TCP/IP Tutorial and Technical Overview - IBM Redbooks

The profiling of TCP for WP-TCP sought to optimize the protocol to compensate
for these scenarios. Specifically, eight optimizations were included in WP-TCP.
These are:
► Large window size: Built in to TCP is the use of the Bandwidth Delay Product
(BDP), calculated from the available bandwidth and the round-trip time, to
calculate the minimum window size. It also calculates the maximum window
size using the smallest value of the send or receive buffers. WP-TCP can opt
to use the maximum window size, relying on the congestion control
mechanisms built into TCP to regulate the data as needed. The WP-TCP
specification indicates that implementation of this optimization is
recommended, but not required.
► The window scale option: Entities that opt to use the large window size are
required to also implement the window scale option defined by RFC 1323. If
an entity does not support the large window size optimization, implementation
of the window scale optimization is optional.
► Round-trip time measurement: Also defined in RFC 1323, round-trip time
measurement (RTTM) is recommended for use by any entity that implements
window sizes larger than 64 K. This also requires the use of the time stamp
option.
► Large initial window: The standard TCP implementation includes an initial
window up to two segments in size. However, WP-TCP employs an extension
defined in RFC 2414 that allows WP-TCP to send an initial window of three or
four segments. This is limited to 4380 bytes. Implementation of this
optimization is optional.
► Path MTU discovery: WP-TCP now supports the procedures, defined in RFCs
1191 and 1981, used to discover the maximum transmission unit (MTU) of
any given path. This allows WP-TCP to optimize the amount of data sent in
each packet to avoid fragmentation or dropped packets. Implementation of
this optimization is recommended, but not required.
► MTU larger than the default IP MTU: If an entity cannot support path MTU
discover (for example, if some of the routers in the path do not support the
needed ICMP messages), a WP-TCP implementation can assume a value
exceeding the default IPv4 or IPv6 values. However, the value assumed must
not exceed 1500 bytes, and must reflect the maximum segment size
negotiated when establishing the TCP connection.
► Selective acknowledgement: The fast recover algorithm, defined in RFC
2581, tends to be less efficient when attempting to recover from multiple
losses in a single window. This scenario is very likely on wireless
connections, and therefore, an alternative to the fast recover algorithm was
implemented. Selective acknowledgement (SACK) was defined in RFC 2018
Chapter 18. Wireless Application Protocol 677