1. Introduction to Distributed Systems and Networking

2. 2 Announcements Homework 4 due today Attempting to schedule Prelim II for Thursday, April 26th

3. 3 Goals for today Introduction to Distributed Systems Introduction to Networking

4. 4 Centralized vs Distributed Systems Centralized System: System in which major functions are performed by a single physical computer –Originally, everything on single computer –Later: client/server model Distributed System: physically separate computers working together on some task –Early model: multiple servers working together Probably in the same room or building Often called a “cluster” –Later models: peer-to-peer/wide-spread collaboration Server Client/Server Model Peer-to-Peer Model

5. 5 Distributed Systems Definition: Loosely coupled processors interconnected by network Distributed system is a piece of software that ensures: –Independent computers appear as a single coherent system Lamport: “A distributed system is a system where I can’t get my work done because a computer that I’ve never heard of has failed”

6. 6 Why use distributed systems? These are now a requirement: –Economics dictate that we buy small computers –Cheap way to provide reliability –We all need to communicate –It is much easier to share resources –Allows a whole set of distributed applications –A whole set of future problems need machine communication Collaboration: Much easier for users to collaborate through network resources (such as network file systems) –…–…

7. 7 Distributed Systems: Issues The promise of distributed systems: –Higher availability: one machine goes down, use another –Better durability: store data in multiple locations –More security: each piece easier to make secure Reality has been disappointing –Worse availability: depend on every machine being up Lamport: “a distributed system is one where I can’t do work because some machine I’ve never heard of isn’t working!” –Worse reliability: can lose data if any machine crashes –Worse security: anyone in world can break into system Coordination is more difficult –Must coordinate multiple copies of shared state information (using only a network) –What would be easy in a centralized system becomes a lot more difficult

8. 8 Distributed Systems Goals Connecting resources and users Transparency: the ability of the system to mask its complexity behind a simple interface –Location: Can’t tell where resources are located –Migration: Resources may move without the user knowing –Replication: Can’t tell how many copies of resource exist –Concurrency: Can’t tell how many users there are –Parallelism: System may speed up large jobs by splitting them into smaller pieces –Fault Tolerance: System may hide various things that go wrong in the system Openness: portability, interoperability Scalability: size, geography, administrative Transparency and collaboration require some way for different processors to communicate with one another

9. 9 Software Concepts SystemDescriptionMain Goal Distributed OSTightly coupled OS for multiprocessors and homogeneous m/cs Hide and manage hardware resources Networked OSLoosely coupled OS for heterogeneous computers, LAN/WAN Offer local services to remote clients MiddlewareAdditional layer atop NOS implementing general-purpose services Provide distribution transparency Local OS Middleware Distributed Applications Network Machine A Machine B Machine C

10. 10 Some Applications Air traffic control Banking, stock markets Military applications Health care, hospital automation Telecommunications infrastructure E-commerce, e-cash …

11. 11 Few Challenges No shared clocks –How to order events No shared memory –Inconsistent system state Scalability Fault tolerance –Availability, recoverability Consensus Self management Security

12. 12 Networking Middleware gives guarantees not provided by networking How do you connect computers? –Local area network (LAN) –Wide area network (WAN) Let us consider the example of the Internet

13. 13 Internet: Example Click -> get page specifies - protocol (http) - location (www.cnn.com)

14. 14 Internet: Locating Resource www.cnn.com –name of a computer –Implicitly also a file (index.html) Map name to internet protocol (IP) address –Domain name system (DNS) hostlocal com cnn.com? a.b.c.d

15. 15 Internet: Connection Http (hyper-text transport protocol) sets up a connection –TCP connection (transmission control protocol) –between the host and cnn.com to transfer the page The connection transfers page as a byte stream –without errors: flow control + error control Hostwww.cnn.com Connect OK Get page Page; close

16. 16 Internet: End-to-end Byte stream flows end to end across many links/switches: –routing (+ addressing) That stream is regulated and controlled by both ends: –retransmission of erroneous or missing bytes; flow control

17. 17 Internet: Packets The network transports bytes grouped into packets Packets are “self-contained”; routers handle them 1 by 1 The end hosts worry about errors and pacing –Destination sends ACKs; Source checks losses

18. 18 Internet: Bits Equipment in each node sends packets as string of bits That equipment is not aware of the meaning of the bits Frames (packetizing) vs. streams

19. 19 Internet: Points to remember Separation of tasks –send bits on a link: transmitter/receiver [clock, modulation,…] –send packet on each hop [framing, error detection,…] –send packet end to end [addressing, routing] –pace transmissions [detect congestion] –retransmit erroneous or missing packets [acks, timeout] –find destination address from name [DNS] Scalability –routers don’t know full path –names and addresses are hierarchical

20. 20 Internet : Challenges Addressing ? Routing ? Reliable transmission ? Interoperability ? Resource management ? Quality of service ?

21. 21 Concepts at heart of the Internet Protocol Layered Architecture Packet Switching Distributed Control Open System

22. 22 Protocol Two communicating entities must agree on: –Expected order and meaning of messages they exchange –The action to perform on sending/receiving a message Asking the time

23. 23 Layered Architectures Human beings can handle lots of complexity in their protocol processing. –Ambiguously defined protocols –Many protocols all at once How computers manage complex protocol processing? –Specify well defined protocols to enact. –Decompose complicated jobs into layers; each has a well defined task

24. 24 Layered Architectures Break-up design problem into smaller problems –More manageable Modular design: easy to extend/modify. Difficult to implement –careful with interaction of layers for efficiency

25. 25 Layered Architecture Web, e-mail, file transfer,... Reliable/ordered transmission, QOS, security, compression,... End-to-end transmission, resource allocation, routing,... Point-to-point links, LANs, radios,... Applications Middleware Routing Physical Links users network

26. 26 The OSI Model Open Systems Interconnect (OSI) –standard way of understanding conceptual layers of network comm. –This is a model, nobody builds systems like this. Each level –provides certain functions and guarantees –communicates with the same level on remote notes. A message –generated at the highest level –is passed down the levels, encapsulated by lower levels –until it is sent over the wire. On the destination –Encapsulated message makes its way up the layers –until the high-level message reaches its high-level destination.

27. 27 OSI Levels Presentation Transport Network Data Link Physical Application Presentation Transport Network Data Link Physical Application Node A Node B Network Session

28. 28 OSI Levels Physical Layer –electrical details of bits on the wire Data Link Layer –sending “frames” of bits and error detection Network Layer –routing packets to the destination Transport Layer –reliable transmission of messages, disassembly/assembly, ordering, retransmission of lost packets Session Layer –really part of transport, typ. Not impl. Presentation Layer –data representation in the message Application –high-level protocols (mail, ftp, etc.)

29. 29 Internet protocol stack HTTP, SMTP, FTP, TELNET, DNS, … TCP, UDP. IP Point-to-point links, LANs, radios,... Application Transport Network Physical users network

30. 30 Air travel Ticket (purchase) Baggage (check) Gates (load) Runway (take off) Passenger Origin Ticket (complain) Baggage (claim) Gates (unload) Runway (landing) Passenger Destination Airplane routing

31. 31 Summary Network: physical connection that allows two computers to communicate –Packet: unit of transfer, sequence of bits carried over the network Protocol: Agreement between two parties as to how information is to be transmitted Internet Protocol (IP) –Used to route messages through routes across globe –32-bit addresses, 16-bit ports Reliable, Ordered, Arbitrary-sized Messaging: –Built through protocol layering on top of unreliable,