Chapter 2 Protocols and Architecture

1
Chapter 2Protocols and Architecture

• William StallingsData and Computer
  Communications 7th Edition
• Leon-Gacia and W. Jaja
• Communication Networks-Fundamental Concepts and
  Key Architecture Chapter 2.1

2
1. The caller picks up the phone triggering the
flow of current in wires that connect to the
telephone office.
1.
2. The current is detected, and a dial tone is
transmitted by the telephone office to indicate
that it is ready to receive the destination
number.
2.
Connection setup
3. The caller sends this number by pushing the
keys on the telephone set. Each key generates a
pair of tones that specify a number. (In the
older phone sets, the user dials a number that in
turn generates a corresponding number of pulses.)
3.
4. The equipment in the telephone office then
uses the telephone network to attempt a
con-nection. If the destination telephone is
busy, then a busy tone is returned to the caller
otherwise ringing signals are sent to both the
originating and destination telephones. The
ringing signals are discontinued when the
destination phone is picked up and communication
can then proceed.
4.
Information transfer
5.
5. The voice signals travel in both directions.
Connection release
6. Either user terminates the call by putting
down a receiver.
6.
Figure 1.5
3
Figure 1.6
4
The user clicks on a link to indicate which
document is to be retrieved. The browser must
determine the Internet address of the machine
that contains the document. To do so, the browser
sends a query to its local name server.
Step
1.
Once the address is known, the browser
establishes a connection to the server process in
the specified machine, usually a TCP connection.
For the connection to be successful, the
specified machine must be ready to accept TCP
connections.
2.
3.
The browser runs a client version of HTTP, which
issues a request specifying both the name of the
document and the possible document formats it can
handle.
The machine that contains the requested document
runs a server version of HTTP. It reacts to the
HTTP request by sending an HTTP response which
contains the desired document in the appropriate
format.
4. 6.
7. 8.
The user may start to view the document. The TCP
connection is closed after a certain timeout
period.
Figure 2.1
5
Need For Protocol Architecture

• E.g. File transfer
• Source must activate comms. Path or inform
  network of destination
• Source must check destination is prepared to
  receive
• File transfer application on source must check
  destination file management system will accept
  and store file for his user
• May need file format translation
• Task broken into subtasks
• Implemented separately in layers in stack
• Functions needed in both systems
• Peer layers communicate

6
Key Elements of a Protocol

• Syntax
• Data formats
• Signal levels
• Semantics
• Control information
• Error handling
• Timing
• Speed matching
• Sequencing

7
Protocol Architecture

• Task of communication broken up into modules
• For example file transfer could use three modules
• File transfer application
• Communication service module
• Network access module

8
Simplified File Transfer Architecture
9
A Three Layer Model

• Network Access Layer
• Transport Layer
• Application Layer

10
Network Access Layer

• Exchange of data between the computer and the
  network
• Sending computer provides address of destination
• May invoke levels of service
• Dependent on type of network used (LAN, packet
  switched etc.)

11
Transport Layer

• Reliable data exchange
• Independent of network being used
• Independent of application

12
Application Layer

• Support for different user applications
• e.g. e-mail, file transfer

13
Protocol Architectures and Networks
14
Addressing Requirements

• Two levels of addressing required
• Each computer needs unique network address
• Each application on a (multi-tasking) computer
  needs a unique address within the computer
• The service access point or SAP
• The port on TCP/IP stacks

15
Protocols in Simplified Architecture
16
Protocol Data Units (PDU)

• At each layer, protocols are used to communicate
• Control information is added to user data at each
  layer
• Transport layer may fragment user data
• Each fragment has a transport header added
• Destination SAP
• Sequence number
• Error detection code
• This gives a transport protocol data unit

17
Protocol Data Units
18
Network PDU

• Adds network header
• network address for destination computer
• Facilities requests

19
Operation of a Protocol Architecture
20
Standardized Protocol Architectures

• Required for devices to communicate
• Vendors have more marketable products
• Customers can insist on standards based equipment
• Two standards
• OSI Reference model
• Never lived up to early promises
• TCP/IP protocol suite
• Most widely used
• Also IBM Systems Network Architecture (SNA)

21
OSI Unified View Protocols

• Layer n in one machine interacts with layer n in
  another machine to provide a service to layer n
  1
• The entities comprising the corresponding layers
  on different machines are called peer processes.
• The machines use a set of rules and conventions
  called the layer-n protocol.
• Layer-n peer processes communicate by exchanging
  Protocol Data Units (PDUs)

n-PDUs
n Entity

n Entity
Layer n peer protocol
22
OSI Unified View Services

• Communication between peer processes is virtual
  and actually indirect
• Layer n1 transfers information by invoking the
  services provided by layer n
• Services are available at Service Access Points
  (SAPs)
• Each layer passes data control information to
  the layer below it until the physical layer is
  reached and transfer occurs
• The data passed to the layer below is called a
  Service Data Unit (SDU)
• SDUs are encapsulated in PDUs

23
Layers, Services Protocols
n1 entity
n1 entity
n-SDU
n-SDU
n-SAP
n-SAP
n-SDU
H
n entity
n entity
n-SDU
H
n-PDU
24
OSI

• Open Systems Interconnection
• Developed by the International Organization for
  Standardization (ISO)
• Seven layers
• A theoretical system delivered too late!
• TCP/IP is the de facto standard

25
OSI - The Model

• A layer model
• Each layer performs a subset of the required
  communication functions
• Each layer relies on the next lower layer to
  perform more primitive functions
• Each layer provides services to the next higher
  layer
• Changes in one layer should not require changes
  in other layers

26
OSI Layers
27
The OSI Environment
28
OSI as Framework for Standardization
29
Layer Specific Standards
30
Layers, Services Protocols
n1 entity
n1 entity
n-SDU
n-SDU
n-SAP
n-SAP
n-SDU
H
n entity
n entity
n-SDU
H
n-PDU
31
Elements of Standardization

• Protocol specification
• Operates between the same layer on two systems
• May involve different operating system
• Protocol specification must be precise
• Format of data units
• Semantics of all fields
• allowable sequence of PCUs
• Service definition
• Functional description of what is provided
• Addressing
• Referenced by SAPs

32
OSI Layers (1)

• Physical
• Physical interface between devices
• Mechanical
• Electrical
• Functional
• Procedural
• Data Link
• Means of activating, maintaining and deactivating
  a reliable link
• Error detection and control
• Higher layers may assume error free transmission

33
OSI Layers (2)

• Network
• Transport of information
• Higher layers do not need to know about
  underlying technology
• Not needed on direct links
• Transport
• Exchange of data between end systems
• Error free
• In sequence
• No losses
• No duplicates
• Quality of service

34
OSI Layers (3)

• Session
• Control of dialogues between applications
• Dialogue discipline
• Grouping
• Recovery
• Presentation
• Data formats and coding
• Data compression
• Encryption
• Application
• Means for applications to access OSI environment

35
Application Upper Layers

• Application Layer Provides services that are
  frequently required by applications DNS, web
  acess, file transfer, email
• Presentation Layer machine-independent
  representation of data
• Session Layer dialog management, recovery from
  errors,

Incorporated into Application Layer
36
Headers Trailers

• Each protocol uses a header that carries
  addresses, sequence numbers, flag bits, length
  indicators, etc
• CRC check bits may be appended for error detection

Application
Application
APP DATA
Application Layer
Application Layer
Transport Layer
Transport Layer
Network Layer
Network Layer
Data Link Layer
Data Link Layer
Physical Layer
Physical Layer
bits
37
Use of a Relay
38
Service Primitives and Parameters

• Services between adjacent layers expressed in
  terms of primitives and parameters
• Primitives specify function to be performed
• Parameters pass data and control info

39
Primitive Types
40
Interlayer Interaction
layer
N1 user
N provider
N provider
N1 user
System A
System B
41
Timing Sequence for Service Primitives
42
Connectionless Connection-Oriented Services

• Connection-Oriented
• Three-phases
• Connection setup between two SAPs to initialize
  state information
• SDU transfer
• Connection release
• E.g. TCP, ATM

• Connectionless
• Immediate SDU transfer
• No connection setup
• E.g. UDP, IP
• Layered services need not be of same type
• TCP operates over IP
• IP operates over ATM

43
TCP/IP Protocol Architecture

• Developed by the US Defense Advanced Research
  Project Agency (DARPA) for its packet switched
  network (ARPANET)
• Used by the global Internet
• No official model but a working one.
• Application layer
• Host to host or transport layer
• Internet layer
• Network access layer
• Physical layer
• 3 Layer Model (4 Layer model

44
Physical Layer

• Physical interface between data transmission
  device (e.g. computer) and transmission medium or
  network
• Characteristics of transmission medium
• Signal levels
• Data rates
• etc.

45
Network Access Layer

• Exchange of data between end system and network
• Destination address provision
• Invoking services like priority

46
Internet Layer (IP)

• Systems may be attached to different networks
• Routing functions across multiple networks
• Implemented in end systems and routers

47
Transport Layer (TCP,UDP)

• Reliable Stream Service
• Reliable delivery of data
• Ordering of delivery
• Unreliable Block Service

48
Application Layer

• Support for user applications
• e.g. http, SMPT

49
OSI v TCP/IP
50
TCP

• Usual transport layer is Transmission Control
  Protocol
• Reliable connection
• Connection
• Temporary logical association between entities in
  different systems
• TCP PDU
• Called TCP segment
• Includes source and destination port (c.f. SAP)
• Identify respective users (applications)
• Connection refers to pair of ports
• TCP tracks segments between entities on each
  connection

51
UDP

• Alternative to TCP is User Datagram Protocol
• Not guaranteed delivery
• No preservation of sequence
• No protection against duplication
• Minimum overhead
• Adds port addressing to IP

52
TCP/IP Concepts
53
Internetworking
Ethernet LAN

• Internetworking is part of network layer and
  provides transfer of packets across multiple
  possibly dissimilar networks
• Gateways (routers) direct packets across networks

H
H
G
Net 1
G
G
G
Net 5
Net 2
G
G
H
H
G gateway H host
54
Addressing level

• Level in architecture at which entity is named
• Unique address for each end system (computer) and
  router
• Network level address
• IP or internet address (TCP/IP)
• Network service access point or NSAP (OSI)
• Process within the system
• Port number (TCP/IP)
• Service access point or SAP (OSI)

55
Trace of Simple Operation

• Process associated with port 1 in host A sends
  message to port 2 in host B
• Process at A hands down message to TCP to send to
  port 2
• TCP hands down to IP to send to host B
• IP hands down to network layer (e.g. Ethernet) to
  send to router J
• Generates a set of encapsulated PDUs

56
Example HTTP

• HTTP is an application layer protocol
• Retrieves documents on behalf of a browser
  application program
• HTTP specifies fields in request messages and
  response messages
• Request types Response codes
• Content type, options, cookies,
• HTTP specifies actions to be taken upon receipt
  of certain messages

57
HTTP Protocol
GET
HTTP Client
HTTP Server
Response

• HTTP assumes messages can be exchanged directly
  between HTTP client and HTTP server
• In fact, HTTP client and server are processes
  running in two different machines across the
  Internet
• HTTP uses the reliable stream transfer service
  provided by TCP

58
Example TCP

• TCP is a transport layer protocol
• Provides reliable byte stream service between two
  processes in two computers across the Internet
• Sequence numbers keep track of the bytes that
  have been transmitted and received
• Error detection and retransmission used to
  recover from transmission errors and losses
• TCP is connection-oriented the sender and
  receiver must first establish an association and
  set initial sequence numbers before data is
  transferred
• Connection ID is specified uniquely by
• (send port , send IP address, receive port ,
  receiver IP address)

59
HTTP uses service of TCP
HTTP server
HTTP client
Response
GET
Port 80
Port 1127
TCP
GET
Response
TCP
60
PDUs in TCP/IP
61
Example Header Information

• Destination port
• Sequence number
• Checksum

62
Some Protocols in TCP/IP Suite
63
TCP/IP Protocol Suite
HTTP
DNS
SMTP
RTP
Distributed applications
User datagram service
Reliable stream service
TCP
UDP
Best-effort connectionless packet transfer
(ICMP, ARP)
Diverse network technologies
64
Internet Names Addresses

• Internet Names
• Each host a a unique name
• Independent of physical location
• Facilitate memorization by humans
• Domain Name
• Organization under single administrative unit
• Host Name
• Name given to host computer
• User Name
• Name assigned to user
• leongarcia_at_comm.utoronto.ca

• Internet Addresses
• Each host has globally unique logical 32 bit IP
  address
• Separate address for each physical connection to
  a network
• Routing decision is done based on destination IP
  address
• IP address has two parts
• netid and hostid
• netid unique
• netid facilitates routing
• Dotted Decimal Notation
• int1.int2.int3.int4
• (intj jth octet)
• 128.100.10.13

DNS resolves IP name to IP address
65
Physical Addresses

• LANs (and other networks) assign physical
  addresses to the physical attachment to the
  network
• The network uses its own address to transfer
  packets or frames to the appropriate destination
• IP address needs to be resolved to physical
  address at each IP network interface
• Example Ethernet uses 48-bit addresses
• Each Ethernet network interface card (NIC) has
  globally unique Medium Access Control (MAC) or
  physical address
• First 24 bits identify NIC manufacturer second
  24 bits are serial number
• 009027966807 12 hex numbers

Intel
66
Chapter 2.1 Applications and Layered
Architectures

• Protocols, Services Layering

67
Layers, Services Protocols

• The overall communications process between two or
  more machines connected across one or more
  networks is very complex
• Layering partitions related communications
  functions into groups that are manageable
• Each layer provides a service to the layer above
• Each layer operates according to a protocol
• Lets use examples to show what we mean

68
Web Browsing Application

• World Wide Web allows users to access resources
  (i.e. documents) located in computers connected
  to the Internet
• Documents are prepared using HyperText Markup
  Language (HTML)
• A browser application program is used to access
  the web
• The browser displays HTML documents that include
  links to other documents
• Each link references a Uniform Resource Locator
  (URL) that gives the name of the machine and the
  location of the given document
• Lets see what happens when a user clicks on a
  link

69
The user clicks on a link to indicate which
document is to be retrieved. The browser must
determine the Internet address of the machine
that contains the document. To do so, the browser
sends a query to its local name server.
Step
1.
Once the address is known, the browser
establishes a connection to the server process in
the specified machine, usually a TCP connection.
For the connection to be successful, the
specified machine must be ready to accept TCP
connections.
2.
3.
The browser runs a client version of HTTP, which
issues a request specifying both the name of the
document and the possible document formats it can
handle.
The machine that contains the requested document
runs a server version of HTTP. It reacts to the
HTTP request by sending an HTTP response which
contains the desired document in the appropriate
format.
4. 6.
7. 8.
The user may start to view the document. The TCP
connection is closed after a certain timeout
period.
Figure 2.1
70
1. DNS
A. 64.15.247.200
Q. www.nytimes.com?

• User clicks on http//www.nytimes.com/
• URL contains Internet name of machine
  (www.nytimes.com), but not Internet address
• Internet needs Internet address to send
  information to a machine
• Browser software uses Domain Name System (DNS)
  protocol to send query for Internet address
• DNS system responds with Internet address

71
2. TCP
ACK
ACK, TCP Connection Request From 64.15.247.200
Port 80 To128.100.11.13 Port 1127
TCP Connection Request From 128.100.11.13 Port
1127 To 64.15.247.200 Port 80

• Browser software uses HyperText Transfer Protocol
  (HTTP) to send request for document
• HTTP server waits for requests by listening to a
  well-known port number (80 for HTTP)
• HTTP client sends request messages through an
  ephemeral port number, e.g. 1127
• HTTP needs a Transmission Control Protocol (TCP)
  connection between the HTTP client and the HTTP
  server to transfer messages reliably

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3. HTTP
Content
200 OK
GET / HTTP/1.1

• HTTP client sends its request message GET
• HTTP server sends a status response 200 OK
• HTTP server sends requested file
• Browser displays document
• Clicking a link sets off a chain of events across
  the Internet!
• Lets see how protocols layers come into play

73
Protocols

• A protocol is a set of rules that governs how two
  or more communicating entities in a layer are to
  interact
• Messages that can be sent and received
• Actions that are to be taken when a certain event
  occurs, e.g. sending or receiving messages,
  expiry of timers
• The purpose of a protocol is to provide a service
  to the layer above

74
Layers

• A set of related communication functions that can
  be managed and grouped together
• Application Layer communications functions that
  are used by application programs
• HTTP, DNS, SMTP (email)
• Transport Layer end-to-end communications
  between two processes in two machines
• TCP, User Datagram Protocol (UDP)
• Network Layer node-to-node communications
  between two machines
• Internet Protocol (IP)

75
Example HTTP

• HTTP is an application layer protocol
• Retrieves documents on behalf of a browser
  application program
• HTTP specifies fields in request messages and
  response messages
• Request types Response codes
• Content type, options, cookies,
• HTTP specifies actions to be taken upon receipt
  of certain messages

76
HTTP Protocol
GET
HTTP Client
HTTP Server
Response

• HTTP assumes messages can be exchanged directly
  between HTTP client and HTTP server
• In fact, HTTP client and server are processes
  running in two different machines across the
  Internet
• HTTP uses the reliable stream transfer service
  provided by TCP

77
Example TCP

• TCP is a transport layer protocol
• Provides reliable byte stream service between two
  processes in two computers across the Internet
• Sequence numbers keep track of the bytes that
  have been transmitted and received
• Error detection and retransmission used to
  recover from transmission errors and losses
• TCP is connection-oriented the sender and
  receiver must first establish an association and
  set initial sequence numbers before data is
  transferred
• Connection ID is specified uniquely by
• (send port , send IP address, receive port ,
  receiver IP address)

78
HTTP uses service of TCP
HTTP server
HTTP client
Response
GET
Port 80
Port 1127
TCP
GET
Response
TCP
79
Example DNS Protocol

• DNS protocol is an application layer protocol
• DNS is a distributed database that resides in
  multiple machines in the Internet
• DNS protocol allows queries of different types
• Name-to-address or Address-to-name
• Mail exchange
• DNS usually involves short messages and so uses
  service provided by UDP
• Well-known port 53

80
Local Name Server
Authoritative Name Server
1
5
4
2
3
6
Root Name Server

• Local Name Server resolve frequently-used names
• University department, ISP
• Contacts Root Name server if it cannot resolve
  query
• Root Name Servers 13 globally
• Resolves query or refers query to Authoritative
  Name Server
• Authoritative Name Server last resort
• Every machine must register its address with at
  least two authoritative name servers

81
Example UDP

• UDP is a transport layer protocol
• Provides best-effort datagram service between two
  processes in two computers across the Internet
• Port numbers distinguish various processes in the
  same machine
• UDP is connectionless
• Datagram is sent immediately
• Quick, simple, but not reliable

82
Summary

• Layers related communications functions
• Application Layer HTTP, DNS
• Transport Layer TCP, UDP
• Network Layer IP
• Services a protocol provides a communications
  service to the layer above
• TCP provides connection-oriented reliable byte
  transfer service
• UDP provides best-effort datagram service
• Each layer builds on services of lower layers
• HTTP builds on top of TCP
• DNS builds on top of UDP
• TCP and UDP build on top of IP

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Utility

• Ping Find the availability of remote device
• Tracert Find the path to remote device
• Route Show routing table
• tcpdump
• Ethereal
• Ipconfig
• Nslookup
• Ethereal

84
Required Reading

• Stallings chapter 2
• Leon-Gacia Chapter 2
• Suggest Reading
• Comer,D. Internetworking with TCP/IP volume I
• Comer,D. and Stevens,D. Internetworking with
  TCP/IP volume II and volume III, Prentice Hall
• Halsall, F. Data Communications, Computer
  Networks and Open Systems, Addison Wesley
• RFCs