(libc.info.gz) Socket Concepts
(libc.info.gz) Communication Styles
16.1 Socket Concepts
When you create a socket, you must specify the style of communication
you want to use and the type of protocol that should implement it. The
"communication style" of a socket defines the user-level semantics of
sending and receiving data on the socket. Choosing a communication
style specifies the answers to questions such as these:
* *What are the units of data transmission?* Some communication
styles regard the data as a sequence of bytes with no larger
structure; others group the bytes into records (which are known in
this context as "packets").
* *Can data be lost during normal operation?* Some communication
styles guarantee that all the data sent arrives in the order it was
sent (barring system or network crashes); other styles occasionally
lose data as a normal part of operation, and may sometimes deliver
packets more than once or in the wrong order.
Designing a program to use unreliable communication styles usually
involves taking precautions to detect lost or misordered packets
and to retransmit data as needed.
* *Is communication entirely with one partner?* Some communication
styles are like a telephone call--you make a "connection" with one
remote socket and then exchange data freely. Other styles are
like mailing letters--you specify a destination address for each
message you send.
You must also choose a "namespace" for naming the socket. A socket
name ("address") is meaningful only in the context of a particular
namespace. In fact, even the data type to use for a socket name may
depend on the namespace. Namespaces are also called "domains", but we
avoid that word as it can be confused with other usage of the same
term. Each namespace has a symbolic name that starts with `PF_'. A
corresponding symbolic name starting with `AF_' designates the address
format for that namespace.
Finally you must choose the "protocol" to carry out the
communication. The protocol determines what low-level mechanism is used
to transmit and receive data. Each protocol is valid for a particular
namespace and communication style; a namespace is sometimes called a
"protocol family" because of this, which is why the namespace names
start with `PF_'.
The rules of a protocol apply to the data passing between two
programs, perhaps on different computers; most of these rules are
handled by the operating system and you need not know about them. What
you do need to know about protocols is this:
* In order to have communication between two sockets, they must
specify the _same_ protocol.
* Each protocol is meaningful with particular style/namespace
combinations and cannot be used with inappropriate combinations.
For example, the TCP protocol fits only the byte stream style of
communication and the Internet namespace.
* For each combination of style and namespace there is a "default
protocol", which you can request by specifying 0 as the protocol
number. And that's what you should normally do--use the default.
Throughout the following description at various places
variables/parameters to denote sizes are required. And here the trouble
starts. In the first implementations the type of these variables was
simply `int'. On most machines at that time an `int' was 32 bits wide,
which created a _de facto_ standard requiring 32-bit variables. This
is important since references to variables of this type are passed to
Then the POSIX people came and unified the interface with the words
"all size values are of type `size_t'". On 64-bit machines `size_t' is
64 bits wide, so pointers to variables were no longer possible.
The Unix98 specification provides a solution by introducing a type
`socklen_t'. This type is used in all of the cases that POSIX changed
to use `size_t'. The only requirement of this type is that it be an
unsigned type of at least 32 bits. Therefore, implementations which
require that references to 32-bit variables be passed can be as happy
as implementations which use 64-bit values.
(libc.info.gz) Communication Styles
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