Cisco Systems 78-11741-02 manual Enabling TCP Selective Acknowledgment, Enables Path MTU Discovery

Configuring IP Services

Configuring TCP Performance Parameters

To enable Path MTU Discovery, use the following command in global configuration mode:

Command	Purpose
Router(config)# ip tcp path-mtu-discovery [age-timer	Enables Path MTU Discovery.
{minutes infinite}]

Customers using TCP connections to move bulk data between systems on distinct subnets would benefit most by enabling this feature. Customers using remote source-route bridging (RSRB) with TCP encapsulation, serial tunnel (STUN), X.25 Remote Switching (also known as XOT or X.25 over TCP), and some protocol translation configurations might also benefit from enabling this feature.

The ip tcp path-mtu-discoveryglobal configuration command is to enable Path MTU Discovery for connections initiated by the router when it is acting as a host. For a discussion of how the Cisco IOS software supports Path MTU Discovery when the device is acting as a router, see the section “Understanding Path MTU Discovery” earlier in this chapter.

The age-timer is a time interval for how often TCP should reestimate the path MTU with a larger maximum segment size (MSS). The default Path MTU Discovery age-timer is 10 minutes; its maximum is 30 minutes. You can turn off the age timer by setting it to infinite.

Enabling TCP Selective Acknowledgment

The TCP selective acknowledgment feature improves performance in the event that multiple packets are lost from one TCP window of data.

Prior to this feature, with the limited information available from cumulative acknowledgments, a TCP sender could learn about only one lost packet per round-trip time. An aggressive sender could choose to resend packets early, but such re-sent segments might have already been successfully received.

The TCP selective acknowledgment mechanism helps improve performance. The receiving TCP host returns selective acknowledgment packets to the sender, informing the sender of data that have been received. In other words, the receiver can acknowledge packets received out of order. The sender can then resend only the missing data segments (instead of everything since the first missing packet).

Prior to selective acknowledgment, if TCP lost packets 4 and 7 out of an 8-packet window, TCP would receive acknowledgment of only packets 1, 2, and 3. Packets 4 through 8 would need to be re-sent. With selective acknowledgment, TCP receives acknowledgment of packets 1, 2, 3, 5, 6, and 8. Only packets 4 and 7 must be re-sent.

Refer to RFC 2018 for more detailed information on TCP selective acknowledgment.

The feature is used only when multiple packets are dropped within one TCP window. There is no performance impact when the feature is enabled but not used. To enable TCP selective acknowledgment, use the following command in global configuration mode:

Command	Purpose
Router(config)# ip tcp selective-ack	Enables TCP selective acknowledgment.

Cisco IOS IP Configuration Guide

IPC-113