Networks and Distributed Systems Andy Wang Operating Systems

Networks and Distributed Systems Andy Wang Operating Systems COP 4610 / CGS 5765

Technology Trends Decade Technology $ per machine 50 s $10 M Sales volume 100 Users per machine 1000 s 60 s Mainframe $1 M 10 K 100 s 70 s $100 K 1 M 10 s 80 s Mini computers PC $10 K 100 M 1 90 s Portables $1 K 10 B 1/10

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

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

Reality of Distributed Systems n Worse availability q n Worse reliability q n One can lose data if any machine crashes Worse security q n 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 n Definitions q q Network: physical connection that allows two computers to communicate Packet: a unit of transfer n q A sequence of bits carried over the network Protocol: An agreement between two parties as to how information is to be transmitted

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

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

Aloha Network q q Arbitration: blind broadcast, with a checksum at the end of a packet Packets might become garbled in the case of simultaneous transmissions

Aloha Network q q Arbitration: blind broadcast, with a checksum at the end of a packet Packets might become garbled in the case of simultaneous transmissions

Aloha Network q q 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 ‘ 80 s) n n By Xerox First practical local area network q q q Uses wire (as opposed to radio) Broadcast network Key advance: a new way for arbitration

Ethernet’s Arbitration Techniques n n Carrier sensing: Ethernet does not send unless the network is idle Collision detection: sender checks if packet is trampled q n 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 n n A generalization of interconnected local area networks Uses machines to interconnect various networks q q q Routers, gateways, bridges, repeaters LAN 2 Act like switches Packets are copied as they transmitted across different networks LAN 1

Routing n n Concerns how a packet can reach its destination Typically, a packet has to go through multiple hops before getting to a destination q q 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. q 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 n 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 n Congestion occurs when everyone sends to the same output link on a switch Crossbar buffers

Solutions 1. No flow control: Packets get dropped when the receiving buffer is full q Downloading 2 GB of movie across the Internet can make many people unhappy Crossbar buffers

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

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