Networks and Distributed Systems

1. Networks and Distributed Systems Mark Stanovich Operating Systems COP 4610

2. Technology Trends

3. Distributed Systems • Allow physically separate computers to work together + Easier and cheaper to mass-produce simple computers • Off-the-shelf components + A company can incrementally increase the computing power

4. Promises of Distributed Systems • Higher availability • If one machine goes down, use another • Better reliability • A user is able to store data in multiple locations • More security • Each simple component is easier to make secure

5. Reality of Distributed Systems • Worse availability • A system may depend on many or all machines being up • Worse reliability • One can lose data if any machine crashes • Worse security • Security is as strong as the weakest component • Coordination is difficult because machines can only use the network medium

6. Network Technologies • Definitions • Network: physical connection that allows two computers to communicate • Packet: a unit of transfer • A sequence of bits carried over the network • Protocol:An agreement between two parties as to how information is to be transmitted

7. Broadcast Networks • A broadcast network uses a shared communication medium • e.g. wireless, Ethernet, cellular phone network • The sender needs to specify the destination in the packet header • So the receiver knows which packet to receive • If a machine were not the intended destination • Discard the packet

8. Arbitration • Concerns the way to share a given resource • In Aloha network (1970s) • Packets were sent through radios on Hawaiian Islands

9. Blind Broadcast Receiver: If a packet is garbled discard else sends an acknowledgment Sender: If the acknowledgment does not arrive resend the packet

10. Ethernet (introduced in the early ‘80s) • By Xerox • First practical local area network • Uses wire (as opposed to radio) • Broadcast network • Key advance: a new way for arbitration

11. Ethernet’s Arbitration Techniques • Carrier sensing: Ethernet does not send unless the network is idle • Collision detection: sender checks if packet is trampled • If so, abort, wait, and retry • Adaptive randomized waiting: a sender picks a bigger wait time (plus some random duration) after a collision

12. The Internet • A generalization of interconnected local area networks • Uses machines to interconnect various networks • Routers, gateways, bridges, repeaters • Act like switches • Packets are copied as they transmitted across different networks LAN 2 LAN 1

13. Routing • Concerns how a packet can reach its destination • Typically, a packet has to go through multiple hops before getting to a destination • Each hop is a router, which directs a packet to the next hop • Routing is achieved through routing tables

14. Routing Table Updates • Each routing entry contains a cost • <destination, next hop, # hops> • Neighbors periodically exchange routing table entries • If the neighbor has a cheaper route, use that one instead

15. Point-to-Point Networks • Instead of sharing a common network medium, all nodes in the network can be connected directly to a router/switch

16. Point-to-Point Networks + Higher link performance (no collisions) + Greater aggregate bandwidth than a single link

17. Point-to-Point Networks + Network capacity can be upgraded incrementally + Lower latency (no arbitration)

18. Crossbar buffers buffers Issues in Point-to-Point Networks • Congestion occurs when everyone sends to the same output link on a switch

19. Crossbar buffers buffers Solutions 1. No flow control: Packets get dropped when the receiving buffer is full • Downloading large files across the Internet can make many people unhappy

20. Solutions 2. Flow control between switches: a switch does not send until the buffer space is available in the next switch • Problem: cross traffic Crossbar buffers buffers

21. Solutions 3. Per-flow flow control: a separate set of buffers is allocated for each end-to-end stream • Problem: fairness ABAB AAAA ACBC BBBB CCCC