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2019 年 TCP/IP 习题答案和复习课提要

403 工作室 @铮哥牛逼

Key point

试卷题型：填空题（12 空）、判断题 5 题、简答题 5 题、综合题 3 题（其中一个是程序注释题）

有几个知识点提醒大家掌握：

1. socket 实现 (掌握会调用哪些函数分别创建以下哪个结构：file、socket、inpcb、tcpcb 等)（课堂练习）

falloc（15.6）、socreate、in_pcballoc(22.1)、tcp_newtcpcb（25.8）

2. udp_input 和 udp_output 函数

3. 满足哪些条件才会发送 ICMP 重定向报文

首先，只有在同一接口( r t_ i f p 和 r c v i f)上接收或重发分组时，才能应用重定向。

其次，被选择的路由本身必须没有被 I C M P 重定向报文创建或修改过 (R T F _ D Y N A M I C | R T F _ M O

D I F I E D)

而且该路由也不能是到默认目的地的( 0 . 0 . 0 . 0 )

全局整数 i p s e n d r e d i r e c t s 指定系统是否被授权发送重定向.

当传给 i p _ f o r w a r d 的参数 s r c r t 指明系统是对分组路由选择的源时，禁止系统重定向

First, redirects are applicable only when a packet is received and resent on the same interface (rt_ifp

and rcvif).

Next, the selected route must not have been itself created or modified by an ICMP redirect message

(RTF_DYNAMIC | RTF_MODIFIED).

Third, the route cannot be to the default destination (0.0.0.0).

Forth, system need to have administrative authority to send redirects, which means the global integer

ipsendredirects is 1.

Five, redirects are not suppressed when the system is source routing a packet as indicated by the srcrt

argument passed to ip_forward, which means the srcrt argument is False.

4. 理解图 24.17 和图 24.18,理解 TCP 的流量控制机制和收发窗口如何滑动，理解每个序号变量的含义

5. 在 IP 处理过程中可能会发生哪些差错，并发送哪些 icmp 差错报文（作业中有,需要查看 ch8 ch10 中的代

码）

1、If ip_ttl has reached 1 (IPTTLDEC), an ICMP time exceeded message is returned to the sender and

the packet is discarded. Otherwise, ip_forward decrements ip_ttl by IPTTLDEC.

2、If no route can be found for the destination, an ICMP host unreachable error is returned and the

packed discarded.

3、If fragmentation is prohibited by the DF bit, ip_output discards the packet and returns

EMSGSIZE. If the datagram was generated on this host, a transport protocol passes the error back to the

process, but if the datagram is being forwarded, ip_forward generates an ICMP destination unreachable

error with an indication that the packet could not be forwarded without fragmentation

P221

4、an ICMP time exceeded error be sent to the source host if the timer expires and the first

fragment of the datagram has been received. P232

2019 年 TCP/IP 习题答案和复习课提要

403 工作室 @铮哥牛逼

Chapter 1. Introduction

6. Which communication protocol families do 4.4BSD support?

1、TCP/IP

2、XNS (Xerox Network Systems)

3、The OSI protocols

4、The Unix domain protocols.

7. Two popular application programming interfaces (APIs) for writing programs to use the Internet protocols are socket

and TLI (Transport Layer Interface).

8. The networking code in the 4.4BSD kernel is organized into three layers: socket layer, protocol layer, interface

layer. The interface layer contains the device drivers that communicate with the network devices.

9. Data structures that are created by the kernel when the process calls socket are: filedesc, socket, vnode.

10. The return value from socket is a descriptor. The socket system calls start with a descriptor. Please describe how a

descriptor leads to a socket structure.

When the process executes a system call,

the kernel starts with the descriptor value and

uses “fd_ofiles” to index into the vector of “file”

structure pointers, ending up with the “file”

structure for the descriptor. The “file”

structure points to “socket” structure.

11. When a UDP datagram(数据包) arrives on a

network interface, how does the kernel find the

corresponding socket structure?

The kernel searches through all the UDP

protocol control blocks to find the appropriate

one, minimally based on the destination UDP

port number and perhaps the destination IP

address, source IP address, and source port

numbers too. Once the inpcb structure is

located, the kernel finds the corresponding

socket structure through the inp_socket

pointer

12. In the call to sendto, the fifth argument points to

an Internet socket address structure (named serv) and the sixth argument specifies its length (which is 16 bytes). One

of the first things done by the socket layer for this system call is to verify that these arguments are valid (i.e., the

pointer points to a piece of memory in the address space of the process) and then copy the socket address structure

into an mbuf. What is the m_type member of the resulting mbuf ? or What is the m_type member of the mbuf

containing socket address structure?

The “m_type” member specifies the type of data contained in the mbuf, which for this example is

MT_SONAME, name of the socket.

13. What is the protocol layer corresponding to a UDP socket descriptor?

剩余12页未读，继续阅读

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