CISC 370 - Class Today

04/20/23 1R. Smith - University of St Thomas - Minnesota

CISC 370 - Class Today CISC 370 - Class Today

• The LabThe Lab• Project ScheduleProject Schedule• Homework 7 and 8Homework 7 and 8• RecapRecap• POTSPOTS

Upcoming HomeworkUpcoming Homework

• I’ll post some Chapter 12/13 homework soonI’ll post some Chapter 12/13 homework soon– Outline: April 23Outline: April 23

• I’ll e-mail comments to your groupI’ll e-mail comments to your group– Revised Outline (optional): Apr 30Revised Outline (optional): Apr 30

– Papers: May 12Papers: May 12– Presentations: May 12 and 14Presentations: May 12 and 14


A Lab ProblemA Lab Problem

• Someone plugged in the FirewallSomeone plugged in the Firewall

• Caused ‘some’ machines to get Net 10 Caused ‘some’ machines to get Net 10 addressesaddresses– The packets are discarded in most ‘real’ routersThe packets are discarded in most ‘real’ routers– This causes alarm bells in some routersThis causes alarm bells in some routers– Fortunately we were not visited by angry IRT admins.Fortunately we were not visited by angry IRT admins.

• I.E. Someone hooked up wires incorrectlyI.E. Someone hooked up wires incorrectly– The connection did NOT match the setup published in the LabThe connection did NOT match the setup published in the Lab

• This was also NAUGHTY.This was also NAUGHTY.


Homework Review: Chapter 7Homework Review: Chapter 7

• 7.1: Mailbagging7.1: Mailbagging– Good things: transmission efficiency, less intermediate Good things: transmission efficiency, less intermediate

storagestorage– Bad things: More work for endpoints (arguable)Bad things: More work for endpoints (arguable)

• 7.3: One message is best7.3: One message is best– SMTP sends one copy per serverSMTP sends one copy per server– ““Spam” vs “Velveeta”Spam” vs “Velveeta”

• An old Internet discussionAn old Internet discussion• Spam = one message with many destinationsSpam = one message with many destinations• Velveeta = many messages to separate destinationsVelveeta = many messages to separate destinations


7.8: Tracing a Header7.8: Tracing a Header

• http://courseweb.stthomas.edu/resmith/c/cisc370/c9sp/headers.txt

• E-mail ForensicsE-mail Forensics– Every e-mail server prepends a ‘Received’ lineEvery e-mail server prepends a ‘Received’ line– Ideally the information is accurateIdeally the information is accurate

• In fact, it’s easy to forgeIn fact, it’s easy to forge– Each server leaves earlier ‘Received’ lines undisturbedEach server leaves earlier ‘Received’ lines undisturbed

– Typical forgery: modifies just the “From:” line.Typical forgery: modifies just the “From:” line.– We can spot fakes by looking for inaccurate detailsWe can spot fakes by looking for inaccurate details

• Do “Received:” lines match the “From:” line?Do “Received:” lines match the “From:” line?• Do domain names match IP addresses?Do domain names match IP addresses?• Do date/time stamps make sense?Do date/time stamps make sense?


http://courseweb.stthomas.edu/resmith/c/cisc370/c9sp/headers.txt

http://courseweb.stthomas.edu/resmith/c/cisc370/c9sp/headers.txt

Homework: Chapter 8Homework: Chapter 8

• 8.1: a=net bits, b=host bits, c=#nets, d=#hosts, e=octet range8.1: a=net bits, b=host bits, c=#nets, d=#hosts, e=octet range– Class A: (a) 8 bits, (b) 24 bitsClass A: (a) 8 bits, (b) 24 bits

• (c) first bit of the first octet in a class A address is 0 (leaving 7 bits), so 2(c) first bit of the first octet in a class A address is 0 (leaving 7 bits), so 277 = = 128 – 2 (0 and 127 are disallowed) = 126 networks,128 – 2 (0 and 127 are disallowed) = 126 networks,

• (d) 2(d) 22424 = 16,777,216 – 2 (host address cannot be all 0’s or all 1’s) = = 16,777,216 – 2 (host address cannot be all 0’s or all 1’s) = 16,777,214 hosts16,777,214 hosts

• (e) range: 1 through 126(e) range: 1 through 126– Class B: (a) 16 bits, (b) 16 bits, Class B: (a) 16 bits, (b) 16 bits,

• (c) first two bits of the first octet in a class B address are 10 (leaving 14 (c) first two bits of the first octet in a class B address are 10 (leaving 14 bits), so 2bits), so 214 14 = 16,384 networks, = 16,384 networks,

• (d) 2(d) 224 24 = 65,536 – 2 (host address cannot be all 0’s or all 1’s) = 65,534 hosts, = 65,536 – 2 (host address cannot be all 0’s or all 1’s) = 65,534 hosts, (e) range: 128 through 191(e) range: 128 through 191

– Class C: (a) 24 bits, (b) 8 bits, Class C: (a) 24 bits, (b) 8 bits, • (c) first three bits in the first octet in a class C address are 110 (leaving 21 (c) first three bits in the first octet in a class C address are 110 (leaving 21

bits), so 2bits), so 221 21 = 2,097,152 networks, = 2,097,152 networks, • (d) 2(d) 28 8 = 256 – 2 (host address cannot be all 0’s or all 1’s) = 254 hosts, = 256 – 2 (host address cannot be all 0’s or all 1’s) = 254 hosts, • (e) range: 192 through 223(e) range: 192 through 223


Chapter 8 homework, continuedChapter 8 homework, continued• 8.5: Address 192.168.100; mask 255.255.192.08.5: Address 192.168.100; mask 255.255.192.0

– Class B-C “CIDR” address – no subnets, 2Class B-C “CIDR” address – no subnets, 21414 hosts (16K) hosts (16K)– #0 bits = 14 – If taken as Class B:#0 bits = 14 – If taken as Class B:

• a) # Subnet bits = 16-15 = 2 bits => 4 subnetsa) # Subnet bits = 16-15 = 2 bits => 4 subnets• b) # Host bits = 14 => 16K hostsb) # Host bits = 14 => 16K hosts

• 8.8: Exhausting address spaces8.8: Exhausting address spaces– 31,557,60031,557,600 seconds per year, per 10 seconds; per second seconds per year, per 10 seconds; per second– IPV4: easy answer = 2IPV4: easy answer = 23232 => 4 billion or so => 1,360 years => 4 billion or so => 1,360 years– IPV4: “book solution” = 2IPV4: “book solution” = 22121 “available” => 242 days “available” => 242 days– IPV6: based on book misprint = 2125 => 35 minIPV6: based on book misprint = 2125 => 35 min– IPV6: for real = 2IPV6: for real = 2125125 => 10 => 103030 years years

• 8.9: Finding mask and gateway8.9: Finding mask and gateway– # of hosts on network# of hosts on network



The Plain Old Telephone SystemThe Plain Old Telephone System

• POTSPOTS– Architecture (recap)Architecture (recap)– SS-7SS-7

• WANs from the POTS folksWANs from the POTS folks– X.25X.25– Frame RelayFrame Relay– ATMATM

Traditional POTS ArchitectureTraditional POTS Architecture

• Provides Provides analoganalog connections to endpoints connections to endpoints– Digital features added atop analog voice-quality signalsDigital features added atop analog voice-quality signals– Digital emphasis has evolved in the past 10-15 yearsDigital emphasis has evolved in the past 10-15 years

• Grandly hierarchicalGrandly hierarchical– ““Routing” is based on the hierarchyRouting” is based on the hierarchy

• Country code -> select the appropriate PTTCountry code -> select the appropriate PTT• 3 digit area code -> regional long distance exchange3 digit area code -> regional long distance exchange• 3 digit exchange -> loop’s central office3 digit exchange -> loop’s central office• 4 digit subscriber number -> local loop wiring4 digit subscriber number -> local loop wiring

– Routing within a regionRouting within a region• Ad-hoc but fixedAd-hoc but fixed• Mostly relies on redundant connections to all destinationsMostly relies on redundant connections to all destinations



Telephone connection sequenceTelephone connection sequence

• Both phones are on-hook, One goes off-hookBoth phones are on-hook, One goes off-hook• End office sends dial toneEnd office sends dial tone• Caller dials a numberCaller dials a number• Switch uses this as the ‘called address’Switch uses this as the ‘called address’• If called address is not busy, make it ringIf called address is not busy, make it ring• Send ring tone to callerSend ring tone to caller• If called phone goes off hook, connect the callIf called phone goes off hook, connect the call• Turn off the ring signalTurn off the ring signal• Continue the connection till a phone goes on-Continue the connection till a phone goes on-

hookhook


Office-to-office connectionOffice-to-office connection

• Originating office finds a free connection on an Originating office finds a free connection on an interoffice trunkinteroffice trunk

• Sends a request for a ‘digit register’ to receive Sends a request for a ‘digit register’ to receive the called numberthe called number

• Destination sends a ‘wink’ when it has a digit Destination sends a ‘wink’ when it has a digit register for originator to useregister for originator to use

• Originator sends the number to the destination Originator sends the number to the destination officeoffice

• The destination connects to the end subscriber The destination connects to the end subscriber loop, or continues through another officeloop, or continues through another office


Signaling System 7 (SS7)Signaling System 7 (SS7)

• Today, trunks use SS7 for control signalingToday, trunks use SS7 for control signaling• Packet technology + POTS office architecturePacket technology + POTS office architecture

– ““Offices” are now called “switches”Offices” are now called “switches”– Highly redundantHighly redundant

• Supports modern capabilitiesSupports modern capabilities– Phone numbers not tied to hardware (subscriber loop)Phone numbers not tied to hardware (subscriber loop)– Phone numbers ‘roam’ geographicallyPhone numbers ‘roam’ geographically– Remote voice mailRemote voice mail– Toll free numbers (800 etc)Toll free numbers (800 etc)– Special charge numbers (900 etc)Special charge numbers (900 etc)


Elements of SS7Elements of SS7

• These devices are deployed redundantlyThese devices are deployed redundantly• Service Switching Points (SSPs)Service Switching Points (SSPs)

– Connect to subscribers – local loopsConnect to subscribers – local loops– Connect to STPs via SS7Connect to STPs via SS7– Sends queries to SCPs to find out how to route a callSends queries to SCPs to find out how to route a call

• Service Transfer Points (STPs)Service Transfer Points (STPs)– A packet switch tailored to handle SS7A packet switch tailored to handle SS7– Routes data based on phone numbersRoutes data based on phone numbers– Firewalling traffic from ‘external’ networksFirewalling traffic from ‘external’ networks

• Service Control Points (SCPs)Service Control Points (SCPs)– Centralized databasesCentralized databases– Links particular phone numbers to particular subscribersLinks particular phone numbers to particular subscribers– Provides routing information for reaching subscribersProvides routing information for reaching subscribers


WANs - The Telcos' parting attempts at WANs - The Telcos' parting attempts at relevancerelevance• They really are mired in an existing business They really are mired in an existing business

model and customer basemodel and customer base• Makes it hard for them to deal with the Makes it hard for them to deal with the

changing data comm landscapechanging data comm landscape• You can almost see how modern services like You can almost see how modern services like

ATM reflect demands by particular (large) ATM reflect demands by particular (large) customers with particular expectationscustomers with particular expectations

• Telcos still exist because they can meet these Telcos still exist because they can meet these demands and charge high tariffs for them.demands and charge high tariffs for them.


Classic WAN LineupClassic WAN Lineup

• "Leased Lines" - dedicated point to point "Leased Lines" - dedicated point to point connections (archaic!!)connections (archaic!!)

• Most of these were a fixed (huge!) cost per Most of these were a fixed (huge!) cost per monthmonth

• Cost tied to distance of connectionCost tied to distance of connection– Analog - an ancient and relatively slow service (56K)Analog - an ancient and relatively slow service (56K)– Digital Data Service - a slightly less ancient and slow service Digital Data Service - a slightly less ancient and slow service

(56K)(56K)– T-1 - the workhorse for early Internet sites 1.54MT-1 - the workhorse for early Internet sites 1.54M– T-3 - something of an improvement: 44.7MT-3 - something of an improvement: 44.7M


Newer ServicesNewer Services

• Frame Relay - more recent service Frame Relay - more recent service – (talk more about it in a minute) (talk more about it in a minute) – 44.7M44.7M– Charge per month for the connecting portCharge per month for the connecting port– Added charge per month for each virtual circuit's capacityAdded charge per month for each virtual circuit's capacity– No extra charge for longer distancesNo extra charge for longer distances

• Synchro Optical Net (SONET) 51.4M to ...Synchro Optical Net (SONET) 51.4M to ...– Standard designation for optical hardware connectionsStandard designation for optical hardware connections– ““OC” numbersOC” numbers

• OC-1 (or STS-1) at 50Mb/secOC-1 (or STS-1) at 50Mb/sec– thruthru

• OC-192 (STS-192) at 9.6Gb/secOC-192 (STS-192) at 9.6Gb/sec• STS-768 at 38 Gb/sec.. etc.STS-768 at 38 Gb/sec.. etc.


"Switched Services”"Switched Services”

• gee, a choice of destinations!gee, a choice of destinations!• Dial-up analog - the classic modem connection 56KDial-up analog - the classic modem connection 56K• X.25 packet switching - now archaic 56KX.25 packet switching - now archaic 56K• ISDN -ISDN -

– a first attempt at integrated ditigtal service: a first attempt at integrated ditigtal service: – up to 1.54Mup to 1.54M– cost per month plus connect time charge + long distance chargescost per month plus connect time charge + long distance charges

• ADSL - something more contemporary, but aging: ADSL - something more contemporary, but aging: – up to 9Mup to 9M

• Frame Relay - see, both switched and unswitchedFrame Relay - see, both switched and unswitched• ATM - the Great White Hope of the telcosATM - the Great White Hope of the telcos

– if this doesn't bring in business, they're historyif this doesn't bring in business, they're history– Pricing structure varies, but is not usually distance sensitivePricing structure varies, but is not usually distance sensitive


•Trade-offs between choicesTrade-offs between choices

• Cost structure: per link, per connection, per Cost structure: per link, per connection, per packet, distance sensitive, etc.packet, distance sensitive, etc.

• Switched vs unswitchedSwitched vs unswitched

• Channels per physical link: all in one, or Channels per physical link: all in one, or multiplexedmultiplexed

• Reliability and flow control: network or Reliability and flow control: network or endpoint responsibility?endpoint responsibility?


X-25 Network ProtocolX-25 Network Protocol

• Telco industry’s first - unsuccessful - attempt Telco industry’s first - unsuccessful - attempt to build a networking protocolto build a networking protocol

• Designed a "smart network“Designed a "smart network“• Misused the notion of a protocol stackMisused the notion of a protocol stack

– used it to establish independence among protocol designers at used it to establish independence among protocol designers at different levels -different levels -

– led to serious inefficienciesled to serious inefficiencies– Flow control and error correction replicated at layers 2 and 3Flow control and error correction replicated at layers 2 and 3


X.25 ArchitectureX.25 Architecture

• Telcos took as an article of faith that Telcos took as an article of faith that connections are fundamentalconnections are fundamental– Embedded per-connection overhead in individual network Embedded per-connection overhead in individual network

switchesswitches– Personally, I implemented X.25 over the Arpanet backbone Personally, I implemented X.25 over the Arpanet backbone

without such foolishness and it worked fine.without such foolishness and it worked fine.– Flow control took some fine-tuning, but that worked, too.Flow control took some fine-tuning, but that worked, too.

• ServicesServices– Cost per packet - I remember this; probably a link cost, tooCost per packet - I remember this; probably a link cost, too– Multiple channels per link possibleMultiple channels per link possible– Switched and unswitched channels possible ('permanent' Switched and unswitched channels possible ('permanent'

virtual circuits)virtual circuits)


Frame RelayFrame Relay

• A "dumber network" than X.25A "dumber network" than X.25– closer to “end to end” Internet architecture conceptcloser to “end to end” Internet architecture concept

• WAN with unreliable datagrams and no flow WAN with unreliable datagrams and no flow controlcontrol– Relies on end-to-end protocols like TCP to handle flow control Relies on end-to-end protocols like TCP to handle flow control

and error correctionand error correction– 'Smarter' than datagrams – 'Smarter' than datagrams –

• retains order of transmission on a channelretains order of transmission on a channel– Stallings argues that this works because modern digital Stallings argues that this works because modern digital

transmission methods are more reliable than the analog transmission methods are more reliable than the analog modem-based techniquesmodem-based techniques

– Greatly increased network efficiency and reduced transmission Greatly increased network efficiency and reduced transmission delays by eliminating "smart network" protocol overheaddelays by eliminating "smart network" protocol overhead


Protocol detailsProtocol details

• Multiple channels – Multiple channels – – channel 0 for linking other channels to endpointschannel 0 for linking other channels to endpoints

• Each channel can have its own endpoint – Each channel can have its own endpoint – – either predefined or on a "per call" basiseither predefined or on a "per call" basis– Like ‘virtual circuits’ on X.25Like ‘virtual circuits’ on X.25

• Individual packets carry a channel number or Individual packets carry a channel number or "Data Link Connection Identifier" (DLCI)."Data Link Connection Identifier" (DLCI).


Setting up a connectionSetting up a connection

• Initating host sends a SETUP packet - crosses the Initating host sends a SETUP packet - crosses the network to the destination, delivered to destination network to the destination, delivered to destination host.host.

• Destination host accepts by sending a CONNECT Destination host accepts by sending a CONNECT packet - goes back to the initiating host.packet - goes back to the initiating host.

• The SETUP/CONNECT protocol establishes a channel, The SETUP/CONNECT protocol establishes a channel, assigns a DLCI.assigns a DLCI.

• When connection finished, send a RELEASE to other When connection finished, send a RELEASE to other endend

• Other end responds with RELEASE COMPLETEOther end responds with RELEASE COMPLETE• No big deal - just different names for the same sort of No big deal - just different names for the same sort of

thing.thing.


Congestion controlCongestion control• Not much.Not much.• "Danger Will Robinson" bit – "Danger Will Robinson" bit –

– says that there's congestion in one direction or the other. says that there's congestion in one direction or the other. – "Forward/Backward Explicit Congestion Notification" FECN or BECN)"Forward/Backward Explicit Congestion Notification" FECN or BECN)

• "Sacrificial Lamb" bit – "Sacrificial Lamb" bit – – says this packet is a good one to discard if things are too congested. says this packet is a good one to discard if things are too congested. – "Discard Eligibility" DE"Discard Eligibility" DE

• Implement multiple transmission rates, based on what Implement multiple transmission rates, based on what is paid foris paid for– Committed Info Rate (CIR) - what's paid forCommitted Info Rate (CIR) - what's paid for– Maximum Rate (MR) - what is acceptedMaximum Rate (MR) - what is accepted– Access Rate – Access Rate –

• what the link accepts – what the link accepts – • excess past MR gets discardedexcess past MR gets discarded


•ATM or "Cell Relay“ATM or "Cell Relay“

• A "cell" is a "frame" only it's supposed to be A "cell" is a "frame" only it's supposed to be transmitted faster.transmitted faster.– Dumber and more efficient than X.25Dumber and more efficient than X.25– Cell sequence is preservedCell sequence is preserved

• Basic Features Basic Features – Virtual channelsVirtual channels– Packet format/featuresPacket format/features– Service categoriesService categories


Virtual paths and virtual channelsVirtual paths and virtual channels

• Users see virtual channels as logical Users see virtual channels as logical connectionsconnections

• Virtual paths are a network level property: Virtual paths are a network level property: – represents a set of virutal channels with a common destination represents a set of virutal channels with a common destination

– – – network handles them as an aggregated entity instead of network handles them as an aggregated entity instead of

handling the channels individuallyhandling the channels individually


Packet formatPacket format

• Packet destination = virtual path + virtual Packet destination = virtual path + virtual channel within pathchannel within path

• Payload type = user data vs system data, Payload type = user data vs system data, – also includes info about congestionalso includes info about congestion– poor flow control againpoor flow control again

• Sacrificial lamb bit - "Cell Loss Priority" (CLP)Sacrificial lamb bit - "Cell Loss Priority" (CLP)• 8-bit checksum for the header8-bit checksum for the header

– since bit errors could cause pain to the networksince bit errors could cause pain to the network


ATM Service categories ATM Service categories

• or, "I'm a big customer and you'd better provide me the or, "I'm a big customer and you'd better provide me the category of service I want or I'm calling in the category of service I want or I'm calling in the competition."competition."– + Constant bit rate (CBR) - traditional connection service+ Constant bit rate (CBR) - traditional connection service

– + Variable Bit Rat (VBR) - gives network more flexibility and lower cost + Variable Bit Rat (VBR) - gives network more flexibility and lower cost to the customerto the customer

– + Unspecified Bit Rate (UBR) - 'best effort' service - give it whatever + Unspecified Bit Rate (UBR) - 'best effort' service - give it whatever bandwidth is left overbandwidth is left over

– + Avaliable bit rate (ABR) - specifies a minimum cell rate required + Avaliable bit rate (ABR) - specifies a minimum cell rate required (MCR) and a peak rate (PCR). Connects LANs across ATM(MCR) and a peak rate (PCR). Connects LANs across ATM

– + Guaranteed Frame Rate (GFR) + Guaranteed Frame Rate (GFR)

• - for connecting to Internet backbone. Has the ATM net understand - for connecting to Internet backbone. Has the ATM net understand frame boundaries, so packets are discareded in "frame" sets frame boundaries, so packets are discareded in "frame" sets instead of individually, possibly from separate frames.instead of individually, possibly from separate frames.


Creative Commons LicenseCreative Commons License

This work is licensed under the Creative This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United Commons Attribution-Share Alike 3.0 United

States License. To view a copy of this license, States License. To view a copy of this license, visit http://creativecommons.org/licenses/by-visit http://creativecommons.org/licenses/by-

sa/3.0/us/ or send a letter to Creative sa/3.0/us/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Commons, 171 Second Street, Suite 300, San

Francisco, California, 94105, USA.Francisco, California, 94105, USA.

Documents

CISC 370 - Class Today