Understanding Sockets: A Deep Dive into TCP and UDP for Network Connections, Slides of Social Work

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Introduction to LAN/WAN

Sockets

Outline

)^ Socket basics )^ TCP sockets )^ Socket details )^ Socket options )^ Final notes

Socket Basics

)^ An

end-point

for a IP network connection

• what the application layer “plugs into”– programmer cares about ApplicationProgramming Interface (API)

)^ End point determined by two things:

• Host address: IP address is

Network Layer

• Port number: is

Transport Layer

)^ Two end-points determine a connection: socketpair

• ex: 206.62.226.35,p21 + 198.69.10.2,p1500– ex: 206.62.226.35,p21 + 198.69.10.2,p

Ports

)^ Numbers (vary in BSD, Solaris):

• 0-1023 “reserved”, must be root– 1024 – 49151 (registered with IANA)– 49152 – 65535 “ephemeral”

)^ /etc/services:

ftp 21/tcptelnet 23/tcpfinger 79/tcpsnmp 161/udp

Transport Layer

)^ UDP: User Datagram Protocol

• no acknowledgements– no retransmissions– out of order, duplicate possible– connectionless

)^ TCP: Transmission Control Protocol

• reliable (in order, all arrive, no duplicates)– flow control– connection– duplex

Socket Details

TCP/IP Sockets in C: Practical Guide for Programmers

M J Donahoo and K Calvert

©2001 Morgan Kaufmann

Sections 6.1.3-6.1.

of text

)^ Socket address structure )^ TCP client-server )^ UDP client server )^ Misc stuff

-^ setsockopt(), getsockopt()

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TCP Client-Server

Server socket()bind()listen()accept()

Client socket()connect()send() recv()

(Block until connection

)^

“Handshake”

recv()send()

Data (request)Data (reply)

close()

End-of-File

recv()close()

“well-known”

port

socket()

int socket(int

family

, int

type

, int

protocol

• Create a socket, giving access to transport layer service. )^ family

is one of

-^ AF_INET (IPv4), AF_INET6 (IPv6), AF_LOCAL (local Unix),–^ AF_ROUTE (access to routing tables), AF_KEY (new, for encryption)

)^ type

is one of

-^ SOCK_STREAM (TCP), SOCK_DGRAM (UDP)–^ SOCK_RAW (for special IP packets, PING, etc. Must be root)

)^ protocol

is 0 (used for some raw socket options)

)^ upon success returns socket descriptor

• like file descriptor => -1 if failure )^ Example:

If (( sockfd = socket (AF_INET, SOCK_STREAM, 0)) < 0)err_sys (“socket call error”);

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listen()

)^ sockfd

is socket descriptor from

socket()

)^ backlog

is maximum number of

incomplete

int^ connections– historically 5– rarely above 15 on a even moderate web server! )^ Sockets default to active (for client) )^ Example:If (listen (sd, 2) != 0)errsys (“listen call error”);

listen(int

sockfd

,^ int

backlog

Announce willingness to accept connections, givequeue size, change socket state for TCP server.

accept()

)^ sockfd

is socket descriptor from

socket()

)^ cliaddr

and

addrlen

return protocol address from client

)^ returns brand new descriptor, created by OS )^ if used with

fork()

,^ can create concurrent server (more

later) ) Example:sfd = accept (s, NULL, NULL);if (sfd == -1) err_sys (“accept error”); int accept(int

sockfd

, struct sockaddr

cliaddr

**socklen_t ***

addrlen

Return next completed connection.

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connect()

)^ sockfd

is socket descriptor from

socket()

)^ servaddr

is a pointer to a structure with:

• Server

port number

and

IP address

• must be specified (unlike

bind()

)^ addrlen

is length of structure

)^ client doesn’t need

bind()

• OS will pick ephemeral port

)^ returns socket descriptor if ok, -1 on error )^ Example:

if ( connect (sockfd, (struct sockaddr *) &servaddr, sizeof (servaddr)) != 0)err_sys(“connect call error”); int connect(int

sockfd

, const struct sockaddr

***** servaddr

, socklen_t

addrlen

Connect to server.

Sending and Receiving

int recv(int

sockfd

, void *

buff

size_t

mbytes

, int

flags

int send(int

sockfd

, void *

buff

size_t

mbytes

, int

flags

)^ Same as

read()

and

write()

but for

flags

)^ flags

examples (see man pages)

-^ MSG_DONTWAIT (this send non-blocking)–^ MSG_OOB (out of band data, 1 byte sent ahead)–^ MSG_WAITALL (don’t give me less than max)–^ MSG_DONTROUTE (bypass routing table)

Concurrent TCP Server

)^ Close

sock

in child,

newsock

in parent

)^ Reference count for socket descriptor

Text segment

sock = socket()/ setup socket /while (1) {

newsock = accept(sock)fork()if child

read(newsock)until exit

Parent int sock;int newsock;

Child int sock;int newsock;

UDP Client-Server

Server socket()bind()recvfrom()

Client socket()sendto()recvfrom()

(Block until receive datagram)^ sendto()

Data (request) Data (reply)

close()

“well-known”port

• No “handshake”- No simultaneous

close()

• Note: usually

fork()

for concurrent servers!

Called

iterative

server