Principles of Modern Communications
Network Standards
based on 2011 lecture series by Dr. S. Waharte.
Department of Computer Science and Technology,
University of Bedfordshire.
11th January 2013
Outline
Modern
Communications
David Goodwin
University of
Bedfordshire
1 Definitions and concepts
Definitions and
concepts
2 Message ordering and reliability
Message ordering and
reliability
Semantics and syntax
3 Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
4 Encoding application messages
Standards
architectures
5 Vertical communication on the source host
6 Standards architectures
84
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
3
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Last Lecture
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ Lecture 1 introduced core concepts and principles.
4
ˆ One of these was the importance of standards.
ˆ In this lecture, we will look at standards in much more depth.
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
ˆ Lecture 1 discussed
Vertical
communication on the
source host
standards in terms of five
layers of operation.
Standards
architectures
ˆ This will also be an
important focus of Lecture
2.
84
Standards
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
5
ˆ Standards allow different systems to work together.
Message ordering and
reliability
ˆ The terms “standard” and “protocol” mean the same thing.
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Standards
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
6
Message ordering and
reliability
Example
Semantics and syntax
Encoding application
messages
ˆ What is the standard language in this classroom?
Vertical
communication on the
source host
ˆ What would happen if you didn’t have this standard?
Standards
architectures
84
Network Standards
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
7
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Physical, Data Link, and Internet Layers
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
8
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Internet, Transport & Application Layers
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
9
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Layers
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
10
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and 11
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Message ordering and
reliability
Network Standards Concepts
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and 12
reliability
ˆ Network Standards
Semantics and syntax
ˆ Network standards are rules that govern the exchange of
Encoding application
messages
messages between hardware or software processes on different
hosts, including messages (ordering, semantics, and syntax),
reliability, and connection orientation.
ˆ What are ordering, semantics, and syntax?
Vertical
communication on the
source host
Standards
architectures
84
Network Standards Concepts
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Message Order
Definitions and
concepts
Message ordering and 13
reliability
Semantics and syntax
Encoding application
messages
ˆ Turn taking (think of telephone conversations)
ˆ Order of messages in a complex transaction
ˆ Who must initiate communication, and so on
ˆ Reliability
Vertical
communication on the
source host
ˆ A reliable protocol both detects and corrects errors during
Standards
architectures
ˆ Error detection alone is not enough.
ˆ Some unreliable protocols detect errors but then only drop
transmission.
incorrect messages.
84
Network Standards Concepts
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Message Order in HTTP
Definitions and
concepts
Message ordering and 14
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
ˆ In HTTP, the client program initiates the communication by
sending an HTTP request message
Network Standards Concepts
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ Message Order in HTTP
Message ordering and 15
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Network Standards Concepts
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and 16
reliability
Semantics and syntax
Encoding application
messages
ˆ Reflection: Message Order in HTTP
ˆ The client must initiate the interaction.
ˆ Other network standards have more complex turn taking; for
instance TCP.
ˆ Human turn taking is loose and flexible.
ˆ But message order for network standards must be rigid
Vertical
communication on the
source host
because computers are not intelligent.
Standards
architectures
ˆ TCP exemplifies more complex message ordering.
84
A Connection
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and 17
reliability
Semantics and syntax
ˆ HTTP is connectionless.
ˆ Every request-response cycle is independent.
ˆ TCP is connection-oriented.
Encoding application
messages
ˆ There is a formal opening of the connection.
ˆ Within the connection, messages are sequenced,
Vertical
communication on the
source host
acknowledged, and retransmitted if necessary.
Standards
architectures
ˆ There is a formal closing of the connection.
84
A TCP Connection
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ A connection opening requires three segments.
Definitions and
concepts
ˆ SYN segments only have headers with the SYN bit set.
Message ordering and 18
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
A TCP Connection
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ HTTP request and response messages are acknowledged if
Definitions and
concepts
Message ordering and 19
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
received correctly.
A TCP Connection
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Unacknowledged segments are retransmitted.
Definitions and
concepts
Message ordering and 20
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
A TCP Connection
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Four messages close connections.
Definitions and
concepts
ˆ FIN segments are headers with the FIN bit set.
Message ordering and 21
reliability
Semantics and syntax
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Semantics and syntax
Message ordering and
reliability
Semantics and syntax 22
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Network Standards Concepts
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
ˆ Message Semantics
Semantics and syntax 23
Encoding application
messages
Vertical
communication on the
source host
ˆ Semantics = the meaning of a message.
ˆ HTTP request message: “Please give me this file.”
ˆ HTTP response message: Here is the file. (Or, I could not
comply for the following reason. . . )
Standards
architectures
84
Network Standards Concepts
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ Message Semantics
Message ordering and
reliability
Semantics and syntax 24
Encoding application
messages
ˆ Network standards normally have a very limited set of
possible message meanings.
ˆ For example, HTTP requests have only a few possible
meanings.
Vertical
communication on the
source host
ˆ GET: Please give me a file.
ˆ PUT: Store this file (not often used).
ˆ A few more.
Standards
architectures
84
General Message Organization
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Message Syntax: Message Layout
ˆ Like human grammar, but more rigid.
ˆ In general, messages may have three parts.
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 25
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
General Message Organization
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Message Syntax
ˆ The data field contains the content being delivered.
ˆ The header is everything before the data field.
ˆ The trailer is everything after the data field.
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 26
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
General Message Organization
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Message Syntax
ˆ The header is usually broken into smaller sections called
Definitions and
concepts
header fields.
ˆ There often is an address field to indicate where to deliver
Message ordering and
reliability
Semantics and syntax 27
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
the message.
General Message Organization
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Most Messages Do Not Have Trailers.
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 28
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
General Message Organization
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 29
ˆ Field lengths may be measured in bits or bytes.
Encoding application
messages
ˆ Another name for byte is “octet.”
Vertical
communication on the
source host
ˆ The term “octet” is used frequently in networking.
Standards
architectures
84
General Message Organization
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Some Messages Do Not Have Data Fields
Definitions and
concepts
ˆ Example: TCP SYN segments only have headers.
Message ordering and
reliability
Semantics and syntax 30
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
Ethernet Frame
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 31
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ The Ethernet frame has a header, data field, and trailer.
The header has multiple fields.
84
Ethernet Frame
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 32
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ There are source and destination address fields.
ˆ Ethernet addresses are called MAC addresses (We will see
84
why in Chapter 6).
ˆ MAC addresses are 48 bits long. (In contrast, IP addresses
in IP packets are 32 bits long.)
Ethernet Frame
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 33
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ The Ethernet data field contains the packet that the frame
is delivering.
84
Ethernet Frame
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 34
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ The trailer contains the Frame Check Sequence field for
error detection.
ˆ The sender computes the value based on all bits. The
receiver redoes the calculation.
ˆ If the receiver gets a different value, it discards the frame.
There is no error correction.
84
ˆ Ethernet is not reliable.
Internet Protocol (IP) Packet
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 35
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ The IP packet is a long string of bits. It is drawn 32 bits on
each line.
ˆ The first line has the bits 0 through 31. The second line has
bits 32 to 63, and so on.
84
Internet Protocol (IP) Packet
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 36
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ For the Header Checksum field, the sender calculates a
number based on other bits and places the number in the
Header Checksum field.
ˆ The receiver redoes the calculation. If the receiver does not
84
get what the sender computed, then there has been an
error, and the packet is dropped.
Internet Protocol (IP) Packet
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 37
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ There are 32-bit fields for the source IP address and the
destination IP address.
84
Internet Protocol (IP) Packet
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 38
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ The data field often contains a TCP segment.
ˆ It may also contain a UDP datagram.
84
Internet Protocol (IP) Packet
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
ˆ The IP packet always has a header.
Semantics and syntax 39
ˆ The IP packet always has a data field.
Encoding application
messages
ˆ The IP packet NEVER has a trailer.
Vertical
communication on the
source host
ˆ In fact, trailers are only found on Layer 2 frames.
ˆ In fact, many Layer 2 frames do not have trailers.
Standards
architectures
84
TCP Segment
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 40
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ The Sequence Number field gives the TCP segment’s order
in the session.
ˆ The Acknowledgement Number field indicates the segment
that this segment is acknowledging.
ˆ These fields are 32 bits long.
84
TCP Segment
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 41
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ Flag fields are one-bit fields.
ˆ If the value is 1, the flag field is Set.
ˆ If the value is 0, the flag field is Not Set.
ˆ Flag bits are SYN, ACK, FIN, and RST.
84
ˆ TCP has six flag fields.
TCP Segment
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 42
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ The TCP Checksum Field is for error correction.
ˆ The sender computes the value in the field.
ˆ If the receiver computes the same value, it sends an ACK.
ˆ If not, the receiver discards the segment and sends nothing.
84
The sender will resend the segment. TCP is reliable.
TCP Segment
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ TCP segments always have headers.
Message ordering and
reliability
Semantics and syntax 43
ˆ SOME TCP segments do not have data fields.
ˆ Supervisory segments, such as SYN, do not carry data
Encoding application
messages
because the information to be conveyed, such as opening a
connection, delivers no data.
Vertical
communication on the
source host
ˆ TCP segments NEVER have trailers.
Standards
architectures
84
UDP Datagram
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 44
Encoding application
messages
Vertical
communication on the
source host
ˆ In UDP, the application message must fit into a single UDP
datagram.
Standards
architectures
ˆ So few header fields are needed.
ˆ The UDP Checksum field is for error detection.
ˆ If there is an error, UDP discards the datagram.
ˆ If it does not detect an error, it accepts the datagram but
does not send an acknowledgement.
84
Perspective on TCP and UDP
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
ˆ If the transport layer process detects an error in an arriving
Semantics and syntax 45
Encoding application
messages
TCP segment, what does it do?
ˆ If the transport layer process detects an error in an arriving
UDP datagram, what does it do?
Vertical
communication on the
source host
ˆ Are these actions different?
Standards
architectures
84
Recap: Port Number Fields
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 46
Encoding application
messages
ˆ Both TCP and UDP headers begin with
Vertical
communication on the
source host
ˆ a source port number field and
ˆ a destination port number field.
Standards
architectures
84
Port Numbers on Servers
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 47
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ Multitasking servers run multiple applications.
ˆ Each application is assigned a port number.
84
Port Numbers on Servers
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax 48
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
ˆ Major applications usually are given well-known port
numbers from 0 to 1023.
ˆ HTTP’s well-known port number is 80.
ˆ The File Transfer Protocol has two: 20 and 21.
84
Hypertext Transfer Protocol Message Syntax
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ HTTP
Message ordering and
reliability
Semantics and syntax 49
Encoding application
messages
Vertical
communication on the
source host
ˆ The application layer is the highest layer.
ˆ It has more standards than any other layer.
ˆ HTTP is not the only application layer standard; it is one of
many.
ˆ Many application layer protocols, such as SMTP for e-mail,
Standards
architectures
are much more complex than HTTP.
84
Hypertext Transfer Protocol Message Syntax
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ HTTP Syntax Uses [CRLF]
Message ordering and
reliability
Semantics and syntax 50
Encoding application
messages
ˆ This stands for carriage return plus line feed.
ˆ On printers, carriage moves the print head back to the left of
the same line, and line feed moves the print head down a line.
Vertical
communication on the
source host
ˆ In simpler terms, [CRLF] begins a new line.
ˆ Question: What will two [CRLFs] in a row do?
Standards
architectures
84
Hypertext Transfer Protocol Message Syntax
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ HTTP Request Message:
Definitions and
concepts
ˆ GET /panko/home.htm HTTP/1.1[CRLF]
ˆ Host: voyager.shidler.hawaii.edu
Message ordering and
reliability
Semantics and syntax 51
Encoding application
messages
ˆ In the first line:
ˆ GET says that this is a request to get a file.
ˆ /panko/home.htm is the location of the file.
ˆ HTTP/1.1 says that the browser speaks HTTP/1.1.
Vertical
communication on the
source host
Standards
architectures
ˆ The second line specifies the host to receive this HTTP
request message.
84
Hypertext Transfer Protocol Message Syntax
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ In HTTP, most lines have this syntax:
Message ordering and
reliability
Semantics and syntax 52
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
ˆ
ˆ
ˆ
ˆ
ˆ
Keyword: Value
Example: Host: voyager.shidler.hawaii.edu
This is like e-mail, with its To:, From:, and so on, fields.
HTTP was based on e-mail header concepts.
Consequently, it feels old-fashioned.
Hypertext Transfer Protocol Message Syntax
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ HTTP Response Message
Message ordering and
reliability
Semantics and syntax 53
Encoding application
messages
Vertical
communication on the
source host
Standards
architectures
84
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
HTTP/1.1 200 OK[CRLF]
Date: Tuesday, 20-MAR-2011 18:32:15 GMT[CRLF]
Server: name of server software[CRLF]
MIME-version: 1.0[CRLF]
Content-type: text/plain[CRLF]
-[CRLF]
File to be downloaded. A string of bytes that may be text,
graphics, sound, video, or other content.
Perspective: Syntax
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ We have spent much time on message syntax because it is
Message ordering and
reliability
extremely important in networking.
Semantics and syntax 54
Encoding application
messages
ˆ Ethernet frames are drawn with fields one below the other.
ˆ IP, TCP, and UDP syntax are drawn with fields beginning
Vertical
communication on the
source host
with bit position.
ˆ HTTP header syntax is shown as a series of lines of text,
Standards
architectures
most of which have the Keyword: Value format.
84
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
55
Vertical
communication on the
source host
Standards
architectures
84
Encoding application
messages
Encoding Application Messages
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Encoding
Definitions and
concepts
ˆ Applications must convert application message content into
Message ordering and
reliability
bits.
ˆ This is necessary because all lower layers have fields
Semantics and syntax
Encoding application
messages
56
Vertical
communication on the
source host
Standards
architectures
84
consisting only of ones and zeros.
ˆ This is called encoding.
Encoding Text
Modern
Communications
ˆ Text often is encoded with 7-bit ASCII.
David Goodwin
University of
Bedfordshire
ˆ The eighth bit in each byte is unused.
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
57
Vertical
communication on the
source host
Standards
architectures
84
Binary Representations of Whole Numbers
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
58
Vertical
communication on the
source host
Standards
architectures
84
Encoding Application Messages
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ Encoding Alternatives
Message ordering and
reliability
ˆ Some information consists of alternatives that have no
particular order:
Semantics and syntax
Encoding application
messages
59
Vertical
communication on the
source host
ˆ Gender (male or female): 2 alternatives
ˆ Sales region (north, south, east, west): 4 alternatives
ˆ City of birth: Many alternatives
ˆ How many bits do you need to represent alternatives?
Standards
architectures
ˆ Must be encoded into a field of fixed length
84
Encoding Application Messages
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Encoding Alternatives
ˆ If a field is N bits long, it can represent 2N alternatives
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
60
Vertical
communication on the
source host
Standards
architectures
84
Encoding Application Messages
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Each additional bit doubles the number of alternatives a field
can represent.
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
61
Vertical
communication on the
source host
Standards
architectures
84
Encoding Application Messages
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Each additional bit doubles the number of alternatives a field
Definitions and
concepts
can represent.
Message ordering and
reliability
Example
Semantics and syntax
Encoding application
messages
62
Vertical
communication on the
source host
ˆ Give the number of possible alternatives for
1
Standards
architectures
2
3
4
5
84
1
2
3
4
5
bit:
bits:
bits:
bits:
bits:
6
7
8
9
10
6 bits:
7 bits:
8 bits:
9 bits:
10 bits:
Encoding Application Messages
Modern
Communications
David Goodwin
University of
Bedfordshire
Example
Definitions and
concepts
Message ordering and
reliability
1
What is the formula for the number of alternatives you can
represent in a field?
2
A field is three bits long. How many alternatives can it
represent?
3
A field is four bits long. How many alternatives can it
represent?
4
If you want to encode the 12 months, how many bits will
you need in the field?
Semantics and syntax
Encoding application
messages
63
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Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
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Vertical
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Vertical communication on the source host
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
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ˆ The application process (in this case a browser) creates an
application message for the application process on the
destination host.
ˆ In this case, the application process is a browser.
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Vertical communication on the source host
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
66
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ˆ The application process passes the HTTP message down to
the transport layer process.
ˆ The transport process encapsulates the HTTP message in
the data field of TCP segment.
ˆ This requires adding a TCP header.
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Vertical communication on the source host
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
67
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Standards
architectures
ˆ The transport process passes the TCP segment down to the
internet process, which encapsulates the segment in an IP
packet.
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Vertical communication on the source host
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
68
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architectures
ˆ The internet process passes the IP packet to the data link
process, which is Ethernet.
ˆ Ethernet adds a header and trailer.
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Vertical communication on the source host
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
69
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architectures
ˆ The data link process passes the frame down to the physical
layer.
ˆ The physical layer does NOT do encapsulation. It merely
converts the bits of the frame into signals.
84
Vertical communication on the source host
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ The application, transport, internet, and data link layer
Message ordering and
reliability
process do the same thing.
Semantics and syntax
ˆ Create a message in all but the originating layer (in this case,
Encoding application
messages
application), by encapsulation.
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ˆ Then pass the message down to the next lower layer.
ˆ The physical layer process is different.
ˆ It merely converts the bits of the frame into signals.
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Standards
architectures
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
Vertical
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architectures
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Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ To build a house, you do not build one room, then decide
Message ordering and
reliability
what to do next.
Semantics and syntax
ˆ You create an architecture identifying the rooms that will be
Encoding application
messages
needed and how the rooms will relate to one another in terms
of flow.
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ˆ Then you begin the design of individual rooms.
Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
ˆ In networking, standards agencies first create standards
architectures that define the categories of standards to be
developed.
Semantics and syntax
Encoding application
messages
ˆ They define categories in terms of layers.
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ˆ They then design standards for the individual categories.
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Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ There are two major standards architectures.
Message ordering and
reliability
ˆ TCP/IP
ˆ This is the standards architecture for the Internet and many
Semantics and syntax
Encoding application
messages
corporate internets.
ˆ Its standards agency is the Internet Engineering Task Force
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(IETF).
74
ˆ As we saw in Chapter 1, most of its documents are called
requests for comment (RFCs).
ˆ Some (but not all) RFCs are official Internet standards.
84
Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ OSI
Message ordering and
reliability
ˆ Its two standards agency are ISO and the ITU-T.
ˆ ISO (the International Organization for Standardization) is a
Semantics and syntax
Encoding application
messages
computer standards agency.
ˆ The ITU-T is the International Telecommunications
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architectures
Union—Transmission Standards Agency.
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ˆ Don’t confuse ISO the organization with OSI the architecture.
Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ OSI divides standards into seven layers:
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
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Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ OSI divided application communication into three layers.
Message ordering and
reliability
ˆ Session (5) creates a managed session between applications.
Semantics and syntax
If there is a break in communication, the communication only
has to go back to the last roll-back point.
ˆ Presentation (6) was designed to translate between data
formats on different computers. In practice, it is used as a
category for file format standards, such as jpg and mp3.
ˆ Application (7) handles other details of application
communication.
Encoding application
messages
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Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ TCP/IP divides standards into four layers.
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
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Hybrid TCP/IP-OSI Standards Architecture
Modern
Communications
David Goodwin
University of
Bedfordshire
ˆ Companies actually use the Hybrid TCP/IP-OSI Standards
Architecture that takes standards at different layers from the
two architectures.
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
Vertical
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architectures
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Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ Dominance
Message ordering and
reliability
ˆ OSI is almost 100% dominant at the physical and data link
Semantics and syntax
Encoding application
messages
layers.
ˆ It is so dominant that the IETF assumes that network users
Vertical
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architectures
will use OSI standards at the physical and data link layers.
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ˆ This means single switched or wireless LANs and WANs.
Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
ˆ Dominance
Message ordering and
reliability
ˆ TCP/IP is about 90% dominant at the internet and transport
Semantics and syntax
layers, but other standards architectures are sometimes used
at these layers.
Encoding application
messages
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ˆ IPX/SPX is used on older Novell Netware file servers.
ˆ SNA is used by many IBM mainframes.
ˆ AppleTalk is used by some Apple servers.
Major Standards Architectures
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
ˆ Dominance
Semantics and syntax
ˆ At the application layer, many standards come from TCP/IP,
Encoding application
messages
but many also come from OSI, the World Wide Web
Consortium, and other standards agencies.
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Recap: Characteristics of Protocols in this
Chapter
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
Encoding application
messages
Vertical
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Standards
architectures
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84
Next Lecture
Modern
Communications
David Goodwin
University of
Bedfordshire
Definitions and
concepts
Message ordering and
reliability
Semantics and syntax
ˆ Lecture 1: Core concepts and principles
Encoding application
messages
ˆ Lecture 2: Standards concepts
ˆ Lecture 3: Physical propagation layer
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