分布式系统第3版第1章习题解答：硬件与资源共享实例

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在《分布式系统》第三版的第1章中，讨论了分布式系统中的资源共享和共享实例。本章的练习题1.1要求列举五种硬件资源和五种数据或软件资源，并给出它们在分布式系统中的实际应用场景。 1. **硬件资源** - **中央处理器 (CPU)**: 在分布式系统中，CPU被用作计算服务器，执行客户端的处理器密集型应用程序。例如，服务器可以为多个远程用户提供并发服务。 - **远程对象服务器**：这种服务器执行客户端请求的方法，将处理任务分发到不同的机器上，提高系统的响应速度和效率。 - **蠕虫程序（Worm Program）**：蠕虫程序可以与桌面计算机共享CPU资源，如病毒或恶意软件，影响系统的性能。 - **内存缓存服务器**：它存储最近访问的网页，利用快速的RAM提升其他本地计算机访问网络资源的速度。 - **磁盘资源**：如文件服务器提供共享文件存储，虚拟硬盘服务器（如在第8章中提及）支持虚拟化存储，视频点播服务器（如在第15章中涉及）则用于流媒体内容。 2. **数据或软件资源** - **内存**：除了CPU，其他服务器可能也需要内存来执行操作，如缓存服务器通过共享RAM资源来加速数据访问。 - **磁盘存储**：包括共享文件系统，允许多台计算机访问和协作编辑同一份文件。 - **屏幕共享**：网络窗口系统如X-11使得远程计算机上的窗口更新内容能在本地屏幕上实时显示。 - **打印机**：网络打印机接受来自多个计算机的任务，通过排队系统管理打印作业，实现资源共享。 - **网络带宽和容量**：包转发、数据传输等操作涉及整个网络的共享，确保多设备间的高效通信。这些资源在分布式环境中至关重要，它们通过网络连接协调工作，实现了系统的灵活性、扩展性和可靠性。理解这些共享模式有助于设计和优化分布式系统架构，以最大化资源利用率和用户体验。

Distributed Systems, Edition 3: Chapter 3 Solutions 3

Last updated: 21 February 2001 3:29 pm

©George Coulouris, Jean Dollimore and Tim Kindberg 2000

v) remote procedure call.

3.7 Ans.

i) The long duration of sessions, the need for reliability and the unstructured sequences of characters

transmitted make connection-oriented communication most suitable for this application. Performance is

not critical in this application, so the overheads are of little consequence.

ii) File calls for the transmission of large volumes of data. Connectionless would be ok if error rates are low

and the messages can be large, but in the Internet, these requirements aren’t met, so TCP is used.

iii)Connectionless is preferable, since messages are short, and a single message is sufficient for each

transaction.

iv)Either mode could be used. The volume of data transferred on each transaction can be quite large, so

TCP is used in practice.

v) RPC achieves reliability by means of timeouts and re-trys. so connectionless (UDP) communication is

often preferred.

3.8 Explain how it is possible for a sequence of packets transmitted through a wide area network to

arrive at their destination in an order that differs from that in which they were sent. Why can’t this

happen in a local network? Can it happen in an ATM network?

3.8 Ans.

Packets transmitted through a store-and-forward network travels by a route that is determined dynamically for

each packet. Some routes will have more hops or slower switches than others. Thus packets may overtake each

other. Connection-oriented protocols such as TCP overcome this by adding sequence numbers to the packets

and re-ordering them at the receiving host.

It can’t happen in local networks because the medium provides only a single channel connecting all of

the hosts on the network. Packets are therefore transmitted and received in strict sequence.

It can’t happen in ATM networks because they are connection-oriented. Transmission is always through

virtual channels, and VCs guarantee to deliver data in the order in which it is transmitted.

3.9 A specific problem that must be solved in remote terminal access protocols such as Telnet is the

need to transmit exceptional events such as ‘kill signals’ from the ‘terminal’ to the host in advance

of previously-transmitted data. Kill signals should reach their destination ahead of any other

ongoing transmissions. Discuss the solution of this problem with connection-oriented and

connectionless protocols.

3.9 Ans.

The problem is that a kill signal should reach the receiving process quickly even when there is buffer overflow

(e.g. caused by an infinite loop in the sender) or other exceptional conditions at the receiving host.

With a connection-oriented, reliable protocol such as TCP, all packets must be received and

acknowledged by the sender, in the order in which they are transmitted. Thus a kill signal cannot overtake other

data already in the stream. To overcome this, an out-of-band signalling mechanism must be provided. In TCP

this is called the URGENT mechanism. Packets containing data that is flagged as URGENT bypass the flow-

control mechanisms at the receiver and are read immediately.

With connectionless protocols, the process at the sender simply recognizes the event and sends a

message containing a kill signal in the next outgoing packet. The message must be resent until the receiving

process acknowledges it.

3.10 What are the disadvantages of using network-level broadcasting to locate resources:

i) in a single Ethernet?

ii) in an intranet?

To what extent is Ethernet multicast an improvement on broadcasting?

3.10 Ans.

i. All broadcast messages in the Ethernet must be handled by the OS, or by a standard daemon process. The

overheads of examining the message, parsing it and deciding whether it need be acted upon are incurred by

every host on the network, whereas only a small number are likely locations for a given resource. Despite this,

Distributed Systems, Edition 3: Chapter 3 Solutions 4

Last updated: 21 February 2001 3:29 pm

©George Coulouris, Jean Dollimore and Tim Kindberg 2000

note that the Internet ARP does rely on Ethernet braodcasting. The trick is that it doesn’t do it very often - just

once for each host to locate other hosts on the local net that it needs to communicate with.

ii. Broadcasting is hardly feasible in a large-scale network such as the Internet. It might just be possible

in an intranet, but ought to be avoided for the reasons given above.

Ethernet multicast addresses are matched in the Ethernet controller. Multicast message are passed up to

the OS only for addresses that match multicast groups the local host is subscribing to. If there are several such,

the address can be used to discriminate between several daemon processes to choose one to handle each

message.

3.11 Suggest a scheme that improves on MobileIP for providing access to a web server on a mobile

device which is sometimes connected to the Internet by mobile phone and at other times has a

wired connection to the Internet at one of several locations.

3.11 Ans.

The idea is to exploit the cellular phone system to locate the mobile device and to give the IP address of its

current location to the client.

3.12 Show the sequence of changes to the routing tables in Figure 3.8 that would occur (according to

the RIP algorithm given in Figure 3.9) after the link labelled 3 in Figure 3.7 is broken.

3.12 Ans.

Routing tables with changes shown in red (grey in monochrome printouts):

Step 1: costs for routes that use Link 3 have been set to ∞ at A, D

Routings from A Routings from B Routings from C

To Link Cost To Link Cost To Link Cost

local

∞

local

Routings from D Routings from E

Link Cost

local

∞

local

Step 2: after first exchange of routing tables

Routings from A Routings from B Routings from C

To Link Cost To Link Cost To Link Cost

local

∞

local

∞

local

Routings from D Routings from E

Link Cost

local

∞

local

Distributed Systems, Edition 3: Chapter 3 Solutions 6

Last updated: 21 February 2001 3:29 pm

©George Coulouris, Jean Dollimore and Tim Kindberg 2000

3.14 Ans.

The basic TCP buffering algorithm described on p. 105 is not very efficient for interactive input. Nagle’s

algorithm is designed to address this. It requires the sending machine to send any bytes found in the output

buffer, then wait for an acknowledgement. Whenever an acknowledgement is received, any additional

characters in the buffer are sent. The effects of this are:

a) For a web server: the server will normally write a whole page of HTML into the buffer in a single

write

When the

write

is completed, Nagle’s algorithm will send the data immediately, whereas the basic

algorithm would wait 0.5 seconds. While the Nagle’s algorithm is waiting for an acknowledgement, the

server process can write additional data (e.g. image files) into the buffer. They will be sent as soon as

the acknowledgement is received.

b) For a remote shell (Telnet) application: the application will write individual key strokes into the buffer

(and in the normal case of full duplex terminal interaction they are echoed by the remote host to the

Telnet client for display). With the basic algorithm, full duplex operation would result in a delay of 0.5

seconds before any of the characters typed are displayed on the screen. With Nagle’s algorithm, the first

character typed is sent immediately and the remote host echoes it with an acknowledgement piggy-

backed in the same packet. The acknowledgement triggers the sending of any further characters that

have been typed in the intervening period. So if the remote host responds sufficiently rapidly, the display

of typed characters appears to be instantaneous. But note that a badly-written remote application that

reads data from the TCP buffer one character at a time can still cause problems - each read will result in

an acknowledgement indicating that one further character should be sent - resulting in the transmission

of an entire IP frame for each character. Clarke [1982] called this the

silly window syndrome

. His

solution is to defer the sending of acknowledgements until there is a substantial amount of free space

available.

c) For a continuous mouse input (e.g. sending mouse positions to an X-Windows application running on a

compute server): this is a difficult form of input to handle remotely. The problem is that the user should

see a smooth feedbvack of the path traced by the mouse, with minimal lag. Neither the basic TCP

algorithm nor Nagle’s nor Clarke’s algorithm achieves this very well. A version of the basic algorithm

with a short timeout (0.1 seconds) is the best that can be done, and this is effective when the network is

lightly loaded and has low end-to-end latency - conditions that can be guaranteed only on local networks

with controlled loads.

See Tanenbaum [1996] pp. 534-5 for further discussion of this.

3.15 Construct a network diagram similar to Figure 3.10 for the local network at your institution or

company.

3.15 Ans.

Left to the reader.

3.16 Describe how you would configure a firewall to protect the local network at your institution or

company. What incoming and outgoing requests should it intercept?

3.16 Ans.

Left to the reader.

3.17 How does a newly-installed personal computer connected to an Ethernet discover the IP addresses

of local servers? How does it translate them to Ethernet addresses?

3.17 Ans.

The first part of the question is a little misleading. Neither Ethernet nor the Internet support ‘discovery’

services as such. A newly-installed computer must be configured with the domain names of any servers that it

needs to access. The only exception is the DNS. Services such as BootP and DHCP enable a newly-connected

host to acquire its own IP address and to obtain the IP addresses of one ore more local DNS servers. To obtain

the IP addresses of other servers (e.g. SMTP, NFS, etc.) it must use their domain names. In Unix, the

nslookup

command can be used to examine the database of domain names in the local DNS servers and a user can select

approriate ones for use as servers.The domain names are translated to IP addresses by a simple DNS request.

Distributed Systems, Edition 3: Chapter 3 Solutions 7

Last updated: 21 February 2001 3:29 pm

The Address Resolution Protocol (ARP) provides the answer to the second part of the question. This is

described on pages 95-6. Each network type must implement ARP in its own way. The Ethernet and related

networks use the combination of broadcasting and caching of the results of previous queries described on page

96.

3.18 Can firewalls prevent denial of service attacks such as the one described on page 96? What other

methods are available to deal with such attacks?

3.18 Ans.

Since a firewall is simply another computer system placed in front of some intranet services that require

protection, it is unlikely to be able to prevent denial of service (DoS) attacks for two reasons:

• The attacking traffic is likely to closely resemble real service requests or responses.

• Even if they can be recognized as malicious (and they could be in the case described on p. 96), a

successful attack is likely to produce malicious messages in such large quantities that the firewall itself

is likely to be overwhelemed and become a bottleneck, preventing communication with the services that

it protects.

Other methods to deal with DoS attacks: no comprehensive defence has yet been developed. Attacks of the

type described on p. 96, which are dependent on IP spoofing (giving a false ‘senders address’) can be prevented

at their source by checking the senders address on all outgoing IP packets. This assumes that all Internet sites

are managed in such a manner as to ensure that this check is made - an unlikely circumstance. It is difficult to

see how the targets of such attacks (which are usually heavily-used public services) can defend themselves

with current network protocols and their security mechanisms. With the advent of quality-of-service

mechanisms in IPv6, the situation should improve. It should be possible for a service to allocate only a limited

amount of its total bandwidth to each range of IP addresses, and routers thorughout the Internet could be setup

to enforce these resource allocations. However, this approach has not yet been fully worked out.

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