Network Sockets

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NETWORK
SOCKETS

INTRODUCTION TO NETWORK SOCKET

PROGRAMMING AND CONCEPTS

Peter R. Egli 2015

Peter R. Egli
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Contents
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.

What is a socket?
Socket = Interface to transport API
Routing in Network Layers
TCP socket spawning
Socket interface functions
Socket calls versus TCP segments
Socket calls versus UDP datagrams
Socket handle
Parameter marshalling / RPC transparency
Low level socket programming
UDP multicast sockets
TCP server socket: C/C++ versus Java example
Client socket: C/C++ versus Java example
IPv6 sockets

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Network Sockets
1. What is a socket?

A socket is an interface for an application to connect to a hosts network stack (part of the OS).
After connecting, an application is able to bidirectionally exchange data with other processes
on the same or another host.

Application

Application

Application

Socket interface

Network
stack (OS)

Network
stack (OS)
IP network

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Network Sockets

2. Socket = Interface to transport API (hosts transport protocols)

A socket has a binding to an NSAP (with an IP address) and a TSAP (with a
TCP/UDP/SCTP port number). The NSAP may have a specific IP address or may represent all IP
addresses of the host (unspecified IP address = wildcard address = 0.0.0.0 = inaddr_any).
App
Socket Interface

Socket

Socket
TSAP (Port #)
OSI Layer 4 (transport)

TCP
NSAP (IP Address)

OSI Layer 3 (network)

IP

OSI Layer 2 (data link)

OSI Layer 1 (physical)

Network Ports
(e.g. Ethernet)

TSAP: Transport Service Access Point

NSAP: Network Service Access Point
Peter R. Egli 2015

Binding to port and

specific IP address

Binding to port and

to inaddr_any
IP layer is router
(between interfaces
and transport layer)
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Network Sockets
3. Routing in Network Layers (1/4)
The routing of packets from and to a socket depends on the bind IP address.
App

Socket

1.
7

1.

2.

80

4567

TCP
IP1

IP2

IP3

IP4

IP

Source port
and IP addr.

Dest. port
and IP addr.

1.

9999

IPn

IP1

1.

9999

IPn

80

IP2

2.

9999

IPn

4567

IP2

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DL

DL

PL

PL

2.

9999

Network Ports
(e.g. Ethernet)

IPn

4567

IP3
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3. Routing in Network Layers (2/4)

1. Specific IP address binding:
UDP socket:
If a UDP socket is bound to a specific IP address, only IP packets with this destination IP
address are routed to and received by this socket.
TCP socket:
In case of a listening TCP socket, only connection requests (inbound connection) addressed to
the bind IP are accepted by the socket.
2. inaddr_any binding:
If a socket is NOT bound to a specific IP address (INADDR_ANY = 0.0.0.0, wildcard IP address),
the socket is bound to all existing interfaces.
UDP socket:
A UDP socket receives any packet that contains the bind port number as target port.
TCP socket:
A listening TCP-socket bound to 0.0.0.0 is able to accept connections on all interfaces provided
that the destination port of the incoming connection request equals the bind port number.
Once the incoming connection is accepted, the created TCP-socket is bound to the destination
IP address of the incoming connection request.

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Network Sockets
3. Routing in Network Layers (3/4)
Localhost binding and routing of outbound packets:
App

App

Socket

3.
7

4567

23

TCP
IP1

IP2

IP3

IP4

127.0.0.1

IP

Dest. port
and IP addr.
Outbound packet

9999

Outbound packet

IPn
9999

Source port
and IP addr.
7

IP1

IPn

4567

DL

DL

PL

PL

IP2

Outbound packet
Peter R. Egli 2015

Network Ports
(e.g. Ethernet)

9999

IPn

4567

IP3
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3. Routing in Network Layers (4/4)

3. localhost binding:
If a socket is bound to localhost=127.0.0.1, then this socket receives only from applications
but not from the network.
Besides the local loopback interface (127.0.0.1 for IPv4, ::1 for IPv6), applications on the same
machine can also use an interface IP address for communication.
4. Outbound IP address:
The source address of outbound packets is either the bound IP address or the address of
the interface over which the packet is sent (if the socket is bound to INADDR_ANY).
N.B.: An outbound packet may also be sent over an interface other than the socket is bound to,
i.e. the routing is based on the IP layers routing table.

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Network Sockets
4. TCP socket spawning

In TCP there exist 2 different socket types: server socket and client socket.
The server socket is used to accept incoming connections. When TCP receives
an incoming connection request on a server socket (SYN) it spawns a new (client) socket
on which the server process can send and receive data (after passing the new socket to a
newly forked server process).
Client
AP

Server
AP

(2) connect()

Socket = API

(1) accept()

(5) Forking of
new process

Server
AP

(4) Spawning of
new TCP client socket

Socket = API

(3) TCP 3-way handshake

TCP

TCP

Full-duplex TCP connection between 2 TCP client sockets

TCP client socket (for sending and receiving data).
TCP server socket (for accepting new connections).
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Network Sockets
5. Socket interface functions (1/2)

TCP Socket Interface Functions:

Depending on the platform (Java, C, Python ...) client and server sockets may be
implemented differently. In C (BSD sockets, Winsock) there is only 1 socket type
while in Java client and server sockets are represented by different classes.
Client:
socket()

Server:
Create client socket

connect()

Create a connection

send()

Send data

receive()

Blocking receive data

close()

Close client socket

Peter R. Egli 2015

serversocket()

Create server socket

bind()

Bind server socket to socket

address (IP+port)

listen()

Create queues for requests

accept()

Block on incoming requests

close()

Close server socket

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5. Socket interface functions (2/2)

UDP Socket Interface Functions:
Client and server have the same socket functions.
There are no functions for connection setup / shutdown since UDP is connectionless.
With one UDP socket it is possible to send to different destination hosts (sendTo() function).
Client & Server:
socket()

create client / server socket

bind()

bind client / server to socket address (IP+port)

send()

send data (client and server)

receive()

receive data (client and server)

close()

close client / server socket

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Network Sockets

socket()

6. Socket calls versus TCP segments (1/3)

Connection establishment:
Client AP

Client Socket

TCP

TCP

Server Socket

Function call
and function
return

Server AP

socket()
bind()
listen()
accept()
socket()
connect()

Client
blocked

Server is blocked
on incoming
requests (listening).

SYN
SYN ACK
unblock
return new
socket handle

ACK

Rx Buffer
receive()

Server directly services

new socket (single thread)
or starts a new thread
(multithreaded).
Server is blocked
on reading.

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Network Sockets

socket()

6. Socket calls versus TCP segments (2/3)

Socket send / receive (symmetric for client and server):
Client AP

Client Socket

TCP

TCP

Server Socket

Function call
and function
return

Server AP

Rx Buffer
receive()

Tx Buffer
send()

Rx Buffer
Data
Segment

Server is blocked
on reading.

ACK

Rx Buffer
unblock
receive
Server handles
the request.

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Network Sockets

socket()

6. Socket calls versus TCP segments (3/3)

Socket close:
Client AP

Client Socket

TCP

TCP

Server Socket

Function call
and function
return

Server AP

receive()

Server is blocked
on reading.
close()

FIN

EOF
Server closes its socket.

ACK
FIN

close()

ACK

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Network Sockets

socket()

7. Socket calls versus UDP datagrams

Client AP

Client Socket

UDP

UDP

Server Socket

Function call
and function
return

Server AP

socket()
socket()

bind()

bind()
receive()
Datagram

send()
receive()

Rx Buffer
Server is blocked
on reading.

Rx Buffer

Rx Buffer
Datagram

send()

Server handles
the request.

Rx Buffer
unblock
read
close()
close()

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Network Sockets
8. Socket handle
In Unix a socket is like a file descriptor.
Same handling as file (open, close, EOF).
Input stream / output stream to read / write to / from socket (like file).

File:
fhdl = fopen(filename,rw);
while not (EOF) {
s = gets(fhdl);
}
puts(fhdl,hello);
fclose(fhdl);

Peter R. Egli 2015

Socket:

Socket
TCP

Socket sock = new Socket(destHostIP,destHostPort);

while not (rx = EOF) {
rx = sock.read();
}
sock.write(Im done);
sock.close

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Network Sockets
9. Parameter marshalling / RPC transparency (1/4)

Problem:
Different implementations (C/Java, processor architecture, compiler) have different
representations of data. A local data structure sent by application on host 1 may look
differently to application on host 2.

Local data
structure

App

App

Socket

Socket

TCP

TCP

IP

IP

Message carrying data structure

Peter R. Egli 2015

Data structure may

look differently
when received by
peer application.

RPC:
Remote Procedure Call
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Network Sockets
9. Parameter marshalling / RPC transparency (2/4)

LSByte Least Significant Byte

MSByte Most Significant Byte

E.g. Endianness:
Endianness is the ordering of bytes of a multibyte data type (integer, long integer etc.).
Network order is the way bytes (and bits) go out to the network. Network order is big endian
(MSByte first).
//the following integer is represented differently on different
//processor architectures / operating systems
int i = 14;
Memory address

n+1

0 (MSByte)
n

0
n+1

14 (LSByte)
n

n+2

n+1

0 (MSByte)
0

int value 14 in Java

14 (LSByte) on 32Bit/64Bit big/little endian
machine

n+3

0
n+6

0
n+1

14

0
n+7

0
n+2

int value 14 in C/C++

0 (MSByte) on 32bit little endian machine

n+3

n+2

n+5

int value 14 in C/C++

14 (LSByte) on 32Bit big endian machine

n+3

n+2

n+1

n+4

n+3

0 (MSByte)

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n+2

14 (LSByte)
n+3

int value 14 in C/C++

on 64bit big endian machine
int value 14 in C/C++
on 8Bit big/little endian machine
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Network Sockets
9. Parameter marshalling / RPC transparency (3/4)

E.g. complex data structures with references:

Complex data structures contain references to to other data structures or objects.
Such references make only sense on the local machine but not on another host.
Local object with
reference to other
local object.

Reference broken (points

to non-existent object).

App

App

Socket

Socket

TCP

TCP

IP

IP

Message carrying object with

detached reference
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Network Sockets
9. Parameter marshalling / RPC transparency (4/4)

Solution:
When sending parameters over the network it is mandatory to bring them into a
standard canonical format. This is called parameter marshalling (or serialization).

Stubs on the client and server marshal parameters into a standard format and vice versa.

App
client stub

Socket

App
Client / server stubs
are linked between app
and socket and perform
parameter / message
marshalling.

TCP
IP

server stub

Socket
TCP

Marshalled messages
between client and server

IP

E.g. IDL/CORBA, Interface Description Language, generates client & server stubs from abstract interface
description. The stubs are then compiled by compiler together with application code.
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Network Sockets
10. Low level socket programming (1/2)

Socket Options (SO):

Socket options allow modifying the behavior of sockets.
Generally such options should be used with caution as this makes applications dependent on
the underlying socket layer (violation of layering principle).
Java (1.6) socket option support:
socket.setSoLinger(boolean on, int linger)

socket.setSoTimeout(int timeout)
socket.setTcpNoDelay(boolean on)
socket.setKeepAlive(boolean on)
socket.setReceivedBufferSize(int size)
socket.setSendBufferSize(int size)
socket.setReuseAddress(boolean on)

SO_LINGER: Define time that socket remains active to

send unsent data after close() has been called
(send data in transmit buffer).
SO_TIMEOUT: Specify a timeout on blocking socket
operations (dont block forever).
SO_NODELAY: Enable/disable Nagles algorithm.
SO_KEEPALIVE: Enable/disable TCP keepalive timer
mechanism.
SO_RCVBUF: Set the size of the receive buffer.
SO_SNDBUF: Set the size of the send buffer.
SO_REUSEADDR: Enable reuse of port number
and IP address so that after a restart an application
can continue using open connections.

C/C++ socket option support:

In C/C++ many more socket options can be set through setsockopt() and getsockopt()
socket API calls.
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Network Sockets
10. Low level socket programming (2/2)

Socket raw interfaces:

A raw socket is directly attached to the network layer without a transport layer
(no TCP, UDP or SCTP layer).
This allows direct access to ICMP (e.g. for traceroute), or IP (e.g. for IPSec).
The raw interface is not available in Java due to security concerns (access to raw interface
requires root access rights since the network stack runs in the kernel space).
App
Socket
Socket
Raw sockets

TSAP (Port #)
TCP / UDP / SCTP
ICMP

NSAP (IP Address)

IP

Data Link

Physical Link

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Network Sockets
11. UDP multicast sockets (1/2)
How does multicasting work?

2.

MCGA 224.0.0.1
if0
host1
if0
host2
if1
host3

if1

Host1

4.

if0

MOSPF or
PIM 5.

1.

Host2

3.

Stream server
(e.g. audio/video)

Host3
4.

1.
2.
3.
4.

5.

3.
Hosts join multicast groups by sending IGMP (Internet Group Management Protocol) membership
reports (on multicast address of interest, e.g. 224.0.1.1).
Multicast routers keep a table to know on which interface multicast packets are to be sent.
Multicast routers send periodic IGMP queries to the multicast hosts to check if they are still member
of the multicast group (again sent on multicast address of interest, e.g. 224.0.1.1).
Upon reception of a multicast packet the multicast router performs a lookup (multicast group table with
multicast group addresses MCGA) and sends the packet to all interfaces that have multicast hosts
attached. The packet is sent using the corresponding multicast link address and is thus received by all
multicast hosts.
The best (and only) route through the network (no loops etc.) is established with
multicast routing protocols such as MOSPF (Multicast OSPF), PIM (Protocol Independent Multicast) etc.

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11. UDP multicast sockets (2/2)

Multicast is only supported on UDP (TCP is connection-oriented and thus not suitable
for multicast).
Multicast addresses:
Multicast addresses are class D IP addresses in the range 224.0.0.0 to 239.255.255.255.
For example:
224.0.0.9
RIP Version 2
224.0.1.1
Network Time Protocol (NTP)
224.0.0.5
All MOSPF routers
Java multicast socket class:
Class MulticastSocket
MulticastSocket(int port)
Creates a multicast socket on specified port.
joinGroup(InetAddress mcastaddr) Join a multicast group.
leaveGroup(InetAddress mcastaddr) Leaves a multicast group (no IGMP report sent,
only for hosts internal bookkeeping).
send() and receive()
Inherited methods from DatagramSocket class.

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Network Sockets
12. TCP server socket: C/C++ versus Java example
#include <sys/socket.h>

import java.net.*;
import java.io.*;

int main()
{
struct sockaddr_in serv, cli;
char
request[REQUEST], reply[REPLY];
int
listenfd, sockfd, n, clilen;
if ((listenfd = socket(PF_INET, SOCK_STREAM, 0)) < 0)
err_sys("socket error");
memset($serv, sizeof(serv), 0);
serv.sin_family = AF_INET;
serv.sin_addr.s_addr = htonl(INADDR_ANY);
serv.sin_port = htons(TCP_SERV_PORT);
if (bind(listenfd, (SA) &serv, sizeof(serv)) < 0)
err_sys("bind error");
if (listen(listenfd, SOMAXCONN) < 0)
err_sys("listen error");

public static void main(String[] args)

{
ServerSocket serv;
Socket
cli;
PrintStream out;
InputStream in;
try {
serv = new ServerSocket(33333);
} catch(IOException e) { ... }

while(true) {
try {
cli = serv.accept();
} catch(IOException e) { ... }
try {
out = cli.getOutputStream();
in = cli.getInputStream();
String request = in.readln();

for (;;) {
clilen = sizeof(cli);
if ((sockfd = accept(listenfd, (SA) &cli, &clilen)) < 0)
err_sys("accept error");
if ((n = read_stream(sockfd, request, REQUEST)) < 0)
err_sys("read error");
// n Bytes in request[] verarbeiten, reply[] erzeugen
if (write(sockfd, reply, REPLY) != REPLY)
err_sys("write error");
close(sockfd);
}

// reply erzeugen...
out.println(reply);
cli.close();
} catch (IOException e) { ... }

}
try {
serv.close();
} catch (IOException e) { ... }
}
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Network Sockets
13. TCP client socket: C/C++ versus Java example

#include <sys/socket.h>

int main(int argc, char *argv[])

{
struct sockaddr_in serv;
char
request[REQUEST], reply[REPLY];
int
sockfd, n;

import java.net.*;
import java.io.*;
public static void main(String[] args)
{
Socket
clnt;
PrintStream out;
InputStream in;

// Prfen der Parameter...

memset(&serv, sizeof(serv), 0);
serv.sin_family = AF_INET;
serv.sin_addr.s_addr = inet_addr(argv[1]);
serv.sin_port = htons(TCP_SERV_PORT);
if (connect(sockfd, (SA) &serv, sizeof(serv)) < 0
err_sys("connect error");

try {
clnt = new Socket("localhost", 33333);
} catch(IOException e) { ... }

// request[] initialisieren...
if (write(sockfd, request, REQUEST) != REQUEST)
err_sys("write error");
if (shutdown(sockfd, 1) < 0)
err_sys("shutdown error");
if ((n = read_stream(sockfd, reply, REPLY)) < 0)
err_sys("read error");

try {
out = clnt.getOutputStream();
in = clnt.getInputStream();
out.print("Hallo Server!");
String reply = in.readln();
clnt.close();
} catch (IOException e) { ... }

// n Bytes von reply[] verarbeiten...

exit(0);

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Network Sockets
14. IPv6 sockets (1/2):
Most host platforms (Linux, Windows, Sun) already support IPv6.

IPv6 sockets with Java:

Java supports IPv6 since version 1.4.
No difference to IPv4 sockets.
IPv6 automatically enabled when detected.
No source code change, no bytecode change required for IPv6 as long as the
application does not use numeric IP addresses.
Java IPv6 API:
java.net.Inet4Address
java.net.Inet6Address
java.net.preferIPv4Stack
java.net.preferIPv6Addresses

IPv4 address class

IPv6 address class
Property to set preference for IPv4.
Property to set preference for IPv6.

N.B.: The properties are only accepted as VM arguments on startup of a program.

They can not be changed at runtime.
Example: -Djava.net.preferIPv4Stack=false -Djava.net.preferIPv6Stack=true

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Network Sockets
14. IPv6 sockets (2/2):
Scenarios:
Dual stack:
Listening
socket on
:: port
12345

Separate stacks:
Listening
socket on
IPv4 Socket
0.0.0.0
port 12345

Socket

TCP

IPv4

IPv6

172.20.92.89

fe80::511a:886c:a8cc:dc66

Data Link
The listening socket accepts connections
to 172.20.92.89 and
fe80::511a:886c:a8cc:dc66 on port 12345.
Windows is dual stack since Windows Vista.

Peter R. Egli 2015

Listening
socket on
IPv6 Socket :: port
12345

TCP

TCP

IPv4

IPv6

172.20.92.89

fe80::511a:886c:a8cc:dc66

Data Link
IP4 socket accepts connections only on
172.20.92.89.
IPv6 socket accepts connections only on
fe80::511a:886c:a8cc:dc66.

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