ICOM 6115 Computer Networks and the WWW Manuel

ICOM 6115 – Computer Networks and the WWW Manuel Rodriguez-Martinez, Ph. D. Lecture 26 ICOM 6115 ©Manuel Rodriguez-Martinez

Lecture Objectives • Introduction to Transport Layer – Layer 4 – User Datagram Protocol (UDP) – Transmission Control Protocol (TCP) ICOM 6115 ©Manuel Rodriguez-Martinez

Transport Layer • Provides reliable, cost-effective way to move data between two applications – independent of type of layer 3 • Application level API often interact with this module • Sockets are built on top of transport layer – TCP Sockets – UDP Sockets ICOM 6115 ©Manuel Rodriguez-Martinez

Layer 3, Layer 4 and Layer 5 Stack ICOM 6115 ©Manuel Rodriguez-Martinez

Transport services • Connectionless – Blocks of data sent over datagrams • Targeted for a specific process on a given machine • Connection-oriented – Reliable stream of bytes sent between two processes – Adds reliability to layer 3 • Transport entity – Module that uses the network layer to move data (inside OS of hosts) ICOM 6115 ©Manuel Rodriguez-Martinez

Typical Operations • LISTEN (usually server side) – Wait for a connection to occur • CONNECT (usually client side) – Request a connection to be established • SEND – Move some amount of data between processes • RECEIVE – Accepts some amount of data • DISCONNECT (bidirectional) – Terminate the connection ICOM 6115 ©Manuel Rodriguez-Martinez

Encapsulation at layer 4 • Data is packaged into TPDU – Transport Protocol Data Unit ICOM 6115 ©Manuel Rodriguez-Martinez

Typical State Transition Diagram ICOM 6115 ©Manuel Rodriguez-Martinez

Addressing • Transport layer identifies process by: – <Layer 3 Address, Port number> • Idea – Layer 3 Address identifies host on which process is running • Network Service Access Point (NSAP) • IP address – Port uniquely identifies a process on that host • Transport Services Access Point (TSAP) • Applications use to port to index data structure where data is placed – Send and receive operations ICOM 6115 ©Manuel Rodriguez-Martinez

Example of NSAPs and TSAPs ICOM 6115 ©Manuel Rodriguez-Martinez

Connection daemon ICOM 6115 ©Manuel Rodriguez-Martinez

Connection daemon • Connection daemon is used to spawn a new process to service a client request • Often used if we do not want to have a server running all the time on background – Ex. inetd can spawns telnet processes • OS must support way to pass variable – Pass context of process – UNIX • Fork() operation – Windows • Threads ICOM 6115 ©Manuel Rodriguez-Martinez

Demultiplexing data Process Port Message Queues Layer 4 Layer 3 ICOM 6115 ©Manuel Rodriguez-Martinez Process

User Datagram Protocol (UDP) • Provides a mechanism to send datagrams between processes – Blocks of data are sent • Just adds port information on top of IP packet – Can arrive delayed, out of order, or get lost • Unreliable services – Need to add ACKs if you want more • Some applications that use it – DNS (Domain Name Services) – NFS (Network File System) ICOM 6115 ©Manuel Rodriguez-Martinez

UDP Header ICOM 6115 ©Manuel Rodriguez-Martinez

Transmission Control Protocol (TCP) • Provides a reliable byte stream between processes • Data is guaranteed to – arrive safely – Be organized in the proper order and read in order • TCP streams provide the abstraction of a byte stream – Like a file ICOM 6115 ©Manuel Rodriguez-Martinez

TCP Byte Stream ICOM 6115 ©Manuel Rodriguez-Martinez

Buffering Data in TCP • Data is buffered until enough is present to be sent or read • Alternatively, the buffer can be flushed to force data out ICOM 6115 ©Manuel Rodriguez-Martinez

TCP Header ICOM 6115 ©Manuel Rodriguez-Martinez

TCP Three Way Handshake • Use to establish a connection • Client sends SYN message with a seq number x • Server replies with – SYN indicating its own sequence numbers y – ACK indicating it expects data with seq x + 1 • Client replies with ACK indicating it expects data with seq y + 1 • Sequence numbers address bytes! ICOM 6115 ©Manuel Rodriguez-Martinez

Timeline diagram ICOM 6115 ©Manuel Rodriguez-Martinez

Some Issues • Need to way for 3 -way handshake before sending data – How long is this? • This is bad for short live connection that send little data – HTPP request with small number of images ICOM 6115 ©Manuel Rodriguez-Martinez