Protocol Data Units Flashcards
PDU (Protocol Data Unit)
- 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
Maximum Transmission Unit (MTU)
• 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
Troubleshooting MTU
• 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