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SocketsDistributed Systems

Philipp Kupferschmied

Universitat Karlsruhe, System Architecture Group

May 6th, 2009

Philipp Kupferschmied Sockets 1/ 16

The Socket AbstractionUser’s Point of View

Sockets in Linux

1 The Socket AbstractionSocket BasicsCharacteristics

2 User’s Point of ViewConnectionsSingle-Threaded ServersSummary

3 Sockets in LinuxData StructuresTraversing LayersConclusion



Sockets in Linux

Socket BasicsCharacteristics

What are Sockets?

Socket Socket

Kernel

ProcessA

ProcessB

Connection oriented IPC

“Phone connection” between processes

Protocol determines how the connection is used



Sockets in Linux


Socket Types

Socket

File descriptor (sock fd)

Protocol family

Unix-domain (AF UNIX)Internet-domain (AF INET)

Socket type

Stream (SOCK STREAM)Datagram (SOCK DGRAM)

Protocol

UNIX STREAM, UNIX DGRAMTCP, UDP



Sockets in Linux


Unix-Domain Sockets

Implemented purely in memory

Users can bind a socket to an “address”

Socket becomes accessible at this “address”

Unix-domain addresses are file names

/tmp/server

sockaddr un

sun family = AF UNIXsun path = /tmp/server Socket Socket

Client Server



Sockets in Linux


Unix-Domain Sockets





/tmp/server

sockaddr un


Client Server

/ tmp/server



Sockets in Linux


Unix-Domain Sockets





/tmp/server

sockaddr un


Client Server

/ tmp/server

request



Sockets in Linux


Unix-Domain Sockets





/tmp/server

sockaddr un


Client Server

/ tmp/server

request

reply



Sockets in Linux


Internet-Domain Sockets

Implemented via the network



Internet-domain addresses consist of IP and port number

192.168.0.1:80

sockaddr in

sin family = AF INETsin port = 80sin addr = 0x0100A8C0

Socket Socket

Client Server



Sockets in Linux







192.168.0.1:80

sockaddr in


Socket Socket

Client Server

192.168.0.1:80192.168.0.1:3245



Sockets in Linux







192.168.0.1:80

sockaddr in


Socket Socket

Client Server

192.168.0.1:80

request

192.168.0.1:3245



Sockets in Linux







192.168.0.1:80

sockaddr in


Socket Socket

Client Server

192.168.0.1:80

request

reply

192.168.0.1:3245



Sockets in Linux


Characteristics

Communication between two sockets

Connection-oriented orPacket-based

Allow for error control (TCP, UNIX STREAM)

Network-capable IPC

Flexible

AX 25, APPLETALK, IPX, HTTP, RPC, . . .



Sockets in Linux

ConnectionsSingle-Threaded ServersSummary

Establishing a Connection

socket() socket



Sockets in Linux



bind() socket



Sockets in Linux



l isten() socket



Sockets in Linux



accept() socket



Sockets in Linux



accept()

socket()socket

socket



Sockets in Linux



accept() connect()socketsocket



Sockets in Linux



socket’

socket

socketread()wri te()close()

wri te()read()close()



Sockets in Linux



accept() socket



Sockets in Linux



socket() socket

bind()

l isten()

accept() connect()

socket()socket

socket

socket

socket’

socket

socketread()wri te()close()

wri te()read()close()

socket



Sockets in Linux


Data Transfer

socket’ socketread()wri te()

wri te()read()

socket

socket’

socket’’ socket

socket

socket connect()

wri te()

wri te()read()

read()

accept()

Blocking syscalls ‘accept’ and ‘read’ require a thread per socket.Philipp Kupferschmied Sockets 9/ 16


Sockets in Linux


Data Transfer

socket’ socketread()wri te()

wri te()read()

socket

socket’

socket’’ socket

socket

socket connect()

wri te()

wri te()

fd_set

select()or poll()

Use ‘select’ or ‘poll’ to watch many sockets in one thread.Philipp Kupferschmied Sockets 9/ 16


Sockets in Linux


Strengths and Weaknesses

+ Encapsulation of data transfer

+ Extensibility with new protocols

+ Flexibility (e.g., X server)

Unix-domain sockets for local clientsInternet-domain sockets for remote clientsSame code, except connect()

– Inconvenient programming interface

– Each process has to close its local socket



Sockets in Linux


Example (Unix-Domain)

Client:

int s = socket ( AF_UNIX, SOCK_STREAM, 0 );

struct sockaddr_un server_addr;

server_addr.sun_family = AF_UNIX;

strcpy( server_addr.sun_path, "/tmp/serversock" );

connect( s, (struct sockaddr *) &server_addr, sizeof(server_addr) );

Server:

s = socket( AF_UNIX, SOCK_STREAM, 0 );

struct sockaddr_un server_addr;

server_addr.sun_family = AF_UNIX;

strcpy( server_addr.sun_path, "/tmp/serversock" );

bind( s, (struct sockaddr *)&server_addr, sizeof(server_addr) );

listen( s, 5 );



Sockets in Linux


Example (Internet-Domain)

Client:

int s = socket( AF_INET, SOCK_STREAM, 0 );

struct sockaddr_in server_addr;

server_addr.sin_family = AF_INET;

server_addr.sin_port = htons(15001);

inet_aton( "127.0.0.1", &server_addr.sin_addr );

connect( s, (struct sockaddr *)&server_addr, sizeof(server_addr) );

Server:

int s = socket( AF_INET, SOCK_STREAM, 0 );

struct sockaddr_in server_addr;

server_addr.sin_family = AF_INET;

server_addr.sin_port = htons(15001);

inet_aton( "127.0.0.1", &server_addr.sin_addr );

bind( s, (struct sockaddr *)&server_addr, sizeof(server_addr) );

listen( s, 5 );



Sockets in Linux

Data StructuresTraversing LayersConclusion

struct socket

struct socket

stateflagsproto opsfasync listfileskwaittype

struct proto ops

inet writeinet read. . .

struct sock

sock commonsk protocolsk type. . .headtail

struct sk buff

nextprevskdevdata. . .



Sockets in Linux


Linux Sockets in Action

write(fd)

sys_write(fd)

sock_write()

inet_sendmsg()

tcp_sendmsg()

tcp_send_skb()

tcp_transmit_skb()

ip_queue_xmit()

ei_start_xmit()

read()

sys_read()

sock_read()

inet_recvmsg()

tcp_recvmsg()

tcp_data()

tcp_rcv()

ip_recv()

ei_recv

ei_interrupt

proto_ops

data USER

VFS

TRANSPORT

NETWORK

DATA LINK

PHYSICAL

socket

sock

data

data



Sockets in Linux



write(fd)

sys_write(fd)

sock_write()

inet_sendmsg()

tcp_sendmsg()

tcp_send_skb()

tcp_transmit_skb()

ip_queue_xmit()

ei_start_xmit()

read(fd)

sys_read(fd)

sock_read()

inet_recvmsg()

tcp_recvmsg()

tcp_data()

tcp_rcv()

ip_recv()

ei_recv

ei_interrupt

proto_ops

data USER

VFS

TRANSPORT

NETWORK

DATA LINK

PHYSICAL

socket

sock

data



Sockets in Linux



write(fd)

sys_write(fd)

sock_write()

inet_sendmsg()

tcp_sendmsg()

tcp_send_skb()

tcp_transmit_skb()

ip_queue_xmit()

ei_start_xmit()

read(fd)

sys_read(fd)

sock_read()

inet_recvmsg()

tcp_recvmsg()

tcp_data()

tcp_rcv()

ip_recv()

ei_recv

ei_interrupt

proto_ops

data USER

VFS

TRANSPORT

NETWORK

DATA LINK

PHYSICAL

socket

sock

data

data



Sockets in Linux



write(fd)

sys_write(fd)

sock_write()

inet_sendmsg()

tcp_sendmsg()

tcp_send_skb()

tcp_transmit_skb()

ip_queue_xmit()

ei_start_xmit()

read(fd)

sys_read(fd)

sock_read()

inet_recvmsg()

tcp_recvmsg()

tcp_data()

tcp_rcv()

ip_recv()

ei_recv

ei_interrupt

proto_ops

data USER

VFS

TRANSPORT

NETWORK

DATA LINK

PHYSICAL

socket

sock

data



Sockets in Linux


Conclusions

Sockets are an abstract communication interface

Hide different protocols (Unix-domain vs. Internet-domain)Protocol dictates communication model

Transient vs. persistent

Unix’ish “everything is a file” paradigm

Linux implementations are similar for different protocolsRealized via function pointers and matching data structures

Homebrew inheritance, “object-oriented C”

(Questionable) solution to both performance and structuredsoftware



Sockets in Linux


Literature

Sources of the Linux kernel 2.4/2.6

Helmut Herold: Linux/Unix Systemprogrammierung

Jurgen Wolf: Linux-Unix-Programmierunghttp://www.pronix.de/pronix-6.html

Beck: Linux Kernelprogrammierung

Online Tutorial:http://www.cs.rpi.edu/academics/courses/netprog/

Programming Assignment

You can gain experience in socket programming by implementingRPC via sockets in the programming assignment!


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