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| -rw-r--r-- | README | 86 |
1 files changed, 41 insertions, 45 deletions
@@ -7,53 +7,49 @@ LITERAL+ [RFC2088] and MULTIAPPEND [RFC3502], these extensions greatly improve performance by reducing the number of required round trips hence are recommended. - -Stateful synchronization is only possible for mailboxes supporting -persistent message Unique Identifiers (UID) and persistent storage of -mod-sequences (MODSEQ); any non-compliant mailbox will cause imapsync to -abort. Furthermore, because UIDs are allocated not by the client but by -the server, imapsync needs to keep track of associations between local -and remote UIDs for each mailbox. The synchronization state of a -mailbox consists of its UIDNEXT and HIGHESTMODSEQ values on each server; -it is then assumed that each message with UID < $UIDNEXT have been -replicated to the other server, and that the metadata (such as flags) of -each message with MODSEQ <= $HIGHESTMODSEQ have been synchronized. -Conceptually, the synchronization algorithm is derived from [RFC4549] -with the [RFC7162, section 6] amendments, and works as follows: - - - 1. SELECT (on both servers) a mailbox the current UIDNEXT or - HIGHESTMODSEQ values of which differ from the values found in the - database (for either server). Use the QRESYNC SELECT parameter from - [RFC7162] to list changes (vanished messages and flag updates) since - $HIGHESTMODSEQ to messages with UID<$UIDNEXT. - - 2. Propagate these changes onto the other server: get the corresponding - UIDs from the database, then a/ issue an UID STORE + UID EXPUNGE - command to remove messages that have not already been deleted on - both servers, and b/ issue UID STORE commands to propagate flag - updates (send a single command for each flag list in order the - reduce the number of round trips). (Conflicts may occur if the - metadata of a message has been updated on both servers with - different flag lists; in that case imapsync issues a warning and - updates the message on each server with the union of both flag - lists.) Repeat this step if the server sent some updates in the - meantime. Otherwise, update the HIGHESTMODSEQ value in the - database. - - 3. Process new messages (if the current UIDNEXT value differ from the - one found in the database) by issuing an UID FETCH command and for - each message RFC822 body received, issue an APPEND command to the - other server on-the-fly. Repeat this step if the server received - new messages in the meantime. Otherwise, update the UIDNEXT value - in the database. Go back to step 2 if the server sent some updates - in the meantime. - - 4. Go back to step 1 to proceed with the next unsynchronized mailbox. - - Consult the manual for more information. imapsync is Copyright© 2015 Guilhem Moulin ⟨guilhem@fripost.org⟩, and licensed for use under the GNU General Public License version 3 or later. See ‘COPYING’ for specific terms and distribution information. + +####################################################################### + + +IMAP traffic is mostly text (beside message bodies perhaps) hence +compresses pretty well: enabling compression can save a great amount of +network resources. + +However establishing a SSL/TLS connection (type=imaps, or type=imap and +STARTTLS=YES) yields a small overhead due to the SSL/TLS handshake. + +On the other hand if SSH access is allowed on the remote server, one can +tunnel the IMAP traffic through SSH and use OpenSSH's ControlPersist +feature to save most of the cryptographic overhead (at the expense of a +local 'ssh' process and a remote 'imap' process). Moreover if the IMAP +user is a valid UNIX user it is possible to use pre-authentication on +the remote server as well, which saves the extra round trip caused by +the AUTHENTICATE command. For instance the following configuration +snippet saves bandwidth and brings a significant speed gain compared to +type=imaps. + + local: $XDG_CONFIG_HOME/imapsync: + [remote] + type = tunnel + command = /usr/bin/ssh user@imap.example.net + + local: ~/.ssh/config: + Host imap.example.net + IdentityFile ~/.ssh/id-imapsync + IdentitiesOnly yes + ControlPath /run/shm/%u@%n + ControlMaster auto + ControlPersist 10m + StrictHostKeyChecking yes + ServerAliveCountMax 3 + ServerAliveInterval 10s + RequestTTY no + Compression yes + + remote: ~user/.ssh/authorized_keys: + command="/usr/lib/dovecot/imap",no-agent-forwarding,no-port-forwarding,no-pty,no-user-rc,no-X11-forwarding ssh-... id-imapsync |