Definition of Network Units: Fragment, Segment, Packet, Frame, Datagram

What units are used in network terminology? Where do we use them?

I know the following - but I'm not sure what their meaning is:

• Fragment
• Segment
• Packet
• Frame
• Datagram

Can they be assigned to a specific OSI-Layer? Is it more complex?

Taking from Section 1.2 in TCP/IP Illustrated: Vol 1 by Richard Stevens et al., consider the 4 layered TCP/IP stack:

        +-------------+-------------------------+
        | Application |  Telnet, FTP, etc       |
        +-------------+-------------------------+
        | Transport   |  TCP, UDP               |
        +-------------+-------------------------+
        | Network     |  IP, ICMP               |
        +-------------+-------------------------+
        | Link        | drivers, interface card | 
        +-------------+-------------------------+

Segment: If the transport protocol is TCP, the unit of data sent from TCP to network layer is called Segment.

Datagram: This is used in 2 layers. If the network protocol is IP, the unit of data is called Datagram. At transport layer, if protocol is UDP, we use datagram there as well. Hence, we differentiate them as UDP Datagram, IP Datagram.

Frame: Physical layer representation.

Packet: It is a more generic term used either transport layer or network layer. TCP Packet, UDP Packet, IP Packet etc. I have not seen it to represent Physical layer data units.

Fragment: My guess here is that when a unit of data is chopped up by a protocol to fit the MTU size, the resultant unit of data is called Fragments. But I am guessing.

These terms are used to refer at which OSI layer we are referring to, the data remain same but the headers and trailers are getting added or removed.

In Phy layer i.e. physical layer the actual data is in "bits" i.e. 0 and 1.

When it reaches to data link layer or Layer 2 it becomes "frame" with source and destination mac address getting added.

When it reaches the third layer or the networking layer it becomes a "packet" with source and destination IP address attached to it.

Finally it become "segment" at Layer 4 or TCP/IP layer. Basically "segment" term is used for TCP connections and "datagrams" for UDP.

So what about other layers like : session, presentation and application ? In these layers the data is termed as "PDU" or "protocol data unit".

So when someone says, that a "frame" is being received by a networking device, then you know that the device is switch.

Router receives a packet and process it.

The answer above is complete. Nevertheless, I think the term packet came from packet switching if I remembered that correctly ..

There are 2 types :

Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel (circuit) through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the duration of the communication session. The circuit functions as if the nodes were physically connected as with an electrical circuit.(Wiki)

Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets.(Wiki)

I would have defined it myself, but just to be sure.

So here is the Packet Term. In regard of fragment I concur with Josua Schmid's answer.

Frame as physical layer representation doesn't sound 100% accurate to me. It is mostly in the data-link layer, which is above the physical.

For a more operational definition, I would say that a frame is a protocol data unit that has means to determine its boundaries. Examples:

• an HDLC frame has flags at the beginning and end and a bit stuffing
  mechanism
• an Ethernet frame uses preamble and SFD
• GFP uses HEC frame delineation, like ATM

A cell, as in ATM, just to introduce one term more, is a fixed-length frame.

A packet is instead a protocol data unit that relies on a lower layer unit to determine its boundaries. It may fit exactly in one frame or be broken into fragments if required. In the latter case fragmentation and reassembly mechanisms must be devised. Datagram is mostly a synonym for packet.

An application data stream can be broken into segments. A segment is carried over a datagram, then segments are reassembled into the original application stream. This is typically done by the TCP layer. So in TCP we have application stream over TCP segment over IP datagram over data-link frame. In UDP there is no segmentation, so the UDP datagram is one-to-one mapped onto the IP datagram.