1 Fall 2005 Hardware Addressing and Frame Identification Qutaibah Malluhi CSE Department Qatar University

2 Identifying a Destination  Data sent across a shared network reaches all attached stations –All stations on shared-media LAN receive all transmissions –True for all LAN topologies  Need to devise a technique for delivering message through LAN medium to single, specific destination computer  To allow sender to specify destination –Each computer is assigned a unique number. Commonly referred to as »hardware address »physical address, or »media access (MAC) address

3 Specifying a Recipient  Sending computer includes in the frame –Destination hardware address –Source hardware address (its own), for most LAN technologies –Sending computer also identifies type of data carried in the frame  Only the computer identified in frame receives copy of frame. –Frames not addressed to me are discarded by my net interface card

4 LAN Interface Hardware  LAN interface handles all details of frame transmission and reception –Adds hardware addresses, error detection codes, etc. to outgoing frames –May use DMA to copy frame data directly from main memory (when sending) or to main memory (when receiving) –Obeys access rules (e.g., CSMA/CD) when transmitting –Checks error detection codes on incoming frames –Checks destination address on incoming frames  If destination address on incoming frame matches the local station's address, a copy of the frame is passed to the attached computer  Frames not addressed to the local computer are ignored –don't affect the local computer in any way – do not interfere with processing or memory of local computer

5 Format of Physical Address  What valid numeric value are used?  What is the size of this value? –In practice 1-6 bytes  How are these addresses assigned?  Where are they located inside the frame?  The specific network technology/standard specifies all of the above questions

6 Assigning Hardware Addresses  Hardware addresses must be unique on a LAN  How can those address be assigned and who is responsible for uniqueness?  Static: Hardware manufacturer assigns permanent address to each interface –Manufacturer must ensure every interface has a unique address  Configurable: Address can be set by end user, either thru switches or jumpers on the interface or thru software –System administrators must coordinate to avoid conflict  Dynamic: Interface automatically assigns hardware address each time it is powered up –Automatic scheme must be reliable to prevent conflicts

7 Broadcast Address  Some applications want to broadcast messages to all stations on the LAN  Because LAN technologies use a shared communication channel, broadcasting is naturally very efficient  Special reserved broadcast address –Station’s network interface hardware accepts its own address as well as the broad cast address.

8 Multicasting  Broadcasting consumes computational resources on the stations because the station’s memory and CPU (not the Net Interface Hardware) are involved in processing and discarding unwanted broadcast frames.  Multicasting –Network interface card »Always accepts unicast and broadcast »Can accept zero or more multicast address –Application software »Determine multicast address to accept »Informs network interface card

9 Identifying Frame Content  Destination must get some clue about how to interpret frame data –E.g., ASCII, ARP, IPX, etc.  Can use: –Explicit frame type - identifying value included with frame describes type of included data (self-identifying frames) –Implicit frame type – sender and receiver must agree on »a content of the frame, or »a mechanism to determine the content from the data

10 Conceptual Frame Format  LAN technology standards define frame format for each technology  All contemporary standards use a format consisting of a header part and a data part.  Header –Has address and other identifying information –Has fixed layout -- fields with fixed size and location  Payload –Data area –May vary in size

11 Ethernet Addressing  Standardized by IEEE  Each station assigned unique 48-bit address  Addresses are static and are assigned when network interface card (NIC) manufactured  All 1s address specifies broadcast  Addresses starting with 1 are used for multicast –Half of address reserved for multicast  Uses self-identifying frames –2-byte explicitly determine frame type

12 Ethernet Frame Format  Preamble is alternating 1’s and 0’s to synchronize the sender and receiver hardware  Sender places –Sender’s address in source –Recipient’s address in destination –Type of data in frame type –Cyclic redundancy check in CRC

13 Ethernet Type Examples

14 LANs without Frame Type  If not frame type field, sending and receiving computers must agree –To only send one type of data (imposes severe limitations) –To put type information in first few octets of payload  Most system need type information  In practice, to ensure interoperability –format of encoding area must be universally agreed upon –format typically set by standards body

15 IEEE LLC/SNAP  IEEE standard includes Logical Link Control (LLC) SubNetwork Attachment Point (SNAP) header  SNAP/LLC header is widely used; e.g., by Ethernet  LLC portion indicates SNAP field to follow  OUI (Organizationally Unique Identifier) identifies –Ethernet specification organization in this example  TYPE field interpreted as in Ethernet –IP type in this example

16 Demultiplexing On Type  Network interface hardware –Received copy of each transmitted frame –Examines address and either discards or accepts –Passes accepted frames to system software  Network device software –Examines frame type –Passes frames to correct software module

17 Network Analyzers  A network analyzer or network monitor or ``network sniffer'' is used to examine the performance of or debug a network –Can report statistics such as capacity utilization, distribution of frame size, collision rate or token circulation time –Can record and display specific frames, to understand and debug packet transmissions and exchanges  Basic idea is a computer with a network interface that receives all frames –This mode of operation is called promiscuous mode –Standard interface cards can be configured for promiscuous mode  Combined with software, a computer can examine any frame on LAN –No privacy over a LAN

18 Filtering Frames  Analyzer can be configured to filter and process frames –Count frames of a specific type or size –Display only frames from or to specific computers –In general, can be configured to match value of any field and capture only those frames meeting the filter specification  Analyzer can display real-time performance by computing running totals over specific time periods

19 Hardware Addressing Summary  LAN technologies use hardware addresses to identify frame destination –Each station must have a unique address on the LAN segment  Hardware addresses may be statically assigned, configurable or automatically assigned  Each LAN technology defines its own hardware format  Frames include a header with fields for destination, source and other information such as frame type  Frame type defines how to interpret frame data  Network analyzer can receive all frames and display statistics or aid in debugging problems