mtu size packet fragmentation

We can understand about Fragmentation - if big size packet is broken into a small size (1500 byte) of packet its called fragmentation.

Data packets have two main parts: the header and the payload. The header contains information about the packet's source and destination addresses, while the payload is the actual contents of the packet

Note:- MTU almost always is used in reference to layer 3* packets

Example to understand the fragmentaion -

Imagine a shipping company E-Kart is handling a package that exceeds the weight limits of one of their facilities. Instead of refusing to deliver the package, the shipping company divides the package contents into three smaller packages. It also duplicates the shipping label for each package and adds a note indicating that each package is one part of a series that must arrive together — the first package is 1 of 3, the second is 2 of 3, etc

Fragmentation not work into below condition -

1- Fragmentation is not permitted in IPv6

2- Fragmentation is also not possible when the "Don't Fragment" flag is activated in a packet's IP header.

What is the 'Don't Fragment' flag

Router receives the packet will analyze the header and check for the Don't Fragment flag. If the flag is on and the packet exceeds the MTU, the router then drops the packet instead of fragmenting it

for example if router got packet with size of 3000 byte with Don't Fragment header, its simple drop the packet because its MTU size set 1500 only, so its not fragment because of Don't fragment flag is on.

Path MTU discovery

Path MTU discovery, or PMTUD, is the process of discovering the MTU of all devices, routers, and switches on a network path. If Computer A and Server A from the example above were to use PMTUD, they would identify Router B's MTU requirements and adjust their packet size accordingly to avoid fragmentation

MSS------->>> maximum segment size

MSS stands for maximum segment size. MSS is used by TCP at layer 4 of the Internet, the transport layer, instead of layer 3. MSS is only concerned with the size of the payload within each packet. It is calculated by subtracting the length of TCP and IP headers from MTU

While packets that exceed a router's MTU are either fragmented or dropped, packets that exceed the MSS are always dropped