Networking

Flags

A collection of six 1-bit fields that signal special conditions through flags. The following flags are available for the sender's use:

Frame

A frame is a structured package for moving data that includes not only the raw data, or "payload," but also the sender's and receiver's network addresses, and error checking and control information.

Segment

A segment is a unit of data that results from subdividing a larger protocol data unit. Transport layer protocols break large data units received from the Session layer into multiple smaller units, called segments. This process is known as segmentation.

Checksum

Allows the receiving node to determine whether the TCP segment became corrupted during transmission. The Checksum field is 16 bits long.

Acknowledgment number (ACK)

Confirms receipt of the data via a return message to the sender. The Acknowledgment number field is 32 bits long.

Data

Contains data originally sent by the source node. The size of the Data field depends on how much data needs to be transmitted, the constraints on the TCP segment size imposed by the network type, and the limitation that the segment must fit within an IP datagram.

Padding

Contains filler information to ensure that the size of the TCP header is a multiple of 32 bits. The size of this field varies; it is often 0.

Sequence number

Identifies the data segment's position in the stream of data segments already sent. The Sequence number field is 32 bits long.

Urgent pointer

Indicates a location in the data field where urgent data resides. This field is 16 bits long.

Sliding-window size (or window)

Indicates how many bytes the sender can issue to a receiver while acknowledgment for this segment is outstanding. This field performs flow control, preventing the receiver from being deluged with bytes.

TCP header length

Indicates the length of the TCP header. This field is four bits long.

Destination port

Indicates the port number at the destination node. The Destination port field is 16 bits long.

Source port

Indicates the port number at the source node. The Source port field is 16 bits long.

TCP (Transmission Control Protocol)

Operates in the Transport layer of the OSI model

Port 80

Port 80, is typically used to accept Web page requests from the HTTP protocol.

Routable

Protocols that can span more than one LAN (or LAN segment) are routable, because they carry Network layer addressing information that can be interpreted by a router. TCP/IP is routable.

RST, SYN, FIN Flags

RST—If set to 1, the sender is requesting that the connection be reset. SYN—If set to 1, the sender is requesting a synchronization of the sequence numbers between the two nodes. This code is used when TCP requests a connection to set the initial sequence number. FIN—If set to 1, the segment is the last in a sequence and the connection should be closed.

Options

Specifies special options, such as the maximum segment size a network can handle. The size of this field can vary between 0 and 32 bits.

TCP (Transmission Control Protocol)

TCP operates in the Transport layer of the OSI model and provides reliable data delivery services. TCP is a connection-oriented subprotocol, which means that a connection must be established between communicating nodes before this protocol will transmit data. TCP further ensures reliable data delivery through sequencing and checksums. Also, provides flow control to ensure that a node is not flooded with data.

URG, ACK, PSH Flags

URG—If set to 1, the Urgent pointer field contains information for the receiver. ACK—If set to 1, the Acknowledgment field contains information for the receiver. (If set to 0, the receiver will ignore the Acknowledgment field.) PSH—If set to 1, it indicates that data should be sent to an application without buffering.

TCP/IP (Transmission Control Protocol/Internet Protocol)

is not simply one protocol, but rather a suite of specialized protocols - including TCP, IP, UDP, ARP, and many others - called subprotocols.