Command History Release Modification

11.2 F This command was introduced.

Usage Guidelines TCP might not experience optimal performance if multiple packets are lost from one window of data.

With the limited information available from cumulative acknowledgments, a TCP sender can learn about only one lost packet per round trip time. An aggressive sender could retransmit packets early, but such retransmitted segments might have already been successfully received.

The TCP selective acknowledgment mechanism helps overcome these limitations. The receiving TCP returns selective acknowledgment packets to the sender, informing the sender about data that has been received. The sender can then retransmit only the missing data segments.

TCP selective acknowledgment improves overall performance. The feature is used only when multiple packets drop from a TCP window. There is no performance impact when the feature is enabled but not used.

This command becomes effective only on new TCP connections opened after the feature is enabled.

This feature must be disabled if you want TCP header compression. You might disable this feature if you have severe TCP problems.

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

Examples The following example enables the router to send and receive TCP selective acknowledgments:

ip tcp selective-ack

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