P protocols
InX.400, communication between the User Agent( UA), Message Stores( MS), Access Units( AU), Physical Delivery Access Units ( PDAU) and Mail Transfer Services( MTS) within the application instances takes place using defined protocols. These protocols, which are specified in X.419, are used to implement the services offered. The P protocols refer to the communication between User Agent (UA) and Mail Transfer Agent( MTA), between two MTAs and between Message Store (MS) and User Agent (UA).
X.400/88 knows the following P protocols:
The P1 protocol is the Message Transfer Protocol (MTS). It is used for communication between the Message Transfer Agents. The partner MTAs have equal rights.
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The P2 protocol is the protocol between the user agents, the Interpersonal Messaging Protocol. The P2 protocol divides the content of an electronic message into a header and a body, where the body can be constructed using different character sets.
The P3 protocol, the Submission and Delivery Protocol, is the access protocol to the message transfer service (MTS). It is used for both user agent and message store access to the message transfer service. In this protocol, all three objects, the User Agent (UA), the Message Store (MS) and MTS can act as initiators
The P5 protocol provides access from a telex interface into the message handling system
The P7 protocol as MS Access Protocol( MSAP), which regulates the access of a user agent to the message store. The communication is triggered exclusively by the user agent, which is always the initiator. The Message Store cannot become active and cannot connect to a UA. The P7 protocol is not included in the 1984 version of X.400.
The P protocols P1, P3 and P7 are used for the X.400-specific application service elements( MASE, MDSE, MSSE, MTSE) and for the application-independent service elements Application Control Service Element ( ACSE), Reliable Transfer Service Element( RTSE) and Remote Operation Service Element( ROSE).