sliding window protocol for datalink layer.ppt
- 1. CS 640 1 Outline RTT estimation using EWMA Stop and Wait Sliding Window Algorithms Transport Layer: Sliding Window Reliability
- 2. CS 640 2 Transport layer functions • Transport protocol defines the pattern/sequence for end hosts sending packets • Transport has to deal with a number of things – Multiplexing/demultiplexing – ports and message queues – Error detection within packets - checksums – Reliable, in order delivery of packets - Today – Flow control – Today – Connection management - TBD – Congestion control – TBD • We’re moving toward understanding TCP
- 3. CS 640 3 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
- 4. CS 640 4 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?
- 5. CS 640 5 Acknowledgements & Timeouts Sender Receiver Frame ACK Timeout Time Sender Receiver Frame ACK Timeout Frame ACK Timeout Sender Receiver Frame ACK Timeout Frame ACK Timeout Sender Receiver Frame Timeout Frame ACK Timeout (a) (c) (b) (d)
- 6. CS 640 6 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?
- 7. CS 640 7 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 = a x EstimatedRTT + b x SampleRTT – where a + b = 1 a between 0.8 and 0.9 b between 0.1 and 0.2 • Set timeout based on EstRTT – TimeOut = 2 x EstRTT
- 8. CS 640 8 Stop-and-Wait Process • 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 Sender Receiver
- 9. CS 640 9 Solution: Pipelining via Sliding Window • Allow multiple outstanding (un-ACKed) frames • Upper bound on un-ACKed frames, called window Sender Receiver Time … …
- 10. CS 640 10 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
- 11. CS 640 11 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 SWS LAR LFS … …
- 12. CS 640 12 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 RWS LFR LFA … …
- 13. CS 640 13 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
- 14. CS 640 14 Another Pipelining Possibility: Concurrent Logical Channels • Multiplex 8 logical channels over a single link • Run stop-and-wait on each logical channel • Maintain three state bits per channel – channel busy – current sequence number out – next sequence number in • Header: 3-bit channel num, 1-bit sequence num – 4-bits total – same as sliding window protocol • Separates reliability from order
- 15. CS 640 15 Stop & wait sequence numbers Sender Receiver Timeout Timeout Sender Receiver Timeout Timeout (c) (d) Sender Receiver (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
- 16. CS 640 16 Sliding Window Example 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 Sender Receiver A3 3 4 5 6 A4 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
- 17. CS 640 17 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