Ethernet frame and MTU

Ethernet Frame Sizes

Standard Ethernet Frame (without 802.1Q):

  • Total Size: 1518 Bytes
  • Breakdown:
    • Destination MAC: 6 Bytes
    • Source MAC: 6 Bytes
    • Type: 2 Bytes
    • Data: 1500 Bytes
    • FCS: 4 Bytes
    • Destination mac(6) + source MAC(6) + type(2) + data(1500) + FCS(4)

Ethernet Frame with 802.1Q Tagging:

  • Total Size: 1522 Bytes
  • Breakdown:
    • Destination MAC: 6 Bytes
    • Source MAC: 6 Bytes
    • Type: 2 Bytes
    • 802.1Q Tag: 4 Bytes
    • Data: 1500 Bytes
    • FCS: 4 Bytes
    • Destination mac(6) + source MAC(6) + type(2) + 802.1q(4) + data(1500) + FCS(4)

Key Header Sizes

  • IP Header: 20 Bytes
  • TCP Header: 20 Bytes
  • UDP Header: 8 Bytes
  • VXLAN Header: 8 Bytes

Defining MTU and MSS

MTU (Maximum Transmission Unit):
The largest size of an IP packet that can be transmitted without needing fragmentation. Typically, this includes the IP packet along with its TCP/UDP headers.

MSS (Maximum Segment Size):
This is derived from the MTU. It is the largest segment of data that TCP can send in a single packet. Calculated as: MSS=MTU−(IP Header+TCP Header)

For a standard MTU of 1500 bytes: MSS=1500−(20+20)=1460

Troubleshooting MTU Issues

When using the ping command with the -f (don’t fragment) flag, the packet size might exceed the MTU, leading to fragmentation issues. For example, an MTU of 1500 bytes results in an MSS of 1472 bytes when using the following calculation: MSS=MTU−(IP Header+ICMP Header)=1500−(20+8)=1472

Best MTU for VPN

The optimal MTU for VPN can vary depending on the specific protocol and network configuration used. It’s often recommended to experiment with values between 1400-1500 bytes and monitor the performance to find the best setting for your environment.

 

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