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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
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- Spring '12