%%% title = "Game State over Real Time Protocol" abbrev = "Game State over RTP" ipr= "trust200902" area = "applications" workgroup = "" keyword = ["realtime","game"]

[seriesInfo] status = "standard" name = "Internet-Draft" value = "draft-jennings-dispatch-game-state-over-rtp-01" stream = "IETF"

[[author]] initials="C." surname="Jennings" fullname="Cullen Jennings" organization = "cisco" [author.address] email = "fluffy@iii.ca" [author.address.postal] country = "Canada"

[[author]] initials="R." surname="Logan" fullname="Rich Logan" organization = "Cisco" [author.address] email = "rilogan@cisco.com" [author.address.postal] country = "United Kingdom"

%%%

.# Abstract

This specification defines an Real Time Protocol (RTP) payload to send game moves and the state of game objects over RTP. This is useful for games as well collaboration systems that use augment or virtual reality.

RTP provide a way to synchronize game state between players with robust technique for recovery from network packet loss while still having low latency.

{mainmatter}

Overview

Many real time applications, such as games, want to to share state about 3D objects across the network. This specification allows an application to define objects with state, and the current values of that state over RTP [@!RFC3550].

The conceptual model is each RTP sender has a small number of objects with state that needs to be synchronized to the other side. The current values are periodically sent over RTP. They MAY be sent when the values change, but they MUST also be sent periodically so that any lost updates are eventually received and state is consistent between the sender and receiver.

The state sent can include a time stamp and rate change estimates that allow the receiver to estimate the current state values even at a point in the future. An application that receives a state update can apply it immediately (often called immediate based), wait a fixed delay and then apply state changes (often called delay based), or apply a predicted value based on overwriting any previous predictions (often called rollback based).

In many cases the state does not have any units but if does, SI units SHOULD be used. Unless otherwise defined by the application, the default coordinate system SHOULD be a left handed coordinate system where Y points up and X points to the right.

Applications can define their own objects or use some of the predefined common objects. Each object is identified by a type and identifier number to uniquely identify the object within the scope of that sender's RTP stream. Multiple updates and objects can be combined in a single RTP packet so that they are guaranteed to be fate shared and either atomically delivered at the same time or not delivered at all to the receiver.

The objects are defined as a series of primitives that define common types. The objects and updates to state are encoded with Tag Length Value (TLV) style encoding so that receivers can skip objects they do not understand. The Objects in an single RTP packet MUST be processed in order. This allows a sender to write state in an old format followed by a new format, allowing the new format to override values in the old format. This allows for easy upgrade of the protocol with backwards compatibility.

Goals:

Support 2D and 3D
Support delay and rollback based synchronization
Relatively compact, simple encoding
Extensible for applications to send custom data
Support for forward and backwards compatibility

Primitives

This section defines primitives that are useful in defining objects. The definition are in W3C style EBNF [https://www.w3.org/TR/2010/REC-xquery-20101214/#EBNFNotation].

Location

Loc1 ::=
 Float32 /* x */
 Float32 /* y */
 Float32 /* z */

Loc1 is simply a 3D location of a point stored in Float32;

Loc2 ::=
 Float32 /* x */
 Float32 /* y */
 Float32 /* z */
 Float16 /* vx */
 Float16 /* vy */
 Float16 /* vz */

Loc2 has a location as Float32 followed by the rate of change in location per second as Float16.

Scale

Scale1 ::=
Float16 /* all dimensions */

Scale1 will scale the object in all dimensions equally.

Scale2 ::=
 Float32 /* x */
 Float32 /* y */
 Float32 /* z */
 Float16 /* vx */
 Float16 /* vy */
 Float16 /* vz */

Scale2 has a scale in each axis as Float32 followed by the rate of change in the scale per second as Float16.

Normal

Norm1 ::=
 Float16 /* nx */
 Float16 /* ny */
 Float16 /* nz */

Normal vector for a point.

TextureUV

TextureUV1 ::=
 Float32 /* u */
 Float32 /* v */

Location in texture map for a point.

Rotation

Rot1 ::=
 Float16 /* i */
 Float16 /* j */
 Float16 /* k */

The non-real parts of a normalized rotation quaternion. The real part can be computed based on how it is normalized.

Rot2 ::=
 Float16 /* s.i */
 Float16 /* s.j */
 Float16 /* s.k */
 Float16 /* e.i */
 Float16 /* e.j */
 Float16 /* e.k */

Rot2 defines the s, the current rotation, and e, an estimated value of the rotation in one second. The rotation e is chosen such that rotation of the object will follow a rotation along the great circle path from s to e with an constant angular rotation rate such that it would reach e in 1 second.

This representation of rate of change of the rotation allows the receiver to use an algorithm such as SLERP [https://en.wikipedia.org/wiki/Slerp] to estimate the current rotation for the object for short periods of time into the future. The representation cannot represent something that is rotating faster than one revolution ever 2 seconds.

Open Issue: Would there be a better way to represent angular velocity?

Child Transform

Transform1 ::=
 Float16 /* tx */
 Float16 /* ty */
 Float16 /* tz */

Defines a linear translation of a child object from a base object.

Texture URL

TextureUrl1 ::= String

URL of image with texture map. JPEG images SHOULD be supported.

Mesh URL

MeshUrl1 ::= String

URL of external mesh.

Open Issue: Any mandatory to implements mesh formats?

Texture Stream

In some cases it is desirable to provide a separate RTP video stream which might have the texture used be the frame from the video stream with the corresponding time to the game object that uses the texture map. To do this there needs to be a way to identity the other RTP video stream. One way to do that is use the "Payload Type" value used for the RTP packets for that video stream.

TextureRtpPT1 ::= UInt8 /* pt */

RTP "Payload Type" value of RTP video stream to use as a texture map.

Open Issue: Is there a better way to identity video stream?

Timestamp

Time1 ::= UInt16 /* time in ms */

Lower 16 bits of number in milliseconds since 00:00:00 UTC on 1 January 1970, not counting leap seconds. The assumption is these timestamps are accurate to about the level of an NTP synchronized clock. In C++20 this can be found with:

duration_cast<milliseconds>(
      system_clock::now().time_since_epoch()
    ).count() % 65536

Objects

All objects must start with a unique tag that defines the object type, a VarUInt length, a VarUInt objectID, and the data for object. Applications can reserve tags for objects in the registry defined in the IANA section. Objects can be made extensible by adding a section that contains optional tag, length, value tuples.

Common Objects

Generic Game Object

It is common to need to describe the location, rotation, scale, and parent for objects in a scene.

Object1 ::= tagObject1 Length ObjectID Time1
 Loc1
 Rot1
 Scale1
 Boolean /* Active */
  ( tagParent1 Length ObjectID )? /* Optional Parent */

Object1 contains a simple 3D location, rotation, scale and an optional ID of a parent object.

Object2 ::= tagObject2 Length ObjectID Time1
 Loc2
 Rot2
 Scale2
 Boolean /* Active */
 ( tagParent1 Length ObjectID )? /* Optional Parent */

Object2 has the same information but with the ability to scale differently in each dimension and derivatives that describe how all the parameters change over time.

Player Head

Head1 ::= tagHead1 Length ObjectID Time1
  Loc2 Rot2
  ( tagHeadIpd1 Length Float16 /* IPD */ )?

Defines location and rotation of head with optional interpupillary distance (IPD).

Mesh

Object with the following variable length arrays:

Mesh1 ::= tagMesh1 Length ObjectID
 ( TextureUrl1 | TextureRtpPT1 )
 VarUInt /* num Vertexes */
 Loc1+ /* vertexes */
 VarUInt /* numNormals */
 Norm1* /* normals */
 VarUInt /*  numTextureCoord */
 TextureUV1* /*  textureCoord */
 VarUInt /* numTrianglesIndex */
 VarUInt+ /* trianglesIndex */

The vertex is an array of at least 3 locations that defines the vertex of a triangle mesh. The normals array can either be empty or the same size as the vertex and defines the normal for each vertex. The uv array must be empty or the same size as vertex array and have the u,v coordinate in the texture map for the vertex.

The texture can be defined by a URL that may refer to some local resource or a resource retrieved over the network. Alternatively, the texture can reference a local RTP video stream, in which case the most recently received frame of video is used as the texture and texture updates with new frames of video.

The triangles array can be of a different size from the vertex array. Each entry defines one triangle in the mesh and contains the index of the three vertexes in the vertexes array. Vertexes MUST be in counter clockwise order.

An important limitation to note is that objects cannot span RTP packets so the Mesh needs to be small enough that it size is less that the MTU. A typical limit might be as low as 50 triangles.

External Mesh

The Mesh2 object allows a mesh to be loaded from an external URL and then moved, rotated, and scaled. An optional texture map may be used.

Mesh2::= tagMesh2 Length ObjectID
  Loc2 Rot2 Scale2
  MeshUrl1
  ( TextureUrl1 | TextureRtpPT1 )? /* Optional Texture Map */
  ( tagParent1 Length ObjectID )? /* Optional Parent */

Player Hand

Hand1 ::= tagHand1 Length ObjectID Time1
 Boolean /* left */
 Loc2 Rot2

The Hand1 identifies a location and rotation of a hand. The left is true for the left hand, false for a right hand.

Hand2 ::= tagHand2 Length ObjectID Time1
 Boolean /* left */
 Loc2 Rot2
 Transform1 /* wrist */
 Transform1 /* thumbTip */
 Transform1 /* thumbIP */
 Transform1 /* thumbMCP */
 Transform1 /* thumbCMC */
 Transform1 /* indexTip */
 Transform1 /* indexDIP */
 Transform1 /* indexPIP */
 Transform1 /* indexMCP */
 Transform1 /* indexCMC */
 Transform1 /* middleTip */
 Transform1 /* middleDIP */
 Transform1 /* middlePIP */
 Transform1 /* middleMCP */
 Transform1 /* middleCMC */
 Transform1 /* ringTip */
 Transform1 /* ringDIP */
 Transform1 /* ringPIP */
 Transform1 /* ringMCP */
 Transform1 /* ringCMC */
 Transform1 /* pinkyTip */
 Transform1 /* pinkyDIP */
 Transform1 /* pinkyPIP */
 Transform1 /* pinkyMCP */
 Transform1 /* pinkyCMC */

Hand2 represents a wired skeletal hand. The boolean is true for the left hand. The location should represent the location of the palm and rotation is from the palm facing the x axis.

The transform points represent the relative location to the main joints in the fingers from the hand location. The location of the wrist is followed by finger joints. The fingers are ordered by thumb, index, middle, ring, then pinky. The joints are ordered by tip of finger (TIP), distal interphalangeal joint (DIP), proximal interphalangeal joint (PIP), metacarpophalangeal joint (MCP), then carpometacarpal joint (CMC). Note the thumb has no middle phalange so the PIP and DIP joint just become the interphalangeal joint (IP).

Note: The Microsoft documentation calls the MCP "knuckle" for the fingers and PIP "middle joint" and the CMC is called "Metacarpal", which is very confusing since this is not the metacarpophalangeal joint. The MCP for the thumb gets called "proximal", not knuckle, and the IP is "middle".

Names of the joints are explained in [https://en.wikipedia.org/wiki/Interphalangeal_joints_of_the_hand]

This is about 175 bytes, so at a 5Hz update rate this will be around 10 Kbps. [TODO: Check.]

Common Controls

Three Degree of Freedom (3DOF) Controller

Defines the location and orientation of a 6DOF controller.

ThreeDOF1 ::= tag3DOF1 Length ObjectID Time1
  bool isLeft
  Rot2

For systems with two controls, the isLeft indicates if it is the left or right controller.

Six Degree of Freedom (6DOF) Controller

Defines the location and orientation of a 6DOF controller.

SixDOF1 ::= tag6DOF1 Length ObjectID Time1
  bool isLeft
  Loc2 Rot2
  ( tag6DOFpointer1 Loc1 /* interrsection point */ )?

For systems with two controls, the isLeft indicates if it is the left or right controller. The intersection point can be used for or systems where the controller can indicate the location that a controller is pointing at.

Game Controller

Transfers the current values and time the value last changed for a left and right analog stick with x and y values ranging from -1.0 to 1.0 and the state of several buttons along with the time a button state last changed.

Bit	Button
1	Menu
2	View
3	A
4	B
5	X
6	Y
7	DPadUp
8	DPadDown
9	DPadLeft
10	DPadRight
11	LeftShoulder
12	RightShoulder
13	LeftStickButton
14	RightStickButton
15	LeftTrigger
16	RightTrigger
17	LeftShoulder
18	RightShoulder
19	Z
20	Pause

Sony style buttons for Square, Triangle ,Cross, Circle map to X,Y,A,B and R1,R2,L1,L2 map to RightShoulder, RightTrigger, LeftShoulder, LeftTrigger.

GameControl1 ::= tagGameControl1 Length ObjectID Time1
   VarInt Time1 /* buttons state and last time state changed */
   Float16 Float16 /* left stick x,y */
   Float16 Float16 /* right stick x,y */

Encoding

Each RTP payload will contain one or more objects. An object cannot be split across two RTP packets. The general design is that if the decoder has not been coded to understand a given object type, the decode can skip over the object to the next object but will not be able to provide any information and the internal format of the data.

The objects are defined such that they always start with a tag that indicates the type followed by a length of the object (so it can be skipped). Any optional or variable parts of the object also use tags so that the decoder can always be implemented as a LL(1) parser.

In general, network byte order encoding is used on the wire.

A length field is encoded to represent the number of bytes following the length field and does not include the size of the length field or information before it.

Tag

Constant values of tags can be found in the IANA section. They are encoded as VarUInt.

Float

Float16, Float32, and Float64 are encoded as IEEE 754 half, single, and double precisions respectively.

The half precision are often useful for things where only a few significant digits are needed such as normals. The internal representation of them will often be single precession (four bytes) in memory but they can be reduced to two bytes when encoded on the wire.

Note there is an example decode for a single precision float at [https://datatracker.ietf.org/doc/html/rfc7049#appendix-D]

Boolean

Encoded as byte with 0 for false or 1 for true.

Integer

UInt8, Int8, UInt16, Int16, UInt32, Int32, UInt16, Int64 encoded as 1, 2, 4, or 8 bytes.

VarInt is encoded as:

Top bits of first byte is 0, then 7 bit signed integer (-64 to 63)
Top bits of first byte is 10, then 6+8 bit signed integer (-8192 to 8191)
Top bits of first byte is 110, then 5+16 bit signed integer (1,048,576 to 1,048,575)
Top bits of first byte is 1110,0001 then next 4 bytes 32 bit signed integer
Top bits of first byte is 1110,0010 then next 8 bytes 64 bit signed integer

VarUInt is encoded as:

Top bits of first byte is 0, then 7 bit unsigned integer
Top bits of first byte is 10, then 6+8 bit unsigned integer
Top bits of first byte is 110, then 5+16 bit unsigned integer
Top bits of first byte is 1110,0001 then next 4 bytes 32 bit unsigned integer
Top bits of first byte is 1110,0010 then next 8 bytes 64 bit unsigned integer

String

Strings are encoded as a VarUInt length in bytes (not characters) followed by a UTF-8 representation of the string.

Blob

Blobs are encoded as a VarUInt length in bytes followed by the binary data that goes in the blob.

Full Intra Request

RTP supports a Full Intra Request (FIR) Feedback Control feedback messages. When an RTP sender receives a FIR, it SHOULD send a copy of all the relevant game state.

RTP and SDP

The media type is application/gamestate and defined in Section (#sec-iana-media). There are no optional or required parameters. The RTP marker bit is not used. The RTP clock MUST be 90 kHz.

An example m= and a= line in SDP could look like:

   m=application 49170 RTP/AVP 98
   a=rtpmap:98 gamestate/90000

Multiple Objects as defined in this specification can be concatenated into one RTP payload.

No storage format is defined.

IANA {#sec-iana}

Media Type Registration {#sec-iana-media}

This specification defines a new media type using the template defined in [@!RFC4288] and following the guidance provided in [@?RFC4855].

   Type name: application
   Subtype name: gamestate
   Required parameters: None.
   Optional parameters: None.
   Encoding considerations: This media is sent as binary data.
   Security considerations: 
        (IANA point to security section of this RFC)
   Interoperability considerations: None.
   Published specification: 
        (IANA to replace with RFC number for this specification)
   Applications that use this media type: N/A
   Fragment identifier considerations: N/A
   Additional information: None
   Person & email address to contact for further information: 
        Cullen Jennings <fluffy@iii.ca>
   Intended usage: COMMON
   Restrictions on usage: N/A
   Author: Cullen Jennings <email:fluffy@iii.ca>
   Change controller: IESG
   Provisional registration? (standards tree only): No

Game State Tag Registry {#sec-iana-game-tags}

The specification defines a new IANA registry called "Game State Type Registry" for tag values. All values MUST be greater than zero.

Values 1-127 are assigned by "IETF Review" as defined in [RFC8126], and should only be used when size is critical, the object is small, and will be used frequently.

Values 127-16383 are assigned by "Specification Required" as defined in [RFC8126].

Values 16384 to 2,097,151 are "First Come First Served" as defined in [RFC8126].

Initial assignments are:

TagName	Value
tagInvalid	0
tagHead1	1
tagHand1	2
tagObject1	3
tagParent1	4
tagMesh1	128
tagHand2	129
tagHeadIPD1	130
tagObject2	131
tagMesh2	132
tagGameControl1	133
tag3DOF1	134
tag6DOF1	135
tag6DOFpointer1	136

Security {#sec-security}

Like most things in RTP, the data can be personal identifying information. For example, the Hand2 type of data when generated by tracking a persons hand might identify that user.

{backmatter}

Acknowledgments

Thanks to Paul Jones for comments and writing an implementation.

Implementations

A C++ open source implementation is available at:

https://github.com/cisco/gse

Test Vectors

Head Location

Head Location type 1 with head at location 1.1, 0.2, 30.0, no rotation (so quaternion 0, 0, 0, 1) and not rotating, an objectID of 4, a time of 5 ms after epoch and an IPD of 0.056.

Field	Type	Value	Hex
head1	Tag	1	0x01
len	VarInt	33	0x21
objID	VarInt	0	0x00
time	UInt16	5	0x0500
loc.x	Float32	1.1	0x3F8CCCCD
loc.y	Float32	0.2	0x3E4CCCCD
loc.z	Float32	30.0	0x41F00000
loc.vx	Float16	0.0	0x0000
loc.vy	Float16	0.0	0x0000
loc.vz	Float16	0.0	0x0000
rot.s.i	Float16	0.0	0x0000
rot.s.j	Float16	0.0	0x0000
rot.s.k	Float16	0.0	0x0000
rot.e.i	Float16	0.0	0x0000
rot.e.j	Float16	0.0	0x0000
rot.e.k	Float16	0.0	0x0000

Encode API

API that take a high level representation of each object where types are all float or int and returns a memory buffer.

Decode API

API that takes binary data and set up objects and updates the objects and returns object ids that were updated.

For each object, an API to get the predicted values at a given time.

EBNF

Head1 ::= tagHead1 Length ObjectID Time1 Loc2 Rot2
  ( tagHeadIpd1 Length Float16 /* IPD */ )?

Mesh1 ::= tagMesh1 Length ObjectID
 ( TextureUrl1 | TextureRtpPT1 )
 VarUInt /* num Vertexes */
 Loc1+ /* vertexes */
 VarUInt /* numNormals */
 Norm1* /* normals */
 VarUInt /*  numTextureCoord */
 TextureUV1* /*  textureCoord */
 VarUInt /* numTrianglesIndex */
 VarUInt+ /* trianglesIndex */

Mesh2::= tagMesh2 Length ObjectID
  Loc2 Rot2 Scale2
  MeshUrl1
  ( TextureUrl1 | TextureRtpPT1 )? /* Optional Texture Map */
  ( tagParent1 Length ObjectID )? /* Optional Parent */

Object1 ::= tagObject1 Length ObjectID Time1
 Loc1
 Rot1
 Scale1
 Boolean /* Active */
  ( tagParent1 Length ObjectID )? /* Optional Parent */

Object2 ::= tagObject2 Length ObjectID Time1
 Loc2
 Rot2
 Scale2
 Boolean /* Active */
 ( tagParent1 Length ObjectID )? /* Optional Parent */

Hand1 ::= tagHand1 Length ObjectID Time1
 Boolean /* left */
 Loc2 Rot2

Hand2 ::= tagHand2 Length ObjectID Time1
 Boolean /* left */
 Loc2 Rot2
 Transform1 /* wrist */
 Transform1 /* thumbTip */
 Transform1 /* thumbIP */
 Transform1 /* thumbMCP */
 Transform1 /* thumbCMC */
 Transform1 /* indexTip */
 Transform1 /* indexDIP */
 Transform1 /* indexPIP */
 Transform1 /* indexMCP */
 Transform1 /* indexCMC */
 Transform1 /* middleTip */
 Transform1 /* middleDIP */
 Transform1 /* middlePIP */
 Transform1 /* middleMCP */
 Transform1 /* middleCMC */
 Transform1 /* ringTip */
 Transform1 /* ringDIP */
 Transform1 /* ringPIP */
 Transform1 /* ringMCP */
 Transform1 /* ringCMC */
 Transform1 /* pinkyTip */
 Transform1 /* pinkyDIP */
 Transform1 /* pinkyPIP */
 Transform1 /* pinkyMCP */
 Transform1 /* pinkyCMC */

ThreeDOF1 ::= tag3DOF1 Length ObjectID Time1
  bool isLeft
  Rot2
  
SixDOF1 ::= tag6DOF1 Length ObjectID Time1
  bool isLeft
  Loc2 Rot2
  ( tag6DOFpointer1 Loc1 /* intersection point */ )?

GameControl1 ::= tagGameControl1 Length ObjectID Time1
   VarInt Time1 /* buttons state and last time state changed */
   Float16 Float16 /* left stick x,y */
   Float16 Float16 /* right stick x,y */
   
Tag ::= VarUInt

tagInvalid ::= #x00
tagHead1 ::= #x01
tagHand1 ::= #x02
tagObject1 ::= #x03
tagParent1 ::= #x04
tagMesh1 ::= #x80 #x00
tagHand2 ::= #x80 #x01
tagHeadIpd1 ::= #x80 #x02
tagObject2 ::= #x80 #x03
tagMesh2 ::= #x80 #x04
tagGameControl1 ::= #x80 #x05
tag3DOF1 ::= #x80 #x06
tag6DOF1 ::= #x80 #x07
tag6DOFpointer1 ::= #x80 #x08

ObjectID ::= VarUInt

Length ::= VarUInt

Loc1 ::=
 Float32 /* x */
 Float32 /* y */
 Float32 /* z */

Loc2 ::=
 Float32 /* x */
 Float32 /* y */
 Float32 /* z */
 Float16 /* vx */
 Float16 /* vy */
 Float16 /* vz */

Scale1 ::=
 Float16 /* all dimensions */

Scale2 ::=
 Float32 /* x */
 Float32 /* y */
 Float32 /* z */
 Float16 /* vx */
 Float16 /* vy */
 Float16 /* vz */

Norm1 ::=
 Float16 /* x */
 Float16 /* y */
 Float16 /* z */

TextureUV1 ::=
 Float32 /* u */
 Float32 /* v */

Rot1 ::=
 Float16 /* i */
 Float16 /* j */
 Float16 /* k */
 /* w computed based on quaternion is normalized */

Rot2 ::=
 Float16 /* s.i */
 Float16 /* s.j */
 Float16 /* s.k */
 Float16 /* e.i */
 Float16 /* e.j */
 Float16 /* e.k */

Transform1 ::=
 Float16 /* tx */
 Float16 /* ty */
 Float16 /* tz */

TextureUrl1 ::= String

MeshUrl1 ::= String

TextureRtpPT1 ::= UInt8 /* pt */

Time1 ::= UInt16 /* time in ms */

Tag ::= VarUInt

Boolean ::=  #x00 | #x01

String ::= VarUInt byte*

Blob ::= VarUInt byte*

Float16 ::= byte byte
Float32 ::= byte byte byte byte
Float64 ::= byte byte byte byte byte byte byte byte

Int8 ::= byte
Int16 ::= byte byte
Int32 ::= byte byte byte byte
Int64 ::= byte byte byte byte byte byte byte byte

UInt8 ::= byte
UInt16 ::= byte byte
UInt32 ::= byte byte byte byte
UInt64 ::= byte byte byte byte byte byte byte byte

VarUInt ::=
 ( [#x0-#x7F] ) |
 ( [#x80-#x87] byte ) |
 ( [#x88-#x8B] byte byte ) |
 ( #xE1 UInt32 ) |
 ( #xE2 UInt64 )

VarInt ::=
 ( [#x0-#x7F] ) |
 ( [#x80-#x87] byte ) |
 ( [#x88-#x8B] byte byte ) |
 ( #xE1 Int32 ) |
 ( #xE2 Int64 )

byte ::= [#x00-#xFF]

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