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TCP Tuning Parameters

 

TCP Advertised Receive Window Size: The TCP Receive Window Size (RWIN) is the amount of data in bytes that can be received before the sender must await confirmation that the data arrived properly. Another relevant element to this parameter is the Maximum Segment Size (MSS), which is the maximum amount of data that may be received in one network segment at one time. It is calculated as the MTU value minus 40 bytes (40 bytes are allocated for basic TCP and IP information in each segment).  Setting the TCP Receive Window size to even multiples of the MSS increases the percentage of full-sized data segments used during transmission, resulting in more efficient downloading, uploading, and network-based data exchanges.

TCP Large Windows: Windows 98 and later Windows operating systems accept larger amounts of data before acknowledgement of receipt is required, which can enhance performance for higher-speed connections on networks with large bandwidth delay products (such as high-speed transcontinental connections or satellite links).

Selective Acknowledgements: Windows 98 and later Windows operating systems support Selective Acknowledgements (SACK). Selective acknowledgements allow the TCP network to recover from IP packet loss without re-sending packets that were already received by the receiver. SACK is most useful when employed with TCP large windows.

Fast Retransmission and Recovery: Windows 98 and later Windows operating systems support Fast Retransmission and Fast Recovery of TCP connections that encounter IP packet loss in the network. These mechanisms allow a sender to quickly infer a single packet loss from the reception of duplicate acknowledgements for a previously sent and acknowledged network packet. This mechanism is useful when the network or Internet is intermittently congested.

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MTU: MTU is the default packet size (amount of data) that Windows sends and receives information over the Internet or across your network. When your computer establishes a connection with another computer over the Internet or your LAN, the two computers must use a common MTU value to send and receive data. The computer with the lowest MTU value determines the value both must use. Enabling this setting causes your system to attempt to discover the MTU over the path to a remote host. By discovering the remote host MTU and limiting TCP segments to this size, TCP can automatically eliminate fragmentation at routers along the path that connect networks with different MTUs.  Fragmentation adversely affects TCP throughput and network congestion.

Blackhole Detection: A blackhole, in relation to Internet communication, is a situation when your system attempts to automatically determine the MTU of a system on the other end, and is unable to do so because the system on the other end does not respond to MTU requests with the proper information. If black hole detection is enabled, your system attempts to discover whether the connected systems support automatic MTU discovery. If they do not support automatic MTU discovery, the MTU will be detected using alternate and less efficient methods, causing performance degradation.

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TCP TTL: Time To Live (TTL) is a parameter included with each segment of transmitted data.  TTL indicates how long the data is allowed to survive before being discarded.  TTL is based how many hops (or network server transitions) the data segment travels over before it is a transmission failure.  The larger the TTL value, the greater chance the data eventually arrives at its destination.  However, a too-large TTL may result in unnecessary delays when data will fail due to network errors.