Before The Canned Slides r What sort of things did you learn from Mikes talk on Friday? r Mini-homework this week: create a wiki page with cross-referenced facts from Mike’s presentation. Will be in the wiki soon! r Did you work on your project? r I won’t be collecting projects until the end of the term! r Threading is a big deal for these projects – we’ll spend a day in Java land on Friday to talk about threading Transport Layer3-1

Questions about RDT r What the heck are these “state machine” things? Why does the book use them? r What does the “big picture” look like? r What extra messages are added to the basic dialog? r What assumptions have been made so far? r Why can we use just 0 and 1 as sequence numbers in our packet stream? r Does this sequence number depend on the behavior of the link? Transport Layer3-2

Questions about RDT r What sort of errors are not detected? r What would happen if an ACK is delayed so much that the sender retransmits unnecessarily? r How do the sender and receiver state machines mesh?  What was the  r How did NACK get removed from the protocol? Transport Layer3-3

Transport Layer3-4 Chapter 3 Transport Layer Computer Networking: A Top Down Approach 4 th edition. Jim Kurose, Keith Ross Addison-Wesley, July A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in 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 mention their source (after all, we’d 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 copyright J.F Kurose and K.W. Ross, All Rights Reserved

Transport Layer3-5 rdt3.0: channels with errors and loss New assumption: underlying channel can also lose packets (data or ACKs) m checksum, seq. #, ACKs, retransmissions will be of help, but not enough Approach: sender waits “reasonable” amount of time for ACK r retransmits if no ACK received in this time r if pkt (or ACK) just delayed (not lost): m retransmission will be duplicate, but use of seq. #’s already handles this m receiver must specify seq # of pkt being ACKed r requires countdown timer

Transport Layer3-6 rdt3.0 sender sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) start_timer rdt_send(data) Wait for ACK0 rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) ) Wait for call 1 from above sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) start_timer rdt_send(data) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,0) ) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1) stop_timer udt_send(sndpkt) start_timer timeout udt_send(sndpkt) start_timer timeout rdt_rcv(rcvpkt) Wait for call 0from above Wait for ACK1  rdt_rcv(rcvpkt)   

Transport Layer3-7 rdt3.0 in action

Transport Layer3-8 rdt3.0 in action

Transport Layer3-9 Performance of rdt3.0 r rdt3.0 works, but performance stinks r example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet: T transmit = 8kb/pkt 10**9 b/sec = 8 microsec m U sender : utilization – fraction of time sender busy sending L (packet length in bits) R (transmission rate, bps) = m 1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link m network protocol limits use of physical resources!

Transport Layer3-10 rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 senderreceiver RTT last packet bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R

Transport Layer3-11 Pipelined protocols Pipelining: sender allows multiple, “in-flight”, yet-to- be-acknowledged pkts m range of sequence numbers must be increased m buffering at sender and/or receiver r Two generic forms of pipelined protocols: go-Back-N, selective repeat