LWIP

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. 

Basic Concepts

 From the application's point of view, data handling in the BSD socket API is done in continuous memory regions. This is convenient for the application programmer since manipulation of data in application programs is usually done in such continuous memory chunks. Using this type of mechanism with lwIP would not be advantageous, since lwIP usually handles data in buffers where the data is partitioned into smaller chunks of memory. Thus the data would have to be  copied into a continuous memory area before being passed to the application. This would waste both processing time and memory since. 

UDP Processing

                     

UDP is a simple protocol used for demultiplexing packets between different processes. The state for each UDP session is kept in a PCB structure . The UDP PCBs are kept on a linked list which is searched for a match when a UDP datagram arrives. The UDP PCB structure contains a pointer to the next PCB in the global linked list of UDP PCBs. A UDP session is defined by the IP addresses and port numbers of the end-points and these are stored in the local ip, dest ip, local port and dest port fields. 

 Abstract

LWIP is an implementation of the TCP/IP protocol stack.. Interest for connecting small devices to existing network infrastructure such as global internet is steadily increasing.  

Queuing And Transmitting Data

Data that is to be sent is divided into appropriate sized chunks and given sequence numbers by the tcp enqueue() function. Here, the data is packeted into pbufs and enclosed in a tcp seg struct The TCP header is build in the pbuf, and filled in with all fields except the acknowledgment number, ackno, and the advertised window, wnd. These fields can change during the queuing time of the segment and are therefore set by tcp output() which does the actual transmission of the segment. After the segments are built, they are queued on the unsent list in the PCB. 

 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.