TCP Sockets

Ø Socket Address Structure

Ø Utility Functions:

ü Byte Order

ü Byte Manipulation

ü Conversion

ü Read/Write

Ø Socket Functions:

ü Connect

ü Bind

ü Listen

ü Accept

ü Close

Ø Value_Result Arguments

Ø Socket Address Structures:

struct  in_addr {
in_addr_t  s_addr ;   // 32-bit, IPv4 net byte order
}

struct sockaddr_in {
sin_len
sin_family
sin_ port
struct   in_addr sin_addr
sin _zero[8]
}

Example:

In TCP daytime Server:
 

struct  sockaddr_in   servaddr;

….
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

If you like to find the value of any constant (e.g., INADDR_ANY) compile the program as:

% gcc –E prog.c > t

and look at t.

Generic socket address structure

Used for args to socket functions
(developed for many protocols, IPv4, IPv6, UNIX, etc..)

struct sockaddr {

sa_len
sa_family
char       sa_data [14]
}

Example:

Struct sockaddr_in      Serv;  // IPv4
Struct sockaddr_in6    Serv;  // IPv6

bind (sockfd,     (struct sockaddr *) &Serv,   sizeof(Serv))

OR
#define  SA      struct  sockaddr

bind (sockdf,    (SA *) &Serv,   sizeof(Serv))

Ø Utility Functions

ü Byte Order Functions

little-endian  &  big-endian

htons    host to network  short
htonl            ...                long
ntohs
ntohl

See: ~/stevens2nd.book/unpv12e/intro/byteorder.c

ü Byte Manipulation Functions

bzero        memset
bcopy       memcpy
bcmp        memcmp

% man -s 3c bstring

% man -s 3c memory

ü Conversion Functions

Convert from dotted-decimal (e.g., 128.82.4.7) to 32-bit network byte order binary value.

inet_pton  presentation to numeric
inet_ntop numeric to presentation

For details see:

% man inet

ü Read/Write Utility Functions

lib/readn.c : Read "n" bytes from a descriptor.
lib/writen.c : Write "n" bytes to a descriptor.
lib/readline.c: Read line from a descriptor.
lib/isfdtype.c: test type of a descriptor

  (e.g.,  if (isfdtype(fd, S_IFSOCK) = = 1) it is socket).

ü Socket Functions

Figure: Socket Functions for elementary TCP client-server

ü Connect:

Send SYN....&

after 6 seconds..&

after 24 seconds

     if  after 75 seconds no SYN,ACK received
          ETIMEOUT is returned.

     if  server responds with RST (no process waiting at port)
          ECONNREFUSED is  returned.

if  a router returns ICMP destination unreachable
          send after 6 and 24 seconds and if no connection after 75 seconds
EHOSTUNREACHE is returned.

NOTE: This is implementation-dependent.

You may use tcpdump to check your particular implementation.

(e.g., use tcpServer0 & tcpClient0 with tcpdump)

somethingelse % tcpServer0 1233

something % tcpClient0 somethingelse 1233

You can't reconnect the socket to another address unless

you close and call socket again.

ü Bind:

Server: Normally bind to a well know port & INADDR_ANY.

- Using port# 0:

kernel chooses a free port and

we use getsockname to find the selected port.

- When a connection is accepted,

the address of the connection is fixed and

we use getsockname to find the interface IP address.

- You can bind to specific IP address instead of INADDR_ANY, 

only connection to this address is accepted.

Client: Normaly do not bind to any specific port or address.

- As part of connect bind is implicitly called.

- Any ephemeral port and interface IP address is filled based on
   the routing table.

- Use getsockname to find out the port and address.

ü Listen:

Figure: Listen Backlog Queues

Figure: Queues Statistics

Statistics:

The queue lengths for an HTTP server with backlog value of 64 never reached.

Example:

out of 90,924 time tested the values of the queues:

90,358 time we found the completed queue empty

(i.e., 99.4% of time it is empty).
3,033 time we found the incomplete queue empty

(i.e,   3.4% of time it is empty).

If for any reason the queue is full,

simply do nothing and the client tries 3 times and then give up.

ü Accept:

S2 = accept (S1, ....)

S1 is the listining socket
S2 is the connected socket.

getsockname return the same port number for S1 and S2. (demultiplexing is done based on the source address and port number of the client since the connection is identified by 4 topple ).

Concurrent Server:

Figure: Exec functions

Figure: Concurrent Server Outline

Figure: Accept

Figure: Fork

ü Close

TCP will send queued data

(unless SO_LINGER option is used to discard unsent data)

and then send FIN segment.

Ø Value_Result Arguments

Value-only: bind, connect, sendto

Vlaue_Result: accept, recvfrom, getsockname, getpeername

Example: value_result

struct sockaddr_in     clientaddr ;
socklen_t                    len;
int                               listenfd, connectfd;

len = sizeof (clientaddr);
connectfd = accept (listenfd,   (SA *) &clientaddr,   &len);