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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 retransmit packets early, but such retransmitted 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 has been received. In other words, the receiver can acknowledge packets received out of order. The sender can then retransmit 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 have to be resent. With selective acknowledgment, TCP receives acknowledgment of packets 1, 2, 3, 5, 6, and 8. Only packets 4 and 7 have to be resent.
Refer to RFC 2018 for more detailed information on TCP selective acknowledgment.
This feature is supported on these platforms:
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, perform the following task in global configuration mode.
| Task | Command |
|---|---|
| Enable TCP selective acknowledgment. | ip tcp selective-ack |
The following example enables the router to send and receive TCP selective acknowledgments:
ip tcp selective-ack
The new ip tcp selective-ack command controls TCP selective acknowledgment.
To enable TCP selective acknowledgment, use the ip tcp selective-ack global configuration command. To disable TCP selective acknowledgment, use the no form of this command.
ip tcp selective-ackThis command has no arguments or keywords.
Disabled
Global configuration
This command first appeared in Cisco IOS Release 11.2 F.
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.
The following example enables the router to send and receive TCP selective acknowledgments:
ip tcp selective-ack
ip tcp header-compression
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