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CS 5565 Network Architecture and Protocols

This lecture covers announcements, problem set submissions, project due dates, office hours, and reading assignments for CS 5565 in Spring 2006. It focuses on the principles of reliable data transfer and the characteristics of an unreliable channel.

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CS 5565 Network Architecture and Protocols

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  1. CS 5565Network Architecture and Protocols Godmar Back Lecture 16

  2. Announcements • Problem Set 1 graded • Project 1A due Wed Feb 22 (today) 11:59pm • Will try for quick turn-around time • Problem Set 2 due Mar 4 12:20pm • Office hours this week • MWR 3-4pm & F 4-5 • Reading Assignment: Chapter 3 • 3.1-3.3 already covered • 3.4 next CS 5565 Spring 2006

  3. 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 CS 5565 Spring 2006

  4. 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 rdt_rcv():called when packet arrives on rcv-side of channel Reliable data transfer: getting started send side receive side CS 5565 Spring 2006

  5. Upcalls vs Downcalls • Consumer/Producer pattern for receive() • deliver_data() is an upcall from k to k+1 • data producer, coordinates with data consumer at application layer or • “receive_data()” as downcall from k+1 to k • data consumer, coordinates with data producer in transport layer • don’t confuse with rdt_rcv() upcall from k-1 to k • Both choices are possible/acceptable for k+1/k interface in project: • difference is who deals with concurrency + signaling • You don’t have a choice for k/k-1 interface: • UDP socket API doesn’t (usually) support callbacks CS 5565 Spring 2006

  6. 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 CS 5565 Spring 2006

  7. 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) rdt_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) sender receiver CS 5565 Spring 2006

  8. 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 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 receiver feedback: control msgs (ACK,NAK) rcvr->sender retransmission rdt2.0: Channel with Bit Errors CS 5565 Spring 2006

  9. Wait for ACK or NAK rdt_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) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for call from above udt_send(sndpkt) rdt_rcv(rcvpkt) && isACK(rcvpkt) L sender rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) CS 5565 Spring 2006

  10. 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) CS 5565 Spring 2006

  11. 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) 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) CS 5565 Spring 2006

  12. What happens if ACK/NAK corrupted? sender doesn’t know what happened at receiver! can’t just retransmit: possible duplicate Handling duplicates: sender adds sequence number to each pkt sender retransmits current pkt if ACK/NAK garbled 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 CS 5565 Spring 2006

  13. Wait for ACK or NAK 0 Wait for call 1 from above Wait for ACK or NAK 1 rdt2.1: Sender FSM, w/ garbled ACK/NAKs rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) Wait for call 0 from above udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) L L rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) rdt_send(data) sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) udt_send(sndpkt) CS 5565 Spring 2006

  14. Wait for 0 from below Wait for 1 from below rdt2.1: Receiver, w/ garbled ACK/NAKs rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) CS 5565 Spring 2006

  15. 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 CS 5565 Spring 2006

  16. same functionality as rdt2.1, using ACKs only instead of NAK, receiver sends ACK for last pkt received OK receiver must explicitly include seq # of pkt being ACKed duplicate ACK at sender results in same action as NAK: retransmit current pkt rdt2.2: a NAK-free protocol CS 5565 Spring 2006

  17. Wait for ACK 0 Wait for call 0 from above Wait for 0 from below rdt2.2: Sender, Receiver fragments rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) ) udt_send(sndpkt) sender FSM fragment rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq1(rcvpkt)) receiver FSM fragment L udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK1, chksum) udt_send(sndpkt) CS 5565 Spring 2006

  18. New assumption:underlying channel can also lose packets (data or ACKs) checksum, seq. #, ACKs, retransmissions will be of help, but not enough Approach: sender waits “reasonable” amount of time for ACK retransmits if no ACK received in this time if pkt (or ACK) just delayed (not lost): retransmission will be duplicate, but use of seq. #’s already handles this receiver must specify seq # of pkt being ACKed requires countdown timer rdt3.0: Channels w/ Errors and Loss CS 5565 Spring 2006

  19. Evolution of Assumptions perfect channel error detection: checksums, receiver feedback: acknowledgements; retransmissions Automatic Repeat reQuest (ARQ protocol) • rdt1-3 are stop-and-waitprotocols rdt1.0 bit errors rdt2.0 (redundant) retransmissions duplicates sequence numbers 0/1: “alternating bit” protocol corrupted acks rdt2.1 eliminate NAK: use ACK+seq num ---- rdt2.2 timeouts lossy channel rdt3.0 lossy channel with bit errors CS 5565 Spring 2006

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