Networks and
Distributed Systems
Andy Wang
Operating Systems
COP 4610 / CGS 5765
Technology Trends
Decade Technology $ per
machine
50s
$10M
Sales
volume
100
Users per
machine
1000s
60s
Mainframe
$1M
10K
100s
70s
$100K
1M
10s
80s
Mini
computers
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


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. 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
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 2GB of movie 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
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