  Overview of the TS5 system
  Lee H <lee@leeh.co.uk>

For the purposes of this document, ircd versions:
  hybrid6.0
  ircd-comstud-1.12
  CSr31pl4

and prior, are TS3.

ircd-hybrid-6.2 and later support TS5.

Whats TS5?
----------

The difference between TS5 and TS3 is what happened on opless channels.  TS
works by establishing which server has the oldest version of the channel,
the version that is oldest, keeps its modes and ops, the version that is
youngest, removes their modes and ops, and accepts the older version.

There was an exception to this rule with opless channels, if a channel was
opless, TS3 would allow anybody to keep their ops and modes on the channel.
TS5 aims to stop this, by removing this exception.

Example1: 

An irc network, with server A (every server is ts3)

UserA is on ServerA, in channel #broken.  This channel is opless, and has a
TS of 800000000.  ServerA splits, and whilst it is split, UserA cycles
channel #broken, recreates the channel and is given ops.  On ServerA #broken
now has a TS of 900000000 and has ops.  ServerA rejoins with the network,
via HubB.  HubB realises #broken is opless, so allows UserA to retain ops.
The TS is moved forward to 900000000.

The network now sees #broken as having a TS of 900000000, with UserA being
opped.

Example2:

An irc network, with server C (every server is ts5)

Same scenario as above.  ServerC splits and UserC cycles channel #broken,
recreating it with a TS of 900000000.  ServerC rejoins with the network via
HubD.  HubD realises #broken has a TS of 800000000 locally, and ServerC is
showing a TS of 900000000, it ignores ServerC's modes and ops.  The channel
remains opless.  ServerC receives HubD's modes, and it notices HubD has a
lower TS of channel #broken.  It removes UserC's ops, removes the channel
modes on #broken, and accepts HubD's status.

The network version of #broken hasn't changed.  It is still opless, with a TS
of 800000000.


As you can see, TS5 makes splitting a server to regain ops useless, as it
cannot be abused to give ops after a netsplit.

The problem with TS5 however, is what happens on a mixed TS5/TS3 network.
Channels where the older TS has ops will behave the same way on TS5 and TS3,
however an opless channel will behave differently, as you can see above.

The result of TS5/TS3 mixed can be a desync:

Example1:

As per Example1 above, except the rest of the network is TS5, ServerA is
TS3.  ServerA would keep its modes and ops, whilst the rest of the network
would remove them.  This means only ServerA would see UserA as opped.  The
desync can be abused, as UserA can send modes.  Hybrid6.0 servers will
accept these modes from the unopped client, so if UserA ops UserB, who then
ops UserA, the channel will be the same across all Hybrid6.0 and Hybrid6.1
servers.

Example2:

As per Example2 above, except the rest of the network is TS3.  ServerC is
TS5.  ServerC would remove its modes and ops, therefore UserC would not be
opped on ServerC, therefore it could not send any mode changes to the
channel.  Although it is opped elsewhere, it isn't opped locally, so the
desync cannot be abused.

As you can see, the desync's that can occur can either be resynced, or are
useless to the user, so a mixed TS5/TS3 network is not a huge problem,
although a desync is NOT a good thing to have.


Why TS5?
--------

We have jumped to TS5 from TS3, because there was a version of ircd that was
TS4, so it was thought better to avoid a clash with an existing version.


Advantages
----------

It's a realistic event that a server will be attacked so it splits off a
network, then used to regain ops in a channel.  TS5 makes this pointless,
the server will never give ops on a netsplit.  TS5 is network wide, so it
leaves individual servers free to choose options like NO_JOIN_ON_SPLIT,
whilst keeping splits useless to users.


Disadvantages
-------------

It's virtually impossible for a user to actively regain ops themselves (some
regard this as an advantage..) because on a large sized channel, its
impossible to get people to leave so it can be recreated, therefore if a
network did not have some form of services, it could possibly end up
requiring oper intervention, as you cant get everybody to leave, and you
cant use splits to regain ops, therefore if the channel is open (an
invite-only channel would gradually destroy itself as noone new can join) it
could be impossible for a user to regain ops.

On a network that has some form of services, The effect of TS5 would be
minimal, however the services must be of sufficient quality to fix opless
channels, as TS5 renders netsplits for ops worthless.


Recommendations
---------------

If your network has good stable services, we recommend TS5 is enabled, as
people have no reason to abuse netsplits anyway.

If your network has no services at all, then TS5 may cause problems with
users being left with a permanently opless channel.

If your network occupies the middle ground, then its a choice between users
needing to be able to use splits to regain ops, or making netsplits that are
caused to regain ops worthless.

If TS5 is chosen, the FULL network must upgrade and this should be done in a
relatively short space of time to minimise the possible desync effects.


Alternatives
------------

There is also NO_JOIN_ON_SPLIT and NO_OP_ON_SPLIT, however these use the
configuration of minimum servers and users, and sometimes a split that is
above these limits is enough to be abused to regain ops, whereas if the
limits are too high, clients will never be able to join anything or be opped
when they create a channel.
