All rights reserved. How A10 Networks Supports Layer 4 Load Balancing Layer 4 load balancing and layer 7 load balancing can each have a role to play in traffic management. Learn about our layer 4 load balancing products. ARP assists IP in directing datagrams to the appropriate receiving host by mapping Ethernet addresses 48 bits long to known IP addresses 32 bits long.
ICMP reports on the following:. Dropped packets — Packets that arrive too fast to be processed. The ping Command contains more information on the operating system commands that use ICMP for error detection. TCP enables applications to communicate with each other as though connected by a physical circuit. TCP sends data in a form that appears to be transmitted in a character-by-character fashion, rather than as discrete packets.
This transmission consists of a starting point, which opens the connection, the entire transmission in byte order, and an ending point, which closes the connection. TCP attaches a header onto the transmitted data.
This header contains a large number of parameters that help processes on the sending machine connect to peer processes on the receiving machine. TCP confirms that a packet has reached its destination by establishing an end-to-end connection between sending and receiving hosts. UDP, the other transport layer protocol, provides datagram delivery service. UDP does not verify connections between receiving and sending hosts.
The application layer defines standard Internet services and network applications that anyone can use. These services work with the transport layer to send and receive data. Many application layer protocols exist.
The following list shows examples of application layer protocols:. The protocol includes the ftp command local machine and the in. FTP enables a user to specify the name of the remote host and file transfer command options on the local host's command line. The in. Unlike rcp , ftp works even when the remote computer does not run a UNIX-based operating system. A user must log in to the remote computer to make an ftp connection unless the remote computer has been configured to allow anonymous FTP.
You can now obtain an enormous amount of materials from anonymous FTP servers that are connected to the Internet. Universities and other institutions set up these servers to offer software, research papers, and other information to the public domain. The ftp 1 man page describes all ftp command options that are invoked through the command interpreter. The ftpd 1M man page describes the services that are provided by the daemon in. The SYN flag is used in the first step of the connection establishment between the two hosts.
This flag is found only in the first packet from the server and the host containing a synchronizing sequence number. For example, to create a three-way handshake between a server and a host, the host sends a SYN flag to a server providing all the necessary information such as its port number source port and destination port number signifying which services it wants access to.
This contains a source port number the port number used as the destination port number on the SYN flag sent by the host and a destination port number the port number that the host used as source port number. In the example above, the host initiates the session termination.
But in practice, any device can terminate a session. TCP provides reliable data transfer, that means that all packet sent from a source reaches its destination without any failure. In a situation where an error occurs along the route, TCP uses a mechanism to resend the faulty segment. By so doing, it achieves reliability. For example, if a web server has bytes of data to send to a requesting web client.
Then, the server sends the first bytes of the data, with a sequence number of 1. The host after receiving this first segment, sends an ACK informing the server that the packet has reached its destination and starts waiting for the arrival of the next segment i.
This mechanism continues up to bytes, with the host sending an acknowledgment for each segment received. The explanation above does not recover any error, it will be the same TCP mechanism used for error recovery. For error recovery, TCP uses the sequence and ACK flag so that the receiving host can notice missing data and request the sending device to resend the segment and uses the ACK flag to acknowledge the received of the missing piece.
Assuming in the example above, the host received the first hundred bytes with a sequence number of 1, and instead of receiving the next bytes with a sequence number of 2, the host receives a segment with bytes with a sequence number of 3, it will send a packet requesting for that missing segment, i. The sending device can also resend a segment if the receiving device does not acknowledge all the data sent.
The sending device, in this case, is a server. The server waits for a few moments using a timer called the retransmission timer, to make sure that no other acknowledgment arrives, after that it then decides to resend that particular segment that it did not receive its acknowledgment flag. Because network host has limited resources such as limited space and processing power, TCP implements a mechanism called flow control using a window concept.
This is applied to the amount of data that can be awaiting acknowledgment at any one point of time. The receiving device uses the windowing concept to inform the sender how much data it can receive at any given time. This allows the sender to either speed up or slow down the sending of segments through a window sliding process. User datagram protocol UDP is considered as a best-effort transport protocol because it is a light-weighted transport protocol. UDP is a connectionless protocol, meaning it provides no reliability or reordering of the data segment and flow control like TCP.
However, according to Cisco , UDP provides some similar benefits to TCP, such as data segmentation and multiplexing using port numbers. UDP is used by applications that are tolerant to loss of data but not delay. But with UDP, if a packet is missing, the streaming will continue unnoticed.
It only becomes apparent when many segments are missing, which is seen in low video quality and lack of synchronization between video and audio. The whole purpose of building an enterprise network or connecting a small office home office SOHO network to the internet, is for applications such as text messaging, email, video streaming, video, and audio conversations to occur.
To manage these multiple simultaneous conversations, TCP and UDP uses a header field that can uniquely identifies these applications running simultaneously. This unique identifier is called port number. Each service running on a device uses a specific well-known port number. These port numbers identify each application or service running on a client uniquely. Source port numbers are port numbers dynamically generated by the sending device transport layer, that identifies each conversation between the two end devices.
In a segment sent by a client, a destination port number is placed within it to tell the destination server, the services that the client is requesting.
This mechanism is possible because, unlike on a client machine in which request can originate from any locally unused port, services provided by a server have a well-known dedicated port assigned to them.
As such, the destination port number is inserted by the client, informs the server. For example, Telnet uses TCP transport protocol and has a destination port number of When a server receives a segment with a destination port number of 23, it knows that the client is requesting a Telnet service.
The source and the destination port numbers placed within a segment, only identifies which application in a client, requests for that service from a server.
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