Datagram
Encyclopedia
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed.
A datagram consists of header and data areas, where the header contains information sufficient for routing from the originating equipment to the destination without relying on prior exchanges between the equipment and the network. The source and destination addresses as well as a type field are found in the header of a datagram.
The term datagram is often considered synonymous to "packet", but there are some nuances. First, the term packet applies to any message formatted as a packet, while the term datagram is generally reserved for packets of an "unreliable" service. An "unreliable" service does not notify the user if delivery fails. For example, IP
itself provides an unreliable service and UDP
over IP also provides an unreliable one. That is why UDP packets are generally called datagrams. Second, if a datagram fragments
, then its fragments may be referred as packets, but not as datagrams. However, TCP
refers to its fragments as TCP segments, not packets, presumably to assert that its fragments are reliable.
A datagram consists of header and data areas, where the header contains information sufficient for routing from the originating equipment to the destination without relying on prior exchanges between the equipment and the network. The source and destination addresses as well as a type field are found in the header of a datagram.
The term datagram is often considered synonymous to "packet", but there are some nuances. First, the term packet applies to any message formatted as a packet, while the term datagram is generally reserved for packets of an "unreliable" service. An "unreliable" service does not notify the user if delivery fails. For example, IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
itself provides an unreliable service and UDP
User Datagram Protocol
The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
over IP also provides an unreliable one. That is why UDP packets are generally called datagrams. Second, if a datagram fragments
IP fragmentation
The Internet Protocol implements datagram fragmentation, so that packets may be formed that can pass through a link with a smaller maximum transmission unit than the original datagram size....
, then its fragments may be referred as packets, but not as datagrams. However, TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
refers to its fragments as TCP segments, not packets, presumably to assert that its fragments are reliable.