Notes On Chapter Nine -- Hardware Addressing And Frame Type Identification

• 9.1 Introduction
  
  
  • How does one facilitate one-pair-at-a-time communication on a shared LAN? This chapter discusses hardware addressing and frame type identifiers.
    
    
• 9.2 Specifying a recipient
  
  
  • (Note: some of the details described below could differ slightly on some LAN's)
    
    
  • Typically each node on a shared LAN has a unique physical address. For example, the ethernet address of altair is: 8:0:20:9f:a0:c2. Note that this physical address is NOT the same thing as the IP address. (The IP address of altair is 130.17.1.50).
    
    
  • When host X wants to send a frame to host Y, X places the physical address of Y in the frame and puts the frame on the LAN.
    
    
  • In a shared LAN, the frame sent by X arrives at the network interface (e.g. ethernet card) of every host on the LAN.
    
    
  • When a frame arrives at the network interface on a host Z, the interface hardware compares the destination address in the frame with Z's physical address. Unless the two addresses match, the interface does not pass the frame up to the software running on host Z. The interface hardware does this screening all by itself. There's no involvment of the host Z's CPU if the addresses do not match. If lots of frames that are not addressed to Z arrive at the network interface of Z, it will not slow down the programs that are running on Z (unless of course they are specifically waiting for something to happen on the network, like the arrival of a packet).
    
    
  • If the addresses do match, the network interface copies the frame to a location in RAM and then interrupts the CPU. Software on the host then examines the frame and does whatever it needs to do with it.
    
    
• 9.3 How LAN Hardware Uses Addresses To Filter Packets
  
  
  • The network interface hardware is sophisticated and powerful. The host operating system does not have to be concerned with details of how packets are transmitted.
    
    
  • Interface hardware checks packet length and CRC. It discards frames containing errors and handles all the details of sending and receiving frames -- including maintaining queues of incoming and outgoing packets.
    
    
  • We see then that hosts X and Y on a shared LAN can communicate without requiring any additional work from other CPU's on the LAN.
    
    
• 9.4 Format of a Physical Address
  
  
  • Each network technology has its own standards for the kind of addresses and frame layouts that will be used on the network.
    
    
  • The physical address is assigned to the network interface, not the host. A host with more than one network interface therefore has (at least) one address for each interface.
    
    
  • a physical address can be assigned statically by the manufacturer of the interface, dynamically at boot time, or it can be a configurable property that the user sets by issuing a software command or by setting hardware switches. Each method of assignment has its advantages and disadvantages.
    
    
• 9.5 Broadcasting
  
  
  • There is a way to "broadcast" on most shared LAN's. This means that a packet can be addressed in such a way that it is treated as if it were addressed to every host on the LAN.
    
    
  • Network interface hardware recognizes a special broadcast address. If a frame arrives at the interface of host Z addressed either specifically to Z, or addressed to the broadcast address, then the interface will pass the frame up to the OS on the host Z.
    
    
• 9.6 Multicasting
  
  
  • Broadcasts are "expensive" to the network because all the hosts CPU's have to process a broadcast packet.
    
    
  • Multicasting is a way to send just one packet to a group of several hosts, yet avoid a broadcast.
    
    
• 9.7 Multicast Addressing
  
  
  • There are special multicast addresses that cannot be used as individual host addresses or as broadcast addresses.
    
    
  • A program can configure a network interface to accept packets addressed to a particular multicast address, or some set of multicast addresses.
    
    
  • Multicasting can be used to allow a group of say four out of fifty hosts on a LAN to receive a streaming audio performance.
    
    
    • This is much preferable to broadcasting the performance, which would force all 50 hosts to process all the packets of the performance.
      
      
    • Also it is better to multicast than to send four individual copies of the performance, one to each of the four hosts. The use of multicast conserves network bandwidth.
      
      
• 9.8 Identifying Packet Contents
  
  
  • Different kinds of information can be carried in frames -- similar to the idea of data types.
    
    
  • Often it is useful to mark a frame with some indicator of its type. A special field within the frame can be used for this.
    
    
  • The type field of a frame can be part of the standard that defines the frame format, or it can just be some part of the frame that the sender and receiver agree will be used for this purpose.
    
    
• 9.9 Frame Headers And Frame Format
  
  
  • Most standards for frames call for a header portion followed by a payload portion for the actual data. Headers contain fields such as addresses, flags, type, and "checksums."
    
    
  • Headers are usually a fixed size. Payloads are often variable in size. Usually the header has a field that tells the total size of the packet.
    
    
• 9.10 An Example Frame Format
  
  
  • Ethernet frame headers have an 8-byte preamble for receiver sync, 6-byte destination address, 6-byte source address, and 2-byte frame type field.
    
    
  • The ethernet frame has a payload section following the header that can contain from 46 to 1500 bytes.
    
    
  • The type field can indicate what is in the payload section of the ethernet packet. For example the payload is often a packet belonging to a higher-level protocol. The type field can indicate that the payload is an IP, ARP, AppleTalk, or Novell Network packet.
    
    
  • The type field can help the receiver determine what to do with the ethernet packet. For example, if the payload is an IP packet the ethernet packt would be passed to a process running Internet Protocol software. On the other hand if the payload is a Novell Network packet, it would be handed to a Novell protocol process.
    
    
  • The ethernet frame has a trailer containing a 4-byte CRC.
    
    
• 9.11 Using Networks That Do Not Have Self Identifying Frames
  
  
  • For network technologies that do not have self-identifying frame standards, there is a LLC/SNAP standard whereby applications can use the first part of the payload section of a frame as a type field.
    
    
• 9.12 Network Analyzers, Physical Addresses, Frame Types
  
  
  • A network analyzer is a special purpose computer that connects to a network and monitors the performance of the network.
    
    
  • The analyzer can report throughput, average frame size, frequency of collision, break down of frame types, and so on.
    
    
  • The network interface on a network analyzer runs in promiscuous mode so that the interface passes all packets received up to the system software on the analyzer.
    
    
  • Note: Most any computer attached to a LAN can be used to read all information in all packets on the LAN.
    
    
  • The network analyzer can be instructed to examine all frames coming from a certain host, or all frames of a certain type, or combinations of these kinds of things, based on matching patterns in header or payload areas of the frames.
    
    
• 9.13 Summary
  
  
• 9.14 Ethernet Address Assignment
  
  
  • Manufacturers of Ethernet hardware apply to the IEEE Registration Authority to get ethernet numbers assigned to the ethernet cards they make.