Page 2

Bionic Buffalo Tech Note #103: Introduction to Pegasus Client Support

servers implementing U-N.

The U-U Protocol is defined using the OMG Interface Definition Language (IDL), along
with textual explanations. OMG provides formal mappings from IDL to protocols as well
as to APIs, so there is an API for the U-U protocol, in addition to a well-defined over-the-
wire behaviour. (The API is defined for a variety of programming languages, but this
document, along with most client implementations and the examples in the DSM-CC
specification, uses C.)

The CORBA specifications map IDL to the General Inter-ORB Protocol (GIOP). The mapping of
GIOP to TCP/IP is called the Internet Inter-ORB Protocol (IIOP). In addition to the IIOP derived from
the IDL mapping, DSM-CC allows other protocols, including Sun's ONC (Open Network Computing)
RPC, to be used for remote object interaction, but IIOP is by far the most common.

In the ensuing discussion, use is made of three terms: server, client, and SRM. We leave “server” and
“client” with the common imprecise but adequate definitions. An SRM, or Session and Resource
Manager, is a third type of entity which manages the network and its resources. The closest common
analogue is the switching office in a telephone company: the client phone uses the SRM (switching
office) to establish a connection, then carries on a conversation with the server. Although that might in
some instances be an oversimplification, it should be an adequate model for this paper. Note that the
SRM may be colocated with the server, or it may be in a separate machine or at some different
location.

Contrasting DSM-CC and Pegasus ISA

In addition to the LSCP and SSP protocols and specifications mentioned previously, there is a third
Pegasus specification: Pegasus Interactive Services Architecture, whose subject matter is commonly
called “ISA”. Although ISA claims some measure of DSM-CC compliance, in some ways it directly
contradicts the DSM-CC standard.

In DSM-CC, there is a local

DSM::Session

object on the client. The client application uses the

local

DSM::Session

object to connect to the server, returning a reference to a local

DSM::

ServiceGateway

object. The

DSM::Session

object can do this in one of two ways:

In systems with U-N signalling, the

DSM::Session

object connects with a local

DSM::SessionUU

object, which performs the U-N signalling. The

DSM::

SessionUU

object communicates with the SRM, using the U-N protocol, to set up a

session between the local

DSM::ServiceGateway

object and a remote

DSM::

ServiceGatewayUU

object on the server. (In the process of setting up the session, the

SRM negotiates with the server's session manager, from which it obtains an object
reference for the

DSM::ServiceGatewayUU

object. That object reference is relayed

back to the client, giving the client access to the

DSM::ServiceGatewayUU

object.)

Page 2 of 11