TRANSFER Programming Manual

P2 Messages—Mail Messages
X400 Services
6–2 069138, Update 1 to 040970 Tandem Computers Incorporated
Figure 6-1 shows the OSI model and X.400 layers. You do not need to understand the
OSI model in order to use the X400 Gateway; this diagram is provided for additional
information.
Figure 6-1. The OSI Model
02
4
Application Laye
r
Presentation Layer
Session Laye
r
Transport Layer
Network Laye
r
Data Link Laye
r
Physical Layer
Message Path
P2
P1
Physical Media
Originator
Recipient
7
6
5
4
3
2
1
UAL
MTL
Note These layers are part of the OSI model. You cannot access them with clients or agents.
P2 Messages—
Mail Messages
X.400 standardized electronic mail service is provided as a type of User Agent (UA)
service by the UAL. The mail service formats mail messages into a protocol called P2,
and then relays the messages to the MTL for additional formatting and subsequent
transport.
Each P2 message consists of a header followed by one or more body parts. The header
contains the message originator; primary, courtesy-copy, and blind-copy recipients;
and other service information. Each body part contains data in one format, such as
binary data, 8-bit ASCII text, or EBCDIC text. Messages can be nested within body
parts.