Internet is a network of networks.

Internet is very complex and ever changing, both in terms of its hardware and software components, as well as in the services it provides.

1.1.1 Nuts-and-Bolts components)

Hosts and End systems

Description

(Basic

hardware and

software

The public Internet is a worldwide computer network, that is, the network that connects millions of computing devices throughout the world. In Internet jargon, all of these devices are called hosts or end systems. Communication Links End systems are connected together by communication links. There are many types of communication links; coaxial cable, copper wire, fiber optics, and radio spectrum. Different links can transmit data at different rates, with the transmission rate of a link measured in bits/second. Packet Switches End systems are indirectly connected to each other through intermediate switching devices known as packet switches. A packet switch takes a chunk of information arriving at one of its incoming communication links and forwards that chunk of information on one of its outgoing communication links. Two most prominent types of packet switches are routers and link-layer switches. In the jargon of Internet, the chunk of information is called a packet. Path From the sending end system to the receiving end system, the sequence of communication links and the packet switches traversed by a packet is known as a route or a path through the network. Packet Switching The Internet uses a technique known as packet switching that allows multiple communicating end systems to share a path, or parts of a path, at the same time. ISPs End systems access the Internet through Internet Service Providers (ISPs). Protocols

End systems, packet switches, and other pieces of Internet, run protocols that control the sending and receiving of information within the Internet. The Transmission Control Protocol (TCP) and the Internet Protocol (IP) are the most important protocols in the Internet. The IP protocol specifies the format of the packets that are sent and received among the routers and end systems. The Internets principal protocols are collectively known as TCP/IP. Internet Standards Internet Standards are developed by the Internet Engineering Task Force (IETF). The IETF documents are called requests for comments (RFCs). Intranet There are many private networks whose hosts cannot exchange messages with hosts outside of the private network. These private networks are often referred to as intranets.

1.1.2 A Service Description (Networking Infrastructure)

The Internet allows distributed programs running on its end systems to exchange data with each other. These applications include Web surfing, instant messaging, audio and video streaming, Internet telephony, distributed games, peer-to-peer (P2P) file sharing, remote login, electronic mail etc. Services Internet Provides The Internet provides two services to its distributed applications: a connectionoriented service and connectionless service. The connection-oriented reliable service guarantees that data transmitted from a sender to a receiver will eventually be delivered to the receiver in order and its entirety. The connectionless service does not make any guarantees about eventual delivery.

1.1.3 What is a Protocol?

All activity in the Internet that involves two or more communicating remote entities is governed by a protocol. Protocols are running everywhere in the Internet. A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

1.2.1 End systems, Clients, and Servers

End systems (Hosts) In computer networking jargon, the computers connected to the Internet are often referred to as end systems, as they sit at the edge of the Internet. End systems are also referred to as hosts because they host application programs such as a Web browser program, a Web server program, an e-mail reader program, or an e-mail server program. Hosts are sometimes further subdivided into two categories: clients and servers. Client-Server Model A client program is a program running on one end system that requests and receives a service from a server program running on another end system. The Web, e-mail, file transfer, remote login, and many other popular applications adopt client/server model. Since a program typically runs on one computer and the server program runs on another computer, client/server Internet applications are distributed applications. Peer-Peer Model The P2P file-sharing applications in the users end system acts as both a client program and a server program. The program running in a peer acts as a client when it requests a file from another peer; and the program acts as a server when it sends a file to another peer. Internet telephony is one of examples of this model.

1.2.2 Connectionless and Connection-Oriented Service

Connection-Oriented Service Control packets are sent before the actual data for the establishment of connection. This process is called Handshaking process. Reliable data transfer; transmission of information without errors and in proper order. Acknowledgements and retransmissions. Flow control; makes sure that neither side of a connection overwhelms the other side by sending too much packets too fast. Congestion control; service helps preventing the Internet from entering the state of gridlock by forcing the end systems to decrease their rate of data transmission. Transmission Control Protocol (TCP) Applications using TCP include remote login, e-mail, file-transfer, and Web etc. Connectionless Service No handshaking like in connection-oriented service by sending control packets before actual data transferring. Since there is no handshaking process, data can be delivered soon.

Ideal for simple transaction-oriented applications. No reliability that the data transferred will be reach entirely or not. No provisions for flow control and congestion control. User Datagram Protocol (UDP) Applications using this protocol are Internet phone, video conferencing, and multimedia applications etc.

1.3.1 Circuit Switching and Packet Switching

Two fundamental approaches to build a network: circuit switching and packet switching. In circuit-switched networks, the resources needed along a path between the end systems are reserved for the duration of the communication. In packet-switched networks, these resources are not reserved. A sessions messages use the resources on demand and as consequence may have to wait for access to a communication link. Multiplexing in Circuit-Switched Networks A circuit in a link is implemented with either frequency-division multiplexing (FDM) or time-division multiplexing (TDM). With FDM, the frequency spectrum of a link is shared among the connections established across the link. This link dedicates a frequency band, known as bandwidth, to each connection for the duration of the connection. For a TDM link, time is divided into frames of fixed duration, and each frame is divided into a number of time slots. When the network establishes a connection across a link, the network dedicates one time slot in every frame to the connection. For TDM, the transmission rate of a circuit is equal to the frames rate multiplied the number of bits in a slot. Packet Switching Applications exchange messages in accomplishing their task. Messages can contain anything the protocol designer wants. Messages may perform a control function or can contain data such as an e-mail message, a JPEG image, or an MP3 audio file. In modern computer networks, the source breaks large messages into smaller chunks of data known as packets. Between source and destination, each of these packets travels through communication links and packet switches. Packets are transmitted over each communication link at a rate equal to the full transmission rate of the link. Store-and-Forward Transmission

Most packet switches use store-and-forward transmission at the inputs of the link. It means that the switch must receive the entire packet before it can begin transmit the first bit of the packet onto the outbound link. So this technique introduces store-and-forward delay at each switch on the path of packet. This delay is proportional to the length of the packet in bits. Store-and-forward delay = L (no. of bits) / R (bps) seconds.

Output Buffer Each packet switch has an output buffer (output queue), which stores the packets to be sent. If an arriving packet needs to be transmitted across a link but finds it busy, then it must have to wait in the output buffer. In this way, packets suffer queuing delay. Packet Loss In the case, that a packet has to wait in the output buffer and the buffer is filled completely with other packets to be transmitted later, then this causes packet loss. Either the arriving packet or one of the already-queued packets will be dropped. Packet Switching Versus Circuit Switching; Statistical Multiplexing Critics of packet switching: packet switching is not suitable for real-time services because of its variable and unpredictable end-to-end delays. Packet Switching proponents: packet switching offers better sharing of bandwidth than circuit switching. It is simpler, more efficient and less costly to implement than circuit switching. The data transferred on a link-layer switch is not transmitted in any fixed pattern; rather the bandwidth is shared on demand. It is called statistical multiplexing.

1.3.2 Packet Switched Networks: Datagram Networks and VirtualCircuit Networks

Two broad classes of packet-switched networks: datagram networks and virtualcircuit networks. They differ in whether their switches use destination addresses or so-called virtual-circuit numbers to forward packets toward their destination. Datagram Network Any network that forwards packets according to the host destination addresses is called datagram network. The routers in the Internet forward packets according to the host destination addresses; hence Internet is a datagram network.

Analogous to postal service. Each packet traversing the network contains the address of the packets destination in its header. Analogous to a car driver who does not use maps but instead prefers to ask for directions. In contrast with VC networks, datagram networks do not maintain connection-state information in their switches. Packet switches make forwarding decisions based upon the packets destination address. Virtual-Circuit Network Any network that forwards packets according to virtual-circuit numbers is called a virtual-circuit network. Examples: X.25, frame relay, ATM (asynchronous transfer mode). A virtual connection between a source and destination host. Setting up and maintaining this VC will involve not only the two end systems but each and every switch along the VCs source-to-destination path. A virtual circuit identifier (VC ID) will be assigned to a VC when a VC is first established between source and destination. The source and destination of a VC are only indirectly identified through the VC ID; the actual addresses of the source and destination are not needed to perform switching. Packet switching can be performed quickly. A switch in a VC network maintains state information for its ongoing connections. Each time a new connection is established across a switch, a new connection entry must be added to the switchs translation table; and each time a connection is released, an entry must be removed from the table.

1.4.1 Access Networks

Access networks are the physical link(s) that connect an end system to its edge router, which is the first router on a path from the end system to any other distant end system. Three categories of access networks: residential access, company access, and wireless access. Residential Access Residential access connecting home end systems into the network. One form: dial-up modem over an ordinary analog telephone (twisted-pair copper wire) line into a residential ISP. 56 kbps.

Another form is the broadband residential access. There are two types of broadband residential access: digital subscriber line (DSL) and hybrid fibercoaxial cable (HFC). DSL access is typically provided by a telephone company. Much higher rates as compared to the dial-up modems. DSL and HFC require special modems, called cable modems. HFC is a shared broadcast medium. Every packet sent by the head end travels downstream on every link to every home; and every packet sent by a home travels on the upstream channel to the head end. As the downstream and upstream channels are shared, a distributed multiple access protocol is needed to coordinate transmissions and avoid collisions. DSL transmission rate is dedicated rather than shared. Always on service provided to subscribers. Telephone calls and Internet facility at the same time. Company Access Local Area Network (LAN) is typically used to connect and end system to the edge router. Ethernet technology is the most prevalent access technology. 10 Mbps to 10 Gbps. It uses either twisted-pair copper wire or coaxial cable to connect a number of end systems with each other and with an edge router. Ethernet uses a shared medium, so that the end users share the transmission rate of the LAN. Switched Ethernet uses multiple twisted-pair Ethernet segments connected at a switch to allow the full transmission rate of an Ethernet to be delivered to different users on the same LAN simultaneously. Wireless Access In a wireless LAN, wireless users transmit/receive packets to/from a base station (wireless access point, WAP) within a radius of few tens of meters. The base station is typically connected to a wired Internet and thus serves to connect wireless users to the wired network. In wide-area wireless access networks, the base station is managed by a telecommunications provider and serves users within a radius of tens of kilometers. Wi-Fi provides a shared transmission rate of 11 Mbps.

1.4.2 Physical Media

A bit, when travelling from source to destination, passes through a series of transmitter/receiver pairs. For each transmitter-receiver pair, the bit is sent by

propagating electromagnetic waves or optical pulses across a physical medium. Examples: twisted-pair copper wire, coaxial cable, multimode fiber-optical cable, terrestrial radio spectrum, and satellite radio spectrum. Physical media fall into two categories: guided media and unguided media. With guided media, the waves are guided along a solid medium, such as fiber-optical cable, a twisted-pair copper wire, or a coaxial cable. With unguided media, the waves propagate in the atmosphere and in outer space, such as in a wireless LAN or a digital satellite channel. Twisted-Pair Copper WireLeast expensive and most commonly used. Consists of two insulated copper wires, each about 1 mm thick, arranged in a regular spiral pattern. Unshielded twisted pair (UTP) is commonly used for computer networks within a building, that is, for LANs. Data rates for LANs using twisted pair today range from 10 Mbps to 1 Gbps. Coaxial Cable Consists of two concentric copper conductors rather than parallel. High bit rates. Can be used as a guided shared medium. Fiber Optics An optical fiber is a thin, flexible medium that conducts pulses of light, with each pulse representing a bit. Can support ten or even hundreds of Gbps. Have very low signal attenuation up to 100 kilometers. Terrestrial Radio Channels Carry signals in electromagnetic spectrum. No need to install physical wire. Can penetrate walls, provide connectivity to a mobile user, and can potentially carry a signal for long distances. Classified into two groups: those that operate in local areas, ten to a few hundred meters coverage; and those that operate in wide area, spanning tens of kilometers. Satellite Radio Channels Links two or more Earth-based microwave transmitters/receivers, known as ground stations. The satellite receives transmissions on one frequency band, regenerates the signal using a repeater, and transmits the signal on another frequency. Can provide transmission rates in the gigabits per second range. Two kinds of satellites used: geostationary satellites and low-altitude satellites.

Geostationary satellites permanently remain above the same spot on Earth. These are used at the backbone of the Internet. Low-altitudes satellites are placed much closer to the Earth and do not remain permanently above one spot on Earth.

1.5 ISPs and Internet Backbones

In the public Internet, access networks situated at the edge of the Internet are connected to the rest of the Internet through a tiered hierarchy of ISPs. Tier-1 ISPs At the very top of the hierarchy is a relatively small number of so-called tier-1 ISPs. Their link speeds are often 622 Mbps or higher, with the large tier-1 ISPs having links in 2.5 to 10 Gbps range. Directly connected to each of the other tier-1 ISPs Connected to a large number of tier-2 ISPs and other customer networks International in coverage Known as Internet backbone networks No group officially sanctions tier-1 status Tier-2 ISPs Typically has regional or national coverage and is connected to only a few of the tier-1 ISPs. A tier-2 ISP is said to be a customer of the tier-1 ISP to which it is connected, and the tier-1 ISP is said to be a provider to its customer. A provider ISP charges its customers ISP a fee depending on the transmission rate of the link connecting the two. At the bottom of the hierarchy are the access ISPs. Connection between ISPs When two ISPs are directly connected to each other, they are said to peer with each other. Within the ISPs network, the points at which the ISP connects to other ISPs are known as Points of Presence (POPs). A POP is simply the group of one or more routers in the ISPs network at which routers in other ISPs or in the networks belonging to the ISPs customers can connect. ISPs are often interconnect at Network Access Points (NAPs), each of which can be owned and operated by either some third-party telecommunications company or by an Internet backbone provider.

The trend is for tier-1 ISPs to interconnect with each other at private peering points, and for tier-2 ISPs to interconnect with other tier-2 ISPs and with tier-1 ISPs at NAPs.

1.6 Delay and Loss in Packet-Switched Networks

The most important delays: nodal processing delay, queuing delay, transmission delay, and propagation delay. Processing Delay The time required to examine the packets header and determine where to direct the packet is part of the processing delay. It also includes the time needed to check for bit-level errors in the packet. Queuing Delay At the queue, the packet experiences queuing delay as it waits to be transmitted onto the link. Depends on the number of already queuing packets waiting for their transmission. If there is no packet in the queue, the queuing delay is zero. Transmission Delay For a packet of size L bits and let the link rate be R bits/second. It is also known as store-and-forward delay is denoted by L/R. It is the amount of time to transmit the entire packet into the link. Propagation Delay The time required to propagate a bit from the beginning to the next switch is the propagation delay. The bit propagates at the propagation speed of the link. Ranges from 2 x 10^8 to 3 x 10^8 meters/sec. It is the distance between the two routers divided by the propagation speed. Nodal Delay d(proc) = processing delay, d(queue) = queuing delay, d(trans) = transmitting delay, d(prop) = propagation delay, then d(nodal) = node-to-node delay is: d(nodal) = d(proc) + d(queue) + d(trans) + d(prop) Comparing Transmission and Propagation Delay The transmission delay is the amount of time required for the router to push out the packet; it is a function of the packets length and the transmission rate of the link, but has nothing to do with the distance between the two routers.

The propagation delay is the time it takes a bit to propagate from one router to the next; it is a function of the distance between the two routers, but has nothing to do with the packets length and the transmission rate of the link. 1.6.2 Queuing Delay and Packet Loss If a is the average rate at which packets arrive at the queue (in packets/sec), R is the transmission rate (in bits/sec), and L is the size of all the packets (in bits). Then, the ratio La/R is called the traffic intensity. If traffic intensity > 1, then the queue size will increase continuously and the delay will approach infinity. So always design your system so that traffic intensity < 1. Packet Loss If a packet arrives to find a full queue in a switch, with no place to store such a packet, a router will drop that packet, that is, the packet will be lost. The fraction of lost packets increases as the traffic intensity increases. End-to-End Delay Assuming there are (N-1) routers between the source and destination hosts, no queuing delay, processing delay to be d(proc), transmitting delay to be L/R bits/sec, and the propagating delay to be d(prop), then, end-to-end delay is accumulated to d(end-end) = N ( d(proc) + d(trans) + d(prop) ) 1.7 Protocol Layers and Their Service Models The protocols of various layers taken together are called the protocol stack. The Internet protocol stack consists of five layers: the physical layer, the data link layer, the network layer, the transport layer, and the application layer. Application Layer Residence of network applications and their application-layer protocols. HTTP, SMTP, FTP etc. Transport Layer Transports the application-layer messages between the client and server sides of an application. TCP and UDP. Transport layer message is referred to as a segment. Network Layer Responsible for moving network-layer packets known as datagrams from one host to another.

It has a protocol (Internet Protocol, IP) that defines the fields in the datagram as well as how the end systems and routers act on these fields. It also has routing protocols that determine the routes that datagrams take between sources and destinations. Link Layer To move a packet from one node to the next node in the route, the network layer relies on the services of the link layer. Examples: Ethernet and Point-to-point Protocol (PPP). Link layer packets are referred to as frames. Physical Layer The job of the physical layer is to move the individual bits within a frame from one node to the next. Examples: twisted-pair copper wire, single-mode fiber optics etc.