Networked applications sockets
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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

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


Socket programming

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


Clients and servers

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”


Client server communication

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


Client and server processes

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


Socket end point of communication

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


Identifying the receiving process

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


Using ports to identify services

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)


Knowing what port number to use

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)


Delivering the data division of labor

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)


Windows socket api

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


Typical client program

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


Socket programming with udp

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


Client server socket interaction udp

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


Creating a socket socket

Creating a Socket: socket()

  • Operation to create a socket

    • SOCKET WSAAPI socket ( int af, int type, int protocol);

  • afAn 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).


Establishing the server s name and port history

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);


Sending data

Sending Data

  • int WSAAPI sendto ( SOCKETs, const char FAR * buf, int len, intflags, const struct sockaddr FAR * to, inttolen );

  • sA descriptor identifying a (possibly connected) socket.

  • bufA buffer containing the data to be transmitted.

  • lenThe length of the data in buf.

  • flagsSpecifies the way in which the call is made.

  • toAn optional pointer to the address of the target socket.

  • tolenThe size of the address in to.

Slides by Jennifer Rexford


Receiving data

Receiving Data

  • int WSAAPI recvfrom ( SOCKETs, char FAR* buf, intlen, intflags,

  • struct sockaddr FAR* from,int FAR* fromlen);

  • sA descriptor identifying a bound socket.

  • bufA buffer for the incoming data.

  • lenThe length of buf.

  • flagsSpecifies the way in which the call is made.

  • fromAn optional pointer to a buffer which will hold the source address upon return.

  • fromlenAn optional pointer to the size of the from buffer.

Slides by Jennifer Rexford


Byte ordering little and big endian

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


Why can t sockets hide these details

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


Servers differ from clients

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


Typical server program

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


Server preparing its socket

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


Putting it all together

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()


Serving one request at a time

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


Tcp info for later in the course

TCP info for later in the course


Socket programming using tcp

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


Putting it all together1

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()


Tcp info for later in the course1

TCP info for later in the course


Serving one request at a time1

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


Blocking and non blocking

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()…


Reading or writing to multiple sockets

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.


Manipulating sets for the select func

Manipulating Sets for the select() func.

Note: fd in Windows in the Socket structure


Additional select info

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.


Wanna see real clients and servers

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/


A final note what is peer to peer communication

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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