This section gives more details on how to configure a network interface for μC/TCP-IP.
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Buffers' Management
This section describe how µC/TCP-IP uses buffers to receive and transmit application data and network protocol control information. You should understand how network buffers are used by µC/TCP-IP to correctly configure your interface(s).
Network Buffers
µC/TCP-IP stores transmitted and received data in data structures known as Network Buffers. Each Network Buffer consists of two parts: the Network Buffer header and the Network Buffer Data Area pointer. Network Buffer headers contain information about the data pointed to via the data area pointer. Data to be received or transmitted is stored in the Network Buffer Data Area.
µC/TCP-IP is designed with the inherent constraints of an embedded system in mind, the most important being the restricted RAM space. µC/TCP-IP defines network buffers for the Maximum Transmission Unit (MTU) of the Data Link technology used, which is most of the time Ethernet. Default Ethernet’s maximum transmit unit (MTU) size is 1500 bytes.
Receive Buffers
Network Buffers used for reception for a Data Link technology are buffers that can hold one maximum frame size. Because it is impossible to predict how much data will be received, only large buffers can be configured. Even if the packet does not contain any payload, a large buffer must be used, as worst case must always be assumed.
Transmit Buffers
On transmission, the number of bytes to transmit is always known, so it is possible to use a Network Buffer size smaller than the maximum frame size. µC/TCP-IP allows you to reduce the RAM usage of the system by defining small buffers. When the application does not require a full size frame to transmit, it is possible to use smaller Network Buffers. Depending on the configuration, up to eight pools of Network Buffer related objects may be created per network interface. Only four pools are shown below and the remaining pools are used for maintaining Network Buffer usage statistics for each of the pools shown.
In transmission, the situation is different. The TCP/IP stack knows how much data is being transmitted. In addition to RAM being limited in embedded systems, another feature is the small amount of data that needs to be transmitted. For example, in the case of sensor data to be transmitted periodically, a few hundred bytes every second can be transferred. In this case, a small buffer can be used and save RAM instead of waste a large transmit buffer. Another example is the transmission of TCP acknowledgment packets, especially when they are not carrying any data back to the transmitter. These packets are also small and do not require a large transmit buffer. RAM is also saved.
Network Buffer Architecture
µC/TCP-IP uses both small and large network buffers:
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