Distributed Component Object Model

Distributed Component Object Model (DCOM) is a proprietary Microsoft technology for communication between software components on networked computers. DCOM, which originally was called "Network OLE", extends Microsoft's COM, and provides the communication substrate under Microsoft's COM+ application server infrastructure.
The addition of the "D" to COM was due to extensive use of DCE/RPC (Distributed Computing Environment/Remote Procedure Calls) – more specifically Microsoft's enhanced version, known as MSRPC.
In terms of the extensions it added to COM, DCOM had to solve the problems of
One of the key factors in solving these problems is the use of DCE/RPC as the underlying RPC mechanism behind DCOM. DCE/RPC has strictly defined rules regarding marshalling and who is responsible for freeing memory.
DCOM was a major competitor to CORBA. Proponents of both of these technologies saw them as one day becoming the model for code and service-reuse over the Internet. However, the difficulties involved in getting either of these technologies to work over Internet firewalls, and on unknown and insecure machines, meant that normal HTTP requests in combination with web browsers won out over both of them. Microsoft, at one point, attempted and failed to head this off by adding an extra http transport to DCE/RPC called "ncacn_http" (Network Computing Architecture connection-oriented protocol). This was later resurrected to support a Microsoft Exchange 2003 connection over HTTP.
DCOM was publicly launched as a beta for Windows 95 September 18, 1996.
DCOM is supported natively in Windows NT 4.0, Windows 2000, Windows XP, and Windows Server 2003, as well as Windows 7, Windows 8, Windows 10, Windows Server 2008, Windows Server 2008 R2, Windows Server 2012, Windows Server 2012 R2 and Windows Server 2016.
Hardening.
As part of the initiative that began at Microsoft as part of Secure Development Lifecycle to re-architect insecure code, DCOM saw some significant security-focused changes in Windows XP Service Pack 2.
Alternative versions and implementations.
COMsource: Its source code is available, along with full and complete documentation, sufficient to use and also implement an interoperable version of DCOM. According to that documentation, COMsource comes directly from the Windows NT 4.0 source code, and even includes the source code for a Windows NT Registry Service.
In 1995, Digital and Microsoft announced Affinity for OpenVMS (also known as NT Affinity) which was intended to allow OpenVMS to serve as the persistence layer for Windows NT client-server applications. As part of this initiative, an implementation of the Distributed Component Object Model (DCOM) was added to OpenVMS Alpha. In order to support DCOM, VMS was provided with implementations of the Windows Registry, NTLM authentication, and a subset of Win32 APIs needed to support COM. DCOM was first added to OpenVMS V7.2-1 for the Alpha. A similar implementation of DCOM was added to Digital Unix as part of the AllConnect program.
The Wine Team is also implementing DCOM for binary interoperability purposes; they are not currently interested in the networking side of DCOM, which is provided by MSRPC. They are restricted to implementing NDR (Network Data Representation) through Microsoft's API, but are committed to making it as compatible as possible with MSRPC.
TangramCOM is a separate project from Wine, focusing on implementing DCOM on Linux-based smartphones.
The Samba Team is also implementing DCOM for over-the-wire interoperability purposes: unlike the Wine Team, they are not currently interested in binary-interoperability, as the Samba MSRPC implementation is far from binary-interoperable with Microsoft's MSRPC.