Finding a way to connect existing computer networks of diverse types was a primary goal in the design of TCP/IP. Achieving of this goal has made TCP/IP particularly successful in the networking of computers that run different operating systems and that are manufactured by different vendors. TCP/IP is also designed to accommodate a very large number of host computers and local networks. A site or organization that uses TCP/IP need not be connected to the Internet, but most sites and organizations are connected.
The open, nonproprietary nature of TCP/IP and its global scope have made it popular among users of the UNIX operating system. Standards for writing communications programs in C have also become widespread. The two most common standards are the BSD UNIX Socket Library interface and the UNIX System V Transport Layer Interface (TLI).
The SAS/C Library currently implements the BSD UNIX Socket Library interface because it is somewhat more common than TLI, and it has better support from the underlying communications software on MVS and CMS systems. The socket library is integrated with SAS/C support for UNIX file I/O to provide the same type of integration between file and network I/O that is available on BSD UNIX systems.
TCP/IP is now a base for higher level protocols that support many popular networking applications. Some of these protocols are:
An IP address is a 32-bit number that specifies both the number for the individual physical network and the number for a given host computer on that network. The term host computer can apply to any end-user computer system that connects to a network. The size of a host can range from an X-terminal or a PC to a large mainframe. Among all the organizations connected to the Internet, the address of each host computer is unique. The address is often written in dotted decimal notation. Dotted decimal notation is the decimal value of each byte (often referred to as an octet in the literature on TCP/IP) separated by a period. For example,
192.22.31.05 is the dotted decimal notation for a machine whose 32-bit address is 0xC0161f05At the IP protocol layer, host computers cannot be referenced by name. Refer to Domain Name System (DNS) for an explanation of name referencing for host computers. All network communication uses IP addresses. The IP layer routes packets of data to their destinations, which may be many physical hops away from the source of the message. A physical hop is a gateway through which the data must pass. The IP layer does not guarantee that a packet will reach its destination, nor does it provide error checking for the data. The IP layer does not provide flow control or any lasting association (connection) between sender and receiver. A higher layer of the protocol, such as the User Datagram Protocol (UDP) or TCP, must provide all these services.
In addition to the communication capabilities of IP, UDP adds checksums for the application data and protocol ports to help distinguish among the different processes that are communicating between sending and receiving machines. A checksum detects errors in the transfer of a packet from one machine to another. A protocol port is an abstraction used to distinguish between multiple destinations within a single host.
A datagram is a basic unit of information transferred across a network. UDP does not guarantee that datagrams reach their destination, nor does it ensure that the datagrams are received in the order in which they are sent. Because UDP does not use connections or sessions, it is called a connectionless protocol.
UDP ports are two-byte integers that specify a particular service or program within a host computer. For example, port 13 is generally used by programs that query the date and time maintained by a particular host. A client-server relationship is usually defined in a UDP transaction. The server waits for messages (listens) at a predefined port. When it receives a datagram from a new client, the server knows where to respond because the datagram contained both the sender's IP address and its port number.
Each organization has the ability to control names within its own domain. Domains are arranged in a hierarchy. For example, the XYZ Company, Inc., may have names all ending in the following:
abcvm.vm.xyz.commight be the primary VM system at the XYZ Company, Inc. DNS enables you to use a File Transfer Program command such as
ftp abcvm.vm.xyz.cominstead of
ftp 188.8.131.52when transferring a file to this VM system.
Although it is possible to locate the mapping of host addresses to
host names in a file (for example,
/etc/hosts on UNIX), DNS is more
versatile than a system that maps addresses to names in a file.
Under a system that maps names to addresses,
the file containing the
mapped names and addresses:
must be replicated on every host, does not have the
capacity to contain the mappings for
all computers on a system as large as the Internet, and cannot be
on a real-time basis.
DNS uses server processes called name servers to stay current with the names assigned within a particular domain. The network administrator provides the name servers with configuration files. Each configuration file contains the mapping for the domain that it controls. Name servers in a particular domain can refer to the addresses of name servers for higher- and lower-level domains if the configuration files that they control do not contain a particular name or address.
Name servers typically run on only a few machines in an organization. Programs can use a set of routines, known as the resolver, to query their organization's name server. The resolver routines are associated with the application and provide all the message formatting and TCP or UDP communications logic necessary to talk to their organization's name server.
DNS is general enough to allow distributed management of other types of information, such as mailbox locations, and it does not require any correspondence between domains and IP addresses or physical network connections.
With Release 6.00, the SAS/C Library supports both integrated and non-integrated sockets. With integrated sockets, the TCP/IP sockets are integrated with OpenEdition support instead of being a direct run-time library interface to the TCP/IP software implemented only in the run-time library.
With non-integrated sockets, the SAS/C Socket Library relies on an underlying layer of TCP/IP communications software, such as IBM TCP/IP Version 2, or higher, for VM and MVS. TCP/IP communications software handles the actual communications. The SAS/C Library adds a higher level of UNIX compatibility, as well as integration with the SAS/C run-time environment.
Copyright (c) 1998 SAS Institute Inc. Cary, NC, USA. All rights reserved.