Ch 9 Hardware Addressing and Frame Type Identification

1. Ch 9 Hardware Addressing and Frame Type Identification

2. Specifying a Recipient • All stations on shared-media LAN receive all transmissions • To allow sender to specify destination • Each station assigned a unique number • Known as station’s physical address, hardware address, or media access address (MAC address) • Each frame contains address of intended recipient in the header (also the sender’s address)

3. Conceptual Frame Format • Each LAN technology defines the exact format • Header • Contains address and typeinformation • Fixed size • Payload • Contains data being sent • Variable in size

4. Illustration Of Ethernet Frame • Sender places • 64-bit alternating 1s and 0s • Sender’s address in source • Recipient’s address in destination • Type of data in frame type • Cyclic redundancy check in CRC

5. Ethernet Addressing • Standardized by IEEE • Each station assigned by unique 48-bit address (demo) • Address assigned when network interface card (NIC) manufactured • Demo: ipconfig /all

6. Use Addresses to Filter Packets • LAN interface HW handles • Sending and receiving frames • Checks frame length and CRC • Compare the destination address in the received frame • Accept or discard(filter packets) Computer CPU Memory LAN NIC

7. Ethernet Address Recognition • Each frame contains destination address • All stations receive a transmission • Station discards any frame addresses to another station • Important: interface hardware, not software, checks address

8. Possible Destinations • Packet can be sent to: • Single destination (unicast) • All stations on network (broadcast) • Subset of stations (multicast) • Address used to distinguish A B C D E F

9. Advantages of Address Alternatives • Unicast • Efficient for interaction between two computers • Broadcast • Efficient for transmitting to all computers • Multicast • Efficient for transmitting to a subset of computers

10. Broadcast on Ethernet • Sender • Places broadcast address in frame • All 1s address specifies a broadcast • Transmits one copy on shared network • All stations receive the copy • Receiver always accepts BC frame CRC

11. Multicast on Ethernet A E B C F D Group 1: AC 1000…01 Group 2: CDF 1000…11

12. Multicast on Ethernet • Half addresses reserved for multicast (start with a 1 bit) • Network interface card • Always accepts unicast and broadcast • Can be programmed to accept zero or more multicast addresses • Software • Determines multicast address to accept • Informs NIC

13. Promiscuous Mode • Allows interface to accept all frames • Designed for testing / debugging • Available on most interface hardware

14. Network Analyzer • Listens in promiscuous mode for testing and maintenance • Produces • Summaries (e.g., % of broadcast frames) • Specific items (e.g., frames from a given address) • Example • Ethereal http://www.ethereal.com/ (WinPcap_3_0.exe and ethereal-setup-0.10.2.exe) • Sniffer

16. Exercise • Use the Ethereal to find the physical address of computers on a LAN

17. End

18. Identifying Frame Contents • Integer type field tells recipient the type of data being carried • For example, e-mail, web pages, or text files • Two possibilities • Self-identifying or explicit type (network hardware includes a type field in each frame) • Implicit type (application sending data must handle type)

19. Example Ethernet Types

20. When Network HardwareDoes Not Include Types • Sending and receiving computers must agree • To only send single type of data • To put type information in first few octets of payload • Questions • What size should the type information be? • Meaning of type value? • Multiple standards

21. A Standard For Type Information • Defined by IEEE, called LLC / SNAP header • LLC portionspecifies that a type field follows • SNAP portion contains 2 fields • Organizationally Unique Identifier • TYPE