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The Address Resolution Protocol (ARP) is a computer networking protocol for determining a network host's link layer or hardware address when only its Internet Layer (IP) or Network Layer address is known. This function is critical in local area networking as well as for routing internetworking traffic across gateways (routers) based on IP addresses when the next-hop router must be determined. ARP was defined by RFC 826 in 1982.[1] It is Internet Standard STD 37. ARP has been implemented in many types of networks, such as Internet Protocol (IP) network, CHAOS, DECNET, Xerox PARC Universal Packet, Token Ring, FDDI, IEEE 802.11 and other LAN technologies, as well as the modern high capacity networks, such as Asynchronous Transfer Mode (ATM). Due to the overwhelming prevalence of IPv4 and Ethernet in general networking, ARP is most frequently used to translate IP addresses to Ethernet MAC addresses. In the next generation Internet Protocol, IPv6, ARP's functionality is provided by the Neighbor Discovery Protocol (NDP).
[edit] Operating scopeThe Address Resolution Protocol is a low level request and answer protocol that is communicated on the media access level of the underlying network. For Ethernet systems, an ARP message is the payload of Ethernet packets. ARP therefore operates only across the local link that a host is connected to. Within the framework of the Internet Protocol Suite, this characteristic makes ARP a Link Layer protocol.[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 since it requires the definitions of network addresses of the Network Layer, it is not unusual to find it referenced at that layer. An example of use in OSI networking, is ATMARP, used to resolve Asynchronous Transfer Mode (ATM) NSAP addresses in IP over ATM deployments. [edit] Packet structureThe Address Resolution Protocol uses a simple message format that contains one address resolution request or response. The size of the ARP message depends on the upper layer and lower layer address sizes, which are given by the type of networking protocol (usually IPv4) in use and the type of hardware or virtual link layer that the upper layer protocol is running on. The message header specifies these types, as well as the size of addresses of each. The message header is completed with the operation code for request (1) and reply (2). The payload of the packet consists of four addresses, the hardware and protocol address of the sender and receiver hosts. The principal packet structure of ARP packets is shown in the following table which illustrates the case of IPv4 networks running on Ethernet. In this scenario, the packet has 48-bit fields for the sender hardware address (SHA) and target hardware address (THA), and 32-bit fields for the corresponding sender and target protocol addresses (SPA and TPA). Thus, the ARP packet size in this case is 28 bytes.
ARP protocol parameter values have been standardized and are maintained by IANA.[3] [edit] ARP probeAn ARP probe is an ARP request constructed with an all-zero sender IP address. The 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. [edit] ARP announcementsARP may also be used as a simple announcement protocol. This is useful for updating other host's mapping of a hardware address when the sender's IP address or MAC address has changed. Such an announcement, also called a gratuitous ARP message, is usually broadcast as an ARP request containing the sender's protocol address (SPA) in the target field (TPA=SPA), with the target hardware address (THA) set to zero. An alternative is to broadcast an ARP reply with the sender's hardware and protocol addresses (SHA and SPA) duplicated in the target fields (TPA=SPA, THA=SHA). An ARP announcement is not intended to solicit a reply; instead it updates any cached entries in the ARP tables of other hosts that receive the packet. The operation code may indicate a request or a reply because the ARP standard specifies that the opcode is only processed after the ARP table has been updated from the address fields.[4][5][6] Many operating systems perform gratuitous ARP during startup. That 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 interface drivers to effect load balancing for incoming traffic. In a team of network cards, it is used to announce a different MAC address within the team that should 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 mediationARP 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] Inverse ARP and Reverse ARPThe Inverse Address Resolution Protocol (Inverse ARP or InARP), is a protocol used for obtaining Network Layer addresses (e.g., IP addresses) of other nodes from Data Link Layer (Layer 2) addresses. It is primarily used in Frame Relay (DLCI) and ATM networks, in which 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. As ARP translates Layer 3 addresses to Layer 2 addresses, InARP may be described as its inverse. In addition, InARP is actually implemented as a protocol extension to ARP. It uses the same packet format from ARP; but has different operation codes. Reverse Address Resolution Protocol (Reverse ARP or RARP), like InARP, also translates Layer 2 addresses to Layer 3 addresses. However, while in InARP the requesting station is querying the Layer 3 address of another node, RARP is used to obtain the Layer 3 address of the requesting station itself for address configuration purposes. RARP is now obsolete. It was replaced by BOOTP, which was later superseded by the Dynamic Host Configuration Protocol (DHCP). [edit] See also
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This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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