Last Updated: Jan 15, 2026
Introduction
This article explains the networking technique known as Dynamic MSS (Maximum Segment Size) Clamping, why it is important for network stability scenarios, and its relevance in environments that use jumbo frames.
Prerequisites
A basic understanding of networking concepts is helpful but not required.
Step-by-Step Instructions
Core Concepts: MTU and MSS
To understand MSS clamping, we first need to define two key terms:
- MTU (Maximum Transmission Unit): This is the largest data packet size (in bytes) that a device can transmit over a network. The standard MTU for Ethernet networks is 1500 bytes. If a packet is larger than the MTU of a network link, it must be broken into smaller pieces (fragmented), which can slow down performance.
- MSS (Maximum Segment Size): This is the largest amount of data that can fit inside a single TCP packet's payload. It is calculated by subtracting the IP and TCP header sizes from the MTU. For a standard 1500-byte MTU, the MSS is typically 1460 bytes (1500-byte MTU - 20-byte IP header - 20-byte TCP header).
During the initial TCP connection setup (the "three-way handshake"), each device communicates its MSS to the other, agreeing on the maximum segment size they can exchange.
How Dynamic MSS Clamping Works
Problems occur when a network path between two devices contains a link with a smaller MTU than the devices assume. This is common when using VPNs or other tunnels, which add extra headers to packets and reduce the effective MTU.
When this happens, a device might send a packet that is too large for the tunnel to handle, leading to dropped packets, high retransmit rates, and severe connection instability.
Dynamic MSS clamping solves this problem. A network device (like a router or firewall) along the path intercepts the TCP handshake packets and intelligently rewrites the MSS value to a lower, safer number. By "clamping" the MSS, it ensures that all transmitted packets are small enough to pass through the entire network path without being dropped or fragmented.
Why It Matters: Jumbo Frames and Network Stability
The concept of MSS clamping is especially relevant for networks that use jumbo frames—Ethernet frames larger than the standard 1500 bytes, often up to 9000 bytes. Jumbo frames can increase network throughput by reducing header overhead.
However, issues arise if a network path includes segments that do not support jumbo frames. In such cases, dynamic MSS clamping is essential to resize packets down to a standard MSS before they hit the incompatible link, preventing communication failures.
If you are experiencing networking instability, such as a high number of packet retransmits (dropping from thousands to hundreds is a significant improvement), enabling dynamic MSS clamping on your network gateways can be a critical step to ensure stable and performant connections.
Troubleshooting and Resolution
Check your VM's MTU
You can verify the setting on your VM's network interface using the ip link show command. The standard configuration should show an of 1500.
Example:
$ ip link show
2: ens7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
link/ether 4e:54:19:6b:b9:54 brd ff:ff:ff:ff:ff:ff
altname enp0s7How Crusoe Cloud Mitigates This Issue
To prevent problems related to mismatches, Crusoe manages Maximum Segment Size () clamping at the infrastructure level. This process helps ensure that TCP packets are sized appropriately to avoid fragmentation and maintain stable connections.
Important Recommendation: Do Not Manually Clamp MSS
It is strongly discouraged for users to apply their own clamping rules on VMs. This can create unnecessary overhead for the VM's CPUs and may lead to significant performance degradation.
Contact Crusoe Support
If you are experiencing persistent network instability or have a specific use case that requires a non-standard (e.g., for Jumbo Frames), please contact Crusoe Support. Our team can investigate the issue and make the necessary adjustments at the infrastructure level to ensure network stability and performance for your environment.