* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chapter 9
Survey
Document related concepts
Airborne Networking wikipedia , lookup
Parallel port wikipedia , lookup
Recursive InterNetwork Architecture (RINA) wikipedia , lookup
Passive optical network wikipedia , lookup
Computer network wikipedia , lookup
Registered jack wikipedia , lookup
Network tap wikipedia , lookup
Cracking of wireless networks wikipedia , lookup
Zero-configuration networking wikipedia , lookup
Wake-on-LAN wikipedia , lookup
IEEE 802.1aq wikipedia , lookup
Point-to-Point Protocol over Ethernet wikipedia , lookup
Transcript
Ethernet Network Fundamentals Chapter 9 Objectives Identify the basic characteristics of network media used in Ethernet. Describe the physical and data link features of Ethernet. Describe the function and characteristics of the media access control method used by Ethernet protocol. Objectives Explain the importance of Layer 2 addressing used for data transmission and determine how the different types of addressing impacts network operation and performance. Compare and contrast the application and benefits of using Ethernet switches in a LAN as opposed to using hubs. Explain the ARP process. Outline Overview of Ethernet Ethernet: standards and implementation Ethernet: layer 1 and layer 2 Logical Link Control: connecting to the upper layers MAC: getting data to the media Physical implementations of Ethernet Ethernet: communication through the LAN Historic Ethernet Legacy Ethernet Current Ethernet Moving to 1Gbps and beyond Outline Ethernet frame Ethernet MAC Frame: encapsulating the packet Ethernet MAC address Hexadecimal numbering and addressing Another layer of addressing Ethernet unicast, multicast and broadcast CSMA/CD: the process Ethernet timing Interframe spacing and backoff Ethernet physical layer 10-Mbps Ethernet Outline Hubs and switches 100-Mbps Ethernet (Fast Ethernet) 1000-Mbps Ethernet (Gigabit Ethernet) 10-Gbps and Future Ethernet Legacy Ethernet: using hubs Ethernet: using switches Switches: selective forwarding Address Resolution Protocol (ARP) Resolving IPv4 addresses to MAC addresses Maintaining a cache of mappings ARP broadcast issues Overview of Ethernet Ethernet is one of the LAN standards. There were several other LAN standards such as Token Ring, FDDI, etc. However, nowadays, Ethernet is the dominant standard used in LANs. The initial Ethernet specifies a 10Mbps LAN standard using coaxial cable. Over time, Ethernet has evolved to use other transmission media as well as higher data rate. Ethernet: Standards and Implementation The first Ethernet standard was published in 1980 by a consortium of Digital Equipment Corporation, Intel and Xerox (DIX). Ethernet was then standardized by IEEE in 1985 under the standard 802.3. Ethernet is also called IEEE 802.3 This standard includes layer 1 (physical layer) and part of layer 2 (data link layer) of the OSI model. Ethernet: Layer 1 and Layer 2 Ethernet: Layer 1 and Layer 2 Ethernet includes two standards: IEEE 802.3 – for physical layer and the MAC sublayer of the data link layer. IEEE 802.2 – for the LLC sublayer of the data link layer. Layer 1 elements specified by Ethernet: Signals Bit streams that travel on the media Physical components that put signals on the media Network topologies Ethernet: Layer 1 and Layer 2 Layer 2 elements specified by Ethernet: The MAC sublayer is concerned with the physical components that are used to communicate information and prepare data for transmission over that media. Provides an interface to the upper-layer protocols Provides an address to identify devices Uses frames to organize bits into meaningful groups Controls the transmission of data from sources Will be different for different media. The LLC sublayer, on the other hand, is independent of the physical equipment used. Logical Link Control: Connecting to the Upper Layers LLC manages the communication between the networking software at the upper layers and hardware at the lower layers. LLC is implemented in software and is independent of the physical equipment. LLC can be considered the driver software for the network interface card (NIC). The driver program interacts directly with the hardware on the NIC to pass the data between the media and the MAC sublayer. LLC takes the Network layer PDU (IPv4 packet) and adds control information to deliver the packet to the destination node. MAC: Getting Data to the Media The MAC sublayer is implemented in hardware (the NIC). It has two main responsibilities: Data encapsulation Performing media access control (MAC) Data encapsulation involves the process of adding a header and a trailer to layer 3 PDUs. There are three functions of data encapsulation: Frame delimiting – to identify the start and end of frame. Addressing – to specify the source and destination MAC address MAC: Getting Data to the Media Media access control is performed to control how and when the nodes gain access to the media. Error detection – to add CRC (cyclic redundancy check) code to the trailer to be used by the receiver to detect bit errors This includes what to do to recover from collision. For historic and legacy Ethernet networks, this is done using CSMA/CD. Media access control also defines when to decide to accept a frame. This is done by examining the destination MAC address inside the frame header. Physical Implementations of Ethernet The first Ethernet was developed back in 1970s. Since then, it has evolved to meet the increased demand for high-speed LANs. Started with 10 Mbps Ethernet. Today, the fastest Ethernet can achieve data rate up to 10 Gbps. Also used in WANs (wide area networks) and MANs (metropolitan area networks). There are various Ethernet standards to support different data rate, transmission media and connector specification. Physical Implementations of Ethernet To ensure compatibility between the various standards, Ethernet uses the same frame structure. As a result, the different Ethernet standards can be used together in a single LAN. Reasons for Ethernet success: Simplicity and ease of maintenance Ability to incorporate new technologies Reliability Low cost of installation and upgrade. Historic Ethernet The original Ethernet design uses a shared bus topology. There were two standards: 10BASE2 (Thinnet) – uses thin coaxial cable. 10BASE5 (Thicknet) – uses thick coaxial cable. These early Ethernet standards were deployed in low bandwidth (10 Mbps) LANs. Due to the use of bus topology, collision may happen. Therefore, access to the media was managed by CSMA, and later CSMA/CD. Historic Ethernet Legacy Ethernet This generation of Ethernet is characterized by the use of UTP cable and physical star topology. However, the logical topology is still a bus. Hosts are connected to a hub using UTP cable. This Ethernet standard is called 10BASE-T. A hub broadcast incoming frame to all outgoing ports. Only one station can transmit at a time (half-duplex). Collision may happen. Access to the media is managed using CSMA/CD. Later, the use of a hub is replaced with a switch. This minimizes the possibility of collisions and increases the performance and reliability of Ethernet. Legacy Ethernet Current Ethernet This Ethernet standard has a data rate of 100 Mbps. The use of hubs has been replaced with switches. This leads to point-to-point logical topology. A switch can forward incoming frame only to the port that leads to the receiver. The standard is called 100BASE-T (Fast Ethernet). This minimizes the possibility of collisions. Eliminates the necessity to perform CSMA/CD. It also enables the hosts to have the full bandwidth of the media. Later switch implementation also support full-duplex communications. Current Ethernet Topology physical: Star Logical: Point-to-point Moving to 1 Gbps and Beyond Ethernet nowadays can operate at the rate of 1 Gbps (1000 Mbps) or even 10 Gbps (10000 Mbps). Switch is still used as the connecting device. These are called Gigabit Ethernet and 10-Gigabit Ethernet respectively. This higher speed is necessary to properly support Voice over IP (VoIP) and multimedia applications. However, the switch must have full-duplex capability. In addition to UTP cable, fiber optic cable can also be used. Fiber optic cable will allow for greater distance. This enables Ethernet to be used in WANs and MANs. Moving to 1 Gbps and Beyond Ethernet Frame All Ethernet standards use the same frame structure. There are three versions of Ethernet frame: Ethernet has a header and a trailer. DIX IEEE 802.3 (Original) IEEE 802.3 (Revised 1997) An Ethernet frame has minimum size of 64 bytes and maximum size of 1518 bytes. Any received frame not within this size will be considered as corrupted and will be discarded. Ethernet Frame Ethernet Frame Preamble and start frame delimiter Destination address The MAC address of the receiver. Source address To synchronize sending and receiving devices. The MAC address of the sender. Length/Type Can either be a length or a type value. Length refers to the length of the data field. Type refers to the upper-layer protocol. Ethernet Frame Data and pad Contains layer 3 PDU (an IPv4 packet). All frames must be at least 64 bytes long. If the data is small, it needs to be pad with zeros so that the frame reaches its minimum size. Frame check sequence (FCS) Contains the error correction code for error detection. The code is generated by the sender using a technique called CRC (cyclic redundancy check). Ethernet MAC Address Each NIC has a unique MAC address. In a LAN with a logical bus topology (such as Ethernet), a frame sent will be received by all hosts in the LAN. This address is burned inside the NIC’s ROM and cannot be changed. The hosts will decide whether they will take or discard the frame based on the destination MAC address inside the header. An Ethernet MAC address is a 48-bit binary value, expressed in 12 hexadecimal digits. Ethernet MAC Address There are two parts to the MAC address: The OUI is assigned by IEEE and is unique for each NIC manufacturer. Organization Unique Identifier (OUI) – 24 bits. Vendor assigned – 24 bits. For example, Cisco is assigned the OUI “00 60 2F”. The other 24 bits are then assigned by the vendor uniquely to every NIC that they produce. From Windows command prompt, you can check your MAC address by typing “ipconfig /all”. Ethernet MAC Address Hexadecimal Numbering and Addressing Hexadecimal (“hex”) is a convenient way to represent binary values. Decimal – base 10 number system Binary – base 2 number system Hex – base 16 number system Each hexadecimal digit represents a 4-bit value. Hex digit ranges from 0 to 9 and A to F. Hex number is normally preceded by 0x (example: 0x73). Hexadecimal Numbering and Addressing Another Layer of Addressing A network host has two addresses: Layer 3 address – IP address Layer 2 address – MAC address Although both addresses are unique for a host, they serve different purpose. IP address is hierarchical and is used to carry packet from source to destination host across the Internet (or any TCP/IP network). MAC address is non-hierarchical and is used to carry frame from source to destination host across the local media (example: a LAN, a point-to-point WAN connection). Ethernet Unicast, Multicast and Broadcast Unicast: sending frame to a single host Broadcast: sending frame to all hosts in a network. IP address: IP address of receiving host. MAC address: MAC address of receiving host. IP address: broadcast address of the targeted network (for directed broadcast) or 255.255.255.255 (for limited broadcast). MAC address: FF-FF-FF-FF-FF-FF Multicast: sending frame to a group of hosts IP address: a multicast IP address that identifies the group. MAC address: a special address that starts with 01-00-5E, followed by the lower 23 bits of the multicast IP address and followed by a 0. Ethernet MAC Ethernet uses the media access control (MAC) method called CSMA/CD. Only used in historical and legacy Ethernet (physical bus topology and physical ring topology using hub). If a switch is used instead of a hub, the use of CSMA/CD is not required. CSMA/CD is used to prevent and recover from collision. Collision refers to the condition where signals from two or more hosts get mixed up in the media. Can happen in any shared media environment. CSMA/CD: The Process There are three steps in CSMA/CD process: Listen before sending Each device must listen to the media before transmitting a frame. Transmission is only done if there is no other signal on the media. Detecting a collision While transmitting, the device keeps listening to the media to check if collision occurs. Collision can be detected when the signal on the media is jumbled up as the result of the collision. CSMA/CD: The Process Jam signal and random backoff If collision is detected, a jam signal is transmitted so that all other devices know about the collision. All devices involved in the collision would then invoke a backoff algorithm which will cause them to stop transmitting for a random amount of time. After the random backoff timer expires, the device would go back to the “listen before sending” mode. The random backoff timer will make sure that the devices would not transmit at the same time again and cause another collision. Hubs and Collision Domain Collision can occur in any shared media topology, even when using CSMA/CD. Conditions that lead to the increase in collision: More devices connected to the network More frequent access to the media Increased cable distances between devices In legacy Ethernet, the size of the network can be increased with the use of hub or repeater. Done by connecting a hub to another hub. Hubs and Collision Domain A series of devices connected using hubs or repeaters creates a collision domain. A collision domain is the area of the network which will be affected when collision occurs. Using hubs and repeaters will increase the size of the collision domain. For good performance, it is desirable to keep the size of the collision domain to be as small as possible. This is achieved by using switch instead of hub to increase the network size (add more devices). Ethernet Timing In order to properly manage collision, there are several issues related to timing that need to be considered. Latency Timing and synchronization Bit time Slot time These issues may put a limit on some of the network characteristics such as number of hubs used, maximum cable length and minimum frame size. Ethernet Timing – Latency Latency refers to the time it takes for the electrical signal to travel down the cable. Each hub in the signal path will increase latency. Longer latency would increase the possibility of collision. This is because it takes a longer time for a device at one end of the network to listen for the signal from another device at the other end of the network. Without being able to detect the signal from the first device soon enough, the second device will transmit and cause collision. Ethernet Timing – Timing and Synchronization Synchronization refers to the method used to get the receiver to synchronize with the signal’s frequency. This is done using 64 bits of timing synchronization information (called preamble bits) in the Ethernet header. Used in 10 Mbps Ethernet. Asynchronous communication. 100 Mbps and faster Ethernet use signaling methods that include a method of synchronization. The preamble bits in the header are not required, although they are retained for compatibility reasons. Synchronous communication. Ethernet Timing – Bit Time Bit time refers to the time it takes for a bit to be placed and sensed on the media. 10 Mbps Ethernet: bit time = 100 ns 100 Mbps Ethernet: bit time = 10 ns 1 Gbps Ethernet: bit time = 1 ns 10 Gbps Ethernet: bit time = 0.1 ns For CSMA/CD to operate, a sending device must become aware of collision before it has completed the transmission of a minimum-sized frame. With shorter bit time, shorter cable is needed for this to happen. This puts a limit on the maximum cable length. Ethernet Timing – Slot Time Slot time refers to the maximum time required to detect a collision. Equal to twice the time it takes for a signal to travel between two most-distant stations on the network. The slot time must be set such that if a collision is going to occur, it will be detected within the transmission time of a minimum-size frame. Ethernet slot times: 10 Mbps Ethernet: 512 bit times (51200 ns) 100 Mbps Ethernet: 512 bit times (5120 ns) 1 Gbps Ethernet: 4096 bit times (4096 ns) Ethernet Timing – Slot Time The slot time specified also includes the time it takes for the signal to travel through cables and hubs. This is then used to define the standard for: The maximum length of network cables The maximum number of hubs that can be used in a shared Ethernet segment. Using cables that are longer than the one specified by the standard will create late collision detection. Collision that is detected too late cannot be managed by CSMA/CD (Ethernet will not handle the frame retransmission). The receiving software would then need to initiate a retransmission. Interface Spacing and Backoff After a node has transmitted, a delay needs to occur before the next transmission. The delay after successful transmission is called interframe spacing. This give time for the signals in the media to dissipate. It is required regardless of whether the frame is transmitted successfully or not (due to collision). Interframe spacing = 96 bit times The delay after a collision is called backoff. Backoff time = interframe spacing + (some random time) The random time will be different for each device to prevent them from transmitting at the same time and cause another collision. Ethernet Physical Layer The differences between Ethernet, Fast Ethernet, Gigabit Ethernet and 10-Gigabit Ethernet occur at the physical layer. 10 Mbps: 10BASE-T Ethernet 100 Mbps: Fast Ethernet 1 Gbps (1000 Mbps): Gigabit Ethernet 10 Gbps: 10-Gigabit Ethernet For each data rate, there are several Ethernet types. There are 3 identifiers in an Ethernet type: Speed (in Mbps) Type of signaling used (baseband or broadband) Cable distance or type of medium used Ethernet Physical Layer Ethernet Type Bandwidth Cable Type Max distance (m) 10BASE5 10Mbps Thick coax 500 10BASE2 10Mbps Thin coax 185 10BASE-T 10Mbps Cat3/Cat5 UTP 100 100BASE-TX 100Mbps Cat5 UTP 100 100BASE-FX 100Mbps Multimode/single mode fiber 400/2000 1000BASE-T 1 Gbps Cat5e UTP 100 1000BASE-TX 1 Gbps Cat6 UTP 100 1000BASE-SX 1 Gbps Multimode fiber 550 1000BASE-LX 1 Gbps Single mode fiber 2000 10GBASE-T 10 Gbps Cat6a/Cat7 UTP 100 10GBASE-LX4 10 Gbps Multimode/single mode fiber 300/10,000 10-Mbps Ethernet There are three 10 Mbps Ethernet standards: 10BASE5 and 10BASE2 are early Ethernet standards. 10BASE5, 10BASE2 and 10BASE-T Used coaxial cable. Connected in physical bus topology. No longer used nowadays. All 10 Mbps Ethernet today uses the 10BaseT standard. 10-Mbps Ethernet 10BASE-T Ethernet has the following characteristics: Uses Cat3 or Cat5 UTP cable. Uses a physical star topology, with a hub at the center. The maximum length for the UTP cable is 100 meter. Uses Manchester encoding over two unshielded twistedpair cable. The UTP cable used has the following characteristics: It has four-pair of copper wires, terminated at each end with an 8-pin RJ-45 connector. Pin 1 and 2 are used for transmitting Pin 3 and 6 are used for receiving. 100-Mbps Ethernet (Fast Ethernet) There are two popular 100-Mbps Ethernet standards: 100BASE-TX and 100BASE-FX 100BASE-TX supports 100 Mbps transmission over UTP cable. Most of the implementation details of 100BASE-TX are similar to 10BASE-T except for the following: The UTP cable used must be Cat5 or better. It uses the 4B/5B encoding. A switch is typically used instead of a hub. 100-Mbps Ethernet (Fast Ethernet) 100BASE-FX is similar to 100BASE-TX except that it uses fiber optic cable instead of UTP cable. 100BASE-FX is commonly used to interconnect two devices in a point-to-point fashion. Between two computers Between a computer and a switch Between two switches A switch supporting 100BASE-FX normally have only one fiber interface (as opposed to many UTP cable interfaces). 1000-Mbps Ethernet (Gigabit Ethernet) There are several 1 Gbps Ethernet standards, that can support both UTP and fiber optic cables. 1000BASE-T, 1000BASE-SX, 1000BASE-LX, etc. 1000BASE-T supports 1 Gbps transmission over UTP cable. All four pair of wires in the Cat5 cable are used. Each wire pair can transmit 125 Mbps. With four pair of wires, the cable can transmit 500 Mbps. Each wire pair can transmit in full-duplex, doubling the data rate from 500 Mbps to 1000 Mbps. 1000-Mbps Ethernet (Gigabit Ethernet) Other modifications made to the 1000BASE-T standard: The encoding used is 4D-PAM5. Each 8-bit of data is converted to 4 code symbols. The 4 code symbols are transmitted simultaneously over the four pair of wires. Each pair of wire can transmit in full-duplex. At any one time, each wire will contain signals from both sides. Special techniques are used so that the receiver can read the received signals correctly. Multiple voltage levels are used. 1000-Mbps Ethernet (Gigabit Ethernet) 1000BASE-SX and 1000BASE-LX support 1 Gbps transmission over fiber optic cable. 1000BASE-SX: used for multimode fiber 1000BASE-LX: used for single-mode fiber (can provide longer distance compared to 1000BASE-SX). Two strands of optical fiber are used. The encoding scheme used is 8B/10B. Using fiber optics offer the following advantages over UTP: Noise immunity Smaller physical size Increased distance 10-Gbps and Future Ethernet 10-Gigabit Ethernet (10GbE) standard is included in the IEEE 802.3ae standard (a revised version of IEEE 802.3). 10GbE is designed not only to be used in LANs, but also in MANs and WANs. Due to the use of the same frame structure, 10GbE can be used with existing Ethernet infrastructure. Makes it easy to upgrade or extend the current network. Standards on 40-, 100-, and even 160-Gbps Ethernet are already being developed. Legacy Ethernet: Using Hubs A hub is a physical layer device, which broadcast the received signal to all the outgoing ports. The use of hubs to increase the size of a network can cause several issues: Limited scalability – there is a limit on the amount of bandwidth that devices can share. Increased latency – can increase collisions. More network failures – a malfunction device can affect the whole network. More collision – due to larger collision domain. Nowadays, hubs are only used in small LANs or in LAN with low bandwidth requirement. Ethernet: Using Switches Switches provide an alternative to the contentionbased environment of legacy Ethernet. Switch allows the segmentation of LAN into separate collision domains. Each port represents a separate collision domain. This reduces collision. The bandwidth is only shared between the nodes inside that collision domain. This increases the average bandwidth for each node. Provides better performance. Ethernet: Using Switches router Single Collision Domain hub hub hub hub hub hub hub hub Ethernet: Using Switches router switch hub hub switch hub Collision Domains hub hub hub Collision Domains Ethernet: Using Switches Devices can also be attached directly to a switch. Each device would then be in its own collision domain. Connecting devices directly to a switch brings the following advantages: Dedicated full bandwidth to each port – the bandwidth is no longer shared between multiple devices. Collision-free environment – there is only one device in the collision domain and therefore collision does not occur, which results in higher throughput. Full-duplex operation – the device can transmit and receive simultaneously (for example, in 100 Mbps LAN, the device and transmit at 100 Mbps and receive at 100 Mbps). Switches: Selective Forwarding The advantages of a switch are due to its ability to do the following: Forward frame only to the port where the destination node is connected. This is called selective forwarding. Free the other ports from transmitting signals unnecessarily. Creates a logical point-to-point connection. Buffer an incoming frame and forward it to the proper port when the port is idle. This is called store and forward. Allows a sender to transmit even though the outgoing port is currently busy. This prevents collision from ever happening. Forwarding Based on Destination MAC Address To perform selective forwarding, the switch maintains a table called the MAC table. This table is also called the switch table or the bridge table. The table keeps a list of MAC addresses together with the switch port number that leads to the device with that MAC address. When a frame comes, the switch will: Read the destination MAC address in the header. Find a matching address in the MAC table. Forward the frame to the appropriate outgoing port as specified in the MAC table. Switch Operation Learning: When a switch is first put on the network, the MAC table is empty. The switch is able to fill in the MAC table automatically. When a frame arrives from a certain port, the switch will check whether the source MAC address is already available in the table. If not, then the source MAC address and its corresponding incoming port number will be recorded in the table. Aging: Each entry in the MAC table is associated with a timer. If the timer expires, the entry will be deleted from the table. A common timer value is 300 seconds (5 minutes). Switch Operation Flooding: Selective forwarding: When a frame arrives at the switch but the destination MAC address is not in the MAC table, the frame will be forwarded to all outgoing ports. When a frame arrives at the switch and the destination MAC address is in the MAC table, it will be forwarded only to the corresponding port. Filtering: A frame will not be forwarded if the receiver is located on the same port on which the frame arrives. Address Resolution Protocol (ARP) When an Ethernet frame is to be transmitted, the sender must put the destination MAC address in the header. How can the sender know what is the destination MAC address? It can refer to a table in its RAM called the ARP table (or ARP cache). On Windows, this table can be viewed with the command ‘arp –a’. This table contains mappings between an IP address and its corresponding MAC address. Address Resolution Protocol (ARP) When a device is first connected to the network, this table is initially empty. If a device needs to send a frame and the receiver’s MAC address is not in the table, it needs to find the mapping. A mapping can be automatically added into the ARP table with the following process: The device broadcasts an ARP request message which contains the IP address of the destination host. All devices in the LAN will receive this request. Address Resolution Protocol (ARP) Each entry in the ARP table is associated with a timer. The device whose IP address matches the address in the ARP request message will send an ARP reply message. This message contains the device’s MAC address. The sending device can then add this information to its ARP table. When the timer expires, the entry will be deleted from the ARP table. The ARP table entries can also be managed manually. On Windows, this is done using the ‘arp’ command. Mapping to Destination Outside the Local Network MAC address is only valid within the local network. If the receiver is on the same network as the sender, the destination MAC address put in the header corresponds to the MAC address of the receiver. But what if the destination is outside the local network? The sender must put the MAC address of the gateway router in the header. Mapping to Destination Outside the Local Network The MAC address of the gateway router can be obtained using the same method as discussed before, or by using proxy ARP. Proxy ARP is protocol that allows a router to reply to an ARP request message asking for the MAC address of a host outside the local network. The router would then reply this message with its own MAC address. When the frame is forwarded to another network, the source and destination MAC address in the header will change accordingly.