Ch. 9 Hardware Addressing & Frame Type Identification

1. Ch. 9 Hardware Addressing & Frame Type Identification Physically, any signal sent across a shared network reaches all attached stations. At a given station, the network interface hardware detects the signal and extracts a copy of the frame Q: How can two computers communicate directly across a shared medium in which all attached stations receive a copy of all signals? A: Use physical address (Hardware address, MAC address • When a sender transmits a frame across the LAN, the sender includes the hardwarehardware address of the intended recipient in the frame header • Although all stations receive a copy of the frame, the LAN interface hardware on each station checks the address in the frame header to determine whether it should accept the frame A unique numeric value associated with the Network interface hardware

2. Organization of the hardware in a computer attached to a LAN Network interface hardware handles all the details of sending/receiving frames: • Checks the length of an incoming frame * to make sure it is within the bound • Checks the CRC * to ensure the bits arrived intact * to discard frames that contain errors • Compares the destination address in the frame to its physical address: * if matched, accepts the frame and passes it to the O.S.. * if not, discards it Note: the network interface hardware does not use the CPU of the computer Network Interface Hardware Processor & Memory LAN connection

3. Format of a Physical Address: • Static * Address assigned by the hardware manufacturer * permanent, ease of use • Configurable * Address set when (configured) when the hardware is first installed * permanent  address remains the same across reboots • Dynamic * Address automatically assigned when the station first boots The station generates a number as the address, then sends a message to that address over the network If no response  O.K. otherwise  address already in use, need to choose another one *Address may vary each time the station reboots

4. Addressing Schemes • Broadcasting * A mode of operation that allows the possibility of addressing a packet to all destinations on the network * Using a special code (the broadcast address) in the address field • Multicasting * A mode of operation that supports packet transmission to a subset of the machines on the network * A restricted form of broadcasting, by reserving some addresses (the multicast address) for multicasting • Unicasting * The traditionalphysical address is classified as a unicast address, which denotes a single station

5. Example Frame Formats Ethernet Frame Format 32-bit CRC 8 6 6 2 46-1500 4 bytes • Preamble (64 bits) * synchronization pattern of alternating 1s and 0s • DA (Destination address, 48 bits) * unicast: 0 followed by 0s & 1s * broadcast : 1 followed by all 1s * multicast: 1 followed by 0s & 1s • SA (Source address, 48 bits) • Frame Type (16 bits) * Allows Ethernet software to route the user data to a specific protocol, application, or process Frame Type Preamble DA SA Data (46-1500) FCS Header Data Trailer

6. 0000-05DC •  Reserved for use with • IEEE LLC/SNAP • 0800  IPv4 • 0805  X.25 • 6559  Frame relay • 8035  RARP • 80D5  ISM SNA •  Novell IPX • … • Frame Type (16 bits) * see P.130 for other types • User Data * must be in [46, 1500] bytes long • FCS (Frame check sequence) * a 32-bit CRC value covering all bits in the packet, excluding the preamble and FCS • Note: * the end-of-frame is determined when no traffic is detected on the Ethernet * a min. frame spacing of 9.6 ms is required for the receiving hardware to examine the frame and pass it to the higher layer of the software

7. Example Frame Formats IEEE 802.3 Frame Format 32-bit CRC 7 1 6 6 2 46-1500 4 byte Preamble SFD DA SA Data (46-1500) FCS Length • Preamble (56 bits) * synchronization pattern of alternating 1s and 0s (1010…10), used by receiver • SFD (Start Frame Delimiter): 10101011 • DA (Destination address, 48 bits) * can be 2 octets (2 bytes) long, depending on implementation • SA (Source address, 48 bits) * can be 2 octets (2 bytes) long, depending on implementation • Length (16 bits): the length of data • Pad: used when data is less than 46 octets • FCS (Frame Check Sequence) * a 32-bit CRC, based on all fields except Preamble, SFD, & FCS Header Data Trailer Type field in the Ethernet Frame format

8. Some network technologies do not include a Type field in the header. • Q: How can two computers on such networks know the type of data in each frame? • A: Two approaches 1. S and R agree to use a single format of data  too restrictive, seldom used 2. S and R agree to use the first few octets of the data field to strore type information  two questions: a) what should the size be? b) who should specify the values?

9. A frame with a frame type is called Self-identifying • For frames that are not self-identifying, the software is free to choose how to interpret type information  does not work well because programmers might accidentally select the same value for different type information • To ensure all software agrees on values used to specify types, standard organizations have defined the meaning of each value  unfortunately, many standard orgs. have made such assignments and they do not always coordinate their efforts  the same value might be assigned to different types by two orgs. • To solve this problem, IEEE has defined a standard that includes two fields: one to identify the org., the other to identify a type as defined by that org.  widely used  the specification is known as a LLC/SNAP header • An example LLC/SNAP header: (Next)

10. LLC LLC-H SNAP-H MAC • An example LLC/SNAP header: (located at the beginning of the frame data area) • LLC: Logical Link Control SNAP: Subnetwork Attachment Point Type 1  unacknowledged connectionless service AA AA 03 00 00 00 08 00 (8 octets) A type field follows OUI Type Type 1 LLC PDU (0: Ethernet org.) (0800: IPv4) (OUI: Org. Unique Identifier)