04 - socket programming

8/4/2019 04 - Socket Programming

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CS4233 Network Programming

Socket Programming

Chen-Lung Chan

Department of Computer Science

National Tsing Hua University

Socket Interface

UNIX domain socket Local IPC AF_UNIX

Filesystem: path name

Internet domain socket

Internet communications AF_INET

Internet address: IP + Port

A Client-Server Exchange

Clientprocess

Serverprocess

1. Client sends request

2. Server

handlesrequest

3. Server sends response4. Client

handlesresponse

Resource

A serverprocess and one or more clientprocesses

Server manages some resource. Server provides serviceby manipulating

resource for clients.

Network Applications

OS

NetworkInterface

ClientAppl.

SocketOS +NetworkAPIs

OS

NetworkInterface

ServerAppl.

SocketOS +NetworkAPIs

Internet

Client MachineClient Machine Server MachineServer Machine

Access to Network via Program Interface Sockets make network I/O look like files Call system functions to control and communicate Network code handles issues of routing, segmentation.


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Internet Connections (TCP/IP)

Connection socket pair(140.114.78.11:3479, 140.114.78.20:80)

Server(port 80)

Client

Client socket address140.114.78.11:3479

Server socket address140.114.78.20:80

Client host address140.114.78.11

Server host address140.114.78.20

Note:3479is an portrandomlyallocatedby the kernel

Note:80is awell-knownportassociated with Web servers

Two common paradigms for clients and servers communication Datagrams (UDP protocol, SOCK_DGRAM)

Connections (TCP protocol, SOCK_STREAM)

Connections are point-to-point, full-duplex (2-waycommunication), and reliable. (TODAYS TOPIC!)

Clients

Examples of client programs

Web browsers, ftp, telnet, ssh How does a client find the server?

The IP address in the server socket address identifies thehost (more precisely, an adaptor on the host)

The (well-known) port in the server socket address identifiesthe service, and thus implicitly identifies the server processthat performs that service.

Examples of well known ports Port 7: Echo server

Port 23: Telnet server Port 25: Mail server

Port 80: Web server

Using Ports to Identify Services

Web server(port 80)

Client host

Server host 140.114.78.20

Echo server(port 7)

Service request for140.114.78.20:80

(i.e., the Web server)

KernelClient

Web server(port 80)

Echo server(port 7)

Service request for140.114.78.20:7

(i.e., the echo server)KernelClient

Servers

See /etc/services for a

comprehensive list of theservices available on aLinux machine.

Servers are long-running processes (daemons). Created at boot-time (typically) by the init process (process 1)

Run continuously until the machine is turned off.

Each server waits for requests to arrive on a well-known port associated with a particular service. Port 7: echo server

Port 23: telnet server

Port 25: mail server

Port 80: HTTP server

A machine that runs a server process is also often

referred to as a server.


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Using netstat

The netstatcommand prints a lot of

useful network information such asnetwork connections, routing tables, andinterface statistics. netstat: display a list of open sockets

netstata: show both listening and non-listening sockets

Using netstatto view the status of yoursocket program!

Sockets

What is a socket?

To the kernel, a socket is an endpoint ofcommunication. To an application, a socket is a file descriptor that

lets the application read/write from/to the network. Remember: All Unix I/O devices, including networks, are

modeled as files.

Clients and servers communicate with each byreading from and writing to socket descriptors.

The main distinction between regular file I/Oand socket I/O is how the application opensthe socket descriptors.

Overview of the SocketsInterface

Client /ServerSession

Client Server

socket socket

bind

listen

read

writeread

write

Connectionrequest

read

close

close EOF

Await connectionrequest fromnext client

open_listenfd

open_clientfd

acceptconnect

struct sockaddr_in serveraddr;

/* fill in serveraddr with an address */

/* Connect takes (struct sockaddr *) as its second argument */

connect(clientfd, (struct sockaddr *) &serveraddr,sizeof(serveraddr));

Network Byte Ordering Network is big-endian, host may be big- or little-endian Functions work on 16-bit (short) and 32-bit (long) values htons()/htonl(): convert host byte order to network byte order ntohs()/ntohl(): convert network byte order to host byte order Use these to convert network addresses, ports,

Structure Casts You will see a lot of structure casts

Socket Programming Cliches


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Echo Client: socket()

int clientfd; /* socket descriptor */

if ((clientfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)

return -1; /* check errno for cause of error */

... (more)

socket() creates a socket descriptor on the

client. AF_INET: indicates that the socket is associated

with Internet protocols. SOCK_STREAM: selects a reliable byte stream

connection.

Echo Client: gethostbyname()

The client then builds the servers Internet address.


struct hostent *hp; /* DNS host entry */

struct sockaddr_in serveraddr; /* servers IP address */

...

/* fill in the server's IP address and port */

if ((hp = gethostbyname(hostname)) == NULL)

return -2; /* check h_errno for cause of error */

bzero((char *) &serveraddr, sizeof(serveraddr));

serveraddr.sin_family = AF_INET;

bcopy((char *)hp->h_addr,(char *)&serveraddr.sin_addr.s_addr, hp->h_length);

serveraddr.sin_port = htons(port);

Echo Client: connect()

Finally, the client creates a connection with the server. Client process suspends (blocks) until the connection is

created.

After resuming, the client is ready to begin exchangingmessages with the server via Unix I/O calls on descriptorsockfd.


struct sockaddr_in serveraddr; /* server address */

...

/* Establish a connection with the server */

if (connect(clientfd, (struct sockaddr *)&serveraddr,

sizeof(serveraddr)) < 0)

return -1;

Echo Client: read() & write()

After the connection is established, we canuse read() and write() to exchange data

between the client and the server. The use of read() and write() are the same as we

mentioned in Direct File I/O.

However, recv() and send() are suggested dueto their compatibility with Winsock. #include

ssize_t recv(int s, void *buf, size_t len, int flags)

ssize_t send(int s, const void *buf, size_t len, intflags)


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Servers and Sockets

client 1 server client 2

call connect call accept

ret connectret accept

call connect

call readwrite

ret readclose

close call accept

ret connect

call read

ret read

close

write

ret accept

close

Server must be able to handle multiple requests

Where should pending connections be queued up?

Echo Server (1/2)

Create a socket

socket() Announce the service

bind()

Wait for connection listen()

accept()

Exchange data read(), write()

recv(), send() Close the socket

close()

Echo Server (2/2)

if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)

return -1;

/* Listenfd will be an endpoint for all requests to porton any IP address for this host */



serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);

serveraddr.sin_port = htons((unsigned short)port);

if (bind(listenfd, (struct sockaddr *)&serveraddr,


return -1;

/* Make it a listening socket ready to accept connection requests */

if (listen(listenfd, LISTENQ) < 0)

return -1;

Echo Server: Initialize SocketAddress

struct sockaddr_in serveraddr; /* server's socket addr */





IP addresses INADDR_ANY: address to accept any incoming messages INADDR_LOOPBACK: 127.0.0.1 (local host) The server is only active on the specified IP address

Ex: public IP: 140.114.78.20, but the program bind at 127.0.0.1

only the clients in the local host can contact the server

IP addr and port stored in network (big-endian) byte order

htonl() converts longs from host byte order to network byte order. htons() converts shorts from host byte order to network byte order.


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Echo Server: bind() & listen()

bind() associates the socket with the socket addresswe just created.

listen() indicates that this socket will accept connection(connect()) requests from clients.





if (bind(listenfd, (struct sockaddr *)&serveraddr,


return -1;if (listen(listenfd, LISTENQ) < 0)

return -1;

Echo Server: accept()

listenfd(3)

Client

1. Server blocks in accept(),

waiting for connection requeston listening descriptor listenfd.clientfd

Server

listenfd(3)

Client

clientfd

Server2. Client makes connectionrequest by calling and blockingin connect().

Connectionrequest

listenfd(3)

Client

clientfd

Server

3. Server returns connfd from

accept(). Client returns fromconnect(). Connection is nowestablished between clientfd andconnfd.

connfd(4)

Echo Server: Main Loop

The server loops endlessly, waiting for connectionrequests, then reading input from the client, andechoing the input back to the client.

main() {

/* create and configure the listening socket */

while(1) {

/* accept(): wait for a connection request */

/* echo(): read and echo input lines from client til EOF */

/* close(): close the connection */

}

}

Echo Server: accept()

int listenfd; /* listening descriptor */

int connfd; /* connected descriptor */

struct sockaddr_in clientaddr;int clientlen;

clientlen = sizeof(clientaddr);

connfd = accept(listenfd, (struct sockaddr_in *)&clientaddr,

&clientlen);

accept() blocks waiting for a connection request.

accept() returns a connected descriptor(connfd) with the sameproperties as the listening descriptor(listenfd) Returns when the connection between client and server is created

and ready for I/O transfers. All I/O with the client will be done via the connected socket.

accept() also fills in clients IP address.


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Summary of Socket APIs

int socket(int domain, int type, int protocol);

int connect(int sockfd, const struct sockaddr *serv_addr, socklen_t

addrlen); int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen);

int listen(int s, int backlog);

int accept(int s, struct sockaddr *addr, socklen_t *addrlen);

ssize_t read(int fd, void *buf, size_t count);

ssize_t write(int fd, const void *buf, size_t count);

ssize_t recv(int s, void *buf, size_t len, int flags);

ssize_t send(int s, const void *buf, size_t len, int flags); int shutdown(int s, int how);

int close(int fd);

uint32_t htonl(uint32_t hostlong); uint16_t htons(uint16_t hostshort); uint32_t ntohl(uint32_t netlong); uint16_t ntohs(uint16_t netshort);

Other Socket APIs - Address

Internet address manipulation struct hostent *gethostbyname(const char *name)

returns a structure of type hostentfor the given host name in_addr_t inet_addr(const char *cp)

convert the Internet host address cp from numbers-and-dots notationinto binary data in network byte order

char *inet_ntoa(struct in_addr in) convert the Internet host address in given in network byte order to a

string in standard numbers-and-dots notation (a.b.c.d) The string is returned in a statically allocated buffer, which subsequent

calls will overwrite.

Socket name int getsockname(int s, struct sockaddr *name, socklen_t *namelen)

get socket name int getpeername(int s, struct sockaddr *name, socklen_t *namelen)

get name of connected peer socket

Other Socket APIs - Socket Options

int setsockopt(SOCKET s, int level, intoptname, const char FAR* optval, int optlen)

int getsockopt(SOCKET s, int level, intoptname, char FAR* optval, int FAR* optlen)

Useful options level = SOL_SOCKET

SO_RCVBUF

SO_SNDBUF

level = IPPROTO_TCP

TCP_NODELAY

Some Helpful Tools

ps The pscommand reports process status program.

telnet The telnetcommand can be used as a client

program for testing. Ex: telnet cs22.cs.nthu.edu.tw 21

Then, we can communicate directly with the FTP server.

netstat The netstatcommand prints a lot of useful network

information such as network connections, routingtables, and interface statistics. Ex: we can see the actual address that our program binds.


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Problem: Close Socket

int sock = socket(AF_INET, SOCK_STREAM, 0);bind(sock, (struct sockaddr *)&name, sizeof(struct sockaddr_in));

listen(sock, 5);int ns = accept(sock, NULL, NULL);char buf[1024] = "Test!!\r\n";send(ns, buf, strlen(buf), 0);close(ns);close(sock);

When you re-execute the above code, you may get an error ofBind: Address Already in Use .

This is because the socket is not released immediately. $ netstat -a

cs22.4233 cs22.50414 5840 0 10136 0 TIME_WAIT

Bind: Address Already in Use (1/2)

LAST_ACK

Local end initiates closure Remote end initiates closure

Closing first guaranteesTIME_WAIT

States of a server socket

receive ACK

close() is called,send FIN

Closing second avoidsTIME_WAIT

CLOSED

TIME_WAIT

FIN_WAIT_2 CLOSING

FIN_WAIT_1

close() is called,send FIN

receive ACK receive FINand send ACK

receive FINand send ACK

receive ACK

timeout completes

CLOSE_WAIT

ESTABLISHED

receive FINand send ACK

Bind: Address Already in Use (2/2)

It is possible for the process to completebefore the kernel has released the associatednetwork resource, and this port cannot bebound to another process until the kernel hasdecided that it is done.

Avoidance SO_REUSEADDR

Use setsockopt() to set the SO_REUSEADDR socketoption, which explicitly allows a process to bind to a portwhich remains in TIME_WAIT

Client closes first

Reduce timeout Shorten the timeout associated with TIME_WAIT

Design a Protocol (1/2)

Avoid passing binary data 32 bit vs 64 bit

Byte order Little endian vs big endian

Ex: FTP telnet cs22.cs.nthu.edu.tw 21

220-

220-=====================================================

220- Department of Computer Science

220- National Tsing Hua University

220-

220- Host: cs22.cs.nthu.edu.tw [140.114.87.72]

220------------------------------------------------------

220- Connected from: vc.cs.nthu.edu.tw [140.114.78.20]

220 cs22 FTP server (Version 4.122 Fri Feb 5 17:14:49 CST 1999) ready.


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Design a Protocol (2/2)

Ex: FTP contd

HELP

214- The following commands are recognized (* =>'s unimplemented).

USER PORT STOR MSAM* RNTO NLST MKD CDUP

PASS PASV APPE MRSQ* ABOR SITE XMKD XCUP

ACCT* TYPE MLFL* MRCP* DELE SYST RMD STOU

SMNT* STRU MAIL* ALLO CWD STAT XRMD SIZE

REIN* MODE MSND* REST* XCWD HELP PWD MDTM

QUIT RETR MSOM* RNFR LIST NOOP XPWD

USER xxxxxxxx

331 Password required for xxxxxxxx.

commands

return code

FTP Protocol

Client Server

accept()

connect()

220 Welcome Message

USER xxxx

331 Password required for xxxx.

PORT a,b,c,d,e,f

200 PORT command successful.

RETR filename

bind() & listen() connect()

accept()

bind() & listen()

QUIT

Hint: CRLF = \r\nPASS yyyy

230 User xxxx logged in.

Notes of TCP Socket (1/3)

The result of recv() on a TCP socket may notbe the same as your prediction.

Ex1: A sends This is a test!!\r\n to B In A: send(sA, This is a test!!\r\n, 18, 0);

In B: recv(sB, buf, sizeof(buf), 0);

What is the result of buf? It may be

This is a test!!\r\n

This is a

This


Ex2: A sends This is a test!!\r\n and Testagain!!\r\n to B

In A: send(sA, This is a test!!\r\n, 18, 0);send(sA, Test again!!\r\n, 14, 0);

In B: recv(sB, buf, sizeof(buf), 0);

What is the result of buf? It may be

This is a test!!\r\n

This is a test!!\r\nTest again!!\r\n

This is a

This is a test!!\r\nTest


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The return value of recv(), i.e., the length,

is unpredictable. Ensure that the message has been

completely received before parsing it.

What should we do?

Append the received data into a buffer Repeat the above step until \r\n is received

Parsing the Message (1/2)

String operations strcmp() strtok() strstr() sprintf()

The above functions only deal with strings, i.e., acharacter array which ends with \0. Ex: char buf[5] = test;

buf[0] = t; buf[1] = e; buf[2] = s; buf[3] = t; buf[4] = \0;

So, if you receive 10 bytes from the socket and storethem into char buf[20], remember to set buf[10] as\0 before you use the above string functions. Hint: always let the content of the buffer be a string

Parsing the Message (2/2)

char buf[256]

256

int count = recv(sock, buf, sizeof(buf), 0);T e s

T e s \0

buf[count] = \0;

char *p = strstr(buf, \r\n);

t \r \n T int len = recv(sock, buf+count, sizeof(buf)-count, 0);

t \r \n T

buf[count+len] = \0;

\0

p = strstr(buf, \r\n);

// get the message from the buffer

memcpy();

Stabilize Your Program

Message parsing Be careful to use string operations

Check return values The socket APIs would return errors bind(), accept(), connect(), recv(), send()

Remember to release unnecessary resources Close the sockets once you do not require them

anymore

Thread synchronization/interprocesscommunications Be careful to design the system flow.


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What is Peer-to-Peer (P2P)?

Peer-to-peer is a way of structuring

distributed applications such that theindividual nodes have symmetric roles.

Rather than being divided into clientsand servers each with quite distinct roles,in P2P applications a node may act as

both a client and a server.

Client-Server vs. P2P

Client-server

Clients are dumb little is assumed Servers do most things (compute, store, control)

Centralization makes things simple

But introduces

Single point of failure/bottleneck, tighter control,

P2P

Peers (typically) have equal functionality A peer can be a client, a server, or a router

04 - socket programming

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