Transcript Basic Concepts

Network Standards
Layered Architectures
Chapter 2
Updated January 2007
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)
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
2-3
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
2-4
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 or
giving an error message
Semantics in HTTP, which governs the Web
2-5
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
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Figure 2-3: Syntax of HTTP Request and
Response Messages, Continued
• HTTP Response Message
–
–
–
–
–
–
–
Syntax is
very rigid
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]
2-7
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
2-8
Figure 2-1: Standards Govern the
Exchange of Messages, Continued
• General Message Syntax (Organization)
– Header and trailer are further divided into fields
Trailer
Message with
all three parts
Data Field
Header
Other
Header
Field
Destination
Address
Field is
Used by Switches and Routers
Like the Address on an Envelope
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Figure 2-4: General Message Organization,
Continued
Data Field
Message without
a trailer
Header
Other
Header
Field
Destination
Address
Field
Usually only data link
layer messages have trailers
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Figure 2-4: General Message Organization,
Continued
Header
Message with
only a header
Other
Header
Field
Destination
Address
Field
e.g.
TCP supervisory messages are pure headers
(there is no data field content to deliver)
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2.
Reliability
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
Carry
HTTP
Req &
Resp
(4)
5. ACK (4)
6. Data = HTTP Response
Request-Response
Cycle for Data Transfer
TCP Segment (Message) 4
Carries an HTTP Request
7. ACK (6)
Segment 5 Acknowledges It
There Is No Need to Resend
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Figure 2-5: A TCP Session, Continued
Client PC
TCP Process
Webserver
TCP Process
8. Data = HTTP Request (Error)
Carry
HTTP
Req &
Resp
(4)
9. Data = HTTP Request (No ACK so Retransmit)
Error Handling
10. ACK (9)
TCP Segment (Message) 8
Lost in Transmission
11. Data = HTTPIsResponse
There Is No Acknowledgment
12. ACK (11)
So the Sender Retransmits It
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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 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
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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
Client PC
TCP Process
In TCP
Webserver
TCP Process
Connection-Opening Messages
Time
Messages During the Connection
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
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
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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
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
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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
Host A
Switch X1
Physical Link
X1-X2
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 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 A
Host B
Switch X1
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
Host A
Switch
3 Physical Links
1 Data Link
2 Switches
Data Link
A-R1
Switch
Physical Link
A-X1
Switch X1
Physical
Link
X1-X2
Mobile Client
Station
Switch
X2
Physical
Link
X2-R1
Server
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 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
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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
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
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Figure 2-10: Internet and Data Link Layers in an
Internet, Continued
Frame X
Packet
Host A
Data Link
A-R1
In Network X:
Switch
Two Destination Addresses:
Packet:Switch
Host B (Destination
Host)
Server
Frame: Router Station
R1
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
Data Link
In Network Y:
R1-R2
Two Destination Addresses:
Packet: Host B (Destination Host)
Frame: Router R2
To
Network Z
Router R2
Frame Y
Packet
Network Y
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Figure 2-10: Internet and Data Link Layers in an
Internet, Continued
Data Link
R2-B
Frame Z
Packet
Switch
Z1
Host
B
Router R2
In Network Z:
Two DestinationSwitch
Addresses:
Packet: Host B (Destination
Host)
Z2
Frame: Host B
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-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
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Figure 2-11: Internet and Transport Layer
Standards, Continued
Client PC
2.
Transport Layer
end-to-end (host-to-host)
TCP is connection-oriented, reliable
UDP is connectionless and unreliable
Server
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
Client PC
Webserver
Application
Webserver
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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
• Application (5)
Be able to repeat
this in your sleep!
• Transport (4)
• Internet (3)
• Data Link (2)
• Physical (1)
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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
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Figure 2-14: Ethernet Frame
Preamble (7 octets) 10101010 …
Header
Start of Frame Delimiter
(1 octet) 10101011
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
Data Field
(variable
length)
LLC Subheader
(usually 7 octets)
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
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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
Bit 0
The IP packet is drawn 32 bits to a line
Header
Version
Length
(4 bits)
(4 bits)
Diff-Serv
(8 bits)
Identification
(16 bits)
Time to Live
Protocol
(8 bits)
(8 bits)
Version is Bits 0-3
Bit 31
Total Length
(16 bits)
Flags
(3 bits)
Fragment Offset
(13 bits)
Header Checksum (16 bits)
Identification is Bits 32-47
Header length is Bits 4-7
Diff Serv is Bits 8-15
Time to live is Bits 48-55
Total Length is Bits 16-31
2-47
Figure 2-15: Internet Protocol (IP) Packet
Bit 0
Bit 31
Version
Header
Length
Diff-Serv
Identification
Time to Live
Protocol
Total Length
Flags
Fragment Offset
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
2-48
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. 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.
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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 217)
– Simple Mail Transfer Protocol (SMTP) for e-mail
– More than a dozen messages must be exchanged to
send an e-mail message
2-52
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
HTTP
Message
Passes Message
Down to Transport Process
Transport
Process
HTTP TCP
Message Hdr
Encapsulation of HTTP Message
in Data Field of TCP Segment
2-54
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
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Figure 2-18: Layered Communication on the
Source Host, Continued
Transport
Process
Internet
Process
HTTP TCP
Message Hdr
HTTP TCP IP
Message Hdr Hdr
Encapsulation of TCP Segment
in Data Field of IP Packet
2-56
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
2-57
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.
2-58
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
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
2-59
Figure 2-19: Decapsulation on the Destination
Host
Eth
HTTP TCP IP Eth
Trlr Message Hdr Hdr Hdr
Data Link
Process
Physical
Process
2-60
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
2-61
Figure 2-19: Decapsulation on the Destination
Host, Continued
HTTP TCP
Message Hdr
HTTP TCP IP
Message Hdr Hdr
Transport
Process
Internet
Process
Decapsulation of TCP Segment
from Data Field of IP Packet
2-62
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
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Figure 2-20: Layered End-to-End Communication
Source and
Destination
Hosts Have
5 Layers
App
Trans
Int
DL
Phy
Source
Host
Switches
Have Two
Layers
--Each Switch
Port
Has One
Layer (1)
Switch
1
Switch
2
Routers
Have Three
Layers
--Each Router
Port
Has Two
Layers (1&2)
Router
1
Switch
3
Router Destination
2
Host
2-64
Figure 2-21: Combining Horizontal and Vertical
Communication
Hypertext Transfer Protocol
App
Transmission Control Protocol
Trans
Int
Internet Protocol
DL
Phy
Source
Host
Switch
1
Switch
2
Router
1
Switch
3
Router Destination
Host
2
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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
Internetworking
Transmission
within a single
LAN or WAN
TCP/IP
Application
Application
(Layer 5)
Presentation
Application
Session
Transport (Layer 4)
Transport
Transport
Internet (Layer 3)
Network
Internet
Data Link (Layer 2)
Data Link
Physical (Layer 1)
Physical
Use OSI
Standards Here
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Figure 2-23: OSI and TCP/IP
Standards
Agency or
Agencies
OSI
TCP/IP
ISO (International
Organization for
Standardization)
IETF (Internet
Engineering Task
Force)
ITU-T (International
Telecommunications
Union—
Telecommunications
Standards Sector)
2-68
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)
2-69
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
2-70
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
2-75
Figure 2-26: Characteristics of Protocols
Discussed in the Chapter
Layer
Protocol
ConnectionOriented
/Connectionless
Reliable/
Unreliable
5 (App)
HTTP
Connectionless
Unreliable
4 (Transport)
TCP
Connectionoriented
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
2-76
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
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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
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Topics Covered
• Hybrid TCP/IP-OSI Standards Architecture
– 5. Application layer (application-to-application)
– 4. Transport layer (host-to-host)
– 3. Internet layer (across an internet)
– 2. Data link layer (across a switched network)
– 1. Physical layer (between adjacent devices)
2-80
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
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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
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Topics Covered
• Not All Devices Have All Layers
– Hosts have all five
– Routers have only the lowest three
– Switches have only the lowest two
2-83
Topics Covered
• OSI Architecture
– Divides application layer into three layers
• Session
• Presentation
• Application
• Other Standards Architectures
– IPX/SPX
– SNA
– AppleTalk
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