Adjusting the TCP Maximum Segment Size

Table 1. TCP Maximum Segment Size (MSS) Adjustment for Fabric Extend
Feature	Product	Release introduced
Ability to adjust the maximum segment size (MSS)	5320 Series	Fabric Engine 8.6
	5420 Series	VOSS 8.5
	5520 Series	VOSS 8.5
	5720 Series	Fabric Engine 8.7
	7520 Series	Fabric Engine 8.10
	7720 Series	Fabric Engine 8.10
	VSP 4450 Series	Not Supported
	VSP 4900 Series	VOSS 8.3.1
	VSP 7200 Series	Not Supported
	VSP 7400 Series	VOSS 8.3.1
	VSP 8200 Series	Not Supported
	VSP 8400 Series	Not Supported
	VSP 8600 Series	Not Supported
	XA1400 Series	VOSS 8.1.8

You can adjust the TCP maximum segment size (MSS) to improve the throughput for the TCP session over a Fabric Extend (FE) adjacency.

TCP MSS Overview

When a client initiates a connection with a server, it uses TCP SYN packets to negotiate the MSS to avoid fragmentation. The client and server use the outgoing maximum transmission unit (MTU) to advertise the MSS.

If a tunnel exists between the client and server, the encapsulation consumes more room in the outer IP header. As a result, the router that performs the tunnel encapsulation fragments the packet to fit over the tunnel. Adjust the MSS to modify the value in the TCP SYN packet so the client and server negotiate a lower number and leave headroom for tunnel encapsulation.

This adjustment functionality applies to IPv4 only.

Important

If you enable this functionality and port mirroring simultaneously, the switch does not mirror CP-generated packets.

VSP 4900 Series Support

TCP MSS adjustment applies unidirectionally when a packet is forwarded from a UNI interface to any other interface. To use this functionality, you must enable TCP MSS adjustment on both sides of the FE tunnel.

VSP 7400 Series Support

TCP MSS adjustment applies unidirectionally when a packet is forwarded into an FE tunnel. To use this functionality, you must enable TCP MSS adjustment on both sides of the FE tunnel.

XA1400 Series Support

The MSS adjustment functionality only activates when at least one Fabric Extend (FE) tunnel with a MTU less than or equal to 1500 is configured. The functionality is inactive if no FE tunnels with MTU less than or equal to 1500 are configured. Deleting the last tunnel with MTU less than or equal to 1500 results in the functionality becoming inactive.

The switch can auto-derive the MSS value based of the tunnel MTUs or you can manually configure the MSS value. The formula for the auto-derived value is

min(Tunnels MTUs) - 200B (size for VXLAN + MIM + IPSEC + IP+TCP headers)

If you configure multiple FE tunnels with MTU less than or equal to 1500, then the switch uses the lowest of all tunnel MTUs to auto-derive the TCP MSS adjust value. The same value applies bidirectionally, to all TCP syn packets that transmit NNI to UNI and UNI to NNI.

Tip

As a best practice, disable this enhancement on the head-end side and enable only at the branch side.

The switch does not support different TCP adjust MSS values if you configure different FE tunnel MTUs on different tunnels.

If you configure FE tunnels and regular NNIs on the same adjacency, then the TCP adjust MSS value applies to all TCP packets traversing across regular NNIs and FE tunnels.