Hardware Addressing and Frame Type Identification the need for addressing addressing in hardware static, dynamic and configurable addressing broadcast addresses and promiscuous mode frame type and frame format Ethernet frame format

The need for addressing A LAN provides a physical path from one computer to all others Most communication is from one computer to one another addressing allows the source to identify the destination hardware addressing prevents unnecessary messages being seen by a computer

Specifying a recipient Each station is assigned a unique numerical address on the network called its hardware address or physical address Each frame transmitted across the network typically includes a destination address field and a source address field the network hardware in each station decodes each frame and uses the destination address to determine whether to accept it including the source address field makes it easy to generate a reply

LAN hardware and filtering LAN hardware is typically completely separate from the computer’s CPU and memory

Static, dynamic and configurable hardware addresses Static - the manufacturer assigns the hardware address which never changes Dynamic - assign the address whenever the station boots up Configurable - the customer sets the address (usually on installation)

Pros and cons Static - easier for customer, addresses never change and computers can easily move network Dynamic - easier for vendor, addresses can be smaller, but conflicts can arise Configurable - allows permanent and smaller addresses

Broadcasting Some applications require that a sender transmits a frame to all stations on the network Use a special reserved broadcast address All stations configured to accept packets for their own address and the broadcast address

Multicasting Restricted form of broadcasting Broadcast capability without wasting CPU resources on all computers E.g. allows some computers to participate in an audio transmission Extends the addressing scheme by reserving some addresses for multicast Extends the network interface to accept an additional set of addresses

Promiscuous mode Network hardware can often be programmed to accept all packets - promiscuous mode Useful for network analysers (sniffers) Security implications Wireless networks can overhear and analyse packets sent within their range Delay Tolerant Networks -- data couriers Mobile Ad Hoc networks – opportunistic data transfers Security: Encryption and/or collaborative reputation

Identifying packet contents Computer may need to know what type a packet is in order to process it Explicit frame type - network hardware designers specify how type information is included in the frame and may define types Implicit frame type - the sending and receiving computers must agree how to specify types in software

Frame format Frames typically include a frame header and a frame data area header has a fixed size data area varies between a minimum and maximum

Example - Ethernet frame format Ethernet uses a 48 bit static addressing scheme and a 16 bit frame type Ethernet standards define standard frame types

Implicit frame types Either all computers agree to use a single standard format for data or they negotiate their own type fields (but these are not seen by the network hardware) IEEE standard to define frame types in software first component identifies a standards organisation second component is defined by that organisation

Example of Frame Types Used with Ethernet

Summary The need for addressing Addressing in hardware network interface cards static, dynamic and configurable addressing broadcast addresses, multicast addresses and promiscuous mode Frame type and frame format Ethernet frame format