| Post Office Protocol |
Post Office Protocol
In computing, local e-mail clients use the Post Office Protocol version 3
(POP3), an application-layer Internet standard protocol, to retrieve
e-mail from a remote server over a TCP/IP connection. Many subscribers to
individual Internet service provider e-mail accounts access their e-mail with
client software that uses POP3.
Overview
POP3 has made earlier versions of the protocol, informally called POP1 and
POP2, obsolete. In contemporary usage, the less precise term POP almost
always means POP3 in the context of e-mail protocols.
The design of POP3 and its procedures supports end-users with intermittent
connections (such as dial-up connections), allowing these users to retrieve
e-mail when connected and then to view and manipulate the retrieved messages
without needing to stay connected. Although most clients have an option to
leave mail on server, e-mail clients using POP3 generally connect, retrieve
all messages, store them on the user's PC as new messages, delete them from the
server, and then disconnect. In contrast, the newer, more capable Internet
Message Access Protocol (IMAP) supports both connected (online) and
disconnected (offline) modes of operation. E-mail clients using IMAP
generally leave messages on the server until the user explicitly deletes them.
This and other aspects of IMAP operation allow multiple clients to access the
same mailbox. Most e-mail clients support either POP3 or IMAP to retrieve
messages; however, fewer Internet Service Providers (ISPs) support IMAP. The
fundamental difference between POP3 and IMAP4 is that POP3 offers access to a
mail drop; the mail exists on the server until it is collected by the client.
Even if the client leaves some or all messages on the server, the client's
message store is considered authoritative. In contrast, IMAP4 offers access to
the mail store; the client may store local copies of the messages, but these are
considered to be a temporary cache; the server's store is authoritative.
Clients with a leave mail on server option generally use the POP3 UIDL
(Unique IDentification Listing) command. Most POP3 commands
identify specific messages by their ordinal number on the mail server. This
creates a problem for a client intending to leave messages on the server, since
these message numbers may change from one connection to the server to another.
For example if a mailbox contains five messages at last connect, and a different
client then deletes message #3, the next connecting user will find the last two
messages' numbers decremented by one. UIDL provides a mechanism to avoid these
numbering issues. The server assigns a string of characters as a permanent and
unique ID for the message. When a POP3-compatible e-mail client connects to the
server, it can use the UIDL command to get the current mapping from these
message IDs to the ordinal message numbers. The client can then use this mapping
to determine which messages it has yet to download, which saves time when
downloading. IMAP has a similar mechanism, a 32-bit unique identifier (UID) that
must be assigned to messages in ascending (although not necessarily consecutive)
order as they are received. Because IMAP UIDs are assigned in this manner, to
retrieve new messages an IMAP client need only request the UIDs greater than the
highest UID among all previously-retrieved messages, whereas a POP client must
fetch the entire UIDL map. For large mailboxes, this difference can be
significant.
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