Datakom høsten Datakommunikasjon høsten 2002 Forelesning nr 8, mandag 7. oktober TCP flytkontroll
Datakom høsten Øvingsoppgaver Oppgave 1 To noder A og B benytter sliding window protokoll med 3 bits sekvensnr. Vindusstørrelse=7 Vis vindussituasjonen for følgende: a)Før A sender noen rammer. b)Etter at A har sendt ramme 0, 1, 2, og 3 og B har bekreftet mottak av 0, 1 og 2. c)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 Credit Scheme zGreater control on reliable network zMore effective on unreliable network zDecouples flow control from ACK yMay ACK without granting credit and vice versa zEach octet has sequence number zEach transport segment has seq number, ack number and window size in header
Datakom høsten Use of Header Fields zWhen sending, sequence number is that of first octet in segment zACK includes AN (Ack Number)=i, W (Window)=j zAll octets through SN (Sequence Number)=i-1 acknowledged yNext expected octet is i zPermission to send additional window of W=j octets yi.e. octets through i+j-1
Datakom høsten TCP Flow Control zreceive side of TCP connection has a receive buffer: z speed-matching service: matching the send rate to the receiving app’s drain rate zapp process may be slow at reading from buffer sender won’t overflow receiver’s buffer by transmitting too much, too fast flow control
Datakom høsten TCP Flow control: how it works (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 yguarantees receive buffer doesn’t overflow
Datakom høsten Credit Allocation 200 oktetter sendes i hvert segment Initial credit = 1400
Datakom høsten Sending and Receiving Perspectives