TCP IP Illustrated

These two options are defined in rfc 1323 jacobson

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Unformatted text preview: . This chapter looks at some proposed modifications to TCP that allow it to obtain the maximum throughput at these higher speeds. We first look at the path MTU discovery mechanism, which we've seen earlier in the text, focusing this time on how it operates with TCP. This often lets TCP use an MTU greater than 536 for nonlocal connections, increasing its throughput. We then look at long fat pipes, networks that have a large bandwidth-delay product, and the TCP limits that are encountered on these networks. Two new TCP options are described that deal with long fat pipes: a window scale option (to increase TCP's maximum window above 65535 bytes) and a timestamp option. This latter option lets TCP perform more accurate RTT measurement for data segments, and also provides protection against wrapped sequence numbers, which can occur at high speeds. These two options are defined in RFC 1323 [Jacobson, Braden, and Borman 1992]. We also look at the proposed T/TCP, modifications to TCP for transactions. The transaction mode of communication features a client request responded to by a server reply. It is a common paradigm for client-server computing. The goal of T/TCP is to reduce the number of segments exchanged by the two ends, avoiding the three-way handshake and the four segments to close the connection, so that the client receives the server's reply in one RTT plus the time required to process the request. What is impressive about these new options-path MTU discovery, the window scale option, the tirnestamp option, and T/TCP - is that they are backward compatible with existing TCP implementations. Newer systems that include these options can still interoperate with all older systems. With the exception of an additional field in an ICMP message that can be used by path MTU discovery, these newer options need only be implemented on the end systems that want to take advantage of them. We finish the chapter by looking at recently published figures dealing with TCP performance. 24.2 Path MTU Discovery file:///D...
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