1. TUNALIData Communications1 Chapter 2 Protocols and Architecture

2. TUNALIData Communications2 Need For Protocol Architecture E.g. File transfer 1.Source must activate comms. Path or inform network of destination 2.Source must check destination is prepared to receive 3.File transfer application on source must check destination file management system will accept and store file for this user 4.May need file format translation High degree of cooperation between two systems Task broken into subtasks Implemented separately in layers in stack Same set of layered functions needed in both systems Peer layers communicate: Protocol

3. TUNALIData Communications3 Key Elements of a Protocol Syntax —Data formats Semantics —Control information —Error handling Timing —Speed matching —Sequencing

4. TUNALIData Communications4 Protocol Architecture Task of communication broken up into modules For example file transfer could use three modules —File transfer application: Task 3 and 4 —Communication service module: Task 2 —Network access module: Task 1

5. TUNALIData Communications5 Simplified File Transfer Architecture Structured set of modules that implements the commun. functions: Protocol architecture

6. TUNALIData Communications6 A Three Layer Model Network Access Layer Transport Layer Application Layer

7. TUNALIData Communications7 Network Access Layer Exchange of data between the computer and the network Sending computer provides address of destination May invoke levels of service (i.e. priority) Dependent on type of network used (LAN, packet switched etc.) Above the network access layer, the remainder functions need not to be concerned about the network type

8. TUNALIData Communications8 Transport Layer Reliable data exchange —All of the data arrived —Data arrived in correct order The mechanisms for realibility: —Independent of network being used —Independent of application

9. TUNALIData Communications9 Application Layer Support for different user applications e.g. e-mail, file transfer

10. TUNALIData Communications10 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

11. TUNALIData Communications11 Protocol Architectures and Networks

12. TUNALIData Communications12 Protocols in Simplified Architecture

13. TUNALIData Communications13 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: data from the next higher layer and control information

14. TUNALIData Communications14 Protocol Data Units

15. TUNALIData Communications15 Network PDU Adds network header —network address for destination computer —Facilities requests

16. TUNALIData Communications16 Operation of a Protocol Architecture

17. TUNALIData Communications17 Standardized Protocol Architectures Required for devices to communicate Different vendors have different data formats and data exchange protocols Special-purpose approach to communications software development is too costly Two standards: —OSI Reference model Never lived up to early promises —TCP/IP protocol suite Most widely used

18. TUNALIData Communications18 OSI The functions should be broken into more managable parts Open Systems Interconnection Developed by the International Organization for Standardization (ISO) Seven layers A theoretical system delivered too late

19. TUNALIData Communications19 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

20. TUNALIData Communications20 OSI Layers There is no direct communication between peer layers except at the physical layer Even at the physical layer, two systems may not be directly connected

21. TUNALIData Communications21 The OSI Environment

22. TUNALIData Communications22 OSI as Framework for Standardization

23. TUNALIData Communications23 Layer Specific Standards

24. TUNALIData Communications24 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 PDUs Service definition —Standard services that each layer provides to the next higher layer —Functional description of what is provided Addressing —Referenced by SAPs

25. TUNALIData Communications25 Service Primitives and Parameters Services between adjacent layers expressed in terms of primitives and parameters Primitives specify function to be performed (i.e. procedure call) Parameters pass data and control info

26. TUNALIData Communications26 Primitive Types REQUESTA primitive issued by a service user to invoke some service and to pass the parameters needed to specify fully the requested service INDICATIONA primitive issued by a service provider either to: indicate that a procedure has been invoked by the peer service user on the connection and to provide the associated parameters, or notify the service user of a provider-initiated action RESPONSEA primitive issued by a service user to acknowledge or complete some procedure previously invoked by an indication to that user CONFIRMA primitive issued by a service provider to acknowledge or complete some procedure previously invoked by a request by the service user

27. TUNALIData Communications27 Timing Sequence for Service Primitives

28. TUNALIData Communications28 OSI Layers (1) Physical —Physical interface between devices Mechanical: physical properties of the interface to a transmission medium Electrical: representation and data rate of bits Functional: functions performed by circuits of physical interface Procedural: which bitstreams are exchanged accross the physical medium Data Link —Means of activating, maintaining and deactivating a reliable link —Error detection and control —Higher layers may assume error free transmission

29. TUNALIData Communications29 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

30. TUNALIData Communications30 OSI Layers (3) Session —Control of dialogues between applications —Dialogue discipline (full or half duplex) —Grouping (data can be marked to define groups) —Recovery (checkpoint mechanism) Presentation —Data formats and coding —Offers app. programs a set of data trans. services —Data compression and encryption Application —Means for applications to access OSI environment

31. TUNALIData Communications31 Use of a Relay

32. TUNALIData Communications32 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 Organizes the communication task into five layers —Application layer —Host to host or transport layer —Internet layer —Network access layer —Physical layer

33. TUNALIData Communications33 Physical Layer Physical interface between data transmission device (e.g. computer) and transmission medium or network Concerned with specifying characteristics of transmission medium Signal levels Data rates etc.

34. TUNALIData Communications34 Network Access Layer Exchange of data between end system and network Destination address provision Invoking services like priority Software used at this layer depends on the type of network to be used —Circuit switching —Packet switching (e.g. frame relay) —LANs (e.g. Ethernet)

35. TUNALIData Communications35 Internet Layer (IP) Systems may be attached to different networks Routing functions across multiple networks Implemented in end systems and routers

36. TUNALIData Communications36 Transport Layer Reliable delivery of data Ordering of delivery Transmission Control Protocol (TCP) is the most commonly used protocol to provide this functionality

37. TUNALIData Communications37 Application Layer Support for user applications e.g. http, SMTP

38. TUNALIData Communications38 OSI v TCP/IP

39. TUNALIData Communications39 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 —Regulates flow of segments —Recover from lost or damaged segments

40. TUNALIData Communications40 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 Some transaction-oriented applications make use of UDP

41. Operation of TCP and IP Total communications facility may consists of multiple networks (subnetworks) Network access protocol (NAP) is used to connect a computer to a subnetwork NAP enables the host send data across the subnetwork IP is implemented in all end systems and the routers —Act as relay TCP is implemented only in the end systems TUNALIData Communications41

42. TUNALIData Communications42 Addressing level Unique address for each entity in the communication system Two levels of addressing are needed Network level address —IP or global internet address (TCP/IP) —To deliver data to the proper host Process within the system —Port number (TCP/IP) —To deliver data to the proper process

43. TUNALIData Communications43 TCP/IP Concepts

44. TUNALIData Communications44 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

45. TUNALIData Communications45 PDUs in TCP/IP

46. TUNALIData Communications46 Example Header Information TCP header and data form a TCP segment —Destination port —Sequence number —Checksum IP header and data form an IP datagram —Destination IP address Network access layer appends its own header —Destination subnetwork address —Facilities requests

47. TUNALIData Communications47 Some Protocols in TCP/IP Suite

48. TUNALIData Communications48 Required Reading Stallings chapter 2 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