3rdEd_Chapter3_March22_2005.ppt

1、Transport Layer,3-1,Chapter 3Transport Layer,Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004.,A note on the use of these ppt slides: Were making these slides freely available to all (faculty, students, readers). Theyre in

2、PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you

3、mention their source (after all, wed like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copy

4、right 1996-2005 J.F Kurose and K.W. Ross, All Rights Reserved,Transport Layer,3-2,Chapter 3: Transport Layer,Our goals: understand principles behind transport layer services: multiplexing/demultiplexing reliable data transfer flow control congestion control,learn about transport layer protocols in t

5、he Internet: UDP: connectionless transport TCP: connection-oriented transport TCP congestion control,Transport Layer,3-3,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-orient

6、ed transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-4,Transport services and protocols,provide logical communication between app processes running on different hosts transport

7、protocols run in end systems send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app layer more than one transport protocol available to apps Internet: TCP and UDP,Transport Layer,3-5,Transport vs. network layer,network layer:

8、 logical communication between hosts transport layer: logical communication between processes relies on, enhances, network layer services,Household analogy: 12 kids sending letters to 12 kids processes = kids app messages = letters in envelopes hosts = houses transport protocol = Ann and Bill networ

9、k-layer protocol = postal service,Transport Layer,3-6,Internet transport-layer protocols,reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP no-frills extension of “best-effort” IP services not available: delay guarantees bandwidth g

10、uarantees,Transport Layer,3-7,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection manageme

11、nt 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-8,Multiplexing/demultiplexing,application,transport,network,link,physical,P1,application,transport,network,link,physical,application,transport,network,link,physical,P2,P3,P4,P1,host 1,host 2,host 3,= process,= socke

12、t,delivering received segments to correct socket,gathering data from multiple sockets, enveloping data with header (later used for demultiplexing),Transport Layer,3-9,How demultiplexing works,host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries

13、 1 transport-layer segment each segment has source, destination port number host uses IP addresses DatagramSocket mySocket2 = new DatagramSocket(99222); UDP socket identified by two-tuple: (dest IP address, dest port number),When host receives UDP segment: checks destination port number in segment d

14、irects UDP segment to socket with that port number IP datagrams with different source IP addresses and/or source port numbers directed to same socket,Transport Layer,3-11,Connectionless demux (cont),DatagramSocket serverSocket = new DatagramSocket(6428);,SP provides “return address”,Transport Layer,

15、3-12,Connection-oriented demux,TCP socket identified by 4-tuple: source IP address source port number dest IP address dest port number recv host uses all four values to direct segment to appropriate socket,Server host may support many simultaneous TCP sockets: each socket identified by its own 4-tup

16、le Web servers have different sockets for each connecting client non-persistent HTTP will have different socket for each request,Transport Layer,3-13,Connection-oriented demux (cont),Client IP:B,server IP: C,SP: 9157,DP: 80,D-IP:C,S-IP: A,D-IP:C,S-IP: B,D-IP:C,S-IP: B,Transport Layer,3-14,Connection

17、-oriented demux: Threaded Web Server,Client IP:B,server IP: C,SP: 9157,DP: 80,P4,D-IP:C,S-IP: A,D-IP:C,S-IP: B,D-IP:C,S-IP: B,Transport Layer,3-15,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data tran

18、sfer,3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-16,UDP: User Datagram Protocol RFC 768,“no frills,” “bare bones” Internet transport protocol “best

19、 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

20、 small segment header no congestion control: UDP can blast away as fast as desired,Transport Layer,3-17,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

21、recovery!,source port #,dest port #,32 bits,Application data (message),UDP segment format,length,checksum,Length, in bytes of UDP segment, including header,Transport Layer,3-18,UDP checksum,Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1s complement sum) of segmen

22、t 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? More later .,Goal: detect “errors” (e.g., flipped bits) in t

23、ransmitted segment,Transport Layer,3-19,Internet Checksum Example,Note When adding numbers, a carryout from the most significant bit needs to be added to the result Example: add two 16-bit integers,1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1

24、1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1,wraparound,sum,checksum,Transport Layer,3-20,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-oriented trans

25、port: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-21,Principles of Reliable data transfer,important in app., transport, link layers top-10 list of important networking topics!,charac

26、teristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt),Transport Layer,3-22,Principles of Reliable data transfer,important in app., transport, link layers top-10 list of important networking topics!,characteristics of unreliable channel will determine comp

27、lexity of reliable data transfer protocol (rdt),Transport Layer,3-23,Principles of Reliable data transfer,important in app., transport, link layers top-10 list of important networking topics!,characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt),Tra

28、nsport Layer,3-24,Reliable data transfer: getting started,send side,receive side,Transport Layer,3-25,Reliable data transfer: getting started,Well: incrementally develop sender, receiver sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer but control info will f

29、low on both directions! use finite state machines (FSM) to specify sender, receiver,event causing state transition,actions taken on state transition,state: when in this “state” next state uniquely determined by next event,Transport Layer,3-26,Rdt1.0: reliable transfer over a reliable channel,underly

30、ing channel perfectly reliable no bit errors no loss of packets separate FSMs for sender, receiver: sender sends data into underlying channel receiver read data from underlying channel,Wait for call from above,packet = make_pkt(data) udt_send(packet),rdt_send(data),extract (packet,data) deliver_data

31、(data),Wait for call from below,rdt_rcv(packet),sender,receiver,Transport Layer,3-27,Rdt2.0: channel with bit errors,underlying channel may flip bits in packet checksum to detect bit errors the question: how to recover from errors: acknowledgements (ACKs): receiver explicitly tells sender that pkt r

32、eceived OK negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK new mechanisms in rdt2.0 (beyond rdt1.0): error detection receiver feedback: control msgs (ACK,NAK) rcvr-sender,Transport Layer,3-28,rdt2.0: FSM specification,Wa

33、it for call from above,snkpkt = make_pkt(data, checksum) udt_send(sndpkt),extract(rcvpkt,data) deliver_data(data) udt_send(ACK),rdt_rcv(rcvpkt) y= 73, so the rcvrwants 73+ ;y SendBase, sothat new data is acked,Transport Layer,3-66,TCP: retransmission scenarios,Host A,Seq=100, 20 bytes data,ACK=100,p

34、remature timeout,Host B,Seq=92, 8 bytes data,ACK=120,Seq=92, 8 bytes data,ACK=120,Seq=92 timeout,SendBase = 100,SendBase = 120,SendBase = 120,Sendbase = 100,Transport Layer,3-67,TCP retransmission scenarios (more),SendBase = 120,Transport Layer,3-68,TCP ACK generation RFC 1122, RFC 2581,Event at Rec

35、eiver Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Arrival of in-order segment with expected seq #. One other segment has ACK pending Arrival of out-of-order segment higher-than-expect seq. # . Gap detected Arrival of segment that partially or complete

36、ly 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, ACKing both in-order segments Immediately send duplicate ACK, indicating seq. # of next expected byte Immediate send ACK, provided that segment starts

37、at lower end of gap,Transport Layer,3-69,Fast Retransmit,Time-out period often relatively long: long delay before resending lost packet Detect lost segments via duplicate ACKs. Sender often sends many segments back-to-back If segment is lost, there will likely be many duplicate ACKs.,If sender recei

38、ves 3 ACKs for the same data, it supposes that segment after ACKed data was lost: fast retransmit: resend segment before timer expires,Transport Layer,3-70,event: ACK received, with ACK field value of y if (y SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer e

39、lse increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) resend segment with sequence number y ,Fast retransmit algorithm:,a duplicate ACK for already ACKed segment,fast retransmit,Transport Layer,3-71,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing an

40、d demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-72,TCP Flow

41、 Control,receive side of TCP connection has a receive buffer:,speed-matching service: matching the send rate to the receiving apps drain rate,app process may be slow at reading from buffer,Transport Layer,3-73,TCP Flow control: how it works,(Suppose TCP receiver discards out-of-order segments) spare

42、 room in buffer = RcvWindow = RcvBuffer-LastByteRcvd - LastByteRead,Rcvr advertises spare room by including value of RcvWindow in segments Sender limits unACKed data to RcvWindow guarantees receive buffer doesnt overflow,Transport Layer,3-74,Chapter 3 outline,3.1 Transport-layer services 3.2 Multipl

43、exing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-75,

44、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

45、 Socket connectionSocket = welcomeSocket.accept();,Three way handshake: Step 1: client host sends TCP SYN segment to server specifies initial seq # no data Step 2: server host receives SYN, replies with SYNACK segment server allocates buffers specifies server initial seq. # Step 3: client receives S

46、YNACK, replies with ACK segment, which may contain data,Transport Layer,3-76,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 con

47、nection, sends FIN.,Transport Layer,3-77,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. Note: with small modification, can handle simultaneous FINs.,client,

48、FIN,server,ACK,ACK,FIN,closing,closing,closed,timed wait,closed,Transport Layer,3-78,TCP Connection Management (cont),TCP client lifecycle,TCP server lifecycle,Transport Layer,3-79,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.

49、4 Principles of reliable data transfer,3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control,Transport Layer,3-80,Principles of Congestion Control,Congestion: informally: “too

50、 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!,Transport Layer,3-81,Causes/costs of congestion: scenario 1,two senders, two receivers

51、 one router, infinite buffers no retransmission,large delays when congested maximum achievable throughput,Transport Layer,3-82,Causes/costs of congestion: scenario 2,one router, finite buffers sender retransmission of lost packet,finite shared output link buffers,Host A,lin : original data,Host B,lo

52、ut,lin : original data, plus retransmitted data,Transport Layer,3-83,Causes/costs of congestion: scenario 2,always: (goodput) “perfect” retransmission only when loss: retransmission of delayed (not lost) packet makes larger (than perfect case) for same,“costs” of congestion: more work (retrans) for

53、given “goodput” unneeded retransmissions: link carries multiple copies of pkt,Transport Layer,3-84,Causes/costs of congestion: scenario 3,four senders multihop paths timeout/retransmit,Q: what happens as and increase ?,finite shared output link buffers,lin : original data,lout,lin : original data, p

54、lus retransmitted data,Transport Layer,3-85,Causes/costs of congestion: scenario 3,Another “cost” of congestion: when packet dropped, any “upstream transmission capacity used for that packet was wasted!,lout,Transport Layer,3-86,Approaches towards congestion control,End-end congestion control: no ex

55、plicit 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 (SNA, DECbit, TCP/IP ECN, ATM) explicit rate sender should send at,Two broad appro

56、aches towards congestion control:,Transport Layer,3-87,Case study: ATM ABR congestion control,ABR: available bit rate: “elastic service” if senders path “underloaded”: sender should use available bandwidth if senders path congested: sender throttled to minimum guaranteed rate,RM (resource management

57、) cells: sent by sender, interspersed with data cells bits in RM cell set by switches (“network-assisted”) NI bit: no increase in rate (mild congestion) CI bit: congestion indication RM cells returned to sender by receiver, with bits intact,Transport Layer,3-88,Case study: ATM ABR congestion control

58、,two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell sender send rate thus minimum supportable rate on path EFCI bit in data cells: set to 1 in congested switch if data cell preceding RM cell has EFCI set, sender sets CI bit in returned RM cell,Transport Layer,3-89,Chapter 3 outline,3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer,3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 P