Thunderbird:IMAP RFC 4551 Implementation

See RFC 4551 and bug 436151 for more details

This page describes my plan for implementing this RFC in the mailnews code.

Overview

This extension to the IMAP protocol is primarily intended for use by mobile devices but it will help Thunderbird users as well. It reduces the amount of data the client needs to sync every time a folder is selected. The server keeps track of modification sequence numbers for every change made to a folder. When a folder is selected, the server returns the highest modification sequence number for that folder. The client can then just do an incremental sync, based on the highest modification sequence number it is aware of. If those two numbers are the same, the client doesn't need to do any synchronization at all.

User Experience

This should all be transparent to the user. If their IMAP server supports this extension, selecting large folders should be quite a bit faster, especially over lower bandwidth connections.

We may need to provide a way for the user to turn off this feature, in case their server implementation has bugs in it. My fervent hope is that if this comes to pass, we can get by with a hidden pref.

How Syncing Works Today

When we open a folder, we list all the message UID's and flags in the folder, e.g.,

UID fetch 1:* (FLAGS)

The IMAP protocol code parses the responses and builds up the nsIImapFlagAndUidState object, which it then passes to nsImapMailFolder::UpdateImapMailboxInfo, via the nsIMailboxSpec argument. UpdateImapMailboxInfo then compares the uids and flags it knows about in the nsIMsgDatabase with the uids and flags the server knows about. It removes headers from the db that don't exist any more on the server, updates the flags of messages that it does know about, and tells the nsImapProtocol object about headers it doesn't know about, so the nsImapProtocol code can issue protocol to fetch those headers.