SEGMENT,FRAME AND PACKET

What is a Frame?

Frame is the name used to represent the Protocol Data Unit(PDU) at one of the lowest level in the reference model. Its the PDU at data link layer. Any communication between two computers or two networking devices is always done using MAC addresses. MAC addresses are nothing but physical addresses associated with the network interface card.

If you want to deliver a message to another machine on a network, it will first search for the physical address of the destination. There is actually something called as ARP, that does the job of translating IP addresses to Physical addresses. ARP(address resolution protocol) is one of the core components in networking.

 Every operating system with networking capability, has this feature called ARP. It does the job of maintaining a mapping between ip addresses and corresponding MAC addresses(Physical address).

PDU of Transport layer is called as a “Segment”, PDU of network layer is called as a “Packet” & PDU of data link layer is called as a “Frame”.

What is segment:?

So applications provide byte stream to TCP, and TCP takes care of creating it into discrete pieces of messages. These discrete pieces of messages at Transport Layer(where TCP comes into picture) are called "Segments".

What is packets?

The Segment (which includes data chunk and its headers) are then passed down from Transport layer to the next layer called Network Layer. Here is where IP addresses steps in. I think out of all three(Segments, Packets & Frames), packets are something that's mostly correctly used in different places. As packets are discrete messages in Network Layer which has IP addresses, they are sometimes referred to as "IP packets".

NOTE:-  A "segment" which is now sitting inside a "packet", is now passed down to the next layer called data link layer. Data link layer will add its own control and functional fields on top of the packet(called headers) to make something called as a "frame".

 

 

SEGMENT	Packet	frame
PDU at transport layer.	PDU at network layer.	PDU at Data-link layer.
Port numbers are part of Segments	IP addresses are part of a Packet	Mac addresses are part of Frames