Computer Networks. Southeast University 东南大学止于至善. Nanjing, Jiangsu China. June 3 rd 23 rd, 2013

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1 Computer Networks Southeast University 东南大学止于至善 Nanjing, Jiangsu China June 3 rd 23 rd, 2013

2 Course Objectives The objective of the course is to provide an indepth understanding of architectural principles, fundamental concepts and basic techniques underlying the design and implementation of the Internet and its protocols Whenever possible and useful, a quantitative approach is used

3 Course Approach The focus is on advanced topics related to the design of large-scale, evolvable, reliable and robust internet In-depth discussion of network design problems, models and approaches to a variety of design and implementation issues related to network protocols for large-scale internets Explore the Internet design principles and tradeoffs and understand adopted solutions in context goals, assumptions and what problems are solved Gain hands-on experience through problem solving and project development

4 Recommended Readings James Kurose and Keith Ross: Computer Networking, A Top Down Approach, Addison Wesley William Stallings: Data & Computer Communications, Prentice Hall Larry Peterson and Bruce Davie, : Computer Networks: A Systems Approach, Morgan Kauffman Douglas E. Comer, David L. Stevens: Internetworking with TCP/IP, Prentice Hall

5 Network Programming Useful References W. Richard Stevens, TCP/IP Illustrated, Volume 1: The Protocols W. Richard Stevens, Bill Fenner and Andrew Rudoff, Unix Network Programming, Volume 1: The Sockets Networking API (3rd Edition), Online Resources Search for Socket Programming

6 Wide Area Networks Architecture, Protocols and Design Principles Overview

7 Communication Networks Network, a collection of nodes capable of transporting data between pairs of attached stations. Host, End System Communication Network Switching node, Intermediate system

8 What is a Computer Network? Software and hardware networked infrastructure It allows access to different types of resources Computing resources, input/output devices, files, databases... It provides a medium through which geographically dispersed users communicate and socialize It is a national information infrastructure for social and economic activities A consensual [environment] experienced daily by billions of operators, in every nation,...

9 WAN Structure domains/autonomous systems transit domains exchange point Hosts End Systems peering border routers routers Access Networks stub domains

10 Network Classification Networks are usually classified according to: Geographical coverage Transmission technology Wired and Wireless Networks Point-to-Point and Broadcast Networks Switching technology Technique used to exchange information between end-points

11 Point-to-Point Networks Multiple connections between individual pairs of machines Packets may visit intermediate nodes before reaching destinations Routing plays a major role

12 Broadcast Networks A single communication channel shared by all network stations Packets are received and processed by all stations No intermediate switching node. Routing is trivial. Only few stations, usually one, are targeted Wasteful for non-intended recipients.

13 Star network topology Hub Network is preserved in case of end- node failures.

14 Ring network topology Computers are connected in a circular fashion Failure of an end-node causes network failure

15 Bus network topology Network is tolerant to end-node failure

16 WAN Switching Techniques

17 Switching Techniques Switched Communication Networks Switched Networks Broadcast Networks Circuit Switched Packet Switched Datagram Virtual Circuit

18 Broadcast Networks Information transmitted by any node is received by every node Typically used in LANs wired and wireless No need for intermediate node, Several limitations Limited transmission range Privacy of communication Media Access Control is required to coordinate access to the shared communication medium

19 Circuit Switching Exclusive dedication of a portion of the available bandwidth to carry traffic between source and destination Bandwidth is allocated using : Frequency Division Multiplexing Time Division Multiplexing Incoming data are switched to the appropriate outgoing channel without delay Telephone network, a typical example

20 Circuit Switching: TDMA and TDMA FDMA Four Users: Frequency TDMA Time Frequency Time

21 Circuit Switched Networks Used in telephone networks POTS All resources needed by a call, such as communication links, buffers and processing, are reserved for the entire duration of the call Resource reservation guarantees quality of service (QoS) A D E C B F

22 Circuit Switching A node (switch) in a circuit switching network incoming links Node outgoing links

23 TDM-based Circuit Switching Multiplexing and De-Multiplexing Relative slot position inside a frame determines which conversation the data belongs to Needs synchronization between sender and receiver Slots may remain empty Dynamic allocation of slot, possible but difficult

24 Circuit Switching Call establishment requires three phases: Connection setup phase A circuit is set up between source and destination, and resources are reserved Transmission phase Traffic is exchanged between the sender and the receiver Connection tear-down phase The call is terminated and resource are returned to the resource pool

25 Circuit Switching Timing Diagram User 1 Switch 1 Switch 2 Switch 3 User 2 Propagation Delay Call Request Connect Processing Delay Data Traffic Call Disconnect Time

26 Circuit Switching Networks Delays for setting up connections can be high. Ordinary telephone lines: Call setup is on the order of 5 to 25 seconds after completion of dialing.

27 Message Switching Networks A physical circuit is shared among multiple users. Leased communication facilities are used. Data enters the network in the form of messages Messages are stored and subsequently forwarded. No circuit switching delays are involved. Queueing delays occur. Message lengths are slightly longer because of headers.

28 Message Switching Timing Diagram User 1 Switch 1 Switch 2 Switch 3 User 2 Message Propagation Delay Message Time Processing and Queueing Delay

29 Packet Switching Networks Equivalent to message switching for short messages Maximum message length for transmission is imposed. Any message exceeding the maximum is broken up into shorter units called packets. Packets traversing a network share netwroks resources with other packets statistical sharing Demand for resources may exceed amount of the resources available Contention

30 Packet Switching Timing Diagram User 1 Switch 1 Switch 2 Switch 3 User 2 Propagation Delay Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Processing and Queueing Delay Time

31 Packet Switching Store-and-Forward Networks At each node the entire packet is received, stored, and then forwarded to the next node incoming links Node Memory outgoing links

32 Packet Switching Multiplexing and Demultiplexing Data from any sender can be transmitted at any given time Headers are used to differentiate traffic from different senders

33 Packet Delays Propagation delay: time for the signal to propagate from the sender to the receiver Determined by the wave propagation speed: 200 m/ s in a wire Transmission delay: time needed to transmit the signal representing a block of data Determined by the data rate and the length of the packet, Processing and queueing delay: time needed to process the packet and the time the packet has to wait until the link becomes available: store-and-forward. Determined by the router processing speed and the network load

34 Performance Tradeoffs Circuit Switching Circuit switching may lead to smaller delays if large amounts of data are exchanged. Elimination of the header, Elimination of intermediate node queueing.

35 Performance Tradeoffs Packet Switching A major benefit is the pipelining effect, achieved by the simultaneous use of communication links Considerable gain in efficiency, Shorter delays, despite inclusion of headers for each packet. Lower probability of retransmission, Shorter messages are less likely to have errors than longer ones, Errors do not cause retransmission of entire messages, but only of relatively shorter packets. Packets can be routed independently, possibly minimizing congestion.

36 Packet-Switching vs. Circuit- Switching Ability of packet-switching to exploit statistical multiplexing: Efficient bandwidth usage ratio between peek and average rate is 3:1 for audio, and 15:1 for data traffic Consider a 1 Mbps communication link, where each user requires 100 kbps when transmitting, but sends 10% of the time, Circuit switching: Each caller is allocated 100 kbps capacity, At most 10 callers are supported. Packet switching: With 35 ongoing calls, probability that 10 or more callers are simultaneously active is about , Can support many more callers, with small probability of contension If user traffic is bursty (on/off), then packet switching can be more efficient than circuit switching.

37 Performance Tradeoffs Packet switching provides flexibility in meeting the user needs. Example: the needed rate is 75 kbps while the channel rate is 64 kbps The use of packet switching meets the demands of the user more easily. However, packet-switching needs to deal with congestion: More complex routers Harder to support QoS: delay and bandwidth guarantees In practice, both techniques are frequently combined IP over SONET, IP over Frame Relay

38 Packet Switching Techiques Two basic approaches to packet switching are common: Datagram packet switching, Virtual circuit packet switching.

39 Virtual Circuit Packet Switching An initial phase is used to setup a fixed route. Similar to circuit switching, except that a delay occurs at each node, Call request and call accept must both wait their turns on transmission.

40 Timing of Virtual Circuit Packet Switching Host 1 Host 2 Node 1 Node 2 VC establishment propagation delay between Host 1 and Node 1 Packet 1 Data transfer Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 VC termination

41 Virtual Circuit Packet Switching Upon path setup, the virtual circuit appears to the user as a dedicated circuit. In reality, the circuit is shared among multiple users. Destination address is no longer required in the packet header. Only a virtual circuit number is needed to identify the destination. Usually, the circuit number is locally defined Packets have shorter headers and fixed routing makes possible fast packet switching.

42 Virtual Circuit Service Characteristics Reliable delivery of information Powerful error control Sequencing of packets Detection and suppresion of duplicates Congestion control minimizes queueing delays Delays, however, are more variable than they are with dedicated circuits Enhanced security

43 Datagram Packet Switching Each packet is independently switched Each packet header contains the intended destination address No resources are pre-allocated (reserved) in advance Packet from different connections compete for the resource Used in IP-based networks

44 Timing of Datagram Packet Switching Host 1 Host 2 Node 1 Node 2 transmission time of Packet 1 at Host 1 Packet 1 Packet 2 Packet 3 propagation delay between Host 1 and Node 2 Packet 1 Packet 2 Packet 3 processing delay of Packet 1 at Node 2 Packet 1 Packet 2 Packet 3

45 Datagram Packet Switching Datagram Packet Switching does not require a call setup For short transactions, it may be faster Individual datagrams are routed independently Increases processing overhead at the router Routing table lookups

46 Datagram Packet Switching Host C Host A Host D Node 1 Node 2 Node 3 Node 5 Host B Node 4 Node 6 Node 7 Host E

47 Datagram Service Characteristics The network makes a best effort attempt to deliver the packets Each packet is treated as a separate entity with no prior route determination Packets may follow different paths to destination No guarantees for reliable delivery Packets may be lost, duplicated, or may arrive out of order The network relies on the user application to enhance the basic datagram service

48 Connection Semantics Literature often uses the term connectionoriented and connectionless to refer to different network services. Virtual circuit transmission is a special case of connection-oriented transmission Datagram service is a special case of connectionless transmission

49 Internetworking

50 Internetworking The main goal is to provide a universal network formed out of physically different networks. Internetworking involves complex issues: Different addressing and naming schemes Different routing techniques Different congestion control techniques Different hardware interfaces Connection oriented vs connectionless services Different data unit sizes Different error control techniques

51 Network Design Layering To reduce design complexity, most networks are organized into layers Each layer enhances the service of the layer below Added value service Two layer-n entities in different computers use a peer-to-peer layer-n protocol to communicate with each other Two adjacent layers within the same computer communicate through an interface The interface defines the primitive operations and services lower layer provides to upper layer

52 Network Architecture The set of layers and protocols define the protocol architecture Neither the details of the implementation nor the specification are part of the protocol architecture The implementation, however, must obey the appropriate protocol Interfaces on all machines need not be the same, as long as the protocols are correctly implemented Interfaces are not visible from the outside

Interface Layer N-2 Layer N-2 protocol Layer N-2 Layer 2 Layer 1

53 Network Architecture Layer N Layer N-1 Layer N Layer N protocol Layer N-1 Interface Layer N-1 protocol Layer N Layer N-1 Layer N Layer N-1 Interface Layer N-2 Layer N-2 protocol Layer N-2 Layer 2 Layer 1 Layer 2 Layer 2 protocol Layer 1 Interface Layer 1 protocol Layer 2 Layer 1

54 Information Flow Data Layer N Protocol Data H Data Layer N-1 Protocol H Data H H Data H H Layer 2 Protocol Data H H Data H H Data Layer 1 Protocol H H H Data H H H Data H H H Data H H H Data Physical Layer

55 Layers Interface Relation Layer N+1 ICI Interface SDU SAP IDU: Interface Data Unit IDI: Interface Control Information SDU: Service Data Unit SAP: Service Access Point PDU: Protocol Data Unit ICI SDU Hdr SDU Layer N N-PDU Exchanged by Layer N Entities

56 Protocol Functions and Design Issues Connection Control Connectionless vs. Connection oriented Segmentation and Reassembly Encapsulation Ordered Delivery Flow Control Error Control Addressing Multiplexing Transmission Services

57 Reassembly Segmentation and Reassembly Fragmentation Segmentation is the process of breaking a PDU into smaller PDUs Reassembly is the process of gathering all fragments into original PDU Original PDU HDR Fragment 1 HDR Fragment 1

58 Reasons for Segmentation Network may impose limits on data size X.25 packet is limited to 128 bytes Ethernet packet is limited to 1526 bytes ATM cell payload is limited to 48 bytes

59 Segmentation Benefits Error control may be more efficient with smaller PDUs In case of errors, only a small PDU need to be retransmitted More equitable access to shared links May result in shorter average delay Smaller PDUs require smaller buffers Checkpoints and restart-recovery operations may be required at different points in time Data transfer must be synchronized to avoid losses

60 Segmentation Disadvantages Each fragment carries a fixed minimum amount of control information Smaller fragments result in larger overhead Fragment arrivals causes interrupts that must be serviced Smaller fragments result in larger number of interrupts Fragments require processing at different levels Smaller fragments result in larger processing overhead

61 Encapsulation Each PDU contains not only data but control information Some PDUs may contain only control information Control information contains Addressing information Error detection code Protocol control functions

62 Ordered Delivery PDUs may not arrive in the order of they have been transmitted Maintaining ordered delivery of data may be achieved through PDU unique identifiers IDs are assigned sequentially in increasing order modulo a Max_Number The value of Max_Number may need to be twice the maximum number of outstanding PDUs Wrap-around problem

63 Flow Control Flow Control regulates the amount of data transfer between sender and receiver Stop-and-Wait, simplest form of flow control Sliding-window, a more efficient credit based mechanism

64 Error Control Error control guards against loss of damage of data and control information Most techniques involve error detection and retransmission Frame Check Sequence (FCS) is used to detect errors

65 Addressing Addressing is a difficult concept that covers a number of issues Addressing level Addressing scope Connection identifiers Addressing Mode

66 Addressing Level Addressing level refers to the level at which an entity is named Typically, a unique address is associated with an end system Network level address (IP address, ISO NSAP) Within a host, applications are assigned unique identifiers

67 Addressing Scope Network level addresses are global in scope Global non-ambiguity Synonyms are permitted Global applicability Global addresses can be identified in any system Addresses above the network level need not be unique globally

68 Addressing Concept Host A Host B Service Access Point (SAP) Port TCP IP Network Access Protocol Physical Logical Transport Connection Global Network Address Router TCP IP Network Access Protocol Physical IP Net 1 Net 2 NAP 1 NAP 2

69 Connection Identifiers In connection oriented mode, it is desirable to use connection names instead of global addresses Routing, connection names can be used to identify a route Reduced overhead, as connection names are shorter than physical network addresses Multiplexing, data can be transmitted over multiple connections simultaneously Use of state information enables such functions as flow control and error control

70 Addressing Mode Most commonly, an address refers to a single system, port, SAP Unicast address Addresses may identify multiple simultaneous recipients Multicast address More generally, addresses may identify all recipients Broadcasting

71 Multiplexing Multiplexing refers to the fundamental concept of sharing a logical or physical system resource among multiple higher level entities Link, CPU, buffer, are typical shared resources

72 Multiplexing and Protocol Connections Lower Level Connection One-to-One Multiplexing Upper Level Connection Upward Multiplexing Downward Multiplexing

Transmission Services Depending on the service, different Quality-of-Services (QoS) are provided Connection Connectionless Service Reliable Message Stream Reliable Byte Stream Unreliable

73 Transmission Services Depending on the service, different Quality-of-Services (QoS) are provided Connection Connectionless Service Reliable Message Stream Reliable Byte Stream Unreliable Connection Unreliable Datagram Acknowledged Datagram Request-Reply Example Sequence of Pages Remote Login Digitized Voice Electronic Regular Mail Electronic Registered Mail Database Query

74 Architectural Design Issues For Next Generation Networks

75 Network Design Principle Layering Revisited Layering is a key concept to implementing communication protocols. Layering provides: Modularity Ease of abstraction Reuse Key design decision: what functionality to put in each layer? Network Design

76 Network Design Guide the organization and assignment of functions within the system Impose a structure on the design space rather than solve a particular design problem The structure provides a basis for analysis of tradeoffs and a rational for design choices

77 Datagrams vs. Virtual Circuits Best Effort vs. Reliable service Can the network be totally trusted? Where should reliability belong?

78 Statefull vs. Stateless Connection oriented networks require full state management Establishment of a new connection causes a state change in the switch A direct side effect, fate sharing Fate of the end-to-end connection depends on the state of intermediate nodes Connectionless network are stateless Simple and more robust Only task required is to maintain routing tables

79 Internet Design Principles To achieve robustness, the emphasis on simplicity was a guiding principle of the Internet design. The result is a light network core that provides minimal packet-level forwarding service This minimalistic approach to network design was later justified and generalized into the so called end-to-end argument.

80 End-to-End Argument Making good a judgment about where to place functionalities in a complex system is what system design is all about. End-to-End argument states that a function, such as reliability or ordered delivery, should not be provided in the lower levels of the system unless it can be completely and correctly implemented at that level Functional redundancy can be allowed if performance optimization is sought Example: error control on a hop-by-hop basis

81 End-to-End Argument In essence, the argument states that functions placed at low levels of a system may be redundant or of little value when the cost of providing them at the low level is factored in A function is provided by a (sub)system only if it can be completely and correctly implemented within it What about performance? sometimes an incomplete version of the function provided by the communication system may be useful as a performance enhancement

82 Reliable File Transfer A Case Study Host A Host B Appl. Appl. OS OK OS Solution 1: make each step reliable, and then concatenate them Solution 2: end-to-end check and retry

83 Discussion Solution 1 may not be satisfactory What happens if the sender or/and the receiver misbehave? The receiver has to do the check anyway! In this case, full functionality can be entirely implemented at application layer; No need for reliability to be implemented at lower layers Is there any need to implement reliability at lower layers?

84 Discussion Yes, but only to improve performance Example: noisy network environment Consider the case of a high error rate communication network Reliable communication service at data link layer enhances performance Errors can be detected and corrected at the frame level

85 End-to-End Arguments Summary If a functionality can be totally supported at the application layer, don t implement it at a lower layer The application knows best what it needs Add functionality in lower layers iff: (1) it is used and improves performances of a large number of applications, and (2) it does not impact negatively the performance of other applications End-to-End argument has been instrumental in the design of the Internet

86 End-to-End Argument and Internet Design The IP network layer provides a simple, best effort, datagram service May not be reliable TCP supports reliable data delivery Performance enhancement for a variety of applications: Telnet, FTP, HTTP, etc Decision does not impact other applications, if UDP can be used Everything else is implemented at application level

87 Internet Architecture Application R O B U S T FRAGILE Internet Protocol Network Physical

88 Conclusion Introduction to Computer Networks Classes of Networks Switching Techniques Network Protocol Architectures Layering

ET4254 Communications and Networking 1

ET4254 Communications and Networking 1 Topic 9 Internet Protocols Aims:- basic protocol functions internetworking principles connectionless internetworking IP IPv6 IPSec 1 Protocol Functions have a small set of functions that form basis of