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CSE 524: Lecture 11

CSE 524: Lecture 11. Transport Layer (Part 2). Administrative. Exam Mean=72 Where are you in class? 80-100 => A 70-80 => Borderline Below 70 => B and below Research paper topic Most have approval Please see me after class if you do not. Transport Layer. Last class

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CSE 524: Lecture 11

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  1. CSE 524: Lecture 11 Transport Layer (Part 2)

  2. Administrative • Exam • Mean=72 • Where are you in class? • 80-100 => A • 70-80 => Borderline • Below 70 => B and below • Research paper topic • Most have approval • Please see me after class if you do not

  3. Transport Layer • Last class • Transport layer functions • This class • Repeat last lecture’s missing section on congestion control…. • Specific transport layers

  4. Specific transport layers • UDP • unreliable (“best-effort”), • unordered • unicast or multicast delivery • TCP • reliable • in-order • unicast • SCTP (will not cover in class) • See http://www.ietf.org/rfc/rfc2960.txt • reliable • optional ordering • unicast

  5. TL: UDP and Transport Layer Functions • Demux to upper layer • UDP port field • Quality of service • none • Security • none • Delivery semantics • Unordered • Unicast or multicast • Flow control • none • Congestion control • none • Reliable data transfer • none, but data integrity provided by checksum

  6. TL: UDP: User Datagram Protocol http://www.rfc-editor.org/rfc/rfc768.txt “no frills,” “bare bones” Internet transport protocol “best effort” service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired

  7. TL: UDP: more often used for streaming multimedia apps loss tolerant rate sensitive other UDP uses (why?): DNS SNMP reliable transfer over UDP: add reliability at application layer application-specific error recovery! many applications re-implement reliability over UDP to bypass TCP new transport protocols? 32 bits source port # dest port # Length, in bytes of UDP segment, including header checksum length Application data (message) UDP segment format

  8. TL: UDP checksum Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field similar to IP’s header checksum Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonethless? More later …. Goal: detect “errors” (e.g., flipped bits) in transmitted segment

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