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Configuring TCP Stack Parameters

Overview

Caches are typically deployed by customers for any of the following reasons:

The fourth reason is an often overlooked and much less understood property of deploying caching, and it can often have a huge benefit on the performance of TCP end to end.

Queries sent between a server and a client and the replies generated are defined as transactions. For data transactions between client and servers, the size of windows and buffers is important, and fine-tuning the TCP stack parameters therefore becomes the key to maximizing this benefit.

The relevant TCP parameters to maximize cache performance and throughput include the ability to tune timeout periods, client and server receive/send buffer sizes, and TCP window scaling behavior.


Note   Because of the complexities involved in TCP parameters, care is advised in tuning these parameters. In nearly all environments, the default TCP settings are adequate. Fine tuning of TCP settings is for network administrators with adequate experience and full understanding of TCP operation details.

Configuring TCP Parameters Using the Content Engine GUI

To view the TCP configuration parameters in the Content Engine, start the Content Engine GUI, click Systems, and then choose TCP. The TCP Configuration window is displayed. (See Figure 12-1.) See Table 12-1 for a description of the TCP configuration parameters and the corresponding CLI commands.


Figure 12-1: TCP Configuration Window



Table 12-1: TCP CLI Configuration Parameters
Fields TCP CLI Commands Descriptions
Send Buffer

To Server

tcp server-send-buffer kbytes

TCP outgoing window size in kilobytes (1-128). The default is 8 KB.

To Client

tcp client-send-buffer kbytes

TCP outgoing window size in kilobytes (1-128). The default is 32 KB.

Receive Buffer

To Server

tcp server-receive-buffer kbytes

TCP incoming window size in kilobytes (1-128). The default is 32 KB.

To Client

tcp client-receive-buffer kbytes

TCP incoming window size in kilobytes (1-128). The default is 8 KB.

R/W Timeout

To Server

tcp server-rw-timeout seconds

Interval after which the Content Engine will time out trying to read or write to the network (1-3600). The default is 120 seconds.

To Client

tcp client-rw-timeout seconds

Interval after which the Content Engine will time out trying to read or write to the network (1-3600). The default is 120 seconds.

Keepalive

Keepalive

tcp keepalive-probe-interval

Length of time that the Content Engine will keep a connection open before disconnecting.

Timeout

tcp keepalive-interval seconds

Length of time that the Content Engine will keep an idle connection open (1-3600 seconds). The default is 300 seconds.

Count

tcp keepalive-probe-cnt count

Number of times the Content Engine will retry a connection (1-10 attempts). The default is 4 attempts.

Congestion Window Base

tcp cwnd-base

Initial congestion window value (1-10 segments). The default is 2 segments.

Initial slow start threshold value

tcp init-ss-threshold

Low watermark threshold for slow start (2-10 segments). The default is 2 segments.

Retransmitter Timer Increment factor

tcp increase-xmit-timer-value

Factor (1-3) used to modify the length of the retransmit timer by 1 to 3 times the base value determined by the TCP algorithm. The default is 1, which leaves the times unchanged.

Note   Modify this factor with caution. It can improve throughput when TCP is used over slow reliable connections but should never be changed in an unreliable packet delivery environment.

Maximum Segment Size

Server

tcp server-mss maxsegsize

Maximum packet size sent to servers. The default is 1460 bytes.

Client

tcp client-mss

Maximum packet size sent to clients. The default is 1432 bytes

Others

Satellite

tcp server-satellite
tcp client-satellite

Server and client TCP compliance with RFC 1323. See the "TCP-Over-Satellite Extensions" section.

Type of Service

type-of-service

TCP Type of Service. The default is disabled.

Ecn

ecn

TCP explicit congestion notification.

TCP-Over-Satellite Extensions

The Content Engine has the ability to turn on TCP-over-satellite extensions (as documented in
RFC 1323) to maximize performance and end-to-end throughput over satellite-type connections.

The large number of satellites available to network infrastructures has increased the amount of bandwidth available in the air. Taking advantage of these connections through satellite-type connections has created new challenges in the use of TCP transactions and acknowledgments:

Use the tcp server-satellite and tcp client-satellite commands to set the TCP connection so that it complies with RFC 1323.

TCP Configuration Examples

To display current TCP configuration information, use the show tcp EXEC command. Note that the default 8 KB incoming window size for the client buffer is used.

ContentEngine# show tcp ==TCP Configuration== TCP keepalive timeout 300 sec TCP keepalive probe count 4 TCP keepalive probe interval 75 sec TCP server R/W timeout 120 sec TCP client R/W timeout 120 sec TCP server send buffer 8 k TCP server receive buffer 32 k TCP client send buffer 32 k TCP client receive buffer 8 k TCP server max segment size 1460 TCP satellite (RFC1323) disabled TCP client max segment size 1432 TCP explicit congestion notification disabled TCP type of service disabled TCP cwnd base value 2 TCP initial slowstart threshold value 2 TCP increase(multiply) retransmit timer by 1 ContentEngine#

In this example, the tcp client-receive-buffer command is used to change the TCP incoming window size to 100 KB.

ContentEngine(config)# tcp client-receive-buffer 100 ContentEngine(config)

You can now verify the configuration change with the show tcp command.

ContentEngine# show tcp ==TCP Configuration== TCP keepalive timeout 300 sec TCP keepalive probe count 4 TCP keepalive probe interval 75 sec TCP server R/W timeout 120 sec TCP client R/W timeout 120 sec TCP server send buffer 8 k TCP server receive buffer 32 k TCP client send buffer 32 k TCP client receive buffer 100 k TCP server max segment size 1460 TCP satellite (RFC1323) disabled TCP client max segment size 1432 TCP explicit congestion notification disabled TCP type of service disabled TCP cwnd base value 2 TCP initial slowstart threshold value 2 TCP increase(multiply) retransmit timer by 1 ContentEngine#

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Posted: Mon Nov 18 11:28:56 PST 2002
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