Introduction

CPSC 363 Computer NetworksEllen WalkerHiram College

(Includes figures from Computer Networking by Kurose & Ross, © Addison Wesley 2002)

What is the Internet?

• INTERconnected NETworks

• Hardware– Hosts (end systems)– Routers– Communication Links

• Cables• Wires• Wireless

• Organization– Standards (RFC’s)– Protocols

“Cool” internet appliances

World’s smallest web serverhttp://www-ccs.cs.umass.edu/~shri/iPic.html

IP picture framehttp://www.ceiva.com/

Web-enabled toaster+weather forecaster

Message Transmission across the Internet

• Source host creates the message to send• Message is transmitted via the local network to the local router (gateway)

• Message travels from router to router until the destination’s gateway is reached

• Message is transmitted across the destination’s LAN to the destination host.

• Destination host receives and acts upon the message.

What is Provided?

• Communication services for distributed applications– Remote file transfer and login– WWW– Peer to Peer (e.g. Gnutella, Kazaa)– Multimedia applications– IM, chat – New applications all the time…

• Types of services– Connection-oriented & reliable (TCP)– Connectionless unreliable (UDP)– No guarantee of how quickly the message is delivered

TCP vs UDP: An analogy

• TCP is like a conversation– Each party makes sure the other party received and understood each message.

– If you don’t understand, I repeat …

• UDP is like shouting from a rooftop– I keep yelling (transmitting info) with no idea of whether you can hear and understand me

What is a Protocol?

• Set of conventions surrounding a conversation, e.g.– Dial phone, hear ring– Hear ring, pick up phone, “Hello”– “Hello, may I speak to…”– “Just a minute…” [New person] “Hello”– “Hello”– [conversation here]– “Goodbye”– “Goodbye”, hang up phone– Hang up phone

• Conversation is aborted if appropriate response is missing!

Examples of Computer Network Protocols

• Application protocols– HTTP (Hypertext transfer protocol)– FTP (File transfer protocol)– SMTP (Simple mail transfer protocol)

• Router and Network protocols– TCP (Transmission Control Protocol)– IP (Internet Protocol)

Human and Computer Protocols with “Handshaking”

End Systems

• Servers– Provide a “service” to others on the network

• Web server• Email server• Database Management System

• Clients– Request “services” from others

• Web browser• Email or DB client program

• Both– Peer to Peer (P2P) hosts act as both client and server.

Network Core

• How is information passed through the network?

• Goal: fast, accurate, avoid “idleness”

• Terms from telephony, information theory– Circuit Switching vs. Packet Switching – Multiplexing (Frequency vs. time)– Bandwidth, Latency, Throughput

Circuit Switching

End-end resources reserved for “call”

• link bandwidth, switch capacity

• dedicated resources: no sharing

• circuit-like (guaranteed) performance

• call setup required• Once setup, transmission

time depends only on message length and distance (not number of switches)

Sharing Links

• Frequency Division Multiplexing (FDM)– Each message travels in a unique Frequency Band (like an FM radio station)

• Time Division Multiplexing (TDM)– Time is divided into Frames, and Frames are divided into Slots. Each message gets one slot.

Freq

Time

Disadvantages of Circuit Switching

• Time to set up the circuit• Once the circuit is connected, bandwidth is reserved -- even if no message is being sent– Telephone “silence”– Reading the message before responding…

• … but it worked pretty well for telephones for years! (Mostly because people aren’t silent much)

Packet Switching

• Divide messages into fixed-size packets

• Each packet uses full bandwidth• Each packet travels its own path (message is reassembled and reordered at the end)

• Switches (now called routers) collect and pass packets along paths toward their final destination

Store and Forward Transmission

• Router has several links• When a packet is received:

– Router waits until the entire packet has been received before doing anything (store-and-forward delay)

– Packet is stored in the output buffer until the outgoing link is available (queuing delay)

– If the buffer is full, the packet is dropped (packet loss)

– Packet is sent on the outgoing link (to the next router)

How Long Does it Take?

• Assume we want to send a packet of L=1000 bits from one host to another across the network

• The packet must pass through Q=4 links to get there.

• Each link can transmit R= 10,000 bps (bits per second)

• From one router to the next will take (1000 bits) / (10,000 bits / second) = 0.1 seconds

• After 0.1 seconds, the full packet is available to send out. Assuming no delays, we can do this 4 times in 0.4 seconds, which is the total time to send the message.

Unit Math

• If you forget the formulas for computing times and distances, remember you can always do the “unit math” for example:Bits / (Bits / Second) =Bits * (Seconds / Bits) =(Bits * Seconds) / Bits =(Seconds * Bits) / Bits =Seconds * (Bits / Bits) =Seconds

Scientific Notation Math

• Another useful “math trick” for these types of calculations is “exponent math”. Start by converting to Scientific Notation.

• Scientific notation represents a number as a value between 1 and 10 (or -1 and -10) multiplied by 10 to an appropriate power– Powers greater than 0 are 10, 100, 1000…– Powers less than 0 are 0.1, 0.01, 0.001, …– Examples:

• 20,000 = 2.0* 104 (2.0e4 in Java notation)• 0.1 = 1 * 10-1 (1.0e-1 in java notation)

Exponent Math

• To multiply, multiply the coefficient and add the exponent– 2.0e4 * 1.6e2 = (2.0*1.6)e (4+2) = 3.2e6– Check: 20,000 * 160 = 3,200,000

• To divide, divide the coefficient and subtract the exponent– 4e3 / 2e4 = (4/2)e(3-4) = 2e-1– Check: 4,000 / 20, 000 = 0.2

• If your coefficient is too large or small, adjust the exponent– 1.2e3 / 4e4 = (1.2/4)e(3-4) = 0.3e-1 = 3e-2(Since the coefficient got larger, the exponent must get smaller)

Another Example

• Link bandwidth is 2400 bits / second

• Message is 120,000 bits long• Message must pass through 3 links

• With no queueing delays or propagation delays, how long will the message take?

Message Switching vs. Packet Switching

• Message switching: 1 packet per message– Store-and-forward delay is per message

• Packet switching: many packets per message– Packets will always be of a fixed size– Store-and-forward delay is per packet– Packet 1 can go from router A to router B while packet 2 is going from host to router A

– This pipelining adds parallelism to the network.

Packet Switching Example

• Link bandwidth is 2400 bits / second• Message is 120,000 bits long, divided into 4 packets of 30,000 bits each

• Message must pass through 3 links

• With no queuing delays or propagation delays, how long will the message take?

Packet Switching Transmission Delay Formula

• Message length: L bits• Packet size: P bits• Number of links: N links• Transmission rate: R bits / second

• Delay = (P/R) * (N+(L/P)-1)• P/R is the time to pass 1 packet• N+(L/P)-1 is the number of “links”: N steps to get the first packet to the end, then (P/L)-1 more steps until the last packet comes out.

• Note: take the ceiling of L/P to get an integer value.

Considering Delay and Loss

• Processing Delay (typically microseconds)– Time to examine header and determine where to store the packet

• Queuing Delay (varies based on traffic)– Time waiting for outbound link to be available

• Transmission Delay (packet length / transmission rate)– Time to push all bits of the packet onto the link

• Propagation Delay (distance / speed)– Time for one bit to travel the length of the link

• Packet Loss– When the queue is completely full, packets are dropped.

Tollbooth Analogy

• Road has tollbooths every 100km.• Messages are convoys; cars are packets

• Transmission delay = time for one car to pass one toll barrier

• Propagation delay = time for one car to travel 100km to the next toll barrier

R and S in Tollbooth Analogy

• R = transmission rate of the router (bits / second)– Corresponds to number of cars that can be served by a toll barrier in a fixed amount of time (say, cars / hour).

– Increase by adding more toll collectors at the barrier

• S = propagation speed of the link (e.g. speed of electrons in copper; speed of light)– Corresponds to car speed– Increase (legally) by raising the speed limit

Some Tradeoffs in Network Design

• We have little control over propagation delay– Can’t move Europe (or Mars) closer– Can’t increase the speed of light!

• Transmission delay is easier to control – Faster hardware, or more hardware in parallel increases transmission rate

• Make sure transmission rate is at least as fast as average incoming bit rate (traffic intensity)– Otherwise, queues are guaranteed to fill and stay full

Javascript Applets

• These can be found at the book’s website– Start at our site (http://cs.hiram.edu/~walkerel/cs363)

– Or go direct to the textbook site(http://www.awl.com/kurose-ross-3)

– All 3 chapter 1 applets are very nice

Finding Real Delays

• Application called “traceroute”– Sends a special message to the destination

– Each host along the way replies back with timing information

• To run in linuxtraceroute <address> , e.g. traceroute www.yahoo.com

How Are Packets Directed?

• Virtual Circuit– All packets take the same path from source to destination

– Packets contain virtual circuit number (VC#) - changes at each hop

– VC# tells router where to send the packet next

– Each router has table of incoming interface# and VC#, and outgoing interface# and VC#

– Special packets are used to establish and break virtual circuits

How are Packets Directed?

• Datagram– Each packet has a header with an address– Address is hierarchically structured– Router looks at appropriate part of the address to select next destination• Paris -> plane to USA [USA]• USA customs -> truck to Akron, OH post office [442]

• Akron -> truck to Hiram, OH post office [44234]• Hiram post office -> Service Center [Hiram College]

• Service center -> my mailbox [Ellen Walker, CS Dept]

Virtual Circuit vs. Datagram

• Virtual Circuit– All packets take the same path, arrive in order

– Circuit, once established, subject to delays, etc.

– Routers must keep connection state information

• Datagram– Packets can take different paths, must be reassembled

– Paths can vary to account for outages; delays– No connection state information maintained in routers

Network Taxonomy

Internet Service Providers (ISPs)

• Tier 1 (“backbone”)– AT&T, Sprint, UUNet, etc.– National & international connections

• Tier 2– Provide regional connections, e.g. Oarnet (Ohio)

– Purchase services from Tier 1 providers

• Tier 3 and local– “Last mile” connections– Purchase services from Tier 2– Sell services directly to individual & small business customers

Message Path through Internet

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP Tier 3

ISP

localISP

localISP

localISP

Residential “gateway” connections

• Dialup modem – Standard analog telephone line– <= 56Kbps

• Digital subscriber line (DSL) – Standard phone lines, restricted distance to modem– 3 Frequency channels (downstream, upstream, voice)– 384K–1.5Mbps downstream, 128K-256Kbps upstream

• Hybrid Fiber Coaxial Cable (HFC)– Fiber optic cable into neighborhood terminal, connected to coax cable to each house

– 2 channels (downstream & upstream)– Shared medium (all messages to all connected modems)

Residential Access: Cable

Local Area Networks

• Ethernet (most common wired technology)– 10 Mbps , 100 Mbps, 1Gbps even now 10Gps– Twisted pair copper wire or coax cable

• Wireless LAN– Base station (access point) connected to wired LAN

– IEEE 802.11b is 11Mbps (802.11g is faster)– Typically good for 10s of meters

• WAP (Europe, US) and I-mode (Japan)– Extend cell-phone network to Internet– Eg. Cingular GPRS, still fairly expensive

Building Links: Guided Media

• Twisted pair copper wire (phone, CAT-5)– 2 wires twisted around each other to help limit interference

– CAT-5 cable has more twists, better insulation

• Coaxial Cable– Cable wrapped by insulation surrounded by another conductor

• Fiber optics– Thin flexible “glass pipe”– Require optical rather than electrical transmitters, receivers, switches, amplifiers, etc.

Unguided Media

• Terrestrial radio link– Bands allocated for wireless transmission

– FCC regulated in US

• Satellite radio channels– Geostationary satellites used for phone network & internet backbone

Dealing with Complexity

• The path a message takes is complex; dealing with hosts, switches, packets, media, etc.

• Therefore we use an abstract model to divide the transmission into layers

• The sender at each layer uses the lower layers (as a black box) to send information directly to the recipient at that layer.

• Each layer considers information from layers above to be “data bits”

Example: Airplane Routing

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Internet Protocol Stack

• Application (web, email)• Transport (TCP, UDP, message transport)

• Network (IP, datagram routing between endhosts)

• Link (Ethernet, datagram transfer between neighboring switches)

• Physical (bits on the “wire”)

Internet Hardware

• End host (all 5 layers)• Intermediate switches (generally 3, sometimes 2 layers only)

Enveloping a Message

• Each layer takes data from the above layer– Breaks it apart if necessary (e.g. packetizing)

– Adds an “envelope” = header and/or footer bits

– Passes it to the next layer below, which will treat envelope as part of the data