This lecture… Motivation for distributed vs. centralized systems Survey of network technologies

Technology Trends

Decade Technology $ per machine

Sales volume

Users per machine

50’s - $10M 100 1000’s

60’s Mainframes $1M 10K 100’s

70’s Minicomputers

$100K 1M 10’s

80’s PCs $10K 100M 1

90’s Laptops $1K - $5K 100M 1

00’s PDA’s, Smart Phones

$1 - $1k 1B 1/10

Centralized vs. Distributed Systems Centralized System: System in which major

functions are performed by a single physical computer– Originally, everything on single computer– Later: client/server model

Distributed System: physically separate computers working together on some task– Early model: multiple servers working together

» Probably in the same room or building» Often called a “cluster”

– Later models: peer-to-peer/wide-spread collaboration

Centralized vs. Distributed Systems Why do we need distributed systems?

– Cheaper and easier to build lots of simple computers

– Easier to add power incrementally– Principle of bureaucracy avoidance– Collaboration: Much easier for users to

collaborate through network resources (such as network file systems)

The added promise of distributed systems:– Higher availability – one machine goes down, use

another– Better reliability – store data in multiple locations– More security – each piece easier to make secure

Centralized vs. Distributed Systems Reality has been disappointing:

– Worse availability – depend on every machine being up

– Worse reliability – can lose data if any machine crashes

– Worse security – anyone in world can break into system

Key idea: Coordination is more difficult, because we have to coordinate multiple copies of shared state information (using only the network).

What would be easy in a centralized system becomes a lot more difficult.

Goals: Transparencies and “ilities” One important goal behind much distributed systems design is

“Transparency”. That is, distributed systems are complex, so you would like to

shield the user from much of this complexity. (Or would you???). Two questions to ask:– Is it desirable?– Is it achieved?

Various types of transparency are:– Location – Can’t tell where resources are located– Migration – Resources may move throughout the system without

the user knowing.– Replication – Can’t tell how many copies of a resource exist.– Concurrency – Can’t tell how many users there are– Parallelism – System may be able to speed up large jobs by

splitting them into smaller pieces that run at the same time.– Failure – System may hide various things that go wrong in the

system.

Goals: Transparencies In addition, there are certain properties that are desirable

of a distributed system. These include:

– Availability – what is the probability of the system being able to accept and process requests?

– Reliability – This is usually stronger than simply availability. » It means not only is the system up, but that it is working

correctly. » Thus, it includes availability, security and fault-tolerance. Fault-

tolerance can be: fail-fast, i.e. stop immediately if data starts to corrupt,

difficult to achieve). Recover, restart Mask – hide (say, fault during plane landing)

Goals: Transparencies Goals contd.

– Performance - various metrics here: » latency, throughput, time to first byte, time to last byte,

scalability, etc. » The key to performance in a distributed system is to avoid

“bottlenecks”. » Possible bottlenecks include: centralized components, centralized

state (e.g., system tables), centralized control, limited connectivity, and need for consistent “global” state.

– Manageability– Flexibility

A great example of a system that meets many of these criteria is the Internet as it has evolved into its current form.

Networking Definitions Network: physical connection that allows two

computers to communicate.

Packet: unit of transfer, sequence of bits carried over network. – Network carries packets from one CPU to

another. – Destination gets interrupt when packet arrives.

Protocol: agreement between two parties as to how information is to be transmitted, i.e. format and semantics.

CPU CPUnetwork

Broadcast Networks Broadcast networks: shared communication

medium. For example, shared medium can be a wire – all

hosts listen to wire. – Inside a computer, this is called a “bus” – a

shared set of wires between the CPU and the memory modules

Ethernet is an example broadcast network (10 Mbits/sec – 10Gbit/s)

More examples: cellular phones, GSM GPRS and EDGE, CDMA 1xRTT and 1EvDO (10Kbit/s – 2 Mbit/sec).

Delivery When you broadcast a packet, how does receiver

know who it is for? Put header on front of packet:

– Destination | Packet For example, header would contain unique

machine # (network address) of target. Everyone gets packet, discards if not the target. In Ethernet, this check is done in hardware; no OS

interrupt if not for you. This is an example of layering: we’re going to

build up complex network protocols by layering more and more stuff on top of the packet.

Arbitration How do your machines arbitrate for use of shared

medium? Aloha network (70’s) – packet radio between

Hawaiian islands Arbitration: blind broadcast, with checksum at

end of packet. – If received ok (not garbled), send back an

acknowledgement. – If not received ok, discard.

Need checksum anyway, in case airplane flies overhead (or maybe a surfer goes by), and packet gets garbled.

Arbitration Sender waits for a while, and if doesn’t get an

acknowledgement, re-transmits. So if two senders try to send at same time, both get

garbled, both simply re-send later. Problem: stability. What if load increases?

» More collisions less gets through more resent more load… More collisions…

» Unfortunately: some sender may have started in clear, get scrambled without finishing

Ethernet – early 80’s, first practical local area network (Xerox PARC). 10 Mb/s – 100 Mb/s.

Most common LAN for UNIX environments, becoming more common in PC’s.

What we have in the department.

Arbitration Used wire instead of radio, but still broadcast: all machines

tap into single wire, listen to all packets. Key advance was in a new way to do arbitration called

CSMA/CD: Carrier sense multiple access/ collision detection.

Three pieces:1. Carrier sense – don’t send unless idle2. Collision detect – sender checks if packet is trampled.

If so, abort, wait, and retry.» How long should I wait, after trying to send and failing? » What if everyone waits the same length of time? » We’d all keep colliding forever.» Also, what if load increases? More likely to get collisions.

3. Adaptive randomized waiting strategy – if try to send, and get collision, means there are a number of people trying to send, so pick bigger mean wait time. – If collide again, pick even bigger wait.

Point-to-point networks Here’s a different way of thinking about all this –

why have a shared bus for Ethernet at all, why not simplify and only have point-to-point links, plus routers/switches?

Central idea behind ATM (asynchronous transfer mode), the first commercial point-to-point LAN.

Inspiration for ATM taken from telephone network. Point-to-point network: a network in which every

physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus

configuration into a point-to-point network. Router: a device that acts as a junction between

two networks to transfer data packets among them.

Point-to-point networks A bunch of advantages:

1. Higher link performance – can drive point-to-point connection faster than broadcast link

2. Greater aggregate bandwidth than single link3. Can add capacity incrementally – add more

links/switches to get more capacity4. Better fault tolerance (as in Internet)5. Lower latency (no need for arbitration to send,

although you do need a buffer in the switch)

Point-to-point networks Disadvantage: more expensive than having everyone share

one bus.– But, technology has been relentlessly driving the costs

down! As a result, point-to-point communications are starting to be

used in everything from workstations, to local area networks, to the Internet.

Workstations: In a few years, open up a workstation, will see its CPU connected to memory and graphics engine by a switched network, instead of a bus.

Multiprocessors are already connected by hooking together lots of small-scale switches. – For instance, in a 2-D mesh, or in a hypercube.

In LAN’s, not only ATM, but now a version of Ethernet called “switched Ethernet” – uses same packet format, analog signaling as Ethernet, but only two machines on each Ethernet.

Flow Control Options Switches look just like computers: inputs,

memory, and outputs.

What if everyone sends to the same output? Congestion. What happens when buffers fill up?

Option 1: no flow control. Packets get dropped if they arrive and there’s no space.

buffers buffers

C ro s s b a r

switch

Inputs Outputs

Flow Control Options (con’t) If I send a lot, I’ll grab the buffers, and then

everyone else is hosed. Option 2: flow control between switches. Don’t

send me more, unless I have room. Problem: cross-traffic. What if path from source to

destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic?A, B A

C B, C, D

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Flow Control Options (con’t) Option 3: per flow control. Allocate a separate set

of buffers to each end-to-end stream, and use “don’t send me more” on each separate end-to-end stream.

Problem: fairness– Throughput of each stream is entirely dependent

on topology, and relationship to bottleneck.aaaaa

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Flow Control Options (con’t) Automobile Analogy

– At traffic jam, one strategy is merge closest to the bottleneck» Why people get off at one exit, drive 50 feet, merge

back into flow» Ends up slowing everybody else a huge amount

– Also why have control lights on-ramps» Try to keep from injecting more cars than capacity

of road (and thus avoid congestion)

The Internet What happens if you need more bandwidth than a

single Ethernet? For example, SUN has > 10000 workstations. Buy two Ethernets? If so, how do machines on

each network talk to each other?

The Internet Put machine that straddles both networks. Lots of different words for this function: router,

gateway, bridge, repeater. But basically, acts like a switch. Machine watches all packets on each Ethernet,

and if packet is for machine on other one, then copies packet over.

Ethernet Ethernet

The Internet The Internet is just a generalization of this.

– Internet = interconnecting local area networks.

Local networks can be anything – Ethernet, AppleTalk, FDDI, even phone company wires, but building blocks are machines that straddle two or more networks.

AT&T AT&TUoT Ethernet

UBC Ethernet

Waterloo Ethernet

The Internet Protocol: “IP” IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of

an IP packet– Often written as four dot-separated integers, with

each integer from 0—255 (thus representing 8x4=32 bits)

– Example: 142.51.23.217 Internet Host: a computer connected to the Internet

– Host has one or more IP addresses used for routing» Some of these may be private and unavailable for routing

– Not every computer has a unique IP address » Groups of machines may share a single IP address » In this case, machines have private addresses behind a

“Network Address Translation” (NAT) gateway

Address ranges and subnets Address Ranges in IP

– IP address space divided into prefix-delimited ranges:» NN.0.0.0/8: Class A (10.0.0.0 private)» NN.NN.0.0/16: Class B (172.[16-31].0.0 private)» NN.NN.NN.0/24: Class C (192.168.xx.0 private)

– Such ranges often owned by organizations Subnet: A network connecting a set of hosts with

related destination addresses – With IP, all the addresses in subnet are related by

a prefix of bits » The number of matching bits is called the “mask”

and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0)

Subnets A subnet is identified by 32-bit value, with the bits

which differ set to zero, followed by a slash and a mask

» E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX

» Same subnet: 128.32.131.0/255.255.255.0

Difference between subnet and complete network range– Subnet is always a subset of address range– Once, subnet meant single physical broadcast

wire; now, less clear exactly what it means (virtualized by switches)

Hierarchical Networking: The Internet How can we build a network with millions of

hosts?– Hierarchy! Not every host connected to every

other one– Use a network of Routers to connect subnets

together» Routing is often by prefix: e.g. first router matches

first 8 bits of address, next router matches more, etc.

Simple Network Terminology Local-Area Network (LAN) – designed to cover

small geographical area.– Multiaccess bus, ring, or star network– Speed 10 – 1000 Megabits/second– Broadcast is fast and cheap– In small organization, a LAN could consist of a

single subnet. In large organizations, a LAN contains many subnets.

Wide-Area Network (WAN) – links geographically separated sites– Point-to-point connections over long-haul lines

(often leased from a phone company)– Speed 1.544 – 45 Megabits/second– Broadcast usually requires multiple messages

Routing How do packets get to their destination? Simple if

there’s a single machine that straddles all networks, but that won’t work in the Internet!

If packet has to go several hops before it gets to destination, and router straddles several networks, how do routers know how to forward packets?

To answer this, some definitions:– Name – Mom, Fraser– Address – phone #, network address– Route – how do we get there from here

Routing Internet solution: routing tables.

– Each router looks at packet header, does table lookup to decide which link to use to get it closer to destination.

– Also, all machines on the same subnet, have common portion of address/machine id.

Routing table contains:– Destination address range –> output link that gets

closer to destination– Default entry (for subnets without explicit entries)

How do you set up the table? – Internet has no centralized state! – No single machine knows entire topology, and

topology is constantly changing!– Topology constantly changing (faults, reconfiguration,

etc)

Routing Instead:

1. Routing table has “cost” – number of hops to destination (in practice, also considers how heavily used each link is)

2. Neighbors periodically exchange routing tables3. If neighbor has cheaper route, use that one

So:– Initially, routers don’t know about any

destination Loop

Get routing table from neighbors Update routing table

Routing Note that since the Internet is made up of many individual

networks, it’s routing is similar but more complicated. Basically these types of algorithms are performed at

different levels of the network, such as within a subnet and then among the routers that route between the subnets.

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