What's in the Protocol?
[ This article appeared on my blog on April 13th, 2010, -Ed. ]
It's no secret VWRAP is heavily influenced by Second Life's next generation OGP protocol. One view of how the OGP effort became the VWRAP working group can be found in "Notes on the History of the VWRAP Working Group." It's not surprising many people believe every feature of Second Life™ will be found in the VWRAP protocol suite.
But this isn't the case; there will be many features of Linden's virtual world that won't be found in VWRAP. in this post, we're going to look at the VWRAP charter, a recent presentation by josh bell [PDF], and talk about why it's not a big deal some things are left out. But let's start with what's in and what's out.
What follows is a list of features that are widely believed to be "in" and "out." This is not an authoritative list; the feature list will only be official when the internet drafts describing VWRAP become RFCs. There's still time to influence the VWRAP protocol suite; join the email@example.com mailing list and make your voice heard.
What's In : Authentication and Trust
The first step a client application should take when dealing with the virtual world is to authenticate themselves. In VWRAP the authentication process begins with the presentation of credentials and ends with the client application receiving a "seed capability" or "seed cap." You can find more information about this process in the "VWRAP Trust Model and User Authentication" draft.
The seed cap is a randomly generated, difficult to guess web address (URL.) When queried it returns information about where to find other services (like how to place your avatar in a region or manipulate your inventory.)
Some members of the VWRAP mailing list have suggested that a virtual world open to anyone without authentication would be a productive use of the protocol. To support this use case, the authenticator in an authentication message is optional. The protocol does not require an authenticator like a hashed password to accompany a login message, but the virtual world you connect to might.
It's also been pointed out that using Transport Layer Security (TLS) client certificates, OAuth or even HTTP authentication may be useful in some situations. It may be more accurate to call the "authentication" phase more of an "identification with optional authentication" phase.
No matter what technique you use to successfully authenticate your client application to your authentication service, after it's over, the client has a seed capability and the authentication service has enough information to identify the client's user.
What's In : Inventory, Assets and Basic Permissions
Developers should expect a "streamlined" asset system. There is a requirement to maintain positive copy control on objects for users who want such control. But the existing inventory / asset system used in Second Life™ is starting to show it's age.
Assets in VWRAP will be represented as LLSD maps, retrieved using resources defined using the LLIDL interface description language, most likely over HTTP. An "asset" in VWRAP is a collection of meta-data and data. The "data" of an asset is tightly constrained by the asset's type: cubes are represented by cube data; textures with texture data; etc. Asset meta-data will identify the creator, describe base permissions, and include textual descriptions of the object. Assets will use URLs to reference other assets, and not UUIDs. for example, when a cube references a texture for each of it's faces, it will use URLs to point to the textures.
The SimianGrid project, which was born out of the Cable Beach Asset Server, has announced they will be participating in the development of the VWRAP inventory and asset system. Linden Research has not made a public statement of support of VWRAP inventory, but is expected to participate in it's development.
It should also be mentioned that regions could have region-specific assets. ground textures, the shape of buildings, etc. All might be stored in an asset server associated with the region and not with the user who created them.
What's In : Spatial, User-to-User and Group Text and Voice
Text chat in Second Life's legacy protocol is mildly broken. Spatial chat has reasonable performance, but uses the same protocol to carry text chat as to carry friend requests, inventory offers and such-like. Group chat performance is much worse, mostly the result of an arcane architecture involving routing chat messages though a simulator on the way to the user's viewer. Simplification and performance enhancement is expected in both spatial and group text chat.
VWRAP will define a service for spatial, user-to-user and group text chat. There has been significant interest in using XMPP (or other established IM protocols) to carry text chat. This is a natural fit for user-to-user and group text chat.
Voice is expected to work in radically the same way, but using SIP and RTP protocols instead of XMPP.
What's In : Basic Agent Information
This does not mean "profile info." Basic information would likely include very basic presence information like "this user is logged in" or "you can find this agent in this region." Basic agent information is there to support other protocol functions (like teleportation and retrieving avatar shape info.)
What's In : Teleport and Region Crossing
Teleportation has changed radically from the Second Life legacy protocol. The biggest change is the idea that an agent's presence information can potentially be stored in two locations. OGP experimented with the idea that agent-oriented services should be segregated from simulation-oriented services. So if you wanted to know what region a user's avatar was rezzed in, you would query a service associated with that user, not with the region.
Remember, in the OGP world, there may be several organizations that operate simulation regions. Querying each of them to ask, "is this avatar located here?" is sub-optimal.
This means that BOTH agent oriented services and region oriented services need to know where the user's avatar is. VWRAP is proposing extending the OGP Draft 5 Teleport specification to manage the intricate dance of moving an agent from one region to another.
What's In : Avatar Shape Information
We mentioned above that an agent's avatar will be available through the basic agent info service. it's unclear now what format avatar meshes will take. The existing Linden legacy format could be used. As could MPEG-V's ADML (avatar data markup language.) Or maybe both.
Whatever the format, a user's avatar will be made available to both client applications and simulation services.
What's In : the Scene Graph and Object Updates
When your avatar is rezzed into a region, the user's client application may pull a description of the scene graph. That is, the list of 3d objects observable in a given field of view.
There are several ways to communicate this information. For instance, the simulator could package up references to all items in the avatar's field of view and send it to the client as one large chunk. Or it could just start sending a stream of messages; one message per object.
In the past, object update messages have been carried over raw UDP. In the future, object updates may be carried over RTP or an HTTP event queue.
What's In : Object Presence and Physics Simulation
Objects in the virtual world persist. once they're rezzed, they should stay where they are until acted upon by an external force. "Object presence" services will allow a client to query objects rezzed in the world for information like internal state, an object control channel (if you have rights to control it), etc.
Objects that are subject to physical simulation may be moved or rotated by the simulation software running on the region they're rezzed in.
One of the fundamental features of VWRAP is that physics simulation happens centrally. That is, we don't do co-simulation where each client simulates physical interactions and then check with each other to see if any clients are out of sync.
What's In : Region Adjacency Information
Second Life regions are rectangles, 256 meters on a side in the x and y dimension and stretching to infinity along the z axis. Second Life itself is a collection of these regions arranged in a grid. VWRAP will likely NOT mandate this shape as the standard for virtual worlds.
Proposals have surfaced that describe the virtual world with spherical, cylindrical and toroidal coordinates. But even flat "grids" may have non-rectangular regions in the future. to help client applications figure out which systems to query for object information, a service is needed to describe the shape and partitioning of the virtual world.
This is not a map service per se, but something more fundamental. The virtual world is composed of multiple regions being stitched together to give the illusion of a seamless multi-dimensional space.
What's In : Object and Avatar Control
A virtual world would be pretty boring if you couldn't move or animate your avatar. VWRAP will provide a simple, extensible protocol for sending control information to your avatar. The same protocol may be used to control other objects; think of virtual vehicles you climb in and drive.
What's being left out of VWRAP is at least as interesting as what's going in. Here's a rundown of major Second Life features you won't find in the spec.
What's Out : Game Script
Do not look for linden dollars in the VWRAP drafts; you won't find them. Or at least you won't find them now. Linden expressed a preference that the linden dollar interface not be made part of the official specification. There are multiple reasons why this could be the case. VWRAP is still a technology under construction; a design flaw in VWRAP could lead to a very real vulnerability costing thousands if not millions of dollars. Linden also charges a fee for linden dollar transactions. It is possible they want to maintain control of the specification for purely mercantile reasons; to prevent VWRAP from letting third parties to wedge into the linden dollar exchange business.
Some OpenSim developers expressed a desire to prevent Linden from encroaching on their turf. Typical of their position is the statement, "why should i let Linden take a cut of my OpenSim based business?" The fear is that if a linden dollar API was included in the specifications, it would be considered mandatory. Many OpenSim grid operators would not accept a mandatory linden dollar economy.
This is one of the most obvious places where business interest meets standards development, and it will be interesting to see what happens in the future. Maybe Linden will open up a private VWRAP-like API for trusted partners. If you're an OpenSim operator not focused on eCommerce, this may be something you would be interested in. Maybe third parties like two fish / live gamer will produce an alternative specification? It will certainly be interesting to watch the market develop.
What's Out : Land Ownership
Before I start any rumors, I should say that Linden is NOT going to eliminate private land ownership on it's grid. Land ownership is not part of the VWRAP protocol suite because it's expected that different virtual worlds will manage land ownership differently.
VWRAP clients are expected to be able to query a region for "information about the place my user's avatar is at right now" and get a URL to a human readable, HTML web page. If the region supports land ownership, the web page will list that info. But the land ownership schema from Second Life will not be forced on all VWRAP virtual worlds.
This may be an issue for viewer developers. Different protocols for land management will require client developers to know how to query a region for information about if and how ownership is represented and how to present it to the user.
What's Out : Building
So far there's been no mention of building as a use case for VWRAP. This doesn't mean it won't be supported, but it may be that building is handled by way of the generic object and avatar control protocol.
What's Out : Media Attached to Prims
Second Life has had "parcel media" for a while and recently added the ability to add media to a prim face. To date, this use case hasn't been discussed at length in the VWRAP working group. This is not to say it's not important, but it may be that the participants are happy to develop this standard later; perhaps after seeing what happens with Linden's "media on a prim" feature.
What's Out : Detailed Avatar and User Profiles
Second Life users are likely familiar with the standard user profile. Imagine the ability to extend your profile to include information YOU think is important. This is what we're hoping to achieve by moving the profile information out of the core protocol and rebuild it as a simple web application. In the future, expect seeing a HTML page with protocol information instead of a fixed "floater" window in your favorite viewer. About the only thing the core protocol will specify is how to find the URL to the HTML profile page.
It sounds scary. but trust me; it'll be better.
Why the Sky is Not Falling
Some Second Life™ users may have an uneasy feeling, hearing that important features like game script, building and profile management WON'T be in the VWRAP protocol.
Just because a feature is not in the VWRAP specification, it doesn't mean that feature will disappear from Second Life or any other virtual world. What it means is that Linden and the OpenSim developers may diverge on how they want to implement those features.
The lack of a feature in the specification means only that the participants could not find consensus on the feature's functionality. VWRAP will be complete enough to provide a rich experience. By agreeing to core functionality, it frees us to experiment with solutions for other, high-margin features.
It will, however, make client application coder's lives a little more complicated. where Second Life and OpenSimulator diverge, viewer developers will have to know how to work with both systems. This isn't as bad as it might seem; Linden's stock viewer already uses well known techniques to cope with protocol divergence. But more on that later...
In conclusion, let me encourage readers to subscribe to the vwrap mailing list where these specifications are being discussed. What to leave in and what to take out will certainly be the subject of continuing discussions. If you don't speak up now, you don't get to complain later.