1. Transport Layer Outline Intro to transport UDP Reliability (stop and wait, sliding window) CS 640

2. Application Transport 5 Network 4 Link 3 Physical 2 1 To Boldly Go Where We Have Yet to Go • Recall Internet Architecture • Layers used to define functionality • Our focus up to now has been layer 5 • Applications demand reliable transport • Application may demand predictable delays • We are now going up to layer 4 • This layer is tricky! • Goal at the end of the next few weeks is an understanding of the Reno version of TCP CS 640

3. End-to-End Protocols • Underlying network is best-effort • drop messages • re-orders messages • delivers duplicate copies of a given message • limits messages to some finite size • delivers messages after an arbitrarily long delay • Common end-to-end services • guarantee message delivery • deliver messages in the same order they are sent • deliver at most one copy of each message • support arbitrarily large messages • support synchronization • allow the receiver to flow control the sender • support multiple application processes on each host CS 640

4. Basic function of transport layer • How can processes on different systems get the right messages? • Ports are numeric locators which enable messages to be demultiplexed to proper process. • Ports are addresses on individual hosts, not across the Internet. • Ports are established using well-know values first • Port 80 = http, port 53 = DNS • Ports are typically implemented as message queues • Simplest function of the transport layer: multiplexing/demultiplexing of messages • Enables processes on different systems to communicate • End-to-end since only processes on end hosts invoke this protocol CS 640

5. Other transport layer functions • Connection control • Setting up and tearing down communication between processes • Error detection within packets – our first focus • Checksums • Reliable, in order delivery of packets – our second focus • Acknowledgement schemes • Flow control • Matching sending and receiving rates between end hosts • Congestion control • Managing congestion in the network CS 640

6. 0 16 31 SrcPort DstPort Checksum Length Data User Datagram Protocol (UDP) • Unreliable and unordered datagram service • Adds multiplexing/demultiplexing • Adds reliability through optional checksum • No flow or congestion control • Endpoints identified by ports • servers have well-known ports • see /etc/services on Unix • Header format • Optional checksum • Computed over psuedo header + UDP header + data CS 640

7. UDP Checksums • Optional in current Internet • Psuedoheader consists of 3 fields from IP header: protocol number (TCP or UDP), IP src, IP dst and UDP length field • Psuedoheader enables verification that message was delivered between correct source and destination. • IP dest address was changed during delivery, checksum would reflect this • UDP uses the same checksum algorithm as IP • Internet checksum CS 640

8. Basics of dealing with errors • Bit errors can be introduced in packets • This problem has been studied for a long time • Error detection (and correction) codes • Cyclic redundancy check (CRC) is a common error detection method • Basic idea of any scheme is to add redundant data • Extreme example – send two identical copies of data • Poor for many reasons • A primary goal is to send minimal amount of redundant data • CRC used in Ethernet has 32 bits for each 1500 byte packet • Another goal is to make generation of checksum fast CS 640

9. Checksum basics contd. • Simple parity is the most basic method for error detection • Odd/even parity • Internet Checksum • Basic idea: sender adds up all words and transmit the sum • Add using 16 bit one’s complement arithmetic then take one’s complement of the result to get checksum • Receiver adds up all words and compares with checksum • It’s very simple and efficient to code this • Reason that this is used instead of CRC • Not really great detecting errors • CRC is much stronger • Forward error correction is another possibility CS 640

10. UDP in practice • Minimal specification makes UDP very flexible • Any kind of end-to-end protocol can be implemented • See programming assignment #1 • TCP can be implemented using UDP • Examples • Most commonly used in multimedia applications • These are frequently more robust to loss • RPC’s • Many others… CS 640

11. Reliability • We’re heading toward TCP • Baby steps first… lets start with looking at minimal support for reliability CS 640

12. Methods of Reliability • Packets can be lost and/or corrupted during transmission • Bit level errors due to noise • Loss due to congestion • Use checksums to detect bit level errors • Internet Checksum is optionally used to detect errors in UDP • Uses 16 bits to encode one’s complement sum of data + headers • When bit level errors are detected, packets are dropped • Build reliability into the transmission protocol • Using acknowledgements and timeouts to signal lost or corrupt frame CS 640

13. Acknowledgements & Timeouts • An acknowledgement (ACK) is a packet sent by one host in response to a packet it has received • Making a packet an ACK is simply a matter of changing a field in the transport header • Data can be piggybacked in ACKs • A timeout is a signal that an ACK to a packet that was sent has not yet been received within a specified timeframe • A timeout triggers a retransmission of the original packet from the sender • How are timers set? CS 640

14. Acknowledgements & Timeouts CS 640

15. Propagation Delay • Propagation delay is defined as the delay between transmission and receipt of packets between hosts • Propagation delay can be used to estimate timeout period • How can propagation delay be measured? • What else must be considered in the measurement? CS 640

16. Exponentially weighted moving average RTT estimation • EWMA was original algorithm for TCP • Measure SampleRTT for each packet/ACK pair • Compute weighted average of RTT • EstRTT = axEstimatedRTT + bxSampleRTT • where a+b = 1 • a between 0.8 and 0.9 • b between 0.1 and 0.2 • Set timeout based on EstRTT • TimeOut=2xEstRTT CS 640

17. Stop-and-Wait Process Sender Receiver • Sender doesn’t send next packet until he’s sure receiver has last packet • The packet/Ack sequence enables reliability • Sequence numbers help avoid problem of duplicate packets • Problem: keeping the pipe full • Example • 1.5Mbps link x 45ms RTT = 67.5Kb (8KB) • 1KB frames imples 1/8th link utilization CS 640

18. Sender Receiver … ime T … Solution: Pipelining via Sliding Window • Allow multiple outstanding (un-ACKed) frames • Upper bound on un-ACKed frames, called window CS 640

19. Buffering on Sender and Receiver • Sender needs to buffer data so that if data is lost, it can be resent • Receiver needs to buffer data so that if data is received out of order, it can be held until all packets are received • Flow control • How can we prevent sender overflowing receiver’s buffer? • Receiver tells sender its buffer size during connection setup • How can we insure reliability in pipelined transmissions? • Go-Back-N • Send all N unACKed packets when a loss is signaled • Inefficient • Selective repeat • Only send specifically unACKed packets • A bit trickier to implement CS 640

20. £ SWS … … LAR LFS Sliding Window: Sender • Assign sequence number to each frame (SeqNum) • Maintain three state variables: • send window size (SWS) • last acknowledgment received (LAR) • last frame sent (LFS) • Maintain invariant: LFS - LAR <= SWS • Advance LAR when ACK arrives • Buffer up to SWS frames CS 640

21. £ RWS … … LFR LFA Sliding Window: Receiver • Maintain three state variables • receive window size (RWS) • largest frame acceptable (LFA) • last frame received (LFR) • Maintain invariant: LFA - LFR <= RWS • Frame SeqNum arrives: • if LFR < SeqNum < = LFA accept • if SeqNum < = LFR or SeqNum > LFA discarded • Send cumulative ACKs – send ACK for largest frame such that all frames less than this have been received CS 640

22. Sequence Number Space • SeqNum field is finite; sequence numbers wrap around • Sequence number space must be larger then number of outstanding frames • SWS <= MaxSeqNum-1 is not sufficient • suppose 3-bit SeqNum field (0..7) • SWS=RWS=7 • sender transmit frames 0..6 • arrive successfully, but ACKs lost • sender retransmits 0..6 • receiver expecting 7, 0..5, but receives the original incarnation of 0..5 • SWS < (MaxSeqNum+1)/2 is correct rule • Intuitively, SeqNum “slides” between two halves of sequence number space CS 640

23. Stop & wait sequence numbers Sender Receiver Sender Receiver Sender Receiver Frame 0 Frame 0 Frame 0 imeout imeout ACK 0 ACK 0 ACK 0 T T Frame 0 Frame 1 Frame 0 imeout ACK 0 imeout ACK 1 T ACK 0 T Frame 0 (c) (d) ACK 0 (e) • Simple sequence numbers enable the client to discard duplicate copies of the same frame • Stop & wait allows one outstanding frame, requires two distinct sequence numbers CS 640

24. 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 10 10 10 10 10 10 10 10 10 10 11 11 11 11 11 11 11 11 11 11 12 12 12 12 12 12 12 12 12 12 13 13 13 13 13 13 13 13 13 13 14 14 14 14 14 14 14 14 14 14 Sliding Window Example Receiver Sender 0 1 2 A3 3 4 5 6 A4 CS 640

25. Sliding Window Summary • Sliding window is best known algorithm in networking • First role is to enable reliable delivery of packets • Timeouts and acknowledgements • Second role is to enable in order delivery of packets • Receiver doesn’t pass data up to app until it has packets in order • Third role is to enable flow control • Prevents server from overflowing receiver’s buffer CS 640