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Reverse Address Resolution Protocol (RARP)

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1. Motivation and Purpose of RARP

In the early days of networked computing, many workstations were diskless machines (computers without a local hard drive or permanent storage).

• When a diskless workstation boots up, its operating system code resides in ROM, and its Physical (MAC) Address is permanently hardcoded on its Network Interface Card (NIC).
• However, the machine does not know its own Logical (IP) Address because it has no local storage drive to read a configuration file from!
• The Reverse Address Resolution Protocol (RARP) (RFC 903) was designed to solve this: it allows a host to discover its unknown IP address given its known physical MAC address.

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2. The RARP Operation

Operational Steps:

1. RARP Request:
   • The booting host broadcasts an RARP request frame (EtherType = 0x8035).
   • The frame contains its own physical hardware MAC address in both the Sender Hardware Address and Target Hardware Address fields, with the IP address fields left blank (0.0.0.0).
2. RARP Reply:
   • One or more dedicated RARP Servers on the local network intercept the broadcast.
   • The server queries its static mapping database (e.g., /etc/ethers).
   • If a matching MAC entry is found, the server replies via unicast with the host's assigned IP address.

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3. Critical Limitations of RARP

Why was RARP declared obsolete and replaced?

1. Data Link Layer Limitation:
   • RARP operates directly on top of the physical/data link layer (using raw Ethernet frames).
   • Because routers do not forward Layer 2 broadcasts, every physical subnet required its own dedicated RARP server! In a large enterprise with 50 subnets, 50 separate RARP servers were required.
2. Incomplete Configuration:
   • RARP only provided the 32-bit IP address.
   • It did not provide the Subnet Mask, the Default Gateway IP, the DNS Server IP, or the Boot Image File Path. The machine still could not communicate outside its local wire without further complex steps.
3. Static Mapping Only:
   • Administrators had to manually register every machine's MAC address in server configuration files. It could not dynamically lease or reclaim addresses.

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4. Comparison: ARP vs. RARP

Parameter	ARP (RFC 826)	RARP (RFC 903)
Input (Known)	Layer 3 Logical IP Address	Layer 2 Physical MAC Address
Output (Desired)	Layer 2 Physical MAC Address	Layer 3 Logical IP Address
Primary Use Case	Standard packet delivery on LAN	Diskless machine booting
Request Type	Broadcast (FF:FF:FF:FF:FF:FF)	Broadcast (FF:FF:FF:FF:FF:FF)
Reply Type	Unicast	Unicast
EtherType	0x0806	0x8035
Status Today	Active (Universal in IPv4)	Obsolete (Replaced by BOOTP & DHCP)

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