CISC 370 - Class Today

1. CISC 370 - Class Today • The Lab • Project Schedule • Homework 7 and 8 • Recap • POTS R. Smith - University of St Thomas - Minnesota

2. Upcoming Homework • I’ll post some Chapter 12/13 homework soon • Outline: April 23 • I’ll e-mail comments to your group • Revised Outline (optional): Apr 30 • Papers: May 12 • Presentations: May 12 and 14 R. Smith - University of St Thomas - Minnesota

3. A Lab Problem • Someone plugged in the Firewall • Caused ‘some’ machines to get Net 10 addresses • The packets are discarded in most ‘real’ routers • This causes alarm bells in some routers • Fortunately we were not visited by angry IRT admins. • I.E. Someone hooked up wires incorrectly • The connection did NOT match the setup published in the Lab • This was also NAUGHTY. R. Smith - University of St Thomas - Minnesota

4. Homework Review: Chapter 7 • 7.1: Mailbagging • Good things: transmission efficiency, less intermediate storage • Bad things: More work for endpoints (arguable) • 7.3: One message is best • SMTP sends one copy per server • “Spam” vs “Velveeta” • An old Internet discussion • Spam = one message with many destinations • Velveeta = many messages to separate destinations R. Smith - University of St Thomas - Minnesota

5. 7.8: Tracing a Header • http://courseweb.stthomas.edu/resmith/c/cisc370/c9sp/headers.txt • E-mail Forensics • Every e-mail server prepends a ‘Received’ line • Ideally the information is accurate • In fact, it’s easy to forge • Each server leaves earlier ‘Received’ lines undisturbed • Typical forgery: modifies just the “From:” line. • We can spot fakes by looking for inaccurate details • Do “Received:” lines match the “From:” line? • Do domain names match IP addresses? • Do date/time stamps make sense? R. Smith - University of St Thomas - Minnesota

6. Homework: Chapter 8 • 8.1: a=net bits, b=host bits, c=#nets, d=#hosts, e=octet range • Class 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 27= 128 – 2 (0 and 127 are disallowed) = 126 networks, • (d) 224= 16,777,216 – 2 (host address cannot be all 0’s or all 1’s) = 16,777,214 hosts • (e) range: 1 through 126 • 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 bits), so 214 = 16,384 networks, • (d) 224 = 65,536 – 2 (host address cannot be all 0’s or all 1’s) = 65,534 hosts, (e) range: 128 through 191 • 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 bits), so 221 = 2,097,152 networks, • (d) 28 = 256 – 2 (host address cannot be all 0’s or all 1’s) = 254 hosts, • (e) range: 192 through 223 R. Smith - University of St Thomas - Minnesota

7. Chapter 8 homework, continued • 8.5: Address 192.168.100; mask 255.255.192.0 • Class B-C “CIDR” address – no subnets, 214 hosts (16K) • #0 bits = 14 – If taken as Class B: • a) # Subnet bits = 16-15 = 2 bits => 4 subnets • b) # Host bits = 14 => 16K hosts • 8.8: Exhausting address spaces • 31,557,600 seconds per year, per 10 seconds; per second • IPV4: easy answer = 232 => 4 billion or so => 1,360 years • IPV4: “book solution” = 221 “available” => 242 days • IPV6: based on book misprint = 2125 => 35 min • IPV6: for real = 2125 => 1030 years • 8.9: Finding mask and gateway • # of hosts on network R. Smith - University of St Thomas - Minnesota

8. The Plain Old Telephone System • POTS • Architecture (recap) • SS-7 • WANs from the POTS folks • X.25 • Frame Relay • ATM R. Smith - University of St Thomas - Minnesota

9. Traditional POTS Architecture • Provides analog connections to endpoints • Digital features added atop analog voice-quality signals • Digital emphasis has evolved in the past 10-15 years • Grandly hierarchical • “Routing” is based on the hierarchy • Country code -> select the appropriate PTT • 3 digit area code -> regional long distance exchange • 3 digit exchange -> loop’s central office • 4 digit subscriber number -> local loop wiring • Routing within a region • Ad-hoc but fixed • Mostly relies on redundant connections to all destinations R. Smith - University of St Thomas - Minnesota

10. Telephone connection sequence • Both phones are on-hook, One goes off-hook • End office sends dial tone • Caller dials a number • Switch uses this as the ‘called address’ • If called address is not busy, make it ring • Send ring tone to caller • If called phone goes off hook, connect the call • Turn off the ring signal • Continue the connection till a phone goes on-hook R. Smith - University of St Thomas - Minnesota

11. Office-to-office connection • Originating office finds a free connection on an interoffice trunk • Sends a request for a ‘digit register’ to receive the called number • Destination sends a ‘wink’ when it has a digit register for originator to use • Originator sends the number to the destination office • The destination connects to the end subscriber loop, or continues through another office R. Smith - University of St Thomas - Minnesota

12. Signaling System 7 (SS7) • Today, trunks use SS7 for control signaling • Packet technology + POTS office architecture • “Offices” are now called “switches” • Highly redundant • Supports modern capabilities • Phone numbers not tied to hardware (subscriber loop) • Phone numbers ‘roam’ geographically • Remote voice mail • Toll free numbers (800 etc) • Special charge numbers (900 etc) R. Smith - University of St Thomas - Minnesota

13. Elements of SS7 • These devices are deployed redundantly • Service Switching Points (SSPs) • Connect to subscribers – local loops • Connect to STPs via SS7 • Sends queries to SCPs to find out how to route a call • Service Transfer Points (STPs) • A packet switch tailored to handle SS7 • Routes data based on phone numbers • Firewalling traffic from ‘external’ networks • Service Control Points (SCPs) • Centralized databases • Links particular phone numbers to particular subscribers • Provides routing information for reaching subscribers R. Smith - University of St Thomas - Minnesota

14. WANs - The Telcos' parting attempts at relevance • They really are mired in an existing business model and customer base • Makes it hard for them to deal with the changing data comm landscape • You can almost see how modern services like ATM reflect demands by particular (large) customers with particular expectations • Telcos still exist because they can meet these demands and charge high tariffs for them. R. Smith - University of St Thomas - Minnesota

15. Classic WAN Lineup • "Leased Lines" - dedicated point to point connections (archaic!!) • Most of these were a fixed (huge!) cost per month • Cost tied to distance of connection • Analog - an ancient and relatively slow service (56K) • Digital Data Service - a slightly less ancient and slow service (56K) • T-1 - the workhorse for early Internet sites 1.54M • T-3 - something of an improvement: 44.7M R. Smith - University of St Thomas - Minnesota

16. Newer Services • Frame Relay - more recent service • (talk more about it in a minute) • 44.7M • Charge per month for the connecting port • Added charge per month for each virtual circuit's capacity • No extra charge for longer distances • Synchro Optical Net (SONET) 51.4M to ... • Standard designation for optical hardware connections • “OC” numbers • OC-1 (or STS-1) at 50Mb/sec • thru • OC-192 (STS-192) at 9.6Gb/sec • STS-768 at 38 Gb/sec.. etc. R. Smith - University of St Thomas - Minnesota

17. "Switched Services” • gee, a choice of destinations! • Dial-up analog - the classic modem connection 56K • X.25 packet switching - now archaic 56K • ISDN - • a first attempt at integrated ditigtal service: • up to 1.54M • cost per month plus connect time charge + long distance charges • ADSL - something more contemporary, but aging: • up to 9M • Frame Relay - see, both switched and unswitched • ATM - the Great White Hope of the telcos • if this doesn't bring in business, they're history • Pricing structure varies, but is not usually distance sensitive R. Smith - University of St Thomas - Minnesota

18. Trade-offs between choices • Cost structure: per link, per connection, per packet, distance sensitive, etc. • Switched vs unswitched • Channels per physical link: all in one, or multiplexed • Reliability and flow control: network or endpoint responsibility? R. Smith - University of St Thomas - Minnesota

19. X-25 Network Protocol • Telco industry’s first - unsuccessful - attempt to build a networking protocol • Designed a "smart network“ • Misused the notion of a protocol stack • used it to establish independence among protocol designers at different levels - • led to serious inefficiencies • Flow control and error correction replicated at layers 2 and 3 R. Smith - University of St Thomas - Minnesota

20. X.25 Architecture • Telcos took as an article of faith that connections are fundamental • Embedded per-connection overhead in individual network switches • Personally, I implemented X.25 over the Arpanet backbone without such foolishness and it worked fine. • Flow control took some fine-tuning, but that worked, too. • Services • Cost per packet - I remember this; probably a link cost, too • Multiple channels per link possible • Switched and unswitched channels possible ('permanent' virtual circuits) R. Smith - University of St Thomas - Minnesota

21. Frame Relay • A "dumber network" than X.25 • closer to “end to end” Internet architecture concept • WAN with unreliable datagrams and no flow control • Relies on end-to-end protocols like TCP to handle flow control and error correction • 'Smarter' than datagrams – • retains order of transmission on a channel • Stallings argues that this works because modern digital transmission methods are more reliable than the analog modem-based techniques • Greatly increased network efficiency and reduced transmission delays by eliminating "smart network" protocol overhead R. Smith - University of St Thomas - Minnesota

22. Protocol details • Multiple channels – • channel 0 for linking other channels to endpoints • Each channel can have its own endpoint – • either predefined or on a "per call" basis • Like ‘virtual circuits’ on X.25 • Individual packets carry a channel number or "Data Link Connection Identifier" (DLCI). R. Smith - University of St Thomas - Minnesota

23. Setting up a connection • Initating host sends a SETUP packet - crosses the network to the destination, delivered to destination host. • Destination host accepts by sending a CONNECT packet - goes back to the initiating host. • The SETUP/CONNECT protocol establishes a channel, assigns a DLCI. • When connection finished, send a RELEASE to other end • Other end responds with RELEASE COMPLETE • No big deal - just different names for the same sort of thing. R. Smith - University of St Thomas - Minnesota

24. Congestion control • Not much. • "Danger Will Robinson" bit – • says that there's congestion in one direction or the other. • "Forward/Backward Explicit Congestion Notification" FECN or BECN) • "Sacrificial Lamb" bit – • says this packet is a good one to discard if things are too congested. • "Discard Eligibility" DE • Implement multiple transmission rates, based on what is paid for • Committed Info Rate (CIR) - what's paid for • Maximum Rate (MR) - what is accepted • Access Rate – • what the link accepts – • excess past MR gets discarded R. Smith - University of St Thomas - Minnesota

25. ATM or "Cell Relay“ • A "cell" is a "frame" only it's supposed to be transmitted faster. • Dumber and more efficient than X.25 • Cell sequence is preserved • Basic Features • Virtual channels • Packet format/features • Service categories R. Smith - University of St Thomas - Minnesota

26. Virtual paths and virtual channels • Users see virtual channels as logical connections • Virtual paths are a network level property: • represents a set of virutal channels with a common destination – • network handles them as an aggregated entity instead of handling the channels individually R. Smith - University of St Thomas - Minnesota

27. Packet format • Packet destination = virtual path + virtual channel within path • Payload type = user data vs system data, • also includes info about congestion • poor flow control again • Sacrificial lamb bit - "Cell Loss Priority" (CLP) • 8-bit checksum for the header • since bit errors could cause pain to the network R. Smith - University of St Thomas - Minnesota

28. ATM Service categories • or, "I'm a big customer and you'd better provide me the category of service I want or I'm calling in the competition." • + Constant bit rate (CBR) - traditional connection service • + Variable Bit Rat (VBR) - gives network more flexibility and lower cost to the customer • + Unspecified Bit Rate (UBR) - 'best effort' service - give it whatever bandwidth is left over • + Avaliable bit rate (ABR) - specifies a minimum cell rate required (MCR) and a peak rate (PCR). Connects LANs across ATM • + Guaranteed Frame Rate (GFR) • - for connecting to Internet backbone. Has the ATM net understand frame boundaries, so packets are discareded in "frame" sets instead of individually, possibly from separate frames. R. Smith - University of St Thomas - Minnesota

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