Protocols

In Information Technology, a communication protocol is a system of rules that allow two or more entities of a communication system to communicate between them to transmit information via any kind of variation of a physical quantity. These are the rules or standard that defines the syntax, semantics and synchronization of communication and possible error recovery methods. Protocols may be implemented by hardware, software or combination of both.

Internet Protocol (IP)

Internet Protocol (IP) essentially is the Internet layer. The other protocols found here merely exist to support it. Every device on the network have a software or logical address called an IP address. IP looks at each packet’s address. Then, using a routing table, it decides where a packet is to be sent next, choosing the best path.
IP receives segments from the Host-to-Host layer and fragments them into datagrams (datagram is basically protocol data units which in case of IP is packets) if necessary. IP then reassembles datagrams back into segments on the receiving side. Each datagram is assigned the IP address of the sender and of the recipient. Each router (layer-3 device) that receives a datagram makes routing decisions based on the packet’s destination IP address.

Internet Control Message Protocol (ICMP)

ICMP works at the Network layer and is used by IP for many different services. ICMP is a management protocol and messaging service provider for IP. Its messages are carried as IP datagrams.
ICMP packets have the following characteristics:
-They can provide hosts with information about network problems.
-They are encapsulated within IP datagrams.

ICMP relates to these events and messages:

Destination Unreachable: If a router can’t send an IP datagram any further, it uses ICMP to send a message back to the sender, advising it of the situation.
Buffer Full/Source Quence: If a router’s memory buffer for receiving incoming datagrams is full, it will use ICMP to send out this message until the congestion abates.
Hops/Time Exceeded: Each IP datagram is allotted a certain number of routers, called hops, to pass through. If it reaches its limit of hops before arriving at its destination, the last router to receive that datagram deletes it. The executioner router then uses ICMP to send an obituary message, informing the sending machine of the demise of its datagram.
Ping: Packet Internet Groper (Ping) uses ICMP echo request and reply messages to check the physical and logical connectivity of machines on a network.
Traceroute: Using ICMP time-outs, Traceroute is used to discover the path a packet takes as it traverses a network.

Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP) finds the hardware address of a host from a known IP address. When IP has a datagram to send, it must inform a Network Access protocol, such as Ethernet or wireless, of the destination’s hardware address on the local network. (It has already been informed by upper-layer protocols of the destination’s IP address.) If IP doesn’t find the destination host’s hardware address in the ARP cache, it uses ARP to find this information. ARP interrogates the local network by sending out a broadcast asking the machine with the specified IP address to reply with its hardware address. So basically, ARP translates the software (IP) address into a hardware address.

Reverse Address Resolution Protocol (RARP)

RARP works opposite to ARP. It resolves MAC addresses to IP addresses. When an IP machine happens to be a diskless machine, it has no way of initially knowing its IP address, but it does know its MAC address. Reverse Address Resolution Protocol (RARP), discovers the identity of the IP address for diskless machines by sending out a packet that includes its MAC address and a request for the IP address assigned to that MAC address. A designated machine, called a RARP server, responds with the answer.

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