0 1 9
8
7
6
5
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2 0 1
0 1 2 3
Payload Data and Padding
VCI
HEC
GFC VPI PT C
Figure 1.12 An ATM cell as used to carry simple data.

padded and it is the responsibility of the network level protocol (for example, IP) to
supply enough information in length fields to determine where the data ends
and where the padding starts.
1.6.4 Packet over SONET
Synchronous Optical Network and Synchronous Digital Hierarchy (SONET and
SDH) are two closely related specifications for carrying data over fiber-optic
links. Originally intended as ways of carrying multiple simultaneous voice con-
nections (telephone calls) over the same fiber, SONET and SDH use a technique
known as time division multiplexing (TDM) to divide the bandwidth of the fiber
between the data streams so that they all get a fair share and so that they all
deliver data at a steady rate (voice traffic is particularly sensitive to data that
arrives in fits and starts).
SONET links (henceforth we will say SONET to cover both SONET and
SDH) are very common because of their use for voice traffic, so it should be no
surprise to discover that much of the Internet is built on SONET links. Data may
be efficiently transported over SONET links using a technique called Packet
over SONET (PoS) which is flexible and offers high bandwidth while still allow-
ing voice data to be carried on the same fibers at the same time. PoS has been
one of the factors enabling the rapid growth of the Internet because it makes
use of existing infrastructure, allows high bandwidth, and offers relatively long
(hundreds of kilometers) links. TDM makes it possible for several smaller data
flows to be combined on a single fiber, allowing several data streams to share a
single physical link. SONET traffic may be further combined using different
wavelengths on a single fiber through wave division multiplexing (WDM) to
increase the amount of traffic carried.
Figure 1.13 shows how a PoS network may be constructed with low-band-
width links at the edges (OC3 is 155.5 Mbps which gives an effective data rate
of 149.76 Mbps), medium bandwidth links approaching the core (OC48 is 2,488
Mbps), and a core trunk link (OC192 is 9,953 Mbps). Connections to desktop
computers (that is, hosts) very rarely use PoS. Instead, they are connected to
dedicated routers using local area network technologies such as Ethernet
or Token Ring. The routers are responsible for directing traffic between
areas of the network and for aggregating the low-bandwidth traffic onto high-
bandwidth links.
The IETF has specified a way to carry data packets over SONET in RFC
2615. This technique uses a general data framing technique for point-to-point
links called the Point-to-Point Protocol (PPP), which is itself described in RFC
1661. The PPP frame, shown in Figure 1.14, is a pretty simple encapsulation of
the data, using a 2-byte field to identify the payload protocol so that the
packet can be delivered to the right application. Before a PPP frame can be sent
over a SONET link it is also encapsulated within a pair of start/end frame bytes

as described in RFC 1662 (shown in gray in Figure 1.14). This makes it easy
for the receiver to spot when a frame starts and to distinguish data from an
idle line.
The frame is now ready to be sent on the SONET link. Note that the overhead
of control information to data for PoS is very low (about 3 percent for large
packets) compared with the other chief technique for sending data over fiber-optic
links (Asynchronous Transfer Mode [ATM]), where the overhead is as much as
15 percent of the data transmitted.
OC3
OC3
OC3
OC3
OC48
OC48
OC48
OC48
OC48
OC192 OC192
OC192
Figure 1.13 A PoS network.
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2 0 1
0 1 2 3
Payload Data
CRC
Flag Address Control Protocol
Protocol
(continued)
Flag
Figure 1.14 A PPP frame encapsulated between start and end frame bytes.

1.6.5 Dial-Up Networking
Dial-up networking is a familiar way of life for many people who use home
computers to access the Internet. A dial-up connection is, of course, point-to-
point with the user’s computer making a direct connection to a dedicated com-
puter at their Internet Service Provider (ISP). These connections run over
normal phone lines and, just as in Packet over SONET, use the Point-to-Point
Protocol with start and end flags to encapsulate the frames. Dial-up networking
should be considered to cover communications over any link that is activated
for the duration of a transaction and then is dropped again. This includes phone
lines, ISDN, cable modems, and so on.
Dial-up networking poses particular challenges in IP and is discussed at
greater length in Chapter 15.
1.6.6 802.2 and Logical Link Control
Within networking protocols such as Ethernet and Token Ring, it is often useful
to employ a simple data wrapper to help determine the type and purpose of the
data. The IEEE defines the 802.2 standard, which inserts three bytes (shown in gray
in Figure 1.15) before the payload data. The Destination Service Access Point
(DSAP) and Source Service Access Point (SSAP) are used to identify the service
(the data application or network layer) to which the data should be delivered.
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Preamble
Payload Data
Start Delimiter
(0×AB)
Source Address
Destination Address
Length
Pad
CRC
DSAP SSAP
Control
Figure 1.15 An 802.3 frame showing 802.2 data encapsulation.

1.7 The Protocols at a Glance 19
Another important encapsulation is the Subnetwork Access Protocol
(SNAP) header. This may be used on its own or in association with an 802.2
header, as shown in Figure 1.16. When a SNAP header is used with 802.2, the
DSAP and SSAP are set to 0 ×AA and the control byte is set to 0×03 to indicate
that the SNAP header follows.
The SNAP header fields are used to identify the payload protocol. The
Organizationally Unique Identifier (OUI) provides a way to manage different
sets of payload types. When the OUI is set to zero, the Type field shows the
payload type using a set of values known as the EtherTypes. The EtherTypes are
maintained by the IEEE and mirrored by IANA, and the EtherType value for
IPv4 is 0 ×800.
The encapsulation syntaxes and any associated semantics are known as
Logical Link Control (LLC) protocols.
1.7 The Protocols at a Glance
Figure 1.17 shows how the main protocols described in this book are related.
Where one protocol is shown vertically above another, it means that the higher
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Payload Data
Start Delimiter
(0×AB)
Source Address
Destination Address
Length
Pad
CRC
DSAP (0×AA) SSAP (0×AA)
Control (0×03) OUI
Type
Figure 1.16 An 802.3 frame showing 802.2 data encapsulation with a SNAP header.

Exploring the Variety of Random
Documents with Different Content

are, I say, and stir not if you value your comfort. Would you warn
your comrade then? That will silence you."
The native who had charge of the prisoner lifted his paddle and hit
at the prostrate figure lying beneath him in the darkness. But,
fortunately for the latter, the blow failed to reach the mark, and,
striking the bottom of the boat within an inch of his head, almost
dashed a hole through the wood-work. But it had the desired effect,
for Tyler at once realized his position and held his tongue, while the
recollection of what had happened came to him like a flash in spite
of his dizzy condition.
"I remember now," he said to himself with something approaching a
groan. "A hook caught me under the arm, and before I could make
out what was up, I was splashing in the water and was being
dragged into another boat. Then some fellow caught me a crack
over the head, and—by Jove! how sore I am! They must have pulled
me about and kicked me pretty savagely, for I feel as though every
bone in my body were broken. And I'm a prisoner."
The thought set him wondering what would be his fate, though that
was a question which required little answering. Then he began to
think of Li Sung, and with a feeling of gladness he realized that the
Chinaman had made good his escape.
"Then, after all, there may be some chance for me," he said to
himself. "Captain Keppel and the Rajah of Sarawak were to put in an
appearance at the mouth of the river on this date, and were to await
our report there. If Li has got clear away he will, no doubt, lie up in
some quiet spot and think the matter out. Then he will see that he
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and will row for all he is worth. The tide will help him, and by the
time the night comes again he will be at the rendezvous. Then the
anchors will be raised, and the schooners will sail up with the flood,
and may, perhaps, be here by the following evening. By then I shall
be—"

Once more he broke off suddenly, realizing with a feeling akin to
despair that he would be dead, for was it likely that the pirates
would keep him a captive for long?
"Hardly," whispered Tyler to himself. "They will be angry, and men
who are in that condition do not pause and allow time to keep them
from their revenge. To-day, within a few hours, they will commence
to torture me, and then all will be over, and Li will have had his
journey for nothing, while I shall be another of the victims to be
added to the long list already set down to the brutality of these
pirates."
The conviction was not a very cheerful one, and for long it occupied
our hero's thoughts to the exclusion of all others. And all the while,
as he lay there prostrate on the floor of the boat, with throbbing
head and limbs which ached in every part, the native who stood
guard over him still rested a naked foot heavily on his chest, while
ever and anon he turned his eyes from the dark surface of the river
to the spot where his prisoner lay.
"One of the dogs at least is in our power," he kept saying to himself,
"and when the day comes, and the sun sails up to the sky, the
second will come beneath our eyes. Then we shall return to Rembas
in great favour, and our comrades will speak well of us. And
afterwards there shall be a feast, when our prisoners shall afford us
some amusement. And supposing the other dog escapes?"
The question set him wondering what would be their reception at
Rembas, and he was bound to confess to himself that the chief who
had sent them out to make the captures would have something to
say.
"He will scowl at us, and call us cowards and dogs," said the native.
"But he will do us no evil, for have we not been partially successful?
For that reason he will talk, and then he will forgive. But we shall
have to incur the laughter of our comrades."
Once more the man lapsed into silence, while the boat sped on its
course. But nowhere was there a sign of Li Sung, though they

searched every foot of that portion of the river, and shouted a
warning to their comrades who had taken up their posts above and
below, in the hope of capturing the fugitives as they passed. Then,
slowly at first, and later with the same swiftness with which it was
wont to leave the earth, the day came full upon them, and they
could see for miles along the surface.
"Not a boat in sight, comrades," said the leader, the native who had
charge of Tyler. "One of the men we have with us, but where is the
other? Has anyone seen or heard aught of him?"
He looked round at his crew with questioning eyes, and, hearing no
response, spoke to them again.
"Then what are we to do?" he demanded fiercely. "You have all
heard what our chief has promised to do to us if we who were so
many returned unsuccessful. Well, we are that. We have but one
captive, while the second is at large. Shall we return at once, or will
it meet your views if we kill this dog who lies beneath my foot, and
then sail for the lower reaches? For myself, I fear to enter Rembas
again, for the thought that our companions will jeer at me is worse
than the fear of death."
"And with us also," cried his companions. "We have fought hard, and
are weary. But rather than go now to our homes to rest, we will row
on if to return is to mean trouble with those who remained behind."
"It was a bad day for all of us when we were chosen for the duty,"
burst in another when his comrades had done and there was silence
in the boat; "but I fail to see why we should fear to return to
Rembas. Tell me, my friends, who can say where the second of the
Chinamen has got to? Is there a man here who set eyes on him or
on his boat after we captured the fool who lies at the bottom of our
own craft?"
The Dyak, a shrivelled-up and aged man, peered at each one of the
crew in turn, his beady eyes passing their faces one by one without
discovering an answer.

"No one responds," he went on with a cunning smile. "Then can our
brothers in Rembas tell more? Can they say that he has escaped
simply because we have not laid hands upon him? Ye are children,
who need a man of my years and experience to nurse you. Listen to
me, and say whether this will suit our case. We were told off for this
matter, and the fates willed it that we should meet with great
trouble; for how were we to guess that these Chinese dogs would
prove so cunning? And how could we foretell the fact that the two
who were caught by the bore would be armed with guns, good
guns, mark you, my brothers, which shot better than our own?
Others would have found the task impossible, but we were not to be
so easily beaten, and though weary we clung to our ground. Then
fortune came our way, and we captured one of the dogs, while with
a blow of the pole to which the hook is attached the boat in which
they rowed was sent to the bottom, with a big hole through the
boards. Tell me, does not the stream on the river Sarebus run fast,
and are not men easily drowned in its waters? Then that is the fate
which has befallen the second of the Chinamen. He is dead, and by
to-night the body will be washing out to sea, there to satisfy the
mouths of the sharks which keep watch there. It is all plain and
simple, and those at Rembas will recognize the truth of the story we
tell."
With another cunning glance the man took his seat, and, dipping his
paddle into the stream, turned the head of the boat towards the
distant town of Rembas.
"Come," he continued persuasively, "believe what I say, and agree to
tell the tale as I have told it. And recollect that we have with us one
prisoner who will help to make us welcome."
For some little time the others, who sat or kneeled in the boat,
looked at one another doubtfully, while they discussed the matter in
low tones. Then they began to see that their comrade had pointed
out the only path which they could take, and on considering it they
saw that the tale was a likely one.

"And besides," said the man who had had the post in the bows, "I
remember that as I thrust a second time at the figure of the man
who still remained to be taken, the hook struck heavily against the
craft, and may well have capsized it or battered a hole in the boards.
Yes, the tale is good, and we should agree to it."
And so, after a deal of discussion and eager conversation, the head
of the boat was turned again to Rembas, for the stream had swung
her round. Then the paddles dipped in the water, and very soon they
were at their journey's end.
"Rise! The chief awaits you!"
The words were shouted in Tyler's ear, while his custodian kicked
him savagely in the ribs. Then signs were made that he was to
mount the slope which led to a formidable-looking stockade, and
was to enter the gate which stood wide open.
"Come," shrieked the man, angered at the delay which had occurred
in the carrying out of his orders, "stir yourself and be quick, or I will
find something better and more persuasive than a foot. Here, stand
on your legs!"
With that he clutched at Tyler's clothing and swung him over the
gunwale of the boat and on to the ground beside the edge of the
water. Then two others came to his aid, and in a moment they had
set him upon his feet and had given him a push in the direction in
which he was to go. But, to their amazement, the prisoner collapsed
at once, and fell heavily upon his face.
"He would make believe that he cannot stand. He is a cunning dog!"
cried one of them. "Let us try again, and set something beneath him
to keep him upright. Ah, perhaps the point of a knife will help him!"
Once more Tyler was hoisted to his feet, while the ruffian who had
spoken last whipped a knife from his waist-cloth, and held it so that
the prisoner would meet with an injury if he was so foolish as to fall.
Then his comrades were in the act of starting aside when a by-
stander interfered.

"You will kill the man and rob us of our fun," he called out suddenly,
starting forward as he did so. "See! Have you no eyes? The fellow is
weak with loss of blood, and here is the spot from which it comes."
He pointed to the arm where the knife had struck some few hours
before, and then to the garments below, which were stained red
with blood. As for Tyler, he made no movement, but watched his
captors through half-closed eyes; for he had a difficult game to play,
and felt that the moment was a critical one.
"If I show fight, or am strong and can walk, they will treat me
badly," he had said to himself as he lay upon the floor of the boat
and thought the matter out. "Then their chief will have me brought
before him, and will endeavour to get some information from me; for
the chances are that he will guess that I have something to do with
the English of Sarawak. I should refuse, of course, and then, seeing
that I was of no further use, he would give the order for my
execution. That will not suit me, for my object is to gain time.
Captain Keppel will be at the mouth of the river by now, and Li Sung
will reach him to-night. To-morrow night at the earliest is the hour
when I may expect them. I must pretend to know a lot, and yet be
too weak to talk. Then in the hope of getting news from me when I
am stronger they will curb their impatience and treat me well. Also,
finding that I am helpless, they will not be so watchful, and perhaps
I may manage to give them the slip."
The plan seemed to be a good one, and as Tyler had thought it out
in the darkness, and had sought for a good excuse for his weakness,
the wound produced by the kriss which had struck him in the arm
occurred to him, and he had at once commenced to tear the
bandage from it.
"That will allow the wound to bleed freely for a time and to stain my
clothing," he said to himself. "I am wet from head to foot, so that a
little blood will spread and look like a lot, and so mislead them. Yes,
when the day comes I shall pretend to be almost on the point of
death, and shall be incapable of standing."

"See!" cried the man again, drawing attention to the prisoner's arm.
"He has a wound, and it has bled freely, which accounts for his
weakness. Let me tie a cloth about it, and then carry him, for we do
not desire to see our captive slip from our fingers, and so rob us of
the pleasure which we hope to have. Stand aside, you who gape and
hold the knife, while I see to the man."
Evidently the one who had spoken was of some consequence, for his
comrades did not demur, and instead stood on one side; while the
one who had drawn his kriss returned it to its place looking abashed
and uncomfortable as he did so. A few moments later the wound
was roughly bound, and Tyler was being carried up into the
stockade. A sheltered spot was found for him, and he was placed
upon the ground, while orders were given for water and food to be
brought to him. Then those who had captured him went in a body to
their chief to tell him the tale which they had agreed upon. As for
Tyler, left alone in the shadow of one of the huts, he dared not so
much as move an arm lest someone should be watching.
"I must remember the part I am playing," he said to himself, "and
must on no account appear to be shamming. When they see that I
am helpless they will leave me alone, and perhaps I shall have the
night to myself. Ah, here comes someone!"
Through his half-closed lids he caught sight of a woman advancing
towards him, and at once made ready to act his part. Closing his
eyes, he lay so still that he might have been dead, and made no
movement when the woman spoke to him. A second later his hand
was grasped and the arm lifted to its fullest height, only to be
dropped again, to see, perhaps, whether it would fall with a crash,
or whether this seemingly unconscious man had power to control it.
However, Tyler guessed the object of the movement, and allowed
the limb to fall with all its weight. Then he felt a gourd placed to his
lips, while a few drops of cold water were allowed to trickle into his
mouth.
"He is but young, and will recover," said the woman in soft tones.
"He is one of a race whom we admire, for their men are hard and

can fight and work well, and by the tale which has come to us this
lad and his comrade, who is dead, made a fine stand against our
men. Well, it is a pity, for he must die. But the chief has sent word
that he is to be carefully tended, for the rogue may have news of
these white people who propose to come up the river and attack us.
Not that we care much for the tale, for Rembas is safe against
thousands. There, I have sent some water down his throat, and in a
little time he will be better and will be sensible. He shall have some
food then, and perhaps to-morrow morning he will be well enough
to be killed."
She did not seem to see the strangeness of her words, but took it
for granted that once her charge was better he would be executed.
Indeed, to her mind such a course seemed only natural, for if the
prisoner were not strong and fully alive, how could he afford
amusement to the pirates, a collection of people who revelled in
cruelty? And therefore, having done her best for him, she left him to
himself and went about her daily work, wondering where the
youthful Chinaman had come from, and how it was that he
happened to be in the river Sarebus. As for Tyler, no sooner had the
woman left him to himself than he gently opened his eyes and
looked about him, carefully taking stock of the buildings and of the
forts which were erected on every hand.
"Evidently making preparations for the attack which is expected," he
said, observing that guns had been placed in position in many
places, so as to command the approach from the river, and that
stockades were being built. "But our guns would quickly send them
flat to the ground, and scatter the pirates. How much I should like to
be present at the engagement, and what would I not give to be able
to get away now and inform my commander of the preparations
being made to resist him? Yes, when the guns have done their work
the real excitement will begin, for the boats will row right in till they
are within gun-shot, and then they will have to surmount the booms
which are outside. And there is another question of importance.
Captain Keppel ought to know the exact position of those booms, so
as to send a boat ahead to blow a hole through them. I must get

away! The very first chance I get I shall take to my heels and make
a bolt for it."
It was a desperate resolve to make, but a natural one under the
circumstances; for what else could he do? To lie there simply meant
that his end was put off for a few hours. Sooner or later, if he but
waited for it, he would be dragged to execution, and then no one
could save him. Why not, then, snatch at the smallest chance which
offered, and trust to his heels to carry him to safety?
So determined was our hero that, seeing that no one was at hand,
he was almost in the act of springing to his feet, when sounds broke
upon his ear, and he became aware of the fact that a number of
men were approaching, and in their midst the leader of the Rembas
pirates. Instantly his eyes closed as if he were still insensible.
"So that is the man who kept you all at bay?" said the chief, looking
critically at the unconscious figure at his feet. "A lanky Chinaman,
you tell me? But—no, surely not one of that country, for see, his
pigtail is almost severed, while the arm which is bandaged is too
white for one of that race. This is no Chinaman, but an Englishman.
I can tell him at a glance, for I have been at Singapore and at
Sarawak."
The news that their prisoner was of greater consequence than they
had imagined caused the Malays and Dyaks the greatest
astonishment and pleasure, and as their chief assured them that he
was an Englishman they danced with delight.
"He will be all the more valuable," said the chief thoughtfully, "for we
will contrive to gather news of the intended attack from him.
Remember, though we of Rembas are not always on terms of
friendship with those who live at Pakoo and at Paddi, yet on this
occasion, when all are to meet the foreigner, we shall bury our
differences and make common cause against the enemy. For that
reason the Dutchman, Hans Schlott, will not refuse if I ask him a
favour, and will come hither at my bidding. We will send to him at
once, for he can speak the language of these Englishmen, and we

will ask him to interview the prisoner. Come, no time must be lost,
for many miles of river lie between him and us."
Fortunate for Tyler was it that he could not understand what was
said, for then he would have realized that his case was almost
hopeless. Hans Schlott to come and interview him as he lay a
prisoner at Rembas! Why, the crafty Dutchman would recognize him
in a moment, and would at once insist on his execution. That such
would be the consequence of a meeting between the man who had
murdered Mr. Beverley and Tyler Richardson could not be doubted,
and had our hero but known of the proposal to send for him, have
but dreamed that ere noon of the following day the Dutchman would
be there in the stockade which surrounded Rembas, there is no
doubt that he would have watched eagerly for the smallest loophole
for escape, and would have snatched at it instantly, however
desperate the chance which it offered. However, perhaps it was as
well that he was ignorant of the facts, for his peace of mind was less
disturbed in consequence, and he was able to devote more attention
to his surroundings and to plans for getting away than would have
been possible had the dread of an interview with Hans Schlott been
before him.
"Yes, he is an Englishman, and comes from Sarawak, I'll be bound,"
went on the chief of the Rembas pirates, closely inspecting the
unconscious prisoner again. "He may have come here by accident,
having been washed in by the flood from the sea, or he may have
come hither with the object of spying upon us. The last is the most
likely. But we shall soon know, for the Dutchman will interrogate
him, and if his tongue wags but slowly in reply, we shall have a
means to quicken it. But let us see to the message. To you," and he
took one of the by-standers by the arm, "I give the post of honour.
Take a boat and crew this instant, and row for Paddi, bearing this
from me: 'The lord of Rembas bids the lord of Paddi greeting, and
asks that he take passage in this boat, or in any one of his own
vessels, and come to Rembas at once, as we are in need of his
services. We have a prisoner, a young Englishman, who is tall and
lanky, and we desire to have him questioned, thinking that he has

been sent to spy upon us.' There, that should be sufficient. Go now,
without loss of time, and nightfall should see you at Paddi. An hour
will do for discussion, and after that you will set out to return. By
noon to-morrow you will be here with our guest, and we shall be
able to look into this matter. When that is done, and the night is
fallen, we will have a bonfire in the centre of the stockade, and there
we will pass the time pleasantly at the expense of the prisoner."
Once more he ran his eye over the apparently unconscious figure at
his feet. Then he turned away and went to see that his orders were
carried out without delay. As for the others, they, too, soon went to
their huts, and Tyler was left lying in the shadow alone.
"They suspect me," he said to himself, opening his eyes and looking
around. "I am not certain, but I think I overheard the word 'Englis',
which would make it appear that the chief recognized that I was not
a Chinaman. If that is so, he is likely to question me very closely,
and he will certainly not be inclined to show me any mercy on
account of the fact that I am one of those who are about to attack
him. Well, I must wait for night to fall, and then I shall make a dash
for it. Ah, they have forgotten to search me for arms!"
With a sudden flush of pleasure he realized that his revolvers still
occupied their place in his waist-cloth, and that he had a means of
defence. Then, as there was nothing more to be done, he looked
about him till he felt that he knew every corner of Rembas, and then
fell into a doze. When he awoke again, the woman who had
previously come to his help was standing beside him, and at once
she lifted the gourd of water to his lips.
"You are better, but still weak," she said. "Drink, and afterwards I
will give you food, for it is only in that way that you will become a
man again. There, lie still while I place the food in your mouth."
In her way she was kind to this forlorn prisoner, and seeing that he
still appeared to be so weak as to be unable to lift his hands, she fed
him with rice, which she conveyed to his lips by means of her
fingers. Then she gave him another drink of water, and having

placed a roll of matting beneath his head, and the gourd beside him,
she left him for the night.
"Sleep," she said, "and when to-morrow comes I will come to you
again to feed you. A fine rest to you!"
A moment later she was out of sight, and Tyler found himself alone
lying in the lengthening shadow of a hut which was situated in the
very centre of the town of Rembas. Other huts clustered about him,
but they seemed to be untenanted, and he soon made sure that
they must contain provisions and arms and ammunition. As for the
inhabitants, numbers were to be seen at work on the fortifications,
busily making them more secure against the expected attack, while
others went about their ordinary business. Every now and again
someone would stroll past the spot where the prisoner lay, perhaps
to satisfy his curiosity; but on each occasion Tyler was lying in
precisely the same spot, his head propped upon the roll of matting,
and his limbs spread out in a manner which showed that he was
helpless. Apparently the pirates were satisfied that there was no
deceit, for when the sun went down, and the long shadow cast by
the hut suddenly became merged into the general darkness, they did
not trouble to move him, but left him there, feeling that he was
secure.
"He can come to no harm, and the night air will serve to revive him,"
said the leader of the Rembas pirates as he passed to his hut.
"These pale-faces are not like us, and a very little takes their
strength away. But you will see that he will revive by the morning,
for he has already taken food. Leave him, and perhaps when the day
comes he will be able to sit up, or even to stand upon his feet with
help. As for escape, it is out of the question in his case."
Two hours later, when all sounds in the stockade had died down,
Tyler prepared to make an attempt to escape, and, as a preliminary,
thrust his hand into his waist-cloth and brought his revolvers out,
one at a time. Carefully running his hands over them, he made sure
that they were ready for use, though whether or not the powder had
been spoilt by his short immersion in the river he could not say.

Then, having stared about him, and listened eagerly for sounds of
any pirate who might happen to be abroad, he rose to his feet and
began to steal away in the dense shadow of the hut.
"I must keep out of the rays of the moon," he said to himself,
glancing at the sky, where a crescent of the orb was calmly floating.
"The light is quite sufficient to show me to an enemy, and, on the
other hand, it will enable me to detect one who may be
approaching. I'll stick close to the huts, and when I am bound to
cross an open space will make a dash for it. Here goes!"
With a revolver in either hand he stole along beside the wall of
beaten clay and bamboo, and soon came to the end of the hut.
Then, having paused for some moments, he flitted across the space
which intervened between it and a second, and once more was
buried in deep darkness.
Ah! As he stood there, looking about him with eager eyes in case
someone should have seen him, a sound, the rustle of a garment,
broke upon his ear, and instantly he became riveted to the spot, his
limbs held rigidly, while he searched the shadows with his eyes.
There it was again, and as he looked he fancied he caught sight of a
dusky figure away on his right. Was it a Dyak sent to watch the
prisoner, and who, seeing that he was about to escape, was
following with the intention of springing upon him as his hopes were
about to be realized? Yes, that must be the solution of this mystery,
and the fellow over there was tracking him, following him like a cat.
At the thought a cold perspiration broke from Tyler's forehead, while
his heart thumped so fiercely against his ribs that he even dreaded
lest the sound was audible. Then, too, his head throbbed, partly with
the rough treatment which he had received when captured, and
partly owing to the excitement under which he laboured. He could
scarcely think, could hardly gather his wits, and stood there for
some seconds scarcely daring to breathe. Then the courage which
had helped him thus far through the many dangers which he had of
late been called upon to face came to his aid, his old spirit of
determination returned to him, and in an instant, it seemed, Tyler

Richardson was himself again, peering into the darkness with all his
senses alert, and judging the situation with that calmness which had
astounded his friends on former occasions.
"Dyak or Malay," he whispered to himself, "it makes no difference to
me, for I have met both before, and have beaten them. I will not
allow this fellow to spoil my hopes, and will shoot him like a dog if
he interferes. But does he see me? I am in the shade, and it is
possible that he has lost sight of my figure. Ah, I will play a prank
upon him!"
Suddenly perceiving that if the man, whoever he was, had been
following, he would have seen him dart across the space lit by the
feeble rays of the moon, and that, having watched the prisoner gain
the shadow of the hut, the pirate would expect him to creep along
beside the latter and emerge again at the farther end, Tyler decided
to remain where he was for a time, and so mislead the man.
"By not moving myself I may force him to disclose his own position,"
he thought, "and then I shall be able to deal with him. And, besides,
it is not at all certain that he is actually following me. Perhaps he has
some other game to carry out, and cares nothing for the prisoner."
However improbable the last might be, Tyler did not mean to lose
sight of it, and, in accordance with his resolution, crouched in the
shadow, and remained perfectly still there, peering out into the
comparative light beyond in the hope that he would catch sight of
the stranger.
There he was. The swish of a linen garment and the patter of a
sandalled foot broke the silence, and a tall figure was seen to glide
along beside a hut across the way and disappear round the corner.
Strange! He was moving away from Tyler, for now the hut stood
between them. But not for long; for ere many moments had passed
the same sounds were heard again, and the same ghostly object
came into view, this time more easily seen, for the reason that a few
stray rays of the moon reached him. Why, he had made a round of
the dwelling, and, as if he had failed to find that for which he was

searching, was now flitting across to another near at hand! Instantly
Tyler turned to follow his movements, wondering what the man
could want. Then he suddenly swung right round, for the muzzle of
one of his weapons had tapped against the woodwork of the hut,
and had given rise to a sharp noise which had instantly brought the
stranger to a stop.
"He heard it, and if he did not know of my presence here before he
suspects it now," thought Tyler. "It was a piece of carelessness and
bad luck, and may cost me my life. Ah, he too is hiding in the
shadow and peering in this direction!"
For many minutes did the two silent figures watch each other, or,
rather, search the shadows in the vain endeavour to recognize who
was hidden there. And more than once did Tyler lift his arm and take
aim at his opponent, thinking that to risk a shot would be better,
perhaps, than to wait there in such uncertainty, and always with the
fear in his heart that another of the pirates might put in an
appearance. But however justified the deed, he could not reconcile
himself to it. It seemed so much like murder, like shooting a man
from behind; and as the thought came to him our hero lowered his
weapon, while he puzzled his brains as to how he should act.
As for the stranger, his patience seemed to swiftly come to an end,
and at length he commenced to creep from the shadow towards the
spot where Tyler was in hiding. A tall, lanky individual, in that half-
light he appeared almost like a giant as he stood for the space of a
second to his full height. Then, snatching at a weapon which was
thrust in a band of linen about his waist, he dropped on hands and
knees and stealthily crawled forward.
"He hopes to attack me in the shadow," thought Tyler, at his wits'
end how to act. "If I stay here he will crawl into the shadow higher
up, and will then come down beside the wall of the hut, where I
shall be unable to see him. That will not do, and as it is clear that I
am discovered, and that I shall have to fight for my life, I too will
make for the open. If he gives a shout I shall fire and then run for
my life."

His mind made up to act in this manner, he threw himself on hands
and knees also, and at once crawled out into the open, one weapon
thrust into his girdle, and the other held in his right hand. And thus,
like two panthers awaiting the moment to spring upon one another,
the two ghostly figures advanced across the open. Rapidly did the
distance between them lessen, till at last only some fifteen feet
separated the combatants. Now was the time to act, and as each
grasped the fact, they sprang to their feet. Instantly Tyler's figure
stiffened, his arm swung up to the horizontal, and he covered his
man with an aim which never left the mark, and which never
trembled in spite of his excitement. As for his opponent, he seemed
even taller and more forbidding than before, and as he stood to his
full height, and raised his naked weapon above his head, the rays of
the moon flashed upon him, increasing the ferocity of his
appearance. But Tyler was not to be frightened so easily, and indeed
scarcely seemed to have taken note of the features of the man. As if
to make more certain of his aim he squinted along the barrel of his
revolver and elevated the muzzle just a little till it lit upon some
object which glinted brightly upon the naked chest of the man. Then
his finger went to the trigger, there was a pause, and slowly the grip
tightened. A second and the haunt of the river pirates would have
been awakened by the report of a shot, when an exclamation of
astonishment burst from our hero.
"What!" he exclaimed in low tones. "Can it be possible, or am I
dreaming?"
As if some sight had dazed him, he passed his hand across his eyes,
and stared again at his opponent, keeping his revolver levelled at
him all the while. Then he advanced a step or two and peered at the
stranger. As for the latter, he, too, was acting in a manner strangely
different from that which one would have expected. As Tyler had
levelled his weapon he had started back a pace. Then he had
suddenly leaned forward and stared into the face of the man who
stood before him. What were his thoughts no one could say, but
there he stood as if spell-bound, not uttering a sound, staring at his
silent opponent. A moment later, however, as Tyler gave vent to the

words, the figure opposite him had sprung forward, the naked
weapon had dropped to the ground, and two hands were groping for
his in the semi-darkness.
"Massa! Massa Tyler Richardson! You! de prisoner!"
"And that is Li Sung!" gasped Tyler. "I cannot believe it possible. I
am escaping. I thought you were a pirate about to stop me, and in
less time than I can think I should have shot you. Then the moon
showed me that the bright point at which I aimed was a small brass
box in which you carry snuff, and instantly I realized that it must be
you. What are you doing here? Speak! We have no time to lose!"
For a few seconds the faithful Li Sung could not respond to his
master, so utterly taken aback and staggered was he by the
extraordinary ending of what had appeared to be a serious danger.
This his master! And the latter had recognized his servant but just in
time, had told him by the snuff-box which dangled about his neck,
and which, had he been away in China, amongst his countrymen,
would never have been there. Yes, it was strange that a habit which
he had learned from the Malays of Borneo should save his life, and
that the wearing of a box of metal about his neck should enable his
master to find him. His delight was beyond everything. This stoical,
placid Chinee was a different being, and for the first time for many a
year he was at a loss for words, while his lips trembled and tears
started to his slit-like eyes.
"Massa!" he managed to gasp at last. "Li he comee here to find you.
He watch de town of Rembas and sneak here when de light die
down. Den he creep close to de stockade, and he hear plenty fine
words from de sentry who talkee to one of de men. He learn dat you
am velly weak, and am wandering in de head. Den Li say dat dat not
right. He must getee you away from de place, and he kill de sentry
Yes, him not wait to tink velly much, but kill him wid de knife. Den
him sneakee into de town, and—"
"And happened to run up against the weak and helpless prisoner,"
burst in Tyler in a whisper. "It is all plain now, and the rest can be

left to later on. Only this I know. You stood by me. You came here at
the risk of your life to save me. I shall not forget, and later on will
thank you."
"No needee to do dat, massa," was the Chinaman's reply. "Li not
alivee to come here if massa not savee him in de river. But time we
left de town. What am de orders?"
"That you lead the way to the river, and that we make for the mouth
as soon as possible."
Without further conversation they turned their faces to the stockade
which surrounded the town of Rembas. At the gate lay the figure of
the sentry who had been keeping watch, and whom Li Sung had
killed, and over his body each stepped in turn. The gate gave to a
slight push, and ere very long they were on the bank of the river
which ran direct into the Sarebus.
"De boat or a prahu? What does massa say?"
"The first till we are in the stream, and then the last," was the sharp
and unhesitating reply. "Lead the way."
Grasping the edge of the Chinaman's cloak, for it was dark beneath
the trees which fringed the bank, Tyler followed Li Sung without a
doubt of his ability to lead him. Then, arrived at the spot where the
boat was moored, each stepped into her, while a thrust from the
Chinaman's brawny leg sent her afloat. There, standing up cutting
the silvery beams with a line of black, was the mast of a small river
prahu, and at once, as if they had chosen it by common consent, the
boat was rowed in that direction. No need to hoist an anchor, for a
rope of rattan alone held the barque, and a swift flash of the knife
severed it Then the two dusky figures went to the ropes, and soon
the prahu was standing down-stream with bellying sail.
"De bore am gone and de tide am falling," said Li Sung, coming aft
to where Tyler stood at the helm. "Keep de ship to de centre of de
river and all am safe. Comee de morning and de sea am in sight."

"And perhaps the friends who are waiting for us. Get along forward,
Li Sung, and keep a bright look-out, for it would be hard if we were
to run on a bank after all that has happened."
Alone upon the wide sweep of water they sailed swiftly towards the
mouth of the river Sarebus, and, just as the sun rose, emerged into
the sea. Instantly a shout left their lips, and they turned to shake
one another by the hand; for anchored behind a tiny sandy
promontory were two vessels, one of European build, which was
undoubtedly the Dido.

CHAPTER XIX
An Attack upon the Stockades
"Back again! We scarcely expected you, and a load of anxiety is
lifted from my mind," cried the captain of the Dido in hearty tones as
Tyler swarmed to the deck of the war vessel, and, leaving Li Sung in
the prahu made fast to the gangway below, went striding to the
poop. "Come, tell me the news, for I am eager to hear it. Here we
are, you see, at the rendezvous appointed, and I may say that all
are eager to be moving. Let us get down into the cabin and have a
chat."
Seizing the Chinaman, who had boarded the vessel, by the hand,
Captain Keppel wrung it with enthusiasm, and then, ere he led the
way to his cabin beneath the poop, took a step backwards, the
better to be able to survey the gallant young fellow who had so
willingly undertaken to ascend the Sarebus and gain tidings of the
enemy. As for the crew of the Dido, they were not slow to guess
what was happening, and, remembering the fact that two Chinamen
had so recently taken possession of a prahu, much to their
mystification, and that the young fellow who had so recently joined
them had been absent ever since, they recognized that this
Chinaman who had just come aboard in his tattered and travel-
stained garments must be the same. For some moments they waited
watching the interview upon the poop. Then, as their commander
was seen to grasp the stranger by the hand, their excitement was
roused to the highest.
"Took 'im by the 'and!" shouted one lusty tar, lifting an enormous
palm to shade his eyes from the sun. "Then if that don't prove that
that feller ain't a Chinee after all, well—bust me!"

He paused for a moment to find an expression adequate for the
occasion, and then, changing his plug to the other cheek, looked
round at his comrades.
"It's the young orfficer what come aboard a few days gone back," he
whispered hoarsely. "Him what arrived at Sarawak with the fleet of
prahus and a crew of darkies. Strike me! but what's he been up to
this time?"
"Been sailing up the river Sarebus," shouted John Marshall, the
boatswain of the old schooner, who had taken up his quarters on the
Dido for the time being, instantly recognizing Tyler as he stood there
on the upper deck. "He's been risking his life again. He's been after
them 'ere pirates and the Dutch bloke as I told yer about. Here, let's
give him a rouser. One, two, now all together!"
Thanks to the fact that John Marshall had a busy and a ready
tongue, the crew of the Dido, and indeed the greater part of the
inhabitants of Sarawak, had long ere this gained news of all the
adventures which had befallen Tyler on his way to join his ship, and
now that something more had happened, and they gathered the fact
that this lad—for he was little more than that—had dared to ascend
the river and spy upon the pirates themselves, their enthusiasm
knew no bounds, and, leaping forward at John's shout, they rent the
air with their cheers, repeating them till Captain Keppel advanced to
the rail and lifted his hand to ask for their silence.
"Nicely done, my lads!" he said, in tones of satisfaction. "He will
appreciate your cheers far more perhaps than the poor thanks which
I can give him. Let me tell you that this officer is a gallant one
indeed, and that although he is still practically a new-comer and a
stranger to most of you, yet that he has already earned distinction.
For the splendid manner in which he brought through the tribe of
Dyaks he will deserve commendation at the hands of his seniors, but
now he has added something more, and I shall strongly recommend
him for reward. There, my lads, I'm even more pleased than are
you, and to celebrate the occasion, and because we have some fun

ahead, I'll give orders for a round of grog to be served. Then you
can drink success to our friend and to the coming expedition."
Turning away from the men as they sent up another cheer, the
commander inspected Tyler closely, and then spoke again.
"You have had a rough time, that I can perceive at a glance," he
said with a start. "You are wounded, and there is a hunted look on
your face. Come below. A good meal and a glass of wine will do you
no harm, and then you can let me have your information."
Taking his junior by the arm he led him to the companion, and
ushered him into the cabin. Then he called for food and drink, and
would not hear a word from Tyler till the latter had had his wants
satisfied. Then the ship's surgeon was called in, and speedily
dressed the wound.
"A clean cut, which would have done better had it been dressed at
once," he said, as he looked at the arm critically. "But that, of
course, is not always possible. Yes, there has been severe
hemorrhage, and by the appearance of your cheeks you are still
somewhat weak from loss of blood. But that is a matter which can
soon be set right. Shall I place Mr. Richardson on the sick-list, sir."
For answer Captain Keppel smiled at our hero, and then turned to
his interrogator.
"Ask the lad himself," he said with a laugh. "He does not look to me
like the fellow who would willingly miss the fun which we have
before us."
"Well, what do you say? Shall it be the sick-list and a bunk in the
sick-bay, or a hammock and a good sleep in your own quarters?"
demanded the doctor.
"The last, please," answered Tyler without hesitation. "As to the
wound, it is really nothing, I assure you. The kriss went through the
arm, and almost dropped out by its own weight. It does not hurt at
all, and all that I have suffered has been from loss of blood. To that

I owe the fact that I am alive at this moment. But I am dog-tired
and want a sleep."
"Then you shall have it, my lad," cried the commander. "But first I
must get your news, for time is precious and we have much to do.
Come, out with it, and then off to your hammock."
Thus bidden, Tyler rapidly outlined the preparations which he had
seen in progress at Rembas, and told Captain Keppel that, having
now seen that stronghold in addition to Paddi, he was sure that the
former would offer a fiercer resistance. Then, urged by those who
were listening, he narrated how he and the Chinaman had held the
rock, and had finally taken flight; how he had been hauled into the
enemy's boat; and how, while making his escape from the
stronghold of Rembas, a strange figure had dogged his footsteps,
and had almost joined in combat with him.
"I call it a wonderful piece of luck," exclaimed Lieutenant Horton,
who had also joined the party in time to hear the tale. "Anyone else
would probably have fired point-blank, and only discovered that the
opponent was a friend in reality when it was too late. How fortunate
that you caught sight of the tin about the neck of this Li Sung!"
For some little time those who had listened to the tidings which Tyler
had brought discussed the matter, commenting on the news and
upon the steps which he had taken to make good his escape.
Indeed, so interested did they become that their attention was
withdrawn from our hero, and when at last they turned to question
him further, they discovered that he was fast asleep, his head
reclining upon his hands on the cabin table, while his breathing was
long and deep, showing how much he was in need of rest.
"Done to a turn," said Captain Keppel in a whisper. "The lad has
worked like a Trojan, and has been at it almost since the time when
he left Sarawak. We know at any rate that he has not had a wink of
sleep for two nights, while he has been fighting most of the time.
Catch hold of his legs, Doctor, while I take his head and shoulders.
We'll pop him into his bunk without disturbing him."

In a trice they had Tyler between them, and in less than a minute he
was between the blankets, snoring heavily and utterly unconscious
of his surroundings. When his eyes opened again, and he looked
about him in bewilderment, it was to discover that sunlight was
streaming down into his quarters, and that his comrades were
hastily donning their clothes.
"Halloo! Still day?" he asked, peering at the others and rubbing his
eyes. "What a sleep I've had!"
"Should say so, old chap," was the laughing answer. "At any rate
you've lain there like a log since you arrived here, and that was
somewhere before noon yesterday."
"Yesterday? Impossible!"
"Not a bit of it. You've slept for twenty hours on end, and if you're
not precious slippy you'll be too late to hear all about the expedition.
All hands are to muster at once while the skipper reads out the
orders."
Tyler was out of his bunk like a shot, and rapidly scrambled into a
suit of clothes, tearing the old ones off in a moment. Then he hastily
washed his face and hands, and darted up on deck in the wake of
his comrades. Above, all were assembled, and listened eagerly as
the captain read the orders for the day. Then each man went to the
magazine, there to be served with arms, which all at once set to
work to clean and put in the best of order. That some big movement
was afoot anyone could see, for there was an air of half-suppressed
excitement about the tars, and they discussed in eager terms the
chances of a hand-to-hand conflict with the enemy.
Early on the following morning all was in readiness, and no sooner
had breakfast been finished and swept away than the shrill notes of
a pipe rang out, while the bugle of the marines awoke the echoes.
Then a boat came pulling alongside, bearing the Rajah of Sarawak.
"We propose to leave Rembas till the last," said Captain Keppel, as
Tyler stood before him and the rajah, having been called on to the
poop to speak with them. "We shall leave the Dido here and pull up

in open boats, taking that tope over there with us. She is well
supplied with food and with ammunition, and must be well guarded.
And now for the force. Lieutenant Wilmot Horton will be in
command, for it is one of the perquisites of his rank to lead an
expedition of this nature. But I propose to go also, in my gig, and
with me will be the rajah. You will accompany us, Mr. Richardson,
and, since you have now been face to face with these pirates on two
occasions, we shall expect you to take good care of us.
"And now for the actual men to be taken," he went on, turning away
from Tyler with a smile as the latter flushed red at his remarks. "The
force of officers and men, sailors and marines, will be approximately
eighty in number. We shall take the pinnace, two cutters, my gig,
and the Jolly Bachelor, which the rajah has kindly placed at my
disposal. She is native built, and admirably suited to our needs, for
she will take thirty men with ease, besides a six-pounder. The
pinnace will be armed with a twelve. The tope will accompany the
expedition with food and ammunition, and strung on to our forces
we shall have nearly a thousand natives, Borneans, Malays, and
Dyaks, but mostly the latter, and a goodly few the same who
accompanied Mr. Richardson from along the coast. They may not be
of much use in the attack, but I promise you that they will be to the
fore if flight is attempted by the enemy, for they do not love them,
and have suffered much at their hands."
"They have indeed," burst in the Rajah. "For a century and more
their children and wives have been enslaved, and the men killed,
while their fruit-trees and their plantations have been cut down and
ruined. But you will have to keep a close hand upon these natives,
Keppel, for they do not know what discipline means."
"I mean to," was the emphatic answer, "and for that purpose I am
placing one of my officers over them, with strict injunctions to watch
them. And now, if you are ready, Rajah, we will set out."
The ruler of Sarawak having assented, the bugles and whistles once
more set the echoes ringing, and very soon the boats of the
expedition had been marshalled. Drawing a rifle from the magazine,

Tyler dropped into the gig and awaited the coming of his
commander. Half an hour later all were in readiness, and having
turned the bows of the boats in that direction they were pulled into
the river Sarebus. In spite of the fact that a deluge of rain poured
down upon their heads not one of the attacking-party seemed to
mind, or to have his high spirits damped. For the weather was
warm, and each one wore a kajan, a mat through which the head
was thrust, which effectually protected them from the wet. Then,
again, who could say what would happen? Perhaps heavy and fierce
fighting was in store for the force, for these pirates of the river had
had their quarters there, father and son, for more than a generation,
and would not be likely to yield them without a struggle. Then,
again, they would be ashore, behind stockades, and would have the
advantage of knowing every inch of the river, while the attackers
would have to come up in the open, exposed to every gun and rifle.
But if the enemy counted upon the fact that the British tars would
be dismayed at the thought, they were doomed to disappointment,
for all that the latter did was to joke and laugh, with an occasional
grumble at the long pull which must intervene between themselves
and the enemy.
"Well, there's one thing about the business that I like," cried one of
them as he pulled at his oar. "A long pull's a long pull, and yer can't
alter it nohow, but yer can have it made easy like if the flood's with
yer. That's what we've got, and yer can feel the rush at every stroke.
With a stream like this well be there against to-morrow night."
"And then the guns'll be popping," burst in another.
"Bet yer a quid o' 'bacca I'm in their show afore you, Billie."
"Done with yer," was the answer, growled in the huskiest of voices.
"It's a fair bet, and our mates'll see who's the winner."
Laughing and chatting as they rowed, the hours swiftly passed away,
Tyler having much to occupy his attention. Indeed, every bend of
the river brought some recollection to his mind. It was there that
they had hidden their prahu, that low sandy bank was the spot

where they had landed in search of inhabitants, while, higher up, the
land became even more familiar.
"That is the rock which Li Sung and I defended," he ventured to
remark, when at last the expedition was close to the branch of the
river upon which Rembas was situated, "and by turning to the left
now we should be under their guns before we could believe it."
"Then we will keep straight on," was the reply. "But that was a
capital site for defence, Mr. Richardson. For two alone it was just the
isolated position which would offer a chance of success, and you
were fortunate to have it so close at hand when you were capsized.
But that reminds me of the bore. We will look out for a spot in which
to pass the night."
When darkness fell the boats of the expedition lay snugly under the
banks, while the men lay in them, smoking and waiting for the meal.
Then fires were lighted and kettles set to boil, while certain of the
men were told off to act as sentries. And thus, pulling cautiously by
day, and tying up to the banks at night, the winding course of the
river was slowly followed and Paddi approached. At length the latter
was close at hand, and one fine morning, after the bore had gone
sweeping past on its course, with its usual accompaniment of
brushwood and drift from the banks of the stream, the expedition
loaded weapons, and, pulling up their moorings, took the flood
which went racing on to Paddi. Had they wished to go slow to their
destination it would have been almost impossible, so strong and
rapid was the stream in these upper reaches. But the pace suited
the spirit of every man, and particularly of those who occupied the
gig. At the helm, sitting in his shirt sleeves, was the Rajah of
Sarawak, as calm as if before his own home at Sarawak, while close
at hand were Captain Keppel and Tyler. Forward of them were the
crew, a set of lusty fellows, whose hands itched to toss their oars
aside and snatch at the cutlass which each carried in his belt. Hark!
A murmur in front, a bend in the river, and nothing but trees and
jungle to be seen. Was it the enemy? The commander turned his
face towards his companion questioningly, and in reply Tyler nodded.

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