TCP/IP Sockets

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TCP/IP Sockets

• ECS 152B

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What is a socket?

• API network programming interface
• Socket abstraction through which an application
  may send/receive data.
• Allows an application to plug-in a network and
  communicates with other apps. in the same
  network.
• Sockets different underlying protocol families.

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TCP/IP

• TCP/IP does not include an API definition.
• There are a variety of APIs for use with TCP/IP
• Sockets (Berkeley sockets)
• TLI (Transport layer interface by ATT)
• Winsock
• MacTCP

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Functions needed

• Specify local and remote communication endpoints
• Initiate a connection
• Wait for incoming connection
• Send and receive data
• Terminate a connection gracefully
• Error handling

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Sockets, protocols, ports
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Creating a Socket

• int socket(int family,int type,int proto)
• family specifies the protocol family (PF_INET for
  TCP/IP).
• type specifies the type of service (SOCK_STREAM,
  SOCK_DGRAM).
• SOCK_STREAM reliable byte-stream
• SOCK_DGRAM best-effort datragram
• protocol specifies the specific protocol
• IPPROTO_TCP, IPPROTO_UDP
• 0 means the default

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socket()

• The socket() system call returns a socket
  descriptor (nonnegative) or -1 on error.
• socket() allocates resources needed for a
  communication endpoint - but it does not deal
  with endpoint addressing.

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Unix Descriptor Table
Descriptor Table
Data structure for file 0
0
1
Data structure for file 1
2
3
Data structure for file 2
4
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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
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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
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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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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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Assigning an address to a socket

• The bind() system call is used to assign an
  address to an existing socket.
• int bind( int sockfd,
• const struct sockaddr myaddr, int
  addrlen)
• bind returns 0 if successful or -1 on error.

const!
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bind()

• calling bind() assigns the address specified by
  the sockaddr structure to the socket descriptor.
• You can give bind() a sockaddr_in structure
• bind( mysock,
• (struct sockaddr) myaddr,
• sizeof(myaddr) )

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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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Uses for bind()

• There are a number of uses for bind()
• Server would like to bind to a well known address
  (port number).
• Client can bind to a specific port.
• Client can ask the O.S. to assign any available
  port number.

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Framing

• TCP stream service, does not conserve boundary
• Framing formatting the information s.t. the
  receiver can parse message
• The beginning and end of the message, boundaries
  between fields within the message
• Fixed message size
• Using delimiter

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TCP Clients

• Create a socket, socket()
• Establish connection to the server, connect()
• Communicate, send() and recv()
• Close connection, close()

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connect()

• int connect( int sockfd,
• const struct sockaddr foreignserver,
• socklen_t addrlen)
• sockfd is an already created TCP socket.
• foreignserver contains the address of the server
  (IP Address and TCP port number)
• connect() returns 0 if OK, -1 on error

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Client Code

• TCP clients can call connect() which
• takes care of establishing an endpoint address
  for the client socket.
• dont need to call bind first, the O.S. will
  take care of assigning the local endpoint address
  (TCP port number, IP address).
• Attempts to establish a connection to the
  specified server.
• 3-way handshake

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Send/receive in a TCP socket

• int send(int sd, const void buf, unsigned int
  messagelen, int flags)
• int recv(int sd, void buf, unsigned int
  bufferlen, int flags)
• sd socket descriptor
• flags change default behavior, 0 default
• Return -1 for error

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TCP Server

• Creating a passive mode (server) socket.
• Establishing an application-level connection.
• send/receive data.
• Terminating a connection.

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listen()

• int listen( int sockfd, int backlog)
• sockfd is the TCP socket (already bound to an
  address)
• backlog is the number of incoming connections the
  kernel should be able to keep track of (queue for
  us).
• listen() returns -1 on error (otherwise 0).

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accept()

• int accept( int sockfd,
• struct sockaddr cliaddr,
• socklen_t addrlen)
• sockfd is the passive mode TCP socket.
• cliaddr is a pointer to allocated space.
• addrlen is a value-result argument
• must be set to the size of cliaddr
• on return, will be set to be the number of used
  bytes in cliaddr.

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accept() return value

• accept() returns a new socket descriptor (small
  positive integer) or -1 on error.
• After accept returns a new socket descriptor, I/O
  can be done using the send() recv() system
  calls.

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Terminating a TCP connection

• Either end of the connection can call the close()
  system call.
• If the other end has closed the connection, and
  there is no buffered data, reading from a TCP
  socket returns 0 to indicate EOF.

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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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A simple client example
/ Create a reliable, stream socket using TCP
/ if ((sock socket(PF_INET, SOCK_STREAM,
IPPROTO_TCP)) lt 0) DieWithError("socket()
failed") / Construct the server address
structure / memset(echoServAddr, 0,
sizeof(echoServAddr))
echoServAddr.sin_family AF_INET
echoServAddr.sin_addr.s_addr inet_addr(servIP)
echoServAddr.sin_port
htons(echoServPort)
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/ Establish the connection to the echo server
/ if (connect(sock, (struct sockaddr )
echoServAddr, sizeof(echoServAddr)) lt 0)
DieWithError("connect() failed")
echoStringLen strlen(echoString) /
Determine input length / / Send the string
to the server / if (send(sock, echoString,
echoStringLen, 0) ! echoStringLen)
DieWithError("send() sent a different number of
bytes than expected")
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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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Under the hook

• Data structure
• Buffering and TCP
• Deadlock
• Performance
• TCP socket life cycle

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Data structure of a TCP socket
Socket descriptor
recved queue
sent queue
protocol state (closed)
Local IP 111.22.3.4 Remote IP
123.45.6.78 Local Port 2249 Remote Port 3726
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UDP

• Local IP 111.22.3.4
• Remote IP 123.45.6.78
• Local Port 2249
• Remote Port 3726

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Buffering and TCP

• Cannot assume correspondence between send()s and
  recv()s
• Ex
• connect(s,)
• send(s, buffer0,1000,0)
• send(s, buffer1,2000,0)
• send(s, buffer2,5000,0)
• close(s)

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Queues

• SendQ
• RecvQ
• Delivered

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Deadlock

• NgtSendQRecvQ
• Notes design the protocol carefully to avoid
  simultaneous send() in both directions.

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Performance

• Effective size of send() is limited by SendQ
  size.
• Can change using SO_RCVBUF SO_SNDBUF socket
  options.

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TCP Socket life cycle

• Client
• Connecting
• Server
• Binding
• Listen
• Accept