Regions

Opensimulator: Documentation: Regions

Subjects related to regions.

Region sizes

Standard sized regions

The standard region size is a 256 meter square. This is the only size supported by Second Life.

Varregions

The varregion is based on the old megaregion concept but with completely rewritten code allowing for much more integration of the 256x256 m "blocks". The dimension limits (quadratic and multiples of 256 m) are the same as for a megaregion but for nearly all other practical purposes a varregion is treated by the simulator as a single unit.

Megaregions

The megaregion was introduced by [Aurora] and later adopted by other opensimulator forks.

A megaregion is essentially made from several standard regions merged into a single simulator with all region "blocks" sharing the same server resources and several other features and with no region borders between them. Since it's based on the standard 256x256 m region, the size of a megaregion has to be a product of 256 m and the region has to be quadratic with the same numebr of "blocks" along the x (west-to-east) and the y (south-to-north) axis.

The megaregion is an outdated concept, replaced by the varregion, but there may still be used by a few grids running on old versions of Opensimulator.

Initial connection to a region

The process by which a viewer connects to a region is rather complicated. It's necessary to accumulate information sent via multiple routes before the viewer can start up a new region.

For the initial region, the reply from the login server provides enough information to get the process started, but not enough to start up the region. Additional data needed include:

• The major capabilities, which are obtained by making an HTTP request of the seed capability, which is a URL answered by the region server.
• The region handshake reply, which is sent by the server to the viewer over the UDP connection, in response to a region handshake request.

Both of those requests must be answered before the region can be created in the viewer.

Login to the first region

The login process starts with an XMLRPC request to the login server. On a successful login, an XML set of key-value pairs is returned, which includes, at least:

seed_capability

the URL for the initial region used to query for more capability URLs.

secure_session_id

a unique UUID for the current session. Unclear what this is for.

circuit_code

a unique ID which identifies the UDP connection

sim_ip

the IP address of the initial region server. This is currently IPv4 only.

sim_port

UDP port, to be used with sim_ip, for receiving and sending UDP messages.

session_id

UUID of the current session.

agent_id

UUID of the avatar logging in. This is the avatar's permanent identity.

message

Message to display to the user at login. Usually a message of the day or some such.

first_name

First name of user/avatar.

last_name

Last name of user/avatar.

start_region [x, y]

world coordinates of the southwest (0,0) corner of the initial region.

look_at [x,y,z]

unit vector indicating the initial direction of view.

Note that this information does not contain the initial location of the avatar. That reaches the viewer via an object update to an object with the same UUID as the agent_id. An implication of this is that the viewer doesn't know where the avatar is at startup, yet it will have to send an agent update before it knows that. This causes a race condition in interest list updates.

Open Simulator logins for grids with varregions provide some additional info:

region_size_x

Region size, meters.

region_size_y

Region size, meters.

If these are absent, 256 is the default value.

Teleports

TBD

Adjacent regions

TBD

How viewers find out about adjacent regions

In both Second Life and Open Simulator, viewers are told about the regions adjacent to regions to which they are already connected. This information is sent over the event poller channel in LLSD format. At some time in the past it was sent as a UDP message (Enable Simulator, Low/151), but that message is now "blacklisted" in the Second Life UDP message_template.msg file.

The data sent in an EnableSimulator message is a subset of that in a login reply:

Handle

region origin in world coordinates, sent as a 64-bit region handle value, big-endian.

IP

IP address of simulator

Port

UDP port for simulator, sent as a 32-bit integer.

RegionSizeX

optional, Open Simulator only, region size in meters, sent as a 4-byte binary string, big-endian.

RegionSizeY

optional, Open Simulator only, region size in meters, sent as a 4-byte binary string, big-endian.

There is also an EstablishAgentCommunication message:

sim-ip-and-port

IP address and UDP port, in string format: "87.101.7.128:15272"

seed-capability

URL of the simulator's capability request server.

agent-id

UUID of agent involved

These fields all have the same meanings as in a login reply.

Once both of these messages have arrived, it is possible to begin talking to an adjacent region's simulator.

Region crossings

TBD