TCP/IP Tutorial and Technical Overview - IBM Redbooks

► User authentication might be necessary.
► The location of the server must be known.
Call message and reply transport
The RPC protocol can be implemented on any transport protocol. In the case of
TCP/IP, it can use either TCP or UDP as the transport vehicle. When using UDP,
remember that this does not provide reliability. Therefore, it is the responsibility
of the caller program to employ any needed reliability (using timeouts and
retransmissions, usually implemented in RPC library routines). Note that even
with TCP, the caller program still needs a timeout routine to deal with exceptional
situations, such as a server crash or poor network performance.
RPC call message structure
The RPC call message consists of several fields:
► Program and procedure numbers
Each call message contains three unsigned integer fields that uniquely
identify the procedure to be executed:
– Remote program number
– Remote program version number
– Remote procedure number
The remote program number identifies a functional group of procedures, for
example, a file system, that includes individual procedures such as read and
write. These individual procedures are identified by a unique procedure
number within the remote program. As the remote program evolves, a version
number is assigned to the different releases.
Each remote program is attached to an internet port. The number of this port
can be freely chosen, except for the reserved well-known-services port
numbers. It is evident that the caller will have to know the port number used
by this remote program.
► Authentication fields
Two fields, credentials and verifier, are provided for the authentication of the
caller to the service. It is up to the server to use this information for user
authentication. Also, each implementation is free to choose the varieties of
supported authentication protocols. These authentication protocols include:
– Null authentication.
– UNIX authentication: The callers of a remote procedure can identify
themselves as they are identified on the UNIX system.
Chapter 11. Application structure and programming interfaces 417