Review Why layer architecture peer entities Protocol and

• Review: – Why layer architecture? – peer entities – Protocol and service interface – Connection-oriented/connectionless service – Reliable/unreliable service – What type of service will a typical end user want? • Why even considering other types of services – layers in the TCP/IP reference model – layers in the ISO/OSI reference model

Application Layer • Problems to be addressed in this layer: – Directly related to the individual application (no common problems) • Design issues – directly related to each application (no common issues). • Introduce application layer concepts. • Examine the service interface between the application layer and the transport layer (socket API) • Give an example showing how to develop network applications.

Some network apps • • • E-mail • Internet telephone Web • Real-time video Instant messaging conference Remote login • Massive parallel P 2 P file sharing computing Multi-user network games • Streaming stored video clips

Network applications – run on different end systems (not routers) – communicate over a network. – view the network as the abstraction provided by the network (the transport layer). • Addressing end points • End-to end communication application transport network data link physical

• Service provided by the network: – Objective: to allow processes on different machines to talk to each other. • Is IP address alone sufficient to address an application entity? – Network end points: • IP address + port number – Why not IP address + PID? • A process can be associated with an end point. – E. g: http -- 80, ssh – 22, telnet— 23, DNS – 53 – Service provided by the network: end-to-end communications: • Reliable connection-oriented byte stream service (TCP) • Unreliable connectionless service (UDP)

– From a network application point of view, what is the logical topology of the network? • A process can bind to an end point and talk to anyone else in the network.

• TCP API: – How to access the end point from a process: – socket: create a new communication end point #include <sys/socket. h> int socket(int domain, int type, int protocol); domain: AF_UNIX file system AF_INET internet address type: SOCK_STREAM reliable connect-oriented, byte stream SOCK_DGRAM unreliable connectionless SOCK_SEQPACKET record stream protocol: 0, non-zero for a specific protocol – bind: attach an address to a socket int bind(int socket, const struct sockaddr *address, size_t address_len) address: contains the port number, address. sin_port

• Service primitives for TCP: – listen: announce willingness to accept connections int listen(int socket, int backlog); backlog: number of outstanding connections in the listen queue – accpet: accept a new connection on a socket int accept (int socket, struct sockaddr *address, size_t *address_len); address: the address of the connecting socket – connect: try to establish a connection int connect(int socket, const struct sockaddr *address, size_t address_len); address: the destination address – write: send data ssize_t write(int fildes, const void *buf, size_t nbyte); – read: receive data ssize_t read(int fildes, void *buf, size_t nbyte); – close: close a connection int close(int fildes);

Server: socket bind listen accept read/write close Client: socket connect read/write close What is the end point (IP + port number) for the server? What is the end point (IP + port number) for the client? When is the TCP connection established?

• One more problem: machines may represent numbers in different ways. – See example 3. c on linprog and program. – use htons/htonl/ntohs routines to solve the problem. • See example 1. c and example 2. c.

• TCP: – Reliable byte stream service. • UDP: – Unreliable datagram service. • Data may get lost – application may need to deal with more details in the communication.

• Basic UDP service interface: – Socket, bind, sendto, recvfrom, close UDP server UDP client TCP server TCP client socket bind socket Bind Listen Accept socket connect recvfrom sendto recvfrom close Read/write close read/write close