IPC with sockets

1
IPC with sockets
Linux Internals

• sockets in the linux operating system

Filip Polsakiewicz
December, 18th 2001
2
topic overview

• motivation why sockets?
• what exactly are sockets?
• working with sockets
• how does the linux kernel implement sockets?
• summary

3
topic overview

• motivation why sockets?
• what exactly are sockets?
• working with sockets
• how does the linux kernel implement sockets?
• summary

4
topic overview

• motivation why sockets?
• what exactly are sockets?
• working with sockets
• how does the linux kernel implement sockets?
• summary

5
topic overview

• motivation why sockets?
• what exactly are sockets?
• working with sockets
• how does the linux kernel implement sockets?
• summary

6
topic overview

• motivation why sockets?
• what exactly are sockets?
• working with sockets
• how does the linux kernel implement sockets?
• summary

7
motivation why sockets?

• UNIX-Operating systems at first only supported
  using directional pipes for ipc
• only related processes on the same machine were
  able to communicate
• only one-way communication

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motivation why sockets?

• UNIX-Operating systems at first only supported
  using pipes for ipc
• only related processes on the same machine were
  able to communicate
• only one-way communication
• networking was difficult since there were no
  standards for network interfaces
• developing applications for network jobs was
  difficult (each network interface had to be
  implemented seperately)

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motivation why sockets?

• many research facilities developed solutions for
  these both problems
• bidirectional pipes, named pipes
• one of these solutions solved both Problems
  sockets

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what are sockets

• sockets are a connection-based type of ipc
• sockets are an abstraction of the network
  interface and the network protocol
• developers do not have to care about implementing
  hardware-specific details

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what are sockets

• sockets are a connection-based type of ipc
• sockets are an abstraction of the network
  interface and the network protocol
• developers do not have to care about implementing
  hardware-specific details

socket layer
Stream Socket
TCP/IP Protocol
network protocols
network interfaces
10 MBit/s Ethernet
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what are sockets
process 1
process 2
KERNEL
socket
socket
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general properties of sockets

• connection-based
• can have an address bound to them
• use packets for data transmission
• allow checking for errors
• UNIX_STREAM and TCP
• allow IPC in a network enviroment
• allrounders
• many protocols such as IPX, AX_25 and Appletalk
  are supported

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general properties of sockets

• connection-based
• can have an address bound to them
• use packets for data transmission
• allow checking for errors
• UNIX_STREAM and TCP
• allow IPC in a network enviroment
• allrounders
• many protocols such as IPX, AX_25 and Appletalk
  are supported

15
general properties of sockets

• connection-based
• can have an address bound to them
• use packets for data transmission
• allow checking for errors
• UNIX_STREAM and TCP
• allow IPC in a network enviroment
• allrounders
• many protocols such as IPX, AX_25 and Appletalk
  are supported

16
general properties of sockets

• connection-based
• can have an address bound to them
• use packets for data transmission
• allow checking for errors
• UNIX_STREAM and TCP
• allow IPC in a network enviroment
• allrounders
• many protocols such as IPX, AX_25 and Appletalk
  are supported

17
general properties of sockets

• connection-based
• can have an address bound to them
• use packets for data transmission
• allow checking for errors
• UNIX_STREAM and TCP
• allow IPC in a network enviroment
• allrounders
• many protocols such as IPX, AX_25 and Appletalk
  are supported

18
general properties of sockets

• connection-based
• can have an address bound to them
• use packets for data transmission
• allow checking for errors
• UNIX_STREAM and TCP
• allow IPC in a network enviroment
• allrounders
• many protocols such as IPX, AX_25 and Appletalk
  are supported

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different types of sockets
SOCKET
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different types of sockets

• UNIX Domain sockets
• realized in memory
• can be bound to an address which appears in the
  local filesystem (e.g. /tmp/mysocket)
• if client and server are on the same machine,
  twice as fast as INET-sockets (see X-Server)
• Internet (INET) sockets
• implemented through the network
• using a combination of 32-bit numbers and a
  port-number as address (e.g. 192.168.0.18088)
• both types have in common once they are bound to
  an address they can be reached by other sockets

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different types of sockets

• UNIX Domain sockets
• realized in memory
• can be bound to an address which is a file
  appears in the local filesystem (e.g.
  /tmp/mysocket)
• if client and server are on the same machine,
  twice as fast as INET-sockets (see X-Server)
• Internet (INET) sockets
• implemented through the network
• using a combination of 32-bit numbers and a
  port-number as address (e.g. 192.168.0.18088)
• both types have in common once they are bound to
  an address they can be reached by other sockets

address
port
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different types of sockets

• UNIX Domain sockets
• realized in memory
• can be bound to an address which appears in the
  local filesystem (e.g. /tmp/mysocket)
• if client and server are on the same machine,
  twice as fast as INET-sockets (see X-Server)
• Internet (INET) sockets
• implemented through the network
• using a combination of 32-bit numbers and a
  port-number as address (e.g. 192.168.0.18088)
• both types have in common once they are bound to
  an address they can be reached by other sockets

23
different types of sockets

• UNIX Domain sockets
• realized in memory
• can be bound to an address which appears in the
  local filesystem (e.g. /tmp/mysocket)
• if client and server are on the same machine,
  twice as fast as INET-sockets (see X-Server)
• Internet (INET) sockets
• implemented through the network
• using a combination of six 32-bit numbers and a
  port-number as address
• both types have in common once they are bound to
  an address they can be reached by other sockets

We will concentrate on UNIX Domain sockets since
INET Sockets will be handled later
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working with sockets (server)
sun_family sun_path
tells the kernel that the socket is now ready to
accept connections
listen(int fd, int backlog)
on an incoming connection
accept(int fd, struct sockaddr upeer_sockaddr,
int upeer_addrlen)
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working with sockets (client)
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communication
SERVER
CLIENT
socket()
bind()
listen()
socket()
connect()
accept()
write
read
read
write
close()
close()
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some additional functions

• socketpair()
• creates two socket file descriptors and connects
  them to each other
• similar to bidirectional pipes
• shutdown(int fd, int how)
• closes the socket file descriptor

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some additional functions

• socketpair()
• creates two socket file descriptors and connects
  them to each other
• simmilar to bidirectional pipes
• shutdown(int fd, int how)
• closes the socket file descriptor

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netstat -a
Active UNIX domain sockets (servers and
established)Proto RefCnt Flags
Type State I-Node
Pathunix 1 STREAM
CONNECTED 2523 _at_0000000bunix 1
STREAM CONNECTED 2689
_at_0000000e unix 0 ACC STREAM
LISTENING 331 /var/run/mysqld/mysqld.so
ckunix 0 ACC STREAM LISTENING
46678 /dev/logunix 1
STREAM CONNECTED 2698
_at_00000010unix 1 STREAM
CONNECTED 225 _at_00000001unix 1
STREAM CONNECTED 237
_at_00000003unix 0 ACC STREAM
LISTENING 2686 /tmp/.X11-unix/X0unix
0 ACC STREAM LISTENING
2514 /var/lib/zope/var/Z2-pcgi.socunix 0
ACC STREAM LISTENING 395
/tmp/.font-unix/fs7100unix 1
STREAM CONNECTED 2699
/tmp/.X11-unix/X0unix 1
STREAM CONNECTED 2693 /tmp/.X11-unix/X0u
nix 1 STREAM
CONNECTED 2541 /dev/log
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how are sockets implemented in the kernel

• each of the socket functions introduced in
  earlier has a corresponding function in the
  kernel
• socket() ? sys_socket()
• bind ? sys_bind()
• when the process calls a function the kernel
  executes the corresponding function which itself
  calls other functions

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how are sockets implemented in the kernel

• each of the socket functions introduced in
  earlier has a corresponding function in the
  kernel
• socket() ? sys_socket()
• bind ? sys_bind()
• when the process calls a function the kernel
  executes the corresponding function which itself
  calls other functions

sock_write()
unix_sendmsg()
sys_write()
stream_sendmsg()
write()
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data-structures
type
proto_ops

data
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data-structures
type
proto_ops

data
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data-structures
type
proto_ops

data
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data-structures
type
proto_ops

data
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summary

• sockets are an abstraction of the actual hardware
  and protocol layers
• sockets are connection-based
• after a socket was bound to an address it can be
  reached by other sockets
• communication of two sockets can be accomplished
  by using standard file IO functions

37
summary

• sockets are an abstraction of the actual hardware
  and protocol layers
• sockets are connection-based
• after a socket was bound to an address it can be
  reached by other sockets
• communication of two sockets can be accomplished
  by using standard file IO functions

38
summary

• sockets are an abstraction of the actual hardware
  and protocol layers
• sockets are connection-based
• after a socket was bound to an address it can be
  reached by other sockets
• communication of two sockets can be accomplished
  by using standard file IO functions

39
summary

• sockets are an abstraction of the actual hardware
  and protocol layers
• sockets are connection-based
• after a socket was bound to an address it can be
  reached by other sockets
• communication of two sockets can be accomplished
  by using standard file IO functions

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thank you for your attention