Troubleshooting guide

1-24
Cisco Wide Area Application Services Configuration Guide
OL-26579-01
Chapter 1 Configuring Network Settings
Configuring TCP Settings
sending rate. However, because the sender is not reducing its sending rate in response to network
congestion, the sender is not able to make any valid assumptions about the current state of the network.
Therefore, in order to avoid congesting the network with an inappropriately large burst of data, the
sender implements the slow start algorithm, which reduces the sending rate to one segment per
transmission. (See the “TCP Slow Start” section on page 1-24.)
You can modify the sender’s retransmit timer by using the Retransmit Time Multiplier field in the WAAS
Central Manager. The retransmit time multiplier modifies the length of the retransmit timer by one to
three times the base value, as determined by the TCP algorithm that is being used for congestion control.
When making adjustments to the retransmit timer, be aware that they affect performance and efficiency.
If the retransmit timer is triggered too early, the sender pushes duplicate data onto the network
unnecessarily; if the timer is triggered too slowly, the sender remains idle for too long, unnecessarily
slowing data flow.
TCP Slow Start
Slow start is one of four congestion control algorithms used by TCP. The slow start algorithm controls
the amount of data being inserted into the network at the beginning of a TCP session when the capacity
of the network is not known.
For example, if a TCP session began by inserting a large amount of data into the network, much of the
initial burst of data would likely be lost. Instead, TCP initially transmits a modest amount of data that
has a high probability of successful transmission. Next, TCP probes the network by sending increasing
amounts of data as long as the network does not show signs of congestion.
The slow start algorithm begins by sending packets at a rate that is determined by the congestion window,
or cwnd variable. (See the “Congestion Windows” section on page 1-23.) The algorithm continues to
increase the sending rate until it reaches the limit set by the slow start threshold (ssthresh) variable.
Initially, the value of the ssthresh variable is adjusted to the receiver’s maximum segment size (RMSS).
However, when congestion occurs, the ssthresh variable is set to half the current value of the cwnd
variable, marking the point of the onset of network congestion for future reference.
The starting value of the cwnd variable is set to that of the sender maximum segment size (SMSS), which
is the size of the largest segment that the sender can transmit. The sender sends a single data segment,
and because the congestion window is equal to the size of one segment, the congestion window is now
full. The sender then waits for the corresponding ACK from the receiving side of the transmission. When
the ACK is received, the sender increases its congestion window size by increasing the value of the cwnd
variable by the value of one SMSS. Now the sender can transmit two segments before the congestion
window is again full and the sender is once more required to wait for the corresponding ACKs for these
segments. The slow start algorithm continues to increase the value of the cwnd variable and therefore
increase the size of the congestion window by one SMSS for every ACK received. If the value of the
cwnd variable increases beyond the value of the ssthresh variable, then the TCP flow control algorithm
changes from the slow start algorithm to the congestion avoidance algorithm.
Path MTU Discovery
The WAAS software supports the IP Path Maximum Transmission Unit (MTU) Discovery method, as
defined in RFC 1191. When enabled, the Path MTU Discovery feature discovers 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 using the largest MTU that the links can handle, the sending device can
minimize the number of packets it must send.