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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 606
SEC. 8.3
DISTRIBUTED SYSTEMS
575
in the range 0–255 separated by dots, as in 192.31.231.65.
When a packet arrives
at a router, the router extracts the IP destination address and uses that for routing.
Since IP datagrams are not acknowledged, IP alone is not sufficient for reliable
communication in the Internet.
To provide reliable communication, another proto-
col,
TCP
(
Transmission Control Protocol
), is usually layered on top of IP.
TCP
uses IP to provide connection-oriented streams.
To use TCP, a process first estab-
lishes a connection to a remote process.
The process required is specified by the IP
address of a machine and a port number on that machine, to which processes inter-
ested in receiving incoming connections listen. Once that has been done, it just
pumps bytes into the connection and they are guaranteed to come out the other end
undamaged and in the correct order. The TCP implementation achieves this guar-
antee by using sequence numbers, checksums, and retransmissions of incorrectly
received packets. All of this is transparent to the sending and receiving processes.
They just see reliable interprocess communication, just like a UNIX pipe.
To see how all these protocols interact, consider the simplest case of a very
small message that does not need to be fragmented at any level. The host is on an
Ethernet connected to the Internet.
What happens exactly? The user process gen-
erates the message and makes a system call to send it on a previously established
TCP connection. The kernel protocol stack adds a TCP header and then an IP
header to the front. Then it goes to the Ethernet driver, which adds an Ethernet
header directing the packet to the router on the Ethernet. This router then injects
the packet into the Internet, as depicted in Fig. 8-31.
Internet
Ethernet 1
header
Headers
Router
Message
Host
Ethernet
TCP
Message
IP
Figure 8-31.
Accumulation of packet headers.
To establish a connection with a remote host (or even to send it a datagram), it
is necessary to know its IP address. Since managing lists of 32-bit IP addresses is
inconvenient for people, a scheme called
DNS
(
Domain Name System
) was in-
vented as a database that maps ASCII names for hosts onto their IP addresses.
Thus it is possible to use the DNS name
star.cs.vu.nl
instead of the corresponding
IP address 130.37.24.6.
DNS names are commonly known because Internet email
Page 607
576
MULTIPLE PROCESSOR SYSTEMS
CHAP. 8
addresses are of the form
user-name@DNS-host-name
.
This naming system al-
lows the mail program on the sending host to look up the destination host’s IP ad-
dress in the DNS database, establish a TCP connection to the mail daemon process
there, and send the message as a file. The
user-name
is sent along to identify which
mailbox to put the message in.
8.3.3 Document-Based Middleware
Now that we have some background on networks and protocols, we can start
looking at different middleware layers that can overlay the basic network to pro-
duce a consistent paradigm for applications and users.
We will start with a simple
but well-known example: the World Wide Web.
The Web was invented by Tim
Berners-Lee at CERN, the European Nuclear Physics Research Center, in 1989 and
since then has spread like wildfire all over the world.
The original paradigm behind the Web was quite simple: every computer can
hold one or more documents, called
Web pages
.
Each Web page contains text,
images, icons, sounds, movies, and the like, as well as
hyperlinks
(pointers) to
other Web pages. When a user requests a Web page using a program called a
Web
browser
, the page is displayed on the screen. Clicking on a link causes the current
page to be replaced on the screen by the page pointed to. Although many bells and
whistles have recently been grafted onto the Web, the underlying paradigm is still
clearly present: the Web is a great big directed graph of documents that can point
to other documents, as shown in Fig. 8-32.
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Figure 8-32.
The Web is a big directed graph of documents.
Each Web page has a unique address, called a
URL
(
Uniform Resource Loca-
tor
), of the form
protocol://DNS-name/file-name
.
The protocol is most commonly
http
(HyperText Transfer Protocol), but
ftp
and others also exist. Then comes the
DNS name of the host containing the file. Finally, there is a local file name telling
which file is needed.
Thus a URL uniquely specifies a single file worldwide
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