Address Resolution Protocol

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In computer networking, the Address Resolution Protocol (ARP) is the method for finding a host's link layer (hardware) address when only its Internet Layer (IP) or some other Network Layer address is known. ARP is defined in RFC 826.[1] It is Internet Standard STD 37.

ARP has been implemented in many types of networks; it is not an IP-only or Ethernet-only protocol. It can be used to resolve many different network layer protocol addresses to interface hardware addresses, although, due to the overwhelming prevalence of IPv4 and Ethernet, ARP is primarily used to translate IP addresses to Ethernet MAC addresses. It is also used for IP over other LAN technologies, such as Token Ring, FDDI, or IEEE 802.11, and for IP over ATM.

In the next generation Internet Protocol, IPv6, ARP's functionality is provided by the Neighbor Discovery Protocol (NDP).

The Internet Protocol Suite
Application Layer
BGP · DHCP · DNS · FTP · GTP · HTTP · IMAP · IRC · Megaco · MGCP · NNTP · NTP · POP · RIP · RPC · RTP · RTSP · SDP · SIP · SMTP · SNMP · SOAP · SSH · Telnet · TLS/SSL · XMPP · (more)
Transport Layer
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Internet Layer
IP (IPv4, IPv6) · ICMP · ICMPv6 · IGMP · IPsec · (more)
Link Layer
ARP · RARP · NDP · OSPF · Tunnels (L2TP) · Media Access Control (Ethernet, MPLS, DSL, ISDN, FDDI) · Device Drivers · (more)
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Contents

[edit] Operating Scope

ARP is a Link Layer protocol because it only operates on the local area network or point-to-point link that a host is connected to.[2]

ARP is also very often discussed in terms of the Open Systems Interconnect (OSI) networking model, because that model addresses hardware-to-software interfaces more explicitly and is preferred by some equipment manufacturers. However, ARP was not developed based on the design principles and strict encapsulation hierarchy of this model and, therefore, such discussions create a number of conflicts as to the exact operating layer within this model. Most often ARP is placed into the Data Link Layer (Layer 2), but it also requires the definitions of network addresses of the Network Layer.

[edit] Packet structure

The following is the packet structure used for ARP requests and replies. On Ethernet networks, these packets use an EtherType of 0x0806, and are sent to the broadcast MAC address of FF:FF:FF:FF:FF:FF. Note that the EtherType (0x0806) is used in the Ethernet header, and should not be used as the PTYPE of the ARP packet. The ARP type (0x0806) should never be used in the PTYPE field of an ARP packet, since a hardware protocol address should never be linked to the ARP protocol. Note that the packet structure shown in the table has SHA, SPA, THA, & TPA as 32-bit words but this is just for convenience — their actual lengths are determined by the hardware & protocol length fields.

+ Bits 0 - 7 8 - 15 16 - 31
0 Hardware type (HTYPE) Protocol type (PTYPE)
32 Hardware length (HLEN) Protocol length (PLEN) Operation (OPER)
64 Sender hardware address (SHA) (first 32 bits)
96 Sender hardware address (SHA) (last 16 bits) Sender protocol address (SPA) (first 16 bits)
128 Sender protocol address (SPA) (last 16 bits) Target hardware address (THA) (first 16 bits)
160 Target hardware address (THA) (last 32 bits)
192 Target protocol address (TPA)
Hardware type (HTYPE) 
Each data link layer protocol is assigned a number used in this field. For example, Ethernet is 1.
Protocol type (PTYPE) 
Each protocol is assigned a number used in this field. For example, IP is 0x0800.
Hardware length (HLEN) 
Length in bytes of a hardware address. Ethernet addresses are 6 bytes long.
Protocol length (PLEN) 
Length in bytes of a logical address. IPv4 address are 4 bytes long.
Operation 
Specifies the operation the sender is performing: 1 for request, 2 for reply, 3 for RARP request, and 4 for RARP reply.
Sender hardware address (SHA) 
Hardware address of the sender.
Sender protocol address (SPA) 
Protocol address of the sender.
Target hardware address (THA) 
Hardware address of the intended receiver. This field is ignored in requests.
Target protocol address (TPA) 
Protocol address of the intended receiver.

[edit] ARP Announcements

An ARP announcement (also known as Gratuitous ARP) is a packet containing a valid Sender HA and SPA for the host which sent it, with TPA equal to SPA (ie destination IP = source IP). Such a request is not intended to solicit a reply, but merely updates the ARP caches of other hosts that receive the packet. This Gratuitous ARP is usually an ARP request [3], but it may also be an ARP reply [4].

Many operating systems perform this during startup. It helps to resolve problems which would otherwise occur if, for example, a network card was recently changed (changing the IP-address-to-MAC-address mapping) and other hosts still have the old mapping in their ARP caches.

Gratuitous ARP is also used by some drivers to ensure load balancing on incoming traffic. In a team of network cards, it is used to announce a different MAC address in the team to receive incoming packets.

ARP announcements can be used to defend link-local IP addresses in the (Zeroconf) protocol (RFC 3927), and for IP address takeover within high-availability clusters.

[edit] ARP Probe

This term is used in the IPv4 Address Conflict Detection specification (RFC 5227). Before beginning to use an IPv4 address (whether received from manual configuration, DHCP, or some other means), a host implementing this specification must test to see if the address is already in use, by broadcasting ARP probe packets. An ARP Request constructed with an all-zero sender IP address is referred to as an ARP Probe.

[edit] ARP mediation

ARP mediation refers to the process of resolving Layer 2 addresses when different resolution protocols are used on multiple connected circuits, e.g., ATM on one end and Ethernet on the others.

[edit] Variants of the protocol

ARP has also been adapted to resolve many types of Layer 2 addresses; for example, ATMARP is used to resolve ATM NSAP addresses in the Classical IP over ATM protocol.

[edit] Inverse ARP and Reverse ARP

The Inverse Address Resolution Protocol, also known as Inverse ARP or InARP, is a protocol used for obtaining Layer 3 addresses (e.g., IP addresses) of other nodes from Layer 2 addresses (e.g. the DLCI in Frame Relay networks). It is primarily used in Frame Relay and ATM networks, where Layer 2 addresses of virtual circuits are sometimes obtained from Layer 2 signaling, and the corresponding Layer 3 addresses must be available before these virtual circuits can be used.

ARP translates Layer 3 addresses to Layer 2 addresses, therefore InARP can be viewed as its inverse. In addition, InARP is actually implemented as an extension to ARP. The packet formats are the same; only the operation code and the certain field values differ.

Reverse ARP (RARP), like InARP, also translates Layer 2 addresses to Layer 3 addresses. However, RARP is used to obtain the Layer 3 address of the requesting station itself, while in InARP the requesting station is querying the Layer 3 address of another node. RARP was obsoleted by BOOTP which itself has been superseded by the Dynamic Host Configuration Protocol (DHCP).

[edit] See also

[edit] References

  1. ^ RFC 826 Address Resolution Protocol, Internet Standard STD-37
  2. ^ RFC 1122 Requirements for Internet Hosts -- Communication Layers
  3. ^ Re: [dhcwg] Gratuitous ARP in DHCP vs. IPv4 ACD Draft
  4. ^ RFC 2002 Section 4.6

[edit] External links

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

Retrieved from "http://en.wikipedia.org/wiki/Address_Resolution_Protocol"
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