Chapter 9 Hardware Addressing and Frame Type Identification

Topics Covered • How LAN HW Uses Addresses To Filter Packets • Format of a Physical Address • Broadcasting, Multicasting & Multicast Addressing • Identifying Packet Contents • Frame Headers And Frame Format • An Example Frame Format • Using NW That Do Not Have Self-Identifying Frames • NW Analyzers, Physical Addresses, Frame Types © 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) • 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 • Recipient (Destination) address • Sender (Source) address • Thus, it is 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 © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Format of A Physical Address The various address forms can be grouped into three broad categories: • Static • HW manufacturer assigns a unique HWA to each NIH. • Configurable • a customer can set a HWA when the HW first installed, can be done through switches or programmable through (EPROM). • Dynamic • automatically assign a HWA to a station when it first boots. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

(+) Advantages & (-) Disadvantages • Static Addressing: (+) Ease of use and permanent (-) Cannot be changed • Dynamic addressing: (+) Manufacturers don’t need to coordinate addresses (+) Address can be smaller (-) Lack of permanence & potential conflict (if LAN fails & restarted possibly 2 nodes picks the same physical addr.) • Configurable Addressing (is a Compromise) (+) They are permanent(like static) (+) They do not need to be large (like dynamic) (+) HW can be replaced without changing its physical addr (-) Cannot be changed (like static) © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Broadcasting • Broadcastmeans to make a copy of the data to all others • Eg. A PC looking for a particular Printer • Most LANs employ shared media meaning all stations receive a copy of the frame. All that is needed for any station is to extract the frame (efficiency issue). • NIH is built to recognize the special broadcast address as well as the station's physical address • The NIH address is sometimes classified as unicast address • If a frame arrives with either of the two addresses in its destination. The NIH accepts the frame and delivers a copy to the station’s 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 (wasting CPU time) for un-communicating nodes • 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 by reserving some addresses for multicast  Extends a NIH to recognize an additional set of addresses • If an application wishes to receive multicast frames, it 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 • 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.

Packet Contents (cont.) 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 • Thus, 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, however 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 & 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 is extended with 0s © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Example Ethernet Frame Format • Figure 9.3 illustrates 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 © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Ethernet Frame Format (Cont.) • 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 to1 is reserved for broadcast • other addresses that start with a1 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 © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Using Networks that Do Not Have Self-Identifying Frames 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? © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Using Networks That Do Not Have Self-Identifying Frames (Cont.1) • Standards organizations have defined the meaning of each value used to specify types. Multiple standards organizations make such assignments, and the organizations do not always coordinate their efforts • To solve the problem of multiple standards organization • 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 © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Using Networks That Do Not Have Self-Identifying Frames (Cont.2) • SNAP portion of the header is divided into two fields. • First, Organization 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 • 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 (Cont.) 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 (spy) on all communication • messages sent over a LAN are not guaranteed to be private © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Chapter 9 Hardware Addressing and Frame Type Identification