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
AAAA
BBBB CCCC
ABABACBC