Protocol Data Units

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 segment, UPD datagram
  • TCP or UDP PDU - TCP segment, UDP datagram
Maximum Transmission Unit (MTU)
  • Maximum IP packet to transmit - but not fragment
  • Fragmentation show 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
    • 1500 bytes - 8 byte ICMP header
    •                   - bytes IP address = 1472 bytes
    • Windows: ping  -f   -l   1472   8.8.8.8

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