Linux内核2.4网络代码结构解析

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"A Map of the Networking Code in Linux Kernel 2.4" 是一份技术报告，详细描绘了Linux内核2.4版本中的网络代码结构。尽管这份文档基于较旧的内核版本，但其提供的信息对于理解Linux网络协议栈的基础架构仍然具有一定的参考价值。在Linux内核中，网络协议栈是核心组件之一，它负责处理网络通信的所有方面，包括数据包的接收、处理和发送。这份报告可能涵盖了以下关键知识点： 1. **网络层**：在Linux内核中，网络层主要处理IP协议。它包括IP包的路由选择、分片和重组等操作。`ip`子系统是这一层的核心，包含如`ip_input`（处理接收到的IP包）和`ip_output`（发送IP包）等函数。 2. **传输层**：传输层主要负责端到端的数据传输，包括TCP（传输控制协议）和UDP（用户数据报协议）。`tcp`和`udp`子系统分别处理这两个协议，提供连接管理、流量控制、错误检测和纠正等功能。 3. **链路层**：这一层处理物理网络接口，如以太网、令牌环等。`dev`子系统是链路层的关键，它管理网络设备，如注册新设备、发送和接收原始数据包。 4. **套接字层**：套接字层提供了应用程序与网络协议栈之间的接口。它定义了一种标准的编程接口，允许应用通过创建、绑定、监听、连接和发送/接收数据来使用网络服务。 5. **协议注册和驱动程序**：在Linux内核中，每种网络协议和硬件驱动都需要注册到相应的子系统。这使得内核能够识别和处理不同的网络协议和硬件设备。 6. **中断处理和底半部**：网络数据包的处理通常涉及中断，这些中断在内核的中断处理程序中被处理。为了避免中断处理程序阻塞其他重要的系统任务，部分处理工作会被推迟到底半部（如软中断或工作队列）。 7. **网络配置和调试工具**：报告可能还涵盖了用于配置网络设备和查看网络状态的命令，如`ifconfig`、`route`和`tcpdump`等。 8. **性能优化**：在2.4内核中，可能会讨论到如中断合并、队列管理策略（如RED或FQ-CoDel）以及TCP窗口缩放等优化技术，以提高网络性能。由于这份报告是基于2.4内核，读者需要注意其中的一些机制可能在新的内核版本中有所变化或已被更新。尽管如此，理解旧版内核的网络架构对于深入学习和调试现代Linux内核的网络功能仍然是有益的。

3 General Data Structures

The networking part of the kernel uses mainly two data structures: one to keep the state of a

connection, called sock (for “socket”), and another to keep the data and status of both incoming

and outgoing packets, called sk_buff (for “socket buffer”). Both of them are described in this

section. We also include a brief description of tcp_opt, a structure that is part of the sock structure

and is used to maintain the TCP connection state. The details of TCP will be presented in

section 6.

3.1 Socket buffers

The sk_buff data structure is defined in include/linux/skbuff.h.

When a packet is processed by the kernel, coming either from user space or from the network

card, one of these data structures is created. Changing a field in a packet is achieved by updating

a field of this data structure. In the networking code, virtually every function is invoked with an

sk_buff (the variable is usually called skb) passed as a parameter.

The first two fields are pointers to the next and previous sk_buff’s in the linked list (packets are

frequently stored in linked lists or queues); sk_buff_head points to the head of the list.

The socket that owns the packet is stored in sk (note that if the packet comes from the network,

the socket owner will be known only at a later stage).

The time of arrival is stored in a timestamp called stamp. The dev field stores the device from

which the packet arrived, if the packet is for input. When the device to be used for transmission is

known (for example, by inspection of the routing table), the dev field is updated correspondingly

(see sections 4.1 and 4.3).

struct sk_buff {

/* These two members must be first. */

struct sk_buff *next; /* Next buffer in list */

struct sk_buff *prev; /* Previous buffer in list */

struct sk_buff_head *list; /* List we are on */

struct sock *sk; /* Socket we are owned by */

struct timeval stamp; /* Time we arrived */

struct net_device *dev; /* Device we arrived on/are leaving by */

The transport section is a union that points to the corresponding transport layer structure (TCP,

UDP, ICMP, etc).

/* Transport layer header */

union

{

struct tcphdr *th;

struct udphdr *uh;

struct icmphdr *icmph;

struct igmphdr *igmph;

struct iphdr *ipiph;

struct spxhdr *spxh;

unsigned char *raw;

} h;

The network layer header points to the corresponding data structures (IPv4, IPv6, ARP, raw, etc).

/* Network layer header */

union

{

struct iphdr *iph;

struct ipv6hdr *ipv6h;

struct arphdr *arph;

struct ipxhdr *ipxh;

unsigned char *raw;

} nh;

The link layer is stored in a union called mac. Only a special case for Ethernet is included. Other

technologies will use the raw fields with appropriate casts.

/* Link layer header */

union

{

struct ethhdr *ethernet;

unsigned char *raw;

} mac;

struct dst_entry *dst;

Extra information about the packet such as length, data length, checksum, packet type, etc. is

stored in the structure as shown below.

char cb[48];

unsigned int len; /* Length of actual data */

unsigned int data_len;

unsigned int csum; /* Checksum */

unsigned char __unused, /* Dead field, may be reused */

cloned, /* head may be cloned (check refcnt

to be sure) */

pkt_type, /* Packet class */

ip_summed; /* Driver fed us an IP checksum */

__u32 priority; /* Packet queueing priority */

atomic_tusers; /* User count - see datagram.c,tcp.c */

unsigned short protocol; /* Packet protocol from driver */

unsigned short security; /* Security level of packet */

unsigned int truesize; /* Buffer size */

unsigned char *head; /* Head of buffer */

unsigned char *data; /* Data head pointer*/

unsigned char *tail; /* Tail pointer */

unsigned char *end; /* End pointer */

3.2 sock

The sock data structure keeps data about a specific TCP connection (e.g., TCP state) or virtual

UDP connection. Whenever a socket is created in user space, a sock structure is allocated.

The first fields contain the source and destination addresses and ports of the socket pair.

struct sock {

/* Socket demultiplex comparisons on incoming packets. */

__u32 daddr; /* Foreign IPv4 address */

__u32 rcv_saddr; /* Bound local IPv4 address */

__u16 dport; /* Destination port */

unsigned short num; /* Local port */

int bound_dev_if; /* Bound device index if != 0 */

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