ARP and RARP
LESSON 7

Outline:
ARP and RARP

The Address Resolution Problem:

• ? How does a host or gateway map an IP address to the correct physical address when it needs to send a packet over a physical network ?
  
• Devise a low-level software that hides physical addresses and allows higher-level programs to work only with internet addresses.
  
• Mapping of high-level to low-level addresses is the address resolution problem.

Physical Addresses:

• Two basic types of physical addresses:
  • Large
    • Ethernet
    • 48 bits
  • Small
    • Token Ring (proNET-10)
    • 8 bits
• proNET-10 - uses Class C addresses with host-id portion = 1, 2, ... , 255

Physical Addresses:
Mapping

• Mapping must be computationally efficient (simply mask all portions of the address excluding the host-id portion)
• When address mapping can only be done via an address table (X.25 to IP), hashing is used to speed up lookup.
• Problem with representation of 48-bit Ethernet addresses within a 32-bit IP address and allowing new machines to be added without recompilation.
• Avoid maintaining a static table of mappings by using the ARP (Address Resolution Protocol).

Address Resolution Protocol (ARP):

• ARP - is a low-level protocol used to bind addresses dynamically.
• ARP allows a host to find a physical address of a target host on the same physical network, given only it’s IP address.
• ARP hides the underlying network physical addressing. It can be thought of as part of physical network system and not the internet protocols.

• ARP broadcasts special packets with the destination’s IP address to ALL hosts.
• The destination host (only) will respond with it’s physical address.
• When the response is received, the sender uses the physical address of destination host to send all packets.

ARP Inefficiencies and Improvements:

• Broadcasting is expensive on network resources, so an ARP cache of recently acquired IP-to-Physical address bindings is kept.
• Other Broadcast Improvements:
  • Include the sender’s IP-to-Physical address binding along with the request, to the destination. This reduces future traffic.
  • During each broadcast, ALL machines can find out the senders physical address and record it locally in it’s ARP cache.
  • When a new machine comes on-line, it immediately broadcasts it’s IP-to-Physical address binding to all nodes.

ARP Functionality:

• There are two main functional parts of the address resolution protocol:
  • Determine the destination’s physical address before sending a packet.
  • Answer requests that arrive for it’s own Physical-to-IP address binding.
• Because of lost/duplicate packets, ARP must handle this to avoid many re-broadcasts.
• Bindings in ARP cache (actual cache table) must be removed after a fixed period of time to ensure validity.
• When a packet is received, the sender’s IP address is stripped and the local table is updated (ARP cache), then the rest of the packet is processed.
• Two types of incoming packets:
  • Those to be processed (correct destination).
  • Stray broadcast packets (can be dropped after updating the ARP cache).
• Application programs may request the destination address many times before the binding is complete. This must be handled, by discarding enqueued requests, when the correct binding returns.

• ARP sets the field "TYPE" for the ID of a frame.
• ARP packets DO NOT have a fixed format header, so they can be used with arbitrary physical addresses and arbitrary protocol addresses.
• The lengths of physical addresses may vary up to 48-bits.

ARP Header Fields:

• Hardware Type: (16-bits) - the type of interface the sender seeks an answer for.
• Protocol Type: (16-bits) - the high-level software address type provided.
• HLEN: (8-bits) - length of arbitrary physical address.
• PLEN: (8-bits) - length of arbitrary protocol address.
• OPERATION: (16-bits) - the specific type of operation requested.
  • ARP.request (1)
  • ARP.response (2)
• SENDER HA: (6-octets) - the sender’s actual hardware address, scalable up to six bytes.
• SENDER IP: (4-octets) - the sender’s IP address, always 32-bits.
• TARGET HA: (6-octets) - the destination node’s hardware address, scalable up to six bytes.
• TARGET IP: (4-octets) - the destination node’s IP address, always 32-bits.

Booting with Physical Addresses:

• Machines without secondary-storage (diskless workstations) that cannot store their IP addresses need to find out by broadcasting their physical address and requesting a binding to the appropriate IP address.
• Terminals and nodes cannot connect to their fileservers without using source and destination IP addresses.
• IP addresses should not be bound into a machines bootstrap image. Not doing so allows for flexibility and convenient changes to configuration information.
• A connection is needed at start-up to allow the diskless workstation to begin functioning (sends request to a server and waits for the response).
• To provide the connection, workstations include a minimal set of TCP/IP protocols in firmware.
• Two advantages of using a unique physical address:
  • Physical address is available from the network interface hardware and does not have to be bound into the operating system.
  • The machine ID depends on the network (not it’s CPU) so all machines will be given uniform and unique IDs.

Reverse Address Resolution Protocol (RARP):

• RARP functionality supports multiple physical network types.
• All machines receive RARP requests but only those authorized to supply RARP services can respond. Machines supplying RARP services are called RARP servers.
• RARP is only used on LANs with a low probability of failure since bootstrapping requires quick responses.

Primary and Backup RARP Servers:

• Primary servers respond while backup servers simply record the time of the incoming requests.
• If the primary server is down, backup server will see reoccurring requests and take over. This take over is done by using a random delay to see if other backup servers respond first.

RARP Header Fields:

• RARP requests and responses use the same frame format as ARP, however the OPERATION field value differs.
• OPERATION: (16-bits) - the specific type of operation requested.
  • RARP.request (3)
  • RARP.response (4)

Last Modification: (Sunday, August 25, 1996)

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1996 Illusion Industries Inc.