Unix Socket – Quick Guide ”; Previous Next What is a Socket? Sockets allow communication between two different processes on the same or different machines. To be more precise, it”s a way to talk to other computers using standard Unix file descriptors. In Unix, every I/O action is done by writing or reading a file descriptor. A file descriptor is just an integer associated with an open file and it can be a network connection, a text file, a terminal, or something else. To a programmer, a socket looks and behaves much like a low-level file descriptor. This is because commands such as read() and write() work with sockets in the same way they do with files and pipes. Sockets were first introduced in 2.1BSD and subsequently refined into their current form with 4.2BSD. The sockets feature is now available with most current UNIX system releases. Where is Socket Used? A Unix Socket is used in a client-server application framework. A server is a process that performs some functions on request from a client. Most of the application-level protocols like FTP, SMTP, and POP3 make use of sockets to establish connection between client and server and then for exchanging data. Socket Types There are four types of sockets available to the users. The first two are most commonly used and the last two are rarely used. Processes are presumed to communicate only between sockets of the same type but there is no restriction that prevents communication between sockets of different types. Stream Sockets − Delivery in a networked environment is guaranteed. If you send through the stream socket three items “A, B, C”, they will arrive in the same order − “A, B, C”. These sockets use TCP (Transmission Control Protocol) for data transmission. If delivery is impossible, the sender receives an error indicator. Data records do not have any boundaries. Datagram Sockets − Delivery in a networked environment is not guaranteed. They”re connectionless because you don”t need to have an open connection as in Stream Sockets − you build a packet with the destination information and send it out. They use UDP (User Datagram Protocol). Raw Sockets − These provide users access to the underlying communication protocols, which support socket abstractions. These sockets are normally datagram oriented, though their exact characteristics are dependent on the interface provided by the protocol. Raw sockets are not intended for the general user; they have been provided mainly for those interested in developing new communication protocols, or for gaining access to some of the more cryptic facilities of an existing protocol. Sequenced Packet Sockets − They are similar to a stream socket, with the exception that record boundaries are preserved. This interface is provided only as a part of the Network Systems (NS) socket abstraction, and is very important in most serious NS applications. Sequenced-packet sockets allow the user to manipulate the Sequence Packet Protocol (SPP) or Internet Datagram Protocol (IDP) headers on a packet or a group of packets, either by writing a prototype header along with whatever data is to be sent, or by specifying a default header to be used with all outgoing data, and allows the user to receive the headers on incoming packets. What is Next? The next few chapters are meant to strengthen your basics and prepare a foundation before you can write Server and Client programs using socket. If you directly want to jump to see how to write a client and server program, then you can do so but it is not recommended. It is strongly recommended that you go step by step and complete these initial few chapters to make your base before moving on to do programming. Unix Socket – Network Addresses Before we proceed with the actual stuff, let us discuss a bit about the Network Addresses − the IP Address. The IP host address, or more commonly just IP address, is used to identify hosts connected to the Internet. IP stands for Internet Protocol and refers to the Internet Layer of the overall network architecture of the Internet. An IP address is a 32-bit quantity interpreted as four 8-bit numbers or octets. Each IP address uniquely identifies the participating user network, the host on the network, and the class of the user network. An IP address is usually written in a dotted-decimal notation of the form N1.N2.N3.N4, where each Ni is a decimal number between 0 and 255 decimal (00 through FF hexadecimal). Address Classes IP addresses are managed and created by the Internet Assigned Numbers Authority (IANA). There are five different address classes. You can determine which class an IP address is in by examining the first four bits of the IP address. Class A addresses begin with 0xxx, or 1 to 126 decimal. Class B addresses begin with 10xx, or 128 to 191 decimal. Class C addresses begin with 110x, or 192 to 223 decimal. Class D addresses begin with 1110, or 224 to 239 decimal. Class E addresses begin with 1111, or 240 to 254 decimal. Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and for internal testing on a local machine [You can test this: you should always be able to ping 127.0.0.1, which points to yourself]; Class D addresses are reserved for multicasting; Class E addresses are reserved for future use. They should not be used for host addresses. Example Class Leftmost bits Start address Finish address A 0xxx 0.0.0.0 127.255.255.255 B 10xx 128.0.0.0 191.255.255.255 C 110x 192.0.0.0 223.255.255.255 D 1110 224.0.0.0 239.255.255.255 E 1111 240.0.0.0 255.255.255.255 Subnetting Subnetting or subnetworking basically means to branch off a network. It can be done for a variety of reasons like network in an organization, use of different physical media (such as Ethernet, FDDI, WAN, etc.), preservation of address space, and security. The most common reason is to control network traffic. The basic idea in subnetting is to partition the host identifier portion of the IP address into two parts − A
Category: Unix Sockets
Aug
10