N6C02-PPT for instructors

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					Network Standards
Layered Architectures
Chapter 2
Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of the book

1. Message Standards (Protocols)

Standards

Standards are rules of operation that allow two hardware or software processes to work together Even if they are from different vendors

2-3

Figure 2-1: Standards Govern the Exchange of Messages • Standards Govern the Exchange of Messages
– Messages must be governed by strict rules
– Because computers are not intelligent

Message

2-4

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 this message easily
2-5

Figure 2-2: Hypertext Transfer Protocol (HTTP) Interactions
1. HTTP Request Message Asking for a File Browser Webserver Application Webserver 2. HTTP Response Message Delivering the File

Client PC

Semantics in HTTP, which governs the Web
2-6

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/1.1[CRLF] • 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.edu[CRLF] • The host to be sent the request message
2-7

Figure 2-3: Syntax of HTTP Request and Response Messages, Continued • HTTP Response Message
– – – – – – – HTTP/1.1 200 OK[CRLF] Date: Tuesday, 20-JAN-2006 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 (byte stream)

• Syntax of fields (lines) after first line:
– Keyword : Content [CRLF]
2-8

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.
Trailer Data Field Header 2-9

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

Message with all three parts

Destination Address Field is Used by Switches and Routers Like the Address on an Envelope 2-10

Other Header Field

Figure 2-4: General Message Organization, Continued

Data Field

Header

Message without a trailer Usually only data link layer messages have trailers

Other Header Field

Destination Address Field

2-11

Figure 2-4: General Message Organization, Continued
Header

Message with only a header e.g. TCP supervisory messages are pure headers (there is no data field content to deliver)

Other Header Field

Destination Address Field

2-12

2. Reliability

Figure 2-5: Reliable Transmission Control Protocol (TCP) Session
• The Transmission Control Protocol (TCP) is an important standard in Internet transmission

• TCP
– If acknowledgments are not sent by the receiver, the sender retransmits the TCP message (called a TCP segment) – This gives reliability: error detection and error correction
2-14

Figure 2-5: Reliable TCP Session, Continued

Client PC TCP Process

Webserver TCP Process

4. Data = HTTP Request
5. ACK (4) 6. Data = HTTP Response TCP Segment (Message) 4 Carries an HTTP Request 7. ACK (6) Segment 5 Acknowledges It There Is No Need to Resend 2-15

Carry HTTP Req & Resp (4)

Request-Response Cycle for Data Transfer

Figure 2-5: A TCP Session, Continued

Client PC TCP Process 8. Data = HTTP Request (Error) Carry HTTP Req & Resp (4)

Webserver TCP Process

9. Data = HTTP Request (No ACK so Retransmit)

10. ACK (9) TCP Segment (Message) 8 11. Data = HTTPIs Lost in Transmission Response
There Is No Acknowledgment 12. ACK (11) So the Sender Retransmits It 2-16

Error Handling

3. Connection-Oriented and Connectionless Protocols

Figure 2-6: Connection-Oriented and Connectionless Protocols
Connection-Oriented Protocol A Open Connection B Connectionless Protocol

A

Message (No Sequence Number)

B

Message 1 (Seq. Num = A1)

Message 3 (Seq. Num B1)
Message 2 (Seq. Num = A2) Close Connection

Connection-oriented protocols Formal openings and closings Also have sequence numbers so that the receiver can put messages in order And so the receiver can send Acknowledgments for specific messages 2-18

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 2-19

Figure 2-6: Connection-Oriented and Connectionless Protocols, Continued

Client PC TCP Process

In TCP

Webserver TCP Process

Connection-Opening Messages Messages During the Connection

Time

Connection-Closing Messages

2-20

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.
2-21

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

2-22

4. The Hybrid TCP/IP-OSI Standards Architecture

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/IPOSI standards architecture shown in the next slide
2-24

Figure 2-8: Hybrid TCP/IP-OSI Architecture

General Purpose
Application-application communication Transmission across an internet

Layer
Application (5) Transport (4) Internet (3)

Specific Layer Purpose
Application-application interworking Host-host communication Packet delivery across an internet Frame delivery across a network Device-device connection 2-25

Transmission across a single network (LAN or WAN)

Data Link (2) Physical (1)

Figure 2-8: Hybrid TCP/IP-OSI Architecture, Continued
• Physical and Data Link Layer Standards
– Govern Communication Through a Single Network – LAN or WAN

2-26

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 Host A

Switch X1 Switch X2

Physical Link X1-X2

2-27

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 network—typically by sending them through several switches along the data link
Data Link A-B Host B

Host A

Switch X1
Switch X2 2-28

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

2-29

Figure 2-9: Physical and Data Link Layer Standards in a Single Network, Continued
3 Physical Links 1 Data Link 2 Switches

Host A Data Link A-R1 Physical Link A-X1 Switch X1 Physical Link X1-X2 Mobile Client Station

Switch

Switch

Server Station

Switch X2

Physical Link X2-R1

Router R1 2-30

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

2-31

Figure 2-10: Internet and Data Link Layers in an Internet, Continued
• Internet Layer
– Internet layer standards govern the transmission of packets across an internet—typically 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

Router 1

Router 2

2-32

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

Host A

Data Link A-R1 R1

Network X 3 Data Links: One per Network 1 Route per Internet Network Z Route A-B Network Y Data Link R1-R2 R2

Host B

Data Link R3-B 2-33

Figure 2-10: Internet and Data Link Layers in an Internet, Continued
Frame X Packet

Host A

Data Link A-R1

Switch In Network X: Two Destination Addresses: Switch Server Packet: Host B (Destination Host) Station Frame: Router R1

Switch X1 Mobile Client Station Switch X2 Route A-B

Router R1

Network X
2-34

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

To Network X
Route A-B Router R1 Frame Y Packet

Data Link In Network Y: R1-R2 Two Destination Addresses: Packet: Host B (Destination Host) Frame: Router R2 To Network Z Router R2

Network Y

2-35

Figure 2-10: Internet and Data Link Layers in an Internet, Continued
Frame Z Packet

Data Link R2-B Host B

Switch Z1

Router R2

In Network Z: Two Destination Addresses: Switch Packet: Host B (Destination Host) Z2 Frame: Host B Mobile Client Stations Network Z 2-36 Switch X2 Router

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) N-1 routers (change frames between each pair of networks) – There usually are many switches within single networks – There usually are many physical links within networks
2-37

Figure 2-11: Internet and Transport Layer Standards • Transport Layer
– Transport layer standards govern aspects of end-toend 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

2-38

Figure 2-11: Internet and Transport Layer Standards, Continued
Transport Layer end-to-end (host-to-host) TCP is connection-oriented, reliable UDP is connectionless and unreliable Internet Layer (usually IP) hop-by-hop (host-router or router-router) connectionless, unreliable

Client PC

Server

Router 1

Router 2

Router 3
2-39

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 2-40

Client PC

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.
2-41

Standards Layers: Recap
• Application (5)

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

2-42

Figure 2-13: Why Layer?

Box

• Breaking up large tasks into smaller tasks and assigning tasks to different individuals is common in all fields • Specialization in standards design (EEs for physical layer, application specialists for application layer, etc.) • Simplification in standards design for individual standards • If you change a standard at one layer, you do not have to change standards at other layers
2-43

5. Syntax Examples for Some Layer Messages

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
2-45

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 … 2-46

Figure 2-14: Ethernet Frame, Continued

Data Field (variable length)

LLC Subheader (usually 7 octets)
Usually IP Packet Encapsulated Packer

PAD (added if data field < 46 octets) Trailer Frame Check Sequence (32 bits)

2-47

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
2-48

Figure 2-15: Internet Protocol (IP) Packet, Continued
Bit 0

The IP packet is drawn 32 bits to a line Diff-Serv (8 bits)
Flags (3 bits)

Bit 31

Header Version Length (4 bits) (4 bits)

Total Length (16 bits)
Fragment Offset (13 bits)

Identification (16 bits) Time to Live Protocol (8 bits) (8 bits) Version is Bits 0-3 Header length is Bits 4-7

Header Checksum (16 bits) Identification is Bits 32-47

Diff Serv is Bits 8-15
Total Length is Bits 16-31

Time to live is Bits 48-55
2-49

Figure 2-15: Internet Protocol (IP) Packet
Bit 0 Version Header Length Diff-Serv Flags Total Length Fragment Offset Header Checksum Bit 31

Identification Time to Live Protocol

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 2-50

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

2-51

Figure 2-16: TCP and UDP at the Transport Layer, Continued
Protocol Layer TCP Transport UDP Transport

Connection-Oriented?
Reliable? Burden on the two hosts Burden on the network

Yes
Yes High High

No
No Low Low
2-52

Why Make TCP Reliable?
• Reliability is a heavy process. The transport layer only involves processing on the two hosts. 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. • The transport layer is the highest layer below the application layer. TCP’s 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.
2-53

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 217)
– Simple Mail Transfer Protocol (SMTP) for e-mail – More than a dozen messages must be exchanged to send an e-mail message
2-54

6. Vertical Communication Between Layer Processes on the Same Host

Figure 2-18: Layered Communication on the Source Host

The process begins when a browser creates an HTTP request message

Application Process Passes Message Down to Transport Process Transport Process

HTTP Message

HTTP TCP Message Hdr

Encapsulation of HTTP Message in Data Field of TCP Segment 2-56

Figure 2-18: Layered Communication on the Source Host, Continued
• When a layer process (N) creates a message, it passes it down to the nextlower-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

2-57

Figure 2-18: Layered Communication on the Source Host, Continued

Transport Process

HTTP TCP Message Hdr

Internet Process

HTTP TCP IP Message Hdr Hdr

Encapsulation of TCP Segment in Data Field of IP Packet 2-58

Figure 2-18: Layered Communication on the Source Host, Continued

Internet Process

HTTP TCP IP Message Hdr Hdr

Data Link Process

Eth HTTP TCP IP Eth Trlr Message Hdr Hdr Hdr

Encapsulation of IP Packet in Data Field of Ethernet Frame 2-59

Figure 2-18: Layered Communication on the Source Host, Continued

Data Link Process

Eth HTTP TCP IP Eth Trlr Message Hdr Hdr Hdr

Physical Process

Physical Layer converts the bits of the frame into signals.

2-60

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

Eth HTTP TCP IP Eth Trlr Message Hdr Hdr Hdr L2 L5 L4 L3 L2

Notice the Pattern: From Right to Left: L2, L3, L4, L5, maybe L2 This makes it easier to remember the order of headers and messages Don’t forget the possible trailing L2 trailer 2-61

Figure 2-19: Decapsulation on the Destination Host

Eth HTTP TCP IP Eth Trlr Message Hdr Hdr Hdr

Data Link Process

Physical Process

2-62

Figure 2-19: Decapsulation on the Destination Host, Continued

HTTP TCP IP Message Hdr Hdr

Internet Process

Eth HTTP TCP IP Eth Trlr Message Hdr Hdr Hdr

Data Link Process

Decapsulation of IP Packet from Data Field of Ethernet Frame 2-63

Figure 2-19: Decapsulation on the Destination Host, Continued

HTTP TCP Message Hdr

Transport Process

HTTP TCP IP Message Hdr Hdr

Internet Process

Decapsulation of TCP Segment from Data Field of IP Packet 2-64

Figure 2-19: Decapsulation on the Destination Host, Continued

HTTP Message

Application Process

HTTP TCP Message Hdr

Transport Process

Decapsulation of HTTP Message from Data Field of TCP Segment 2-65

Figure 2-20: Layered End-to-End Communication

Source and Destination Hosts Have 5 Layers
App Trans Int DL Phy

Switches Have Two Layers --Each Switch Port Has One Layer (1)

Routers Have Three Layers --Each Router Port Has Two Layers (1&2)

Source Host

Switch 1

Switch 2

Router 1

Switch 3

Router Destination 2 Host 2-66

Figure 2-21: Combining Horizontal and Vertical Communication

Hypertext Transfer Protocol App Trans Int DL Phy Source Host Switch 1 Switch 2 Router 1 Switch 3 Router Destination Host 2 2-67 Transmission Control Protocol Internet Protocol

7. OSI, TCP/IP, and Other Standards Architectures

Figure 2-22: The Hybrid TCP/IP-OSI Architecture

Broad Purpose Hybrid TCP/IP-OSI OSI Communication between applications

TCP/IP

Application
Application (Layer 5) Transport (Layer 4) Internet (Layer 3) Data Link (Layer 2) Physical (Layer 1) Presentation Session Transport Network Data Link Physical Transport Internet Use OSI Standards Here Application

Internetworking
Transmission within a single LAN or WAN

2-69

Figure 2-23: OSI and TCP/IP

OSI Standards Agency or Agencies ISO (International Organization for Standardization) ITU-T (International Telecommunications Union— Telecommunications Standards Sector)

TCP/IP IETF (Internet Engineering Task Force)

2-70

Figure 2-23: OSI and TCP/IP, Continued

OSI Dominance Nearly 100% dominant at physical and data link layers Various

TCP/IP 70%-80% dominant at the internet and transport layers. Mostly RFCs (requests for comments)

Documents are Called

2-71

Figure 2-23: OSI and TCP/IP, Continued • Notes
– Do not confuse OSI (the architecture) with ISO (the organization)

– The acronyms for ISO and ITU-T do not match their names, but these are the official names and acronyms

2-72

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

2-73

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
2-74

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

2-75

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
2-76

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

2-77

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
2-78

Figure 2-26: Characteristics of Protocols Discussed in the Chapter
Layer Protocol ConnectionOriented /Connectionless Connectionless Connectionoriented Connectionless Connectionless Reliable/ Unreliable

5 (App)

HTTP

Unreliable

4 (Transport)
4 (Transport) 3 (Internet)

TCP
UDP IP

Reliable
Unreliable Unreliable

2 (Data Link)

Ethernet

Connectionless

Unreliable

Note: Only TCP is connection-oriented and reliable 2-79

8. Topics Covered

Topics Covered
• Standards govern the semantics and syntax of messages
– HTTP: Text request and response messages – Data field, header, and trailer – Header and trailer subdivided into fields

• Reliability
– In TCP, receiver sends ACKs

– Senders retransmit non-acknowledged segments
2-81

Topics Covered
• Connection-oriented versus connectionless
– TCP is connection-oriented
– HTTP is connectionless

• Hybrid TCP/IP-OSI Architecture
– OSI is nearly 100% dominant at Layers 1 and 2 – TCP/IP is 70% to 80% dominant at Layers 3 and 4 – Situation at Layer 5 is complex

2-82

Topics Covered • Hybrid TCP/IP-OSI Standards Architecture
– 1. Physical layer (between adjacent devices) – 2. Data link layer (across a switched network) – 3. Internet layer (across an internet) – 4. Transport layer (host-to-host) – 5. Application layer (application-to-application)

2-83

Topics Covered • Ethernet
– Source and destination addresses are 48 bits long – Switches forward packets by destination addresses – Data field encapsulates an IP packet – Unreliable: if detects an error, drops the frame

• Internet Protocol (IP)
– 32-bit addresses

– Show 32 bits on each line
– Unreliable: checks headers for errors but discards
2-84

Topics Covered • Vertical Communication on the Source Host
– Layer process creates message and then sends the message to the next-lower layer – Next-lower layer encapsulates the message in its own message

– This continues until the final frame at the data link layer

• Vertical Communication on the Destination Host
– Decapsulation and passing up
2-85

Topics Covered

• Not All Devices Have All Layers
– Hosts have all five
– Routers have only the lowest three – Switches have only the lowest two

2-86

Topics Covered

• OSI Architecture
– Divides application layer into three layers • Session • Presentation • Application

• Other Standards Architectures
– IPX/SPX – SNA – AppleTalk
2-87


				
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