DLL: Flow Control DLL. Simplex. Fast sender / slow receiver scenario. Various protocols used. Simplified examples implemented in C.

Transcription

1 DLL: Flow Control Fast sender / slow receiver scenario Feedback mechanism reqd. Various protocols used Stop and Wait Sliding window protocols Hugh Melvin, Dept. of IT, NUI,G 1 DLL Simplified examples implemented in C Simplex Simplex : Stop and Wait Simplex : Noisy channel Hugh Melvin, Dept. of IT, NUI,G 2 Simplex Assumptions No errors => ack, seq_nr not used. Only event possible is successful frame_arrival Infinite buffers Sender D.L.L continuously fetching packets from N.L, inserting them in frames and sending them. Receiver D.L.L continuously awaiting frames, takes them from the phy. layer, extracts the packet from the frame and passes it to the N.L. Hugh Melvin, Dept. of IT, NUI,G 3 1

2 Simplex: Stop and Wait Assumptions No errors Receiver cannot accept frames continuously require feedback from receiver to sender half-duplex channel (one-directional data flow) wait_for_event( ) fn in sender to_physical_layer( ) fn in receiver Hugh Melvin, Dept. of IT, NUI,G 4 Simplex protocol for noisy channel Assumptions Frames may be damaged/lost en route Acknowledge frames Use of timer to detect lost frames/ack Require sequence numbers for correct ack Timer must be set to minimise premature timeout (processing time plus ack time +?) Hugh Melvin, Dept. of IT, NUI,G 5 More complicated protocols Duplex data transmission Data plus acknowledgements in both directions Ack frames represent hi overhead Piggybacking acknowledgements are attached to outgoing data frames => bandwidth saving More advanced protocols: Sliding Window Hugh Melvin, Dept. of IT, NUI,G 6 2

3 Sliding Window protocols Sender / receiver : list of seq_nr s that it is permitted to send /accept size of sending / receiving window Stop-and-wait size = 1 3 bit seq_nr => buffer size is 8. Eg. h/o Window size 1, buffer size 8 Window size 7, buffer size 8 Hugh Melvin, Dept. of IT, NUI,G 7 Hugh Melvin, Dept. of IT, NUI,G 8 Hugh Melvin, Dept. of IT, NUI,G 9 3

4 Sliding Window Sender receives an ack (within its window) => advance window by one => can send one more Sender buffer space = size of window in case any frame is lost etc.. Sending window > 1 useful where round-trip time or processing time significant => pipelining How are lost frames retransmitted? Hugh Melvin, Dept. of IT, NUI,G 10 Retransmission policy Go-back-n : sender has to retransmit all frames starting with the lost frame Selective repeat: only the lost frame is retransmitted. requires storage buffer (receiver window >1) Hugh Melvin, Dept. of IT, NUI,G 11 Hugh Melvin, Dept. of IT, NUI,G 12 4

5 Example DLL protocols HDLC (Hi-Level D.L Control) SLIP (Serial line IP) PPP (Point-to Point) Hugh Melvin, Dept. of IT, NUI,G 13 Hugh Melvin, Dept. of IT, NUI,G 14 Frame Format Flag Byte Checksum (CRC) Control : frame type Information : Supervisory : Unnumbered : HDLC Hugh Melvin, Dept. of IT, NUI,G 15 5

6 HDLC Information Sliding Window : 3 bit seq_nr 3 bit ack (Next frame expected) Supervisory: various functions Retransmit from Next (Go-back-n) Retransmit single frame (Next) Unnumbered Various functions Unreliable data transfer Hugh Melvin, Dept. of IT, NUI,G 16 Hugh Melvin, Dept. of IT, NUI,G 17 Internet Point-to-point lines Dial-up access (home use) Leased lines..subnet Point-to-point data-link protocol SLIP (RFC 1055)..encapsulated IP PPP (RFC 1661) Request For Comment: Internet specifications Hugh Melvin, Dept. of IT, NUI,G 18 6

7 Hugh Melvin, Dept. of IT, NUI,G 19 PPP Similar to HDLC Features Error detection (checksum) Flag Byte for framing Dynamic IP address allocation (home user/isp) Support protocols LCP (Link Control Protocol): setting up connection NCP (Net. Ctrl. Proto): PPP supports diff protocols at N.L (diff. NCP for each NL supported) Hugh Melvin, Dept. of IT, NUI,G 20 Hugh Melvin, Dept. of IT, NUI,G 21 7

8 PPP Frame format Flag Control default unnumbered => unreliable service NB. Reliability provided elsewhere (eg. TCP) Protocol LCP, NCP => DLL issues IP / IPX => data from NL Hugh Melvin, Dept. of IT, NUI,G 22 Hugh Melvin, Dept. of IT, NUI,G 23 Hugh Melvin, Dept. of IT, NUI,G 24 8