发布时间 : 星期四 文章Computer network questions更新完毕开始阅读f56bca0102768e9951e738dd
21. A router can process 2 million packets/sec. The load offered to it is 1.5 million packets/sec on
average. If a route from source to destination contains 10 routers, how much time is spent being queued and serviced by the router?
22. Suppose that host A is connected to a router R 1, R 1 is connected to another router, R 2, and
R 2 is connected to host B . Suppose that a TCP message that contains 900 bytes of data and 20 bytes of TCP header is passed to the IP code at host A for delivery to B. Show the Total length, Identification, DF, MF, and Fragment offset fields of the IP header in each packet
transmitted over the three links. Assume that link A-R1 can support a maximum frame size of 1024 bytes including a 14-byte frame header, link R1-R2 can support a maximum frame size of 512 bytes, including an 8-byte frame header, and link R2-B can support a maximum frame size of 512 bytes including a 12-byte frame header.
答：最初的IP数据包将分成两个IP数据包。不会产生其他碎片。 Link A-R1:
Length = 940; ID = x; DF = 0; MF = 0; Offset = 0 Link R1-R2: （1) Length = 500; ID = x; DF = 0; MF = 1; Offset = 0 (2) Length = 460; ID = x; DF = 0; MF = 0; Offset = 60 Link R2-B:
(1) Length = 500; ID = x; DF = 0; MF = 1; Offset = 0 (2) Length = 460; ID = x; DF = 0; MF = 0; Offset = 60
23. A network on the Internet has a subnet mask of 255.255.240.0. What is the maximum
number of hosts it can handle? 答：对于一个B 类网络，高端16 位形成网络号，低端16 位是子网或主机域。在子网掩码的低端16 位中，最高有效4 位为1111，因此剩下12 位用于主机号。因此，存在4096 个主机地址。但由于全0 和全1 是特别地址，因此最大的主机数目为4094。
24. A router has just received the following new IP addresses: 57.6.96.0/21, 57.6.104.0/21,
57.6.112.0/21, and 57.6.120.0/21. If all of them use the same outgoing line, can they be aggregated? If so, to what? If not, why not?
25. A router has the following (CIDR) entries in its routing table:
Address/mask Next hop 135.46.56.0/22 Interface 0 135.46.60.0/22 Interface 1
192.53.40.0/23 Router 1 default Router 2
For each of the following IP addresses, what does the router do if a packet with that address arrives?
(a) 135.46.63.10 (b) 135.46.57.14 (c) 135.46.52.2 (d) 192.53.40.7 (e) 192.53.56.7
答：包按照以下路径被路由 (a) Interface 1 (b) Interface 0 (c) Router 2 (d) Router 1 (e) Router 2
26. The functions of ARP and how does it work?
基本功能：根据目的主机的IP地址，获得其MAC地址。这就是ARP协议要做的事情。所谓地址解析（address resolution）就是主机在发送帧前将目标IP地址转换成目标MAC地址的过程。 当发送主机和目的主机不在同一个局域网中时，即便知道目的主机的MAC地址，两者也不能直接通信，必须经过路由转发才可以。所以此时，发送主机通过ARP协议获得的将不是目的主机的真实MAC地址，而是一台可以通往局域网外的路由器的某个端口的MAC地址。于是此后发送主机发往目的主机的所有帧，都将发往该路由器，通过它向外发送。这种情况称为ARP代理（ARP Proxy）。
工作原理：在每台安装有TCP/IP协议的电脑里都有一个ARP缓存表，表里的IP地址与MAC地址是一一对应的。
27. Compare the differences between Distance Vector Routing and Link State Routing.
IP路由选择协议一般有三种。1。距离矢量协议。2。链路状态协议。3.两者混合。距离向量协议是基于距离矢量算法的,通过判断路径查找到最佳路由。代表协议有RIP，IGRP等。链路状态协议代表协议则是OSPF等。1.距离矢量协议支持自动汇总，数据链路则需要手动汇总。2.距离矢量会聚慢，数据链路会聚快。3.路由更新前者是路由表，后者事件触发。还有用跳数来度量。15跳最大。后者不限用带宽等度量。距离矢量最大的缺点是会产生路由环路。可以用跳数，水平分割等避免。
28. Primitives of transport service assume asymmetry between the two end points during
connection establishment, one end (server) executes LISTEN while the other end (client) executes CONNECT. However, in peer to peer applications such file sharing systems, e.g. BitTorrent, all end points are peers. There is no server or client functionality. How can transport service primitives be used to build such peer to peer applications?
29. Imagine that a two-way handshake rather than a three-way handshake were used to set up
connections. In other words, the third message was not required. Are deadlocks now
possible? Give an example or show that none exist.
答：死锁是可能的。例如，一个数据包到达了蓝色的，和A承认它。确认丢失，但是A现在是开放的，而B什么都不懂发生了什么事。现在，同样的事情发生在B，无一不是开放的，但期待不同的序列号。超时必须被引入，以避免死锁。
30. Discuss the advantages of sliding window protocols.
解答：滑动窗口协议比较简单，仅需要管理窗口边缘一组参数，而且，对于到达顺序 有错的TPDU不会引起窗口增加和减少方面的问题。
31. Discuss Additive Increase Multiplicative Decrease (AIMD) policies in terms of convergence
and stability.
32. Both UDP and TCP use port numbers to identify the destination entity when delivering a
message. Give two reasons why these protocols invented a new abstract ID (port numbers), instead of using process IDs, which already existed when these protocols were designed. 答：这里有三个原因。首先，进程ID是特定于操作系统的。使用进程ID会作出这些协议依赖于操作系统的。其次，单个进程可以建立通信的多个通道。（每个处理）一个单一的进程ID作为目的地标识符不能用于这些信道之间进行区分。第三，有流程监听众所周知的端口是容易的，但众所周知的进程ID是不可能的。
33. In TCP header, we saw that in addition to the 32-bit acknowledgement field, there is an ACK
bit in the fourth word. Does this really add anything? Why or why not?
34. Discuss the processes of TCP connection and release
TCP协议的主要特点
TCP是一种面向连接的、可靠的传输层协议；
TCP协议建立在不可靠的网络层IP协议之上，IP不能提供任何可靠性机制，TCP的可靠性完全由自己实现；
TCP采用的最基本的可靠性技术是： 确认与超时重传 流量控制
35. DNS uses UDP instead of TCP. If a DNS packet is lost, there is no automatic recovery. Does this
cause a problem, and if so, how is it solved?
答：是的。当DNS的数据包丢失，或者是DNS的服务器无法回复的时候，这就会对用户产生不便。DNS通常会显示检验器的IP地址，否则用户很难重新建立相应的连接。 两个解决办法可以把影响降到最小：第一，建议使用多台DNS服务器，这种情况下，如果最先的DNS的服务器失效，备份的DNS服务器就会用来处理数据；第二，直接通过IP地址定位，而不是通过主服务器转换。
36. Describe the progress of DNS iterative query.
37. A host is in a local network, it wants to open a web page in Internet. Describe what protocols
it will use to accomplish this task. (The more detail the better).