WELL KNOWN PROTOCOL

Address Resolution Protocol (ARP) 

For two machines on a given network to communicate, they must know the other machine’s physical 

(orMAC) addresses. By broadcasting Address Resolution Protocols (ARPs), a host can dynamically 

discover the MAC-layer address corresponding to a particular IP network-layer address. 

After receiving aMAC-layer address, IP devices create an ARP cache to store the recently acquired 

IP-to-MAC address mapping, thus avoiding having to broadcast ARPS when they want to recontact 

a device. If the device does not respond within a specified time frame, the cache entry is flushed. 

In addition to the Reverse Address Resolution Protocol (RARP) is used to map MAC-layer addresses 

to IP addresses. RARP, which is the logical inverse of ARP, might be used by diskless workstations 

that do not know their IP addresses when they boot. RARP relies on the presence of a RARP server 

with table entries of MAC-layer-to-IP address mappings. 

Internet Control Message Protocol (ICMP) 

The Internet Control Message Protocol (ICMP) is a network-layer Internet protocol that provides 

message packets to report errors and other information regarding IP packet processing back to the 

source. 

ICMP Messages 

ICMPs generate several kinds of useful messages, including Destination Unreachable, Echo Request 

and Reply, Redirect, Time Exceeded, and Router Advertisement and Router Solicitation. If an ICMP 

message cannot be delivered, no second one is generated. This is to avoid an endless flood of ICMP 

messages. 

When an ICMP destination-unreachable message is sent by a router, it means that the router is unable 

to send the package to its final destination. The router then discards the original packet. Two reasons 

exist for why a destination might be unreachable. Most commonly, the source host has specified a 

nonexistent address. Less frequently, the router does not have a route to the destination. 

Destination-unreachable messages include four basic types: network unreachable, host unreachable, 

protocol unreachable, and port unreachable. Network-unreachable messages usually mean that a 

failure has occurred in the routing or addressing of a packet. Host-unreachable messages usually 

indicates delivery failure, such as a wrong subnet mask. Protocol-unreachable messages generally 

mean that the destination does not support the upper-layer protocol specified in the packet. 

Port-unreachable messages imply that the TCP socket or port is not available. 

An ICMP echo-request message, which is generated by the ping command, is sent by any host to test 

node reachability across an internetwork. The ICMP echo-reply message indicates that the node can 

be successfully reached. 

An ICMP Redirect message is sent by the router to the source host to stimulate more efficient 

routing. The router still forwards the original packet to the destination. ICMP redirects allow host 

routing tables to remain small because it is necessary to know the address of only one router, even if 

that router does not provide the best path. Even after receiving an ICMP Redirect message, some 

devices might continue using the less-efficient route. 

ICMP Router-Discovery Protocol (IDRP) 

IDRP uses Router-Advertisement and Router-Solicitation messages to discover the addresses of 

routers on directly attached subnets. Each router periodically multicasts Router-Advertisement 

messages from each of its interfaces. Hosts then discover addresses of routers on directly attached 

subnets by listening for these messages. Hosts can use Router-Solicitation messages to request 

immediate advertisements rather than waiting for unsolicited messages. 

IRDP offers several advantages over other methods of discovering addresses of neighboring routers. 

Primarily, it does not require hosts to recognize routing protocols, nor does it require manual 

configuration by an administrator. 

Router-Advertisement messages enable hosts to discover the existence of neighboring routers, but 

not which router is best to reach a particular destination. If a host uses a poor first-hop router to reach 

a particular destination, it receives a Redirect message identifying a better choice. 

Transmission Control Protocol (TCP) 

The TCP provides reliable transmission of data in an IP environment. TCP corresponds to the 

transport layer (Layer 4) of the OSI reference model. Among the services TCP provides are stream 

data transfer, reliability, efficient flow control, full-duplex operation, and multiplexing. 

With stream data transfer, TCP delivers an unstructured stream of bytes identified by sequence 

numbers. This service benefits applications because they do not have to chop data into blocks before 

handing it off to TCP. Instead, TCP groups bytes into segments and passes them to IP for delivery. 

TCP offers reliability by providing connection-oriented, end-to-end reliable packet delivery through 

an internetwork. It does this by sequencing bytes with a forwarding acknowledgment number that 

indicates to the destination the next byte the source expects to receive. Bytes not acknowledged 

within a specified time period are retransmitted. The reliability mechanism of TCP allows devices 

to deal with lost, delayed, duplicate, or misread packets. A time-out mechanism allows devices to 

detect lost packets and request retransmission. 

User Datagram Protocol (UDP) 

The User Datagram Protocol (UDP) is a connectionless transport-layer protocol (Layer 4) that 

belongs to the Internet protocol family. UDP is basically an interface between IP and upper-layer 

processes. UDP protocol ports distinguish multiple applications running on a single device from one 

another. 

Unlike the TCP, UDP adds no reliability, flow-control, or error-recovery functions to IP. Because of 

UDP’s simplicity, UDP headers contain fewer bytes and consume less network overhead than TCP. 

UDP is useful in situations where the reliability mechanisms of TCP are not necessary, such as in 

cases where a higher-layer protocol might provide error and flow control. 

UDP is the transport protocol for several well-known application-layer protocols, including Network 

File System (NFS), Simple Network Management Protocol (SNMP), Domain Name System (DNS), 

and Trivial File Transfer Protocol (TFTP). 

Internet Protocols Application-Layer Protocols 

The Internet protocol suite includes many application-layer protocols that represent a wide variety 

of applications, including the following: 

• File Transfer Protocol (FTP)—Moves files between devices 

• Simple Network-Management Protocol (SNMP)—Primarily reports anomalous network 

conditions and sets network threshold values 

• Telnet—Serves as a terminal emulation protocol 

• X Windows—Serves as a distributed windowing and graphics system used for communication 

between X terminals and UNIX workstations 

• Network File System (NFS), External Data Representation (XDR), and Remote Procedure Call 

(RPC)—Work together to enable transparent access to remote network resources 

• Simple Mail Transfer Protocol (SMTP)—Provides electronic mail services 

• Domain Name System (DNS)—Translates the names of network nodes into network addresses

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