[Opensim-dev] new idea about simcrossing (maybe)

Tue Jul 22 05:48:21 UTC 2008

I was borered recently at the office and try to see how we can have a smooth
simcrossing.
I don't know how actual simcrossing works, but here is my tough :

The idea is using the concept used in GSM cellphone to handle handover
between two cells without droping the communication.

Concept:

The SimCrossing (SC) is based one several messages exchanged between the
object (Object), the actual sim (OriginSim) and the next sim (TargetSim).
The main exchange are located in the HandOverArea (HOarea) located on the
OriginSim. This area is computed by the viewer for each TargetSim (maximum 4
).

 Definition

Name

Description

Comment

Object (OBJ)

Any prims/avt rezzed InWorld that could trigged a SIMCROSSING event

   OriginSim (ORIGSIM)

Actual simulator where the object stand BEFORE simcrossing

   TargetSim (TARGET)

Simulator where the OBJECT could go

   ActiveSet

List containing all the neighborhoods sims and their data (location,ping…)

   HOarea

Logical area defined on the object level where all the messages exchange
should occured

See figure 1 for more details

 Definition HOArea

In order to have a smooth transition, we need to determine an HandOver area
where all the simcrossing messages exchange should happens. This area is
computed on the object level regarding the ping latency between the Target
and the object velocity. This area contains a RedZone area where all the
Simcrossing conditions should be satisfied, otherwise the simcrossing won't
happen.

Phase 1 : Login or rezzing

During the login phase, the object asks the surrounding sims list to its
actual simulator. We can call this list the ActiveSet .  The ActiveSet
contains the sim data and their "ping" time.

The object compute the SimCrossing boundaries and determine an "HandOver"
area with a Redzone area based on the timers (ping + delta). Normaly the
Handover should be ready before entering the Redzone.

*From*

*To*

*Message*

*Comment*

*Object*

Simulator

SCGetActiveSetReq

Ask the OrigSim a list containing all the neighborhood sims and their
coordinate, status…

*OrigSim*

Object

SCGetActiveSetSent

The Sim send all the data to the object

Once the ActiveSet received, the object start pinging all the TargetSims and
compute its own HOarea based on its velocity and the ping results. This
action should be performed on regular basis to update the ActiveSet.

 Phase 2:  Object logged and alive entering in the HandOver Area

On periodic basis, the object send a ping to each simulator on its activeset
and update their status  accordingly in order to keep an up to date
neighbored simulator list.

When the object enter in the HandOver area, a message "SimCrossing Request"
is sent to the Source simulator.  The simulator will relay the message to
the next Simulator (Target Sim) and will start downloading the object
context. Those data should include all the running code, the values of the
code's variables like a memory dump (heap,stack and such).

Case without problem in the HOarea

*From*

*To*

*Message*

*Comment*

*Object*

OrigSim

SCRequest

Contains the TargetSim Id.

*OrigSim*

TargetSim

SCObjReq

Send a simcrossing request to the target sim with all the credentials,
number of script running…

*TargetSim*

OrigSim

SCObjAck

If all the credentials and data are ok, the TargetSim sent that message.

*OrigSim*

Object

SCObjAck

The OrigSim send an ACK to the object.

*OrigSim*

TargetSim

SCObjDataSend

The OrigSim start sending the object's data to the TargetSim

*TargetSim*

OrigSim

SCObjDataACK

The TargetSim acknowledge the data and give a sim crossing clearance.

*TargetSim*

OrigSim

SCHoReady

Give the final clearance once the context is ready on the target sim.

 Case with problem

*From*

*To*

*Message*

*Comment*

*Object*

OrigSim

SCRequest

Contains the TargetSim Id.

*OrigSim*

TargetSim

SCObjReq

Send a simcrossing request to the target sim with all the credentials,
number of script running…

*TargetSim*

OrigSim

SCObjReqNAK

The TargetSim reject the request due to xxx reason.

*OrigSim*

Object

SCRequestNAK

The OrigSim sent a rejection message and the object can't perform the sim
crossing.

 In any case :

If the object reach the RedZone area before receiving *SCObjDataAck*, a
velocity slowdown should be trigged to allow the end of the data download.
We can eventually trigged an LSL state change (tbd).

If the object reach the end of the RedZone before receiving *SCHoReady* the
object stop the sim crossing and a major failure is trigged. This case
shouldn't happened.

 Case when the object didn't want to simcross

If the object leave the HOarea, a message is trigged to release all the data
on the TargetSim.

*From*

*To*

*Message*

*comment*

*Object*

OrigSim

SCabort

Send an abort message to the OrigSim

*OrigSim*

TargetSim

SCobjAbort

The OrigSim send this message to the TargetSim. The Targetsim clear the
previous object request data.

 Sim Crossing

When the object reach the simcrossing limit, the viewer switch to the Target
Sim and the TargetSim send an HandoverCompleted message to the SourceSim.

The SourceSim release the object context.

*From*

*To*

*Message*

*comment*

*OrigSim*

TargetSim

SCperforming

The OrigSim send the last actualized dump from the object to the target sim

*TargetSim*

OrigSim

SCperformed

Acknowledge the sim crossing. The simulator clear the object. The Ho is
completed
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