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Network StandardsLayered Architectures
Chapter 2
Panko’s Business Data Networks and Telecommunications, 6th edition
Copyright 2007 Prentice-HallMay only be used by adopters of the book
1.Message Standards (Protocols)
2-3
Standards
Standards are rules of operationthat allow two hardware or software processes
to work together
Even if they are from different vendors
2-4
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
2-5
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
2-6
Figure 2-2: Hypertext Transfer Protocol (HTTP) Interactions
Client PC Webserver
Browser WebserverApplication
1.HTTP Request Message
Asking for a File
2.HTTP Response Message
Delivering the File
Semantics in HTTP, which governs the Web
2-7
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
2-8
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]
2-9
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.
HeaderData FieldTrailer
2-10
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
OtherHeader
FieldDestination
AddressField is
Used by Switches and RoutersLike the Address on an Envelope
Message withall three parts
2-11
Figure 2-4: General Message Organization, Continued
Data Field Header
OtherHeader
Field
DestinationAddress
Field
Message withouta trailer
Usually only data linklayer messages have trailers
2-12
Figure 2-4: General Message Organization, Continued
Header
OtherHeader
Field
DestinationAddress
Field
Message withonly a header
e.g.TCP supervisory
messages arepure headers
(there is no data field content to deliver)
2.Reliability
2-14
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
2-15
Figure 2-5: Reliable TCP Session, Continued
Client PCTCP Process
WebserverTCP Process
4. Data = HTTP Request
5. ACK (4)
6. Data = HTTP Response
7. ACK (6)
CarryHTTPReq &Resp
(4)
Request-ResponseCycle for Data Transfer
TCP Segment (Message) 4Carries an HTTP Request
Segment 5 Acknowledges It
There Is No Need to Resend
2-16
Figure 2-5: A TCP Session, Continued
Client PCTCP Process
WebserverTCP Process
CarryHTTPReq &Resp
(4)
8. Data = HTTP Request (Error)
9. Data = HTTP Request (No ACK so Retransmit)
10. ACK (9)
11. Data = HTTP Response
12. ACK (11)Error Handling
TCP Segment (Message) 8Is Lost in Transmission
There Is No Acknowledgment
So the Sender Retransmits It
3.Connection-Oriented andConnectionless Protocols
2-18
Figure 2-6: Connection-Oriented and Connectionless Protocols
Message(No Sequence Number)
Connectionless Protocol
A B
Message 1 (Seq. Num = A1)
Message 2 (Seq. Num = A2)
Close Connection
Connection-Oriented Protocol
Open ConnectionA B
Message 3 (Seq. Num B1)
Connection-oriented protocolsFormal openings and closings
Also have sequence numbersso that the receiver can putmessages in order
And so the receiver can sendAcknowledgments for specificmessages
2-19
Figure 2-6: Connection-Oriented and Connectionless Protocols, Continued
Client PCBrowser
WebserverApplication
HTTP Request
HTTP is connectionless
No OpeningsNo Closings
No Sequence NumbersNo Acknowledgments
2-20
Figure 2-6: Connection-Oriented and Connectionless Protocols, Continued
Client PCTCP Process
WebserverTCP Process
Connection-Opening Messages
Time
Connection-Closing Messages
Messages During the Connection
In TCP
2-21
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.
2-22
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
4.The Hybrid TCP/IP-OSI Standards Architecture
2-24
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
2-25
Figure 2-8: Hybrid TCP/IP-OSI Architecture
General Purpose Layer Specific Layer Purpose
Application-application communication
Application (5) Application-application interworking
Transmission across an internet
Transport (4) Host-host communication
Internet (3) Packet delivery across an internet
Transmission across a single network (LAN or WAN)
Data Link (2) Frame delivery across a network
Physical (1) Device-device connection
2-26
Figure 2-8: Hybrid TCP/IP-OSI Architecture, Continued
• Physical and Data Link Layer Standards
– Govern Communication Through a Single Network
– LAN or WAN
2-27
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
Switch X1
Physical LinkA-X1
Host A
Switch X2Physical LinkX1-X2
2-28
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
Switch X1Host A
Switch X2
Host BData LinkA-B
2-29
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
2-30
Figure 2-9: Physical and Data Link Layer Standards in a Single Network, Continued
Host A
Mobile ClientStation
ServerStation
Switch
SwitchX2
Switch X1
Switch
Data LinkA-R1
Physical LinkA-X1
PhysicalLink
X1-X2
Router R1
PhysicalLink
X2-R1
3 Physical Links1 Data Link2 Switches
2-31
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
2-32
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
2-33
Figure 2-10: Internet and Data Link Layers in an Internet, Continued
Host B
Host A
Network XNetwork Y
Network Z
R1
R2
Data Link A-R1
Data Link R3-B
DataLink
R1-R2Route A-B
3 Data Links: One per Network1 Route per Internet
2-34
Figure 2-10: Internet and Data Link Layers in an Internet, Continued
Host A
Mobile ClientStation
ServerStation
Switch
SwitchX2
SwitchX1
Switch
Data LinkA-R1
Router R1
Packet
Frame X
Network X
RouteA-B
In Network X:Two Destination Addresses:
Packet: Host B (Destination Host)Frame: Router R1
2-35
Figure 2-10: Internet and Data Link Layers in an Internet, Continued
Router R1
Router R2
Packet
Frame Y
ToNetwork X
ToNetwork Z
Network Y
Data LinkR1-R2
RouteA-B
In Network Y:Two Destination Addresses:
Packet: Host B (Destination Host)Frame: Router R2
2-36
Figure 2-10: Internet and Data Link Layers in an Internet, Continued
Host B
Mobile ClientStations
SwitchZ1
SwitchX2
SwitchZ2
PacketFrame Z
Network Z
Router R2
Router
Data LinkR2-B
In Network Z:Two Destination Addresses:
Packet: Host B (Destination Host)Frame: Host B
2-37
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
2-38
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
2-39
Figure 2-11: Internet and Transport Layer Standards, Continued
Transport Layerend-to-end (host-to-host)
TCP is connection-oriented, reliableUDP is connectionless and unreliable
Internet Layer(usually IP)
hop-by-hop (host-router or router-router)connectionless, unreliable
Router 1 Router 2 Router 3
Client PCServer
2-40
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
Webserver
Browser WebserverApplication
Client PC
2-41
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.
2-42
Standards Layers: Recap
• Application (5)
• Transport (4)
• Internet (3)
• Data Link (2)
• Physical (1)
2-43
Figure 2-13: Why Layer?
• 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
Box
5.Syntax Examples for Some Layer Messages
2-45
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 Bits10010111
2-46
…
Figure 2-14: Ethernet Frame
Preamble (7 octets) 10101010 …
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
Header
2-47
Figure 2-14: Ethernet Frame, Continued
Data Field(variablelength)
PAD (added if data field < 46 octets)
Frame Check Sequence (32 bits)
LLC Subheader(usually 7 octets)
UsuallyIP Packet
Trailer
EncapsulatedPacker
2-48
Figure 2-14: Ethernet Frame, Continued
• 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
Frame Check Sequence (32 bits)
2-49
Figure 2-15: Internet Protocol (IP) Packet, Continued
Total Length(16 bits)
Version(4 bits)
Diff-Serv(8 bits)
HeaderLength(4 bits)
Identification(16 bits)
Flags(3 bits)
Fragment Offset(13 bits)
Header Checksum (16 bits)Protocol(8 bits)
Time to Live(8 bits)
Bit 0 Bit 31
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
The IP packet is drawn 32 bits to a line
2-50
Figure 2-15: Internet Protocol (IP) Packet
Total LengthVersion Diff-ServHeaderLength
Source IP Address (32 bits)
Identification Flags Fragment Offset
Header ChecksumProtocolTime to Live
Bit 0 Bit 31
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
2-51
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
2-52
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
2-53
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.
2-54
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
6.Vertical Communication Between Layer Processes on the Same Host
2-56
Figure 2-18: Layered Communication on the Source Host
ApplicationProcess
HTTPMessage
TransportProcess
HTTPMessage
TCPHdr
Encapsulation of HTTP Messagein Data Field of TCP Segment
Passes MessageDown to Transport Process
The process begins when a browser creates an HTTP request message
2-57
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
2-58
Figure 2-18: Layered Communication on the Source Host, Continued
TransportProcess
HTTPMessage
InternetProcess
HTTPMessage
TCPHdr
TCPHdr
IPHdr
Encapsulation of TCP Segmentin Data Field of IP Packet
2-59
Figure 2-18: Layered Communication on the Source Host, Continued
InternetProcess
HTTPMessage
TCPHdr
IPHdr
Data LinkProcess
HTTPMessage
TCPHdr
IPHdr
EthHdr
EthTrlr
Encapsulation of IP Packetin Data Field of Ethernet Frame
2-60
Figure 2-18: Layered Communication on the Source Host, Continued
Data LinkProcess
HTTPMessage
TCPHdr
IPHdr
EthHdr
EthTrlr
Physical Process
Physical Layer converts the bits of the frame into signals.
2-61
Figure 2-18: Layered Communication on the Source Host, Continued
The following is the final frame for aan HTTP message on an Ethernet LAN
HTTPMessage
TCPHdr
IPHdr
EthHdr
EthTrlr
L5 L4 L3 L2L2
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
2-62
Figure 2-19: Decapsulation on the Destination Host
HTTPMessage
TCPHdr
IPHdr
EthHdr
EthTrlr
Data LinkProcess
Physical Process
2-63
Figure 2-19: Decapsulation on the Destination Host, Continued
HTTPMessage
TCPHdr
IPHdr
EthHdr
EthTrlr
Data LinkProcess
InternetProcess
HTTPMessage
TCPHdr
IPHdr
Decapsulation of IP Packetfrom Data Field of Ethernet Frame
2-64
Figure 2-19: Decapsulation on the Destination Host, Continued
InternetProcess
HTTPMessage
TCPHdr
IPHdr
TransportProcess
HTTPMessage
TCPHdr
Decapsulation of TCP Segmentfrom Data Field of IP Packet
2-65
Figure 2-19: Decapsulation on the Destination Host, Continued
TransportProcess
HTTPMessage
TCPHdr
ApplicationProcess
HTTPMessage
Decapsulation of HTTP Messagefrom Data Field of TCP Segment
2-66
Figure 2-20: Layered End-to-End Communication
Int
App
DL
Trans
Phy
SourceHost
DestinationHost
Switch1
Switch2
Router1
Switch3
Router2
Source andDestinationHosts Have
5 Layers
SwitchesHave Two
Layers---
Each SwitchPort
Has OneLayer (1)
RoutersHave Three
Layers---
Each RouterPort
Has TwoLayers (1&2)
2-67
Figure 2-21: Combining Horizontal and Vertical Communication
Int
App
DL
Trans
Phy
SourceHost
DestinationHost
Switch1
Switch2
Router1
Switch3
Router2
Hypertext Transfer Protocol
Transmission Control Protocol
Internet Protocol
7.OSI, TCP/IP, and Other Standards Architectures
2-69
Figure 2-22: The Hybrid TCP/IP-OSI Architecture
TCP/IPOSIHybrid TCP/IP-OSIBroad Purpose
Application
Application
Presentation
Session
Application(Layer 5)
Communicationbetweenapplications
Transport
Internet
Transport
Network
Transport (Layer 4)
Internet (Layer 3)Internetworking
Use OSI Standards Here
Data Link
Physical
Data Link (Layer 2)
Physical (Layer 1)
Transmissionwithin a singleLAN or WAN
2-70
Figure 2-23: OSI and TCP/IP
OSI TCP/IP
StandardsAgency or Agencies
ISO (InternationalOrganization for Standardization)
ITU-T (InternationalTelecommunicationsUnion—TelecommunicationsStandards Sector)
IETF (InternetEngineering TaskForce)
2-71
Figure 2-23: OSI and TCP/IP, Continued
OSI TCP/IP
Dominance Nearly 100% dominant at physical and datalink layers
70%-80% dominantat the internet and transportlayers.
Documents areCalled
Various Mostly RFCs (requestsfor comments)
2-72
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
2-73
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
2-74
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
2-75
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
2-76
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
2-77
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
2-78
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
2-79
Figure 2-26: Characteristics of Protocols Discussed in the Chapter
Layer Protocol Connection-Oriented/Connectionless
Reliable/Unreliable
5 (App) HTTP Connectionless Unreliable
4 (Transport) TCPConnection-oriented
Reliable
3 (Internet) IP Connectionless Unreliable
2 (Data Link) Ethernet Connectionless Unreliable
Note: Only TCP is connection-oriented and reliable
4 (Transport) UDP Connectionless Unreliable
8.Topics Covered
2-81
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
2-82
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
2-83
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)
2-84
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
2-85
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
2-86
Topics Covered
• Not All Devices Have All Layers
– Hosts have all five
– Routers have only the lowest three
– Switches have only the lowest two
2-87
Topics Covered
• OSI Architecture– Divides application layer into three layers
• Session• Presentation• Application
• Other Standards Architectures– IPX/SPX
– SNA
– AppleTalk
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