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Configuring IP Services
Configuring TCP Performance Parameters
IPC-113
Cisco IOS IP Configuration Guide
To enable Path MTU Discovery, use the following command in global configuration mode:
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 path-mtu-discovery [age-timer
{minutes | infinite}]
Enables Path MTU Discovery.
Command Purpose
Router(config)# ip tcp selective-ack
Enables TCP selective acknowledgment.