(We describe the mechanism in terms of a client sending a fragmented request message to the server the same mechanism applies when a server sends a fragment response to the client.)įirst, each fragment that makes up the request message contains both a unique FragmentNum and a flag indicating whether this packet is a fragment of a call ( frag ) or the last fragment of a call ( last_frag ) request messages that fit in a single packet carry a no_frag flag. Both the client and server implement a selective acknowledgment mechanism, which works as follows. Each DCE-RPC fragment is assigned a unique fragment number (0, 1, 2, 3, and so on). The FragmentNum field uniquely identifies each fragment that makes up a given request or reply message. As noted above, even if an underlying protocol such as IP provides fragmentation/reassembly, a more sophisticated algorithm implemented as part of RPC can result in quicker recovery and reduced bandwidth consumption when fragments are lost. To distinguish between replies sent before and after a server machine reboots, DCE-RPC uses a ServerBoot field to hold the machine's boot ID.Īnother design choice made in DCE-RPC that differs from SunRPC is the support of fragmentation and reassembly in the RPC protocol. Unlike SunRPC, DCE-RPC keeps track of the last sequence number used as part of a particular activity, so as to ensure at-most-once semantics. A SequenceNum field then distinguishes between calls made as part of the same activity it serves the same purpose as SunRPC's XID (transaction id) field. The activity to which a message belongs is identified by the message's ActivityId field. Like the concurrent logical channel approach described above and in Section 2.5.3, the application programs have to open multiple channels if they want to have more than one request/ reply transaction between them at the same time. At any given time, there can be only one message transaction active on a given channel. An activity is a logical request/reply channel between a pair of participants. Typical DCE-RPC message exchange.Įach request/reply transaction in DCE-RPC takes place in the context of an activity. Also, the server can respond to a Request message with a Reject message (indicating that a call has been rejected), and it can respond to a Ping message with a Nocall message (indicating that the server has never heard of the caller).įigure 5.19. For example, the client can send a Quit message to the server, asking it to abort an earlier call that is still in progress the server responds with a Quack (quit acknowledgment) message. Although not shown in the figure, other message types are also supported. If the server's reply is received reasonably quickly, no Ping s are sent. Instead of the server acknowledging the request messages, however, the client periodically sends a Ping message to the server, which responds with a Working message to indicate that the remote procedure is still in progress. The client sends a Request message, the server eventually replies with a Response message, and the client acknowledges ( Ack ) the response. (In truth, DCE-RPC supports multiple call semantics, including an idempotent semantics similar to SunRPC's, but at-most-once is the default behavior.) There are some other differences between the two approaches, which we will highlight in the following paragraphs.įigure 5.19 gives a timeline for the typical exchange of messages, where each message is labeled by its DCE-RPC type. Unlike SunRPC, however, DCE-RPC implements at-most-once call semantics.
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It is also similar to SunRPC in that it defines a two-level addressing scheme: the transport protocol demultiplexes to the correct server, DCE-RPC dispatches to a particular procedure exported by that server, and clients consult an “endpoint mapping service” (similar to SunRPC's Port Mapper) to learn how to reach a particular server. It can be used with the Network Data Representation (NDR) stub compiler described in Chapter 7, but it also serves as the underlying RPC protocol for the Common Object Request Broker Architecture (CORBA), which is an industry-wide standard for building distributed, object-oriented systems.ĭCE-RPC, like SunRPC, can be implemented on top of several transport protocols including UDP and TCP. Davie, in Computer Networks (Fifth Edition), 2012 DCE-RPCĭCE-RPC is the RPC protocol at the core of the DCE system and was the basis of the RPC mechanism underlying Microsoft's DCOM and ActiveX.
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