Computer Networks Lecture 01-02 Networks F24 PDF

Computer Networks Lecture 01: Introduction Prof. Dr. Sahar M. Ghanem Associate Professor Computer and Systems Engineering Department Faculty of Engineering, Alexandria University Course Outline Textbook: Computer Networking: A Top-Down Approach, 8th ed., Kurose...

Computer Networks Lecture 01: Introduction Prof. Dr. Sahar M. Ghanem Associate Professor Computer and Systems Engineering Department Faculty of Engineering, Alexandria University Course Outline Textbook: Computer Networking: A Top-Down Approach, 8th ed., Kurose & Ross Grading: attendance & participation: 5-7 assignments & quizzes: 40 midterm: 15 final: 40 Join with code: l42tcab Course materials and discussions will be on MS Teams. TA: Eng. Mohamed Essam Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 2 Chapter 1 Computer Networks and the Internet Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 3 Outline What Is the Internet? The Network Edge The Network Core Delay, Loss, and Throughput in Packet-Switched Networks Protocol Layers and Their Service Models Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 4 What Is the Internet? Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 5 Overview We’ll learn that the Internet is a network of networks, and we’ll learn how these networks connect with each other. We’ll use the public Internet, a specific computer network, as our principal vehicle for discussing computer networks and their protocols. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 6 A Nuts-and-Bolts Description (1/3) The Internet is a computer network that interconnects billions of computing devices throughout the world. All of these devices are called hosts or end systems. By some estimates, there were about 18 billion devices connected to the Internet in 2017, and the number will reach 28.5 billion by 2022. End systems are connected together by a network of communication links and packet switches. A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 7 A Nuts-and-Bolts Description (2/3) The transmission rate of a link measured in bits/second (bps). The two most prominent types of packet switches in today’s Internet are routers and link-layer switches. The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system is known as a route or path through the network. End systems access the Internet through Internet Service Providers (ISPs). Each ISP is in itself a network of packet switches and communication links. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 8 A Nuts-and-Bolts Description (3/3) End systems, packet switches, and other pieces of the Internet run protocols. The Transmission Control Protocol (TCP) and the Internet Protocol (IP) are two of the most important protocols in the Internet. The Internet’s principal protocols are collectively known as TCP/IP. Internet standards are developed by the Internet Engineering Task Force (IETF). The IETF standards documents are called requests for comments (RFCs). There are currently nearly 9000 RFCs. Other bodies also specify standards for network components, e.g. the IEEE 802 LAN Standards Committee. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 9 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 10 A Services Description The Internet is an infrastructure that provides services to distributed applications. Internet applications run on end systems—they do not run in the packet switches in the network core. End systems attached to the Internet provide a socket interface that specifies how a program asks the Internet infrastructure to deliver data to another end system. The Internet provides multiple services to its applications. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 11 What Is a Protocol? It takes two (or more) communicating entities running the same protocol in order to accomplish a task. In a human protocol, there are specific messages we send, and specific actions we take in response to the received reply messages or other events (such as no reply within some given amount of time). Much of this course is about computer network protocols. 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. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 12 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 13 The Network Edge Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 14 End Systems The Internet’s end systems include desktop computers (e.g., desktop PCs, Macs, and Linux boxes), servers (e.g., Web and e-mail servers), and mobile devices (e.g., laptops, smartphones, and tablets). Furthermore, an increasing number of non-traditional “things” are being attached to the Internet as end systems. End systems are also referred to as hosts because they host (that is, run) application programs. Hosts are sometimes further divided into two categories: clients and servers. Most of the servers reside in large data centers. For example, as of 2020, Google has 19 data centers on four continents, collectively containing several million servers. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 15 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 16 Access Networks Home Access: DSL, Cable, FTTH, and 5G Fixed Wireless Access in the Enterprise (and the Home): Ethernet and WiFi Wide-Area Wireless Access: 3G and LTE 4G and 5G Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 17 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 18 Home Access: DSL (1/2) When digital subscriber line (DSL) is used, a customer’s telco is also its ISP. A DSL modem uses the existing telephone line to exchange data with a digital subscriber line access multiplexer (DSLAM) located in the telco’s local central office (CO). The residential telephone line carries both data and traditional telephone signals simultaneously, which are encoded at different frequencies: A high-speed downstream channel, in the 50 kHz to 1 MHz band A medium-speed upstream channel, in the 4 kHz to 50 kHz band An ordinary two-way telephone channel, in the 0 to 4 kHz band Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 19 Home Access: DSL (2/2) On the customer side, a splitter separates the data and telephone signals arriving to the home and forwards the data signal to the DSL modem. On the telco side, in the CO, the DSLAM separates the data and phone signals and sends the data into the Internet. Hundreds or even thousands of households connect to a single DSLAM. Downstream transmission rates of 24 Mbs and 52 Mbs upstream rates of 3.5 Mbps and 16 Mbps the newest standard provides for aggregate upstream plus downstream rates of 1 Gbps DSL is designed for short distances between the home and the CO. located within 5 to 10 miles of the CO. (1 mile=1.6 km) Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 20 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 21 Other Home Access Cable Internet access makes use of the cable television company’s existing cable television infrastructure. It is often referred to as hybrid fiber coax (HFC) and is a shared broadcast medium. downstream bitrates of 40 Mbps and 1.2 Gbps, and upstream rates of 30 Mbps and 100 Mbps. Fiber to the home (FTTH) provides even higher speeds is that can potentially provide Internet access rates in the gigabits per second range. 5G fixed wireless promises high-speed residential access, without installing costly and failure-prone cabling from the telco’s CO to the home. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 22 Access in the Enterprise/Home: Ethernet A local area network (LAN) is used to connect an end system to the edge router. Ethernet users use twisted-pair copper wire to connect to an Ethernet switch. With Ethernet access: users typically have 100 Mbps to tens of Gbps access to the Ethernet switch servers may have 1 Gbps to 10 Gbps access Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 23 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 24 Access in the Enterprise/Home: WiFi Wireless LAN access based on IEEE 802.11 technology (WiFi) is now just about everywhere. A wireless LAN user must typically be within a few tens of meters of the access point. 802.11 today provides a shared transmission rate of up to more than 100 Mbps. e.g. home network a roaming laptop, multiple home appliances, as well as a wired PC a base station (WiFi access point) that communicates with the wireless PC and other wireless devices in the home a home router that connects the wireless access point, and any other wired home devices, to the Internet. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 25 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 26 Wide-Area Wireless Access: 3G and LTE 4G and 5G Mobile devices employ the same wireless infrastructure used for cellular telephony to send/receive packets through a base station that is operated by the cellular network provider. A user need only be within a few tens of kilometers (as opposed to a few tens of meters) of the base station. 4G wireless provides real-world download speeds of up to 60 Mbps. 5G will provide even higher-speed. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 27 Physical Media A bit traveling from source to destination, passes through a series of transmitter-receiver pairs and it is sent by propagating electromagnetic waves or optical pulses across a physical medium. Physical media fall into two categories: guided media and unguided media. With guided media, the waves are guided along a solid medium, such as a fiber-optic 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. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 28 The Network Core Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 29 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 30 Packet Switching (1/3) In a network application, end systems exchange messages with each other. The source breaks long messages into smaller chunks of data known as packets. Each packet travels through communication links and packet switches. routers and link-layer switches a router will typically have many incident links Most packet switches use store-and-forward transmission at the inputs to the links. That is it must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 31 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 32 Packet Switching (2/3) Each packet consisting of 𝑳 bits; Transmission rate is 𝑹 bits/sec. Sending one packet from source to destination over a path consisting of 𝑵 links (𝑵 − 𝟏 routers) each of rate 𝑹, the delay 𝒅𝒆𝒏𝒅−𝒕𝒐−𝒆𝒏𝒅 = 𝑵 𝑳/𝑹 ignoring propagation delay For each attached link, the packet switch has an output buffer/queue, which stores packets that the router is about to send into that link. In addition to the store-and-forward delays, packets suffer output buffer queuing delays that depend on the level of congestion in the network Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 33 Packet Switching (3/3) The amount of buffer space is finite, therefore packet loss will occur—either the arriving packet or one of the already-queued packets will be dropped Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 34 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 35 Forwarding Tables and Routing Protocols How does the router determine which link it should forward the packet onto? Every end system has an address called an IP address that has a hierarchical structure. The destination’s IP address is in the packet’s header. Each router has a forwarding table. A router uses a packet’s destination address to index a forwarding table and determine the appropriate outbound link. the Internet has a number of special routing protocols that are used to automatically set the forwarding tables. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 36 Circuit Switching Traditional telephone networks are examples of circuit-switched networks. In circuit-switched networks, the resources needed along a path (buffers, link transmission rate) are reserved for the duration of the communication session between the end systems. When two hosts want to communicate, the network establishes a dedicated end-to-end connection between the two hosts. The sender can transfer the data to the receiver at the guaranteed constant rate. The Internet makes its best effort to deliver packets in a timely manner, but it does not make any guarantees. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 37 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 38 Multiplexing in Circuit-Switched Networks A circuit in a link is implemented with either frequency-division multiplexing (FDM) or time-division multiplexing (TDM). e.g. FM radio stations use FDM to share the frequency spectrum between 88 MHz and 108 MHz, with each station being allocated a specific frequency band. For a TDM link, time is divided into frames of fixed duration, and each frame is divided into a fixed number of time slots. Circuit switching is wasteful because the dedicated circuits are idle during silent periods. Establishing end-to-end circuits is complicated and requires complex signaling software to coordinate the operation of the switches along the end-to-end path Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 39 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 40 Example #1 How long it takes to send a file of 640,000 bits from Host A to Host B over a circuit-switched network. All links use TDM with 24 slots and have a bit rate of 1.536 Mbps. It takes 500 msec to establish an end-to-end circuit. Answer Each circuit has a transmission rate of (1.536 Mbps)/24 = 64 kbps. It takes (640,000 bits)/(64 kbps) = 10 seconds to transmit the file Adding the circuit establishment time, giving 10.5 seconds to send the file. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 41 Packet Switching Versus Circuit Switching Packet switching is not suitable for real-time services (telephone calls and video conference calls). Packet switching offers better sharing of transmission capacity and is simpler, more efficient, and less costly to implement. Circuit switching pre-allocates use of the transmission link regardless of demand, with allocated but unneeded link time going unused. Packet switching on the other hand allocates link use on demand. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 42 Example #2 (1/2) Suppose users share a 1 Mbps link. A user is active only 10 percent of the time Each user alternates between periods of activity, when a user generates data at a constant rate of 100 kbps. With circuit switching, 100 kbps must be reserved for each user at all times. Circuit-switched link can support only 10 (= 1 Mbps/100 kbps) simultaneous users. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 43 Example #2 (2/2) 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 active users, users’ packets flow through the link without delay. Because the probability of having more than 10 simultaneously active users is minuscule in this example, packet switching provides essentially the same performance as circuit switching, but does so while allowing for more than three times the number of users. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 44 Example #3 (1/2) 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 be 10 seconds before all of the active user’s one million bits of data has been transmitted. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 45 Example #3 (2/2) In the case of packet switching, the active user can continuously send its packets at the full link rate of 1 Mbps. All of the active user’s data will be transmitted within 1 second. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 46 A Network of Networks (1/4) Over the years, the network of networks that forms the Internet has evolved into a very complex structure. Much of this evolution is driven by economics and national policy, rather than by performance considerations. One naive approach would be to have each access ISP directly connect with every other access ISP. (hundreds of thousands access ISPs all over the world) Network Structure 1: interconnects all of the access ISPs with a single global transit ISP. (costly global ISP) Network Structure 2 (two-tier hierarchy): hundreds of thousands of access ISPs and multiple global transit ISPs. (competing global transit providers as a function of their pricing and services) Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 47 A Network of Networks (2/4) Network Structure 3 (multi-tier hierarchy): In any given region, there is a regional ISP to which the access ISPs in the region connect. Each regional ISP then connects to tier-1 ISPs that do not have a presence in every city in the world. (a dozen tier-1 ISPs) Each access ISP pays the regional ISP to which it connects, and each regional ISP pays the tier-1 ISP to which it connects. There may be a larger regional ISP to which the smaller regional ISPs in that region connect. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 48 A Network of Networks (3/4) Network Structure 4: To build a network that more closely resembles today’s Internet, we must add to the hierarchical Network Structure 3 points of presence (PoPs): a group of one or more routers in the provider’s network where customer ISPs can connect into the provider ISP (not at the access level) multi-homing: connect to two or more provider ISPs peering: pair of nearby ISPs at the same level of the hierarchy can directly connect their networks together Internet exchange points (IXPs): a third-party company can create an IXP that is a meeting point where multiple ISPs can peer together Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 49 A Network of Networks (4/4) Network Structure 5: builds on top of Network Structure 4 by adding content-provider networks. e.g.: The Google data centers are all interconnected via Google’s private TCP/IP network, which spans the entire globe but is nevertheless separate from the public Internet. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 50 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 51 Delay, Loss, and Throughput Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 52 Overview the physical laws introduce delay and loss as well as constrain throughput throughput is the amount of data per second that can be transferred between end systems The packet suffers from several types of delays at each node along the path processing delay (microseconds or less) queuing delay (microseconds to milliseconds) (depend on the number of earlier- arriving packets) transmission delay is 𝑳/𝑹 (packet length 𝐿 bits; 𝑅 transmission rate in bps) (amount of time required to push all of the packet’s bits into the link) propagation delay (𝒅/𝒔 distance between two routers divided by the propagation speed) (depends on the physical medium) (milliseconds) 2 × 108 meters/sec to 3 × 108 meters/sec Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 53 Nodal Delay 𝒅𝒏𝒐𝒅𝒂𝒍 = 𝒅𝒑𝒓𝒐𝒄 + 𝒅𝒒𝒖𝒆𝒖𝒆 + 𝒅𝒕𝒓𝒂𝒏𝒔 + 𝒅𝒑𝒓𝒐𝒑 The contribution of these delay components can vary significantly. e.g. LAN:𝒅𝒑𝒓𝒐𝒑 is negligible e.g. routers interconnected by a geostationary satellite link:𝒅𝒑𝒓𝒐𝒑 is hundreds of milliseconds (dominant) The processing delay,𝒅𝒑𝒓𝒐𝒄 , is often negligible however, it strongly influences a router’s maximum throughput Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 54 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 55 Queuing Delay and Packet Loss (1/2) The queuing delay can vary from packet to packet (uses statistical measures such as average, variance, probability) Queuing delay depends on the rate at which traffic arrives (𝒂 packets/sec) (assume each is 𝑳 bits), the transmission rate of the link (𝑹 bps), and the nature of the arriving traffic (periodically or in bursts; or random) Traffic intensity = 𝑳𝒂/𝑹 If 𝑳𝒂/𝑹 > 𝟏 the queue will tend to increase without bound and the queuing delay will approach infinity! Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 56 Queuing Delay and Packet Loss (2/2) 𝑳𝒂/𝑹 < 𝟏: the nature of the arriving traffic impacts the queuing delay If packets arrive periodically, then every packet will arrive at an empty queue and there will be no queuing delay If packets arrive in bursts but periodically, there can be a significant average queuing delay e.g. 𝑵 packets arrive simultaneously every (𝑳/𝑹)𝑵 seconds, 𝒏th packet transmitted has a queuing delay of (𝒏 − 𝟏)𝑳/𝑹 seconds A small percentage increase in the intensity will result in a much larger percentage-wise increase in delay. Performance at a node is often measured not only in terms of delay, but also in terms of the probability of packet loss. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 57 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 58 End-to-End Delay Assume , 𝑵 − 𝟏 routers, no queuing delay 𝒅𝒆𝒏𝒅−𝒕𝒐−𝒆𝒏𝒅 = 𝑵 (𝒅𝒑𝒓𝒐𝒄 + 𝒅𝒕𝒓𝒂𝒏𝒔 + 𝒅𝒑𝒓𝒐𝒑 ) Traceroute is a simple program, when the user specifies a destination hostname, the program in the source host sends multiple, special packets toward that destination. (graphical interface PingPlotter) The source sends 𝟑 × 𝑵 packets to the destination. As these packets work their way toward the destination, they pass through a series of routers. When a router receives one of these special packets, it sends back to the source a short message that contains the name and address of the router. The source can reconstruct the route taken by packets flowing from source to destination, and the source can determine the round-trip delays to all the intervening routers. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 59 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 60 Additional Delays Delay of the transmission as part of its protocol for sharing the medium with other end systems as in a WiFi. Packetization delay (to fill a packet), which is present in Voice-over-IP (VoIP) applications. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 61 Throughput (1/2) Use the speedtest application to measure the end-to-end delay and download throughput between a host and servers If a file consists of 𝑭 bits and the transfer takes 𝑻 seconds for Host B to receive all 𝑭 bits, then the average throughput of the file transfer is 𝑭/𝑻 bits/sec. We may think of bits as fluid and communication links as pipes. In a simple two-link network, the throughput is min{𝑹𝒄 , 𝑹𝒔 }, that is, it is the transmission rate of the bottleneck link. For a network with 𝑵 links between the server and the client, with the transmission rates of the 𝑵 links being 𝑹𝟏 , 𝑹𝟐 , … , 𝑹𝑵. The throughput for a file transfer from server to client is min 𝑹𝟏 , 𝑹𝟐 , … , 𝑹𝑵. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 62 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 63 Throughput (2/2) When there is no other intervening traffic, the throughput can simply be approximated as the minimum transmission rate along the path between source and destination. Links in the core of the communication network have very high transmission rates. The constraining factor for throughput in today’s Internet is typically the access network. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 64 Example #4 (1/2) There are 10 simultaneous downloads: 10 servers and 10 clients connected to the core of the computer network. Server access links have the same rate 𝑹𝒔 , all client access links have the same rate 𝑹𝒄. There is a link in the core that is traversed by all 10 downloads with 𝑹 transmission rate. The transmission rates of all other links in the core are much larger than 𝑹𝒔 , 𝑹𝒄 , and 𝑹. If the rate of the common link, 𝑹, is very large, then the throughput for each download will bemin{𝑹𝒔 , 𝑹𝒄 }. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 65 Example #4 (2/2) If the rate of the common link is of the same order as 𝑹𝒔 and 𝑹𝒄 , bottleneck is now the shared link in the core. e.g. 𝑅𝑠 = 2 𝑀𝑏𝑝𝑠, 𝑅𝑐 = 1 𝑀𝑏𝑝𝑠, 𝑅 = 5 𝑀𝑏𝑝𝑠 each download has 𝟓𝟎𝟎 𝒌𝒃𝒑𝒔 of throughput. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 66 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 67 Protocol Layers Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 68 Layered Architecture There are many pieces to the Internet: numerous applications and protocols, various types of end systems, packet switches, and various types of link-level media. Given this enormous complexity, is there any hope of organizing a network architecture, or at least our discussion of network architecture? A layered architecture allows us to discuss a well-defined, specific part of a large and complex system. Each layer provides its service by performing certain actions and using the services of the layer directly below it. Modularity makes it much easier to change the implementation of the service provided by a layer without affecting other components. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 69 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 70 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 71 Protocol Layering (1/4) Network designers organize protocols in layers. A protocol layer can be implemented in software, in hardware, or in a combination of the two. Application-layer protocols are almost always implemented in software and so are transport-layer protocols The physical layer and data link layers are responsible for handling communication over a specific link, they are typically implemented in a network interface card The network layer is often a mixed implementation of hardware and software. Potential drawbacks of layering is that one layer may duplicate lower-layer functionality and the functionality at one layer may need information that is present only in another layer. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 72 Protocol Layering (2/4) The application layer is where network applications and their application-layer protocols reside. With the application in one end system using the protocol to exchange packets of information (called messages) with the application in another end system. e.g. HTTP, SMTP, FTP, DNS The transport layer transports application-layer messages between application endpoints. (a transport-layer packet is refered as a segment) The UDP protocol provides a connectionless service to its applications. TCP provides a connection-oriented service to its applications: guaranteed delivery; flow control; congestion-control. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 73 Protocol Layering (3/4) The network layer (IP layer) is responsible for moving network-layer packets known as datagrams from one host to another. IP protocol defines the fields in the datagram as well as how the end systems and routers act on these fields. This layer contains routing protocols that determine the routes that datagrams take between sources and destinations. The link layer delivers the datagram to the next node along the route. At this next node, the link layer passes the datagram up to the network layer. A datagram may be handled by different link-layer protocols at different links along its route. The link-layer packets are refereed as frames. e.g. Ethernet, WiFi Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 74 Protocol Layering (4/4) The job of the physical layer is to move the individual bits within the frame from one node to the next. Depends on the actual transmission medium of the link. e.g. Ethernet has many physical-layer protocols: one for twisted-pair copper wire, another for coaxial cable, another for fiber, and so on Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 75 Encapsulation The transport layer takes the message and appends additional information. The transport-layer segment encapsulates the application-layer message. The network layer adds network-layer header information such as source and destination end system addresses, creating a network- layer datagram. The datagram is then passed to the link layer, which (of course!) will add its own link-layer header information and create a link-layer frame. Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 76 Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 77 Summary What Is the Internet? The Network Edge The Network Core Delay, Loss, and Throughput in Packet-Switched Networks Protocol Layers and Their Service Models Computer Networks, 2024 (c) Dr. Sahar M. Ghanem 78