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

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networks and distributed systems

Networks and Distributed Systems

Andy Wang

Operating Systems

COP 4610 / CGS 5765

distributed systems
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
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
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
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
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
Arbitration
  • Concerns the way to share a given resource
  • In Aloha network (1970s)
    • Packets were sent through radios on Hawaiian Islands
aloha network
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 network1
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 network2
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
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
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
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
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

routing
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
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
point to point networks
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 networks1
Point-to-Point Networks

+ Higher link performance (no collisions)

+ Greater aggregate bandwidth than a single link

point to point networks2
Point-to-Point Networks

+ Network capacity can be upgraded incrementally

+ Lower latency (no arbitration)

issues in point to point networks

Crossbar

buffers

buffers

Issues in Point-to-Point Networks
  • Congestion occurs when everyone sends to the same output link on a switch
solutions

Crossbar

buffers

buffers

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
solutions1
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

solutions2
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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