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Internet Applications Theory & Applications. Internet Application - Ibrahim Otieno - +254-0722-429297 SCI/ICT Building 2 nd Floor.

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Presentation on theme: "Internet Applications Theory & Applications. Internet Application - Ibrahim Otieno - +254-0722-429297 SCI/ICT Building 2 nd Floor."— Presentation transcript:

1 Internet Applications Theory & Applications

2 Internet Application - Ibrahim Otieno SCI/ICT Building 2 nd Floor Rm. 201

3 Error Reporting Mechanism Best-Effort Semantics and Error Detection IP defines ‘best-effort’ communication service Datagrams can be lost, duplicated, delayed or delivered out of order Nonetheless IP attempts to avoid errors and report problems when they occur Checksum used to detect transmission error Host creates datagram, includes a checksum on header and upon receipt, verified to ensure header is intact

4 Best-Effort Semantics and Error Detection Best-Effort Semantics and Error Detection In case of error, datagram discarded Receiver does not send error message to sender because it cannot trust source address in header Likewise, receiver does not forward datagram because cannot trust destination address in header and therefore discards damaged datagram

5 Problems less severe than transmission errors are reported In case some path on the internet is broken; datagram sent from a host to recipient cannot be delivered TCP/IP suite includes ICMP that IP uses to send such error messages ICMP required for standard implementation of IP These two protocols are co-dependent: IP uses ICMP to send error message, and ICMP uses IP to transport messages Internet Control Message Protocol (ICMP) Internet Control Message Protocol (ICMP)

6 Examples of ICMP error messages include: 1. Source Quench – Router sends whenever it has no more buffer space available for datagrams Source reduces transmission rate upon receipt 2. Time Exceeded – Sent in two cases: ◦ When router reduces ‘time to live’ field in a datagram to zero, it discards datagram and sends a time exceeded message ◦ Send by host if reassembly timer expires before all fragments from datagram arrives Internet Control Message Protocol (ICMP) Internet Control Message Protocol (ICMP)

7 3. Destination Unreachable – Sent when a router determines that datagram cannot be delivered to final destination 4. Redirect – Sent when host creates a datagram destined for a remote network and sends to wrong router and a router realizes that it should have been sent to different router 5. Parameter problem – One of the parameters specified in a datagram is incorrect Internet Control Message Protocol (ICMP) Internet Control Message Protocol (ICMP)

8 ICMP defines information messages that include: 1. Echo Request/Reply - Echo request message sent to ICMP software and ICMP software required to send an ICMP echo reply message The reply carries the same data as the request 2. Address Mask Request/Reply – Host broadcasts address mask request when it boots, and routers that receive request send address mask reply containing correct 32-bit subnet mask being used on the network Internet Control Message Protocol (ICMP) Internet Control Message Protocol (ICMP)

9 ICMP Message Transport ICMP uses IP to transport messages Router creates datagram and encapsulates ICMP message in datagram Datagram then sent by being encapsulated in frame for transmission ICMP messages created in response to datagram Either datagram has a problem or datagram carries an ICMP request message to which destination replies Both ways ICMP error message/reply sent to source

10 Datagram carry source address Router extracts source address from header of datagram and places it in destination header of datagram with ICMP message Datagrams with ICMP message forwarded like other datagram except if a an error is encountered, no error message is sent Avoids internet congestion with error message ICMP Message Transport

11 Ping uses ICMP echo request/reply messages When invoked, sends IP datagram containing ICMP echo request message to specified destination and waits for reply If no reply, retransmits request and if no reply arrives, declares that remote machine not reachable ICMP software on remote machine replies to echo request ICMP Message Transport

12 We can summarize that ICMP includes both error and informational messages ICMP is integrated with IP: ICMP encapsulates messages in IP for transmission and IP uses ICMP to report problems ICMP Message Transport

13 TCP: Reliable Transport TCP: Reliable Transport The need for Reliable Transport Reliability is fundamental in computers Example, programmer application sending data to I/O device does not have to verify that it is intact Application relies on underlying computer system for reliable transfer (no data loss, duplication or delivery out of order) Applications using internet require that too Internet must provide same semantics as a conventional computer system i.e. no data loss, duplication or delivery out of order

14 The Translation Control Protocol The Translation Control Protocol Reliability achieved by transport protocol; Applications interact with it to send/receive data In TCP/IP, TCP provides reliable transport service TCP solves a difficult problem well – better than other general-purpose protocols Consequently, most internet applications built to use TCP

15 The Translation Control Protocol The Translation Control Protocol From application perspective, TCP has seven major features: 1.Connection Orientation –provides connection-oriented service in which applications request a connection to a destination, then use it to transfer data 2.Point-To-Point Communication – Each TCP connection has exactly two end points 3.Complete reliability – guarantees that data sent will be delivered exactly as sent, with no data missing or out of order 4.Stream Interface–application sends continuous sequence of bytes across connection

16 5. Full Duplex communication – allows data to flow in either direction at any time TCP can buffer data, making it possible for application to send data then continue with computation while data being transferred 6. Reliable Connection Startup – requires both applications agree to new connection; packets used in previous connections will not interfere with new connection 7. Graceful Connection Shutdown - application program open connection, send data, then request connection be shut down gracefully The Translation Control Protocol The Translation Control Protocol

17 In summary, TCP provides a completely reliable connection-oriented, full-duplex stream service Allows two applications to form a connection, send data in either direction and then terminate the connection TCP connection started reliably and terminated gracefully The Translation Control Protocol The Translation Control Protocol

18 End-to-End Service and Datagrams End-to-End Service and Datagrams TCP an end-to-end protocol; provides connection from host to remote application Applications request TCP to form a connection, send, receive data, and close connection Connections provided by TCP are virtual TCP software module on two machines exchange messages to achieve illusion of a connection TCP uses IP to carry messages Datagram arrive on destination, IP passes to TCP

19 TCP uses IP to carry messages, IP doesn’t read TCP treats IP as packet communication system connecting hosts at two endpoints and IP treats TCP message as data to be transferred Fig below - internet with two hosts and router illustrating relationship between TCP & IP End-to-End Service and Datagrams End-to-End Service and Datagrams

20 Some problems in communication are: unreliable delivery and computer reboot Two communicating applications using TCP can lose, duplicate, delay or deliver data out of order Messages must be unambiguous, or duplicate messages will be accepted from old connection & interfere with new connection Computer reboot poses challenge to TCP Protocol should reject packets from previous reboot Achieving Reliability Achieving Reliability

21 Packet Loss and Retransmission Packet Loss and Retransmission TCP uses variety of techniques for reliability A common technique is retransmission Sender compensates for packet loss by implementing a retransmission scheme Receiver sends acknowledgement to the sender TCP starts timer on sending and if timer expires before acknowledgement, the sender retransmits How long TCP should wait before retransmitting? Time for LAN and WAN different Complicated by bursts of data that cause congestion, causing delays

22 Packet Loss and Retransmission Packet Loss and Retransmission To summarize, delay for data to reach destination and acknowledgement to return depends on: ◦ traffic in internet ◦ distance to destination TCP allows multiple applications to communicate with multiple destinations concurrently Traffic conditions affect delay, TCP must handle a variety of delays that can change rapidly

23 Adaptive Retransmission Adaptive Retransmission Before TCP most protocols used fixed timeout Designers realized that this would not operate well for internet thus chose to make it adaptive TCP monitors current delay connection, and adapts retransmission timer to accommodate changing conditions TCP estimates round-trip delay, measuring time needed to receive response and records time TCP generates sequence of round-trip estimates and uses statistical function to produce weighted average

24 In addition to weighted average, TCP keeps an estimate of variance and uses linear combination of estimated mean and variance as a value for retransmission Adaptive retransmission helps TCP react quickly when delay: ◦ increases following burst of packets ◦ returns to a lower value after a temporary burst Adaptive Retransmission Adaptive Retransmission

25 Buffers, Flow Control and Windows Buffers, Flow Control and Windows When connection established, each host allocates buffer to hold data and send size to other end As data arrives, receiver sends acknowledgement, which also specify remaining buffer size Amount of buffer space available at any time is called window, and notification that specifies size called window advertisement If receiver reads data as fast as it arrives, will send positive window advertisement If sender operates faster than receiver data will fill receiver’s buffer and advertise a zero window. Sender that receives zero window must stop sending until receiver advertises positive window

26 Three-Way Handshake Three-Way Handshake TCP uses a 3-way handshake – 3 messages 3-way exchange is necessary and sufficient to ensure unambiguous agreement despite packet loss, duplication and delay TCP uses term synchronization segment to describe messages used to create a connection And to describe messages in a 3-way handshake finish segment used to close a connection.

27 Three-Way Handshake Three-Way Handshake Figure below illustrate 3-way handshake used to close connection

28 As other messages, TCP retransmit lost SYN or FIN segments Handshake guarantees TCP will not open or close connection until both ends have interacted 3-way handshake for creating connection requires each end to generate random 32-bit number If application tries to establish new connection after reboot, TCP chooses new random number Pair of applications can use TCP to communicate, close connection then establish new connection without interference from duplicate or delayed packets Three-Way Handshake Three-Way Handshake

29 Congestion Control Congestion Control Packet loss or long delays more likely to be caused by congestion than hardware failure Protocol that retransmit can worsen congestion by injecting additional copies of a message Excessive retransmission, entire system can reach state of congestion collapse (traffic jam). TCP uses packet loss as measure of congestion, and responds by reducing rate of retransmission TCP starts congestion control if a message is lost Instead of retransmitting data to fill the receiver’s window size, TCP begins by sending a single message containing data

30 Congestion Control Congestion Control If acknowledgement arrives without loss, TCP doubles data sent and sends two more messages If acknowledgements arrive for those two, TCP sends four more and so on Exponential increase continues until TCP is sending half of receiver’s advertised window then slows down rate of increase By backing off, TCP is able to alleviate congestion Scheme avoids retransmissions to a congested internet, helping prevent congestion collapse.

31 Internet Routing Internet Routing Static vs. Dynamic Routing IP routing divided into two: static and dynamic Routes are static if they do not change Static routing table loaded with values when system boots, and routes do not change unless an error detected Dynamic routing refers to system that can change routing table information over time

32 Internet Routing Internet Routing Static vs. Dynamic Routing Dynamic routing begins like static routing by loading routes into routing table on booting System also starts route propagation software Routing software interacts with routing software on other routers to learn about optimal routes The software then updates the local routing table to ensure that datagrams follow optimal routes

33 Static Routing in Hosts Static Routing in Hosts Static routing does not require routing software Does not consume bandwidth and no CPU cycles Relatively inflexible; it cannot accommodate network failures and changes in topology Static routing used mostly in cases where host has one network connection and a single router connects network to rest of Internet When application generates a datagram for computer on local net, an entry in routing table directs IP deliver datagram directly to destination When a datagram is destined for any other network, another entry in the table directs IP to send the datagram to router

34 Most PCs on internet use static routing When configuring IP software on a PC, user enters a network prefix, a subnet mask and the IP address of the default router The three items comprise the information needed to create the static routing table Host’s routing table contains two entries: one for network to which network attaches and a default entry that directs all other traffic to router Static Routing in Hosts Static Routing in Hosts

35 Internet Routing Internet Routing The routing table is illustrated in the figure below:

36 Dynamic Routing and Routers Most routers use dynamic routing Assume figure above corresponds to a customer of an ISP and traffic leaving customer’s site through router R 1 must travel to the ISP Because routes never change, routing table in R 1 can be static just as in the routing table of a host Static routing & default routes do not suffice for most routers

37 Dynamic Routing When two ISPs interconnect, both need to exchange routing information dynamically Consider the figure below:

38 Dynamic Routing and Routers Each of two routers belongs to a separate ISP Network labeled Net 2 belongs to corporate customer of ISP1 and network labeled Net 3 belongs to corporate customer of ISP2 Both routers know about network labeled Net 1 However, router R1 dos not know about Net 3 because there is no direct connection Similarly, router R2 does not know about Net 2

39 Dynamic Routing and Routers How can a router in one ISP have routes to networks owned by customers of another ISP? With only 3 networks, static routing suffices However, scheme does not scale to ISPs with thousands of customers Each time a new customer is added, the information must be passed to a person at the other ISP, who then updates the routing table Slow to accommodate failures/congestion For example, if a network interface card fails or router is accidentally unplugged, routing software needs to detect this and find alternative path

40 Example, assume that R1 & R2 in previous figure each run routing software; Routing software uses route propagation protocol to exchange routing information across Net 1 Software running on R2 installs a route to Net 2. If R2 crashes, the routing software in R1 will detect that Net 3 is no longer accessible and will remove the route from R1’s table. Later when R2 comes back on line, the routing software in R1 will determine that Net 3 is reachable again and will reinstall the route. Dynamic Routing and Routers

41 To summarize, each router runs routing software that learns about destinations other routers can reach, and informs other routers about destinations that it can reach The routing software uses incoming information to update the local routing table continuously Dynamic Routing and Routers

42 Client-Server Interaction Functionality of Application Software Physical connections & communication protocols useful but most useful function provided by application software Applications provide high-level services and determine how users perceive the capabilities of the internet e.g. , browsing, transfer of files

43 Client-Server Interaction Functionality of Application Software Applications determine formatting and access of information They also define symbolic names used to identify physical & abstract resources like computers, printers, mailboxes etc Symbolic names allow access or use of services without understanding low-level details

44 Functionality of an Internet Internet provides communication infrastructure but does not specify services offered Internet like a telephone system - provides ability to communicate but doesn’t know what computers do with communication service Network h/w and protocol s/w do not know when to initiate contact with remote computer Communication across internet requires pair of application programs to cooperate like placing a telephone call and it is received on other side

45 The Client-Server paradigm Network applications use client-server paradigm Server wait passively for contact and client initiate communication actively Client and server refer to two applications involved in a communication In general, client s/w has following characteristics: ◦ Is application that becomes client temporarily when remote access is needed, and performs computations locally

46 The Client-Server paradigm In general, client s/w has following characteristics: ◦ Invoked by user and executes for one session ◦ it runs locally on a user’s personal computer ◦ actively initiates contact with a server ◦ actively contacts one remote server at a time but can access many ◦ Does not require special h/w or special O/S

47 In contrast, server software: ◦ Is a special-purpose, privileged program dedicated to providing one service, but can handle multiple remote clients at same time ◦ invoked automatically at system boot ◦ runs on a shared computer ◦ waits passively for contact from remote clients ◦ Requires powerful h/w and a sophisticated O/S Server is not computer (with power h/w, O/S etc) on which the server process runs, the computer is referred to as server-class computer The Client-Server paradigm

48 Info flows in either or both directions For example, client may request a file and server sends copy or it may send a copy of a file to server for storage Client may send series of requests & server issues series of responses Like most application programs, a client and server use transport protocol to communicate A server-class computer can offer multiple services at same time to utilize resources; a separate server program needed for each service Requests, Responses and Direction of Data Flow

49 Client-Server Interaction From figure, Client or Server application interacts directly with transport layer protocol Transport protocol uses lower layer protocols to send and receive individual messages Thus a computer needs a complete stack of protocols to run either a client or server

50 Identifying a Particular Service TP provides way for client to specify service Mechanism assigns each service a unique id, and requires both client and server to use the id Server registers with local protocol software by specifying identifier for service it offers Client’s protocol specifies id for required service TP software on server’s machine uses the id to determine the server program to handle request

51 Identifying a Particular Service TCP uses a 16-bit integer value known as protocol port number (PPN) to identify services and client specifies PPN of desired service. A server computer can allow multiple copies of a server for a single service, i.e. support concurrency Concurrency is fundamental to client-server model of interaction Concurrent server offers service to multiple clients at same time, without requiring each to wait for clients to finish


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