Cisco TCP Tuning

ip tcp ecn

The TCP Explicit Congestion Notification (ECN) feature provides a method for an intermediate router to notify the end hosts of impending network congestion. It also provides enhanced support for TCP sessions associated with applications that are sensitive to delay or packet loss including Telnet, web browsing, and transfer of audio and video data. The benefit of this feature is the reduction of delay and packet loss in data transmissions.

ip tcp selective-ack

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).

ip tcp window-size 12384

This is where you set the Rwin. This is the single most important tweak, and raising the value from the Windows default will greatly improve download speeds. My Rwin is set to 65535.

ip tcp queuemax 15

The queue for outgoing TCP packets
Default 5 for TTY (async and console) interfaces; 20 for others
This command sets the size of the outgoing TCP queue to packets. The queue is maintained per-connection; i.e., every connection has its own queue.

ip tcp mss

Interacts with the ip tcp path-mtu-discovery command and not the ip tcp header-compression command. The ip tcp path-mtu-discovery command changes the default MSS to 1460 even for nonlocal nodes.

The maximum segment size (MSS) is an option of the TCP protocol that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single, unfragmented piece. It does not count the TCP header or the IP header^[1]. For optimum communications, the number of bytes in the data segment and the headers must not add up to more than the number of bytes in the maximum transmission unit (MTU).

The MSS is an important consideration in Internet connections. As data is routed over the Internet, it must pass through multiple gateway routers. Ideally, each TCP segment can pass through every router without being fragmented. If the data segment size is too large for any of the routers through which the data passes, the oversized segments are fragmented. This decreases throughput and increases latency, in some cases dramatically^[2]. The likelihood of such fragmentation can be minimized by keeping the MSS as small as reasonably possible. Every host is required to be able to handle an MSS of at least 536 bytes.

For most computer users, the MSS option is set automatically by the operating system on the SYN packet during the TCP handshake. Each direction of data flow can use a different MSS.