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|| The TCP/IP Guide|
9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
9 TCP/IP Key Applications and Application Protocols
9 TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
9 TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
9 TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME
9 TCP/IP Electronic Mail Standard Message Format: RFC 822
TCP/IP Electronic Mail RFC 822 Standard Message Format Header Field Definitions and Groups
(Page 2 of 3)
Header Field Groups
The RFC 822 message format specifies
many types of headers that can be included in e-mail messages. A small
number of headers are mandatory, meaning they must be included
in all messages. Some are not mandatory but are usually present, because
they are fundamental to describing the message. Others are optional
and are included only when needed.
To help organize the many headers,
the RFC 2822 standard categorizes them into header field groups (as
did RFC 822, though the groups are a little different in the older standard):
- Origination Date Field: Specifies the
date and time that the message was made ready for delivery; see below
for details. (This field is in its own group for reasons that are unclear
to me; perhaps just because it is important.)
- Originator Fields: Contain information
about the sender of the message.
- Destination Address Fields: Specify the
recipient(s) of the message, which may be in one of three different
- Identification Fields: Contain information
to help identify the message.
- Informational Fields: Contain optional
information to help make more clear to the recipient what the message
- Resent Fields: Used to preserve the original
originator, destination and other fields when a message is resent.
- Trace Fields: Used to show the path taken
by mail as it was transported.
In addition, the format allows other,
user-defined fields to be specified, as long as they correspond to the
standard <header name>: <header value> syntax.
This can be used to provide additional information of various sorts.
For example, sometimes the e-mail client software will include a header
line indicating the name and version of the software used to compose
and send the message. We'll also see that MIME
uses new header lines to encode information
about MIME messages.
Key Concept: Each RFC 822 message begins with a set of headers that carry essential information about the message. These headers are used to manage how the message is processed and interpreted, and also describe the contents of the message body. Each header consists of a header name and a header value. There are over a dozen different standard RFC 822 headers, which are organized into groups; it is also possible for customized user headers to be defined.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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