Protocol Data Units Flashcards

1
Q

PDU (Protocol Data Unit)

A
  • A unit of transmission
    • A different group of data at different OSI layers
  • Ethernet operates on a frame of data
    • It has no idea what’s inside
  • IP operates on a packet of data
    • Inside is TCP or UDP, but IP doesn’t know that

• TCP or UDP PDU - TCP segment, UDP datagram

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2
Q

Maximum Transmission Unit (MTU)

A

• Maximum IP packet to transmit - but not fragment

  • Fragmentation slows things down
    • Losing a fragment loses an entire packet
    • Requires overhead along the path

• Difficult to know the MTU all the way through the
path
• Automated methods are often inaccurate, especially
when ICMP is filtered

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3
Q

Troubleshooting MTU

A

• MTU sizes are usually configured once
• Based on the network infrastructure and don’t
change often

• A significant concern for tunneled traffic
• The tunnel may be smaller than your local Ethernet
segment

• What if you send packets with Don’t Fragment (DF)
set?
• Routers will respond back and tell you to fragment
• Hope you get the ICMP message!

• Troubleshoot using ping
• Ping with DF and force a maximum size of 1472
bytes

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