Networks and Distributed Systems Mark Stanovich Operating Systems COP 4610

Technology Trends DecadeTechnology$ per machine Sales volume Users per machine 50s$10M s 60sMainframe$1M10K100s 70sMini computers $100K1M10s 80sPC$10K100M1 90s – 00s Portables<$1K>10B1/10

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  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

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

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

Blind Broadcast Receiver: If a packet is garbled discard else sends an acknowledgment Sender: If the acknowledgment 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  Act like switches  Packets are copied as they transmitted across different networks LAN 1 LAN 2

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 1.Each routing entry contains a cost  2.Neighbors periodically exchange routing table entries 3.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 buffers Crossbar

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 buffers Crossbar

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

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