What is ARP?
The Address Resolution Protocol(ARP) is a communication protocol used to discover the data-link layer address(Layer 2 address like Media Access Control(MAC) address) associated with an Internet layer address(Layer 3 address like IPv4 address). ARP was defined in 1982 by RFC 826.
ARP is a request-response or request-reply protocol in which one device sends a request to another device asking for some information, to which the other device will reply with the required information. It is a message exchange pattern. ARP packets are encapsulated by link layer and are distributed only in a particular network. As a result, ARP is said to be a link layer protocol.
The software of Tonmind Network Speaker: IPTool and Tonmind Audio Manager have applied ARP Protocol to scan IP Speaker rapidly for SIP, RTP Multicast configuration, which is also convenient for bulk setting.
Packet structure
The Address Resolution Protocol uses a simple message format containing one address resolution request or response. The size of the ARP message depends on the link layer and network layer address sizes. The message header specifies the types of network in use at each layer as well as the size of addresses of each. The message header is completed with the operation code for request and reply . 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). The ARP packet size in this case is 28 bytes.
Hardware type (HTYPE)
This field specifies the network link protocol type. Example: Ethernet is 1.
Protocol type (PTYPE)
This field specifies the internetwork protocol for which the ARP request is intended. For IPv4, this has the value 0x0800. The permitted PTYPE values share a numbering space with those for EtherType.
Hardware length (HLEN)
Length (in octets) of a hardware address. Ethernet address length is 6.
Protocol length (PLEN)
Length (in octets) of internetwork addresses. The internetwork protocol is specified in PTYPE. Example: IPv4 address length is 4.
Operation
Specifies the operation that the sender is performing: 1 for request, 2 for reply.
Sender hardware address (SHA)
Media address of the sender. In an ARP request this field is used to indicate the address of the host sending the request. In an ARP reply this field is used to indicate the address of the host that the request was looking for.
Sender protocol address (SPA)
Internetwork address of the sender.
Target hardware address (THA)
Media address of the intended receiver. In an ARP request this field is ignored. In an ARP reply this field is used to indicate the address of the host that originated the ARP request.
Target protocol address (TPA)
Internetwork address of the intended receiver.
ARP protocol parameter values have been standardized and are maintained by the Internet Assigned Numbers Authority (IANA).
The EtherType for ARP is 0x0806. This appears in the Ethernet frame header when the payload is an ARP packet and is not to be confused with PTYPE, which appears within this encapsulated ARP packet.
How does ARP work?
When a new computer joins a LAN, it is assigned a unique IP address to use for identification and communication. When an incoming packet destined for a host machine on a particular LAN arrives at a gateway, the gateway asks the ARP program to find a MAC address that matches the IP address. A table called the ARP cache maintains a record of each IP address and its corresponding MAC address.
All operating systems in an IPv4 Ethernet network keep an ARP cache. Every time a host requests a MAC address in order to send a packet to another host in the LAN, it checks its ARP cache to see if the IP to MAC address translation already exists. If it does, then a new ARP request is unnecessary. If the translation does not already exist, then the request for network addresses is sent and ARP is performed.
ARP broadcasts a request packet to all the machines on the LAN and asks if any of the machines are using that particular IP address. When a machine recognizes the IP address as its own, it sends a reply so ARP can update the cache for future reference and proceed with the communication.
Host machines that don't know their own IP address can use the Reverse ARP (RARP) protocol for discovery.
ARP cache size is limited and is periodically cleansed of all entries to free up space. Addresses tend to stay in the cache for only a few minutes. Frequent updates enable other devices in the network to see when a physical host changes their requested IP addresses. In the cleaning process, unused entries are deleted along with any unsuccessful attempts to communicate with computers that are not currently powered on.
Tonmind IP Paging Speakers Software will be provided for customers to use free of charge. There is no licensing fees. Our engineers team have been devoted to working on upgraded version for better compatibility.
Tonmind also supplies SIP VoIP Gateway, SIP Board Module. The integrators can use them to configure according to project needs. Our engineers will provide professional support and advice.
