Common MTU/MSS Values

Reference for Maximum Transmission Unit (MTU) and Maximum Segment Size (MSS) values across different network types.

MTU vs MSS

MTU (Maximum Transmission Unit) is the largest packet size that can be transmitted over a network link without fragmentation. MSS (Maximum Segment Size) is the largest amount of TCP data that can be sent in a single segment. The relationship is: MSS = MTU - IP Header - TCP Header For IPv4: MSS = MTU - 20 - 20 = MTU - 40 bytes

Key Formula

MSS = MTU - IP Header - TCP Header
 For IPv4: MSS = MTU - 20 - 20 = MTU - 40 bytes

Common MTU/MSS Values

Medium	MTU	MSS	Usage	Notes
Ethernet	`1500`	`1460`	Most common in LANs and internet	Standard Ethernet II frame size
PPPoE (DSL)	`1492`	`1452`	Common for DSL broadband connections	PPP over Ethernet adds 8 bytes overhead
Jumbo Frames	`9000`	`8960`	Data centers, storage networks	Larger frames for high-performance networks
802.11 WiFi	`1500`	`1460`	Wireless LANs	Same as Ethernet for compatibility
VPN (typical)	`1436`	`1396`	IPSec, OpenVPN, WireGuard tunnels	Varies by VPN protocol and encryption
GRE Tunnel	`1476`	`1436`	Generic routing encapsulation	GRE adds 24 bytes of overhead
IPv6 over IPv4	`1480`	`1440`	IPv6 transition mechanisms	6in4 tunneling overhead
MPLS	`1496`	`1456`	Service provider networks	MPLS label adds 4 bytes per label

MTU/MSS Calculation Examples

Standard Ethernet

MTU: 1500 bytes

IP Header: 20 bytes

TCP Header: 20 bytes

Resulting MSS: 1460 bytes

Calculation: 1500 - 20 (IP) - 20 (TCP) = 1460

IPv6 Ethernet

MTU: 1500 bytes

IP Header: 40 bytes

TCP Header: 20 bytes

Resulting MSS: 1440 bytes

Calculation: 1500 - 40 (IPv6) - 20 (TCP) = 1440

PPPoE Connection

MTU: 1492 bytes

IP Header: 20 bytes

TCP Header: 20 bytes

Resulting MSS: 1452 bytes

Calculation: 1492 - 20 (IP) - 20 (TCP) = 1452

Protocol Overheads

Protocol/Header	Overhead (Bytes)	Notes
Ethernet Header	`14`	Destination MAC, Source MAC, EtherType
IPv4 Header	`20`	Minimum size, can be larger with options
IPv6 Header	`40`	Fixed size header
TCP Header	`20`	Minimum size, can be larger with options
UDP Header	`8`	Fixed size header
PPPoE	`8`	PPP over Ethernet encapsulation
GRE	`24`	Generic Routing Encapsulation
IPSec ESP	`24-32`	Varies by encryption and authentication
MPLS Label	`4`	Per MPLS label in stack

Path MTU Discovery

Mechanism to find the largest MTU along a network path

PMTU Discovery Process

Send packets with Don't Fragment (DF) bit set
If packet is too large, router sends ICMP Fragmentation Needed
Sender reduces packet size and tries again
Continue until packet goes through successfully
Cache the discovered MTU for the destination

Common Issues

Firewalls blocking ICMP can break PMTU discovery
Results in 'black hole' where large packets are silently dropped
TCP MSS clamping can work around this issue

Troubleshooting Common Issues

Slow file transfers over VPN

Cause: Large packets being fragmented due to VPN overhead

Solution: Reduce MTU on VPN interface or enable MSS clamping

Web pages not loading completely

Cause: PMTU discovery failing, large packets dropped

Solution: Check firewall ICMP rules or reduce interface MTU

Poor performance on jumbo frame network

Cause: One device not supporting jumbo frames

Solution: Ensure all devices in path support same MTU size

Useful Commands

Checking MTU Settings

Platform	Command	Purpose
Windows	`netsh interface ipv4 show subinterfaces`	Show MTU for all interfaces
Linux	`ip link show`	Display interface MTU settings
macOS	`ifconfig`	Show interface configuration including MTU
Cisco IOS	`show interfaces`	Display interface MTU values

Testing MTU Size

Platform	Command	Purpose
Windows	`ping -f -l 1472 8.8.8.8`	Test maximum packet size (1472 + 28 headers = 1500 MTU)
Linux	`ping -M do -s 1472 8.8.8.8`	Test with Don't Fragment bit set
Any	`tracepath destination`	Discover path MTU to destination

Best Practices

Use consistent MTU sizes across network segments
Test MTU discovery with ping -f -l [size] (Windows) or ping -M do -s [size] (Linux)
Configure MSS clamping on routers for VPN/tunneled traffic
Monitor for fragmentation in network statistics
Document MTU requirements for different network segments
Consider jumbo frames for high-throughput applications

Performance Tip

Mismatched MTU sizes can cause significant performance issues. Always ensure consistent MTU values across your
          network path, especially for high-throughput applications.

Quick Reference

Common Values to Remember

Standard Ethernet: 1500 MTU / 1460 MSS

PPPoE/DSL: 1492 MTU / 1452 MSS

Jumbo Frames: 9000 MTU / 8960 MSS

VPN (typical): 1436 MTU / 1396 MSS

Important Note

When troubleshooting connectivity issues, especially with VPNs or tunnels, MTU/MSS mismatches are often the culprit. Test with smaller packet sizes if large transfers fail but small ones succeed.