CMU 15441 Computer Networking - Lecture (11 pages)

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Lecture



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Lecture

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Lecture Notes


Pages:
11
School:
Carnegie Mellon University
Course:
15441 Computer Networking -
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Good Ideas So Far Flow control Stop wait Parallel stop wait Sliding window 15 441 Computer Networking Loss recovery Lecture 17 TCP Congestion Control Timeouts Acknowledgement driven recovery selective repeat or cumulative acknowledgement 10 26 2006 Outline Lecture 17 TCP Congestion Control 2 Sequence Numbers reminder How large do sequence numbers need to be Must be able to detect wrap around Depends on sender receiver window size TCP flow control E g Max seq 7 send win recv win 7 If pkts 0 6 are sent succesfully and all acks lost Congestion sources and collapse Receiver expects 7 0 5 sender retransmits old 0 6 Max sequence must be send window recv window Congestion control basics 10 26 2006 Lecture 17 TCP Congestion Control 3 10 26 2006 Lecture 17 TCP Congestion Control 4 1 Sequence Numbers TCP Flow Control 32 Bits Unsigned for bytes not packets TCP is a sliding window protocol Circular Comparison For window size n can send up to n bytes without receiving an acknowledgement When the data is acknowledged then the window slides forward b a a b Max 0 Max 0 b a Each packet advertises a window size a b Indicates number of bytes the receiver has space for Why So Big Original TCP always sent entire window For sliding window must have Sequence Space Sending Window Receiving Window Congestion control now limits this No problem Also want to guard against stray packets With IP packets have maximum lifetime of 120s Sequence number would wrap around in this time at 286MB s 10 26 2006 Lecture 17 TCP Congestion Control 5 Window Flow Control Send Side 10 26 2006 Lecture 17 TCP Congestion Control Window Flow Control Send Side Packet Sent Source Dest Dest Port Port SourcePort Port Sequence SequenceNumber Number window Sent and acked Sent but not acked 6 Packet Received Source Dest SourcePort Port Dest Port Port Sequence SequenceNumber Number Acknowledgment Acknowledgment HL Flags Window Window HL Flags D UrgentPointer Pointer D Checksum Checksum Urgent Acknowledgment Acknowledgment



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