Lecture 17: Sockets

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Lecture 17 Sockets

• Prev. summary
• Sockets
• TCP client functions

Application
Transport
Network
Link

• Todays lecture
• TCP server functions

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Client/server socket interaction TCP
Server
Client
create socket, portx, for incoming request
welcomeSocket socket() bind(welcomeSocket,) list
en(welcomeSocket,..)
create socket, connect to hostid, portx
clientSocket socket() connect(clientSocket,
)
send request using socket send(clientSocket)
read request from socket recv(connectionSocket..)
write reply to socket send(connectionSocket)
read reply from socket recv(clientSocket,)
close close(connectionSocket)
close close(clientSocket)
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Bind Function

• Bind function is used to assign a local protocol
  address to a socket
• uses a socket descriptor (sockfd) created by the
  socket function
• uses protocol-specific address and its size
  (myaddr, addrlen)

include ltsys/socket.hgt int bind (int
sockfd, const struct sockaddr myaddr, socklen_t
addrlen) Returns 0 if OK,-1 if error
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Bind Function

• Servers bind their well-known ports when they
  start
• Normally, a TCP client does not bind a port
  number. Why?
• include ltsys/socket.hgt
• int bind (int sockfd, const struct sockaddr
  myaddr, socklen_t addrlen)
• Returns 0 if OK,-1 if error

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Bind Function

• binding IP address to a socket
• TCP server bind restricts the socket to receive
  connections
• destined only to that IP address (i.e., has
  multiple IP addresses).
• TCP client assigns source IP address for
  sending datagrams

Result
Process specifies
IP address
Port
wildcard
0
kernel chooses IP address and port
wildcard
nonzero
kernel chooses IP address, process specifies port
0
local IP addr
process specifies IP address, kernel chooses port
nonzero
local IP addr
process specifies IP address and port
wildcard INADDR_ANY (IPv4)
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Allocation of Port Numbers
5001 65535
1 1023
1024 5000
BSD servers (nonprivilidged)
BSD reserved ports
BSD ephemeral (short lived) ports
49152 65535
1 1023
1024
49151
IANA dynamic or private
IANA registered ports
IANA well-known ports
IANA Internet Assigned Numbers Authority
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bind() Example

• int mysock,err
• struct sockaddr_in myaddr
• memset(myaddr, 0, sizeof(myaddr))
• mysock socket(PF_INET,SOCK_STREAM,IPPROTO_T
  CP)
• myaddr.sin_family AF_INET
• myaddr.sin_port htons( portnum )
• myaddr.sin_addr htonl( ipaddress)
• errbind(mysock, (sockaddr ) myaddr,
  sizeof(myaddr))
• if (errlt0)
• HandleError

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Network Byte Order

• All values stored in a sockaddr_in must be in
  network byte order.
• sin_port a TCP/IP port number.
• sin_addr an IP address.

Common Mistake Ignoring Network Byte Order
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Byte Ordering

• Big-Endian machine the most significant byte has
  the lowest address
• Motorola 68000 and Sparc
• Little-Endian machine the least significant byte
  has the lowest address
• Intel x86 and DEC Alpha architectures
• Network byte order Big-Endian

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Network Byte Order Functions

• h host byte order n network byte
  order
• s short (16bit) l long
  (32bit)
• uint16_t htons(uint16_t)
• uint16_t ntohs(uint_16_t)
• uint32_t htonl(uint32_t)
• uint32_t ntohl(uint32_t)

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Byte ordering

• Whenever sending a multibyte, binary value from
  one machine to another machine
• Passing values to an API function in sockaddr
  structure
• myaddr.sin_port htons( portnum )
• myaddr.sin_addr htonl( ipaddress)
• Receiver needs to convert before using the values

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Client/server socket interaction TCP
Server
Client
create socket, portx, for incoming request
welcomeSocket socket() bind(welcomeSocket,) list
en(welcomeSocket,..)
create socket, connect to hostid, portx
clientSocket socket() connect(clientSocket,
)
send request using socket send(clientSocket)
read request from socket recv(connectionSocket..)
write reply to socket send(connectionSocket)
read reply from socket recv(clientSocket,)
close close(connectionSocket)
close close(clientSocket)
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Listen Function

socket
active socket
passive socket
(default)
a socket used to wait for an incoming
connection
a socket used for initiating a
connection

• Listen function
• converts an unconnected socket into a passive
  socket indicating that the kernel should accept
  incoming connection requests to this socket (a
  created socket is assumed to be active i.e., a
  connect will follow)
• is called only by a TCP server. Why?

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Listen Function
client
complete connection queue
TCP clients connection request
complete
server
TCP server will accept completed connection
incomplete connection queue
include ltsys/socket.hgt int listen (int
sockfd, int backlog) Returns 0 if OK,-1 if
error

• the second argument (backlog) specifies the
  maximum number of connections the kernel should
  queue for this socket

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Accept Function

• Accept function returns the next completed
  connection from the front of the completed
  connection queue.
• If the completed connection queue is empty,
  process is put to sleep
• if accept is succesful, it returns a new socket
  descriptor (newsockfd)

include ltsys/socket.hgt int accept (int sockfd,
const struct sockaddr cliaddr , socklen_t
addrlen) Returns nonnegative descriptor if
OK,-1 if error
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Socket Descriptor Data Structure
Descriptor Table
Family PF_INET Service SOCK_STREAM Local IP
111.22.3.4 Remote IP 123.45.6.78 Local Port
2249 Remote Port 3726
0
1
2
3
4
All fields are filled for socket descriptors
returned by the connect function.
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A simple TCP Server
/ Create socket for incoming connections /
if ((servSock socket(PF_INET, SOCK_STREAM,
IPPROTO_TCP)) lt 0) DieWithError("socket()
failed") / Construct local address
structure / memset(echoServAddr, 0,
sizeof(echoServAddr)) / Zero out structure
/ echoServAddr.sin_family AF_INET
/ Internet address family /
echoServAddr.sin_addr.s_addr htonl(INADDR_ANY)
/ Any incoming interface /
echoServAddr.sin_port htons(echoServPort)
/ Local port / / Bind to the local
address / if (bind(servSock, (struct
sockaddr ) echoServAddr, sizeof(echoServAddr))
lt 0) DieWithError("bind() failed")
/ Mark the socket so it will listen for incoming
connections / if (listen(servSock,
MAXPENDING) lt 0) DieWithError("listen()
failed")
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for () / Run forever / / Set
the size of the in-out parameter /
clntLen sizeof(echoClntAddr) / Wait
for a client to connect / if ((clntSock
accept(servSock, (struct sockaddr )
echoClntAddr,
clntLen)) lt 0) DieWithError("accept()
failed") / clntSock is connected to a
client! / printf("Handling client
s\n", inet_ntoa(echoClntAddr.sin_addr))
HandleTCPClient(clntSock) / NOT
REACHED /
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Summary
communication end-point
communication end-point
socket
socket
client
server

• socket Create a descriptor
• connect Initiate message exchange for
  connection
• bind Set local IP address and port of the
  socket
• listen Place socket in passive mode, set queue
  len
• accept Accept the next incoming connection
  (handshake completed)
• close Terminate communication, deallocate
  descriptor

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Metaphor for Good Relationships

• To succeed in relationships
• you need to establish your own identity.
• you need to be open accepting.
• you need to establish contacts.
• you need to take things as they come, not as you
  expect them.

bind()
accept()
connect()
read might return 1 byte
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Sending Data Through A Socket
sendto
sendmsg

• int write (int sockfd, char buff , int
  nbytes)
• int send (int sockfd, const void buff , int
  nbytes , int flag)
• int sendto (int sockfd, void buff , int
  nbytes , int flag ,
• struct sockaddr to , socklen_t addrlen)
• int sendmsg (int sockfd, struct msghdr msg,
  int flag )
• Returns number of bytes written if OK,-1 if
  error

send
Can specify the destination addr
write
only for connected sockets
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Receiving Data from a Socket
recvfrom
recvmsg

• int read (int sockfd, char buff , int
  nbytes)
• int recv (int sockfd, void buff , int
  nbytes , int flag)
• int recvfrom (int sockfd, void buff , int
  nbytes , int flag ,
• struct sockaddr to , socklen_t addrlen)
• int recvmsg (int sockfd, struct msghdr msg,
  int flag )
• Returns number of bytes read if OK,-1 if
  error

recv
Can specify the destination addr
read
only for connected sockets
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Why Generic Socket Address?
Used to cast pointers to protocol specific
structures
For instance kernel takes callers pointer,
casts it to sockaddr and then looks at the
value of sa_family to determine the type of
the structure. include ltsys/socket.hgt struct
sockaddr uint8_t sa_len sa_family_t sa_famil
y / address family AF_xxx / char sa_data14
/ protocol specific address /
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Various Socket Address Structures
Datalink sockaddr_dl()
IPv6 sockaddr_in6()
IPv4 sockaddr_in()
Unix sockaddr_un()
length
AF_LINK
length
AF_INET
length
AF_INET6
length
AF_LOCAL
interface index
16-bit port
16-bit port
Pathname (up to 104 bytes)
type
name len
32-bit IPv4 address
32-bit flow label
addr len
sel len
(unused)
128-bit IPv6 address
interface name and link-layer address
variable length
fixed length (16-bytes)
fixed length (24-bytes)
variable length
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Socket API

• explicitly created, used, released by
  applications

hostB
hostA
controlled by application developer
controlled by application developer
controlled by operating system
controlled by operating system
internet
server
client
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Necessary Background Information POSIX data
types

• int8_t signed 8bit int
• uint8_t unsigned 8 bit int
• int16_t signed 16 bit int
• uint16_t unsigned 16 bit int
• int32_t signed 32 bit int
• uint32_t unsigned 32 bit int

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More POSIX data types

• sa_family_t address family
• socklen_t length of struct
• in_addr_t IPv4 address
• in_port_t IP port number

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Generic socket addresses

• struct sockaddr
• unsigned short sa_family
• char sa_data14
• sa_family specifies the address type
• AF_INET (Internet protocol address family)
• sa_data specifies the address value.

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struct sockaddr_in (IPv4)

• struct sockaddr_in
• unsigned short sin_family
• unsigned short sin_port
• struct in_addr sin_addr
• char sin_zero8
• struct in_addr
• unsigned long s_addr

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Address

• sockaddr_in is another view of the data in
  sockaddr structure.
• Fill, cast, and pass to the socket functions
• Socket functions look at the sa_family
• PF_INET and AF_INET
• allow maximum flexibility, e.g., the same
  protocol family can use different address
  families.

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sockaddr_in
sockaddr
sa_family
sa_data