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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 the Internet: UDP: connectionless transport TCP: connection-oriented transport

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Transport Layer

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  1. Our goals: understand principles behind transport layer services: multiplexing/demultiplexing reliable data transfer flow control congestion control learn about transport layer protocols in the Internet: UDP: connectionless transport TCP: connection-oriented transport TCP congestion control Transport Layer Ref: slides by J. Kurose and K. Ross Xin Liu

  2. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management TCP congestion control Outline Xin Liu

  3. provide logical communication between app processes running on different hosts transport 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 services and protocols 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 Xin Liu

  4. network layer: 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 network-layer protocol = postal service Transport vs. network layer Xin Liu

  5. 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 guarantees Internet transport-layer protocols 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 Xin Liu

  6. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Principles of reliable data transfer Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management Principles of congestion control TCP congestion control Outline Xin Liu

  7. Multiplexing/demultiplexing application application application transport transport transport P4 P2 P1 P1 P3 network network network link link link physical physical physical Multiplexing at send host: Demultiplexing at rcv host: host 3 host 2 host 1 delivering received segments to correct socket gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) = socket = process Xin Liu

  8. host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries 1 transport-layer segment each segment has source, destination port number (recall: well-known port numbers for specific applications) host uses IP addresses & port numbers to direct segment to appropriate socket How demultiplexing works 32 bits source port # dest port # other header fields application data (message) TCP/UDP segment format Xin Liu

  9. Create sockets : sock=socket(PF_INET,SOCK_DGRAM, IPPROTO_UDP); bind(sock,(struct sockaddr *)&addr,sizeof(addr)); sendto(sock,buffer,size,0); recvfrom(sock,Buffer,buffersize,0); UDP socket identified by two-tuple: (dest IP address, dest port number) When host receives UDP segment: checks destination port number in segment directs UDP segment to socket with that port number IP datagrams with different source IP addresses and/or source port numbers directed to same socket Connectionless demultiplexing Xin Liu

  10. 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-tuple Web servers have different sockets for each connecting client non-persistent HTTP will have different socket for each request Connection-oriented demux Xin Liu

  11. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management TCP congestion control Outline Xin Liu

  12. “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 UDP: User Datagram Protocol [RFC 768] Xin Liu

  13. DHCP client-server scenario DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 arriving client DHCP server: 223.1.2.5 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 Lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs time DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs Xin Liu

  14. Applications and protocols Xin Liu

  15. often used for streaming multimedia apps loss tolerant rate sensitive reliable transfer over UDP: add reliability at application layer application-specific error recovery! UDP: more 32 bits source port # dest port # Length, in bytes of UDP segment, including header checksum length Application data (message) UDP segment format Xin Liu

  16. Checksum • Goal: detect “errors” (e.g., flipped bits) in transmitted segment • UDP header and data • Pseudo header • Source/dest IP address • Protocol, length • Same procedure for TCP Xin Liu

  17. 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? may pass the damaged data UDP checksum Xin Liu

  18. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management TCP congestion control Outline Xin Liu

  19. 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 TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 Xin Liu

  20. TCP segment structure 32 bits source port # dest port # sequence number acknowledgement number head len not used Receive window U A P R S F checksum Urg data pnter Options (variable length) application data (variable length) URG: urgent data (generally not used) counting by bytes of data (not segments!) ACK: ACK # valid PSH: push data now # bytes rcvr willing to accept RST, SYN, FIN: connection estab (setup, teardown commands) Urgent data pointer Internet checksum (as in UDP) Xin Liu

  21. 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 connect(); server: contacted by client 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 SYNACK, replies with ACK segment, which may contain data TCP Connection Management Xin Liu

  22. Closing a connection: client closes socket: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. TCP Connection Management (cont.) client server close FIN ACK close FIN ACK timed wait closed Xin Liu

  23. 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. TCP Connection Management (cont.) client server closing FIN FIN_WAIT_1 ACK closing FIN FIN_WAIT_2 TIME_WAIT ACK timed wait closed closed Xin Liu

  24. TCP Connection Management (cont) TCP server lifecycle TCP client lifecycle Xin Liu

  25. TCP Connection Management • Allow half-close, i.e., one end to terminate its output, but still receiving data • Allow simultaneous open • Allow simultaneous close • Crashes? Xin Liu

  26. [root@shannon liu]# tcpdump -S tcp port 22 tcpdump: listening on eth0 23:01:51.363983 shannon.cs.ucdavis.edu.60042 > weasel.cs.ucdavis.edu.ssh: S 3036713598:3036713598(0) win 5840 <mss 1460,sackOK,timestamp 13989220 0,nop,wscale 0> (DF) 23:01:51.364829 weasel.cs.ucdavis.edu.ssh > shannon.cs.ucdavis.edu.60042: S 2462279815:2462279815(0) ack 3036713599 win 24616 <nop,nop,timestamp 626257407 13989220,nop,wscale 0,nop,nop,sackOK,mss 1460> (DF) 23:01:51.364844 shannon.cs.ucdavis.edu.60042 > weasel.cs.ucdavis.edu.ssh: . ack 2462279816 win 5840 <nop,nop,timestamp 13989220 626257407> (DF) 23:01:51.375451 weasel.cs.ucdavis.edu.ssh > shannon.cs.ucdavis.edu.60042: P 2462279816:2462279865(49) ack 3036713599 win 24616 <nop,nop,timestamp 626257408 13989220> (DF) 23:01:51.375478 shannon.cs.ucdavis.edu.60042 > weasel.cs.ucdavis.edu.ssh: . ack 2462279865 win 5840 <nop,nop,timestamp 13989221 626257408> (DF) 23:01:51.379319 shannon.cs.ucdavis.edu.60042 > weasel.cs.ucdavis.edu.ssh: P 3036713599:3036713621(22) ack 2462279865 win 5840 <nop,nop,timestamp 13989221 626257408> (DF) 23:01:51.379570 weasel.cs.ucdavis.edu.ssh > shannon.cs.ucdavis.edu.60042: . ack 3036713621 win 24616 <nop,nop,timestamp 626257408 13989221> (DF) Xin Liu

  27. 23:01:51.941616 shannon.cs.ucdavis.edu.60042 > weasel.cs.ucdavis.edu.ssh: P 3036714373:3036714437(64) ack 2462281065 win 7680 <nop,nop,timestamp 13989277 626257462> (DF) 23:01:51.952442 weasel.cs.ucdavis.edu.ssh > shannon.cs.ucdavis.edu.60042: P 2462281065:2462282153(1088) ack 3036714437 win 24616 <nop,nop,timestamp 626257465 13989277> (DF) 23:01:51.991682 shannon.cs.ucdavis.edu.60042 > weasel.cs.ucdavis.edu.ssh: . ack 2462282153 win 9792 <nop,nop,timestamp 13989283 626257465> (DF) 23:01:54.699597 shannon.cs.ucdavis.edu.60042 > weasel.cs.ucdavis.edu.ssh: F 3036714437:3036714437(0) ack 2462282153 win 9792 <nop,nop,timestamp 13989553 626257465> (DF) 23:01:54.699880 weasel.cs.ucdavis.edu.ssh > shannon.cs.ucdavis.edu.60042: . ack 3036714438 win 24616 <nop,nop,timestamp 626257740 13989553>(DF) 23:01:54.701129 weasel.cs.ucdavis.edu.ssh > shannon.cs.ucdavis.edu.60042: F 2462282153:2462282153(0) ack 3036714438 win 24616 <nop,nop,timestamp 626257740 13989553> (DF) 23:01:54.701143 shannon.cs.ucdavis.edu.60042 > weasel.cs.ucdavis.edu.ssh: . ack 2462282154 win 9792 <nop,nop,timestamp 13989553 626257740> (DF) 26 packets received by filter 0 packets dropped by kernel Xin Liu

  28. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management TCP congestion control Outline Xin Liu

  29. 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 TCP seq. #’s and ACKs time Host B Host A User types ‘C’ Seq=42, ACK=79, data = ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ Seq=79, ACK=43, data = ‘C’ host ACKs receipt of echoed ‘C’ Seq=43, ACK=80 simple telnet scenario Xin Liu

  30. Q: how to set TCP timeout value? longer than RTT but RTT varies 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 SampleRTT will vary, want estimated RTT “smoother” average several recent measurements, not just current SampleRTT TCP Round Trip Time and Timeout Xin Liu

  31. TCP Round Trip Time and Timeout EstimatedRTT = (1- )*EstimatedRTT + *SampleRTT • Exponential weighted moving average • influence of past sample decreases exponentially fast • typical value:  = 0.125 Xin Liu

  32. Example RTT estimation: Xin Liu

  33. Setting the timeout EstimtedRTT plus “safety margin” large variation in EstimatedRTT -> larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT: TCP Round Trip Time and Timeout DevRTT = (1-)*DevRTT + *|SampleRTT-EstimatedRTT| (typically,  = 0.25) Then set timeout interval: TimeoutInterval = EstimatedRTT + 4*DevRTT Xin Liu

  34. RTT • Timestamp can be used to measure RTT for each segment • Better RTT estimate • NO synchronization required Xin Liu

  35. TCP creates reliable service on top of IP’s unreliable service Pipelined segments Cumulative acks TCP uses single retransmission timer Retransmissions are triggered by: timeout events duplicate acks Initially consider simplified TCP sender: ignore duplicate acks ignore flow control, congestion control TCP reliable data transfer Xin Liu

  36. data rcvd from app: Create segment with seq # seq # is byte-stream number of first data byte in segment start timer if not already running (think of timer as for oldest unacked segment) expiration interval: TimeOutInterval timeout: retransmit segment that caused timeout restart timer Ack rcvd: If acknowledges previously unacked segments update what is known to be acked start timer if there are outstanding segments TCP sender events: Xin Liu

  37. TCP sender(simplified) NextSeqNum = InitialSeqNum SendBase = InitialSeqNum loop (forever) { switch(event) event: data received from application above create TCP segment with sequence number NextSeqNum if (timer currently not running) start timer pass segment to IP NextSeqNum = NextSeqNum + length(data) event: timer timeout retransmit not-yet-acknowledged segment with smallest sequence number start timer event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } } /* end of loop forever */ • Comment: • SendBase-1: last • cumulatively ack’ed byte • Example: • SendBase-1 = 71;y= 73, so the rcvrwants 73+ ;y > SendBase, sothat new data is acked Xin Liu

  38. TCP: retransmission scenarios Host A Host B Seq=92, 8 bytes data ACK=100 Seq=92 timeout timeout X loss Seq=92, 8 bytes data ACK=100 time time lost ACK scenario Host A Host B Seq=92, 8 bytes data Seq=100, 20 bytes data ACK=100 ACK=120 Seq=92, 8 bytes data Sendbase = 100 SendBase = 120 ACK=120 Seq=92 timeout SendBase = 100 SendBase = 120 premature timeout Xin Liu

  39. TCP retransmission scenarios (more) Host A Host B Seq=92, 8 bytes data ACK=100 Seq=100, 20 bytes data timeout X loss ACK=120 time Cumulative ACK scenario SendBase = 120 Xin Liu

  40. TCP ACK generation[RFC 1122, RFC 2581] 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 startsat lower end of gap Event at Receiver 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 completely fills gap Xin Liu

  41. receive side of TCP connection has a receive buffer: speed-matching service: matching the send rate to the receiving app’s drain rate TCP Flow Control flow control sender won’t overflow receiver’s buffer by transmitting too much, too fast • app process may be slow at reading from buffer Xin Liu

  42. (Suppose TCP receiver discards out-of-order segments) spare 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 doesn’t overflow TCP Flow control: how it works Xin Liu

  43. More • Slow receiver • Ack new window • Long fat pipeline: high speed link and/or long RTT • Window scale option during handshaking Xin Liu

  44. Header 32 bits source port # dest port # sequence number acknowledgement number head len not used Receive window U A P R S F checksum Urg data pnter Options (variable length) application data (variable length) Xin Liu

  45. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management TCP congestion control Outline Xin Liu

  46. Congestion: informally: “too many sources sending too much data too fast for network to handle” different from flow control! Who benefits? manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) a top-10 problem! Principles of Congestion Control Xin Liu

  47. end-end control (no network assistance) sender limits transmission: LastByteSent-LastByteAcked  cwnd Roughly, cwnd is dynamic, function of perceived network congestion How does sender perceive congestion? loss event = timeout or 3 duplicate acks TCP sender reduces rate (cwnd) after loss event mechanisms: slow start congestion avoidance AIMD TCP Congestion Control cwnd rate = Bytes/sec RTT Xin Liu

  48. When connection begins, cwnd = 1 MSS Example: MSS = 500 bytes & RTT = 200 msec initial rate = 20 kbps available bandwidth may be >> MSS/RTT desirable to quickly ramp up to respectable rate TCP Slow Start • When connection begins, increase cwnd when an ack received Xin Liu

  49. When connection begins, increase rate exponentially until first loss event: incrementing cwnd for every ACK received double cwnd every RTT Summary: initial rate is slow but ramps up exponentially fast TCP Slow Start (more) Host A Host B one segment RTT two segments four segments time Xin Liu

  50. Congestion Avoidance • ssthresh: when cwnd reaches ssthresh, congestion avoidance begins • Congestion avoidance: increase cwnd by 1/cwnd each time an ACK is received • Congestion happens: ssthresh=max(2MSS, cwnd/2) Xin Liu

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