MailNews:Minimizing Bandwidth Usage

Make Thunderbird more friendly for unreliable and low-bandwidth conditions.

Thunderbird currently has a number of features that could help in these conditions, such as 'offline folders', 'MIME parts on-demand' and caching. But these features do not always work well in both unreliable and low-bandwidth conditions or when combined together.

This wiki pulls together inter-related issues in one place for discussion and to propose a solution.

User Experience (UX)

The desired user experience in unreliable/low-bandwidth conditions could be described as:

Only download what I ask for and only download it once.

There are a number of bugs/features at the moment that are stopping this from happening, namely bug 405437, bug 345832 and bug 439731. More on these later.

Desired Behaviour

1) Once a message or part of a message (MIME part) has been downloaded it should always be stored locally:

• in offline storage if folder is __offline__ and storage rules allow (e.g. diskspace) OR
• in cache

2) A message should always be retrieved from local storage if possible.

Current Behaviour

NOTE: This refers to TB3, however currently the only difference is the new auto-sync feature in TB3.

1) For __offline__ folders:

• If auto-sync is on and message is not excluded (see strategies):
  • it is downloaded and stored automatically
• If auto-sync is off or message is excluded:
  • All non-inline MIME parts < ~30KB:
    • It is downloaded and stored only when viewed
  • One or more non-inline MIME parts > ~ 30KB:
    • MIME parts are downloaded on-demand and cached sometimes (there are constraints such as cache size)

For non __offline__ folders:

• Messages or MIME parts are downloaded and cached sometimes (there are constraints such as cache size)

2) Messages in offline store are retrieved from local storage. Cached MIME parts are retrieved from cache, however large MIME parts (i.e. attachments are rarely cached)

NOTE: Effectively this means that messages with large MIME parts (i.e. attachments) are not cached and not available offline.