Lecture15-Transport

Lecture15-Transport - CSCE 313 Introduction to Computer...

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CSCE 313 Introduction to Computer Systems Instructor: Dr. Ronnie Ward Based on material from Dr.
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2 Transport Layer Flow control Connection management TCP, UDP
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Copyright Jorg Liebeherr 3 Introduction Transport layer protocols are end-to-end protocols Transport layer is only implemented at the hosts Application Transport Network HOST Data Link Network T ransport Network HOST
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Copyright Jorg Liebeherr 4 Functions of the Transport Layer Reliability Connection Establishment Connection Termination Flow control Congestion control
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Copyright Jorg Liebeherr 5 Reliability How to achieve it? Send data in chunks (called packets) Number the packets sequentially at the sender Make sure all numbers are received without gaps
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Copyright Jorg Liebeherr 6 Reliability How to achieve it? Send data in chunks (called packets) Number the packets sequentially at the sender Make sure all numbers are received without gaps Acknowledge each packet number If sender sees an ack gap  sender retransmits
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Copyright Jorg Liebeherr 7 Reliability How to achieve it? Send data in chunks (called packets) Number the packets sequentially at the sender Make sure all numbers are received without gaps Acknowledge each packet number If sender sees an ack gap  sender retransmits Optimization: Cumulative acks (ACK N: acknowledge that all packets up to but not including packet N have been received).
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Copyright Jorg Liebeherr 8 Reliability How to achieve it? Send data in chunks (called packets) Number the packets sequentially at the sender Make sure all numbers are received without gaps Acknowledge each packet number If sender sees an ack gap  sender retransmits Optimization: Cumulative acks (ACK N: acknowledge that all packets up to but not including packet N have been received). What about the number of the first packet?
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Copyright Jorg Liebeherr 9 Connection Establishment Connection establishment is asymmetric: one side puts itself in a LISTEN state ( server ) one side issues a request for connection ( client )
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Copyright Jorg Liebeherr 10 Simple Solution (which has problems) CR (SeqNo = x) Connection Request , A wants to start with SeqNo = x ACK (SeqNo = y) Acknowledge request, B will wants to start with SeqNo = y DATA (SeqNo = x) Data transmission with SeqNo x Two Way Handshake Host A Host B CR (SeqNo = x) ACK (SeqNo = y) DATA(SeqNo = x)
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Copyright Jorg Liebeherr 11 Problems with Two-Way Handshake B responds to CR(SeqNo = z), an old duplicate connection requests from A In the shown sce- nario, A believes that the ACK is for the connection request CR(SeqNo = y) Result: A starts to send data with Sequence x. B will throw the data away since it expects SeqNo = z Host A Host B C R (S eq N o = x ) A K ( S e q = y D TA (Se qN o = x) z
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Copyright Jorg Liebeherr 12 Three-Way Handshake Note: A and B acknowledge the sequence number from the other side This solution provides protection from old duplicate connection requests Host A Host B CR (SeqNo = x) ACK (SeqNo = y, ACK = x ) DATA(SeqNo = x, ACK = y
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Lecture15-Transport - CSCE 313 Introduction to Computer...

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