Network packet

Intelecommunicationsandcomputer networking,anetwork packetis a formatted unit ofdatacarried by apacket-switched network.A packet consists of control information and user data;the latter is also known as thepayload.Control information provides data for delivering the payload (e.g., source and destinationnetwork addresses,error detectioncodes, or sequencing information). Typically, control information is found in packetheadersandtrailers.

Inpacket switching,thebandwidthof thetransmission mediumis shared between multiple communication sessions, in contrast tocircuit switching,in which circuits are preallocated for the duration of one session and data is typically transmitted as a continuousbit stream.

Terminology

In the seven-layerOSI modelofcomputer networking,packetstrictly refers to aprotocol data unitat layer 3, thenetwork layer.A data unit at layer 2, thedata link layer,is aframe.In layer 4, thetransport layer,the data units aresegmentsanddatagrams.Thus, in the example of TCP/IP communication overEthernet,aTCP segmentis carried in one or moreIP packets,which are each carried in one or moreEthernet frames.

Architecture

The basis of the packet concept is the postal letter: the header is like the envelope, the payload is the entire content inside the envelope, and the footer would be your signature at the bottom.

Network designcan achieve two major results by using packets:error detectionandmultiple host addressing.

Framing

Communications protocolsuse various conventions for distinguishing the elements of a packet and for formatting the user data. For example, inPoint-to-Point Protocol,the packet is formatted in 8-bit bytes, and special characters are used to delimit elements. Other protocols, like Ethernet, establish the start of the header and data elements by their location relative to the start of the packet. Some protocols format the information at abitlevel instead of abytelevel.

Contents

A packet may contain any of the following components:

Addresses

Theroutingof network packets requires twonetwork addresses,the source address of the sending host, and the destination address of the receiving host.

Error detection and correction

Error detection and correctionis performed at various layers in theprotocol stack.Network packets may contain achecksum,parity bitsorcyclic redundancy checksto detect errors that occur during transmission.

At the transmitter, the calculation is performed before the packet is sent. When received at the destination, the checksum is recalculated, and compared with the one in the packet. If discrepancies are found, the packet may be corrected or discarded. Anypacket lossdue to these discards is dealt with by the network protocol.

In some cases, modifications of the network packet may be necessary while routing, in which cases checksums are recalculated.

Hop limit

Under fault conditions, packets can end up traversing aclosed circuit.If nothing was done, eventually the number of packets circulating would build up until the network was congested to the point of failure.Time to liveis a field that is decreased by one each time a packet goes through anetwork hop.If the field reaches zero, routing has failed, and the packet is discarded.

Ethernet packets have no time-to-live field and so are subject tobroadcast radiationin the presence of aswitching loop.

Length

There may be a field to identify the overall packet length. However, in some types of networks, the length is implied by the duration of the transmission.

Protocol identifier

It is often desirable to carry multiple communication protocols on a network. A protocol identifier field specifies a packet's protocol and allows the protocol stack to process many types of packets.

Priority

Some networks implementquality of servicewhich can prioritize some types of packets above others. This field indicates which packet queue should be used; a high priority queue is emptied more quickly than lower-priority queues at points in the network where congestion is occurring.

Payload

In general, the payload is the data that is carried on behalf of an application. It is usually of variable length, up to a maximum that is set by the network protocol and sometimes the equipment on the route. When necessary, some networks can break a larger packet into smaller packets.

Examples

Internet protocol

IP packets are composed of a header and payload. The header consists of fixed and optional fields. The payload appears immediately after the header. An IP packet has no trailer. However, an IP packet is often carried as the payload inside an Ethernet frame, which has its own header and trailer.

Per theend-to-end principle,IP networks do not provide guarantees of delivery, non-duplication, or in-order delivery of packets. However, it is common practice to layer areliabletransport protocolsuch asTransmission Control Protocolon top of the packet service to provide such protection.

NASA Deep Space Network

The Consultative Committee for Space Data Systems (CCSDS) packet telemetry standard defines the protocol used for the transmission of spacecraft instrument data over the deep-space channel. Under this standard, an image or other data sent from a spacecraft instrument is transmitted using one or more packets.

MPEG packetized stream

Packetized elementary stream(PES) is a specification associated with theMPEG-2standard that allows anelementary streamto be divided into packets. The elementary stream is packetized by encapsulating sequential data bytes from the elementary stream between PES packet headers.

A typical method of transmitting elementary stream data from a video or audio encoder is to first create PES packets from the elementary stream data and then to encapsulate these PES packets inside anMPEG transport stream(TS) packets or anMPEG program stream(PS). The TS packets can then be transmitted using broadcasting techniques, such as those used in anATSCandDVB.

NICAM

In order to providemonocompatibility,theNICAMsignal is transmitted on asubcarrieralongside the sound carrier. This means that theFMorAMregular mono sound carrier is left alone for reception by monaural receivers. The NICAM packet (except for the header) is scrambled with a nine-bit pseudo-random bit-generator before transmission. Making the NICAM bitstream look more likewhite noiseis important because this reduces signal patterning on adjacent TV channels.

See also

• Anti-replay
• Fast packet switching
• Mangled packet
• Packet analyzer
• Packet generation model
• Statistical time-division multiple xing
• Tail drop