Computer Communications DIT 420 EDA343

Transcription

1 Computer Communications DIT 420 EDA343 Summary Computer Communication 1

2 Important for the exam When/where:Friday Dec 21, , H You may have with you: English-X dictionary no calculators, PDAs, etc (if/where numbers matter, do rounding) Grading 30-40, 41-50, (out of 60)= 3, 4, 5 (CTH) 30-50, (out of 60) = G, VG (GU) To think during last, summary-study Overview; critical eye; explain: why is this so? / How does it work? Computer Communication 2

3 Flashback Computer Communication 3

4 Principles, Organisation Network Problems (in the order faced in the 1st intro): producer-consumer problems, flow and error control, manage to shared (broadcast) transmission media, routing, congestion, connecting transparently different works, serving different types of traffic, performance, mobility security Layering : principle, why Computer Communication 4

5 Highlights types of delay; performance reliable data transfer (flow, error control) datagram vs VC end-to-end communication, congestion control routing, also with mobility multiple protocols (wired, wireless) LANs and related technologies work security issues TCP/IP protocol stack (also applications), evolution (p2p applications, overlays, NAT, streaming apps) Computer Communication 5

6 Types of delay; performance Computer Communication 6

7 Types of delay; performance Propagation, transmission, queueing, processing Throughput (effective bandwidth) Utilization (efficiency) Packet-switching: impact of store&forward TCP s slow start Sliding windows performance initiate TCP connection request object RTT first window = S/R second window = 2S/R third window = 4S/R fourth window = 8S/R object delivered complete transmission A transmission propagation time at client time at server B nodal processing queuing Computer Communication 7

8 Reliable data transfer Computer Communication 8

9 Reliable data transfer Guaranteed, in-order, correct delivery: Flow control: stop&wait sliding windows sequence numbers window sizes dynamic windows (TCP) performance Error detection: checksums, CRC (*) Error control: go-back-n, selective repeat, FEC methods Computer Communication 9

10 Datagram vs VC end-to-end comm. Congestion Control RT traffic Computer Communication 10

11 Datagram vs VC end-to-end communication Conceptual differences Decisions, comparison, why ATM Network/ MPLS(*) Ether LANs Computer Communication 11

12 Congestion control (CC) why, how congestion occurs CC in TCP and performance; implied weaknesses CC in other ways, e.g. VC-based works RT-traffic resource reservation: traffic shaping and policing rate-based initiate TCP connection request object RTT object delivered time at client time a serve Computer Communication 12

13 RT/streaming traffic Conceptual needs: packet/flow marking Admission control Traffic shaping & policing Packet scheduling (switches) Inter context Application-level solutions (FEC, playout delay, caching-cdn) Intserv, Diffserv Computer Communication 13

14 Routing, also with mobility Computer Communication 14

15 Routing, also with mobility Routing algorithms Forwarding Resource, policy issues Addressing mobility, tunneling 2 A 1 5 B 2 D C E F Complementary video links - IP addresses and subs &list=ple9f3f05c381ed8e8&feature=plcp - How does BGP choose its routes &feature=plcp Mobile Switching Center Mobile Switching Center Computer Communication 15 Public telephone work, and Inter

16 Multiple algorithms Computer Communication 16

17 Medium : multiple methods Strategies: (functionality, appropriateness) Contention-based (random ), wired/wireless: Aloha, CSMA(CD/CA) (collision-delay trade-off) Collision-free: Channel partitioning: TDMA, FDMA, CDMA Taking turns: token-passing, reservation-based A B C A s signal strength C s signal strength Computer Communication 17 space

18 LANs & related technologies Computer Communication 18

19 LANs & related link technologies Protocol Examples: wired, wireless Ether, ( wimax), GSM: Functionality, performance under low/high load Connecting devices; functionalities and differences (Hubs, switches) Algorithms for switch- routing : learning& forwarding of packets ARP switch Computer Communication 19

20 BTW: A little backpatching Computer Communication 20

21 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields Error detection not 100% reliable! protocol may miss some errors, but this should happen only rarely larger EDC field yields better detection and correction 5: DataLink Layer 5-21

22 Inter checksums TCP (UDP) s checksum: segment contents = sequence of 16-bit integers checksum: addition (1 s complement sum) of segment contents sender puts checksum value into UDP (TCP) checksum field Cyclic redundancy check (CRC) more powerful error-detection coding data bits, D = binary number consider r+1 bit pattern (generator), G goal: compute r CRC bits, R, such that <D,R> exactly divisible by G (modulo 2) receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! can detect errors on less than r+1 bits International standards for G (CRC polynomials) 5: DataLink Layer 5-22

23 Recall we want: D. 2 r XOR R = ng equivalently: if we divide D. 2 r by G, want remainder R CRC Example r = 3 D R. 2 r = remainder[ ] G 5: DataLink Layer 5-23

24 Security issues Computer Communication 24

25 Security issues C, I, A and methods to achieve them Instantiation in Inter: RSA, PGP, authentication Firewalls and packet filtering Alice channel data, control messages Bob data secure sender s secure receiver data Trudy Computer Communication 25

26 TCP/IP protocol stack (also applications), evolution Computer Communication 26

27 TCP/IP protocol stack (also applications), evolution Instantiation of worksolutions (Routing, Congestion Control, Flow & error control, applications, link layer technologies) Limitations, advantages, updates Application-layer working (P2P applications, overlays, CDNs, multimedia/streaming application protocols) application transport work link physical Computer Communication 27

28 Items we discussed in less detail But exist in the slides you received Good to read (remember: reading is from the book! The slides cannot replavce the book reading) for own benefit; but need not consider for the exam next week CRC s Intserv, Diffserv, RSVP MPLS Computer Communication 28

29 Synthesis: a day in the life of a web request putting-it-all-together: synthesis! goal: identify, review protocols (at all layers) involved in seemingly simple scenario: requesting www page scenario: student attaches laptop to campus work, requests/receives

30 A day in the life: scenario browser DNS server Comcast work /13 school work /24 web page web server Google s work /

31 A day in the life connecting to the Inter DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP connecting laptop needs to get its own IP address: use DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy router (runs DHCP) DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in Ether Ether frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server Ether demux ed to IP demux ed to UDP demux ed to DHCP 5: DataLink Layer 5-31

32 A day in the life connecting to the Inter DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy DHCP server formulates DHCP ACK containing client s IP address (and also IP address of firsthop router for client, name & IP address of DNS server) DHCP DHCP DHCP DHCP DHCP DHCP UDP IP Eth Phy router (runs DHCP) frame forwarded (switch learning) through LAN, demultiplexing at client DHCP client receives DHCP ACK reply Client now has IP address, knows name & addr of DNS server, IP address of its first-hop router 5: DataLink Layer 5-32

33 A day in the life ARP (before DNS, before HTTP) DNS DNS DNS ARP query DNS UDP IP Eth Phy ARP ARP reply ARP Eth Phy before sending HTTP request, need IP address of DNS DNS query created, encapsulated in UDP, encapsulated in IP, encasulated in Eth. In order to send frame to router, need MAC address of router interface: ARP ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router, so can now send frame containing DNS query 5-33

34 A day in the life using DNS DNS DNS DNS DNS DNS DNS UDP IP Eth Phy DNS DNS DNS DNS DNS UDP IP Eth Phy Comcast work /13 DNS server IP datagram containing DNS query forwarded via LAN switch from client to 1 st hop router IP datagram forwarded from campus work to destination (DNS-server) work, routed (tables created by RIP, OSPF and BGP routing protocols) to DNS server demux ed to DNS server DNS server replies to client with IP address of 5: DataLink Layer 34

35 A day in the life TCP connection carrying HTTP HTTP SYNACK SYNACK SYNACK HTTP TCP IP Eth Phy SYNACK SYNACK SYNACK TCP IP Eth Phy web server to send HTTP request, client first opens TCP socket to web server TCP SYN segment (step 1 in 3-way handshake) interdomain routed to web server web server responds with TCP SYNACK TCP connection established! 5-35

36 A day in the life HTTP request/reply HTTP HTTP HTTP HTTP HTTP HTTP TCP IP Eth Phy web page finally (!!!) displayed HTTP HTTP HTTP HTTP HTTP TCP IP Eth Phy web server HTTP request sent into TCP socket IP datagram containing HTTP request routed to web server responds with HTTP reply (containing web page) IP datgram containing HTTP reply routed back to client 5-36

37 Synthesis cont. 5-37

38 Inter structure: work of works Question: given millions of ISPs, how to connect them together?

39 Inter structure: work of works Option: connect each ISP to every other ISP? connecting each ISP to each other directly doesn t scale: O(N 2 ) connections.

40 Inter structure: work of works Option: connect each ISP to a global transit ISP? Customer and provider ISPs have economic agreement. global ISP

41 Inter structure: work of works But if one global ISP is viable business, there will be competitors. ISP A ISP B ISP C

42 Inter structure: work of works But if one global ISP is viable business, there will be competitors. which must be interconnected Inter exchange point (ca 300 in the world; ISP A multiple ISPs peering/switching; 3 rd company) IXP ISP B ISP C peering link (no payment to each-other)

43 Inter structure: work of works and regional works may arise to connect s to ISPS ISP A IXP ISP B ISP C regional

44 Inter structure: work of works and content provider works (e.g., Google, Microsoft, Akamai ) may run their own work, to bring services, content close to end users ISP A ISP B Content provider work IXP regional ISP B

45 Inter structure: work of works Tier 1 ISP Tier 1 ISP Google IXP Regional ISP IXP Regional ISP IXP ISP ISP ISP ISP ISP ISP ISP ISP at center: small # of well-connected large works tier-1 commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage A new form of content provider work (e.g, Google): private work that connects it data centers to Inter, often bypassing tier-1, regional ISPs Introduction 1-45

46 Thank you & good luck in the exam!! When/where:Friday Dec 21, , H You may have with you: English-X dictionary no calculators, PDAs, etc (if/where numbers matter, do rounding) Grading 30-40, 41-50, (out of 60)= 3, 4, 5 (CTH) 30-50, (out of 60) = G, VG (GU) To think during last, summary-study Overview; critical eye; explain: why is this so? / How does it work? Computer Communication 46