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 type information 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) NIC CPU Memory LAN Computer

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 1 s 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 BC D E F Group 1: AC1000…01 Group 2: CDF1000…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 (WinPcap_3_0.exe and ethereal-setup exe) Sniffer

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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, , 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 Hardware Does 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 portion specifies that a type field follows  SNAP portion contains 2 fields Organizationally Unique Identifier TYPE