Data Packet- In information technology, a packet is a formatted block of data carried
by a packet mode computer network. Computer communications links that do not
support packets, such as traditional point-to-point telecommunications links, simply
transmit data as a series of bytes, characters, or bits alone. When data is
formatted into a packet, the network can transmit long messages more efficiently
and reliably.
Packet mechanism A packet consists of two kinds of data: control information
and user data (also known as payload). The control information provides data the
network needs to deliver the user data, for example: source and destination
addresses, error detection codes like checksums, and sequencing information.
Typically, control information is found in packet headers and trailers, with
user data in between.
Different communications protocols use different conventions for distinguishing
between the elements and for formatting the data. In Binary Synchronous
Transmission, the packet is formatted in 8-bit bytes, and special characters are
used to delimit the different 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 a bit level
instead of a byte level.
A good analogy is to consider a packet to be like a letter: the header is like
the envelope, and the data area is whatever the person puts inside the envelope.
A difference, however, is that some networks can break a larger packet into
smaller packets when necessary (note that these smaller data elements are still
formatted as packets).
A network design can achieve two major results by using packets: error detection
and multiple host addressing.
Error detection It is more efficient and reliable to calculate a
checksum or cyclic redundancy check over the contents of a packet than to check
errors using character-by-character parity bit checking.
The packet trailer often contains error checking data to detect errors that
occur during transmission.
Host addressing Modern networks usually connect three or more host
computers together; in such cases the packet header generally contains
addressing information so that the packet is received by the correct host
computer.
In complex networks constructed of multiple routing and switching nodes, like
the ARPANET and the modern Internet, a series of packets sent from one host
computer to another may follow different routes to reach the same destination.
This technology is called packet switching.
Packets vs. datagrams In general, the term packet applies to any message
formatted as a packet, while the term datagram is generally reserved for the
packets of an unreliable service. A reliable service is one where the user is
notified if delivery fails. An unreliable one is where the user is not notified
if delivery fails.
For example, IP provides an unreliable service. TCP uses IP to provide a
reliable service, whereas UDP uses IP to provide an unreliable one. All these
protocols use packets, but UDP packets are generally called datagrams.
When the ARPANET pioneered packet switching, it provided a reliable packet
delivery procedure to its connected hosts via its 1822 interface. A host
computer simply arranged the data in the correct packet format, inserted the
address of the destination host computer, and sent the message across the
interface to its connected IMP. Once the message was delivered to the
destination host, an acknowledgement was delivered to the sending host. If the
network could not deliver the message, it would send an error message back to
the sending host.
Meanwhile, the developers of CYCLADES and of ALOHAnet demonstrated that it was
possible to build an effective computer network without providing reliable
packet transmission. This lesson was later embraced by the designers of
Ethernet.
If a network does not guarantee packet delivery, then it becomes the host's
responsibility to provide reliability by detecting and retransmitting lost
packets. Subsequent experience on the ARPANET indicated that the network itself
could not reliably detect all packet delivery failures, and this pushed
responsibility for error detection onto the sending host in any case. This led
to the development of the end-to-end principle, which is one of the Internet's
fundamental design assumptions.
Example: IP packets IP packets are composed of a header and Payload. The
IPv4 packet header consists of:
4 bits that contain the version, that specifies if it's an IPv4 or IPv6 packet,
4 bits that contain the Internet Header Length which is the length of the header
in multiples of 4 bytes. Ex. 5 is equal to 20 bytes. 8 bits that contain the
Type of Service, also referred to as Quality of Service (QoS), which describes
what priority the packet should have, 16 bits that contain the length of the
packet in bytes, 16 bits that contain an identification tag to help reconstruct
the packet from several fragments, 3 bits that contain a zero, a flag that says
whether the packet is allowed to be fragmented or not (DF: Don't fragment), and
a flag to state whether more fragments of a packet follow (MF: More Fragments)
13 bits that contain the fragment offset, a field to identify which fragment
this packet is attached to, 8 bits that contain the Time to live (TTL) which is
the number of hops (router, computer or device along a network) the packet is
allowed to pass before it dies (for example, a packet with a TTL of 16 will be
allowed to go across 16 routers to get to its destination before it is
discarded), 8 bits that contain the protocol (TCP, UDP, ICMP, etc...) 16 bits
that contain the Header Checksum, a number used in error correction, 32 bits
that contain the source IP address, 32 bits that contain the destination
address. After those, optional flags can be added of varied length, which can
change based on the protocol used, then the data that packet carries is added.
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.
Delivery not guaranteed Many networks do not provide guarantees of
delivery, nonduplication of packets, or in-order delivery of packets, e.g., the
UDP protocol of the Internet. However, it is possible to layer a transport
protocol on top of the packet service that can provide such protection; TCP and
UDP are the best examples of layer 4, the Transport Layer, of the seven layered
OSI model.
The header of a packet specifies the data type, packet number, total number of
packets, and the sender's and receiver's IP addresses.
The term frame is sometimes used to refer to a packet exactly as transmitted
over the wire or radio.
Attributes and Credits
The information and facts supplied on this subject
derive from http://en.wikipedia.org/wiki/Main_Page
by a packet mode computer network. Computer communications links that do not
support packets, such as traditional point-to-point telecommunications links, simply
transmit data as a series of bytes, characters, or bits alone. When data is
formatted into a packet, the network can transmit long messages more efficiently
and reliably.
Packet mechanism A packet consists of two kinds of data: control information
and user data (also known as payload). The control information provides data the
network needs to deliver the user data, for example: source and destination
addresses, error detection codes like checksums, and sequencing information.
Typically, control information is found in packet headers and trailers, with
user data in between.
Different communications protocols use different conventions for distinguishing
between the elements and for formatting the data. In Binary Synchronous
Transmission, the packet is formatted in 8-bit bytes, and special characters are
used to delimit the different 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 a bit level
instead of a byte level.
A good analogy is to consider a packet to be like a letter: the header is like
the envelope, and the data area is whatever the person puts inside the envelope.
A difference, however, is that some networks can break a larger packet into
smaller packets when necessary (note that these smaller data elements are still
formatted as packets).
A network design can achieve two major results by using packets: error detection
and multiple host addressing.
Error detection It is more efficient and reliable to calculate a
checksum or cyclic redundancy check over the contents of a packet than to check
errors using character-by-character parity bit checking.
The packet trailer often contains error checking data to detect errors that
occur during transmission.
Host addressing Modern networks usually connect three or more host
computers together; in such cases the packet header generally contains
addressing information so that the packet is received by the correct host
computer.
In complex networks constructed of multiple routing and switching nodes, like
the ARPANET and the modern Internet, a series of packets sent from one host
computer to another may follow different routes to reach the same destination.
This technology is called packet switching.
Packets vs. datagrams In general, the term packet applies to any message
formatted as a packet, while the term datagram is generally reserved for the
packets of an unreliable service. A reliable service is one where the user is
notified if delivery fails. An unreliable one is where the user is not notified
if delivery fails.
For example, IP provides an unreliable service. TCP uses IP to provide a
reliable service, whereas UDP uses IP to provide an unreliable one. All these
protocols use packets, but UDP packets are generally called datagrams.
When the ARPANET pioneered packet switching, it provided a reliable packet
delivery procedure to its connected hosts via its 1822 interface. A host
computer simply arranged the data in the correct packet format, inserted the
address of the destination host computer, and sent the message across the
interface to its connected IMP. Once the message was delivered to the
destination host, an acknowledgement was delivered to the sending host. If the
network could not deliver the message, it would send an error message back to
the sending host.
Meanwhile, the developers of CYCLADES and of ALOHAnet demonstrated that it was
possible to build an effective computer network without providing reliable
packet transmission. This lesson was later embraced by the designers of
Ethernet.
If a network does not guarantee packet delivery, then it becomes the host's
responsibility to provide reliability by detecting and retransmitting lost
packets. Subsequent experience on the ARPANET indicated that the network itself
could not reliably detect all packet delivery failures, and this pushed
responsibility for error detection onto the sending host in any case. This led
to the development of the end-to-end principle, which is one of the Internet's
fundamental design assumptions.
Example: IP packets IP packets are composed of a header and Payload. The
IPv4 packet header consists of:
4 bits that contain the version, that specifies if it's an IPv4 or IPv6 packet,
4 bits that contain the Internet Header Length which is the length of the header
in multiples of 4 bytes. Ex. 5 is equal to 20 bytes. 8 bits that contain the
Type of Service, also referred to as Quality of Service (QoS), which describes
what priority the packet should have, 16 bits that contain the length of the
packet in bytes, 16 bits that contain an identification tag to help reconstruct
the packet from several fragments, 3 bits that contain a zero, a flag that says
whether the packet is allowed to be fragmented or not (DF: Don't fragment), and
a flag to state whether more fragments of a packet follow (MF: More Fragments)
13 bits that contain the fragment offset, a field to identify which fragment
this packet is attached to, 8 bits that contain the Time to live (TTL) which is
the number of hops (router, computer or device along a network) the packet is
allowed to pass before it dies (for example, a packet with a TTL of 16 will be
allowed to go across 16 routers to get to its destination before it is
discarded), 8 bits that contain the protocol (TCP, UDP, ICMP, etc...) 16 bits
that contain the Header Checksum, a number used in error correction, 32 bits
that contain the source IP address, 32 bits that contain the destination
address. After those, optional flags can be added of varied length, which can
change based on the protocol used, then the data that packet carries is added.
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.
Delivery not guaranteed Many networks do not provide guarantees of
delivery, nonduplication of packets, or in-order delivery of packets, e.g., the
UDP protocol of the Internet. However, it is possible to layer a transport
protocol on top of the packet service that can provide such protection; TCP and
UDP are the best examples of layer 4, the Transport Layer, of the seven layered
OSI model.
The header of a packet specifies the data type, packet number, total number of
packets, and the sender's and receiver's IP addresses.
The term frame is sometimes used to refer to a packet exactly as transmitted
over the wire or radio.
Attributes and Credits
The information and facts supplied on this subject
derive from http://en.wikipedia.org/wiki/Main_Page
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