Transcript Document
Networks and Distributed Systems Sarah Diesburg Operating Systems COP 4610 Technology Trends Decade Technology $ per machine 50s $10M Sales volume 100 Users per machine 1000s 60s Mainframe $1M 10K 100s 70s Mini computers PC $100K 1M 10s $10K 100M 1 Portables <$1K >10B 1/10 80s 90s – 00s 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 Promises of Distributed Systems Higher availability Better reliability If one machine goes down, use another A user is able to store data in multiple locations More security Each simple component is easier to make secure Reality of Distributed Systems Worse availability Worse reliability One can lose data if any machine crashes Worse security A system may depend on many or all machines being up Security is as strong as the weakest component Coordination is difficult because machines can only use the network medium 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 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 Arbitration Concerns the way to share a given resource In Aloha network (1970s) Packets were sent through radios on Hawaiian Islands Aloha Network Arbitration: blind broadcast, with a checksum at the end of a packet Packets might become garbled in the case of simultaneous transmissions Aloha Network Arbitration: blind broadcast, with a checksum at the end of a packet Packets might become garbled in the case of simultaneous transmissions Aloha Network Arbitration: blind broadcast, with a checksum at the end of a packet Packets might become garbled in the case of simultaneous transmissions Blind Broadcast Receiver: If a packet is garbled discard else sends an acknowledgement Sender: If the acknowledgement does not arrive resend the packet 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 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 The Internet A generalization of interconnected local area networks Uses machines to interconnect various networks Routers, gateways, bridges, repeaters LAN 2 Act like switches Packets are copied as they transmitted across different networks LAN 1 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 Routing Table Updates Each routing entry contains a cost 1. 2. 3. <destination, next hop, # hops> Neighbors periodically exchange routing table entries If the neighbor has a cheaper route, use that one instead Point-to-Point Networks Instead of sharing a common network medium, all nodes in the network can be connected directly to a router/switch Point-to-Point Networks + Higher link performance (no collisions) + Greater aggregate bandwidth than a single link Point-to-Point Networks + Network capacity can be upgraded incrementally + Lower latency (no arbitration) Issues in Point-to-Point Networks Congestion occurs when everyone sends to the same output link on a switch 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 Crossbar buffers buffers 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 Solutions 3. Per-flow flow control: a separate set of buffers is allocated for each end-to-end stream Problem: fairness AAAA BBBB ABAB CCCC ACBC