ITEC801 Distributed Systems

1. ITEC801Distributed Systems Protocols Coulouris chapter 3

2. Important Things • Things to get from this set of slides • What is a protocol? • What are protocols for? • Why we have many protocols • What is the difference between connection-oriented and connectionless Protocols

3. Protocols • Inter-process communication is at the heart of distributed systems • IPC depends on protocols • a protocol is simply the rules by which the communication is carried out • a network communications protocol defines all the messages (and their ordering) for a communication Protocols

4. Protocol • Protocol definitions include • Error detection • Message length • Message structure • etc • Protocol implementations may be in hardware or software Protocols

5. How Do Protocols Work? • Transmission of data over network broken down into discrete systematic steps • At each step, certain actions take place • Each step has its own rules and procedures which cannot take place at any other step Protocols

6. Protocol Functioning • Steps must be carried out in a consistent order on every computer in the network • Sending Computer: Steps carried out-Top down. • Receiving Computer: Steps carried out bottom-up. Protocols

7. Protocols Protocols

8. Protocol • There are many protocols • Each protocol has a different purpose and accomplishes different task • Several protocols work together in what is known as a Protocol Stack • Levels of the protocol stack map on to layers of the OSI model Protocols

9. Layered Protocols • Computer network protocols are layered • each layer has its own function • for example: • Network level: IP • Transport level: TCP. • Application level • RPC • Remote method Invocation (RMI) • Message oriented Middleware (MOM) Protocols

10. Internet Protocols (some) Upper Layers Telnet SNMP User apps User apps HTTP TFTP FTP TCP UDP Transport IP ICMP Network Protocols

11. Need for a Protocol Stack • Require a high degree of cooperation between two computer systems that communicate • Activities include: • Path determination/conveying control information to network • Ascertaining destination status • Ascertaining destination application status • Determining format Protocols

12. Need for a Protocol Stack • Single module versus multiple modules. • We could implement all communication in a single piece of software • too unwieldy and hard to maintain Protocols

13. Internet Protocol Stack: TCP/IP • Standard routable enterprise networking protocol • Technology for connecting dissimilar systems Protocols

14. TCP/IP layers: • Application: user interface to Internet. • Transport: • Transmission Control Protocol (TCP): Connection oriented reliable communications for applications. • User Datagram Protocol (UDP): connectionless • Network: Internet Protocol: For routing and addressing. (RFC 791) • Network Interface layer: Inserting/removing frames from network. Protocols

15. OSI Reference Model • Defines seven layers • Physical: Concerned with transmission of unstructured bit stream over physical medium. • Data link: Reliable transmission of information across physical link. • Network: Provides upper layer independence from data transmission/switching technologies. Protocols

16. OSI Reference Model • Transport: reliable transfer of data between end points. • Session: Control structure for communication between application. • Presentation: Independence to application process from data representation. • Application: Access to environment. Protocols

17. OSI Reference Model Protocols

18. Connection-Oriented versus Connectionless • Protocols can be connection–oriented OR Connectionless • Connection oriented: A logical connection is set up between sender and receiver before they actually communicate • Connectionless: Sender and receiver communicate without establishing a logical connection Protocols

19. Connection-Oriented • Setup phase when the connection is created • Communication phase during which the communications happens • Pull-down phase when the connection is terminated Protocols

20. Protocol Terminology • Entity: active elements in each layer • Peer Entities: Entities in same layer but on different machines • Entities in layer N implement a service used by layer N+1 • N layer: Service provider • N+1 layer: service user Protocols

21. Protocols • Each layer provides service at an access point : SAP (Service Access Point) • Each SAP has a unique address that identifies it. • Agreed set of rules at Interface between any two layers Protocols

22. Protocols • Information block passed by layer N+1 to layer N through SAP: Interface Data Unit (IDU) • IDU Block= SDU (Service Data Unit) + Control (ICI) • ICI: Interface Control Information • SDU information passed along network to peer entity up to layer N+1 Protocols

23. Protocols • Control: some guidelines for layer N to do its Job. • Processing on SDU by layer N results in PDU (Protocol Data Unit) • PDU = SDU+ Header • PDUs generated at different layers given a different name Protocols

24. Relation Between Layers at an Interface ICI SDU Layer N+1 SAP Interface ICI SDU Layer N Header SDU N-PDU Protocols