Protocol Data Units

Question 1

PDU (Protocol Data Unit)

Answer 1

• 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

Question 2

Maximum Transmission Unit (MTU)

Answer 2

• 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

Question 3

Troubleshooting MTU

Answer 3

• 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