Configuring TCP Stack Parameters

Overview

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

To save bandwidth
To accelerate the delivery of content
To apply policies on what content is viewed (content filtering)
To increase the throughput of HTTP streams over TCP end to end

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:

Latency—Round trip times to satellites orbiting 24,000 miles above the earth are 550 milliseconds for a single satellite hop. Window size must be set to prevent low-throughput connections.
Bit errors—Packet loss can occur in a land-based- device-to-satellite connection in addition to the losses caused by regular network congestion.
Asymmetric bandwidth—Return bandwidth from satellites can be narrower than receiving bandwidth, thereby affecting performance.

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#