Network Standards Layered Architectures

1
Network StandardsLayered Architectures

• Chapter 2

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1.Message Standards (Protocols)
3
Figure 2-1 Standards Govern the Exchange of
Messages

• Standards
• Rules of operation that allow two hardware or
  software processes to work together
• Even if they are from different vendors
• Standards Govern the Exchange of Messages
• Messages must be governed by strict rules
• Because computers are not intelligent

Message
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Figure 2-1 Standards Govern the Exchange of
Messages (Continued)

• Standards Govern Syntax
• Syntax the organization of the message
• Human example Susan thanked Tom
• This sentence has a subject-verb-object syntax
• Standards Govern Semantics
• Semantics The meaning of the message
• Human example Susan thanked Tom
• Humans understand the meaning of this easily

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Figure 2-2 Hypertext Transfer Protocol (HTTP)
Interactions
1. HTTP Request Message Asking for a File
Browser
Webserver Application
Webserver
Client PC
2. HTTP Response Message delivering the File
orgiving an error message
Semantics in HTTP, which governs the Web
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Figure 2-3 Syntax of HTTP Request and Response
Messages

• CRLF
• Carriage return and line feed (starts a new line)
• HTTP Request Message
• GET /reports/project1/final.htm HTTP/1.1CRLF
• GET is the method (others exist)
• Next comes the path to the file to be retrieved
• Last comes the version of the HTTP standard
• Host voyager.cba.Hawaii.eduCRLF
• The host to be sent the request message

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Figure 2-3 Syntax of HTTP Request and Response
Messages, Continued
Syntax is very rigid

• HTTP Response Message
• HTTP/1.1 200 OKCRLF
• Date Tuesday, 20-JAN-2006 183215 GMTCRLF
• Server name of server softwareCRLF
• MIME-version 1.0CRLF
• Content-type text/plainCRLF
• CRLF
• File to be downloaded (byte stream)
• Syntax of fields (lines) after first line
• Keyword Content CRLF

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Figure 2-1 Standards Govern the Exchange of
Messages, Continued

• General Message Syntax (Organization)
• General Message Organization (Figure 2-4)
• Primary parts of messages
• Data Field (content to be delivered)
• Header (everything before the data field)
• Trailer (everything after the data field)
• The header and trailer act like a delivery
  envelope for the data field.

Header
Data Field
Trailer
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Figure 2-1 Standards Govern the Exchange of
Messages, Continued

• General Message Syntax (Organization)
• Header and trailer are further divided into fields

Trailer
Data Field
Header
Other Header Field
Destination Address Field is Used by Switches and
Routers Like the Address on an Envelope
Message with all three parts
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Figure 2-4 General Message Organization,
Continued
Data Field
Header
Other Header Field
Destination Address Field
Message without a trailer Usually only data
link layer messages have trailers
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Figure 2-4 General Message Organization,
Continued
Header
Other Header Field
Destination Address Field
Message with only a header e.g. TCP supervisory
messages are pure headers (there is no data field
content to deliver)
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2.Reliability
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Figure 2-5 Reliable Transmission Control
Protocol (TCP) Session

• The Transmission Control Protocol (TCP) is an
  important standard in Internet transmission
• TCP
• Receiver acknowledges each correctly-received TCP
  segment.
• If an acknowledgments is not received by the
  sender, the sender retransmits the TCP message
  (called a TCP segment)
• This gives reliability error detection and error
  correction

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Figure 2-5 Reliable TCP Session, Continued
Client PC TCP Process
Webserver TCP Process
4. Data HTTP Request
5. ACK (4)
Carry HTTP Req Resp (4)
6. Data HTTP Response
TCP Segment (Message) 4Carries an HTTP
Request Segment 5 Acknowledges It There Is No
Need to Resend
7. ACK (6)
Request-Response Cycle for Data Transfer
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Figure 2-5 A TCP Session, Continued
Client PC TCP Process
Webserver TCP Process
8. Data HTTP Request (Error)
9. Data HTTP Request (No ACK so Retransmit)
Carry HTTP Req Resp (4)
10. ACK (9)
TCP Segment (Message) 8Is Lost in
Transmission There Is No Acknowledgment So the
Sender Retransmits It
11. Data HTTP Response
12. ACK (11)
Error Handling
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3.Connection-Oriented andConnectionless
Protocols
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Figure 2-6 Connection-Oriented and
Connectionless Protocols
Connectionless Protocol
Connection-Oriented Protocol
A
B
Message (No Sequence Number)
Message 1 (Seq. Num A1)
Connection-oriented protocols have Formal
openings and closings like Telephone calls Also
have sequence numbers so that the receiver can
put messages in order And so the receiver can
send Acknowledgments for specific messages
Close Connection
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Figure 2-6 Connection-Oriented and
Connectionless Protocols, Continued
Client PC Browser
Webserver Application
HTTP Request
HTTP is connectionless No Openings No
Closings No Sequence Numbers No Acknowledgments
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Figure 2-6 Connection-Oriented and
Connectionless Protocols, Continued
In TCP
Client PC TCP Process
Webserver TCP Process
Connection-Opening Messages
Messages During the Connection
Time
Connection-Closing Messages
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Figure 2-7 Advantages and Disadvantages or
Connection-Oriented Protocols

• Advantages
• Thanks to sequence numbers, the parties can tell
  if a message is lost.
• Error messages, such as ACKs can refer to
  specific messages.
• Long messages can be fragmented into many smaller
  messages that can fit inside packets.
• Fragmentation followed by reassembly on the
  destination host is an important concept in
  networking.

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Figure 2-7 Advantages and Disadvantages or
Connection-Oriented Protocols, Cont.

• Disadvantages
• The presence of many supervisory messages
  consumes existing bandwidth
• The processing of connection information places a
  heavy processing load on computers connected to
  the network

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4.The Hybrid TCP/IP-OSI Standards Architecture
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Standards Architecture

• A Standards Architecture is a Broad Plan for
  Creating Standards
• Break the problem of effective communication into
  smaller pieces for ease of development
• Develop standards for the individual pieces
• Just as a building architect creating a general
  plan for a house before designing the individual
  rooms in detail
• The dominant architecture today is the hybrid
  TCP/IP-OSI standards architecture shown in the
  next slide

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Figure 2-8 Hybrid TCP/IP-OSI Architecture
General Purpose(Core Later) Layer Specific Layer Purpose
Application-application communication Application (5) Application-application interworking
Transmission of a packet across an internet Transport (4) Host-host communication
Transmission of a packet across an internet Internet (3) Packet delivery across an internet
Transmission of a frame across a single network (LAN or WAN) Data Link (2) Frame delivery across a network
Transmission of a frame across a single network (LAN or WAN) Physical (1) Device-device connection
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Figure 2-8 Hybrid TCP/IP-OSI Architecture,
Continued

• Physical and Data Link Layer Standards
• Govern Communication Through a Single Network
• LAN or WAN

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Figure 2-9 Physical and Data Link Layer
Standards in a Single Network

• Physical Layer
• Physical layer standards govern transmission
  between adjacent devices connected by a
  transmission medium

Physical Link A-X1
Switch X1
Host A
Switch X2
Physical Link X1-X2
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Figure 2-9 Physical and Data Link Layer
Standards in a Single Network, Continued

• Data Link Layer
• Data link layer standards govern the transmission
  of frames across a single networktypically by
  sending them through several switches along the
  data link

Frame
Host B
Data Link A-B
Switch X1
Host A
Switch X2
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Figure 2-9 Physical and Data Link Layer
Standards in a Single Network, Continued

• Data Link Layer
• Data link layer standards also govern
• Frame organization
• Switch operation

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Figure 2-9 Physical and Data Link Layer
Standards in a Single Network, Continued
3 Physical Links 1 Data Link 2 Switches
Host A
Switch
Data Link A-R1
Switch
Physical Link A-X1
Server Station
Switch X1
Physical Link X1-X2
Physical Link X2-R1
Switch X2
Mobile Client Station
Router R1
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Figure 2-10 Internet and Data Link Layers in an
Internet

• Internet and Transport Layers
• An internet is a group of networks connected by
  routers so that any application on any host on
  any network can communicate with any application
  on any other host on any other network
• Internet and transport layer standards govern
  communication across an internet composed of two
  or more single networks

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Figure 2-10 Internet and Data Link Layers in an
Internet, Continued

• Internet Layer
• Internet layer standards govern the transmission
  of packets across an internettypically by
  sending them through several routers along the
  route
• Messages at the internet layer are called packets
• Internet layer standards also govern packet
  organization and router operation

Packet
Router 1
Router 2
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Figure 2-10 Internet and Data Link Layers in an
Internet, Continued
Host A
Data Link A-R1
R1
Network X
Network Y
3 Data Links One per Network 1 Route per Internet
Data Link R1-R2
Route A-B
Network Z
R2
Host B
Data Link R3-B
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Figure 2-10 Internet and Data Link Layers in an
Internet, Continued
Frame X
Packet
In Network X Two Destination Addresses Packet
Host B (Destination Host) Frame Router R1
Data Link A-R1
Switch
Host A
Switch
Server Station
Switch X1
Mobile Client Station
Switch X2
Route A-B
Router R1
Network X
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Figure 2-10 Internet and Data Link Layers in an
Internet, Continued
To Network X
Route A-B
Router R1
Frame Y
Data Link R1-R2
In Network Y Two Destination Addresses Packet
Host B (Destination Host) Frame Router R2
Packet
Router R2
To Network Z
Network Y
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Figure 2-10 Internet and Data Link Layers in an
Internet, Continued
Frame Z
Packet
Data Link R2-B
Switch Z1
Host B
Router R2
In Network Z Two Destination Addresses Packet
Host B (Destination Host) Frame Host B
Switch Z2
Mobile Client Stations
Switch X2
Router
Network Z
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Frames and Packets

• In an internet with hosts separated by N
  networks, there will be
• 2 hosts
• One packet (going all the way between hosts)
• One route (between the two hosts)
• N frames (one in each network)
• There usually are many switches within single
  networks
• There usually are many physical links within
  networks

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Figure 2-11 Internet and Transport Layer
Standards

• Transport Layer
• Transport layer standards govern aspects of
  end-to-end communication between two end hosts
  that are not handled by the internet layer
• These standards allow hosts to work together even
  if the two computers are from different vendors
  and have different internal designs

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Figure 2-11 Internet and Transport Layer
Standards, Continued
2. Transport Layer end-to-end (host-to-host) TCP
is connection-oriented, reliable UDP is
connectionless and unreliable
Server
Client PC
1. Internet Layer (usually IP) hop-by-hop
(host-router or router-router) connectionless,
unreliable
Router 1
Router 2
Router 3
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Figure 2-12 Application Layer Standards

• Application Layer
• The application layer governs how two
  applications work with each other, even if they
  are from different vendors

Browser
Webserver Application
Webserver
Client PC
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Figure 2-12 Application Layer Standards

• There are more application layer standards than
  any other type of standard because there are many
  applications
• HTTP
• E-Mail
• Database
• Instant Messaging
• FTP
• Etc.

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Standards Layers Recap
Be able to repeat this in your sleep!

• Application (5)
• Transport (4)
• Internet (3)
• Data Link (2)
• Physical (1)

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5.Syntax Examples for Some Layer Messages
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Octets

• Field length may be measured in octets
• An octet is a group of eight bits
• In computer science, an octet is called a byte

Octet 8 Bits 10010111
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Figure 2-14 Ethernet Frame
Preamble (7 octets) 10101010
Start of Frame Delimiter (1 octet) 10101011
Header
Destination Ethernet (MAC) Address (48 bits)
Source Ethernet (MAC) Address (48 bits)
Length (2 octets) Length of Data Field
The Ethernet frame has 48-bit destination and
source address fields.
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Figure 2-14 Ethernet Frame, Continued
Data Field (variable length)
LLC Subheader (usually 7 octets)
Data Field
Usually IP Packet
Encapsulated Packet
PAD (added if data field lt 46 octets)
Frame Check Sequence (32 bits)
The Ethernet frames data field contains a IP
packet(preceded by an LLC subheader). PAD is
added if the data field is less than 46 octets
longPAD length is set to keep the data field
plus PAD 46 octets
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Figure 2-14 Ethernet Frame, Continued
Frame Check Sequence (32 bits)

• Sender computes the frame check sequence field
  value based on contents of other fields
• Receiver recomputes the field value
• If the values match, there have been no errors
• If the values do not match, there has been an
  error
• The receiver simply discards the frame
• Unreliable error detection but not error
  correction

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Figure 2-15 Internet Protocol (IP) Packet,
Continued
The IP packet is drawn 32 bits to a line
Bit 0
Bit 31
Total Length (16 bits)
Version (4 bits)
Diff-Serv (8 bits)
Header Length (4 bits)
Identification (16 bits)
Flags (3 bits)
Fragment Offset (13 bits)
Header Checksum (16 bits)
Protocol (8 bits)
Time to Live (8 bits)
Version is Bits 0-3 Header length is Bits
4-7 Diff Serv is Bits 8-15 Total Length is Bits
16-31
Identification is Bits 32-47 Time to live is
Bits 48-55
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Figure 2-15 Internet Protocol (IP) Packet
Bit 0
Bit 31
Total Length
Version
Diff-Serv
Header Length
Identification
Flags
Fragment Offset
Header Checksum
Protocol
Time to Live
Source IP Address (32 bits)
Destination IP Address (32 bits)
Options (if any)
Padding (to 32-bit boundary)
Data Field (dozens, hundreds, or thousands of
bits) Often contains a TCP segment
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Figure 2-16 TCP and UDP at the Transport Layer

• TCP is reliable
• Not all applications need reliability
• Voice over IP cannot wait for lost or damaged
  packets to be transmitted
• Network management protocols need to place as low
  a burden on the network as possible
• Both types of applications use the simpler User
  Datagram Protocol (UDP) instead of TCP

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Figure 2-16 TCP and UDP at the Transport Layer,
Continued
Protocol TCP UDP
Layer Transport Transport
Connection-Oriented? Yes No
Reliable? Yes No
Burden on the two hosts High Low
Burden on the network High Low
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Why Make TCP Reliable?

• Two reasons
• 1. The transport layer only involves processing
  on the two hosts.
• Reliability is a heavy process.
• It would be far more expensive to make the
  internet or data link layer reliable because this
  would require complex processing on many routers
  or switches, respectively.
• 2. TCPs reliability fixes errors at the
  transport layer and all lower layers in the
  process. This allows the transport layer to give
  the application clean data.

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Figure 2-17 A Complex Application Protocol The
Simple Mail Transfer Protocol (SMTP)

• Some application protocols are simple
• HTTP Simple request-response message cycle shown
  in Figure 2-2
• Some application protocols are complex (Figure
  2-17)
• Simple Mail Transfer Protocol (SMTP) for e-mail
• More than a dozen messages must be exchanged to
  send an e-mail message

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6.Vertical Communication Between Layer Processes
on the Same Host
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Figure 2-18 Layered Communication on the Source
Host
The process begins when a browser creates an HTTP
request message
Application Process
HTTP Message
Passes Message Down to Transport Process
Transport Process
HTTP Message
TCP Hdr
Encapsulation of HTTP Message in Data Field of
TCP Segment
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Figure 2-18 Layered Communication on the Source
Host, Continued

• When a layer process (N) creates a message, it
  passes it down to the next-lower-layer process
  (N-1) immediately
• The receiving process (N-1) will encapsulate the
  Layer N message, that is, place it in the data
  field of its own (N-1) message

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Figure 2-18 Layered Communication on the Source
Host, Continued
Transport Process
HTTP Message
TCP Hdr
Internet Process
HTTP Message
TCP Hdr
IP Hdr
Encapsulation of TCP Segment in Data Field of IP
Packet
57
Figure 2-18 Layered Communication on the Source
Host, Continued
Internet Process
HTTP Message
TCP Hdr
IP Hdr
Data Link Process
HTTP Message
TCP Hdr
IP Hdr
Eth Hdr
Eth Trlr
Encapsulation of IP Packet in Data Field of
Ethernet Frame
58
Figure 2-18 Layered Communication on the Source
Host, Continued
Data Link Process
HTTP Message
TCP Hdr
IP Hdr
Eth Hdr
Eth Trlr
Physical Process
Physical Layer converts the bits of the frame
into signals.
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Figure 2-18 Layered Communication on the Source
Host, Continued
The following is the final frame for a an HTTP
message on an Ethernet LAN
HTTP Message
TCP Hdr
IP Hdr
Eth Hdr
Eth Trlr
L5
L4
L3
L2
L2
Notice the Pattern From Right to Left L2, L3,
L4, L5, maybe L2 Start with the highest-layer
message (in this case, 5) Add headers for each
lower layer (L4, L3, and L2, in this case) Dont
forget the possible trailing L2 trailer
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Figure 2-19 Decapsulation on the Destination Host
HTTP Message
TCP Hdr
IP Hdr
Eth Hdr
Eth Trlr
Data Link Process
Physical Process
61
Figure 2-19 Decapsulation on the Destination
Host, Continued
Internet Process
HTTP Message
TCP Hdr
IP Hdr
HTTP Message
TCP Hdr
IP Hdr
Eth Hdr
Eth Trlr
Data Link Process
Decapsulation of IP Packet from Data Field of
Ethernet Frame
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Figure 2-19 Decapsulation on the Destination
Host, Continued
Transport Process
HTTP Message
TCP Hdr
Internet Process
HTTP Message
TCP Hdr
IP Hdr
Decapsulation of TCP Segment from Data Field of
IP Packet
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Figure 2-19 Decapsulation on the Destination
Host, Continued
Application Process
HTTP Message
Transport Process
HTTP Message
TCP Hdr
Decapsulation of HTTP Message from Data Field of
TCP Segment
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Figure 2-20 Layered End-to-End Communication
Routers Have Three Layers --- Each
Router Port Has Two Layers (12)
Switches Have Two Layers --- Each Switch Port Has
One Layer (1)
Source and Destination Hosts Have 5 Layers
Source Host
Destination Host
Switch 1
Switch 2
Router 1
Switch 3
Router 2
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Figure 2-21 Combining Horizontal and Vertical
Communication
Hypertext Transfer Protocol
Transmission Control Protocol
Internet Protocol
Destination Host
Source Host
Switch 2
Router 1
Switch 3
Router 2
Switch 1
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7.OSI, TCP/IP, and Other Standards Architectures
67
Figure 2-22 The Hybrid TCP/IP-OSI Architecture
TCP/IP
OSI
Hybrid TCP/IP-OSI
Broad Purpose
Application
Application
Application (Layer 5)
Communication between applications
Presentation
Session
Transport
Transport
Transport (Layer 4)
Internetworking
Internet
Network
Internet (Layer 3)
Use OSI Standards Here
Data Link
Data Link (Layer 2)
Transmission within a single LAN or WAN
Physical
Physical (Layer 1)
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Figure 2-23 OSI and TCP/IP
OSI
TCP/IP
Standards Agency or Agencies
ISO (International Organization for
Standardization) ITU-T (International Telecommun
ications Union Telecommunications Standards
Sector)
IETF (Internet Engineering Task Force)
69
Figure 2-23 OSI and TCP/IP, Continued
OSI
TCP/IP
Dominance
Nearly 100 dominant at physical and data link
layers
70-80 dominant at the internet and
transport layers.
Documents are Called
Various
Mostly RFCs (requests for comments)
70
Figure 2-24 OSI Layers

• Layer 1 OSI Physical Layer Standards
• Nearly always used in the hybrid TCP/IP-OSI
  architecture
• Layer 2 OSI Data Link Layer Standards
• Nearly always used in the hybrid TCP/IP-OSI
  architecture

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Figure 2-24 OSI Layers, Continued

• Layer 3 OSI Network Layer Standards
• Same function as internet layer standards in
  TCP/IP
• But OSI network layer standards are incompatible
  with TCP/IP internet layer standards
• Rarely used
• Layer 4 OSI Transport Layer Standards
• Same function as transport layer in TCP/IP
• But OSI transport layer standards are
  incompatible with TCP/IP transport layer
  standards
• Rarely used

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Figure 2-24 OSI Layers, Continued

• Layer 5 OSI Session Layer Standards
• Initiate and maintain a connection between
  application programs on different computers
• Nothing like this layer in TCP/IP
• Rarely used because OSI is rarely used above the
  data link layer and below the application layer

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Figure 2-24 OSI Layers, Continued

• Layer 6 OSI Presentation Layer Standards
• Designed to handle data formatting differences
  between the computers, data compression, and
  encryption.
• Rarely used this way because OSI standards are
  rarely used above the data link layer and below
  the application layer
• In practice, a category for general OSI file
  format standards used in multiple applications
• JPEG, etc.
• These standards are widely used

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Figure 2-24 OSI Layers, Continued

• Layer 7 OSI Application Layer
• For other application-specific matters
• Some OSI application layer standards are used
• Run over TCP/IP transport/internet layer
  processes
• Almost always without actual session and
  presentation layer processes

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Figure 2-25 Other Major Standards Architectures

• IPX/SPX
• Used by older Novell NetWare file servers
• Popular option for newer Novell NetWare file
  servers
• SNA (Systems Network Architecture)
• Used by IBM mainframe computers
• AppleTalk
• Used by Apple Macintoshes

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Figure 2-26 Characteristics of Protocols
Discussed in the Chapter
Layer
Protocol
Connection- Oriented /Connectionless
Reliable/ Unreliable
5 (App)
HTTP
Connectionless
Unreliable
4 (Transport)
TCP
Connection- oriented
Reliable
4 (Transport)
UDP
Connectionless
Unreliable
3 (Internet)
IP
Connectionless
Unreliable
2 (Data Link)
Ethernet
Connectionless
Unreliable
Note Only TCP is connection-oriented and reliable