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Networked Applications: Sockets

Networked Applications: Sockets. Assembled by Ossi Mokryn, based on Slides by Jennifer Rexford, Princeton, And on data from beej’s guide : http://beej.us/guide/bgnet. a host-local , application-created , OS-controlled interface (a “door”) into which

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Networked Applications: Sockets

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  1. Networked Applications: Sockets Assembled by Ossi Mokryn, based on Slides by Jennifer Rexford, Princeton, And on data from beej’s guide : http://beej.us/guide/bgnet

  2. a host-local, application-created, OS-controlled interface (a “door”) into which application process can both send and receive messages to/from another application process socket Socket programming Socket API • introduced in BSD4.1 UNIX, 1981 • explicitly created, used, released by apps • client/server paradigm • two types of transport service via socket API: • unreliable datagram • reliable, byte stream-oriented Goal: learn how to build client/server application that communicate using sockets Application Layer

  3. Clients and Servers • Client program • Running on end host • Requests service • E.g., Web browser • Server program • Running on end host • Provides service • E.g., Web server GET /index.html “Site under construction”

  4. Client-Server Communication • Client “sometimes on” • Initiates a request to the server when interested • E.g., Web browser on your laptop or cell phone • Doesn’t communicate directly with other clients • Needs to know the server’s address • Server is “always on” • Services requests from many client hosts • E.g., Web server for the www.cnn.com Web site • Doesn’t initiate contact with the clients • Needs a fixed, well-known address

  5. Client and Server Processes • Program vs. process • Program: collection of code • Process: a running program on a host • Communication between processes • Same end host: inter-process communication • Governed by the operating system on the end host • Different end hosts: exchanging messages • Governed by the network protocols • Client and server processes • Client process: process that initiates communication • Server process: process that waits to be contacted

  6. Socket: End Point of Communication • Sending message from one process to another • Message must traverse the underlying network • Process sends and receives through a “socket” • In essence, the doorway leading in/out of the house • Socket as an Application Programming Interface • Supports the creation of network applications User process User process socket socket Operating System Operating System

  7. Identifying the Receiving Process • Sending process must identify the receiver • Name or address of the receiving end host • Identifier that specifies the receiving process • Receiving host • Destination address that uniquely identifies the host • An IP address is a 32-bit quantity • Receiving process • Host may be running many different processes • Destination port that uniquely identifies the socket • A port number is a 16-bit quantity

  8. Using Ports to Identify Services Server host 128.2.194.242 Service request for 128.2.194.242:80 (i.e., the Web server) Client host Web server (port 80) OS Client Echo server (port 7) Service request for 128.2.194.242:7 (i.e., the echo server) Web server (port 80) OS Client Echo server (port 7)

  9. Knowing What Port Number To Use • Popular applications have well-known ports • E.g., port 80 for Web and port 25 for e-mail • Well-known ports listed at http://www.iana.org • Well-known vs. ephemeral ports • Server has a well-known port (e.g., port 80) • Between 0 and 1023 • Client picks an unused ephemeral (i.e., temporary) port • Between 1024 and 65535 • Uniquely identifying the traffic between the hosts • Two IP addresses and two port numbers • Underlying transport protocol (e.g., TCP or UDP)

  10. Delivering the Data: Division of Labor • Network • Deliver data packet to the destination host • Based on the destination IP address • Operating system • Deliver data to the destination socket • Based on the protocol and destination port # • Application • Read data from the socket • Interpret the data (e.g., render a Web page)

  11. Windows Socket API • Socket interface • Originally provided in Berkeley UNIX • Later adopted by all popular operating systems • Simplifies porting applications to different OSes • In UNIX, everything is like a file • All input is like reading a file • All output is like writing a file • File is represented by an integer file descriptor • System calls for sockets • Client: create, connect, write, read, close • Server: create, bind, listen, accept, read, write, close • Winsock Programmer's FAQ: http://tangentsoft.net/wskfaq/newbie.html#interop

  12. Typical Client Program • Prepare to communicate • Create a socket • Determine server address and port number • Initiate the connection to the server • Exchange data with the server • Write data to the socket • Read data from the socket • Do stuff with the data (e.g., render a Web page) • Close the socket

  13. UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server application viewpoint Socket programming with UDP UDP: no “connection” between client and server • no handshaking • sender explicitly attaches IP address and port of destination to each packet • server must extract IP address, port of sender from received packet UDP: transmitted data may be received out of order, or lost Application Layer

  14. Client create socket, port=x, for incoming request: serverSocket = DatagramSocket() create socket, clientSocket = DatagramSocket() Create, address (hostid, port=x, send datagram request using clientSocket read request from serverSocket write reply to serverSocket specifying client host address, port number read reply from clientSocket close clientSocket Client/server socket interaction: UDP Server (running on hostid) Application Layer

  15. Creating a Socket: socket() • Operation to create a socket • SOCKET WSAAPI socket ( int af, int type, int protocol); • af An address family specification. only format currently supported is PF_INET, which is the ARPA Internet address format. • Type: semantics of the communication • SOCK_STREAM: reliable byte stream • SOCK_DGRAM: message-oriented service • Protocol: specific protocol • 0: unspecified • (PF_INET and SOCK_STREAM already implies TCP, PF_INET and SOCK_DGRAM implies UDP).

  16. Establishing the server’s name and port - history • struct sockaddr_in server; • server.sin_family = AF_INET; • server.sin_addr.s_addr = inet_addr(IPstring); • server.sin_port = htons(ServerPort);

  17. Sending Data • int WSAAPI sendto ( SOCKETs, const char FAR * buf, int len, intflags, const struct sockaddr FAR * to, inttolen ); • s A descriptor identifying a (possibly connected) socket. • buf A buffer containing the data to be transmitted. • len The length of the data in buf. • flags Specifies the way in which the call is made. • to An optional pointer to the address of the target socket. • tolen The size of the address in to. Slides by Jennifer Rexford

  18. Receiving Data • int WSAAPI recvfrom ( SOCKETs, char FAR* buf, intlen, intflags, • struct sockaddr FAR* from, int FAR* fromlen); • s A descriptor identifying a bound socket. • buf A buffer for the incoming data. • len The length of buf. • flags Specifies the way in which the call is made. • from An optional pointer to a buffer which will hold the source address upon return. • fromlen An optional pointer to the size of the from buffer. Slides by Jennifer Rexford

  19. Byte Ordering: Little and Big Endian • Hosts differ in how they store data • E.g., four-byte number (byte3, byte2, byte1, byte0) • Little endian (“little end comes first”)  Intel PCs!!! • Low-order byte stored at the lowest memory location • Byte0, byte1, byte2, byte3 • Big endian (“big end comes first”) • High-order byte stored at lowest memory location • Byte3, byte2, byte1, byte 0 • IP is big endian (aka “network byte order”) • Use htons() and htonl() to convert to network byte order • Use ntohs() and ntohl() to convert to host order

  20. Why Can’t Sockets Hide These Details? • Dealing with endian differences is tedious • Couldn’t the socket implementation deal with this • … by swapping the bytes as needed? • No, swapping depends on the data type • Two-byte short int: (byte 1, byte 0) vs. (byte 0, byte 1) • Four-byte long int: (byte 3, byte 2, byte 1, byte 0) vs. (byte 0, byte 1, byte 2, byte 3) • String of one-byte charters: (char 0, char 1, char 2, …) in both cases • Socket layer doesn’t know the data types • Sees the data as simply a buffer pointer and a length • Doesn’t have enough information to do the swapping

  21. Servers Differ From Clients • Passive open • Prepare to accept connections • … but don’t actually establish one • … until hearing from a client • Hearing from multiple clients • Allow a backlog of waiting clients • ... in case several try to start a connection at once • Create a socket for each client • Upon accepting a new client • … create a new socket for the communication

  22. Typical Server Program • Prepare to communicate • Create a socket • Associate local address and port with the socket • Wait to hear from a client (passive open) • In TCP: Indicate how many clients-in-waiting to permit • Accept an incoming connection from a client • Exchange data with the client over new socket • Receive data from the socket • Do stuff to handle the request (e.g., get a file) • Send data to the socket • Close the socket • Repeat with the next connection request

  23. Server Preparing its Socket • Bind socket to the local address and port number (Associate a local address with a socket) • #include <winsock2.h> • int WSAAPI bind ( SOCKET s, structsockaddr* name, intnamelen);Arguments: socket descriptor, server address, address length • sA descriptor identifying an unbound socket. • nameThe address to assign to the socket. Often, you bound your listening socket to the special IP address INADDR_ANY. This allows your program to work without knowing the IP address of the machine it was running on, or, in the case of a machine with multiple network interfaces, it allows your server to receive packets destined to any of the interfaces. When sending, a socket bound with INADDR_ANY binds to the default IP address, which is that of the lowest-numbered interface. • NamelenThe length of the name. • Returns 0 on success, and -1 if an error occurs

  24. Putting it All Together Server socket() bind() Client listen() socket() establish connection accept() connect() block send request write() read() process request send response write() read()

  25. Serving One Request at a Time? • Serializing requests is inefficient • Server can process just one request at a time • All other clients must wait until previous one is done • Need to time share the server machine • Alternate between servicing different requests • Do a little work on one request, then switch to another • Small tasks, like reading HTTP request, locating the associated file, reading the disk, transmitting parts of the response, etc. • Or, start a new process to handle each request • Allow the operating system to share the CPU across processes • Or, some hybrid of these two approaches

  26. TCP info for later in the course

  27. process process TCP with buffers, variables TCP with buffers, variables socket socket Socket-programming using TCP Socket: a door between application process and end-end-transport protocol (UCP or TCP) TCP service: reliable transfer of bytesfrom one process to another controlled by application developer controlled by application developer controlled by operating system controlled by operating system internet host or server host or server Application Layer

  28. Putting it All Together Server socket() bind() Client listen() socket() establish connection accept() connect() block send request write() read() process request send response write() read()

  29. TCP info for later in the course

  30. Serving One Request at a Time? • Serializing requests is inefficient • Server can process just one request at a time • All other clients must wait until previous one is done • Need to time share the server machine • Alternate between servicing different requests • Do a little work on one request, then switch to another • Small tasks, like reading HTTP request, locating the associated file, reading the disk, transmitting parts of the response, etc. • Or, start a new process to handle each request • Allow the operating system to share the CPU across processes • Or, some hybrid of these two approaches

  31. Blocking and non blocking • "block" is a techie jargon for "sleep“ • Lots of functions block. accept() blocks. All the recv() functions block. • Why? Because we programmed them this way:When you first create the socket with socket(), the kernel sets it to blocking. • Can we set the socket to be non-blocking? What does it mean? • Yes. It means you have to poll it for data (check on it) • If there’s no data yet, you get -1 (err) and have to try again. • Isn’t that CPU intensive? Yes! • So, you can use select()…

  32. Reading or writing to multiple sockets • A server wants to listen for incoming connections as well as keep reading from the connections it has. • Select – a way to monitor several sockets at the same time, and know their situation. • select()—Synchronous I/O Multiplexing • Manipulates setsof sockets and lets you know: • which ones are ready for reading • which are ready for writing • How? By creating and handling of sets of sockets, and obtaining additional information for each.

  33. Manipulating Sets for the select() func. Note: fd in Windows in the Socket structure

  34. Additional select() info • What happens if a socket in the read set closes the connection? Well, in that case, select() returns with that socket descriptor set as "ready to read". When you actually do recv() from it, recv() will return 0. That's how you know the client has closed the connection. • One more note of interest about select(): if you have a socket that is listen()ing, you can check to see if there is a new connection by putting that socket's file descriptor in the readfds set.

  35. Wanna See Real Clients and Servers? • Apache Web server • Open source server first released in 1995 • Name derives from “a patchy server” ;-) • Software available online at http://www.apache.org • Mozilla Web browser • http://www.mozilla.org/developer/ • Sendmail • http://www.sendmail.org/ • BIND Domain Name System • Client resolver and DNS server • http://www.isc.org/index.pl?/sw/bind/ • …

  36. A final note: what is Peer-to-Peer Communication • No always-on server at the center of it all • Hosts can come and go, and change addresses • Hosts may have a different address each time • Example: peer-to-peer file sharing • Any host can request files, send files, query to find where a file is located, respond to queries, and forward queries • Scalability by harnessing millions of peers • Each peer acting as both a client and server

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