fajri.freebsd.or.id - Fragmentation

Fragmentation

Before talking about fragmentation, we have to understand MTU.
MTU is the maximum size of every packet, which contain IP Header and data.

So I can say that MTU is Header (which is 20 bytes for IP header) + MSS. MSS is the masimum size of data frame in byte.

Then we have to understand Path MTU.
Path MTU is the smallest MTU of any hops from the source to the destination. Look at the sample below.

For the sample above, the path MTU is 1000.

After understand what path MTU is, we have to know what Path MTU Discovery is.
Path MTU Discovery is a method or technique to determine Path MTU. The method is by sending ICMP packet with DF as set and specific packet size. If the respond is “fragmentation needed” or something like that(tm), then the process repeats with lower packet size until get path MTU.
Here’s the ilustration.

That’s the introduction…

After got PathMTU, then when Host A want to send packet to host B, the packet will be fragmented by several packets/frames. Every packet has maximum size as Path MTU. Host A then sends every frames to host B. After host B receives all frames, the host B reassembly every frames to be the original data. Below is the ilustration.

Below is the sample dataflow for this thing.

Then the header that shows the sequence will be like the graph below.

Since TCP/IP is packet switching, so it uses best effort to send the packet. it means that the 2nd packet might be arrive earlier then 1st packet. and soon. Below is the illustration.