```
Filename: 303-protover-removal-policy.txt
Title: When and how to remove support for protocol versions
Author: Nick Mathewson
Created: 21 May 2019
Status: Open

1. Background

   With proposal 264, added support for "subprotocol versions" -- a
   means to declare which features are required for participation in the
   Tor network.  We also created a mechanism (refined later in proposal
   297) for telling Tor clients and relays that they cannot participate
   effectively in the Tor network, and they need to shut down.

   In this document, we describe a policy according to which these
   decisions should be made in practice.

2. Recommending features (for clients and relays)

   A subprotocol version SHOULD become recommended soon after all
   release series that did not provide it become unsupported (within a
   month or so).

   For example, the current oldest LTS release series is 0.2.9; when it
   becomes unsupported in 2020, the oldest supported release series will
   be 0.3.5.  Suppose that 0.2.9 supports a subprotocol Cupcake=1, and
   that all stable 0.3.5.x versions support Cupcake=1-3.  Around one
   month after the end of 0.2.9 support, Cupcake=3 should become a
   _recommended_ protocol for clients and relays.

   Additionally, a feature can become _recommended_ because of security
   reasons.  If we believe that it is a terrible idea to run an old
   protocol, we can make it _recommended_ for relays or clients or both.
   We should not do this lightly, since it will be annoying.

3. Requiring features (for relays)

   We regularly update the directory authorities to require relays to
   run certain versions of Tor or later.  We generally do this after a
   short outreach campaign to get as many relays as possible to upgrade.

   We MAY make a feature required for relays one month after every
   version without it is obsolete and unsupported, though it is better
   to wait three months if possible.

   We SHOULD make a feature required for relays within 12 months after
   every version without it is obsolete and unsupported.

4. Requiring features (for clients)

   Clients take the longest time to update, and are often the least able
   to fetch upgrades. Because of this, we should be very careful about
   making subprotocol versions required on clients, and should only do
   so for fairly compelling reasons.

   We SHOULD NOT make a feature required for clients until it has been
   _recommended_ for clients for at first 9 months.

   We SHOULD make a feature required for clients if it has been
   _recommended_ for clients for at least 18 months.


```
