1 Introduction to Computer Networks Foundation Ilam University Mozafar Bag-Mohammadi

2 Outline Introduction Statistical Multiplexing Inter-Process Communication Network Architecture Performance Metrics

3 Introduction Building a network to support diverse ranges of applications Distributed computing. Multimedia. Telecommunication. E-commerce, etc. What kind of technology do we need? Hardware. Software.

4 First Step What is computer Network? Different views. Differences from other networks, Its generality. What is requirements? Different perspective: Network provider Network designer Application programmer

5 Design goals Connectivity Scalability Simplicity For designers. Most importantly for users. Efficiency cost performance Support for common user services.

6 Building Blocks Nodes: PC, special-purpose hardware… hosts switches, routers and gateways Links: coaxial cable, optical fiber… point-to-point multiple access …

7 Switched Networks two or more nodes connected by a link, or  two or more networks connected by two or more nodes A network can be defined recursively as...

8 Strategies Circuit switching: carry bit streams Connection oriented. Original telephone network Dedicated resource. Packet switching: store-and-forward messages Connectionless (IP) or connection oriented (ATM) Shared resource. Packet switching is the focus of computer Networks.

9 Addressing and Routing Address: byte-string that identifies a node usually unique Routing: process of forwarding messages to the destination node based on its destination address Types of addresses unicast: node-specific broadcast: all nodes on the network multicast: some subset of nodes on the network

10 Multiplexing (resource sharing) Time-Division Multiplexing (TDM) Frequency-Division Multiplexing (FDM) L1 L2 L3 R1 R2 R3 Switch 1Switch 2

11 Statistical Multiplexing On-demand time-division Schedule link on a per-packet basis Packets from different sources interleaved on link scheduling fairness, quality of service Buffer packets that are contending for the link Buffer (queue) overflow is called congestion …

12 Packet Switching A node in a packet switching network incoming linksoutgoing links Node Memory

13 Inter-Process Communication Turn host-to-host connectivity into process-to-process communication regardless where the process are. Give a unified view and fill gaps between what applications expect and what the underlying technology provides. Host Application Host Applicatio n Host Channel

14 IPC Abstractions Request/Reply (Client-server) Guarantee delivering data, and might protect privacy and integrity. distributed file systems (NFS) digital libraries (web) File Transfer (FTP) Stream-Based- sequence or stream of bits. Video on demand: sequence of frames. Delay constrained, but can be fetched before hand. For example, a 1/4 NTSC with 352x240 pixels and 24 bit color. (352 x 240 x 24)/8=247.5KB Assuming 30 frame per second => 7500KBps = 60Mbps Video Conferencing- tightly delay bounded. VIC From Berkeley. Both application can tolerate packet loss.

15 Reliability in the network? What Goes Wrong in the Network? Bit-level errors (electrical interference), a bit is corrupted or a burst error. Packet-level errors (congestion) Messages are delayed Messages are deliver out-of-order Packet loss Third parties eavesdrop Link and node failures

16 Performance Metrics Bandwidth (throughput) data transmitted per time unit link versus end-to-end notation KB = 2 10 bytes Mbps = 10 6 bits per second Latency (delay) time to send message from point A to point B one-way versus round-trip time (RTT) components Latency = Propagation + Transmit + Queue Propagation = Distance / c (light speed) Transmit = Size / Bandwidth

17 Bandwidth versus Latency Relative importance Latency bounded- sending 1-byte by client, 1ms vs 100ms dominates sending a message on a 1Mbps or 100Mbps link Bandwidth Bounded- sending 25MB image: 1Mbps vs 100Mbps dominates 1ms vs 100ms delayed channel. Infinite bandwidth RTT dominates Throughput = TransferSize / TransferTime TransferTime = RTT + 1/Bandwidth x TransferSize 1-MB file to 1-Gbps link the same as 1-KB packet to 1- Mbps link.

18 Delay x Bandwidth Product Amount of data “in flight” or “in the pipe” Example: 100ms x 45Mbps = 560KB We are usually more interested in 2 times of this value Since it take RTT to hear from receiver.

19 Layering Use abstractions to hide complexity and decompose to manageable components. Abstraction naturally lead to layering Alternative abstractions at each layer Request/reply channel Message stream channel Application programs Hardware Host-to-host connectivity

20 Layering Advantages Modularity – protocols easier to manage and maintain Abstract functionality –lower layers can be changed without affecting the upper layers Reuse – upper layers can reuse the functionality provided by lower layers Disadvantages Information hiding – inefficient implementations

21 Protocols Building blocks of a network architecture, or layer abstraction. Each protocol object has two different interfaces service interface: operations on this protocol peer-to-peer interface: messages exchanged with peer Term “protocol” is overloaded specification of peer-to-peer interface module that implements this interface

22 Host 1 Protocol Host 2 Protocol High-level object High-level object Service interface Peer-to-peer interface Interfaces

23 Protocol Machinery Protocol Graph Nodes are protocols and edge are depends on. most peer-to-peer communication is indirect peer-to-peer is direct only at hardware level Digital library application Video application RRP MSP HHP File application Host 1 Digital library application Video application RRP MSP HHP File application Host 2

24 Protocol Machinery (cont) Multiplexing and Demultiplexing (demux key) Encapsulation (header/body) RRPData HHP Application program Application program Host 1 Host 2 Data RRP Data HHP Data RRP Data HHP

25 ISO OSI Reference Model ISO – International Standard Organization OSI – Open System Interconnection Started to 1978; first standard 1979 ARPANET started in 1969; TCP/IP protocols ready by 1974 Goal: a general open standard allow vendors to enter the market by using their own implementation and protocols

26 ISO Architecture Application Presentation Session Transport End host One or more nodes within the network Network Data link Physical Network Data link Physical Network Data link Physical Application Presentation Session Transport End host Network Data link Physical Telnet, FTP, TFTP MSB, integer Manage TCP streams Message, P2P(process) Packet, routing Frame, CRC Raw bit pipe The last 3 protocols are implemented in all elements in the Network.

27 Encapsulation A layer can use only the service provided by the layer immediate below it Each layer may change and add a header to data packet data

28 OSI Model Concepts Service – says what a layer does Interface – says how to access the service Protocol – says how is the service implemented a set of rules and formats that govern the communication between two peers

29 Physical Layer (1) Service: move the information between two systems connected by a physical link Interface: specifies how to send a bit Protocols: coding scheme used to represent a bit, voltage levels, duration of a bit Examples: coaxial cable, optical fiber links; transmitters, receivers

30 Datalink Layer (2) Service: framing, i.e., attach frame separators send data frames between peers others: arbitrate the access to common physical media ensure reliable transmission provide flow control Interface: send a data unit (packet) to a machine connected to the same physical media Protocols: physical layer addresses, implement Medium Access Control (MAC) (e.g., CSMA/CD)…

31 Network Layer (3) Service: deliver a packet to specified destination perform segmentation/reassemble others: packet scheduling buffer management Interface: send a packet to a specified destination Protocols: define global unique addresses; construct routing tables

32 Transport Layer (4) Services: provide an error-free and flow-controlled end-to-end connection multiplex multiple transport connections to one network connection split one transport connection in multiple network connections Interface: send a packet to specified destination Protocols: implement reliability and flow control Examples: TCP and UDP

33 Session Layer (5) Service: full-duplex access management, e.g., token control synchronization, e.g., provide check points for long transfers Interface: depends on service Protocols: token management; insert checkpoints,

34 Presentation Layer (6) Service: convert data between various representations Interface: depends on service Protocol: define data formats, and rules to convert from one format to another

35 Application Layer (7) Service: any service provided to the end user Interface: depends on the application Protocol: depends on the application Examples: FTP, Telnet, WWW browser

36 Internet Architecture Defined by Internet Engineering Task Force (IETF). Developed in mid 60s in the ARPANET project. No assumption about the network tech. … FTPHTTPNV TFTP TCP UDP IP NET 1 2 n

37 Internet Architecture Hourglass Design, IP is the focal point. Delivery is separated from end-to-end process channel. No restrict layering Application vs Application Protocol (FTP, HTTP) Network IP TCP UDP Application

38 OSI vs. TCP/IP OSI: conceptually define services, interfaces, protocols Internet: provide a successful implementation Application Presentation Session Transport Network Datalink Physical Internet Host-to- network Transport Application IP LAN Packet radio TCPUDP TelnetFTPDNS OSITCP