© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.1 Computer Networks and Internets with Internet Applications, 4e By Douglas E. Comer Lecture PowerPoints By Lami Kaya,

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.2 Chapter 9 Hardware Addressing and Frame Type Identification

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.3 Topics Covered 9.1 Introduction 9.2 Specifying A Recipient 9.3 How LAN HW Uses Addresses To Filter Packets 9.4 Format Of A Physical Address 9.5 Broadcasting 9.6 Multicasting 9.7 Multicast Addressing 9.8 Identifying Packet Contents 9.9 Frame Headers And Frame Format 9.10 An Example Frame Format 9.11 Using NW That Do Not Have Self-Identifying Frames 9.12 NW Analyzers, Physical Addresses, Frame Types

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Introduction This chapter considers transmission across a shared LAN in more detail explains how a single pair of computers communicate across a LAN without forcing others to receive/process a copy of each message describes HW addressing (HWA) shows how a computer uses a HWA to identify which computer or computers should receive a copy of a given frame explains how network interface hardware (NIH) uses such addresses to filter incoming packets examines a mechanism that allows a sender to identify the type of the data in each frame explains the conceptual purpose of frame type identifiers

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Specifying A Recipient When a frame is transmitted across a LAN –the electrical signals carrying the bits reach all stations How can two computers communicate directly across a shared medium in which all attached stations receive a copy of all signals? –Each station on the LAN is assigned a unique numeric value called a physical address, HWA or media access address (MAC) address When a sender transmits a frame across the LAN, the sender includes HWA of the intended recipient Although sharing allows all stations to receive a copy of the bits –each station checks the address of each incoming frame to determine whether it should accept the frame Each frame includes two addresses –intended recipient –the sender it easy for a recipient to generate a reply

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved How LAN Hardware Uses Addresses To Filter Packets The NIH handles all the details of sending and receiving –checks the length of an incoming frame –checks the CRC to ensure that the bits arrived intact –and discards frames that contain errors –compares the destination address to the station's HWA If it matches the station's HWA, it accepts the frame If it does not match the station's HWA, it discards a frame addressed to a nonexistent station is ignored. NIH can perform the functions of sending and receiving frames without using the computer's CPU Figure 9.1 illustrates a LAN Network Interface hardware

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8 9.4 Format Of A Physical Address (1) The various address forms can be grouped into three broad categories: Static –HW manufacturer to assign a unique HWA to each NIH Configurable –a customer can use to set a HWA Dynamic –automatically assign a HWA to a station when the station first boots

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Format Of A Physical Address (2) The chief advantages of static addressing are –ease of use and permanence. Dynamic addressing has two advantages: –HW manufacturers do not need to coordinate in assigning addresses –it allows each address to be smaller The chief disadvantages of dynamic addressing are –lack of permanence and potential conflict Configurable addresses provide a compromise between the static and dynamic schemes: –They are permanent –They do not need to be large

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Broadcasting Broadcast means to make a copy of the data available to all others Broadcast has many uses Most LANs employ shared media –they can make broadcasting extremely efficient –All stations receive a copy the frame –All that is needed for all stations to extract the frame NIH is built to recognize –the special broadcast address as well as the station's HWA The NIH address is sometimes classified as –unicast address If a frame arrives with either of the two addresses in its destination –NIH accepts the frame and delivers a copy to the OS

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Multicasting Broadcasting is extremely inefficient –processing and discarding a frame requires computational resources How to take advantage of the broadcast without wasting computing resources (CPU, memory)? –Use a restricted form of broadcasting  multicasting –NIH does not forward multicast frames to the CPU –Instead, NIH must be programmed with specifications of which multicast frames to accept and which to reject –NIH programmed to accept frames that match the specification

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Multicast Addressing How is a NIH programmed to accept some multicast packets and reject others? –The mechanism is an extension of the basic addressing scheme –Multicasting extends the addressing scheme by reserving some addresses for multicast Extends a NIH to recognize an additional set of addresses –If an application on the computer wishes to receive multicast frames, the application must inform the NIH which multicast address to use –The interface adds the address to the set it will recognize, and begins accepting frames sent to that address Multicasting offers the advantages –of only sending a single copy of each frame and allowing arbitrary computers to receive the transmission

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Identifying Packet Contents (1) The address does not specify what the packet contains Many data items use the same representation –a receiver cannot use data in the packet to determine what the packet contains To inform the receiver about its contents –each frame contains additional information that specifies the type of the contents

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Identifying Packet Contents (1) Two methods are used to identify the contents of a frame: explicit frame type –specify how type information is included in the frame and the values used to identify various frame types –The bits of a frame used to identify the contents called the frame type field –The frame is called self-identifying Implicit frame type –NIH does not include a type field in each frame –Instead, the frame carries only data the sender and receiver must agree on the contents of a frame

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Frame Headers And Frame Format Each LAN technology defines the exact frame format Most LAN technologies define a frame to consist of two parts –a frame/packet header that contains information such as the source and destination addresses –a “payload” or “data area” that contains the information being sent Figure 9.2 illustrates the general format In most LAN technologies each field in the frame header has a fixed size and location –all frames used with the technology have the same header size –the data area of a frame does not have a fixed size –the amount of data to be sent determines the size of the data area –HW sometimes enforces a minimum as well as a maximum frame size If the sender has less than the minimum, the data can be extended with zeroes

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© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved An Example Frame Format (1) Figure 9.3 illustrates the frame format used with Ethernet An Ethernet frame begins with a header that contains three fields –The 64-bit preamble alternating 1 s and 0 s that allow the receiver's hardware to synchronize with the incoming signal –The first two fields of the header contain HWA, Destination Address Source Address –The third field of the header consists of a 16-bit Ethernet “frame type” Ethernet uses a 48-bit static addressing scheme

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© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved An Example Frame Format (2) The Digital-Intel-Xerox Ethernet standard –specifies the values that can be used in the header fields and their meanings For example, the standard specifies –address with all 48 bits set to 1 is reserved for broadcast –other addresses that start with a 1 bit are used for multicast –and a 8137h in the Frame Type field specifies that data in the frame follows a Novell Corporation protocol known as IPX Hundreds of Ethernet type values have been assigned –the table in Figure 9.4 contains a few examples

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© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Using Networks That Do Not Have Self-Identifying Frames (1) How can computers connected to such NW know the type of data in each frame? There are two possible approaches: First, the sender and receiver agree to use a single format for data –The first technique is seldom used because it limits a pair of computers to exactly one form of data Second, the sender and receiver agree to use the first few octets of the data field to store type information –Figure 9.5 illustrates how the second technique –Using second technique raises some questions what size should the type information be? who should specify values allowed in the type field and their meaning?

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© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Using Networks That Do Not Have Self-Identifying Frames (2) To ensure that all software agrees on values used to specify types –standards organizations have defined the meaning of each value Multiple standards organizations make such assignments, –and the organizations do not always coordinate their efforts To solve the problem of multiple standards organizations assigning –IEEE has defined a standard that includes a field to identify the standards organization –as well as a field to identify a type as defined by that organization Part of IEEE's standard, the specification is known as –Logical Link Control (LLC) –Sub Network Attachment Point SNAP header The IEEE LLC / SNAP header is widely accepted Figure 9.6 shows an example LLC / SNAP header

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© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Using Networks That Do Not Have Self-Identifying Frames (3) SNAP portion of the header is divided into two fields. –First, Organizationally Unique Identifier (OUI) which is used to identify a particular standards organization –Second, a type value defined by that organization Ex: Figure 9.6 shows that –OUI value of all zeroes is assigned to the organization that specifies Ethernet types The LLC / SNAP type field makes it possible for all computers on a shared NW to broadcast frames –When a frame arrives at a given computer it looks for LLC/SNAP information at the beginning of the frame –If the receiver does not recognize the OUI or does not have software to handle the type of data being sent it discards the frame

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Network Analyzers, Physical Addresses, Frame Types (1) NW analyzer/monitor or packet analyzer –is a device used to determine how well a network system is performing A NW analyzer is sometimes called a “NW sniffer” After it has been attached to a network –an analyzer can monitor specific events –and can report certain statistics or events Most analyzers are flexible, it can be configured to –observe the frames sent by a specified machine –observe traffic of a specific type –compute the percentage of frames of each type

© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Network Analyzers, Physical Addresses, Frame Types (2) How does a network analyzer work? The analyzer program –begins by allowing a user to configure parameters –and then uses the parameters to analyze packets To read packets, analyzer software places the computer's network interface hardware into –promiscuous mode Promiscuous mode means –the NIH is configured to accept all frames any computer attached to a LAN can eavesdrop on all communication messages sent over a LAN are not guaranteed to be private