IP Datagrams/IP Packets

Hosts on an IP network exchange information using IP datagrams, which include
both the units of data that contain whatever information is being exchanged and
the header fields that describe that information (as well as describing the datagram
itself). Whenever a device needs to send data to another system over an IP
network, it will do so by creating an IP datagram, although the datagram is not
what gets sent by IP, at least not in the literal sense.
Instead, IP datagrams get sent as IP packets, which are used to relay the IP datagrams
to the destination system, one hop at a time. Although in many cases an IP
datagram and an IP packet will be exactly the same, they are conceptually different
entities, which is an important concept for understanding how IP actually works.

IP datagrams contain whatever data is being sent (and the associated IP headers),
while IP packets are used to get the datagram to the destination system (as specified
in the IP headers). These IP packets are sent using the framing mechanisms
defined for the specific network medium in use on the local network, and are subject
to network events such as fragmentation or loss. However, the datagram itself
will always remain as the original piece of data that was sent by the original
sender, regardless of anything that happens to any of the packets that are used to
relay the datagram.



This model is necessary due to the way that IP provides a virtual network on top
of the different physical networks that make up the global Internet. Since each of
those networks have different characteristics (such as addressing mechanisms,
frame sizes, and so forth), IP has to provide a mechanism for forwarding datagrams
across those different networks reliably and cleanly. The datagram concept
allows a host to send whatever data needs to be sent, while the IP packet allows
the datagram to actually get sent across the different networks according to the
characteristics of each of the intermediary networks.