Networks and Distributed Systems

Andy Wang

Operating Systems

COP 4610 / CGS 5765

Technology Trends

Decade Technology $ per machine

Sales volume

Users per machine

50s $10M 100 1000s

60s Mainframe $1M 10K 100s

70s Mini computers

$100K 1M 10s

80s PC $10K 100M 1

90s Portables $1K 10B 1/10

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. wire, 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

discardelse

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 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 <destination, next hop, # hops>

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 buffers

Crossbar

Solutions

1. No flow control: Packets get dropped when the receiving buffer is full Downloading 2GB of movie across the Internet

can make many people unhappy

buffers 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 buffers

Crossbar

Solutions

3. Per-flow flow control: a separate set of buffers is allocated for each end-to-end stream Problem: fairness

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