Computer Communication Networks - Unit 1 - PES University PDF

COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering...

COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Introduction What is a computer network? It is like a graph consisting of end-systems or hosts connected to one another via communication links and some packet switches. End-systems or hosts run applications which generate or receive data in the form of packets (i.e., collection of bits) A sequence of packet switches and communication links is called route or path A computer network is usually administered by one entity which configures and maintains the operation Examples of computer networks include home networks, enterprise networks, mobile networks, etc. Hosts 3 connect to the internet via ISPs Introduction What is internet? Internet is a computer network that interconnects billions of computing devices throughout the world. Internet is an interconnected architecture that provides services to distributed applications. How did it come about? History of the internet: DARPA, ARPANET, Packet switched networks, killer applications, TCP/IP, Ethernet, DNS, NSFNET program, IANA, ICANN, RFC, IETF and IESG, IAB 4 COMPUTER COMMUNICATION NETWORKS INTERNET - VISUALIZATION ▪ Internet is viewed as a graphical network that provides services to distributed applications. ▪ End systems are referred to as hosts because they host (that is, run) application programs. ▪ End systems are at the edge of the network. ▪ Hosts are further divided into two categories: clients and servers 5 COMPUTER COMMUNICATION NETWORKS INTERNET - VISUALIZATION Network edges are depicted as shaded regions Network core is highlighted in dark blue Host or End Systems 6 Introduction Notable inventions and inventors World wide web: Tim Berners Lee, MIT laboratory 1989-90 Email: Ray Tomlinson, BBN 1972 DNS: Paul Mockapetris, USC 1982 RFC: Stephen Crocker, UCLA 1969 Packet switching: Leonard Kleinrock, UCLA 1961 TCP/IP: Bob Khan and Vincent Cerf, DARPA and SRI 1972-73 Ethernet: Bob Metcalfe, Xerox PARC 1973 7 Introduction Who owns/controls the internet? ISP (Internet Service Provider) is a business entity or company which provides internet access to the end-systems in return for a subscription fee The place where end-systems connect to an ISP is referred to as point-of-presence (PoP). The number of PoPs (typically in 1000s) held by an ISP tells about its outreach in the internet. PoP consists of routers, link layer switches, MPLS and communication links. ISP examples: Telecom operators, Cable TV operators, Fiber (optic) operators 8 COMPUTER COMMUNICATION NETWORKS Application Layer ISP Architecture ISP architecture of wired network (copper, fibre) is shown here. Subscribers can be anyone of the following: ▪ Home, ▪ Enterprise, ▪ Community, ▪ Business The ISP architecture based on wireless networks like GSM, 4G etc 9 will be relatively different. COMPUTER COMMUNICATION NETWORKS Application Layer Internet Service Provider (ISP) ▪ End systems access the Internet through Internet Service Providers (ISPs). Different types of ISPs are given as : ▪ Residential ISPs such as local cable or telephone companies ▪ Corporate ISPs ▪ University ISPs ▪ ISPs that provide Wi-Fi access in airports, hotels, coffee shops, and other public places ▪ Cellular data ISPs providing mobile access to our smartphones and other devices ▪ The place where end users or access networks connect to an ISP is referred to as Point-of-Presence (PoP). ▪ PoP consists of routers, Ethernet switches and servers. 1 4 0 COMPUTER COMMUNICATION NETWORKS Application Layer Types of ISPs Regional ISP- usually provides internet National ISP- It is a business that access to a specific geographic area provides internet access nation wide 1 4 1 COMPUTER COMMUNICATION NETWORKS Network Core Global Transit ISP Customer Examples of Tier 1 ISPs: AT&T, Sprint, Verizon etc. in the US. Bharti, Reliance, Tata and VSNL are Tier-1 ISPs in India Examples of Regional ISPs: BSNL, Airtel, Vodafone, Reliance communications, etc. Organization of Network Core: ISP hierarchy 4 COMPUTER COMMUNICATION NETWORKS Network Core PoPs of various ISPs in the hierarchy may be interconnected using multi- homing, peering, and Internet exchange points (IXPs). Multihoming : Any ISP (except for Tier-1 ISPs) may choose to multi-home, that is, to connect to two or more provider ISPs. For example, an access ISP may multi-home with two regional ISPs, or it may multi-home with two regional ISPs and also with a tier-1 ISP. Similarly, a regional ISP may multi-home with multiple tier-1 ISPs. 13 Introduction PoP 14 Introduction 15 COMPUTER COMMUNICATION NETWORKS Network Core Peering: ▪ ISPs at the same level of the hierarchy can peer, that is, they can directly connect their networks together so that all the traffic between them passes over the direct connection rather than via upstream intermediaries. Internet Exchange Point (IXP): ▪ A third-party company can create an Internet Exchange Point (IXP) (typically in a stand-alone building with its own switches), which is a meeting point where multiple ISPs can peer together. 1 6 COMPUTER COMMUNICATION NETWORKS Application Layer Services Provided by ISP ▪ ISPs provide a variety of types of network access to the end systems. ▪ ISPs provide Internet access to content providers. ▪ ISPs that provide access to end systems must be interconnected: Lower-tier ISPs are interconnected through national and international upper-tier ISPs. Upper-tier ISPs consists of high-speed routers interconnected with high-speed fiber-optic links. 4 COMPUTER COMMUNICATION NETWORKS Application Layer End systems, packet switches, and other pieces of the Internet run protocols that control the sending and receiving of information within the Internet. The two major protocols are as follows: 1. Transmission Control Protocol (TCP) 2. Internet Protocol (IP) The IP protocol specifies the format of the packets that are sent and received among routers and end systems. The Internet’s principal protocols are collectively known as TCP/IP. 1 4 8 Introduction ISP hierarchy (contd.) 19 Introduction Revenue generation is as follows: End users pay access ISPs Access ISPs pay to regional ISPs Regional ISPs pay to Tier 1 ISPs Tier 1 ISPs may have several bilateral agreements to share resources such as bandwidth and routers Besides, content service providers can enter into bilateral agreements with an ISP at any stage ISPs which perform peering or multi-homing share some of their revenue based on equipment and resource utilization 20 Introduction How does the internet provide services for distributed applications (i.e., programs written in Java, C, etc.)? Distributed means that applications run independently on the hosts or end systems Messages are exchanged by the hosts using the internet socket interfaces of their respective applications Protocols define the format and the order of messages exchanged between two or more hosts Protocols also define the actions taken on the transmission and/or receipt of a message or other event Services (e.g., reliability, guaranteed rate) are provided by hardware or software associated with the devices 21 Classification by topography and functionality 22  THANK YOU Department of Electronics and communication engineering 2 3 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Network edge Computer networks that sit on the periphery of the internet constitute the network edge or the access network End-systems can be further classified as clients and servers Router which connect an access network to a regional/access ISP is referred to as gateway Access network nomenclature Based on size: Local area networks, home networks, wide area networks, etc. Based on topology: Tree, star, ring, bus, point-to-point. Based on physical media: Wired (DSL, Cable, Fiber to the home (FTTH)) or wireless 3 Network edge Home access networks The devices in the home are connected to the internet via a LAN or Wifi router Different physical media could be provided by different access ISPs to connect the home network with the internet 4 Network edge Home access networks An infrastructure handled by a telecom or cable or fiber operator General architecture is given below Home Central office Modem Local multiplexer Core Multiplexer Network Router Home Modem 5 Network edge Feature DSL based access Cable TV based FTTH based network access network access network Modem DSL modem Cable modem Optical modem Local multiplexer Splitter Fiber node Optical network terminator Central office DSL access Cable modem Optical line (CO) multiplexer terminating terminator (OLT) (DSLAM) system (CMTS) Downlink rates 12 Mbps [ITU DOCSIS 2.0 100 Mbps (cable 1999] and 24 standard 42.8 length based) Mbps [ITU 2003] Mbps Uplink rates 1.8 Mbps [ITU DOCSIS 2.0 30 Mbps (cable 1999] and 2.5 standard 30.7 length based) Mbps [ITU 2003] Mbps 6 Network edge Enterprise access network ISP can be telecom operator Built using Ethernet cables, switches and hubs Ethernet switches are preferred over routers in a LAN Routers are used for separating the network into subnets 7 Network edge Wireless networks: Classified according to radio access technologies Spread spectrum, frequency hopping, random access, polling methods, etc. More complex compared to wired access networks Packet losses and time varying wireless channel characteristics Wireless networks can be WiFi-based or cellular-based Wireless networks are usually supported by telecom ISPs Span of wireless networks can be few meters to several kilo meters Wireless networks have undergone tremendous evolution especially with the exploding data requirements of the users 8 Network edge Satellite access networks: Remote end systems get access to the internet via satellite links Implemented when other access networks are not feasible Has lowest data rates among access networks The delays are higher. It depends on the distance between the satellite and the users and the type of satellite Types of satellites: geostationary satellites and low-earth orbiting (LEO) satellites 9  THANK YOU Department of Electronics and communication engineering 1 0 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Network core Also known as backbone network Consists of high speed routers and high speed links (Gigabit Ethernet/optical fibers) Cisco NCS6000 router 3 Network core Network core is part of the internet which is composed of high-speed packet switches and high- speed communication links Network core is constructed using the interconnection of ISPs The packet switches (routers) perform store and forward operation 4 Network core Traffic from access ISPs are aggregated using multiplexers Multiplexers are interconnected to more distant switches through a backbone network Network core follows mesh topology with lot of redundancy Some design problems in network core include: Satisfy delay and reliability constraints Routing Assigning capacity (Flow maximization problem) Cost improvement 5 Network core Routers in the network core perform switching Routers have several links on which packets arrive and depart Switching involves transfer of an incoming packet from one link to an appropriate outgoing link based on IP protocol The switching operation can be done by hardware and/or software Different types of switching performed in the network core Circuit switching Packet switching 6 Network core Circuit switching: Requires connection establishment before data transfer Resources are allocated by every intermediate switch/router between the source and destination hosts Resource example: Fixed link bandwidth, internal memory In telephony, when a path is established between the source and destination we can say a circuit is formed After data transfer, the circuit is closed by releasing the reserved resources at each intermediate router No waiting time and no loss of data at intermediate routers Throughput reduces with resource sharing 7 Network core Circuit switching: A circuit in a link is established either by frequency division multiplexing (FDM) or time division multiplexing (TDM) 8 Network core TDM- Time division multiplexing: Time is divided into frames and frames into slots Slots in a frame are reserved for the transmitting hosts Each slot ends with a guard time to prevent ISI Duration of frame, slot, guard time are fixed 9 Network core FDM- Frequency division multiplexing: Bandwidth is divided into channels All channels reserved for transmitting hosts in a fixed slot time Channel reservation done slot-by-slot-basis Channels separated by guard band to prevent adjacent channel interference 10 Network core Numerical #1: How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? All links are 1.536 Mbps Each link uses TDM with 24 slots/sec Guard time is equal to (1/8)th of the slot time 500 msec to establish end-to-end circuit 11 Network core The user needs one slot per frame Frame size =1 s Number of slots per frame = 24 Slot duration = 1/24 sec = 41.67 msec Effective transmission time per slot = 41.67 * 7/8 = 36.458 msec Number of bits transmitted by a user per frame (Nbs) = link rate * effective transmission time per slot = 1.536M * 36.458m = 56 kilobits Nbs is also bits per slot Number of frames needed to transmit (Nf) = file size/bits per slot = 640000/56000 = 11.42 frames = 12 frames (even if the fraction of a slot is required, the entire slot is meant for that user) Total delay = connection setup time + (Nf-1) * frame duration + 1 slot duration = 500m + 11 * 1sec + 1/24 = 11.0916 sec 12 Network core Numerical #2: How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? Available link rate is 1.536 Mbps Link rate is distributed across 10 channels of 200 kHz Guard band of 50 Hz is used 500 msec to establish end-to-end circuit 13 Network core The user needs one frequency channel per slot Total delay = connection setup time + transmission time = connection setup time + file size/link rate of one channel = 500 msec + 640000/0.1536M = 4.667 sec 14 Network core Packet switching: Data broken into smaller chunk called packets No reservation of resources Suited for bursty traffic Better link utilization Packets are stored in buffer and then forwarded one at a time Requires protocols for link access and reliable packet delivery 15 Network core Packet switching (contd.): Packets may suffer queuing delays and get lost at the routers This happens when rate of arrivals exceeds the rate of departure 16 Network core Packet switching versus Circuit switching – Case 1: Suppose users share a 1 Mbps link. Also suppose that each user alternates between periods of activity when a user generates data at a constant rate of 100 kbps, and periods of inactivity when a user generates no data. Suppose further that a user is active only 10 percent of the time. With circuit switching, 100 kbps must be reserved for each user at all times. For example, with TDM, if a one-second frame is divided into 10 time slots of 100 ms each, then each user would be allocated a one-time slot per frame. Thus, the circuit-switched link can support only 10 (= 1 Mbps/100 kbps) simultaneous users. 17 Network core Packet switching versus Circuit switching – Case 1: With packet switching, the probability that a specific user is active is 0.1. If there are 35 users, the probability that there are 11 or more simultaneously active users is approximately 0.0004. When there are 10 or fewer simultaneously active users (which happens with probability 0.9996), the aggregate arrival rate of data is less than or equal to 1 Mbps. When there are more than 10 simultaneously active users, then the aggregate arrival rate of packets exceeds the output capacity of the link, and the output queue will begin to grow. Thus, packet switching performs same as circuit switched TDM but serves more than three times the number of users. 18 Network core Packet switching versus Circuit switching – Case 2: Suppose there are 10 users and that one user suddenly generates one thousand 1,000-bit packets, while other users remain quiescent and do not generate packets. Under TDM circuit switching with 10 slots per frame and each slot consisting of 1,000 bits, the active user can only use its one-time slot per frame to transmit data, while the remaining nine-time slots in each frame remain idle. It will take 10 seconds Under packet switching, the active user can continuously send its packets at the full link rate of 1 Mbps, since there are no other users has packets for transmission. In this case, it will take 1 second 19  THANK YOU Department of Electronics and communication engineering 2 0 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Delay, loss and throughput The different delays that occur in packet switched transmission are depicted below 3 Delay, loss and throughput ❑ Processing delay: Time taken to inspect (or make) a packet at a packet switch (or source host). Range: Microseconds ❑ Queuing delay: Time spent by a packet in the queue before processing. Depends on the number of packets waiting ahead, traffic intensity and distribution of the arrival process. Range: Microseconds to milliseconds ❑ Transmission delay: Time taken to push a packet on to the link. Depends on length of the packet (L bits) and link rate (R bits/sec). Expressed as L/R ❑ Propagation delay: Time taken by a bit to travel over a link. Depends on the length of the link and the physical medium’s propagation speed (e.g., 2×108 to 3×108 m/s). 4  Delay, loss and throughput ❑ Traffic intensity versus queuing delay ❖ Suppose arrival rate is a packets per sec and departure rate is L/R seconds per packet, then traffic intensity is given by La/R ❖ Let buffer size be infinite ❖ When La/R < 1, every new packet sees an empty queue ❖ When La/R ≥ 1, queue starts to When buffer is finite build up and mean queuing and La/R ≥ 1, then delay could approach infinity packet losses occur 5 Delay, loss and throughput End-to-end delay (dend-end): The total time spent by a packet to travel from the source to the destination. End-to-end delay is the sum of the delays at the source, delays at each packet switch and the propagation delays on each communication link along the path. Delay at a packet switch equals the sum of queuing delay, processing delay and transmission delay Consider N–1 identical and uncongested routers between the source and destination. Let all N–1 links be identical. Let propagation delay on any link, transmission delay and processing delay at any router and source be denoted by dprop, dtrans and dproc respectively. What is the end-to-end delay? 6 Delay, loss and throughput Numerical #3: For the scenario given below, assume the queuing delay, propagation delay and processing delay to be negligible. Suppose packet length L = 7.5 Mb and link rate R = 1.5 Mbps. Calculate the end-to-end delay. 7 Delay, loss and throughput Throughput is the rate (bits/sec) at which the destination host receives the packets. Instantaneous throughput is the throughput at a given time instant whereas average throughput is throughput over the entire file transfer time (e.g., F/T where F is file size and T is file transfer time). Example: What is the maximum achievable throughput? 8 Delay, loss and throughput Example: What is the maximum achievable throughput in the following cases? Let the link rate in the access networks and the bottleneck link in the network core be R=10 Mbps 9  THANK YOU Department of Electronics and communication engineering 1 0 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2  Numerical 4: Consider a 3 Mbps link being shared by 10 users. Suppose we want to achieve maximum throughput using circuit switching. What should be the maximum data packet size for a user assuming 10% guard interval? Assuming a user wants to transmit 64000 bits of data how much time will he take to complete full data transmission? Solution: Throughput is maximized so link rate is divided equally among the users. So per user gets (R) 0.3Mbps. Slot time = 1/10 = 0.1s Transmission delay (dt) = 0.09 sec (i.e., excluding the 10% of the slot time). So the maximum packet size L = dt * R =0.3M * 0.09 = 27 kb To transfer 64000 bits of data, the user spends 64kb / 27kb = 2.37 sec 3  Numerical 4 (contd.): Consider a 3 Mbps link being shared by 10 users. Suppose packet switching is used. What is the maximum achievable throughput? Assuming every user is active only 10% of the time and transmits at a rate of 1Mbps. What is the probability of no queuing? Solution: Maximum achievable throughput is 3Mbps. However, user only transmits at the rate of 1Mbps. Probability of one user being active (p) is 0.1. As long as there are less than or equal to 3 active users, the rate of transmission will not exceed the link rate so no queuing occurs. Therefore, 4  Solution (contd.): Numerical 5: Suppose there is a 10 Mbps microwave link between a geostationary satellite and its base station on Earth. Every minute the satellite takes a digital photo and sends it to the base station. Assume a propagation speed of 2.4×108 m/s. What is the propagation delay of the link? What is the bandwidth-delay product, R · dprop? Let x denote the size of the photo. What is the minimum value of x for the microwave link to be continuously transmitting? Solution: Propagation delay is (36000 km)/(2.4×108 m/s) = 150 ms Bandwidth-delay product 1500 kb Time between photo transmission is 60s therefore, transmit 600 Mb5  Numerical 6: Consider the figure below where transmission delay is the only significant delay. Each link is 2Mbps. Suppose the number of links N is 3. Calculate the end to end delay for the two cases given below. Note that each switch is a store and forward switch. 1. If message of size 8 Mb is transferred without segmentation. 2. If the message is segmented into 800 packets of 10 kb length. Solution: 1. L = 8 Mb, End to end delay = 3 × L/R = 12 sec 2. L = 10 kb, End to end delay = 800 × 3 × L/R =12 sec What about throughput of the connection? 6 What if the links have different rates?  THANK YOU Department of Electronics and communication engineering 7 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Protocol layers and their service Data exchange between two hosts over a communication network is a complex task The complex task is divided into smaller sub-tasks Maintain simplicity for network devices Put burden on the hosts The sub-tasks are completed sequentially The entire process can be visualized as layers arranged top to bottom, where Each layer performs its own unique sub-task On the sender side, each layer waits till the above layer finished its sub-task On the receiver side, each layer waits till the below layer finished its sub-task 3 Protocol layers and their service Communication between two hosts requires the same layers to be implemented in both hosts The peer layers (i.e., sub-task in sender and its counterpart in the receiver) communicate with one other using formatted blocks of data that obey a set of rules or conventions known as a protocol Layers implement protocols in hardware or software Basics requirements of a Protocol: Syntax: Concerns the format of the data blocks Semantics: Includes control information for coordination and error handling Timing: Includes speed matching and sequencing 4 Protocol layers and their service Arranged vertically, the layers on the systems collectively constitute the protocol architecture Two types of protocol architecture were proposed TCP/IP model OSI model TCP/IP model or TCP/IP protocol suite Resulted from protocol research under ARPANET Consists of large collection of protocols issued as Internet standards issued by IAB It consists of 5 layers namely, Application layer, Transport (host-to-host) layer, Network layer (IP layer), Link layer (network access layer), Physical layer 5 Protocol layers and their service Application layer : Applications running on hosts generate/receive data Data is referred to as message A process initiates communication with another by sending a query/request Message is formatted according to the application layer protocol Messages can be big in size Applications can have QoS requirements 6 Protocol layers and their service Transport layer: Responsible for providing QoS for messages Performs multiplexing at the sender Performs demultiplexing at the receiver Maps each message to a corresponding process Appends a new header to each message Message plus header is called segment 7 Protocol layers and their service Network layer: Fragments segments into packets Moves packets hop-by-hop E.g., router to router Uses source and destination IP addresses Path between source host and destination host is discovered Appends a new header to each packet Packet plus header is called datagram 8 Protocol layers and their service Link layer: Pushes the packets onto a link Using link layer protocols Can forward frames using MAC address Appends a new header to the packet Packet plus header is called frame Provides synchronization at receiver Checks for errors in frame 9 Protocol layers and their service Physical layer: Provides physical interface between the host and the link Example: Modem and Ethernet card, wireless adapter Converts binary data into signals Performs modulation and demodulation Performs transmission, reception and filtering of signals 10 Protocol layers and their service Encapsulation happens before departure Decapsulation happens after arrival Encapsulation Decapsulation 11  THANK YOU Department of Electronics and communication engineering 1 2 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Principles of network applications Applications run on end-systems (e.g., computers, servers) Examples: Skype, Whatsapp, Apple Pay, Youtube, Netflix Application developers often build a pair of programs which are coded in C, Java or Python One program is referred to as client program while the other is referred to a server program (e.g., web browser and web server program) These programs are also referred to as processes From the application developer’s perspective, the network architecture is fixed and provides a specific set of services to applications. 3Services: Reliability, throughput, security, timing, etc. Application layer architectures Application architecture dictates how the application developer views the interaction between the applications running on the end-systems 4 Application layer architectures Client-server architecture Client initiates the process communication Server responds to requests from the clients Server is always ON Server is well defined (e.g., IP address) Server can handle concurrent connections Examples: Search engines, Internet commerce, Web- based email, Social media 5 Application layer architectures Peer-to-peer architecture Any host can send and receive data Hosts can join and leave the network any time Hosts allocate resources to help each other P2P architectures are self scalable Distributed algorithms are used for a) Maintaining state information and b) For file sharing Examples: Bit Torrent, Skype 6 Processes communicating Processes exchange messages with one another using the rules governed by the end-systems operating system How to read/write a message? When to read/write a message? 7 Transport layer services 8 Transport layer services Applications and the supported protocols 9 Application–layer protocols Application layer protocols define the following The types of messages exchanged, for example, request messages and response messages The syntax of the various message types, such as the fields in the message and how the fields are delineated The semantics of the fields, that is, the meaning of the information in the fields Rules for determining when and how a process sends messages and responds to messages 10  THANK YOU Department of Electronics and communication engineering 1 1 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Web and HTTP: Overview Web servers store objects embedded in HTML pages The primary object (i.e., HTML page) is called webpage Web applications communicate using the HTTP Client fetches a webpage using a web browser (aka client process) Client process sends a HTTP request message specifying the object requested (aka URL) Web server process sends a HTTP response message which may contain the requested object Web browser: Microsoft Edge, Google Chrome, etc. Web server: Apache, Microsoft Internet Information Server, etc. HTTP 3 is a stateless protocol Web and HTTP: Overview HTTP request-response behaviour What transport layer protocol is used? How many ways can the request and response happen? 1. Persistent TCP 2. Non-persistent TCP 4 Web and HTTP: Non-persistent Separate TCP connection to fetch each object (including base webpage) Assume negligible size for HTTP request message Total access delay per object = Transmission delay at the server + 2 × RTT Socket number of web server is 80 Used in HTTP/1.0 5 Web and HTTP: Persistent Compared to non-persistent connections, persistent HTTP we save total access time and the efforts in establishing TCP connections For each of these connections, TCP buffers must be allocated and TCP variables must be kept in both the client and server. In persistent HTTP connection, only one TCP connection is established (for base webpage) and all objects are fetched back-to-back Server closes connection after some specified time of inactivity Used in HTTP/1.1 (allows up to 6 parallel TCP connections) Used in HTTP/2 (includes multiplexing, message prioritization and server pushing) 6 Web and HTTP Numerical #7: Consider accessing the webpage ww.someSchool.edu/someDepartment/Schoolpage.html which contains two embedded objects. Suppose the Web server and client are connected by a long link of rate R. Let RTT denote the two way propagation delay. Suppose the length (bits) of the webpage and two objects are L1, L2 and L3 respectively. Suppose the HTTP request message is of negligible length and can be piggybacked with acknowledgements. Calculate separately, the total access delay under a persistent TCP connection and non- persistent TCP connections. Show the timing diagram. 7  THANK YOU Department of Electronics and communication engineering 8 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Web and HTTP: Message format HTTP Request message Entity body is empty (download) or non-empty (upload) 3 Web and HTTP: Message format Example-GET message: Request webpage www.gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark- file.html 4 Web and HTTP: Message format Example-GET message (contd.): Inspecting the raw data of the TCP segment 5 Web and HTTP: Message format Web server sends the response message which could have the requested object 6 Web and HTTP: Message format Example-HTTP response message: 7 Web and HTTP: Message format Example-HTTP response (contd.): 8  THANK YOU Department of Electronics and communication engineering 9 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Web and HTTP: Cookies Most commercial websites provide access through user identification. These special identities are called cookies 3 Web and HTTP: Web caching Web cache (aka proxy server) is a network entity that satisfies HTTP requests on the behalf of an origin web server Typically a web cache is purchased and installed by an ISP or an organization Web cache has its own disk storage and keeps copies of recently requested objects in this storage A user’s browser can be configured so that all of the user’s HTTP requests are first directed to the web cache A web cache reduces the infrastructure cost and access delay in large organizations Content delivery networks (CDN) provide web caching too! 4 Web and HTTP: Web caching When object is present on the web cache client simply fetches it; Otherwise web cache initiates a connection to the Origin server. The object is stored and forwarded to the client 5  THANK YOU Department of Electronics and communication engineering 6 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 DNS: Domain Name System ❑Defined in IETF documents RFC 1034 and 1035 ❑Why do we prefer using hostnames to IP addresses? ❑How to get the IP address corresponding to a host name? ❖gaia.cs.umass.edu → 128.119.245.12 ❑A distributed architecture of DNS servers ❑Unix machines running Berkeley Internet Name Domain (BIND) software ❑Uses UDP for transport layer protocol ❑Operates on port 53 of the DNS server 3 DNS: Domain Name System Example: Sending HTTP request to a web server 1st time TCP header: Dst. port 80 UDP header: Dst. port 53 Network layer header Dst. DNS server IP address Link layer header 4 Host A chooses a new source port number for each application DNS: Domain Name System Example: Sending HTTP request to a web server 1st time DNS server performs decapsulation and reads the DNS query. Then, it generates a DNS reply having the IP address of the web server DNS server encapsulates the reply in a UDP segment and passes it 5 DNS: Domain Name System Example: Sending HTTP request to a web server 1st time Upon receiving the DNS reply, the encapsulation of the TCP handshake (i.e., TCP connection request) segment resumes using the IP address obtained for the web server. This TCP segment is passed to the web server 6 DNS: Domain Name System Example: Sending HTTP request to a web server 1st time Web server replies with a TCP handshake (i.e., TCP connection grant) of its own from port 80 7 DNS: Domain Name System Example: Sending HTTP request to a web server 1st time Upon receiving the TCP handshake from the web server, the host performs encapsulation of the HTTP request and then sends it to the web server 8 DNS: Domain Name System Distribution of DNS servers 9 DNS: Domain Name System Hierarchy of DNS servers 10 DNS: Domain Name System Root DNS servers Root DNS servers are the first level of DNS servers which are contacted by the clients to query DNS resource records. http://www.root-servers.org/ offers a map view of the root DNS servers around the world The name, IP address and location of the root DNS servers can be obtained from the above link 13 root DNS servers (actually 247 servers) across the world are maintained by 12 independent organizations https://www.iana.org/domains/root/servers provides list of root server zones 11 DNS: Domain Name System Root DNS servers 12 DNS: Domain Name System TLD DNS servers TLD DNS servers maintain domain level information. Verisign Global Registry Services maintains the TLD servers for the com top-level domain, and the company Educause maintains the TLD servers for the edu top-level domain https://domainpunch.com/tlds/ gives list of TLD servers and their associated domains Authoritative DNS servers maintain various DNS records corresponding to the registered hosts Local DNS servers are proxy servers which reside in an access network They query the DNS hierarchy on behalf of the respective clients 13 DNS: Domain Name System ❑Summary of services: ❖Provides IP address for a given host name ❖Host aliasing ❖Mail server aliasing ❖Load distribution 14  THANK YOU Department of Electronics and communication engineering 1 5 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 DNS: Domain Name System What is queried? A resource record is queried Name can be host name or domain name Value can be host name or IP addresses Type maps Name and Value TTL gives the time to live for a record Type Name Value A Hostname IP address NS Domain Host name of Authoritative DNS CNAME Alias host name Canonical hostname MX Alias host name Canonical mail server name 3 DNS: Domain Name System How is a resource record queried? 4  Type MX query and response 5 DNS: Domain Name System DNS servers and the types of records they maintain Authoritative DNS server → Type A, MX Root DNS server → Type NS TLD server → Type A and NS Local DNS server → All types 6 DNS: Domain Name System 7 Iterative DNS query mechanism DNS: Domain Name System 8 Recursive DNS query mechanism DNS: Domain Name System How to update your website with DNS? Find a registrar Available at http://www.internic.net Registrars are authorized by ICANN Submit names and IP address of your primary authoritative DNS server and secondary DNS (if any) Registrar creates Type NS and Type A records One each for primary and secondary servers Registrar inserts these records into the TLD DNS server You can insert records into your authoritative DNS servers Type A records of your web servers Type A record and MX record of your mail server 9 DNS caching and vulnerabilities Caching Reduces network traffic Reduces delay in DNS response Vulnerabilities Denial of service attack Attackers are distributed Client cannot query to the DNS server as it is choked with DNS queries from attackers Spoofing Attackers mimic a client and send DNS queries Client is choked with DNS responses Man-in-the-middle attack Client-to-server message and/or server-to-client message is altered by malicious users Digital signatures can be used as a remedy 10  THANK YOU Department of Electronics and communication engineering 1 1 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Electronic email Email: Brief overview Interaction between user mailbox and user agent happens using POP3, IMAP or HTTP Every user’s mailbox is hosted on a unique mail server Outgoing messages in a 3 mailbox are handled by SMTP Electronic email Overview: Email message is composed by a sender using the user agent (e.g., web-browser or Outlook). The email is uploaded on to the mailbox of the sender using the Simple Main Transfer Protocol (SMTP) over TCP A mail server hosts the mailboxes of many clients A TCP connection is established between the mail servers of the sender and the recipient of the email message SMTP pushes (moves) the message from the sender to the recipient's mailbox (i.e., sender’s mail server to recipient’s mail server). The socket number is 25 The recipient pulls the message from his/her mailbox using mail access protocols in order to read it. 4 Electronic email SMTP is defined in RFC 5321 and is much older than HTTP SMTP is invoked by sender’s mail server Messages between mail servers are encoded in ASCII 5 SMTP Operation 6 SMTP Comparison with HTTP HTTP allows other encoding formats but SMTP strictly follows ACSII SMTP is a “push” type protocol while HTTP is “pull” type protocol SMTP does not distinguish between object types in its data exchange. 7  THANK YOU Department of Electronics and communication engineering 8 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Video Streaming and Content Distribution Networks On-demand video streaming constitutes about 80% of the internet traffic according CISCO Annual report 2020 Popular on-demand content providers include Netflix, Hotstar, Amazon, Youtube, etc. On-demand video streaming faces two main challenges Bandwidth bottlenecks in the end-to-end path between any server and client Video availability at the bit rate desired by the client Solutions to overcome the above challenges Dynamic adaptive streaming over HTTP (DASH) Content distribution network (CDN) 3 Video Streaming and Content Distribution Networks – Internet video A video is a sequence of images, typically being displayed at a constant rate (e.g., 24-30 images/sec) An uncompressed, digitally encoded image consists of an array of pixels Each pixel is encoded into a number of bits to represent luminance and colour Compression algorithms can achieve any bit rate desired Video quality Bit rate Resolution SD 800-1000 kbps 480p HD 1.2-2 Mbps 720p FHD 1.9-4.5 Mbps 1080p 4 UHD 10 Mbps 2160p Video Streaming and Content Distribution Networks – DASH Youtube was the earliest to adopt DASH Dynamic adaptive streaming refers to varying the video resolution (bit rate) in response to the changes in the available bandwidth at the client Multiple video resolutions are made available at the content server (low resolution ⇒ low bit rate) Each video for a given resolution has an associated URL and a set of chunks (e.g., 4 sec video fragment) A client makes a TCP connection to the content server and requests for the manifest file corresponding to the video A manifest file which provides a URL for each version along with 5 its bit rate Video Streaming and Content Distribution Networks – DASH Upon learning the available versions, it chooses the version of the chunk to request using HTTP GET This depends on the rate adaption algorithm and available bandwidth The content server sends the requested chunk using the HTTP response message The client’s application buffers the received chunks up to a threshold before play out DASH has to ensure that the chunks are maintained above the threshold of the receive buffer 6 Video Streaming and Content Distribution Networks – CDN The objective of DASH is to ensure the quality of experience for the client after a server is chosen The objective of CDN is to maintain the videos closer to the clients and resolve server assignment for video streaming The CDN is a distributed architecture of server clusters placed on which the contents are placed by a pull (Youtube) or push (Netflix) approach Enter Deep Bring Home Large number of small clusters Small number of large clusters Deployed in access ISPs (e.g., Deployed in IXPs (e.g., Akamai) Limelight) Challenge of maintenance and Challenge of delay and overhead 7 throughput Video Streaming and Content Distribution Networks – CDN Operation Content providers distribute the video (different versions) to the CDN company When a client wants to access a video on the content providers webpage, the DNS servers help locate the server cluster under the CDN, and locate the appropriate server Following this the TCP connection is established to the server and then HTTP based DASH takes over during streaming Example, let KingCDN distribute videos of Netcinemas Let a client accesses Transformers 7, bearing the URL http://video.netcinema.com/6Y7B23V, from NetCinema webpage. 8 See the sequence of operations next Video Streaming and Content Distribution Networks – CDN Operation 9  THANK YOU Department of Electronics and communication engineering 1 0 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Transport layer: Services Provides logical connection between the processes Here, logical communication means that the applications in the end-systems overlook the role of the underlying physical interfaces, switches, routers and communication links. Transport-layer protocols are implemented in the end systems but not in network routers. Breaks the application layer message into segments Performs multiplexing and de-multiplexing of segments inside a host Perform error detection and in-order assembly of segments Can provide QoS support for the applications 3 Transport layer: Services UDP Connectionless protocol Does not acknowledge transmitted segments No throughput regulation(i.e., could cause network congestion) No service guarantees Example applications? TCP Connection oriented protocol Adapts throughput according to network congestion Supports flow control at the receiving node Guarantees reliable data transfer under the unreliable network layer Example applications? 4 Transport layer: Sockets Each active application is associated by one or more sockets assigned by the operating system Sockets allow matching the transport layer segments to their respective sockets Sockets are used in multiplexing and demultiplexing of segments Analogy for sockets: You can perform various transactions with your bank account (application) through an ATM (end- system) and bank-side server (end-system). You have various options (sockets) such as balance enquiry, withdrawal, change of ATM PIN, etc. 5 Transport layer: Multiplexing and demultiplexing Every segment exchanged over process communication specifies two sockets The socket for the client process is referred to as source port The socket for the server process is referred to as destination port 6 Transport layer: Multiplexing and demultiplexing Multiplexing: Segments leaving different sockets in a end-system are interleaved so that the network layer can assign the source IP address Demultiplexing: Segments arriving from the network layer with the same destination IP address, corresponding to the end-system, are separated and delivered to respective sockets The multiplexing and demultiplexing requires further information in connection oriented transport where a server handles multiple simultaneous connections 7 Transport layer: Multiplexing and demultiplexing Connectionless transport (one way) 8 Socket examples Connection oriented transport (two-way) 9  THANK YOU Department of Electronics and communication engineering 1 0 COMPUTER COMMUNICATION NETWORKS Department of Electronics and Communication Engineering 1  COMPUTER COMMUNICATION NETWORKS UNIT 1: INTERNET ARCHITECTURE AND APPLICATIONS Department of Electronics and Communication Engineering 2 Connectionless transport: UDP User datagram protocol (UDP): Defined in RFC 768 Simple to implement No connection establishment No connection state Small packet header overhead (header is just 8 bytes long) Suited for applications which are not delay tolerant Suited for real time multimedia applications (e.g., Internet phone, real-time video conferencing, and streaming of stored audio and video.) Suited for applications where old messages have little meaning (e.g., DNS, RIP, SNMP) 3 Connectionless transport: UDP Segment structure: ❖ Source port is used for multiplexing and demultiplexing at the sender ❖ Destination port is used for multiplexing and demultiplexing at the receiver ❖ Length specifies message length plus header length in bytes ❖ Checksum (RFC1701) is used for error detection at the receiver Sender side: Split the segment into 16-bit numbers and sum them. Wrap around carry (if any). Take 1’s complement of the sum (call this UDP checksum) Receiver side: Recompute checksum including UDP checksum. If answer is all 1s then 4it means no error has occurred Connectionless transport: UDP Segment structure (contd.): Checksum example: Assume three16-bit words of the form 5  THANK YOU Department of Electronics and communication engineering 6