Category: Email

Posted on

POP3 – The ideas that didn’t make it.

This is part of series of posts documenting extensions to the POP3 protocol.

I had a few ideas along the way. This post collects some that didn’t quite make it. I present these so the time I spent won’t have been a complete waste. 🐘

Multi-line Response Indication

Good software engineering employs reusable code.

If you’re developing a library to interact with a POP3 service as a client, you’d observe that the protocol operates on an exchange of command and response. This calls for a single function that can be called to send any command to the server and return the response when it arrives. Your function would look like:

var retrResponse = pop3.Command("RETR 4");

Except you can’t do that. There are two distinct classes of response in POP3. One where the response is a single line and another where the response is multiple lines. If all you have is the first line, you have no clear indication that’s the complete response or if there are more lines coming. You, the developer, need to know what kind of response you’re expecting from the server and have the caller pass that information along.

var retrResponse = pop3.CommandMulti("RETR 4");
var deleResponse = pop3.CommandSingle("DELE 4");

Wouldn’t it be nice if there was a clear an unambiguous way for a server to indicate if there are more lines to follow? That way, client code could have that single function that just knows what to do.

When the client calls CAPA, if the response includes “MULTI-LINE-IND”, the client can know what kind of response is coming from the server from the first line, because the server is making these promises:

  1. All multi-line responses will always have a first line that ends with a ” _”.
  2. All single-line responses will never be a line that ends with a ” _”.
C: RETR 1
S: +OK This line ends with an underscore so keep reading. _
(Message goes here.)
S: .
C: DELE 1
S: +OK This line has no underscore so send the next command.

I chose the underscore character as this would technically be encroaching into the human readable section of a response, so it would need to be ignorable by any humans passing by. I had flirted with using “…” as the indicator as it could be included in the text anyway, but that might not work for all languages. My inclination was to keep it as small as possible when displayed, printable ASCII, but also unlikely to be included in an English sentence.

The first issue I stumbled upon with this idea was that the underscore character could be included in the set of possible unique-ids. The command UIDL (n) returns a single line response with the message’s unique-id on the end as a single line response. Any servers implementing this idea would have to exclude underscores from their unique-ids.

The final nail in the coffin was when I took a step back and thought about the developers of POP3 client libraries. Would they make use of my extension?

No. Servers not implementing my new extension will still exist for a long time and people will still want to connect to those servers. As such, client libraries are still going to be passing a flag down to its command/response layer, indicating if the response is going to be multi-line or not. I won’t have saved the developer any effort.

My example implementation still includes this extension, for now.

Wait, there’s more!

Keepa your CAPA!

One thing that bothered me about reconnecting to a POP3 service was the necessity to call CAPA on reconnecting every time. Each time, the server would send the same response back. Wouldn’t it be nice if the client could store the response once and have some sort of notification if it needed to be checked?

My idea was to use the banner that the service sends immediately on connection, together with a new CAPA response.

S: Welcome to my POP3 service version 1.2.3.
C: CAPA
S: +OK Capabilities follow...
S: CAPA-VERSION 1.2.3.
S: UIDL
S: .

Because the CAPA response included this capability, the next time it connected, it could look in the connection banner and see the token “1.2.3.” included. With this, the client is assured that the response from last time is still good and the client need not ask for it again.

Even if the banner changed (which it would if it implemented APOP), so long as this one token was included, the response is still good.

I saw a problem that the response to a CAPA command might change in the course of a connection. The capabilities might be different after going through TLS. Different users might have different capabilities that only reveal themselves once you’ve logged in. Should the response to the PASS command also include a version token? What about other ways of logging in?

I tried to come up with RFC style wording that would have placed different CAPA responses into different domains, but it all got too complicated and too prone to error.

And for what? To save having to reissue a single command and brief response on top of all the TCP and TLS handshakes? Not worth it.

My demonstration implementation implements this capability.

Safe!

Pick up from where we left off.

The RETR command allows the client to download a message, while the TOP command allows the top part of a message. There’s a gap for a command that retrieves the end of a message. If you’ve downloaded part of a message but the connection broke, you could use this to resume from where you left off.

The END command would have worked in the same way as TOP. You select a message and the line you want to start from, and the server would continue from that point.

Selecting a line rather than a byte index was necessary. POP3 is line orientated protocol and any new commands would need to work within that restriction. If you selected a byte index instead, what if you wanted to start from the middle of a CRLF? What if the selected starting byte is a dot but in the middle of a line, should that be dot-padded?

I told myself that servers would need to keep track of what byte indexes each line started at, but that was an unsatisfactory answer.

It was about the same time as another idea. Email files are highly compressible thanks to their large blocks of base-64. I pictured an alternative form of RETR (RETZ?) that would return the +OK line normally, but the multi-line response would be inside a new GZIP stream. Inside that stream, the CRLF lines and dot-padding would still be present. A lone dot would complete the message as normal as the GZIP stream concludes and the normal uncompressed exchange of commands and responses continues.

It was at this point that I remembered there’s already a very well established protocol for downloading large files with compression, chunking, resumption and all that good stuff. HTTP. I’m still thinking about how that could work in practice but that road seems a lot more productive than trying to force HTTP into a POP3 shaped hole.

Picture Credits. (CC)
📷 “More Selvatiche” by Cristina Sanvito.
📷 “Fun with Cling Film” by Elizabeth Gomm.

Posted on

Write Your Own POP3 Service

So, you want to write a POP3 service? That’s great. In this post, we’ll walk through building a simple POP3 service that uses a folder full of EML files as a mailbox and serves them to anyone logging in.

Getting Started

I’m assuming you are already set-up to be writing and building C# code. If you have Windows, the free version of Visual Studio 2019 is great. (Or use a more recent version if one exists.) Visual Studio Code is great on Linux too.

Download and build billpg industries POP3 Listener. Open up a new console app project and include the billpg,POP3Listener.dll file as a reference. You’ll find the code for this project on the same github in its own folder.

using System;
using System.IO;
using System.Collections.Generic;
using System.Net;
using System.Linq;
using billpg.pop3;

namespace BuildYourOwnPop3Service
{
    class Program
    {
        static void Main()
        {
            /* Launch POP3. */
            var pop3 = new POP3Listener();
            pop3.ListenOn(IPAddress.Loopback, 110, false);

            /* Keep running until the process is killed. */
            while (true) System.Threading.Thread.Sleep(10000);
        }
    }
}

This is the bare minimum to run a POP3 service. It’ll only accept local connections. If you’re running on Linux, you may need to change the port you’re listening on to 1100. Either way, try connecting to it. You can set up your mail reader or use telnet to connect in and type commands.

Accepting log-in requests.

You’ll notice that any username and password combination fails. This is because you’ve not set up your Provider object yet. If you don’t set one up, the default null-provider just rejects all attempts to log in. Let’s write one.

/* Add just before the ListenOn call. */
pop3.Provider = new MyProvider();

/* New class separate from the Program class. */
class MyProvider : IPOP3MailboxProvider
{
}

This won’t compile because MyProvider doesn’t meet the requirements of the interface. Let’s add those.

/* Inside the MyProvider class. */
public string Name => "My Provider";

public IPOP3Mailbox Authenticate(
    IPOP3ConnectionInfo info, 
    string username, 
    string password)
{
    return null;
}

Now, the service is just as unyielding to attempts to log-in, but we can confirm our provider code is running by adding a breakpoint to the Authenticate function. Now, when we attempt to log-in, we can see that the service has collected a username and password and is asking us if these are correct credentials or not. Returning a NULL means they’re not.

This might be a good opportunity to take a look at the info parameter. All of the functions where the listener calls to the provider will include this object, providing you with the client’s IP address, IDs, user names, etc. You don’t have to make use of them but your code may find the information useful.

A basic mailbox with no messages.

We can change our Authenticate function to actually test credentials. For our play project we’ll just accept one combination of user-name and password. 

if (username == "me" && password == "passw0rd")
    return new MyMailbox();
else
    return null;

This will fail compilation because we’ve not written MyMailbox yet. Let’s go ahead and do that.

class MyMailbox : IPOP3Mailbox
{
}

Again, we’ll need to write all the requirements of the interface before we can run. So we can move on quickly, let’s provide just the minimum.

The first thing we’ll need is a list of the available messages. We’ll return an empty collection for now.

public IList<string> ListMessageUniqueIDs(
    IPOP3ConnectionInfo info)
    => new List<string>();

The service needs to know if a mailbox is read-only or not. Let’s say it isn’t.

public bool MailboxIsReadOnly(
    IPOP3ConnectionInfo info)
    => false;

The service might sometimes need to know is a message exists or not. For now, it doesn’t.

public bool MessageExists(
    IPOP3ConnectionInfo info,
    string uniqueID)
    => false;

The client might request the size of a message before it downloads it and the service will pass the request along to the provider. I’ve often suspected that clients don’t really need this so let’s just return your favorite positive integer.

public long MessageSize(
   IPOP3ConnectionInfo info, 
   string uniqueID)
   => 58;

The client will, in due course, request the contents of a message, but won’t because both the list-messages and message-exists will deny the existence of any messages, so for now, we can just return null.

public IMessageContent MessageContents(
    IPOP3ConnectionInfo info, 
    string uniqueID)
    => null;

Finally, we need to handle message deletion. Again, we don’t need to do anything just yet.

public void MessageDelete(
    IPOP3ConnectionInfo info, 
    IList<string> uniqueIDs)
{}

And we’re done. Run the code and log-in. Your mailbox will be perpetually empty but you can add breakpoints and confirm everything is running.

List the messages.

Now, let’s actually start with something useful. Let’s change our ListMessageUniqueIDs to return a list of filenames from a folder. You’ll want to replace the value of FOLDER with something that works for you.

const string FOLDER = @"C:\MyMailbox\";

public IList<string> ListMessageUniqueIDs(
    IPOP3ConnectionInfo info)
    => Directory.GetFiles(FOLDER)
           .Select(Path.GetFileName)
           .ToList();

public bool MessageExists(
    IPOP3ConnectionInfo info, 
    string uniqueID)
    => ListMessageUniqueIDs(info)
           .Contains(uniqueID);

Let’s also place an EML file into our mailbox folder. If you don’t have an EML file to hand, you can write your own using notepad. (It doesn’t care if the file has a “.txt” extension.) 

Subject: I'm a very simple EML file.
From: me@example.com
To: you@example.com

Message body goes after a blank line.

If we save that into our mailbox folder and run up the POP3 service, we’ll see there’s a message available. It won’t be able to download it though.

Download the message,

The MessageContents function expects an new object that implements the IMessageContent interface.

/* Replace the MessageContents function. */
public IMessageContent MessageContents(
    IPOP3ConnectionInfo info, 
    string uniqueID)
{
    if (MessageExists(info, uniqueID))
        return new MyMessageContents(
                       Path.Combine(FOLDER, uniqueID));
    else
        return null;
}

/* New class. */
class MyMessageContents : IMessageContent
{
    List<string> lines;
    int index;

    public MyMessageContents(string path)
    {
        lines = File.ReadAllLines(path).ToList();
        index = 0;
    }

    public string NextLine()
        => (index < lines.Count) ? lines[index++] : null;

    public void Close()
    {
    }
}

This shows the requirements of the object that regurgitates a single message’s contents. A function that returns the next line, one-by-one, and another that’s called to close down the stream. The Close function could close opened file streams or delete temporary files, but we don’t need it to do anything in our play project.

Note that the command handling code inside this library has an extension that allows the client to ask for a message by an arbitrary unique ID. Make sure your code doesn’t allow, for example, “../../../../my-secret-file.txt”. Observe the code above checks that the requested unique ID is in the list of acceptable message IDs by going through MessageExists.

Delete messages.

The interface to delete messages passes along a collection of string IDs. This is necessary because the protocol requires that a set of messages are deleted in an atomic manner. Either all of them are deleted or none of them are deleted. We can’t have a situation where some of messages are deleted but some are still there.

But since this is just a play project, we can play fast and loose with such things.

public void MessageDelete(
     IPOP3ConnectionInfo info, 
     IList<string> uniqueIDs)
{
    foreach (var toDelete in uniqueIDs)
        if (MessageExists(info, toDelete))
            File.Delete(Path.Combine(FOLDER, toDelete));
}

What now?

I hope you enjoyed building your very own POP3 service using the POP3 Listener component. The above was a simple project to get you going.

billpg.com

Maybe think about your service could handle multiple users and how you’d check their passwords. What would be a good way to achieve atomic transactions on delete? What happens if someone deletes the file in a mailbox folder just as they’re about to download it?

If you do encounter an issue or you have a question, please open an issue on the project’s github page.

Posted on

POP3 – Goodbye to numeric message IDs!

This is the second post in my series describing a number of extensions to the POP3 protocol. The main one is a mechanism to refresh an already opened connection to allow newly arrived messages to be downloaded, which I’ve described on a separate post. This one is a lot simpler in scope but if we’re doing work in this protocol anyway, this may as well come as a package.

I am grateful to the authors of POP4 for the original idea.

This post is part of series on POP3. See my project page for more.

Enumeration

To recap, when a client wishes to interact with a mailbox, it first needs to send a UIDL command to retrieve a list of messages in the form of pairs of message-id integers and unique-id strings. (I’ve written before how UIDL really should be considered a required command for both client and server.) 

C: UIDL
S: +OK Unique-ids follow...
S: 1 AAA
S: 2 AAB
S: 3 AAJ
S: .

The numeric message-ids are only valid for the lifetime of this connection while the string unique-ids are persistent between connections. All of the commands (prior to this extension) that deal with messages use the numeric message-ids, requiring the client to store the UIDL response so it has a map from unique-id to message-id.

This extension allows the client to disregard the message-ids entirely, modifying all commands that have a message-id parameter (RETR, TOP, DELE, LIST, UIDL) to use a unique-id parameter instead.

“UID:”

If the server lists UID-PARAM in its CAPA response, the client is permitted to use this alternative form of referencing a message. If a message-id parameter to a command is all numeric, the server will interpret that parameter as a numeric message-id as it always has done. If the parameter instead begins with the four characters “UID:”, the parameter is a reference to a message by its unique-id instead.

C: DELE 1
S: +OK Message #1 (UID:AAA) flagged for deletion.
C: DELE UID:AAB
S: +OK Message #2 (UID:AAB) flagged for deletion.

(The POP4 proposal used a hyphen to indicate the parameter was a unique-id reference. I decided against adopting this as it could be confused for a negative number, as if numeric message-ids extended into the negative number space. A prefix is a clear indication we’re no longer in realm of numeric identifiers and may allow other prefixes in future.)

If a client has multiple connections to a single mailbox, it would normally need to perform a UIDL command and store the response for each connection separately. If the server supports unique-id parameters, the client is permitted to skip the UIDL command unless it needs a fresh directory listing. Additionally, the client is able to use a multiple connections without having to store the potentially different unique-id/message-id maps for each connection.

RFC 1939 requires that unique-ids are made of “printable” ASCII characters, 33 to 126. As the space (32) is explicitly excluded, there is no ambiguity where a unique-id parameter ends, either with a space (such as with TOP) or at the end of the line.

Not-Found?

If a requested unique-id is not present, the server will need to respond with a “-ERR” response. To allow the client to be sure the error is due to a bad unique-id rather than any other error, the error response should inside a [UID] response code. (The CAPA response should also include RESP-CODES.)

S: RETR UID:ABC
C: -ERR [UID] No such message with UID:ABC.
S: RETR UID:ABD
C: -ERR [SYS/PERM] A slightly more fundamental error.

It should be noted that a [UID] error might not necessarily mean the message with this unique-id has been deleted. If a new message has arrived since this particular connection opened, the server may or may not be ready to respond to requests for that message. A client should only make the determination that a message has gone only if it can confirm it with either a new or refreshed connection.

Extensions yet to come

I’ve pondered about if this extension, once its written up in formal RFC language, should modify any future extensions that use message-id parameters. Suppose next year, someone writes a new RFC without having read mine that adds a new command “RUTA” that rutabagas the message specified on its command line.

(What? To rutabaga isn’t a verb? Get that heckler out of here!)

The wording could be: “Any command available on a service that advertises this capability in its CAPA response, that accepts a message-id parameter that is bounded on both sides by either the space character or CRLF, and normally only allows numeric values in this position, MUST allow a uid-colon-unique-id alternative in place of the message=id parameter.”

(In other words, this capability, only changes commands where a unique-id with a prefix can unambiguously be distinguished from a numeric message-id.

My inclination is for the RFC defining this capability exhaustively lists the commands it modifies to just the ones we know about. (RETR, TOP, DELE, LIST, UIDL.) I would add a note that strongly encourages authors of future extensions to allow UID: parameters as part of their standard. If someone does add a RUTA command without such a note, then strictly speaking, the client shouldn’t try and use a UID: parameter with the RUTA command, but probably will.

I’m on the fence. What do you think?

Restrictions

RFC 1939 that defines POP3, makes a couple of allowances with the UIDL command that would make UID: parameters problematic. A server is allowed to reuse unique-ids in a single mailbox, but only if the contents of two messages are identical. A server is also allowed to reuse a unique-id once the original message using that unique-id has been deleted.

Since these allowances would introduce a complication to which message is being referenced, any server advertising this capability (in RFC language) MUST NOT exercise these two allowances. If a server advertises UID parameters, it is also promising that its unique-ids really are unique.

Fortunately, all mail servers I’ve looked at can already make this promise, either they use a hash but add their own “Received:” header or they assign an incrementing ID to each incomming message.

billpg.com

Resources
New extension: Mailbox Refresh Mechanism
New extension: Delete Immediately
Prototype POP3 Service

Posted on

POP3 – A Commit and Refresh Mechanism

POP3 is a popular protocol for accessing a mailbox full of email messages. While small devices have moved their mail reading apps to IMAP and proprietary protocols, POP3 remains the preferred protocol for moving email messages between big servers where a no-frills, download-and-delete system is preferred.

A problem with this protocol is embodied in this question: “How often should we poll for new messages?” There’s a non-trivial overhead to connecting. Poll too quickly and you overload the system. Poll too far apart and messages take too long to arrive.

Over my head!

To recap, let’s take a look at what needs to happen every time a POP3 client wants to check for new messages.

  • The client and server handshake TCP as the underlying connection.
  • The client and server handshake TLS for security.
  • The client authenticates itself to the server.
  • The client finally gets to ask if there are any new messages.

“Oh, no new messages? Okay, I’ll go through all that again in five seconds.”

POP3 doesn’t have a way to avoid this continual opening and closing of connections, but it does have a mechanism to add extensions to the protocol. All it needs is for someone to write the new extension down and to develop a working prototype. Which I have done.

“I have no time for your prototype!”

billpg industries POP3 service

On my github account, you’ll find a prototype POP3 server that implements this extension. Download it, compile it, run it. Go nuts. The service is written using the “listener” model. You set it up to listen for incomming connections and it talks the protocol until you shut it down. The library deals with the complexities while requests for messages are passed onto your provider code.

You don’t need to write that provider code if you only want to try it out. I’ve written a basic Windows app where you can type in new messages into a form, ready for the client to connect and download them. If Linux is more your thing or you prefer your test apps to work autonomously, I’ve also written a command-line app that sits there randomly populating a mailbox with new messages, waiting for a client to come along and get them.

To Sleep, and Goodnight…

Now for the new extension itself. It comes in two parts, the SLEE command put a connection to sleep and the WAKE command brings it back.

If a server normally locks a mailbox while a connection is open, then SLEE should release that lock. In fact, SLEE is defined to do everything that QUIT does, except actually close down the connection. Crucially, this includes committing any messages deleted with DELE.

During a sleeping state, you’re no longer attached to the mailbox. None of the normal commands work. You can only NOOP to keep the underlying connection alive, QUIT to shut it down or WAKE to reconnect with your mailbox.

If the server responds to WAKE with a +OK response, a new session has begun. The refreshed connection needs to be viewed as if it is a new connection, as if the client had QUIT and reconnected. The numeric message IDs from before will now be invalid and so the client will need to send a new STAT or UIDL command to update them.

In order to save the client additional effort, the server should include a new response code in the +OK response.

  • [ACTIVITY/NEW] indicates there is are new messages in the mailbox that were not accessible in the earlier session on this connection.
  • [ACTIVITY/NONE] indicates there are no new messages this time, but it does serve as an indication that this server has actively checked and that it is not necessary for the client to send a command to check.
  • (No “ACTIVITY” response indicates the server is not performing this test and the client will need to send a STAT or UIDL command to retrieve this information.)

A server might give an error response to a WAKE command, which may include a brief error message. In this situation where a connection can’t be refreshed for whatever reason, a client might chose to close the underlying connection and open a new one.

The error might include the response code [IN-USE] to indicate that someone else is connected to the mailbox, or [AUTH] to indicate that the credentials presented earlier are no longer acceptable.

Note that the SLEE command is required to include a commit of DELE commands made. If the client does not want the server to commit message deletes, it should send an RSET command first to clear those out.

How would a client use this?

To help unpack this protocol extension, here is description of how the process would work in practice with a server that implements this extension.

The client connects to the server for the first time. It sends a CAPA request and the response includes SLEE-WAKE, which means this connection may be pooled later on.

S: +OK Welcome to my POP3 service!
C: CAPA
S: +OK Capabilities follow...
S: UIDL
S: RESP-CODES
S: SLEE-WAKE
S: .

The client successfully logs in and performs a UIDL which reveals three messages ready to be downloaded. It successfully RETRs and DELEs each message, one by one.

C: USER me@example.com
S: +OK Send password.
C: PASS passw0rd
S: +OK Logged in. You have three messages.
C: UIDL
(Remainder of normal POP3 session redacted for brevity.)

Having successfully downloaded and flagged those three messages for deletion, the client now sends a SLEE command to commit those three DELE commands sent earlier. The response acknowledges that the deleted messages have finally gone and the connection has entered a sleeping state.

C: SLEE
S: +OK Deleted 3 messages. Sleeping.

The client can now put the opened connection in a pool of opened connections until needed later. It is not a problem if the underlying connection is closed without ceremony in this state, but it may be prudent for the pool manager to periodically send a NOOP command to keep the connection alive and to detect if any connections have since been dropped.

C: NOOP
S: +OK Noop to you too!

Time passes and the client wishes to poll the mailbox for new messages. It looks in the pool for an opened connection to this mailbox and takes the one opened earlier. It sends a WAKE command to refresh the mailbox. 

C: WAKE
S: +OK [ACTIVITY/NONE] No new messages.

Because the server supports the ACTIVITY response code and the client recognized it, the client immediately knows that there is nothing left to do. The client immediately sends a second SLEE command to put right back into the sleeping state.

C: SLEE
S: +OK Deleted 0 messages. Sleeping.

(Incidentally, the client is free to ignore the “ACTIVITY” response code and instead send a STAT or UIDL command to make its own conclusion. It will need to do this anyway if the server does not include such a response code.)

More time passes and the client is ready to poll the mailbox again. As before, it finds a suitable opened connection in the pool and it sends another WAKE command.

C: WAKE
S: +OK [ACTIVITY/NEW] You've got mail.

Having observed the notification of new mail, the client sends a new sequence of normal POP3 commands.

This was all within one connection. All the additional resources needed to repeatedly open and close TCP and TLS are no longer needed.

billpg.com

I’ll be doing the job of turning all this informal text into a formal RFC on my github project.

Picture Credits:
📷 Keyboard Painting Prototype by Rodolphe Courtier.

Posted on

POP3 – UIDL is a required command!

RFC 1957 observes, discussing mail reading software that implements the popular POP3 protocol: “two popular clients require optional parts of the RFC. Netscape requires UIDL, and Eudora requires TOP.”

This reads like a complaint, but this tell me that Netscape’s mail reader (which these days is called Thunderbird) is well designed.

The rot started with  RFC 1939, the standard for this protocol. This document specifies that UIDL is optional. This was a mistake. Without UIDL, the protocol is not reliable. I write this in the hope of persuading you that UIDL should not only be considered a requirement for a POP3 server, but that any client software that doesn’t require UIDL should not be trusted. I’m looking at you, Eudora!

What is UIDL and how does it fit into POP3?

UIDL is the “directory listing” command in POP3. When a client issues this request, the server responds with a list of “unique-id” strings that may as well be considered file names.

Opening a POP3 connection, authenticating and performing a “directory listing”.

Each unique-id is paired with a numeric id, starting from 1. The other commands to download and delete messages all use these numeric ids. Each time the client reconnects, it will need to repeat the UIDL command so it knows which numeric ids refer to which messages.

For something as fundamental as a directory listing, it seems odd for that to be optional.

Without UIDL, the client needs to fall-back onto those numeric message ids alone. Instead of UIDL, the STAT command returns the number of messages in a mailbox. With that, the client can loop from 1 to n, downloading and deleting each one, leaving the mailbox empty once they have all been downloaded. As POP3 is explicitly designed for download-and-delete operation and not keeping the messages on the server, you might consider that UIDL is not necessary. So let us follow that road where we don’t have UIDL.

Living in a world without UIDL.

Operating POP3 without UIDL only works in an ideal world. If you had 100% reliable connections to the server then you might get away with it. Reality tells us the world is not ideal.

Let’s think about the step of deleting a message once you’ve downloaded it. You might think that DELE is the request to delete messages you’ve downloaded (or don’t want), but the request to actually delete messages is QUIT.

The client flags the messages to delete with DELE, but those deletes aren’t committed until the client later issues a QUIT request. If the connection stops before a QUIT, the server has to forget about those DELE commands and the messages all have to remain in the mailbox for when you reconnect. This is by design as you wouldn’t want your messages deleted if your client is in an unstable environment that can’t keep a connection open.

Consider though, what would happen if the underlying connection was dropped just as the client issued a QUIT request. You sent the request but no response came back.

Download and delete a single message, but the connection fails a critical point.

What happened? We don’t know. We can’t know. There are three reasonable possibilities…

  • The QUIT command never arrived at the server. The server just saw the connection drop.
  • The server couldn’t process the delete and responded with an error, which got lost.
  • The server successfully deleted the messages, but the response got lost.

You asked for some messages to be deleted, but you don’t know if your instruction was processed or not. The only way to find out is to reconnect (when you can) and see if the messages you asked the server to delete has gone or not.

Let’s say that time has passed and the client is finally able to reconnect to the server again. Last time, the client downloaded a single message and may or may not have deleted it. Now we’ve reconnected we find a single message in the mailbox. Is this the one we deleted before or a new one that’s arrived in the interim? A handy directory listing would be real useful right about now!

This is why I would mistrust any mail reading software that didn’t require that a mail server implements UIDL. Messages might get downloaded twice or wrongly deleted if the wrong assumptions are made.

“Come back!”

The alternatives to UIDL are all unreasonable.

If the above doesn’t convince you that UIDL is necessary, this section is to answer anticipated responses that UIDL is not necessary. Nuh huh!

(If you are already convinced and you don’t want to read my responses to anticipated arguments, you can skip this section.)

“That scenario you describe won’t ever happen in reality.”

Stage one: Denial.

Where is this perfect world where connections don’t stop working at the worst possible time? Where database updates happen instantly? I want to live there!

Think about what a server needs to do to process a QUIT command. Many flagged messages will need to be modified in an atomic transaction such that they won’t be included next time. Indexes will need to be updated and the dust needs to settle before the server can send its acknowledgement. During this time, the underlying TCP connection will be sitting there idle, looking just like a timeout error.

“We wouldn’t have a problem if mail servers were better engineered!”

Stage two: Anger

If your requirements of a mail server include underlying connections over the public internet that never fail, I think your requirements are a little unreasonable.

“So I occasionally see two copies of a message in my mailbox. Big whoop!”

Stage three: Bargaining.

If that started happening in software I was using, I’d file a bug report.

“There are other ways POP3 can resolve this issue.”

Stage four: Depression.

Alas, all of these alternatives that POP3 provide are unreasonable.

You could use the response to LIST as a fall-back? This command requests the size in bytes of each message. Most messages are long enough that they will have a unique size, but this isn’t reliable. Messages are often going to have the same size as others just by accident.

You could use TOP to retrieve just the header and extract something from that to track messages? Problem there is that no single header is a reliable identity. Two adjacent messages might have the same date or the same subject. The closest candidate for a suitable identity is Message-ID but this is generated by the sender, who might not include it or might reuse IDs. If we’re relying on the POP3 server to add them or modify duplicates provided by a sender, we’re back to relying on optional features.

You could use the TOP response and hash the entire header? This could work except message headers can change. I first saw this when experimenting with a mail server and observed that if I connected to a mailbox using IMAP, it would leave IMAP’s version of a unique identity in the header which wasn’t there before. As well as that, anti-spam systems might re-examine a mailbox’s contents and update the anti-spam or anti-virus headers. Any of these changes would look like a new message.

(As well as all that, TOP is itself an optional command, just like UIDL.)

You could download the entire message again and ignore it if you already have it? This would be ultimate fall-back. While I’ve seen headers change, the message body seems to be immutable. This is still an unreasonable situation. We’re downloading the whole message again, just because the server chose not to implement a simple directory-listing command.

Am I certain that the message body is immutable? No, not at all. If someone commented that mail server XYZ updates messages in the form of a MIME attachment, I wouldn’t be at all surprised.

Update – A digression on the Message-ID header

(Added 28/Jan/2021)
I am grateful to commenter “theamk” on Hacker News, who responded to me when I shared this post. To my dismissal of Message-ID as a means of de-duplication, they noted that RFC standards require that Message-IDs must be generated as unique.

I have experienced senders who have broken the protocol, sending many different messages with the same Message-ID. I do not argue these senders were in the wrong but that the POP3 server is not in a reasonable position to correct the situation.

If the server actively corrected the situation and replaced the reused Message-ID header with its own unique value, the message would not be a faithful reproduction of the message as sent any more and further damage any scope for auditing.

If the server discarded or rejected the message with a reused Message-ID, it would open up means for an attacker to predict the Message-ID a legitimate sender is going to use and send a message with that ID first, causing the legitimate sender’s message to be lost. There’s nothing stopping a sender from using someone else’s Message-ID pattern. (Maybe senders should use only unpredictable strings, but wishing it so won’t make it happen.)

This is also to say nothing of the situation when the messages served up don’t have any Message-ID, which I’ve seen happen with messages exchanged within the local server only. (IE. Not routed over the public internet’s mail servers.) None of the small number of services inside the box from the original composer to the POP3 delivery agent supplied a Message-ID when it was missing, so the message turned up with the basic To/From/Subject/etc headers and a Received header, but no Message-ID.

Acceptance?

Because the alternatives are so unreasonable, I consider UIDL a requirement for handling POP3. Servers that don’t implement UIDL are bad servers. Clients that can work without UIDL are unreliable.

Still not convinced? Please leave a comment where you saw this piece posted.

“I’ve seen the future! I’ve seen the future! I’ve seen the future and it’s now!”

IMAP does it wrong.

The other popular mail-reading protocol is IMAP. In contrast to POP3’s download-and-delete model, IMAP’s model is that messages to stay on the server and are only downloaded when the client wishes to read it. This model enables mail readers on low-storage devices such as smartphones.

With IMAP, the IDs are restricted to numeric values and always go upwards, in contrast to the free-for-all “any printable ascii except spaces” allowed by POP3. While this may be nice for the client, by requiring a single source of incrementing ID numbers, it complicates matters for anyone wishing to implement an IMAP server using a distributed database as a back-end.

But the worse thing about IMAP’s message identity system is that the standard permits the server to discard any IDs it has assigned by updating a mailbox’s UIDVALIDITY property. If this value ever changes, it is a signal to the client that any unique IDs it may have remembered are no longer valid.

A client needs a reliable way to identify messages between connections to recover from an unknown state. It does not need for servers to have a license to be unreliable.

If a mail server that implements IMAP wants any respect from me, it would document that its UIDVALIDITY value is fixed and will never change and that the unique-ids it generates are reliable.

POP3 does it wrong too.

If I’m going to criticize IMAP for flaws in its unique ID system, I should address flaws in POP3’s system too, having spent most of this article praising it.

Quoth RFC 1939: “The server should never reuse an unique-id in a given maildrop,” (good) “for as long as the entity using the unique-id exists.” (no!)

Consider that worst case scenario. The client flags a single message to be deleted and finally issues a QUIT command to complete the translation. The server successfully processes the request but the response to the client is lost. As far as the server is concerned, the message is gone and there’s no problem, but as far as the client knows, the continued existence of that message is unknown.

Now consider a new message arrives on the mail server and because the RFC says it can, it assigns the same unique ID to this new message as the one that was just deleted. The client eventually reconnects and requests the list of unique IDs and finds the ID of the message it wanted to delete is still there. It doesn’t know the server used its right to reuse unique IDs and that this is actually a new message!

Now, I’ve never seen a mail server actually reuse a unique ID. The clever people who have developed mail servers in the real world seem to understand that reusing IDs is not something you ever want to do, even if the RFC says you can.

RFC 1939 also says, “this specification is intended to permit unique-ids to be calculated as a hash of the message. Clients should be able to handle a situation where two identical copies of a message in a maildrop have the same unique-id.”

Unique IDs don’t have be unique? Ugh.

This allowance only applies to identical messages. In reality, messages are never identical. After bouncing around the internet and going through various anti-spam and anti-virus servers, messages do accumulate a frightening number of Received: headers left behind from each intermediate hand-over. Each one with a time-stamp and its own ID number. Any one of these is enough to produce a distinct hash.

Picture Credits. (All Creative-Commons licensed.)
“Listening to Radio Karnali” by “BBC World Service”.
“List 84” by “Weisbaden 2010”.
“The Time of Sunset” by Joy Sarah Nawati.
“Future” by “Legosz”.
“PuTTY screen-shots” by me.