P-series protocols

Definition of P-series protocols in The Network Encyclopedia.

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What is P-series protocols?

A group of protocols that are part of the X.400 messaging standards. Five P-series protocols relate to messaging systems that support X.400, such as Microsoft Exchange Server:

  • P1 Protocol:
    Specifies the layout of messages transferred from one Message Transfer Agent (MTA) to another. This protocol specifies that X.400 messages consist of two parts: a P1 header, which acts as an envelope and must contain a globally unique recipient address for message routing and control purposes, and a P2 message, which is the actual content of the message.


  • P2 Protocol:
    Defines the format for transmitting the content of an X.400 message. This format includes a P2 header (which is not used because the P1 header provides the necessary routing information for the message) and a P2 body, which is the actual content of the message and consists of one or more body parts of various types (such as text, images, voice, or telex).


  • P3 Protocol:
    Specifies how a user agent (UA) communicates directly with an MTA for sending or receiving a message. This protocol is not used as often as the P7 Protocol for the same reason that Post Office Protocol version 3 (POP3) is used instead of Simple Mail Transfer Protocol (SMTP) to receive Internet mail.


  • P7 Protocol:
    Specifies how a UA communicates with a message store (MS) in order to selectively retrieve messages from the store and delete unwanted messages without downloading them.


  • P22 Protocol:
    A 1988 revision of the P2 Protocol that clarifies and extends certain features of P2.


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