High-level programming through remote procedure calls (RPC) provides logical client-to-server communication for network application development -- without the need to program most of the interface to the underlying network. With RPC, the client makes a remote procedure call that sends requests to the server, which calls a dispatch routine, performs the requested service, and sends back a reply before the call returns to the client.
RPC does not require the caller to know about
the underlying network.
For example, a program can simply call
rnusers
(a C routine that returns the number of users on a remote machine) much
like making a system call to
malloc.
You can make remote procedure calls from any language, and
between different processes on the same machine.
The remote procedure call model is similar to that of the local model, which works as follows:
The remote procedure call is similar, in that one thread of control logically winds through two processes -- that of the caller and that of the server:
On the server side, a process is dormant -- awaiting the arrival of a call message. When one arrives, the server process computes a reply that it then sends back to the requesting client. After this, the server process becomes dormant again. Figure 1-1 illustrates this basic form of network communication with the remote procedure call.
This figure shows a synchronous RPC call, in which only one of the two processes is active at a given time. The remote procedure call hides the details of the network transport. However, the RPC protocol does not restrict the concurrency model. For example, RPC calls may be asynchronous so that the client can do another task while waiting for the reply from the server. Another possibility is that the server could create a task to process a certain type of request automatically, freeing it to service other requests. Although RPC provides a way to avoid programming the underlying network transport, it still allows this where necessary.
Each RPC procedure is uniquely defined by program and procedure
numbers. The
program number
specifies a group of related remote procedures,
each of which has a different
procedure number.
Each program also has a
version number so that, when a minor change is made to a remote service (adding
a new procedure, for example), a new program number does not have to be
assigned. When you want to call a procedure to find the number
of remote users, you must know the appropriate program, version,
and procedure numbers to use to contact the service.
This information can be found in several sources. For example, the
/etc/rpc
file lists some RPC programs and the
rpcinfo
command lists the registered RPC programs and corresponding version numbers
running on a particular system.
Typically, a service provides a protocol description so that you can write client applications that call the service. The RPC Administrator at Sun Microsystems, Inc. has a list of programs that have been registered with them (that is, have received port numbers from them) but you can write your own local RPC programs. Knowing the program and procedure numbers is useful only if the program is running on a system that you have access to.
The
portmap
network service command starts
automatically when a machine is booted.
As part of its initialization, a server program calls its
host portmap
to create a portmap entry for its program and version number.
To find the port of a remote program, a client sends
an RPC call message to a server
portmap. If the remote program is
registered with the
portmap, it returns the relevant port number
in an RPC reply message. The client program can then send RPC call
message packets to that remote program port.
The
portmap
network service has a well-known (dedicated)
port. Other network service port numbers can be
assigned statically or dynamically when they
register their ports with the portmap of their host.
Refer to the
portmap(8)
reference page for more information about the port mapping service.
The RPC protocol is concerned only with the specification and interpretation of messages; it is independent of transport protocols because it needs no information on how a message is passed among processes.
Also, RPC does not implement any kind of reliability -- the application itself must be aware of the transport protocol type underlying RPC. With a reliable transport, such as TCP/IP, the application need not do much else. However, an application must use its own retransmission and time-out policy if it is running on top of an unreliable transport, such as UDP/IP.
Because of transport independence, the RPC protocol does not actively interpret anything about remote procedures or their execution. Instead, the application infers required information from the underlying protocol (where such information should be explicitly specified). For example, if RPC is running on top of an unreliable transport (such as UDP/IP), and the application retransmits RPC messages after short time-outs, and if the application receives no reply, then it can infer only that a certain procedure was executed zero or more times. If it receives a reply, then the application infers that the procedure was executed at least once.
With a reliable transport, such as TCP/IP, the application can infer from a reply message that the procedure was executed exactly once, but if it receives no reply message, it cannot assume the remote procedure was not executed.
Note
Even with a connection-oriented protocol like TCP, an application still needs time-outs and reconnection to handle server crashes.
ONC RPC is currently supported on both UDP/IP and TCP/IP transports. The selection of the transport depends on the application requirements. The UDP transport, which is connectionless, is a good choice if the application has the following characteristics:
TCP (connection-oriented) is a good transport choice if the application has any of the following characteristics:
RPC can handle arbitrary data structures, regardless of the byte order or structure layout convention on a machine. It does this by converting them to a network standard called External Data Representation (XDR) before sending them over the wire. XDR is a machine-independent description and encoding of data that can communicate between diverse machines, such as a VAX, Sun workstation, IBM-PC, or Cray.
Converting from a particular machine representation to XDR format is called serializing; the reverse process is deserializing.
The
rpcinfo
command reports current RPC registration information
known to
portmap;
administrators can use it to delete registrations.
The
rpcinfo
command can also
find the RPC services registered on a specific host
and report their port numbers and the transports for which the services
are registered. You can also use it to call (through
ping)
a program version on a specific host using the TCP or UDP transport, and to
report whether the response was received. For more information,
see the
rpcinfo(8)
reference page.
Program numbers are assigned in groups of
0x20000000
according to the following chart:
| 0x0 - 0x1fffffff | Defined by Sun |
| 0x20000000 - 0x3fffffff | Defined by user |
| 0x40000000 - 0x5fffffff | Transient |
| 0x60000000 - 0x7fffffff | Reserved |
| 0x80000000 - 0x9fffffff | Reserved |
| 0xa0000000 - 0xbfffffff | Reserved |
| 0xc0000000 - 0xdfffffff | Reserved |
| 0xe0000000 - 0xffffffff | Reserved |
Sun Microsystems administers the first range of numbers,
which should be identical for all ONC users.
An ONC RPC application for general use should
have an assigned number in this first range.
The second range of numbers is for specific, user-defined customer
applications, and is
primarily for debugging new programs. The third, called the
Transient
group, is reserved for
applications that generate program numbers dynamically. The
final groups are reserved for future use, and are not
used.
To register a protocol specification, send a request by network
mail to
rpc@sun.com
or write to:
Include a compilable
rpcgen
.x
file describing your protocol;
you will then receive a unique program number.
See
rpcgen(1)
for more information.
Some of the RPC program numbers are in
/etc/rpc.