1. Network Standards Layered Architectures
Chapter 2

2. 1. Message Standards (Protocols)

3. Figure 2-1: Standards Govern the Exchange of Messages
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

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
Humans understand the meaning of this easily

5. Figure 2-2: Hypertext Transfer Protocol (HTTP) Interactions1.
HTTP Request Message
Asking for a File
Browser
Webserver
Application
Client PC
Webserver 2.
HTTP Response Message
delivering the File or giving an error message
Semantics in HTTP, which governs the Web

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

7. Figure 2-3: Syntax of HTTP Request and Response Messages, Continued
Syntax is
very rigid
HTTP Response Message
HTTP/ OK[CRLF]
Date: Tuesday, 20-JAN :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]

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

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
Other
Header
Field
Destination
Address
Field is
Used by Switches and Routers
Like the Address on an Envelope
Message with
all three parts

10. 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

11. 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)

12. 2. Reliability

13. 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

14. Figure 2-5: Reliable TCP Session, Continued
Client PC
TCP Process
Webserver
TCP Process
4. Data = HTTP Request
Carry
HTTP
Req &
Resp
(4)
5. ACK (4)
6. Data = HTTP Response
TCP Segment (Message) 4 Carries an HTTP Request
Segment 5 Acknowledges It
There Is No Need to Resend
7. ACK (6)
Request-Response
Cycle for Data Transfer

15. 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) 8 Is Lost in Transmission
There Is No Acknowledgment
So the Sender Retransmits It
11. Data = HTTP Response
12. ACK (11)
Error Handling

16. 3. Connection-Oriented and Connectionless Protocols

17. Figure 2-6: Connection-Oriented and Connectionless Protocols
Connection-Oriented Protocol
Connectionless Protocol
Open Connection A B 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
Message 3 (Seq. Num B1)
Message 2 (Seq. Num = A2)
Close Connection

18. HTTP is connectionless
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

19. 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

20. Figure 2-7: Advantages and Disadvantages or Connection-Oriented Protocols
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.

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

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

23. 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

24. 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
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
Physical (1)
Device-device connection

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

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
Switch X1
Host A
Switch X2
Physical Link
X1-X2

27. Figure 2-9: Physical and Data Link Layer Standards in a Single Network, Continued
Data link layer standards govern the transmission of frames across a single network—typically by sending them through several switches along the data link
Frame
Data Link
A-B
Host B
Switch X1
Host A
Switch X2

28. Figure 2-9: Physical and Data Link Layer Standards in a Single Network, Continued
Data link layer standards also govern
Frame organization
Switch operation

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
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

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

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
Packet
Router 1
Router 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 Y
Data
Link
R1-R2
Network Z
Route A-B R2
Host B
Data Link R3-B

33. 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

34. Figure 2-10: Internet and Data Link Layers in an Internet, ContinuedTo
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 To
Network Z
Router R2
Network Y

35. 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

36. In an internet with hosts separated by N networks, there will be:
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

37. Figure 2-11: Internet and Transport Layer Standards
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

38. Figure 2-11: Internet and Transport Layer Standards, Continued2.
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

39. Figure 2-12: Application Layer Standards
The application layer governs how two applications work with each other, even if they are from different vendors
Browser
Webserver
Application
Client PC
Webserver

40. Figure 2-12: Application Layer Standards
There are more application layer standards than any other type of standard because there are many applications
HTTP
Database
Instant Messaging
FTP
Etc.

41. Standards Layers: Recap
Be able to repeat
this in your sleep!
Application (5)
Transport (4)
Internet (3)
Data Link (2)
Physical (1)

42. 5. Syntax Examples for Some Layer Messages

43. Octet = 8 Bits 10010111 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

44. Figure 2-14: Ethernet Frame
Preamble (7 octets) …
Start of Frame Delimiter
(1 octet)
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.

45. 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 < 46 octets)
Frame Check Sequence (32 bits)
The Ethernet frame’s data field contains a IP packet (preceded by an LLC subheader).
PAD is added if the data field is less than 46 octets long PAD length is set to keep the data field plus PAD 46 octets

46. 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

47. Figure 2-15: Internet Protocol (IP) Packet, Continued
The IP packet is drawn 32 bits to a line
Bit 0
Bit 31
Version
(4 bits)
Header
Length
(4 bits)
Diff-Serv
(8 bits)
Total Length
(16 bits)
Identification
(16 bits)
Flags
(3 bits)
Fragment Offset
(13 bits)
Time to Live
(8 bits)
Protocol
(8 bits)
Header Checksum (16 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

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

49. 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

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

51. 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. 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.

52. 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
More than a dozen messages must be exchanged to send an message

53. 6. Vertical Communication Between Layer Processes on the Same Host

54. 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

55. 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

56. 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
Eth
Trlr
HTTP
Message
TCP
Hdr IP
Hdr
Eth
Hdr
Encapsulation of IP Packet
in Data Field of Ethernet Frame

58. Figure 2-18: Layered Communication on the Source Host, Continued
Data Link
Process
Eth
Trlr
HTTP
Message
TCP
Hdr IP
Hdr
Eth
Hdr
Physical
Process
Physical Layer converts the bits of the frame into signals.

59. 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
Trlr
HTTP
Message
TCP
Hdr IP
Hdr
Eth
Hdr L2 L5 L4 L3 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)
Don’t forget the possible trailing L2 trailer

60. Figure 2-19: Decapsulation on the Destination Host
Eth
Trlr
HTTP
Message
TCP
Hdr IP
Hdr
Eth
Hdr
Data Link
Process
Physical
Process

61. Figure 2-19: Decapsulation on the Destination Host, Continued
HTTP
Message
TCP
Hdr IP
Hdr
Internet
Process
Eth
Trlr
HTTP
Message
TCP
Hdr IP
Hdr
Eth
Hdr
Data Link
Process
Decapsulation of IP Packet
from Data Field of Ethernet Frame

62. Figure 2-19: Decapsulation on the Destination Host, Continued
HTTP
Message
TCP
Hdr
Transport
Process
HTTP
Message
TCP
Hdr IP
Hdr
Internet
Process
Decapsulation of TCP Segment
from Data Field of IP Packet

63. Figure 2-19: Decapsulation on the Destination Host, Continued
HTTP
Message
Application
Process
HTTP
Message
TCP
Hdr
Transport
Process
Decapsulation of HTTP Message
from Data Field of TCP Segment

64. Figure 2-20: Layered End-to-End Communication
Routers
Have Three
Layers
---
Each Router
Port
Has Two
Layers (1&2)
Switches
Have Two
Layers
---
Each Switch
Port
Has One
Layer (1)
Source and
Destination
Hosts Have
5 Layers
Int
App DL
Trans
Phy
Source
Host
Switch 1
Switch 2
Router 1
Switch 3
Router 2
Destination
Host

65. Figure 2-21: Combining Horizontal and Vertical Communication
Hypertext Transfer Protocol
Int
App DL
Trans
Phy
Transmission Control Protocol
Internet Protocol
Source
Host
Switch 1
Switch 2
Router 1
Switch 3
Router 2
Destination
Host

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

67. Figure 2-22: The Hybrid TCP/IP-OSI Architecture
Broad Purpose
Hybrid TCP/IP-OSI
OSI
TCP/IP
Communication
between
applications
Application
(Layer 5)
Application
Application
Presentation
Session
Internetworking
Transport (Layer 4)
Transport
Transport
Internet (Layer 3)
Network
Internet
Transmission
within a single
LAN or WAN
Data Link (Layer 2)
Data Link
Use OSI
Standards Here
Physical (Layer 1)
Physical

68. Figure 2-23: OSI and TCP/IP
Standards
Agency or
Agencies
ISO (International
Organization for
Standardization)
ITU-T (International
Telecommunications
Union—
Standards Sector)
IETF (Internet
Engineering Task
Force)

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

71. 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

72. 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

73. 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

74. 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

75. 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

76. 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