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Computer Networks Transport Layer. Topics F Introduction  F Connection Issues F TCP.

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Presentation on theme: "Computer Networks Transport Layer. Topics F Introduction  F Connection Issues F TCP."— Presentation transcript:

1 Computer Networks Transport Layer

2 Topics F Introduction  F Connection Issues F TCP

3 Introduction F Efficient, reliable and cost-effective service to users (application layer) –despite limitations of network layer F Features (sounds like network layer?) –Connection oriented vs. Connectionless –Addressing and flow control F But transport layer makes lower subnet reliable, gives standard interface F Major boundary between user and network! –Few users write code for network layer –Many write code for transport layer

4 Transport Entity F Logical location of transport entity F Physical: OS, separate process, network card

5 Quality of Service F Typical networks do not do all

6 Transport Protocol F Like data link layer: –error control, sequencing, flow control… F But different: –must specify router (data link layer always same) –destination may be down (harder than data link) –subnet may store packets –many lines and variance make buffering and flow control different

7 Finding a Server F Problems locating service –name server

8 Finding a Server F Standard servers wait at well-known port –but what if infrequently used?

9 Establishing a Connection F Subnet can delay, lose, duplicate packets –Connection can happen twice! –Use unique sequence numbers to avoid F When establish connection, exchange sequence numbers –three-way handshake –prevents establishment of unwanted connection

10 Three-Way Handshake CR = Connection Request ACC = Connection Accepted

11 Three-Way Handshake Problems

12 Releasing a Connection F Asymmetric release can result in data loss F Symmetric release easy? –“I’m done” –“Me, too”

13 Two-Army Problem F No safe solution F Use 3-way handshake with timers (fig 6-14)

14 TCP F Each connection is a 32-bit number F Data exchanged via segments –20 byte header plus data F Max is 65,535 bytes (IP payload) –MTU usually smaller F Sliding window protocol

15 TCP Segment Header

16 TCP Transmission Policy

17 F Do not have to send immediately –avoid many small packets F Nagle Algorithm –only 1 outstanding byte at a time –fill up, then send –time delay, then send –bad for some apps (X - with mouse movements)

18 Silly Window Syndrome F Application reads 1 byte at a time F Fix: only send window when 1/2 full

19 TCP Congestion Control F Even if sender and receiver agree, still problems

20 TCP Congestion Control F “Reciver buffer” via receiver’s window F “Network buffer” via congestion window –starts at 1 segment –increases exponentially –until timeout or receiver’s window reached –or threshold, then increases linearly –slow start F When timeout, cut threshold in half and restart

21 TCP Congestion Control

22 Timer Management F Difficult when much variance F RTT =  RTT + (1-  )M (  = 7/8, M ack time) F + add variance, don’t update on retransmits


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