MTU / MSS Calculator

Understanding MTU, MSS, and Network Overhead

In computer networking, optimizing the Maximum Transmission Unit (MTU) and the Maximum Segment Size (MSS) is critical for ensuring efficient data transfer and avoiding packet fragmentation. Fragmentation occurs when a data packet exceeds the MTU of a network interface, forcing the router to break it into smaller pieces. This process increases CPU load on network devices and can severely degrade throughput.

Mathematical Formulas

The relationship between MTU and MSS is defined by the overhead introduced by various protocol headers. The basic formula for calculating MSS is:

$$ MSS = MTU - (Header\_{IP} + Header\_{TCP} + Header\_{Encap}) $$

Where:

• MTU: The total size of the frame (e.g., 1500 bytes for standard Ethernet).
• Header_IP: 20 bytes for IPv4 or 40 bytes for IPv6.
• Header_TCP: Typically 20 bytes, but can increase with options like Timestamps (+12 bytes).
• Header_Encap: Overhead from tunneling protocols (e.g., PPPoE adds 8 bytes, GRE adds 24 bytes).

Common MTU Values and Scenarios

Network engineers often encounter specific MTU values tailored to different environments:

• 1500 Bytes: The standard MTU for Ethernet networks and the Internet.
• 1492 Bytes: Common for PPPoE connections (DSL) to accommodate the 8-byte PPP header.
• 9000 Bytes: Known as "Jumbo Frames," used in data centers to increase efficiency by reducing the number of headers processed for the same amount of data.
• 576 Bytes: A conservative minimum MTU often used in routing protocols or legacy X.25 links to ensure delivery across any network path.

Why MSS Clamping is Important

In tunneling scenarios (like VPNs or GRE tunnels), the additional headers reduce the available space for the payload. If a sender assumes a standard 1500-byte MTU but the path includes a tunnel, packets may be dropped or fragmented. MSS Clamping is a technique configured on routers to rewrite the TCP MSS value in the SYN packet, forcing endpoints to negotiate a smaller segment size that fits inside the tunnel without fragmentation.