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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

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Chapter 3: 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 Chapter 3: Transport Layer Transport Layer

  2. 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 Principles of congestion control 3.7 TCP congestion control Chapter 3 outline Transport Layer

  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 application transport network data link physical application transport network data link physical logical end-end transport Transport services and protocols Transport Layer

  4. network layer: logical communication between hosts transport layer: logical communication between processes relies on, enhances, network layer services Transport vs. network layer C Sport:8050 Dport: 25 A D Sport:4625 Dport: 80 B Transport Layer

  5. reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP services not available: delay guarantees bandwidth guarantees 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 Transport Layer

  6. 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 Principles of congestion control 3.7 TCP congestion control Chapter 3 outline 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 transport transport network network network link link link physical physical physical host 3 host 2 host 1 Transport Layer

  8. host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries transport-layer segment each segment has source, destination port number 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 Transport Layer

  9. Create a socket binding to a port number 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/port can be directed to same socket Connectionless demultiplexing (UDP) Transport Layer

  10. P2 P1 P1 P3 client IP: A SP: 6428 SP: 6428 SP: 5775 DP: 5775 DP: 6428 DP: 9157 Connectionless demux (cont) SP: 9157 DP: 6428 Client IP:B server IP: C Port: 6428 Socket tuple: (dest IP address, dest port number) Two clients’ traffic can be mixed together at server Transport Layer

  11. 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 Two connections cannot mixed together at the receiver host 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 Remember the fork() and new socket generated by accept() Connection-oriented demux (TCP) Transport Layer

  12. SP: 9157 SP: 5775 P1 P1 P2 P4 P3 P6 P5 client IP: A DP: 80 DP: 80 Connection-oriented demux (cont) S-IP: B D-IP:C SP: 9157 DP: 80 Client IP:B server IP: C S-IP: A S-IP: B D-IP:C D-IP:C Transport Layer

  13. SP: 9157 SP: 5775 P1 P1 P2 P3 client IP: A DP: 80 DP: 80 Connection-oriented demux: Threaded Web Server P4 S-IP: B D-IP:C SP: 9157 DP: 80 Client IP:B server IP: C Port: 80 S-IP: A S-IP: B D-IP:C D-IP:C Transport Layer

  14. 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 Principles of congestion control 3.7 TCP congestion control Chapter 3 outline Transport Layer

  15. “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 worm (Slammer) UDP: User Datagram Protocol [RFC 768] Transport Layer

  16. UDP-based Worm: Slammer • Bandwidth-limited worm • Severely congested Internet • Stopped ATM, Flight checking, … • Worm code flow: • Exploit code (buffer overflow) • Generate random target IP address x: • Sendto() worm code to x on udp port 1434 • Fast spreading worm code (Jan. 2003) • Single UDP packet: 376 bytes • Average scan rate: 4000 scans/sec • Infect 90% in 10 minutes • ~ 100,000 infected in an hour • TCP-based worm is much slower • TCP connection setup • Connect() is a blocking call • Multiple threads for spreading Transport Layer

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

  18. Sender: treat segment contents as sequence of 16-bit integers checksum: 1’s complement of addition of segment contents sender puts checksum value into UDP checksum field Receiver: Add all received 16-bit segments, including checksum check if result is 1111 1111 1111 1111: NO - error detected YES - no error detected. But maybe errors nonetheless? More later …. UDP checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment Transport Layer

  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 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

  20. Internet Checksum Example 2 • Suppose a 6-bytes packet content is • 0xABCC, 0x960B, 0x5A3D What is the checksum for this packet? 0x is a hexadecimal representation that each symbol (0-9, A-F) represents 4 bits binary within the value of 0-15. For more details see: http://en.wikipedia.org/wiki/Hexadecimal Normal summation: 0xABCC+0x960B+0x5A3D = 0x19C14 Wrap up carry-out value: 0x9C14 + 0x1 = 0x9C15 So the checksum is: 0xFFFF – 0x9C15 = 0x63EA Transport Layer

  21. 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 Principles of congestion control 3.7 TCP congestion control Chapter 3 outline Transport Layer

  22. 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) Principles of Reliable data transfer u Network layer Transport Layer

  23. rdt_send():called from above, (e.g., by app.). Passed data to deliver to receiver upper layer deliver_data():called by rdt to deliver data to upper udt_send():called by rdt, to transfer packet over unreliable channel to receiver udt_rcv():called when packet arrives on rcv-side of channel Reliable data transfer: getting started send side receive side u Transport Layer

  24. We’ll: incrementally develop sender, receiver sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer but control info will flow on both directions! use finite state machines (FSM) to specify sender, receiver event state 1 state 2 actions Reliable data transfer: getting started event causing state transition actions taken on state transition state: when in this “state” next state uniquely determined by next event Transport Layer

  25. Assumption: underlying 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 Rdt1.0: reliable transfer over a reliable channel rdt_send(data) udt_rcv(packet) Wait for call from below Wait for call from above extract (packet,data) deliver_data(data) packet = make_pkt(data) udt_send(packet) Only need to chop bit-stream data into packets and send receiver sender Modern Internet packet has Maximum Transition Unit (MTU) of 1500 Bytes (Ethernet) Transport Layer

  26. Assumption #1: underlying channel may flip bits in packet checksum to detect bit errors Assumption # 2: no packet will be lost the question: how to recover from errors: acknowledgements (ACKs): receiver explicitly tells sender that pkt received 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 (checksum) Receiver feedback: control msgs (ACK,NAK) rcvr->sender Sender retransmit if NAK Rdt2.0: channel with bit errors Transport Layer

  27. Wait for ACK or NAK udt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(NAK) Wait for call from below rdt2.0: FSM specification rdt_send(data) receiver snkpkt = make_pkt(data, checksum) udt_send(sndpkt) udt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for call from above udt_send(sndpkt) udt_rcv(rcvpkt) && isACK(rcvpkt) L sender udt_rcv(rcvpkt) && notcorrupt(rcvpkt) L : means no action extract(rcvpkt,data) deliver_data(data) udt_send(ACK) Transport Layer

  28. Wait for ACK or NAK rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(NAK) rdt2.0: operation with no errors rdt_send(data) snkpkt = make_pkt(data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for call from above udt_send(sndpkt) rdt_rcv(rcvpkt) && isACK(rcvpkt) Wait for call from below L rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) Transport Layer

  29. Wait for ACK or NAK rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(NAK) rdt2.0: error scenario rdt_send(data) snkpkt = make_pkt(data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for call from above udt_send(sndpkt) udt_rcv(rcvpkt) && isACK(rcvpkt) Wait for call from below L udt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) Transport Layer

  30. What happens if ACK/NAK corrupted? sender doesn’t know what happened at receiver! Time-out and retransmit can’t just retransmit: possible duplicate Handling duplicates: sender retransmits current pkt if ACK/NAK garbled sender adds sequence number to each pkt receiver discards (doesn’t deliver up) duplicate pkt stop and wait rdt2.0 has a fatal flaw! Sender sends one packet, then waits for receiver response Transport Layer

  31. Wait for ACK or NAK 0 Wait for call 1 from above Wait for ACK or NAK 1 rdt2.1: sender, handles garbled ACK/NAKs rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) udt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) Wait for call 0 from above udt_send(sndpkt) udt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) udt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) L L udt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) rdt_send(data) sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) udt_send(sndpkt) Transport Layer

  32. Wait for 0 from below Wait for 1 from below rdt2.1: receiver, handles garbled ACK/NAKs udt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) udt_rcv(rcvpkt) && (corrupt(rcvpkt) udt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) udt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt) udt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) udt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) Why ACK for wrong sequence packet? Transport Layer

  33. Sender: seq # added to pkt two seq. #’s (0,1) will suffice. Why? must check if received ACK/NAK corrupted twice as many states state must “remember” whether “current” pkt has 0 or 1 seq. # Receiver: must check if received packet is duplicate state indicates whether 0 or 1 is expected pkt seq # note: receiver can not know if its last ACK/NAK received OK at sender rdt2.1: discussion Transport Layer

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