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 4900 Series	VOSS 8.3.1
	VSP 7400 Series	VOSS 8.3.1

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.

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.

For 7520 Series and 7720 Series 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.