Troubleshooting guide

1-23
Cisco Wide Area Application Services Configuration Guide
OL-26579-01
Chapter 1 Configuring Network Settings
Configuring TCP Settings
To configure TCP settings from the CLI, you can use the tcp global configuration command.
To enable the MTU discovery utility from the CLI, you can use the ip path-mtu-discovery enable global
configuration command.
This section contains the following topics:
• Explicit Congestion Notification, page 1-23
• Congestion Windows, page 1-23
• Retransmit Time Multiplier, page 1-23
• TCP Slow Start, page 1-24
• Path MTU Discovery, page 1-24
Explicit Congestion Notification
The TCP Explicit Congestion Notification (ECN) feature allows 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. The major issue with ECN is that the
operation of both the routers and the TCP software stacks needs to be changed to accommodate the
operation of ECN.
Congestion Windows
The congestion window (cwnd) is a TCP state variable that limits the amount of data that a TCP sender
can transmit onto the network before receiving an acknowledgment (ACK) from the receiving side of the
TCP transmission. The TCP cwnd variable is implemented by the TCP congestion avoidance algorithm.
The goal of the congestion avoidance algorithm is to continually modify the sending rate so that the
sender automatically senses any increase or decrease in available network capacity during the entire data
flow. When congestion occurs (manifested as packet loss), the sending rate is first lowered then
gradually increased as the sender continues to probe the network for additional capacity.
Retransmit Time Multiplier
The TCP sender uses a timer to measure the time that has elapsed between sending a data segment and
receiving the corresponding ACK from the receiving side of the TCP transmission. When this retransmit
timer expires, the sender (according to the RFC standards for TCP congestion control) must reduce its
Keepalive Timeout Length of time that the WAAS device keeps a connection open
before disconnecting. The range is 1 to 120 seconds. The default is
90 seconds.
Enable Path MTU Discovery Enables discovery of the largest IP packet size allowable between
the various links along the forwarding path and automatically sets
the correct value for the packet size. By default, this option is
disabled. For more information, see the “Path MTU Discovery”
section on page 1-24.
Table 1-1 TCP Settings (continued)
TCP Setting Description