RFC 918

POST OFFICE PROTOCOL 💌

Title block, the tea ☕

• RFC: 918
• Title: POST OFFICE PROTOCOL
• Author: J. K. Reynolds, ISI
• Date: October 1984
• Status: experimental, a proposed protocol for the ARPA-Internet community 🧪
• Obsoletes / Updates: nothing listed, so this one isn't replacing or patching any other RFC 🫶

Status of this Memo

Okay so this RFC is basically suggesting a simple way for workstations to dynamically grab mail from a mailbox server 📬 It's a proposed protocol for the ARPA-Internet community, and the author is literally begging for discussion and suggestions to make it better 🥹

Introduction

The whole point of the Post Office Protocol (POP) is to let a user's workstation access mail from a mailbox server 🦋 Mail is expected to be posted from the workstation to the mailbox server through the Simple Mail Transfer Protocol (SMTP). If you want the deets on that, check out RFC-821 [1] and RFC-822 [2] 📖

Its status is experimental, and it depends on TCP 🧵

The Protocol

The server is the one waiting around for a connection 👀 When a connection opens, the server sends a greeting and then waits for commands. When commands come in, it acts on them and sends back replies 💅

The client opens a connection and waits for that greeting, then sends USER and then PASS to get authorized to access mailboxes 🔑 Next, the client starts a mail reading transaction with either RDEL (read and delete all messages from a mailbox) or RETR (just read all the messages without deleting anything) 📥 The server opens and locks the mailbox and replies with the number of characters in it 🔒 Then the client asks for the data by sending RCEV, and the server sends the mail data 🎁 Once all the data has arrived, the client sends RCVD. If the transaction started with RDEL, the server now deletes the mail data from the mailbox 🗑️ Either way, the server closes and unlocks the mailbox 🔓 Finally, the client ends the session with QUIT 👋

The Normal Scenario

It's giving sequence diagram energy 🤩

        Client                    Server
        ------                    ------
                             Wait for Connection
   Open Connection  -->
                        <--  +OK
                             Wait for Command
   USER Fred        -->
                        <--  +OK
                             Wait for Command
   PASS password    -->
                        <--  +OK
                             Wait for Command
   RDEL mailbox     -->      (open and lock mailbox)
                        <--  #xxx
                             Wait for Command
   RCEV             -->
                        <--  (send a copy of mail)
                             Wait for Command
   RCVD             -->      (deletes mail from mailbox, unlock
                             and close mailbox)
                        <--  +OK
                             Wait for Command
   QUIT             -->
                        <--  +OK
   Close connection --> <--  Close connection
                             Wait for Connection (go back to start)

Definitions of Commands and Replies

Here's where it gets spicy 🌶️ This is the official list of everything you can say and everything you can get back 💖

Summary of Commands and Replies

   Commands                          Replies
   --------                          -------
   USER name                         +OK
   PASS password                     -Error
   RETR mailbox                      #xxx
   RDEL mailbox
   RCEV
   RCVD
   QUIT
   NOOP
   RSET

Commands

USER name

This command identifies the user to the server 🪪 It MUST be followed by the PASS command, no exceptions bestie 🚫

Possible responses: +OK or -ERR ✨

PASS password

PASS carries the password that authenticates this user 🔐 Together, the USER name and PASS password are how the server controls access to the mailboxes 🏰

Possible responses: +OK or -ERR ✨

RETR mailbox

This command starts a mail reading transaction 📖 RETR is for reading all the messages in a mailbox without deleting them 🌸 It MUST be followed by the RCEV command 💅

Possible responses: #xxx or -ERR ✨

RDEL mailbox

This command also starts a mail reading transaction 🧹 RDEL reads all the messages in a mailbox AND deletes them, so be careful with it 🙈 It MUST be followed by the RCEV command 💅

Possible responses: #xxx or -ERR ✨

RCEV

This one is the client's "I'm ready, send it" moment 🙌 It confirms the client is ready to receive the mail data. It MUST be followed by the RCVD command 💅

Possible responses: +OK or (connection aborted) 💀

RCVD

RCVD confirms that the client has received and accepted the mail 📬 It ends the mail reading transaction 🏁 With an RDEL transaction, the mail is not necessarily deleted. That shows up as an error reply, so check your replies 👀

Possible responses: +OK or -ERR ✨

QUIT

QUIT means the client is done with the session 👋 The server sends an OK, then closes the connection 🚪

Possible response: +OK then Close 🦋

NOOP

NOOP is the no operation command, which means it does absolutely nothing except send back an OK 😴 Iconic behavior, honestly 💅

Possible response: +OK ✨

RSET

RSET makes the server abort the current transaction and go back to waiting for a command (one of RDEL, RETR, QUIT, NOOP, or RSET) 🔄 When aborting, the server MUST take care to properly close and unlock the mailbox 🔓 No leaving things hanging, periodt 💀

Possible response: +OK ✨

Acknowledgment

The author wants to give a big shoutout to Jon Postel, Joel Goldberger, Dale Chase, and Michael Butler for helping develop the Post Office Protocol 🫶🌈

References

[1] Postel, J., "Simple Mail Transfer Protocol", RFC-821, USC/Information Sciences Institute, August 1982. 📚

[2] Crocker, D., "Standard for the Format of ARPA-Internet Text Messages", RFC-822, University of Delaware, August 1982. 📚

Diagram

Here's the whole flow in one picture, and it's giving main character energy 🦋

         +---+      +---+       +---+       +---+       +---+
   Open  |   | +OK  |   |       |   | +OK   |   |       |   | +OK
   ----->| S |--+-->| C |------>| S |------>| C |------>| S |---->+
         |   |  ^   |   | USER  |   |       |   | PASS  |   |     |
         +---+  |   +---+       +---+       +---+       +---+     |
                |                 | -ERR                  | -ERR  |
                |                 V                       V       |
   +<-----------+<----------------+<----------------------+       |
   |                                                              |
   V      +---+       +---+                                       |
   +----->|   | QUIT  |   | +OK                                   |
          | C |------>| S |----->Close                            |
   +----->|   |       |   |                                       |
   ^      +---+       +---+                                       |
   |                                                              V
   +<-------------------------------------------------------------+
   |                                                              ^
   |  +---+     +---+      +---+     +---+     +---+     +---+    |
   |  |   |RETR |   | #xxx |   |RECV |   |DATA |   |RCVD |   |+OK |
   +->| C |---->| S |----->| C |---->| S |---->| C |---->| S |--->+
   |  |   | or  |   |      |   |     |   |     |   |     |   |    ^
   |  +---+ RDEL+---+      +---+     +---+     +---+     +---+    |
   |              | -ERR     |         |         |         | -ERR |
   |              V          |         V         |         |      |
   +<-------------+          |       Abort       |         |      |
   |                         V                   V         V      |
   |        +<---------------+-------------------+         +----->+
   |        V
   |      +---+       +---+
   |      |   | RSET  |   | +OK
   +----->| C |------>| S |----->+
   |      |   |       |   |      |
   |      +---+       +---+      |
   |                             V
   +<----------------------------+
   |                             ^
   |      +---+       +---+      |
   |      |   | NOOP  |   | +OK  |
   +----->| C |------>| S |----->+
          |   |       |   |
          +---+       +---+