Welcome back everyone.
This module focuses on IP packet fragmentation and overcoming
the problems it can create in your network.
Fragmentation is a topic that IT engineers despise
because it is ugly and inconvenient.
We will tackle it head on.
We'll begin by understanding what fragmentation is,
why it happens, and how it impacts the network.
To fix it,
I'll show you two simple methods for manually discovering
a network's maximum transmission unit, or MTU,
the bracketing method and the sweeping method.
Both are great ways to prevent fragmentation.
We'll finish up with a deep dive into the packet
details on a new ICMP unreachable, the packet too big message.
Okay, I know definitions can be dry, but bear with me.
Fragmentation is designed to break large packets into smaller packets,
which are called fragments,
so the resulting pieces can pass through a link with a
smaller MTU than the original packet size.
For those unfamiliar with the term MTU,
it refers to the upper bound on the size of a packet
that can be sent out of a link.
When a network has a variety of MTUs along a path,
the path MTU is the smallest MTU along the path.
You can think of it like the bottleneck in the path that governs
the MTU that should be used by applications.
I'll briefly touch on how fragmentation works on a packet level.
Because fragmentation is not directly related to ICMP, I won't belabor it.
IPv4 has a bit in the IP header which grants or denies
permission to a router to fragment a packet.
The bit is called the Don't Fragment bit, or DF for short.
When the bit is 0, the packet is allowed to be fragmented,
but when the bit is 1, the packet cannot be fragmented.
Let's do some high‑level packet walks.
If a large packet with DF set to 0 arrives at a router that has an
egress link with an MTU smaller than the packet,
the router will break the packet up into fragments.
Since each fragment is a new IP packet,
the target host has to reassemble them once they are all received.
The fragments have identifiers known as offsets that
specify their sequence for reassembly.
Now imagine that same large packet arrives at the router with DF set to 1.
The router is no longer allowed to fragment this packet.
The router will do one of two things.
It could send a packet too big unreachable back to the sender,
which is usually the preferred behavior.
This packet indicates that the packet was too large
and fragmentation was required, but was disallowed because DF was set to 1.
In plain English, the packet says, look,
I want to route this packet, but I can't fit it over my link,
and you specifically told me not to break it up.
The alternative behavior is for the router to silently discard the packet,
which happens when ICMP unreachables are filtered from the network.
I'm depicting the former case here since our main focus for
this module is the packet too big message.
What does any of this have to do with ICMP, you ask?
I'm going to show you a variety of techniques for
discovering the MTU along a path using ICMP ping.
Whether the routers in the network send back packet too big
unreachables or not is irrelevant since the techniques I'll
show you work 100% of the time.
Let's briefly discuss the advantages and disadvantages of fragmentation in the next clip before diving into some demos.