Configuring Waas; Overview; Tfo - HP FlexNetwork MSR2003 Configuration Manual

Configuring WAAS

Overview

The Wide Area Application Services (WAAS) feature is a set of services that can optimize WAN
traffic. WAAS solves WAN issues such as high delay and low bandwidth by using optimization
services. WAAS provides the following optimization services:
•
Transport Flow Optimization (TFO).
•
Data Redundancy Elimination (DRE).
•
Lempel-Ziv compression (LZ compression).

TFO

TFO optimizes TCP traffic without modifying packet header information. TFO includes the following
optimization methods:
•
Slow start optimization.
•
Increased buffering.
•
Congestion algorithm optimization.
•
Selective acknowledgement.
Slow start optimization
The initial congestion window size for TCP slow start is one TCP segment. During slow start, TCP
doubles the congestion window size for each received ACK that acknowledges new data. In this
manner, the congestion window will reach an appropriate value by examining the congestion status.
In a WAN environment, the congestion window takes a long time to reach an appropriate size
because of high delay.
Slow start optimization shortens the slow start process by increasing the initial congestion window
size.
Increased buffering
TCP has a maximum buffer size of 64 KB. After the sender sends 64 KB data, it must wait for an ACK
from the receiver before continuing to send data. This mechanism wastes bandwidth on the WAN
link.
Increased buffering increases the TCP buffer size to a maximum of 16384 KB, improving link
efficiency.
Congestion algorithm optimization
TCP uses the congestion window to control congestion. The window size indicates the size of data
that can be sent out before an ACK is received. The window size changes with the congestion status.
The greater the window size, the faster the data rate, which more likely causes congestion. The
smaller the window size, the lower the data rate, which causes low link efficiency.
Congestion algorithm optimization achieves a trade-off between the data rate and congestion by
selecting the optimum window size.
Selective acknowledgement
TCP uses a cumulative acknowledgement scheme. This scheme forces the sender to either wait a
roundtrip time to know each lost packet, or to unnecessarily retransmit segments that have been
correctly received. When multiple nonconsecutive segments are lost, this scheme reduces overall
TCP throughput.
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