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The Socket API
- Prof. Chuan-Ming Liu
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The Socket API Prof. Chuan-Ming Liu Computer Science and - - PowerPoint PPT Presentation
The Socket API Prof. Chuan-Ming Liu Computer Science and - - PowerPoint PPT Presentation
Mobile Computing & Software Engineering Lab The Socket API Prof. Chuan-Ming Liu Computer Science and Information Engineering National Taipei University of Technology Taipei, TAIWAN NTUT, TAIWAN 1 Mobile Computing & Software
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Specifying a Protocol Interface
Two approaches when designing functions to OS that provide applications to access TCP/IP protocols
Specific functions for TCP/IP communication Flexible functions in general with parameters to specify
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Specifying a Protocol Interface
Example:
First approach may use maketcpconnection The other may use makeconnection with a parameter to specify the TCP protocol
Second approach is preferable
To accommodate multiple sets of communication protocols TCP/IP protocols are one single family (family PF_INET)
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Specifying a Protocol Interface
Applications specify operations using a type of service instead of the protocol name
Example: using stream transfer type of service to specify a TCP connection
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Specifying a Protocol Interface
The socket API provides generalized functions that support network communication using many possible
- protocols. Socket calls refer to all TCP/IP
protocols as a single protocol family. The calls allow the programmer to specify the type of service required rather than the name of a specific protocol.
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Socket Abstraction
File descriptors
When performing I/O on a file, open( ) function is called
- pen( ) function will create a file descriptor
deal with the files Are stored in a file descriptor table Each process has one separate descriptor table
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File Descriptor Table
1 2 3 descriptor table (one per process) internal data structure for file 0 internal data structure for file 1 internal data structure for file 2 internal data structure for file 3
Operation System
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Socket Abstraction
Similarly, socket API adds new abstraction for network communication, the socket Each socket is identified by an small integer, socket descriptor
same with the file descriptor An application cannot have both a file descriptor and a socket descriptor with the same value Note: Windows OS using a separate table for socket descriptor
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Socket Abstraction
Socket API contains a function, socket to create a socket. On UNIX, file and socket descriptors use the same descriptor table In Windows OS, a file descriptor and a socket descriptor may have the same value.
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Socket Data Structure
When socket() is called, the OS
allocates a new data structure for socket fills in a new descriptor table entry leaves most of the fields in the structure unfilled
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Socket Descriptor Table
OS
Remote port: Local port: Remote IP: Local IP: Service: SOCK_STREAM Family: PF_INET data structure for a socket pointers to
- ther
socket structures 1 2 3 socket descriptor table (one per process) 4
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Socket Abstraction
When a socket has been created,
The socket is completely general and can be used for arbitrary communication Application specifies how it will be used
Tell the role of a socket in
Passive Active
The only difference between an active socket and a passive one lies in how the applications use it; initially all sockets are created in the same way
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Specifying an Endpoint Address
When a socket is created
contains no information about how to use it no information about the port number and IP address, …, etc
Before using a socket, it must specify some addresses
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Specifying an Endpoint Address
Communication endpoint
In TCP/IP, consists of an IP address and a protocol port number Each protocol family has its own specification for endpoint address
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Specifying an Endpoint Address
The socket abstraction defines an address family for each type of address
Each protocol family can use one or more address families to define address representations TCP/IP protocols all use a single address representation with the symbolic constant AF_INET
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Specifying an Endpoint Address
Note:
Confusion arises between the TCP/TP protocol family, PF_INET and its address family, AF_INET Both have the same numeric value (2)
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Address Structure
Socket system defines a generalized format that all endpoint addresses used for flexibility
(address family, endpoint address in that family) Address family: one of preassigned address types The standard representation for the specified address type
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Sockadd Structure
The most general structure, sockaddr structure
struct sockaddr { u_char sa_len; /* total length */ u_short sa_family; /* type of address*/ char sa_data[14]; /* value of address*/ };
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Sockadd Structure
Does not fit for all address families
Some address families have longer endpoint address Accommodates address in the AF_INET family, so TCP/IP software works well To keep the portability and maintainable, do not use this address structure
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Sockadd_in Structure
TCP/IP protocols exclusively use structure sockaddr_in instead
struct sockaddr_in { u_char sin_len; /* total length */ u_short sin_family; /* type of address*/ u_short sin_port; /* port number*/ struct in_addr sin_addr; /* IP address*/ char sin_zero[8]; /* unused*/ };
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Sockadd_in Structure
When representign a TCP/IP communication endpoint, an application program uses structure sockaddr_in, which contains both an IP address and a protocol port number. Programmers must be careful when writing programs that use a mixture of protocols because some non-TCP/IP endpoint addresses require a larger structure
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Major System Calls in the Socket API
Two groups for socket calls:
Primary calls Utility routines
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Primary Socket Calls
Socket Connect Recv Close Bind Listen Accept Read and write
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Socket Routines
Particularly, we discuss the ones for integer conversion, including
htons ntohs htonl ntohl
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Socket Function
Create a new socket for network communication Return a descriptor for the newly created socket
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Connect Function
On client side Following the socket call Establish an active connection to a remote server
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Send Function
Clients and servers use it to transfer data across a TCP connection Client usually uses send to transmit request; server uses it to transmit replies
Which data should be sent The address of the data to be sent The length of the data
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Recv Function
Clients and serves use it to receive data from a TCP connection Usually, servers use it to receive requests; clients use if to receive replies
Which socket descriptor to use The address of a buffer The length of the buffer
The amount of data received depends on the buffer size defined
Also works for receiving message using UDP
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Close Function
To finish using a socket A socket may be used by many processes, close terminates the connection for the process issues the close call and reduces the reference count by 1 until to 0; when the counter is 0, the socket is deallocated No socket reference count for the threads in a process
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Bind Function
When socket created, no endpoint information inside Bind is used to specify the local endpoint address
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Listen Function
Connection-oriented (TCP) servers use it to place a socket in passive mode and make the server ready to accept incoming connection Set the number of incoming TCP connections the system will enqueue
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Accept Function
After using listen to place a socket in passive mode, the server calls accept to extract the next incoming connection request Accept creates a new socket for each new connection request, and returns the descriptor of the new socket to its caller Server uses the original socket to accept additional connect requests After finishing using the new socket, the server closes it
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Utility Routines – integer conversion
TCP/IP uses network byte order to represent integer with the most significant byte first To be portable, the local machine’s byte
- rder should be consistent with the network
byte order
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Utility Routines – integer conversion
Routines: (h: host, n: network, s: short, l: long)
htons, ntohs htonl, ntohl
For safety, converting the number anyway
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Flow chart for client
socket connect send recv close
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Flow chart for server
bind accept listen socket send recv close
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