The Size of the Payload
Deconstructing MTU and MSS
1. MTU: The Layer 2 Limit
The Maximum Transmission Unit (MTU) is the largest sized frame that can be sent over a physical network medium. For standard Ethernet, this is 1500 bytes. If a packet exceeds the MTU of any switch or router in the path, it must either be Fragmented or Dropped.
2. MSS: The TCP Agreement
The Maximum Segment Size (MSS) is negotiated during the TCP 3-Way Handshake. It tells the other side: "This is the largest amount of raw data I can accept in a single TCP segment."
MSS = MTU - (IP Header) - (TCP Header)3. The Fragmentation Tax
When a router encounters a packet larger than its MTU, it splits the data into two or more packets.
4. Path MTU Discovery (PMTUD)
How does a computer know if a router 5 hops away has a smaller MTU? PMTUD. It sends a packet with the "Don't Fragment" (DF) bit set. If a router cannot pass it, the router sends back an ICMP message: "Fragmentation Needed but DF set." The computer then shrinks its packet size and tries again.
5. Jumbo Frames: The Data Center Exception
Inside high-speed data centers (10G/40G/100G), we often use Jumbo Frames (9000 bytes). This significantly reduces the CPU load because the computer only has to process one header for every 9KB of data, instead of 6 headers.
6. MSS Clamping: Fixing the VPN Tunnel
VPNs and PPPoE add extra headers (overhead), which shrinks the available MTU (e.g., from 1500 down to 1420). To prevent fragmentation, we use MSS Clamping. The router "lies" to both sides of the TCP connection, forcing them to use a smaller MSS (like 1380) so the packets always fit through the tunnel.
Frequently Asked Questions
What happens if MSS is too small?
The network will be "chatty." You will send more headers relative to actual data, increasing overhead and reducing the maximum effective throughput of the link.
Where do I set the MTU?
MTU is set on the network interface level. However, usually, the OS handles this automatically via DHCP.
"You are our partner in accuracy. If you spot a discrepancy in calculations, a technical typo, or have a field insight to share, don't hesitate to reach out. Your expertise helps us maintain the highest standards of reliability."
Contributors are acknowledged in our technical updates.