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1DT057 Distributed Information System Chapter 3 Transport Layer

1DT057 Distributed Information System Chapter 3 Transport Layer. Chapter 3: Transport Layer. Our goals: understand principles behind transport layer services: multiplexing/demultiplexing reliable data transfer flow control congestion control.

carminda-ruiz
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1DT057 Distributed Information System Chapter 3 Transport Layer

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  1. 1DT057Distributed Information SystemChapter 3Transport Layer 2: Application Layer

  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 the Internet: • UDP: connectionless transport • TCP: connection-oriented transport Transport Layer

  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-oriented transport: TCP • segment structure • reliable data transfer • flow control • Congestion control Transport Layer

  4. Transport vs. network layer • network layer: logical communication between hosts • transport layer: logical communication between processes • relies on, enhances, network layer services Transport Layer

  5. 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 network data link physical logical end-end transport 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 guarantees Transport Layer

  6. 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 • Congestion control Transport Layer

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

  8. How demultiplexing works • 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 • host uses IP addresses & port numbers to direct segment to appropriate socket 32 bits source port # dest port # other header fields Transport Layer application data (message) TCP/UDP segment format

  9. P2 P1 P1 P3 SP: 9157 client IP: A DP: 6428 Client IP:B server IP: C SP: 6428 SP: 5775 SP: 6428 DP: 5775 DP: 6428 DP: 9157 Connectionless demux (cont) DatagramSocket serverSocket = new DatagramSocket(6428); Transport Layer SP provides “return address”

  10. 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 • congestion control Transport Layer

  11. 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 Transport Layer

  12. 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 recovery! 32 bits source port # dest port # Length, in bytes of UDP segment, including header checksum length Transport Layer Application data (message) UDP segment format

  13. 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 • congestion control Transport Layer

  14. 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) Transport Layer

  15. 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) Transport Layer

  16. 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) Transport Layer

  17. rdt3.0 example Transport Layer

  18. rdt3.0 example Transport Layer

  19. Performance of rdt3.0 • rdt3.0 works, but performance stinks • ex: 1 Gbps link, 15 ms prop. delay, 8000 bit packet: • U sender: utilization – fraction of time sender busy sending Transport Layer • 1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link • network protocol limits use of physical resources!

  20. rdt3.0: stop-and-wait operation sender receiver first packet bit transmitted, t = 0 last packet bit transmitted, t = L / R first packet bit arrives RTT last packet bit arrives, send ACK Transport Layer ACK arrives, send next packet, t = RTT + L / R

  21. Pipelined protocols Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts • range of sequence numbers must be increased • buffering at sender and/or receiver • Two generic forms of pipelined protocols: go-Back-N, selective repeat Transport Layer

  22. Pipelining: increased utilization sender receiver first packet bit transmitted, t = 0 last bit transmitted, t = L / R first packet bit arrives RTT last packet bit arrives, send ACK last bit of 2nd packet arrives, send ACK last bit of 3rd packet arrives, send ACK ACK arrives, send next packet, t = RTT + L / R Transport Layer Increase utilization by a factor of 3!

  23. Pipelining Protocols Go-back-N: overview • sender: up to N unACKed pkts in pipeline • receiver: only sends cumulative ACKs • doesn’t ACK pkt if there’s a gap • sender: has timer for oldest unACKed pkt • if timer expires: retransmit all unACKed packets Selective Repeat: overview • sender: up to N unACKed packets in pipeline • receiver: ACKs individual pkts • sender: maintains timer for each unACKed pkt • if timer expires: retransmit only unACKed packet Transport Layer

  24. Go-Back-N Sender: • k-bit seq # in pkt header • “window” of up to N, consecutive unACKed pkts allowed Transport Layer • 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

  25. GBN inaction Transport Layer

  26. GBN Demo http://www.cs.mum.edu/courses/cs450/GoBackN.htm Transport Layer

  27. 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 Transport Layer

  28. Selective repeat: sender, receiver windows Transport Layer

  29. receiver sender Selective repeat 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 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 # Transport Layer

  30. Selective repeat in action Transport Layer

  31. 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? Transport Layer

  32. Selective Repeat Demo http://www.eecis.udel.edu/~amer/450/TransportApplets/SR/SRindex.html Transport Layer

  33. 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 • Congestion control Transport Layer

  34. TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 • full duplex data: • connection-oriented: • flow controlled: • point-to-point: • reliable, in-order byte steam: • pipelined: • send & receive buffers Transport Layer

  35. time TCP seq. #’s and ACKs Host B Host A Seq. #’s: • byte stream “number” of first byte in segment’s data ACKs: • seq # of next byte expected from other side • cumulative ACK User types ‘C’ Seq=42, ACK=79, data = ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ Seq=79, ACK=43, data = ‘C’ Transport Layer host ACKs receipt of echoed ‘C’ Seq=43, ACK=80 simple telnet scenario

  36. TCP reliable data transfer • TCP creates rdt 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 Transport Layer

  37. 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 TCP: retransmission scenarios Host A Host B Seq=92, 8 bytes data Seq=100, 20 bytes data ACK=100 ACK=120 Transport Layer Seq=92, 8 bytes data Sendbase = 100 SendBase = 120 ACK=120 Seq=92 timeout SendBase = 100 SendBase = 120 premature timeout

  38. 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 TCP retransmission scenarios (more) Transport Layer SendBase = 120

  39. flow control sender won’t overflow receiver’s buffer by transmitting too much, too fast (currently) unused buffer space application process IP datagrams TCP data (in buffer) TCP Flow Control • receive side of TCP connection has a receive buffer: • speed-matching service: matching send rate to receiving application’s drain rate Transport Layer • app process may be slow at reading from buffer

  40. 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 • congestion control Transport Layer

  41. 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) Transport Layer

  42. TCP congestion control: • goal: TCP sender should transmit as fast as possible, but without congesting network • Q: how to find rate just below congestion level • decentralized: each TCP sender sets its own rate, based on implicit feedback: • ACK: segment received (a good thing!), network not congested, so increase sending rate • lost segment: assume loss due to congested network, so decrease sending rate Transport Layer

  43. loss, so decrease rate X TCP congestion control: bandwidth probing • “probing for bandwidth”: increase transmission rate on receipt of ACK, until eventually loss occurs, then decrease transmission rate • continue to increase on ACK, decrease on loss (since available bandwidth is changing, depending on other connections in network) ACKs being received, so increase rate Transport Layer X X X TCP’s “sawtooth” behavior X sending rate time

  44. Chapter 3: Summary • principles behind transport layer services: • multiplexing, demultiplexing • reliable data transfer • flow control • congestion control • instantiation and implementation in the Internet • UDP • TCP Next: • leaving the network “edge” (application, transport layers) • into the network “core” Transport Layer

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