1. EEL 5718 Computer Communications
Chapter 2: Layering Architecture

2. Outline
Examples of layering
OSI Reference Model
TCP/IP suite

3. Reiteration of Design Philosophy
Overall design: must be considered using system theory—top-down approach
Goal or objective
Functionalities or features or break-down
Tools/budget
Systems diagram/flow chart
Implementation: bottom-up
Design all tasks
Interactions among different tasks
Fine-tuning phases

4. Example 1
How two philosophers communicates

5. Observations Vertical interfaces Horizontal interfaces
Interpreter and philosopher must understand each other
Philosopher submits task while interpreter provides services
Horizontal interfaces
Portability: two interpreters are identical in terms of their functionality
Peer-to-peer protocol: it seems that a peer talks to the same peer directly

6. Example 2 Web services: client-server interaction HTTP HTTP server
Request
HTTP
server
HTTP
client
Response

7. Layering Design HTTP HTTP server client Port # Port 80 TCP TCP
GET , #
TCP
TCP
#, STATUS

8. Simplified File Transfer

9. Protocol Architecture

10. Addressing: Entity ID Every device must have a unique ID: an address
Two levels of addressing required
Each computer needs a unique network address
allows network to deliver data to the right computer
Each application on a computer needs a unique address within the computer
known as service access point or SAP
allows each application to access the transport layer individually

11. Clarification: Network ID
Two ID system: logical ID and physical ID
The same network interface can be used by multiple devices
The same device can be used in other networks
Identify yourself: globally unique IP address
IP address identifies the network interface rather than the host/entity itself, the same “host” may have more than one IP, e.g., router
IP address=(network ID,host ID)
Physical ID: MAC address
Each network card has a unique number, e.g., 48 bits

12. Protocol Data Units (PDU)
Message unit: the format
Protocols are used to communicate at each layer
Control information is added to user data at each layer to tell the peer entity how to handle the data unit
Data from next higher layer + control information = PDU of the current layer

13. Transport PDU Transport layer may fragment user data
Each fragment has a transport header added
Destination SAP (port number)
Sequence number
Error detection code
This gives a transport protocol data unit which is sent down to network layer

14. Network PDU Adds network header to data received from transport layer
network addresses for source and destination computers
PDU contains information how to be handled
This gives a network protocol data unit

15. Protocol Data Units

16. Operation Overview

17. OSI Reference Model Open Systems Interconnection (OSI)
Developed by the International Organization for Standardization (ISO)
A theoretical framework for developing protocol standards
Seven layers
Standardization too slow to be useful in practice!
Elegant protocol structure for better understanding!

18. OSI Model Application Presentation Session Transport Network Data Link
Physical

19. Application Layer
provides means for applications to access OSI environment
provides management functions and useful mechanisms to support distributed applications
hosts general-purpose applications such as telnet, ftp, , http, dns etc

20. Presentation Layer defines data formats between applications
provides data transformation services, e.g. data compression and encryption

21. Session Layer
provides mechanism for controlling dialogues between applications in end systems
key services
dialogue discipline: duplexing
grouping: flow marking
failure recovering: checkpointing

22. Transport Layer
provides reliable, transparent data transfer between end systems
provides end-to-end error recovery and flow control
provides security or end-to-end QoS etc

23. Network Layer
provides a mechanism for transferring data across a subnet (routing algorithms and routing protocols)
relieves higher layers of the need to know anything about the underlying network technologies
provides congestion control and load balancing

24. Data link Layer provides reliable data transfer from point to point
provides error recovery mechanism (FEC and ARQ)

25. Physical Layer provides physical interface between devices
sets up rules by which bits are passed from one to another
deals about mechanical, electrical, functional, procedural characteristics of interface and devices
Mechanical: specification of pluggable connector
Electrical: representation of bits (e.g., voltages)
Functional: spec. of functions of all circuits
Procedural: spec. of events for a bit transmission
REAL COMMUNICATION!

26. Modular Design Philosophy
Tasks of communication broken up into modules, each of which will have a protocol (peer-to-peer protocol)
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

27. Peer Communications

28. Internet Any gateway/router must have at least the lowest three layers
G = gateway/router G
net 5
net 2 G
net 4 G

29. TCP/IP Suite Developed by the US DARPA for ARPANET
Used by the Internet, the de facto standard
Not official model but a working one
Defined by Vint Cerf and Robert Kahn (1974)
First tested in UCLA and BBN
First message sent over Internet: from UCLA to BBN by Leonard Kleinrock (first incomplete transmission from UCLA to SRI)

30. Five Layers Model of TCP/IP
Application layer
Host-to-host or transport layer (transmission control protocol (TCP))
Internet layer (Internet Protocol (IP))
Network access layer
Physical layer

31. Physical Layer
Physical interface between data transmission device (e.g. computer) and transmission medium or network
Specifies:
characteristics of transmission medium
nature of signals
data rates
etc.

32. Network Access Layer
Exchange of packets between end-system (computer) and network (via X.25, Ethernet etc)
Deals with parts of OSI network layer and data link layer
Encapsulation of packets via network frame structure may be used
Software depends on the physical network

33. Internet Layer (IP) Systems may be attached to different networks
Routing functions across multiple networks
Implemented in end-systems and routers
Router:
processor that connects two networks
primary function is to relay data from one network to the other

34. Transport Layer Ensures Independent of the nature of application
reliable delivery of data (to the satisfactory of applications)
right ordering of delivery
Independent of the nature of application
End-to-end protocol supporting interaction between two end-systems
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)

35. Application Layer
Supports user applications, e.g. http, telnet, ftp, SNMP, ...
Depends on applications

36. Two Machines Talk Machine B Machine A Application Application
Transport
Internet
Network Interface
Transport
Router/Gateway
Internet
Internet
Network Interface
Network Interface
Network 1
Network 2

37. Example Physical layout (1,1) (2,1) s PPP (1,3) r w Ethernet (1,2)
router s
(1,3) r
PPP w
Ethernet
(1,2)

38. Example (cont) HTTP PC HTTP TCP Router TCP IP IP IP Net Interface
Ethernet
PPP

39. TCP/IP Suite

40. TCP/IP Upper Layers

41. OSI vs TCP/IP

42. New Trend: Paradigm Shift
A word of caution: strict layering design may NOT work well (Microsoft clumsy system)
QoS forces us to use cross layer information (Internet real-time service support, diffserv)
Wireless network design philosophy tends to break down the layer design tradition (active networks) (IEEE/ACM MobiCom’99)
Buzzword: cross-layer design, still hot particularly for wireless networks

43. Further Reading Textbook: chapter 2
Stallings, Data and Computer Communications (6th ed.), Prentice Hall, 2000, Chapter 1 & 2
Web sites for IETF, IEEE, ITU-T, ISO
Internet Requests for Comment (RFCs)