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  Advanced TCP Options February 8, 1999 | |||||||||||||||
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By Eric A. Hall Considering its importance to the Internet, TCP has experienced surprisingly little change over the years. It has shown itself sufficiently able to ensure that data reaches its destination intact and error-free, and has done a good job of providing flow-control and circuit-management services. Yet TCP has been woefully inadequate in many situations, particularly on modern networks that were unimaginable when TCP was designed. TCP's designers knew they couldn't predict the future, so they wisely allowed for modifications and enhancements that don't break the fundamental interoperability that drives Internet growth. These enhancements are incorporated as "options" within the TCP header and allow new fields to be added, preserving backward-compatibility with older systems. Many new TCP options have been developed and deployed, with a few proving to be extremely useful. These options have been introduced on a wide variety of systems, though typically they're found on high-end Unix systems.
On the Bandwagon However, with the release of Windows 98, Microsoft is bringing these options to the masses, once they are enabled. To do so, add a string value called "Tcp1323opts" to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Vxd\MSTCP registry branch, with one of the following values: It's important to note that Windows98 won't be the last OS to support these options. While none of the options are provided in any shipping version of Novell NetWare, Microsoft Windows NT or Linux, the latter two support the options in releases under development. Even many high-end Unix systems don't support all of them: SunSoft's Solaris 7 is the first major release to incorporate them all, while Hewlett-Packard Co.'s HP-UX and Silicon Graphics' Irix support only a couple. As products that support these options are developed and deployed, it will become increasingly important for network managers to understand how these options work and how they will impact corporate networks; to that end, we present an explanation of these options below. To help convey this information, we'll study a typical exchange of data between a Windows98 client and a Solaris 7 server. In the screen capture at left, you can see the first TCP segment sent from the Windows98 client. The first TCP option shown is the Maximum Segment Size (MSS); this well-known and widely used option is used for publishing the Maximum Transfer Unit (MTU) size of the local network (minus IP and TCP header data). Also scattered throughout the option space are No-Operation options, which are used to internally pad the option space. Neither the MSS or No-Op option are new--both appear in virtually every networked device on the planet. However, the remaining options are new to Windows98. Windows Scale RFC 793, the document that defines TCP, mandates use of a "Window" field in the TCP header of every packet sent across a TCP connection. The Window field provides a 16-bit integer that advertises the number of bytes available in a recipient's receive buffer. This information is used by the sending system's flow-control service to slow down and speed up the amount of data being transferred according to the recipient's capabilities. Technically, the Window field defines the maximum number of bytes that can be sent without requiring the sender to stop transmitting and wait for an acknowledgment. But because most corporate networks use low-latency topologies, such as Ethernet and token ring, the Window field's flow-control mechanism rarely comes into play on the LAN. Data is received and acknowledged quickly, allowing the sender to transmit more data. Thus, the Window field's maximum amount is never reached, and data flows smoothly across the network.
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