Nera SatLink 1000, User Manual

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Nera SatLink Terminal User Guide – Revision P
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Selective Acknowledgement: Basic TCP uses the goback-N acknowledgement scheme. This means
that an acknowledgement applies to all transmissions up to the ack number. This means that when
multiple packets are lost, all packets transmitted after the erroneous transmission must be
retransmitted. By employing a selective acknowledgement scheme, the ACK only applies to a single
segment. The SACK option as described in RFC 2018 points out which segments need to be
retransmitted. This has effect only for large windows (probability for substantial retransmissions).
K.2 Tuning of TCP parameters
TCP is a packet-based protocol where data is transmitted in variable sized blocks. The TCP Receive
Window has a default value of 8 kB in Windows 95/98/NT and about 16 kB in Windows ME/2000/XP,
which is adequate for relatively slow dialup modems and high-speed networks with relatively low latency
(round-trip delay less than about100 ms). Increasing the TCP Receive Window above the default setting
can in certain configurations improve the throughput on satellite connections were the round trip delay
typically is in the order of 600 ms.
Some free tools are available on the Internet to “tweak” various TCP/IP network settings that affect
downstream performances. They basically provide a short-cut to modify registries of Windows based
PCs.
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Tcp Tune at http://moat.nlanr.net/Software/TCPtune/ and
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Dr. Tcp at http://www.dslreports.com/front/drtcp.html are widely used freeware.
Microsoft has confirmed a TCP/IP transmission bug in Windows 95, 98, and NT that can adversely affect
upload throughput over high-delay networks. However, the problem is corrected in NT Service Pack 6.
Windows NT and Windows 95 have older TCP/IP stacks that do not support large windows or selective
acknowledgement. Reportedly there are some patches available to fix this for Windows 95. The window
size should be increased to 65kB.
Windows 98/ME/2000/XP support selective acknowledgement, window scaling, and large windows. The
default window size for Windows 98 is 8k. The Dr. TCP tool should be used to set large receive buffers
(65 kB or above), maximum TCP receive windows shall be set to 115000, as well as enabling windows
scaling/time stamps (RFC1323) and SACK (RFC 2018) functionality.
Figure 29: TCPTune utility