清华大学：TCP and Congestion Control（1）

TCP and Congestion Control (1) Dan li CS Department, Tsinghua University 2021/2/8

TCP and Congestion Control (1) 2021/2/8 1 Dan LI CS Department, Tsinghua University

Today's Lecture w Transport-Layer Introduction Error Recovery s Window flow control Analysis of TCP Congestion Control TCP Variations Beyond TCP Congestion Control w Reading list 2021/2/8

Today’s Lecture  Transport-Layer Introduction  Error Recovery  Window Flow Control  Analysis of TCP Congestion Control  TCP Variations  Beyond TCP Congestion Control  Reading list 2021/2/8 2

Transport Protocols Lowest level end-to end protocol 7 Header generated by sender is interpreted 7-6-5 6 5 only by the destination Transport Transport Routers view transport header as part of the P P P payload Datalink /----o-22--d/ Datalink Physical Physical router 2021/2/8

Transport Protocols  Lowest level end-to￾end protocol.  Header generated by sender is interpreted only by the destination  Routers view transport header as part of the payload 2021/2/8 3 7 6 5 7 6 5 Transport IP Datalink Physical Transport IP Datalink Physical IP router 2 2 1 1

Functionality Split Network provides best-effort delivery End-systems implement richer functions Reliability k In-order delivery Demultiplexing Message boundaries k connection abstraction Congestion control 2021/2/8 4

Functionality Split  Network provides best-effort delivery  End-systems implement richer functions  Reliability  In-order delivery  Demultiplexing  Message boundaries  Connection abstraction  Congestion control  … 2021/2/8 4

Transport Protocols UDP provides just integrity and demultiplex TCP adds k connection abstraction Reliability Ordered delivery Byte stream Flow and congestion control 2021/2/8

Transport Protocols  UDP provides just integrity and demultiplex  TCP adds…  Connection abstraction  Reliability  Ordered delivery  Byte stream  Flow and congestion control 2021/2/8 5

UDP: User Datagram Protocol [RFC 768 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 2021/2/8

UDP: User Datagram Protocol [RFC 768]  “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 2021/2/8 6

Why is there a UDP? No connection establishment(which can add delay Simple: no connection state at sender recelver small header No congestion control: UDP can blast away as fast as desired 2021/2/8

Why is there a UDP?  No connection establishment (which can add delay)  Simple: no connection state at sender, receiver  Small header  No congestion control: UDP can blast away as fast as desired 2021/2/8 7

UDP Applications w Often used for streaming multimedia apps k Loss tolerant Rate sensitive w other UdP based apps k DNs, SNMP Reliable transfer over UdP Must be at application layer Application-specific error recovery 2021/2/8

UDP Applications  Often used for streaming multimedia apps  Loss tolerant  Rate sensitive  Other UDP based apps  DNS, SNMP  Reliable transfer over UDP  Must be at application layer  Application-specific error recovery 2021/2/8 8

UDP Segment Format 32 bits Source port dest port Length, in bytes ength Checksum of UDP segment, including header Application data (message UDP segment format 2021/2/8

UDP Segment Format 2021/2/8 9 Source port # Dest port # 32 bits Application data (message) UDP segment format Length Checksum Length, in bytes of UDP segment, including header

UDP Checksum Goal: detect errors"(e.g flipped bits) in transmitted segment Sender Treat segment contents as sequence of 16-bit integers Checksum: addition of segment contents Sender puts checksum value into UDP checksum field 2021/2/8

UDP Checksum  Goal: detect “errors” (e.g., flipped bits) in transmitted segment  Sender  Treat segment contents as sequence of 16-bit integers  Checksum: addition of segment contents  Sender puts checksum value into UDP checksum field 2021/2/8 10