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Datakommunikasjon høsten 2002

Datakommunikasjon høsten 2002. Forelesning nr 8, mandag 7. oktober TCP flytkontroll. Øvingsoppgaver. Oppgave 1 To noder A og B benytter sliding window protokoll med 3 bits sekvensnr. Vindusstørrelse=7 Vis vindussituasjonen for følgende: Før A sender noen rammer.

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Datakommunikasjon høsten 2002

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  1. Datakommunikasjon høsten 2002 Forelesning nr 8, mandag 7. oktober TCP flytkontroll Datakom høsten 2002

  2. Øvingsoppgaver Oppgave 1 To noder A og B benytter sliding window protokoll med 3 bits sekvensnr. Vindusstørrelse=7 Vis vindussituasjonen for følgende: • Før A sender noen rammer. • Etter at A har sendt ramme 0, 1, 2, og 3 og B har bekreftet mottak av 0, 1 og 2. • Etter at A har sendt ramme 4 og 5, og B har bekreftet 3. Oppgave 2 Filen ftplog_øving 10. oktober viser en FT oppkobling. For Frame 7, 8 og skal du forklare alt som vises. For resten kun hva som skjer med FTP. Datakom høsten 2002

  3. Credit Scheme • Greater control on reliable network • More effective on unreliable network • Decouples flow control from ACK • May ACK without granting credit and vice versa • Each octet has sequence number • Each transport segment has seq number, ack number and window size in header Datakom høsten 2002

  4. Use of Header Fields • When sending, sequence number is that of first octet in segment • ACK includes AN (Ack Number)=i, W (Window)=j • All octets through SN (Sequence Number)=i-1 acknowledged • Next expected octet is i • Permission to send additional window of W=j octets • i.e. octets through i+j-1 Datakom høsten 2002

  5. receive side of TCP connection has a receive buffer: speed-matching service: matching the send rate to the receiving app’s drain rate flow control sender won’t overflow receiver’s buffer by transmitting too much, too fast TCP Flow Control • app process may be slow at reading from buffer Datakom høsten 2002

  6. (Suppose TCP receiver discards out-of-order segments) spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead] Rcvr advertises spare room by including value of RcvWindow in segments Sender limits unACKed data to RcvWindow guarantees receive buffer doesn’t overflow TCP Flow control: how it works Datakom høsten 2002

  7. Credit Allocation 200 oktetter sendes i hvert segment Initial credit = 1400 Datakom høsten 2002

  8. Sending and Receiving Perspectives Datakom høsten 2002

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