Free Tool
MTU / MSS Calculator
Calculate the effective Maximum Segment Size (MSS) from your MTU, accounting for IP headers, TCP headers, and various network encapsulations.
Configuration
Overhead Breakdown
Common MTU Values
| Network Type | MTU | Notes |
|---|---|---|
| Ethernet | 1500 | Standard for most networks |
| Jumbo Frames | 9000 | Datacenter / high-throughput LAN |
| PPPoE (DSL) | 1492 | 8 bytes PPPoE overhead |
| VPN (IPsec) | ~1400 | Varies by cipher suite |
| GRE Tunnel | 1476 | 24 bytes GRE overhead |
| VXLAN | 1446 | 54 bytes VXLAN overhead |
| IPv4 Minimum | 576 | RFC 791 minimum |
| IPv6 Minimum | 1280 | RFC 8200 minimum |
Understanding MTU and MSS
The Maximum Transmission Unit (MTU) is the largest packet size that can be sent over a network link without fragmentation. The Maximum Segment Size (MSS) is the largest amount of data in a single TCP segment -- calculated as MTU minus IP header (20 bytes for IPv4, 40 bytes for IPv6) minus TCP header (20 bytes).
For a standard Ethernet connection with 1500-byte MTU: MSS = 1500 - 20 (IP) - 20 (TCP) = 1460 bytes. When encapsulation is added (VPN, VXLAN, GRE), the effective MTU decreases, reducing the MSS further.
Path MTU Discovery (PMTUD)
Path MTU Discovery is the process of determining the maximum packet size that can traverse a network path without fragmentation. It works by sending packets with the "Don't Fragment" (DF) bit set. If a router along the path has a smaller MTU, it sends back an ICMP "Fragmentation Needed" message.
You can test Path MTU on Linux with: ping -M do -s 1472 target.com (1472 + 28 bytes IP/ICMP header = 1500). Decrease the size until pings succeed to find the path MTU. On Windows, use: ping -f -l 1472 target.com.
Misconfigured MTU is a common cause of mysterious connection issues, especially with VPNs and tunnels. If TCP connections hang after the handshake or large packets get dropped, MTU mismatch is often the culprit. Flowtriq's PCAP capture feature can help diagnose these issues by capturing the actual packet sizes traversing your network.