@Yuan Xue A special acknowledge goes to J.F Kurose and K.W. Ross Some of the slides used in this lecture are adapted from their original slides that accompany the book “Computer Networking, A Top-Down Approach” All material copyright J.F Kurose and K.W. Ross, All Rights Reserved CS 283Computer Networks Spring 2012 Instructor: Yuan Xue

@Yuan Xue Transport Layer Outline Overview 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 UDP: Connectionless transport 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management Congestion control 3.6 Principles of congestion control 3.7 TCP congestion control

@Yuan Xue UDP: User Datagram Protocol [RFC 768] “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

@Yuan Xue UDP: more often used for streaming multimedia apps loss tolerant rate sensitive other UDP uses DNS SNMP reliable transfer over UDP: add reliability at application layer application-specific error recovery! source port #dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header

@Yuan Xue DNS over UDP: A Demo Query Reply Information to pay attention to: 1.UDP format 2.Port numbers in the UDP segments

@Yuan Xue 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 Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected  Discard the damaged segment or pass to the application with a warning, no recovery is performed YES - no error detected. Goal: detect “errors” (e.g., flipped bits) in transmitted segment

@Yuan Xue Internet Checksum Example Checksum is the 16-bit one's complement of the one's complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded with zero octets at the end (if necessary) to make a multiple of two octets.one's complement Note When adding numbers, a carryout from the most significant bit needs to be added to the result Example: add two 16-bit integers wraparound sum checksum

@Yuan Xue End-to-End Principle Link layer has error detection.. Why UDP does error detection again at transport layer? End-to-end guarantee