"lwIP手册-英文原版1：lwIP TCP/IP堆栈设计与实现" - CSDN文库

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"lwIP手册-英文原版1"是关于TCP/IP协议栈lwIP实现的文档。lwIP协议栈的主要目标是减少内存使用和代码大小，使得lwIP适用于具有非常有限资源（如嵌入式系统）的小型客户端。为了降低处理和内存需求，lwIP使用了一个定制的API，不需要进行任何数据复制。这个报告描述了lwIP协议栈的设计和实现。报告首先介绍了lwIP的背景和目标。lwIP的目标是在资源受限的环境中提供可靠的TCP/IP通信。它通过减少内存使用和代码大小来实现这一目标。报告还介绍了lwIP的特性，包括基于事件驱动的系统架构、轻量级的线程管理和内存管理机制等。报告接着详细讨论了lwIP协议栈的架构和功能。lwIP采用了分层的设计，将协议栈划分为不同的功能层。每个层都有特定的功能和接口，使得协议栈模块化和易于扩展。报告还描述了lwIP中各个层的功能和实现细节，包括网络接口层、网络协议层、传输层和应用层。报告还介绍了lwIP特有的特性和优化技术。lwIP使用了一些特殊的技术来减少内存使用和代码大小。其中之一是使用非阻塞式IO操作，这可以减少内存拷贝的次数。另一个特点是lwIP使用了精简的数据结构和算法，以最小化内存占用和处理需求。报告还介绍了lwIP的内存管理机制，包括内存池的设计和实现。报告还讨论了lwIP的性能评估和测试。通过一系列的实验和基准测试，报告评估了lwIP在不同环境下的性能和资源消耗。结果显示，lwIP在资源受限的环境中表现出色，具有较低的内存使用和处理需求。最后，报告总结了lwIP的特点和优势，并提出了一些未来的研究方向。lwIP的主要特点是其适应性和高效性，使其成为嵌入式系统和其他资源受限环境中的理想选择。未来的研究方向包括进一步优化内存使用和处理需求，以及支持更多的网络协议和应用层协议。总之，"lwIP手册-英文原版1"是一份关于lwIP协议栈设计和实现的详细文档。该文档介绍了lwIP的背景、目标、架构、功能、特性和性能评估。它还总结了lwIP的特点和优势，并提出了未来的研究方向。该文档对于对lwIP感兴趣的研究人员和开发人员具有很高的参考价值。

6 BUFFER AND MEMORY MANAGEMENT

of lw IP that precludes it from later b eing implemented in an operating system kernel.

5 The operating system emulation layer

In order to make lwIP portable, operating system speciﬁc function calls and data structures are not

used directly in the code. Instead, when such functions are needed the operating system emulation

layer is used. The operating system emulation layer provides a uniform interface to operating

system services such as timers, process synchronization, and message passing mechanisms. In

principle, when porting lw IP to other operating systems only an implementation of the operating

system emulation layer for that particular operating system is needed.

The operating system emulation layer provides a timer functionality that is used by TCP. The

timers provided by the operating system emulation layer are one-shot timers with a granularity of

at least 200 ms that calls a registered function when the time-out occurs.

The only process synchronization mechanism provided is semaphores. Even if semaphores are

not avaliable in the underlying operating system they can be emulated by other synchronization

primitives such as conditional variables or locks.

The message passing is done through a simple mechanism which uses an abstraction called

mailboxes. A mailbox has two operations: post and fetch. The post operation will not block the

process; rather, messages posted to a mailbox are queued by the operating system emulation layer

until another process fetches them. Even if the underlying operating system does not have native

support for the mailbox mechanism, they are easily implemented using semaphores.

6 Buﬀer and memory management

The memory and buﬀer management system in a communication system must be prepared to

accommodate buﬀers of very varying sizes, ranging from buﬀers containing full-sized TCP segments

with several hundred bytes worth of data to short ICMP echo replies consisting of only a few bytes.

Also, in order to avoid copying it should be possible to let the data content of the buﬀers reside in

memory that is not managed by the networking subsystem, such as application memory or ROM.

6.1 Packet buﬀers — pbufs

A pbuf is lwIP’s internal representation of a packet, and is designed for the special needs of

the minimal stack. Pbufs are similar to the mbufs used in the BSD implementations. The pbuf

structure has support both for allocating dynamic memory to hold packet contents, and for letting

packet data reside in static memory. Pbufs can be linked together in a list, called a pbuf chain so

that a packet may span over several pbufs.

Pbufs are of three types, PBUF

RAM, PBUF ROM, and PBUF POOL. The pbuf shown in

Figure 1 represents the PBUF

RAM type, and has the packet data stored in memory managed by

the pbuf subsystem. The pbuf in Figure 2 is an example of a chained pbuf, where the ﬁrst pbuf

in the chain is of the PBUF

RAM typ e, and the second is of the PBUF ROM type, which means

that it has the data located in memory not managed by the pbuf system. The third type of pbuf,

PBUF

POOL, is shown in Figure 3 and consists of ﬁxed size pbufs allocated from a pool of ﬁxed

size pbufs. A pbuf chain may consist of multiple types of pbufs.

The three types have diﬀerent uses. Pbufs of type PBUF

POOL are mainly used by network

device drivers since the operation of allocating a single pbuf is fast and is therefore suitable for

use in an interrupt handler. PBUF

ROM pbufs are used when an application sends data that is

located in memory managed by the application. This data may not be modiﬁed after the pbuf

has been handed over to the TCP/IP stack and therefore this pbuf type main use is when the

data is located in ROM (hence the name PBUF

ROM). Headers that are prepended to the data

in a PBUF

ROM pbuf are stored in a PBUF RAM pbuf that is chained to the front of of the

PBUF

ROM pbuf, as in Figure 2.

3

7 NETWORK INTERFACES 6.2 Memory management

next

payload

len

tot_len

flags ref

next

payload

len

tot_len

flags ref

next

payload

len

tot_len

flags ref

Figure 3. A chained PBUF POOL pbuf from the pbuf pool.

are 16 bit unsigned integers and the flags and ref ﬁelds are 4 bit wide. The total size of the

pbuf structure depends on the size of a pointer in the processor architecture being used and on the

smallest alignment possible for the processor architecture. On an architecture with 32 bit pointers

and 4 byte alignment, the total size is 16 bytes and on an architecture with 16 bit pointers and 1

byte alignment, the size is 9 bytes.

Thepbufmoduleprovidesfunctionsformanipulationofpbufs.Allocationofapbufisdone

bythefunctionpbuf_alloc()whichcanallocatepbufsofanyofthethreetypesdescribedabove.

Thefunctionpbuf_ref()increasesthereferencecount.Deallocationismadebythefunction

pbuf_free(),whichﬁrstdecreasesthereferencecountofthepbuf.Ifthereferencecountreaches

zerothepbufisdeallocated.Thefunctionpbuf_realloc()shrinksthepbufsothatitoccupies

justenoughmemorytocoverthesizeofthedata.Thefunctionpbuf_header()adjuststhe

payloadpointerandthelengthﬁeldssothataheadercanbeprependedtothedatainthepbuf.

Thefunctionspbuf_chain()andpbuf_dechain()areusedforchainingpbufs.

6.2 Memory management

The memory manager supporting the pbuf scheme is very simple. It handles allocations and

deallocations of contiguous regions of memory and can shrink the size of a previously allocated

memory block. The memory manager uses a dedicated portion of the total memory in the system.

This ensures that the networking system does not use all of the available memory, and that the

operation of other programs is not disturbed if the networking system has used all of it’s memory.

Internally, the memory manager keeps track of the allocated memory by placing a small struc-

ture on top of each allocated memory block. This structure (Figure 4) holds two pointers to the

next and previous allocation block in memory. It also has a used ﬂag which indicates whether the

allocation block is allocated or not.

Memory is allocated by searching the memory for an unused allocation block that is large

enough for the requested allocation. The ﬁrst-ﬁt principle is used so that the ﬁrst block that is

large enough is used. When an allocation block is deallocated, the used ﬂag is set to zero. In order

to prevent fragmentation, the used ﬂag of the next and previous allocation blocks are checked. If

any of them are unused, the blocks are combined into one larger unused block.

7 Network interfaces

In lwIP device drivers for physical network hardware are represented by a network interface

structure similar to that in BSD. The network interface structure is shown in Figure 5. The

network interfaces are kept on a global linked list, which is linked by the next pointer in the

structure.

Each network interface has a name, stored in the name ﬁeld in Figure 5. This two letter

name identiﬁes the kind of device driver used for the network interface and is only used when the

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