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Introduction
Over
the last few years, the interest for connecting computers and computer supported
devices to wireless networks has steadily increased. Computers are becoming more
and more seamlessly integrated with everyday equipment and prices are dropping.
At the same time wireless networking technologies, such as Bluetooth and IEEE
802.11b WLAN , are emerging. This gives rise to many new fascinating scenarios
in areas such as health care, safety and security, transportation, and processing
industry. Small devices such as sensors can be connected to an existing network
infrastructure such as the global Internet, and monitored from anywhere.
The Internet technology has proven itself flexible enough to incorporate the changing
network environments of the past few decades. While originally developed for low
speed networks such as the ARPANET, the Internet technology today runs over a
large spectrum of link technologies with vastly different characteristics in terms
of bandwidth and bit error rate. It is highly advantageous to use the existing
Internet technology in the wireless networks of tomorrow since a large amount
of applications using the Internet technology have been developed. Also, the large
connectivity of the global Internet is a strong incentive.
Since small devices such as sensors are often required to be physically small
and inexpensive, an implementation of the Internet protocols will have to deal
with having limited computing resources and memory. This report describes the
design and implementation of a small TCP/IP stack called lwIP that is small enough
to be used in minimal systems.
Overview
As in many other TCP/IP implementations, the layered protocol design has served
as a guide for the design of the implementation of lwIP. Each protocol is implemented
as its own module, with a few functions acting as entry points into each protocol.
Even though the protocols are implemented separately, some layer violations are
made, as discussed above, in order to improve performance both in terms of processing
speed and memory usage. For example, when verifying the checksum of an incoming
TCP segment and when demultiplexing a segment, the source and destination IP addresses
of the segment has to be known by the TCP module. Instead of passing these addresses
to TCP by the means of a function call, the TCP module is aware of the structure
of the IP header, and can therefore extract this information by itself.
lwIP consists of several modules. Apart from the modules implementing the TCP/IP
protocols (IP, ICMP, UDP, and TCP) a number of support modules are implemented.
The
support modules consists of :-
" The operating
system emulation layer (described in Chapter3)
"
The buffer and memory management subsystems
(described in Chapter 4) "
Network interface functions (described in Chapter 5) "
Functions for computing Internet checksum (Chapter 6) "
An abstract API (described in Chapter 8 )
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