review fundamental concepts of networking. (read chapter 1 and chapter 7, tanenbaum)
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ReviewReview
Fundamental Concepts of Networking.
(Read chapter 1 and chapter 7, Tanenbaum)
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COMP 6320 Basics of Computer Networks Saad Biaz 2
IntroductionIntroduction
A network is a set of independent devices which can communicate.
Implications– Vendor independent– Hardware independent– OS independent– …..
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Principle!Principle!
To communicate, we must use– Common language– Common set of rules
Different levels of communications
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A NetworkA Network
Rou.
telnet photon….
R
R
R
DNS Server Rou.
R photon…
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Two Types of Communication EntitiesTwo Types of Communication Entities End-to-end:
– Used only at the end points of communication sessions Point to point:
– Used on every relaying device
Server
RoutersClient
E2E
E2EPt2Pt
Pt2Pt
Pt2PtPt2Pt
Pt2Pt
Pt2Pt Pt2Pt
Pt2PtPt2PtPt2Pt
Switch/Bridge
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End-to-End Protocols End-to-End Protocols (Examples)(Examples)
Applications: HTTP, FTP, Telnet, DNS
Communication tools: TCP, UDP..
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Point to PointPoint to Point
Protocols: IEEE 802.x., HDLC, Bluetooth, SLIP, PPP
Devices: Ethernet NIC (IEEE 802.3),
wireless card (IEEE 802.11), DSL, modem, ATM switch
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Some RemarksSome Remarks
The transmission of one packet involves– Many different agents– At many different levels
Agents on the same host may request services from agents immediately close to them
Agents on different machines communicate ONLY PEER TO PEER
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A Logical View
Rou.
R
DNS Server Rou.
R photon…
DNS
DNS
UDP
UDP
Network Int. Card
NIC1 NIC2
R R
IP
IP
IP
IPNetwork Int. Card NIC1 NIC2
telnet
telnet
TCP
TCP UDP
ARP IP
Network Int. Card
ARP
ARP
ARP ARP
E2E
E2E
E2E
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Review OutlineReview Outline
End-to-End:– General principles and techniques– Focus on Internet– Upper level lower level
Point to point– General principles– Focus on Internet– Lower level – upper level
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ConclusionConclusion
Communications and Networking are complex
To solve the problem and design good solutions, the problem must broken in different levels.
The difficulty is : What are the different levels? What are the functions of different levels?
The OSI Reference ModelThe OSI Reference Model((Open Systems InterconnectionOpen Systems Interconnection))
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Communications: Communications: A Complex Problem?A Complex Problem?Divide and Conquer..Divide and Conquer..
Communication should be broken in layers such that:– Each layer has a well defined function– Adjacent layers have neat interfaces– Adjacent layers exchange minimal information– Each layer hides details from upper layers– A layer contains similar functions
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Decomposing Communication Decomposing Communication in Layersin Layers
International effort led by ISO (International Standards Organization)
Economical conflicting interests Result: no viable stack implementation, but a nice
formal model good for education and networking experts communication
Neat definition of:– Functions at each layer– Interfaces between adjacent layers
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OSI Reference ModelOSI Reference ModelApplication Application
Presentation Presentation
Interface Interface
Session Session
Interface Interface
Transport Transport
Interface Interface
Protocol
Protocol
Protocol
Protocol
Interface Interface
Interface Interface
Network Network
Link Link
Interface Interface
Phys. Phys
InterfaceInterfaceLink Link
InterfaceInterface
Phys. Phys
NetworkNetwork
User/Client User/Server
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RemarksRemarks
End-to-end layers (protocols):– Application– Presentation– Session– Transport
Point-to-point layers (protocols):– Network– Link– Physical
Application
Presentation
Session
Transport
Network
Link
Physical
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ImplicationsImplications
End-to-end layers: implemented (active) only on the end points.
Point-to-point: implemented throughout ALL the path, on endpoint nodes and intermediary nodes
Application
Presentation
Session
Transport
Network
Link
Physical
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Application LayerApplication Layer
End to endCommunication services provided directly
to the user (or by server):– File transfer– Email– http– X400….
Application
Presentation
Session
Transport
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Presentation LayerPresentation LayerEnd to endThe two communicating entities may be
different (different vendors, different OS, different CPUs)
Application
System M1
Application
System M2
Presentation
Communication System
PresentationM1 Representation M2 Representation
COMMON Representation COMMON Representation
Translation Translation
Application
Presentation
Session
Transport
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Presentation Layer (Cont’d):Presentation Layer (Cont’d):Examples of Common Rep’sExamples of Common Rep’s
Network byte order: (on then wire order)– Little endian– Big endian
Floating point numbers: IEEE 754
Application
Presentation
Session
Transport
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Session LayerSession Layer
End to endAllows establishing communication
sessions:– File transfers under bad network conditions.– Optimize the use of a system
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Transport LayerTransport Layer End to end Hides/adapts defects and limitations of the network sys.
Upper Layers
1
2
3
4
5
Transport Layer
Network System(may loose, duplicate, reorder)
Upper Layers
1
5
3
Transport Layer
4
3
Too big message
Application
Presentation
Session
Transport
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Network LayerNetwork Layer
Point to point
– Addressing
– Routing
– Congestion control
– Administration
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Link LayerLink Layer
Point to point, has two fundamental functions:– Data Link Layer
Framing Error control Flow control
– Medium Access Control Layer
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Physical LayerPhysical Layer
Transmission of raw bitsDeal with:
– Electrical specs– Mechanical specs
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ConclusionConclusion
The OSI reference model is a nice conceptual model
There is no large scale implementation of an OSI protocol stack
The OSI model is largely used by networking people (conceptually)
A Network Protocol StackA Network Protocol Stack(Example)(Example)
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GeneralitiesGeneralities
Most existing protocol stacks have only 5 layers (set before OSI model):– TCP/IP– Novell
SNA (IBM proprietary architecture) has 7 layers
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TCP/IP: “Running code and some consensus”
Physical Layer Ethernet Token Ring 802.11
Link Layer Ethernet Token Ring 802.11
Network Layer IP (and some “helpers”)
Transport Layer TCP UDP
Application Layer SMTP FTP HTTP DNS Telnet ..
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TCP/IP StackTCP/IP StackApplication layer:
– SMTP: Simple Mail Transfer Protocol (Port 25)– FTP : File Transfer Protocol (Port 21)– Telnet : Terminal… (Port 23)– HTTP: HyperText Transfer Protocol (Port 80)– DNS : Domain Name System (port 53)
Transport layer:– UDP: User Datagram Protocol (Protocol # 17)– TCP: Transmission Control P. (Protocol # 6)
Network layer:– IP : Internet Protocol
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ConclusionConclusion
You must understand:– Layering, its purpose, its use in networking– At least role and “position” of each layer in the
stack – Peer to peer communication– Layer- to immediate layer communication– Distinction between end-to-end and point-point
Application LayerApplication Layer
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Where in the OSI Reference Where in the OSI Reference Model?Model?
Application Application
Presentation Presentation
Interface Interface
Session Session
Interface Interface
Transport Transport
Interface Interface
Protocol
Protocol
Protocol
Protocol
Interface Interface
Interface Interface
Network Network
Link Link
Interface Interface
Phys. Phys
InterfaceInterfaceLink Link
InterfaceInterface
Phys. Phys
NetworkNetwork
User/Client User/Server
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How Do Lower Layers Appear to How Do Lower Layers Appear to the Application Layer?the Application Layer?
Application Application
Interface Interface
ProtocolUser/Client User/Server
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Functions of the Application Functions of the Application LayerLayer
Specialized service to the userSet services not offered by lower layers Set services offered by lower layers but not
reliable or not trustedImprove services offered by lower layers
but incomplete
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Specialized Service To the Specialized Service To the UserUser
Virtual chat roomWeb browserFile transferEmailRemote terminalMapping host names to IP addresses…………
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Set Services Not Offered by Lower Set Services Not Offered by Lower LayersLayers
Want to set a reliable connectionless service over the internet that corrects only errors “recoverable” in less than 20ms:– Use TCP? Bad idea because of handshake, congestion
control, of own retransmissions that cannot be controlled, coarse timer…
Enforce secrecy (do not want nosy people to see my messages)
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Set Services Offered by Lower Set Services Offered by Lower Layers but Not Reliable or not Layers but Not Reliable or not
TrustedTrustedBill Gates offers the DoJ:A FREE special version to XP to guarantee
the secrecy of proceedings relative to the Antitrust with encryption at TCP level.
ANDFree Internet access through MSNAND an innocent smile
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Improve Services Offered by Improve Services Offered by Lower Layers but IncompleteLower Layers but Incomplete
Suppose that we notice that an audio or video stream sent over UDP suffers batches of corrupted bytes up to 5 contiguous bytes every 100 bytes…..
Internet Applications Internet Applications Protocols:Protocols:
DNS, Telnet, SMTP, HTTPDNS, Telnet, SMTP, HTTP
Domain Name System (DNS)Domain Name System (DNS)RFC 1034 and RFC 1035RFC 1034 and RFC 1035
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DNS: What ?
• It is a service used to map host names into IP addresses for:– Web URLs– Email addresses
– Based on a Client-Server architecture– Each machine connected to the net must know the
IP address of its DNS server.
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DNS: Why ?DNS: Why ?
Human do not like to use IP addresses (as numbers)
Human want prefer to designate machines with names
Interconnecting devices on the net use IP addresses (as numbers)
Huge number of hosts on the Internet
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DNS: How ?DNS: How ?
Use a file with all machine names and mapping?
Use a local server that contains the database related to all hosts on the Internet?
Use a distributed database system
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DNS: A Hierarchical Distributed DNS: A Hierarchical Distributed DatabaseDatabase
Root server administered by the Internet Corporation for Assigned Names and Numbers (ICANN)
Who administers the rest?
int com edu org gov mil net us fr jp ……….
tamu auburn
cs eng
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DNS Servers DNS Servers (Port 53)(Port 53)
One DNS server per zoneA DNS server is organized as:
– A primary server (with a database) – One to many secondary servers (reliability and
distribution of load)May be delegated or not: a CS department
may want to maintain its own DNS. General network administrator does not want!!!!
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What Does the DNS Client Do?What Does the DNS Client Do?
It prepares a name resolution request based on the DNS protocol– What is the IP address of “photon.cs.tamu.edu”?
It specifies :– The port number of the DNS server (53 well-known)
– The IP address of the DNS server (configuration)
DNS Request
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What Does the UDP Transport Agent What Does the UDP Transport Agent Do?Do?
It encapsulates the DNS request with:– The DNS client port number (source)– The DNS server port number (53 destination)– Some extra information used by UDP
DNS RequestUDP Header
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What Does the IP Network Agent Do?What Does the IP Network Agent Do?
IP encapsulates the UDP datagram with:– The DNS client IP address (configuration)– The DNS server IP address (configuration)– UDP stamp (The payload of the IP packet is a UDP
datagram)– Some extra information used by IP
It finds the next host to which to send the packet
DNS RequestUDP HeaderIP Header
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What Does the IP Network Agent Do? What Does the IP Network Agent Do? (2)(2)
To find the next host:– It checks if the final destination IPD is local (attached to
local segment)– If the final destination is not local:
Then IP looks into a routing table to find the IP address of next hop
Else… IP must find the local host that has IP address IPD
DNS RequestUDP HeaderIP Header
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strangelove.eng.auburn.edu host
Telnet software (Client)
Address Resolver
DNS software (Client)
Forms a DNS Request
UDP transport agent
Encapsulates DNS Request
IP Network agent
Finds next host
How to Resolve IP Address Local Address?
ARPIP add.
Local add.
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Name Resolution (Name Name Resolution (Name IP IP address)address)
Telnet software (Client)
Address Resolver
strangelove.eng.auburn.edu host
DNS software (Client)
Forms a DNS Request
UDP transport agent
Encapsulates DNS Request
IP Network agent
Finds next host
DNS Request
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strangelove.eng.auburn.edu host
Telnet software (Client)
Address Resolver
DNS software (Client)
Forms a DNS Request
UDP transport agent
Encapsulates DNS Request
IP Network agent
Finds next host
How to Resolve IP Address Local Address?
ARPIP add.
Local add.
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DNS software (Server)
Process DNS Request
UDP transport agent
Decapsulates DNS Request
Port?
IP Network agent
Is it for me?What stamp? UDP?
Query gets to the DNS Server
DNS RequestUDP HdrIP Header
DNS RequestUDP Hdr
DNS Request
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DNS Server Responds
DNS software (Server)
Forms DNS Response DNS Response
UDP Hdr
UDP transport agent
Encapsulates DNS Request
DNS Response
IP Header
IP Network agent
Is it for me?What stamp? UDP?
UDP HdrDNS Response
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UDP transport agent
Decapsulates DNS Request
Port?
IP Network agent
Is it for me?What stamp? UDP?
On Host strangelove.eng.auburn.edu
DNS ResponseUDP HdrIP Header
DNS ResponseUDP Hdr
DNS Response
Telnet software (Client)
Address Resolver
DNS software (Client)
Process DNS Response
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Requesting the IP of Requesting the IP of bach.cs.tamu.edu from bach.cs.tamu.edu from mallard.eng.auburn.edumallard.eng.auburn.edu
mallard dns.duc.auburn.edu dns.auburn.edu
Server for .edu Domaindns.tamu.edudns for cs.tamu.edu
Request
Response
TelnetTelnetRFC 854RFC 854
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Telnet: the Big PictureTelnet: the Big Picture(Port 23)(Port 23)
TelnetClient
Terminal Driver
TCP/IP
User at terminal
TelnetServer
Pseudo-Terminal Driver
TCP/IP
One TCP connection
Login Shell
Kernel Kernel
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Telnet to a Port other than 23Telnet to a Port other than 23
TelnetClient
Terminal Driver
TCP/IP
User at terminal
Any stream server
TCP/IP
One TCP connection
Kernel Kernel
SMTPSMTPRFC 822RFC 822
Read 7.2
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Electronic MailElectronic MailUser
User AgentQueue of mail
to be sent
Local Mail Transfer Agent
RelayMTA
Sending Host
RelayMTA
Local Mail Transfer Agent
Queue of mail to be sent
User Agent
UserReceiving Host
Organization 1
Organization 2
Internet
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ConclusionConclusion
You must know– The key Internet application protocols– Their functions– Port number binding– Role in the networking stack
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