ND SatCom SKYWAN IDU 7000, Manual

Page: 128 / 154

124 Network Design and Engineering Guide 2010-10-26
Data Networking
SkyWAN
®
Internet Protocol Features
errors are more common on satellite links compared to terrestrial fibre optic links, this feature
can reduce TCP throughput even further.
To overcome these protocol drawbacks the Sky WAN
®
IDU supports a TCP acceleration func-
tionality which is characterized by two main features:
- Large window size (600 kByte);
- Selective Acknowledgements instead of cumulative acknowledgements.
Host-to-host TCP connections which include the satellite link will be intercepted by the Sky-
WAN
®
IDUs and replaced over the satellite link by the optimized TCP-A procedures as shown
in the figure 4-9.
Figure 4-9 TCP-A Feature Overview
Note that for the hosts this procedure is fully transparent. No changes to the host’s operating
system or applications are necessary. The TCP acknowledgments will be sent by the local IDU
with low delay, thus the satellite delay has no impact on the TCP throughput.
Up to 32 TCP connections can be accelerated by any IDU. Connections exceeding this limit
will be supported as transparent TCP connection without acceleration.
An IP flow to which TCP acceleration should be applied must have a definition in the forwarding
aggregate table with the forwarding behavior “Gold-TCPA”. The simplest configuration would
be to define an aggregate which maps all TCP flows to the Gold-TCPA behavior. However, in
this scenario there is a risk that the 32 accelerable TCP connections may be consumed by less
important services like e.g. user web surfing. This might prevent important connections like e.g.
large file uploads to servers from getting the benefits of acceleration. In such a situation a more
specific definition of Gold-TCPA aggregates which e.g. includes specific IP addresses will be
more reasonable.