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CE80N Introduction to Networks & The Internet Dr. Chane L. Fullmer UCSC Winter 2002.

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Presentation on theme: "CE80N Introduction to Networks & The Internet Dr. Chane L. Fullmer UCSC Winter 2002."— Presentation transcript:

1 CE80N Introduction to Networks & The Internet Dr. Chane L. Fullmer UCSC Winter 2002

2 January 24, 2002CE80N -- Lecture #72 Next Week… Jan 29 (T) Inside the Internet –Chapter 17, Clients + Servers = Distributed Computing –Chapter 18, Names for Computers Due Jan 29: Written Assignment #1 Web search engine evaluation (10% of course grade) Jan 31 (Th) Midterm Exam (20% of course grade)

3 January 24, 2002CE80N -- Lecture #73 Reading Chapter 16 – –TCP: Software For Reliable Communications Chapter 19 – –Why The Internet Works Well

4 January 24, 2002CE80N -- Lecture #74 A Packet Switching System Can Be Overrun Packet switching allows multiple computers to communicate without initial delay. –Requires that the computers divide data into small packets –Requires additional communication software to ensure that data is delivered reliably.

5 Figure 16.1 An example internet with four networks connected by routers. Figure 16.2 Cars from two roads merging onto another road are analogous to packets from two networks merging onto a third network.

6 January 24, 2002CE80N -- Lecture #76 Transport Layer Our goals: understand principles behind transport layer services: –multiplexing –demultiplexing –reliable data transfer –flow control –congestion control instantiation and implementation in the Internet Overview: transport layer services multiplexing/demultiplexing connectionless transport: UDP principles of reliable data transfer connection-oriented transport: TCP –reliable transfer –flow control –connection management principles of congestion control TCP congestion control

7 January 24, 2002CE80N -- Lecture #77 Transport services and protocols provide logical communication between processes running on different hosts transport protocols run in end systems transport vs network layer services: –network layer: data transfer between end systems –transport layer: data transfer between processes relies on, enhances, network layer services application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical logical end-end transport

8 January 24, 2002CE80N -- Lecture #78 Transport-layer protocols Internet transport services: reliable, in-order unicast delivery (TCP) –congestion –flow control –connection setup unreliable (“best-effort”), unordered unicast or multicast delivery: UDP services not available: –real-time –bandwidth guarantees –reliable multicast application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical logical end-end transport

9 January 24, 2002CE80N -- Lecture #79 application transport network M P2 application transport network Multiplexing/demultiplexing Segment Segment - unit of data exchanged between transport layer entities –aka TPDU: transport protocol data unit receiver H t H n Demultiplexing: delivering received segments to correct app layer processes segment M application transport network P1 MMM P3 P4 segment header application-layer data

10 January 24, 2002CE80N -- Lecture #710 Multiplexing/demultiplexing multiplexing/demultiplexing: based on sender, receiver port numbers, IP addresses –source, dest port #s in each segment –Note: well-known port numbers for specific applications gathering data from multiple app processes, encapsulating data with header (later used for demultiplexing) source port #dest port # 32 bits application data (message) other header fields TCP/UDP segment format Multiplexing:

11 January 24, 2002CE80N -- Lecture #711 source port: x dest. port: 23 Multiplexing/demultiplexing: examples host A server B source port:23 dest. port: x port use: simple telnet app Web client host A Web server B Web client host C Source IP: C Dest IP: B source port: x dest. port: 80 Source IP: C Dest IP: B source port: y dest. port: 80 port use: Web server Source IP: A Dest IP: B source port: x dest. port: 80

12 January 24, 2002CE80N -- Lecture #712 UDP: User Datagram Protocol [RFC 768] “no frills,” “bare bones” Internet transport protocol “best effort” service, UDP segments may be: –lost –delivered out of order to app connectionless: –no handshaking between UDP sender, receiver –each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired

13 January 24, 2002CE80N -- Lecture #713 UDP: more often used for streaming multimedia apps –loss tolerant –rate sensitive other UDP uses: –DNS –SNMP reliable transfer over UDP: add reliability at application layer –application-specific error recover! source port #dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header

14 January 24, 2002CE80N -- Lecture #714 UDP checksum Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: –NO - error detected –YES - no error detected. But maybe errors nonetheless? Goal: detect “errors” (e.g., flipped bits) in transmitted segment

15 January 24, 2002CE80N -- Lecture #715 TCP Helps IP Guarantee Delivery When hardware in a router or network system fails, other routers start sending datagrams. As a result, some datagrams: –Arrive in a different order than they were sent –Checked by TCP –Put in order –Checked for duplicates by TCP

16 January 24, 2002CE80N -- Lecture #716 TCP Provides A Connection Between Computer Programs TCP software makes it possible for two computer programs to communicate across the Internet. –Establishes a connection –Exchanges data –Terminates communication

17 January 24, 2002CE80N -- Lecture #717 The Magic Of Recovering Lost Datagrams TCP includes an identification of each datagram. –Ignores duplicate copies –Recovers lost datagrams Uses timers Sends an acknowledgement back to the source

18 January 24, 2002CE80N -- Lecture #718 TCP Retransmission Is Automatic TCP adapts to work everywhere on the Internet –Retransmits after a short time if destination computer is close –Retransmits after a longer period if destination computer is far –Measures delays –Adjusts the timeout factor automatically

19 January 24, 2002CE80N -- Lecture #719 Go-Back-N Sender: k-bit seq # in pkt header “window” of up to N, consecutive unack’ed pkts allowed ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” –may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n): retransmit pkt n and all higher seq # pkts in window

20 January 24, 2002CE80N -- Lecture #720 GBN: receiver receiver simple: ACK-only: always send ACK for correctly- received pkt with highest in-order seq # –may generate duplicate ACKs –need only remember expectedseqnum out-of-order pkt: –discard (don’t buffer) -> no receiver buffering! –ACK pkt with highest in-order seq #

21 January 24, 2002CE80N -- Lecture #721 GBN in action

22 January 24, 2002CE80N -- Lecture #722 Selective Repeat receiver individually acknowledges all correctly received pkts –buffers pkts, as needed, for eventual in-order delivery to upper layer sender only resends pkts for which ACK not received –sender timer for each unACKed pkt sender window –N consecutive seq #’s –again limits seq #s of sent, unACKed pkts

23 January 24, 2002CE80N -- Lecture #723 Selective repeat: sender, receiver windows

24 January 24, 2002CE80N -- Lecture #724 Selective repeat data from above : if next available seq # in window, send pkt timeout(n): resend pkt n, restart timer ACK(n) in [sendbase,sendbase+N]: mark pkt n as received if n smallest unACKed pkt, advance window base to next unACKed seq # sender pkt n in [rcvbase, rcvbase+N-1] send ACK(n) out-of-order: buffer in-order: deliver (also deliver buffered, in-order pkts), advance window to next not-yet-received pkt pkt n in [rcvbase-N,rcvbase-1] ACK(n) otherwise: ignore receiver

25 January 24, 2002CE80N -- Lecture #725 Selective repeat in action

26 January 24, 2002CE80N -- Lecture #726 Selective repeat: dilemma Example: seq #’s: 0, 1, 2, 3 window size=3 receiver sees no difference in two scenarios! incorrectly passes duplicate data as new in (a) Q: what relationship between seq # size and window size?

27 27 TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 full duplex data: –bi-directional data flow in same connection –MSS: maximum segment size connection-oriented: –handshaking (exchange of control msgs) init’s sender, receiver state before data exchange flow controlled: –sender will not overwhelm receiver point-to-point: –one sender, one receiver reliable, in-order byte steam: –no “message boundaries” pipelined: –TCP congestion and flow control set window size send & receive buffers

28 January 24, 2002CE80N -- Lecture #728 TCP segment structure source port # dest port # 32 bits application data (variable length) sequence number acknowledgement number rcvr window size ptr urgent data checksum F SR PAU head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) # bytes rcvr willing to accept counting by bytes of data (not segments!) Internet checksum (as in UDP)

29 January 24, 2002CE80N -- Lecture #729 TCP seq. #’s and ACKs Seq. #’s: –byte stream “number” of first byte in segment’s data ACKs: –seq # of next byte expected from other side –cumulative ACK Q: how receiver handles out-of-order segments –A: TCP spec doesn’t say, - up to implementor time Host A Host B Seq=42, ACK=79, data = ‘C’ Seq=79, ACK=43, data = ‘C’ Seq=43, ACK=80 User types ‘C’ host ACKs receipt of echoed ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ simple telnet scenario

30 January 24, 2002CE80N -- Lecture #730 TCP: reliable data transfer simplified sender, assuming wait for event wait for event event: data received from application above event: timer timeout for segment with seq # y event: ACK received, with ACK # y create, send segment retransmit segment ACK processing one way data transfer no flow, congestion control

31 January 24, 2002CE80N -- Lecture #731 TCP: reliable data transfer 00 sendbase = initial_sequence number 01 nextseqnum = initial_sequence number loop (forever) { 04 switch(event) 05 event: data received from application above 06 create TCP segment with sequence number nextseqnum 07 start timer for segment nextseqnum 08 pass segment to IP 09 nextseqnum = nextseqnum + length(data) 10 event: timer timeout for segment with sequence number y 11 retransmit segment with sequence number y 12 compue new timeout interval for segment y 13 restart timer for sequence number y 14 event: ACK received, with ACK field value of y 15 if (y > sendbase) { /* cumulative ACK of all data up to y */ 16 cancel all timers for segments with sequence numbers < y 17 sendbase = y 18 } 19 else { /* a duplicate ACK for already ACKed segment */ 20 increment number of duplicate ACKs received for y 21 if (number of duplicate ACKS received for y == 3) { 22 /* TCP fast retransmit */ 23 resend segment with sequence number y 24 restart timer for segment y 25 } 26 } /* end of loop forever */ Simplified TCP sender

32 January 24, 2002CE80N -- Lecture #732 TCP ACK generation [RFC 1122, RFC 2581] Event in-order segment arrival, no gaps, everything else already ACKed in-order segment arrival, no gaps, one delayed ACK pending out-of-order segment arrival higher-than-expect seq. # gap detected arrival of segment that partially or completely fills gap TCP Receiver action delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK immediately send single cumulative ACK send duplicate ACK, indicating seq. # of next expected byte immediate ACK if segment starts at lower end of gap

33 January 24, 2002CE80N -- Lecture #733 TCP: retransmission scenarios Host A Seq=92, 8 bytes data ACK=100 loss timeout time lost ACK scenario Host B X Seq=92, 8 bytes data ACK=100 Host A Seq=100, 20 bytes data ACK=100 Seq=92 timeout time premature timeout, cumulative ACKs Host B Seq=92, 8 bytes data ACK=120 Seq=92, 8 bytes data Seq=100 timeout ACK=120

34 January 24, 2002CE80N -- Lecture #734 TCP Flow Control receiver: explicitly informs sender of (dynamically changing) amount of free buffer space –RcvWindow field in TCP segment sender: keeps the amount of transmitted, unACKed data less than most recently received RcvWindow sender won’t overrun receiver’s buffers by transmitting too much, too fast flow control receiver buffering RcvBuffer = size or TCP Receive Buffer RcvWindow = amount of spare room in Buffer

35 January 24, 2002CE80N -- Lecture #735 TCP Round Trip Time (RTT) & Timeout Q: how to set TCP timeout value? longer than RTT –note: RTT will vary too short: premature timeout –unnecessary retransmissions too long: slow reaction to segment loss Q: how to estimate RTT? SampleRTT : measured time from segment transmission until ACK receipt –ignore retransmissions, cumulatively ACKed segments SampleRTT will vary, want estimated RTT “smoothed” –use several recent measurements, not just current SampleRTT

36 January 24, 2002CE80N -- Lecture #736 TCP Round Trip Time and Timeout EstimatedRTT = (1-x)*EstimatedRTT + x*SampleRTT –Exponential weighted moving average –influence of given sample decreases exponentially fast –typical value of x: 0.1 Setting the timeout EstimtedRTT plus “safety margin” large variation in EstimatedRTT -> larger safety margin Timeout = EstimatedRTT + 4*Deviation Deviation = (1-x)*Deviation + x*|SampleRTT-EstimatedRTT|

37 January 24, 2002CE80N -- Lecture #737 TCP Connection Management Recall: TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: –seq. #s –buffers, flow control info (e.g. RcvWindow ) client: connection initiator Socket clientSocket = new Socket("hostname","port number"); server: contacted by client Socket connectionSocket = welcomeSocket.accept(); Three way handshake: Step 1: client end system sends TCP SYN control segment to server –specifies initial seq # Step 2: server end system receives SYN, replies with SYNACK control segment –ACKs received SYN –allocates buffers –specifies server-> receiver initial seq. Step 3: client receives SYNACK, replies with ACK –ACKs SYN

38 January 24, 2002CE80N -- Lecture #738 TCP Connection Management (cont.) Closing a connection: client closes socket: clientSocket.close(); Step 1: client end system sends TCP FIN control segment to server Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN. client FIN server ACK FIN close closed timed wait

39 January 24, 2002CE80N -- Lecture #739 TCP Connection Management (cont.) Step 3: client receives FIN, replies with ACK. –Enters “timed wait” - will respond with ACK to received FINs Step 4: server, receives ACK. Connection closed. client FIN server ACK FIN closing closed timed wait closed

40 January 24, 2002CE80N -- Lecture #740 TCP Connection Management (cont) TCP client lifecycle TCP server lifecycle

41 January 24, 2002CE80N -- Lecture #741 Principles of Congestion Control Congestion: informally: “too many sources sending too much data too fast for network to handle” different from flow control! manifestations: –lost packets (buffer overflow at routers) –long delays (queueing in router buffers) a top-10 problem!

42 January 24, 2002CE80N -- Lecture #742 Causes/costs of congestion: scenario 1 two senders, two receivers one router, infinite buffers no retransmission large delays when congested maximum achievable throughput

43 January 24, 2002CE80N -- Lecture #743 Causes/costs of congestion: scenario 2 four senders multihop paths timeout/retransmit in Q: what happens as and increase ? in

44 January 24, 2002CE80N -- Lecture #744 Causes/costs of congestion: scenario 2 Another “cost” of congestion: when packet dropped, any “upstream transmission capacity used for that packet was wasted!

45 January 24, 2002CE80N -- Lecture #745 Approaches towards congestion control End-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP Network-assisted congestion control: routers provide feedback to end systems –single bit indicating congestion –explicit rate sender should send at Two broad approaches towards congestion control:

46 January 24, 2002CE80N -- Lecture #746 TCP Congestion Control end-end control (no network assistance) transmission rate limited by congestion window size, Congwin, over segments: w segments, each with MSS bytes sent in one RTT: throughput = w * MSS RTT Bytes/sec Congwin

47 January 24, 2002CE80N -- Lecture #747 TCP congestion control: two “phases” –slow start –congestion avoidance important variables: –Congwin –threshold: defines threshold between two slow start phase, congestion control phase “probing” for usable bandwidth: –ideally: transmit as fast as possible ( Congwin as large as possible) without loss –increase Congwin until loss (congestion) –loss: decrease Congwin, then begin probing (increasing) again

48 January 24, 2002CE80N -- Lecture #748 TCP Slowstart exponential increase (per RTT) in window size (not so slow!) loss event: timeout (Tahoe TCP) and/or or three duplicate ACKs (Reno TCP) initialize: Congwin = 1 for (each segment ACKed) Congwin++ until (loss event OR CongWin > threshold) Slowstart algorithm Host A one segment RTT Host B time two segments four segments

49 January 24, 2002CE80N -- Lecture #749 TCP Congestion Avoidance /* slowstart is over */ /* Congwin > threshold */ Until (loss event) { every w segments ACKed: Congwin++ } threshold = Congwin/2 Congwin = 1 perform slowstart Congestion avoidance 1 1: TCP Reno skips slowstart (fast recovery) after three duplicate ACKs

50 January 24, 2002CE80N -- Lecture #750 AIMD: additive increase, multiplicative decrease –increase window by 1 per RTT –decrease window by factor of 2 on loss event TCP congestion avoidance

51 January 24, 2002CE80N -- Lecture #751 TCP Fairness Fairness goal: if N TCP sessions share same bottleneck link, each should be able to get 1/N of link capacity TCP connection 1 bottleneck router capacity R TCP connection 2

52 January 24, 2002CE80N -- Lecture #752 Why is TCP fair? Two competing sessions: Additive increase gives slope of 1, as throughput increases multiplicative decrease decreases throughput proportionally R R equal bandwidth share Connection 1 throughput Connection 2 throughput congestion avoidance: additive increase loss: decrease window by factor of 2 congestion avoidance: additive increase loss: decrease window by factor of 2

53 January 24, 2002CE80N -- Lecture #753 TCP And IP Work Together TCP handles the problems that IP does not handle without duplicating the work that IP does. –Designed at the same time to work as a unified system –Engineered to: Cooperate with each other Compliment each other

54 January 24, 2002CE80N -- Lecture #754 The Internet Works Well The Internet is a marvel of technical accomplishment. TCP/IP: – Accommodates growth and change not imagined

55 January 24, 2002CE80N -- Lecture #755 IP Provides Flexibility IP provides the flexibility to accommodate underlying hardware. –Allows multiple vendors to produce: computers routers that can communicate and interoperate

56 January 24, 2002CE80N -- Lecture #756 TCP Provides Reliability TCP provides reliability. –Handles communication problems –Communicates in applications –Handles rapid changes in performance –Adapts to congestion automatically

57 January 24, 2002CE80N -- Lecture #757 TCP/IP Software Was Engineered For Efficiency TCP/IP sends only the minimum network packets. Runs well on all computers. TCP/IP

58 January 24, 2002CE80N -- Lecture #758 TCP/IP Research Emphasized Practical Results Scientists built, tested, and measured working communication systems. All new ideas were based on experimental evidence.

59 January 24, 2002CE80N -- Lecture #759 Rough Consensus and Working Code TCP/IP arose from a consensus among researchers. –Accepted ideas only after implementation and demonstration –Allowed designers to correct problems early

60 January 24, 2002CE80N -- Lecture #760 Formula For Success The Internet was extremely successful. –Due to: Talented, dedicated research team Experimentation without economic gain Built to operate efficiently Each part worked well in practice before adopted as a standard

61 January 24, 2002CE80N -- Lecture #761 Summary principles behind transport layer services: –multiplexing/demultiplexing –reliable data transfer –flow control –congestion control instantiation and implementation in the Internet –UDP –TCP

62 January 24, 2002CE80N -- Lecture #762 Glossary Congestion –A problem in computer networks that occurs when too many packets arrive at a point in the network. Transmission Control Protocol –(TCP) One of the two major TCP/IP protocols. TCP handles the difficult task of ensuring that all data arrives at the destination in the correct order.

63 January 24, 2002CE80N -- Lecture #763 Glossary IP –Specification for the format of packets computers use when they communicate across the Internet TCP –The name of protocols that specify how computer communicate on the Internet UDP –User Datagram Protocol – UDP is a connectionless protocol, providing unreliable end-to-end packet delivery


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