Connected: An Internet Encyclopedia
RPC Protocol Overview
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RPC Protocol Overview
The Remote Procedure Call (RPC) protocol, documented in
RFC 1831,
is designed to augment IP in a different way than TCP. While TCP
is targeted at the transfer of large data streams (such as a file download),
RPC is designed for network programing, allowing a program to make
a subroutine call on a remote machine. The most important application of
RPC is the NFS file sharing protocol.
The key features of RPC are:
- Request-reply. RPC is a request-reply protocol, and can
exhibit the "ping-pong" behavior typical of such protocols. This
is not fundamental to RPC's design (since multiple requests can
be outstanding, and replys can come in any order), but most programs
will issue a single RPC request, and then block awaiting a reply.
- UDP or TCP transport. RPC actually operates over UDP or TCP.
RPC/UDP is a connection-less, stateless protocol. RPC/TCP is slower,
but provides a reliable, stateful connection.
- Standardized data representation. RPC encodes its data
using the eXternal Data Representation (XDR) protocol, documented in
RFC 1832, which
standardizes the format of integers, floating point numbers, and
strings, permitting different types of computers to enhance
information seamlessly.
- Authentication. RPC provides support for authenticating
the calling program on one machine to the target subroutine on the
other. Authentication can operate in several different modes.
During NFS operations, authentication usually takes the form
of relaying UNIX user and group IDs to the file server for
permission checking.
RPC Service Rendevous
Both TCP and UDP rely on well-known port numbers to perform
service rendevous. For example, TCP TELNET service is available on port 21,
FTP on port 23, SNMP on port 25, and so on. Connecting to
TCP and UDP services simply requires connecting to the right port number.
RPC introduces another step in this process, to divorce services from
being tied to a given port number. It does so using a special RPC
service called PORTMAP or RPCBIND. These binding protocols,
documented in
RFC 1833 and
often refered to as the portmapper,
are unique among RPC services since they
have an assigned port of their own (port 111). Other RPC services,
running on any port number, can register themselves
using an RPC call to port 111.
The portmapper offers other RPC calls to permit service lookup.
The most important consequence of this design is that
the portmapper must be the first RPC program started, and must
remain in constant operation.
Connected: An Internet Encyclopedia
RPC Protocol Overview