Protocol Data Units (PDUs) Flashcards

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Questions and Answers

What is a Protocol Data Unit (PDU)?

A unit of information sent by a protocol at a particular OSI layer.

What is the PDU of a switch?

Ethernet PDU

What is the PDU of a router?

IP PDU

What is a layer 4 PDU?

TCP segment or UDP datagram Signup and view all the answers

How is the size of a PDU determined?

By the maximum transmission unit (MTU) Signup and view all the answers

How is performance affected by fragmenting PDUs?

It takes time to fragment the packet into smaller pieces and send across the network. If any are lost, you have to retransmit them all. Signup and view all the answers

How is the MTU usually determined?

Via automation when the session is established. Signup and view all the answers

When is the MTU determined manually?

If there's some kind of filtering between the devices, and you might have to set it manually. Signup and view all the answers

What protocol sometimes makes you determine the MTU manually?

ICMP Signup and view all the answers

What is the max Ethernet frame size?

1500 bytes Signup and view all the answers

What is the 'Don't fragment' bit?

A special bit in the IP header that tells all devices in a path of data not to fragment any data. Signup and view all the answers

Why would an app use the 'Don't fragment' bit?

Some apps don't like to have their data fragmented. Signup and view all the answers

What protocol might fail if the 'Don't fragment' bit is enabled?

ICMP Signup and view all the answers

What's a good way to determine what the size of an MTU should be?

Use ping - start with the max size of the IP packet and work down until you can ping a device on the other side of the network. Signup and view all the answers

When you ping to determine the size of an MTU, what should the max size be when you start?

1427 bytes Signup and view all the answers

When you ping to determine the size of an MTU, why should you start with a max size of 1427 bytes?

8-byte ICMP header + 20-byte IP header will be added to it. Signup and view all the answers

What does ping -f do?

Tells it not to fragment. Signup and view all the answers

What does ping -i do?

Specifies what the length might be. Signup and view all the answers

What should you do if you set an MTU and ping doesn't work?

Lower the amount of data and send it again until you get exactly the right amount. Signup and view all the answers

Study Notes

Protocol Data Units (PDUs) Overview

A Protocol Data Unit (PDU) is an information unit transmitted by a protocol at a specific OSI layer.

PDU Types

Switch uses Ethernet as its PDU.
Router utilizes IP as its PDU.
Layer 4 PDUs are known as TCP segments or UDP datagrams.

Determining PDU Size

The size of a PDU is determined by the Maximum Transmission Unit (MTU).
MTU typically established automatically during session setup but may require manual configuration if network filtering is present.

Fragmentation and Performance

Fragmenting PDUs can degrade performance due to the time required to split packets and retransmit lost fragments.
Maximum Ethernet frame size is 1500 bytes.

"Don't Fragment" Bit

The "Don't" fragment bit in the IP header instructs devices to avoid fragmenting the data.
Applications may use this option as fragmentation can be problematic.
ICMP may fail if the "Don't" fragment option is enabled.

MTU Determination Methods

To determine the appropriate MTU size, initiate a ping starting with a maximum size of 1427 bytes to account for ICMP and IP header sizes.
Use ping with the option to set the "don't fragment" bit for accurate MTU testing.
If ping fails after setting an MTU, reduce the data size and retry until the optimal size is found.

Ping Commands

The ping -f command disables fragmentation.
The ping -i command allows specification of the packet length.

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Description

Test your knowledge of Protocol Data Units (PDUs) with these informative flashcards. Each card provides a definition and specific examples related to OSI layers, such as switches and routers. Perfect for students and professionals looking to sharpen their understanding of network protocols.