•INTERNETWORKING
•Introduction and Overview
Motivation for Internetworking
Internetworking – technology forms the basis for the internet by accommodating multiple diverse underlying hardware technologies , providing a way to interconnect the networks, and defining a set of communication conventions that the network used to interoperate
•The TCP/IP Internet
•DARPA (Defence Advanced Research Projects Agency)- It includes a set of standards that specify the details of how computers communicate
- a set of conventions for interconnecting networks and forwarding traffic
-Officially named TCP/IP Internet Protocol Suite and commonly referred to as TCP/IP
-TCP/IP can be used to interconnect a set of networks within a single building, within a physical campus or among a set of campuses
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-TCP/IP forms the base technology for the global internet that connects over 650 million individuals in homes, schools, corporations and government labs in virtually all populated areas
•Internet Services
•Focus on standards called protocols . Protocols like TCP and IP provide the syntactic and semantic rules for communication.
- They contain details of the message formats, describe how a computer responds when a message arrives and specify how a computer handles errors or other abnormal conditions.
- communication protocol allows to specify or understand data communication independent on detailed knowledge of any particular vendor’s network hardware
- Hiding the low-level details of communication helps improve productivity in several ways.
1. Programmers deal with higher-level protocol abstractions, they do not need to learn or remember as many details about a given hardware configuration. Thus, they create new programs quickly
2. Since programs are not restricted to a particular computer architecture or a particular network hardware, the programs need not be changed when computers or networks are replaced or reconfigured
3. As application programs built using higher-level protocols independent of underlying hardware, they can provide direct communication between an arbitrary pair of computers and no need to build an application software for each type of computer or each type of network.
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•Application Level Internet Services
•Internet application programs exhibit a high degree of interoperability (refers to the ability of diverse computing systems to cooperate in solving computational problems).
•World wide web(www)- The web allows users to view documents that contain text and graphics and to follow hypermedia links from one document to another
•E-Mail- Allows user to compose a message and send a copy to individuals or groups and also read memos that they have received. One reason for popularity is : protocol makes delivery reliable
•File Transfer- The file transfer application allows users to send or receive a copy of a data file.
•Remote Login and Remote Desktop- allow users sitting at one computer to connect to a remote machine as local
ie keystrokes are sent to the remote machine and display from the remote machine appears on the user’s screen (either in a window or across the entire screen)
•Network-Level Internet Services
•At the network level, an internet provides 2 broad types of services that all application programs use.
•Connectionless Packet Delivery Service: TCP/IP internet forwards small messages from one computer to another based on address information carried in the message.
- It forwards each packet separately , so does not guarantee reliable, in-order delivery
- It maps directly onto the underlying hardware, the connectionless service is extremely efficient.
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•Reliable Stream Transport Service:
- It allows an application on one computer to establish a “connection” with an application on another computer and then to send a large volumes of data across the connection as it were a permanent, direct connection.
- The underneath communication protocol divide the stream of data into small messages and send them one at a time, waiting for the receiver to acknowledge reception
- It handles problems like recover automatically from transmission errors, lost packets or failures of intermediate switches along the path between the sender and receiver
•The features of TCP/IP
§Network Technology Independence: independent of any particular brand or type of hardware . TCP/IP protocol define unit of data transmission called a datagram and specify how to transmit datagrams on a particular network
§Universal Interconnection- allows any pair of computers to communicate. Each computer is assigned an address that is universally recognized throughout the network. Each datagram carries the address of the source and destination
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§End-to-End Acknowledgements: provide acknowledgements between the original source and ultimate destination instead of successive machines along the path.
§Application Protocol Standards: In addition to the basic transport-level services like reliable stream connections), the TCP/IP protocols include standards for common applications including e-mail, file transfer and remote login
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•Network Technologies
Two Approaches to Network Communication
1. Connection oriented(sometimes called circuit switched)
2. Connectionless(sometimes called packet switched)
•Connection oriented networks
•It operates by forming a dedicated connection or circuit between two points eg. Traditional telephone system
Adv: Guaranteed capacity of the network(ie. Once a circuit is established, no other network activity decrease the capacity of the network)
Disadv: Cost(circuit costs are fixed and independent of use)
•Connectionless networks
•Data to be transferred across the network is divided into small pieces called packets and are sent at a time.
•A packet contains few hundred bytes of data, carries identification that enables the network h/w to know how to send to destination
•The network h/w delivers to the destination and the s/w reassembles them into a single file again
Adv
1. Multiple communication among computers can proceed concurrently with intermachine connections shared by all pairs of computers that are communicating
2. low cost and performance(since multiple computers shares network bandwidth, fewer connections are required)
Disadv: less network capacity (ie, as activity increases, network becomes overloaded, computers should wait before they send additional packets
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Packet switched networks are divided into
1. WAN 2. LAN
WAN(called long haul networks):
1. provide communication over long distances
2. Do not limit the distance spanned
3. operates at slower speeds(1.5Million bits per sec to 2.4 billion bits per sec) than LANs and have much greater delay( from few milliseconds to several tenths of a second) between connections
4. a network contains a series of complex computers called packet switches interconnected by long-distance communication lines.
5. The size of the network can be extended by adding a new switch and another communication line. Attaching a user’s computer to a WAN means connecting it to one of the packet switches
•LAN
•Provide high speed connections among the computers and operates between 100Mbps and 10Gbps
•Spans a small area like single building or a single campus. So, lower delays( as short as few tenths of a millisecond or as long as 10 milliseconds) than WANs
•Each computer contains a device known as Network Interface Card(NIC) that connects the machine directly to the network.
•Network Hardware Addresses
•Each hardware technology defines an addressing mechanism that computers use to specify the destination for a packet
•Every computer connected to network is assigned an unique address usually integer
•Every packet contains destination address field that appears in the same location in all packets
•Each hardware technology specifies how computers are assigned addresses
eg. h/w specifies the no: of bits in the address as well as the location of the destination address field in the packet.
•Ethernet Technology
•Ethernet is the name given to popular packet-switched LAN technology invented at Xerox PARC in the early 1970s.
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•Xerox Corporation, Intel Corporation and Digital Equipment Corporation standardized ethernet in 1978.
•IEEE released a compatible version of the standard using the standard number 802.3
•Ethernet has become the most popular LAN technology and the original wiring is coaxial cable has been out-of-date
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•The current technology is twisted pair Ethernet and it allows computers to access ethernet using conventional unshielded copper wires(similar to wires used to connect telephones)
Adv of twisted pair wiring (category 5 cable) is, it reduces costs and is easier to install than coaxial cable
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•The first twisted pair Ethernet known as 10Base-T operated at 10Mbps exactly like orginal ethernet
•A set of 8 wires(4 pairs) is used to connect each computer to a central Ethernet hub or switch
•Only 4 of 8 wires are used : one pair carries data from the computer to the hub and another pair carries data from the hub to the computer
•Hub is an electronic device and it is small box that resides in a wiring closet. A connection between a hub and a computer must be less than 100m long. Hub requires power and allow authorized personnel to monitor and control its operation over the network.
•Ethernet capacity
•The initial twisted pair Ethernet design operated at the same rate as the original Ethernet : data could be transmitted at 10 million bits per second
•High bandwidth(amount of data that can be carried from one point to another within a given period of time) carries heavy traffic loads
•In the late 1970s when ethernet was standardized , a LAN operating at 10 MBps has more sufficient capacity
•For more CPU speeds , ethernet operating at 10Mbps and so ethernet becomes bottleneck
•ATM
•Name given to connection oriented networking for both LAN and WAN
•Designed to permit high-speed data switching
and it is achieved thru special-purpose h/w and s/w
•ATM network consists of one or more high speed switches that each connect to computers and to other switches
•The lowest layers of ATM network uses fixed size frames called cells
•Each cell is exactly of fixed size and ATM switch h/w can process cells quickly
•Each ATM cell is only 53 octets long(5 octets of header followed by 48 octets of data
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•Connection Oriented Networking
•First computer interacts with the switch Connection (telephone call) to specify the destination and waits until switch finds the path thru the n/w and establishes the connection
•Once the connection succeeds , ATM switch chooses an identifier for the connection and passes it to the computer for sending/recieing cells
•When it finishes using an ATM connection, a computer again communicates with the ATM switch to break the connection
•The switch then disconnects the two computers(like hanging up the telephone call)
•Identifiers used for connection can be recycled for the another connection
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•Wide area point-to-point networks
•From TCP/IP point of view, any communication
system that connects exactly two computer is known as point-to-point network.
eg Dialup IP- Dialup internet access
dialup modem in a computer is used to place a phone call to another modem(two modem use audio tones to send data)
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Internetworking Concept and Architectural Model
•Application-Level Interconnection
•Two approaches to hide network details
1. using application programs to handle heterogenity (application-Level Interconnection)
2. Operating system (application-Level Interconnection)
•Each heterogeneous network interconnections provided uniformity through application programs called application gateways
•Application programs executing on each computer understands the details of the network connections for that computer, and interoperates across those connections with application programs on other computers
eg Electronic mail systems
Disadv: 1. Limited and cumbersome communication (If n/w grows to 100s or 1000s n/ws , no one can build all the necessary application programs)
2. No guaranteed reliable communication( intermediate program fails, both source and destination unable to detect or control the problem)
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•Network-Level Interconnection
•It provides the mechanism to deliver packets of data from their original source to ultimate destination without using intermediate application programs.
Switching small packets has many advantages
1. It maps directly onto the underlying n/w hardware making it extremely efficient
2. n/w level interconnection separates data communication activities from application programs, permitting intermediate computers to handle traffic without understanding the applications that are sending or receiving.
3. Using n/w connections keeps the entire system flexible (making possible to build general purpose communication facilities)
4. Allows Network managers to add new technologies by modifying or adding a single piece of new network level software while application programs remains unchanged.
The key to design universal network-level interconnections can be found in an abstract communication system concept known as internetworking
•Properties of the Internet
•All computers in the internet share a universal set of machine identifiers as names or addresses
•Users or application programs need not want to understand the details of hardware interconnections to use the internet
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•Internet Architecture
•How are networks interconnected to form an internetwork?
Internet Gateways or Internet routers are computers that interconnect two networks and pass packets from one to the other
•Interconnection Through IP routers
•In actual internet containing many networks and routers, each router need to know the topology of the internet beyond the networks to which it connects.
Ex. 3 networks interconnected by two routers(R1 sends packets to either n/w1 or N/w2
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•Routers use the destination network, not the destination computer when forwarding a packet.
•Routers have little disk storage and modest main memories
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Internet Protocol: Connectionless Datagram Delivery(IPv4)
•Internet Architecture and Philosophy
•TCP/IP internet provides three sets of services
1. Application Services
2. Reliable Transport Services
3. Connectionless packet delivery Services
Adv: 1. Robust and adaptable
2. Possible to replace one service without disturbing others
•Connectionless Delivery System
•It is packet delivery system
•It is defined as unreliable(delivery is not guaranteed), best-effort(internet makes earnest attempt to deliver packets i.e it will not discard packets suddenly, unreliability occurs when resources are exhausted or n/w fails), connectionless( each packet is treated independently from all others)
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•Purpose of Internet Protocol
•The protocol that defines the unreliable, connectionless delivery mechanism is called Internet Protocol(current version is 4 referred as IPV4
•IP provides three definitions
•Defines the basic unit of data transfer (specifies format of all data across the internet)
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•IP software performs forwarding function(choosing a path to sent to a packet)
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•It includes the set of rules that embody the idea of unreliable delivery
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•The IPv4 Datagram
•The basic transfer unit of internet is Internet Datagram (IP datagram or datagram)
•A datagram is divided into
1. Header –contains source address ,destination address and type field that identifies the contents of the datagram
2. Data Areas-contains arbitrary data
•Datagram Format
•VERS- 4bit field contains the version of the IP protocol that was used create a datagram and to verify b/w the sender/receiver to agree upon the format of the datagram. All IP software checks the version field before processing a datagram whether it matches format of the s/w it expects. If no match, reject datagrams
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•HLEN: Header Length Field also 4 bits gives the header length measured in 32-bit words
All fields in the header is fixed except for IP OPTIONS and PADDING
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•TOTAL LENGTH(16 bits long)-Gives length of the IP datagram measured in octets(both header and data)
The maximum possible size of the datagram is 65,535 octets
Datagram type of Services(TOS) and Differentiated Services
SERVICE TYPE(8 Bit)- how the datagram should be handled that accommodate a set of differentiated services
codepoint- first 6 bits, maps to the service (can define 64 services)
unused - 2 bits
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•The first 3 bits used for precedence(high number-high priority, low number- low priority)
•When last 3 bits are zero, the precedence bits define eight broad classes of service
eg codepoint values xxx000
x – either 1 0r 0
The if the last 3 bits are zero, the router would map the codepoint with precedence 6 or 7 for high prority classes
The 64 codepoint values are divided into 3 administrative sets as
POOL CODEPOINT ASSIGNED BY
1 xxxxx0 Standard Organization
2 xxxx11 Local or experimental
3 xxxx01 Local or experimental
•Datagram encapsulation
•Carrying one datagram in one network frame is called encapsulation
•The datagram is sent in the data area of the frame and travels from one machine to another machine thru the underlying physical network
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•Datagram size, Network MTU and Fragmentation
•Packet switching technology places a fixed upper bound on the amount of data that can be transferred in one physical frame
eg \Ethernet limits the maximum transfer unit(MTU) sizes can be larger than 1500 or smaller . Other 128
TCP/IP software chooses a convenient initial datagram size and arranges to divide large datagrams into smaller pieces when it needs to traverse that has a small MTU
The small pieces into which datagram is divided are called fragments( has same format as original datagram) and the process of dividing a datagram is known as fragmentation
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Fragmenation usually occurs at the router. The fragment size is chosen
That each fragment is sent in single frame. IP represents the offset of data in multiples of eight octets(so fragment size is chosen as multiple of eight)
Reassembly of fragments
Fragments are reassembled at the destination.
Disadv: 1.Reassembling at destination leads to inefficiency
2. If any fragments are lost, datagram cannot be reassembled.The receiver machine starts timer as it recieves an initial fragment . If the timer expiers before all the fragments arrive, then reciever discards the pieces without processing the datagram.
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•Fragmentation Control
•Three fields
INDENTIFICATION, FLAGS, FRAGMENT OFFSET
INDENTIFICATION: Contains a unique integer that identifies the datagram .
-It allows the destination to know which fragment belong to which datagram
- IP sofware keeps a global counter in memory and increments each time a new datagram is created and assigns the result as the datagram’s IDENTIFICATION field
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FRAGMENT OFFSET:
For a fragment, this field specifies the offset in the original datagram of the data being carried
•Time to Live(TTL)(fail-safe mechanism)
•It specifies how long(seconds) the datagram is allowed to remain in the internet system .
•When a datagram is injected into the internet , it sets a max time that it should survive
•Routers or host that processes datagrams must decrement TTL as time passes and removes the datagram from the internet when the timer expires and sends a error message to the source.(It avoids traveling forever)
•Other Datagram header fields
•PROTOCOL: specifies the format of the data area and specifies which high-level protocol was used to create the message in the data area
•SOURCE IP ADDRESS/DESTINATION IP ADDRESS:32-bit address
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