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?
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How is the size of a PDU determined?
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How is performance affected by fragmenting PDUs?
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How is the MTU usually determined?
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When is the MTU determined manually?
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What protocol sometimes makes you determine the MTU manually?
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What is the max Ethernet frame size?
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What is the 'Don't fragment' bit?
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Why would an app use the 'Don't fragment' bit?
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What protocol might fail if the 'Don't fragment' bit is enabled?
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What's a good way to determine what the size of an MTU should be?
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When you ping to determine the size of an MTU, what should the max size be when you start?
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When you ping to determine the size of an MTU, why should you start with a max size of 1427 bytes?
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What does ping -f do?
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What does ping -i do?
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What should you do if you set an MTU and ping doesn't work?
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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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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.