Computer Networks. Chapter 1 - Fundamentals. CEN 5501C - Computer Networks - Spring UF/CISE - Newman 1

Transcription

1 Computer Networks Chapter 1 - Fundamentals CEN 5501C - Computer Networks - Spring UF/CISE - Newman 1

2 Computer Networks Need to share Information Resources Communication vs. Storage Transmission across: Space (communication) Time (storage) CEN 5501C - Computer Networks - Spring UF/CISE - Newman 2

3 Virtuality Architecture Layered Hierarchical Algorithms Information hiding, ADTs, objects Protocols Distributed coordination algorithms Programs Modularity CEN 5501C - Computer Networks - Spring UF/CISE - Newman 3

4 Layered vs. Hierarchical Both Peer-to-peer communication Encapsulation Protocol = common language/behaviors Layered Layer i serves layer i+1 ONLY Layer i gets service from layer i-1 ONLY Hierarchical Lower layers serve higher layers CEN 5501C - Computer Networks - Spring UF/CISE - Newman 4

5 Layered vs. Hierarchical Issues Flexibility Efficiency Modularity Maintainability Scalability/manageability Future adaptability CEN 5501C - Computer Networks - Spring UF/CISE - Newman 5

6 Comparative Architectures 7 - Application 6 - Presentation 5 - Session 4 - Transport 3 - Network 2 Data Link Application Functional Management Data Services Data Flow Control Transmission Ctl Path Control Data Link Control User Network Application Network Services Protocol Transport DDCMP 1 - Physical Physical 1 - Physical ISO - OSI IBM - SNA DEC - DECnet CEN 5501C - Computer Networks - Spring UF/CISE - Newman 6

7 OSI Reference Architecture Physical (L1 = PHY) Data Link (L2 = MAC/Link) Network (L3) Transport (L4) Session (L5) Presentation (L6) Application (L7) CEN 5501C - Computer Networks - Spring UF/CISE - Newman 7

8 PHY Layer Hardware Physical manipulation of medium (modulation) Physical sensing of medium (detection) Low level synchronization (bits/symbols/frames) Forward error correction/error detection Mechanical/electrical interconnect and medium CEN 5501C - Computer Networks - Spring UF/CISE - Newman 8

9 Link Layer Firmware Framing Addressing Medium access control (MAC) Backward error detection/correction Reliable delivery of frames from one STA to a directly connected STA Pacing Upward multiplexing CEN 5501C - Computer Networks - Spring UF/CISE - Newman 9

10 Network Layer Software/firmware Packets/cells Routing Packet fragmentation/reassembly Backward error correction Delivery of frames from source to an indirectly connected destination Congestion control CEN 5501C - Computer Networks - Spring UF/CISE - Newman 10

11 Transport Layer Software on end host: End-to-end layer Reliable communication stream Messages Byte stream Ordering BEC Upward Multiplexing Delivery of messages/byte stream from source process to destination process Congestion control CEN 5501C - Computer Networks - Spring UF/CISE - Newman 11

12 Session Layer Software on end host: end-to-end layer Stream management Dialog control Packet chaining (atomic delivery) Downward Multiplexing Authentication Connection-oriented CEN 5501C - Computer Networks - Spring UF/CISE - Newman 12

13 Presentation Layer Software on end host Common utilities Encryption Compression Uniform formatting (XML, ASN.1, ) Standardized representations Interfacing to local resources CEN 5501C - Computer Networks - Spring UF/CISE - Newman 13

14 Application Layer Software on end host Specific application programs FTP Remote terminal (rlogin, telnet, ssh, ) HTTP May also be layered in distributed software system CEN 5501C - Computer Networks - Spring UF/CISE - Newman 14

15 Encapsulation Receive SDU from higher layer Hide uninterpreted SDU as payload of PDU source destination Hl Ht HnHt HnHt M M M M application transport network link physical application transport network link physical Hl Ht HnHt HnHt M M M M message segment datagram frame (thanks Kurose & Ross) CEN 5501C - Computer Networks - Spring UF/CISE - Newman 15

16 End-to-end Data Transport (thanks Kurose & Ross) CEN 5501C - Computer Networks - Spring UF/CISE - Newman 16

17 Service Models Interface Connectionless Connection-oriented Reliability Best effort Reliable Combinations CEN 5501C - Computer Networks - Spring UF/CISE - Newman 17

18 Service Interface Interface Connectionless: memoryless Send packet Receive packet Connection-oriented: stateful Initialize (set up connection) Use (send/receive) Close (release state) CEN 5501C - Computer Networks - Spring UF/CISE - Newman 18

19 Service Reliability Reliability Best effort Lost packets Duplicate packets Delayed/reordered delivery Damaged packets Reliable Undamaged packets All packets sent delivered in timely fashion Delivered in order sent CEN 5501C - Computer Networks - Spring UF/CISE - Newman 19

20 Message Conventions Request/Confirm Indication/Response 1-REQ 8 - CNF 5 - RSP 4 - IND 2,3 6,7 2 - Tx 7 - Rx 6 - Tx 3 - Rx CEN 5501C - Computer Networks - Spring UF/CISE - Newman 20

21 Combining Service Models Best Effort Reliable Connectionless UDP, IP, IPX, CLNP, DECnet, Appletalk, CLNS,??? Connection-Oriented ATM TCP, X.25, CONS CEN 5501C - Computer Networks - Spring UF/CISE - Newman 21

22 Network Service vs. Implementation Implementation Service Connectionless Connection-Oriented Connectionless UDP, CLNS??? Connection-Oriented TCP, DNA X.25, ATM, CONS, SNA CEN 5501C - Computer Networks - Spring UF/CISE - Newman 22

23 Network Properties Scope Scalability Robustness Autoconfigurability Tweakability Determinism Migration CEN 5501C - Computer Networks - Spring UF/CISE - Newman 23

24 Network Properties - Robustness Types of Errors Link/node failure Data errors (esp. undetected!) S/W errors H/W errors Human Errors Features CEN 5501C - Computer Networks - Spring UF/CISE - Newman 24

25 Network Properties - Robustness Types of Errors Features Safety Barriers Self-stabilization Fault detection Byzantine robustness CEN 5501C - Computer Networks - Spring UF/CISE - Newman 25

26 Reliable Data Transfer Models Errors Receiver capacity Requirements Duplexity Timers State CEN 5501C - Computer Networks - Spring UF/CISE - Newman 26

27 Data Transfer Model Events What can happen at node, channel Frames What do they hold Duplexity Simplex, half duplex, full duplex Time costs What does it take to complete transfer Metrics How do we measure the costs CEN 5501C - Computer Networks - Spring UF/CISE - Newman 27

28 Data Transfer - Events Node New frame to send from HLE Frame/ACK arrival good frame Frame/ACK arrival damaged frame Timeout Attempt to receive next frame by HLE Channel Error damage frame Error lose frame CEN 5501C - Computer Networks - Spring UF/CISE - Newman 28

29 Data Transfer - Frames Forward Control Info Type Sequence number Timestamp Length Addressing Error Detection (FCS) Reverse Control Info ACKs Flow control/pacing Piggybacking Information payload Dst Src Type SN TS Len Payload FCS CEN 5501C - Computer Networks - Spring UF/CISE - Newman 29

30 Channel Model - Duplexity A B A B A B Simplex Half Duplex Full Duplex Simplex only one way Half duplex one way at a time Full duplex simultaneously both ways CEN 5501C - Computer Networks - Spring UF/CISE - Newman 30

31 Processing Data Transfer - Delays Source Destination Transmission Time to put bits on wire Propagation Time for bit to traverse channel Src Dest Tx REQ Src Proc Transmission Propagation Propagation Tx CNF Data frame Dest Proc ACK frame ACK Tx Rx REQ Src Proc Rx CNF CEN 5501C - Computer Networks - Spring UF/CISE - Newman 31

32 Data Transfer - Metrics Utilization Time sending info/total time Storage requirements At source At destination Channel type duplexity Timers Retransmission ACK transmission CEN 5501C - Computer Networks - Spring UF/CISE - Newman 32

33 Reliable Data Xfer - Utopia Infinitely fast receiver Simplex channel No errors 100% utilization by protocol Src Dest CEN 5501C - Computer Networks - Spring UF/CISE - Newman 33

34 Stop&Wait Data Xfer - Pacing Finitely fast receiver Half duplex channel No errors <100% utilization by protocol U Protocol = T Tx /T cycle Data time Src OK Dest T Tx = L (bits)/r (bps) T cycle = T Tx +T prop +T proc + T prop +T proc Cycle time OK CEN 5501C - Computer Networks - Spring UF/CISE - Newman 34

35 Reliable Data Xfer PAR (Positive ACK and Retransmit) Finitely fast receiver Full duplex channel Channel errors Utilization factor due to errors - Src ACK X Dest U errors = T good / T good + T bad U = U protocol x U errors * ACK CEN 5501C - Computer Networks - Spring UF/CISE - Newman 35 -

36 Reliable Data Xfer PAR Src need for sequence numbers Dest Src Dest First Frame Frame 1 - ACK - ACK Second Frame X Frame 2 * Missing ACK Second Frame - duplicate * X Missing ACK Frame 2 - duplicate - ACK - ACK CEN 5501C - Computer Networks - Spring UF/CISE - Newman 36

37 Reliable Data Xfer 1-bit ARQ Src Dest Src Dest Frame 0 Frame 0 - ACK 1 accept - ACK 1 accept * Frame 1 Missing ACK Frame 1 X * Frame 1 ACK 0 X Missing ACK Frame 1 accept - ACK 0 accept - ACK 0 discard duplicate CEN 5501C - Computer Networks - Spring UF/CISE - Newman 37

38 Protocol Utilization 1-bit ARQ Protocol util. Src Dest U proto = T/(T+2τ) T = Tx time = D/R D = size (bits/frame) T τ τ ACK 1 Frame 0 Cycle Time R = data rate (bps) τ = propagation delay Frame 1 U proto = 1/(1+2α) ACK 0 α = τ /T = τ /(D/R) CEN 5501C - Computer Networks - Spring UF/CISE - Newman 38

39 Protocol Utilization 1-bit ARQ 1-bit ARQ Efficiency Efficiency Normalized delay CEN 5501C - Computer Networks - Spring UF/CISE - Newman 39

40 Reliable Data Xfer GBN ARQ Multiple sequence # s Channel errors Discard out of order frames on Rx Resend all frames from missing on * forward Rx buffer size of 1 - Tx buffer size of N Src ACK 1 ACK 2 ACK 3 ACK 3 ACK 3 Dest CEN 5501C - Computer Networks - Spring UF/CISE - Newman 40 D 0 D 1 D 2 D 4 D 5 D 3 (resent) D 4 D 5 ACK 4 ACK 5 ACK 6 Accept D0 Accept D1 Accept D2 Missing D3 Discard D4 Discard D5 Accept D3 Accept D4 Accept D5

41 Reliable Data Xfer SR ARQ Src Dest Multiple sequence # s Channel errors Buffer out of order frames on Rx Resend only missing * frame Rx buffer size of N - Tx buffer size of k ACK 1 ACK 2 ACK 3 ACK 3 ACK 3 ACK 6 ACK 7 ACK 8 Accept D0 Accept D1 Accept D2 Missing D3 CEN 5501C - Computer Networks - Spring UF/CISE - Newman 41 D 0 D 1 D 2 D 4 D 5 D 3 (resent) D 6 D 7 Buffer D4 Buffer D5 Accept D3 Accept D6 Accept D7

42 Protocol Utilization ARQ Protocol util. depends on k U proto = kt/(t+2τ) T = Tx time = D/R τ τ = propagation delay U proto = min(1,k/(1+2α)) α = τ /T = τ /(D/R) Utilization factor due to errors different for GBN and SR kt data Src T Dest τ T cycle CEN 5501C - Computer Networks - Spring UF/CISE - Newman 42

43 Protocol Utilization ARQ ARQ Protocol Efficiency efficiency k=1 k=2 k=3 k=5 k=9 k= a = normalized delay CEN 5501C - Computer Networks - Spring UF/CISE - Newman 43

44 Error Utilization Factor SR-ARQ Only resend missing frames U err,sr = 1-p p = frame error prob GBN-ARQ Send all frames after missing frame U err,gbn =(1-p)/(1+2αp) when k>1+2α U err,gbn =(1-p)/(1-p+kp) when k<1+2α Depends on k CEN 5501C - Computer Networks - Spring UF/CISE - Newman 44

45 Error Utilization Factor Efficiency Loss due to Errors (large a) 1.2 Efficiency GBN k=1 GBN k=2 GBN k=4 GBN k=8 GBN k=16 GBN k= Frame Error Rate p CEN 5501C - Computer Networks - Spring UF/CISE - Newman 45

46 Overall ARQ Utilization SR-ARQ U SR = min{(1-p),k(1-p)/(1+2α)} GBN-ARQ U GBN = (1-p)/(1+2αp) when k>1+2α U GBN = k(1-p)/(1+2α)(1-p+kp) when k<1+2α CEN 5501C - Computer Networks - Spring UF/CISE - Newman 46

47 Overall Utilization ARQ Efficiency, p= Efficiency Normalized delay (a) both k=1 GBN k=9 GBN k=65 GBN k=129 SR k=9 SR k=65 SR k=129 CEN 5501C - Computer Networks - Spring UF/CISE - Newman 47

48 Sequence Numbers Sequence numbers are finite (usually) Assume n bits 2 n -1 Hence, they wrap around i.e., 0 follows 2 n -1 Need to use circular < relation i.e., 0 > 2 n -1 2 n n n-1-1 Need to limit range usable by sender and acceptable to receiver (SW and RW) SW + RW <= 2 n CEN 5501C - Computer Networks - Spring UF/CISE - Newman 48