Arp and rarp

Transmission on LAN’s requires knowledge of destination hardware address (physical address, MAC address).

On a LAN hardware addresses are unique. Address resolution is translation of destination IP

address to destination hardware address

Ex 1: A->B tranmission: A resolves B’s MAC address based on B’s IP address

Ex 2: A->D transmission: A resolves Y1’s MAC address based on Y1’s IP address, Y1 resolves Y2’s MAC address based on Y2’s IP address, Y2 resolves D’s MAC address based on D’s IP address.

Network1 Network2 Network3

Table look-up

◦ Each node keeps a separate table◦ Network adress part of IP address need not be stored.◦ If address mapping changes all nodes update their tables

IP address Physical address (Hardware address)

Computation ◦ Derivation from the host address part of IP address◦ Useful in network that employ reconfigurable (dynamical) port

address.

IP address Physical address(Hardware address)

Message exchange◦ A server responds to address resolution request that it receives◦ Any machine that knows the hardware address responds to

address resolution request broadcasts

Is based on message exchange

Is simple. Optimizations:

◦ Caching◦ A machine broadcasts its

own IP-Ethernet binding At boot time With address resolution

request ARP message is

placed into the payload field of LAN(DLL) frame.

Hardware addr. type Protocol addr. type

Hardware addr. length

Protocol addr. length

Operation

Source hardware addr. (first 4 bytes)

Source hardware addr. (last 2 bytes)

Source protocol addr. (first 2 bytes)

Source protocol addr. (last 2 bytes)

Dest. Hardware addr. (first 2 bytes)

Dest. Hardware addr. (last 4 bytes)

Dest. Protocol addr.

ARP message

Three interconnected /24 networks: two Ethernets and an FDDI ring.

Two possibilities for transmissions to other networks:• 65.7 broadcasts ARP request for 63.8, to which 65.1 responds with(63.8-E3)2. 65.7 realizes 63.8 is on a different network and binds (63.8-E3)

ARP request message (broadcast)

ARP response message (unicast) 0x1 (Ethernet) 0x0800 (IP)

0x100 0x100000000

2 (Response)

0x0

0xE6 192.31

63.8 0x0

0xE1

192.31.65.7

0x1 (Ethernet) 0x0800 (IP)

0x100 0x100000000

1 (Request)

0x0

0xE1 192.31

65.7 0x0

0x0

192.31.63.8

A diskless workstation learns its IP address based on its Ethernet address◦ Workstation broadcasts its Ethernet address.◦ RARP server responds by table look-up and sending back

the corresponding IP address◦ Since the operating system image is IP address independent,

a single image kept at a remote server can be downloaded at boot time.

Since broadcasts are not forwarded by routers a single RARP server is needed on each network (or subnet)