Definition

A message is the basic communications unit for the Second Life world. Messages are used to pass serialized information about almost every part of the system back and forth between two hosts on a network, through connections known as circuits.

There are many hundreds of messages in the Second Life message system. A list of them is available at Category:Messages.

Message Format

Messages are defined in the Message Template. They follow the format:

{
	[Message Name] [Message Frequency] [Message Trust Level] [Message Encoding]
	{
		[Block 1 Name]		[Block 1 Quantity]
		{	[Block 1 Parameter 1 Name]		[Block 1 Parameter 1 Type]	}
		{	[Block 2 Parameter 2 Name]		[Block 2 Parameter 2 Type]	}
                [...]
	}
        [...]
	{
		[Block n Name]		[Block n Quantity]
		{	[Block n Parameter 1 Name]		[Block n Parameter 1 Type]	}
                [...]
	}
}

For example

{
	TestMessage Low 1 NotTrusted Zerocoded
	{
		TestBlock1		Single
		{	Test1		U32	}
	}
	{
		NeighborBlock		Multiple		4
		{	Test0		U32	}
		{	Test1		U32	}
		{	Test2		U32	}
	}
}

Preamble

The preamble contains information about the message as a whole.

Name
- Values: [string]
- Name of the message. Follows C variable naming restrictions.
Frequency
- Values: High|Medium|Low|Fixed [U32]
- Determines whether the message ID is 8, 16, or 32 bits. There can be 254 messages in the High or Medium, 32K in Low. A message with a "Fixed" frequency defines its own ID and is considered to be a signal. For more information, see Packet Layout
Trust Level
- Values: Trusted|NotTrusted
- Denotes the minimum trust level needed for the circuit between the sender and receiver in order for the message to send successfully. For more information, see Circuits.
Compression
- Values: Unencoded|Zerocoded
- Zerocoding will attempt to compress sequences of zeros in the message in order to reduce network load. If there is no major size difference after the compression, it is discarded and the message is sent uncompressed.

Blocks

Blocks are delimiters for groups of data in a message.

Name
- Values: [string]
- Name of the block. Follows C variable naming restrictions.
Quantity
- Values: Single|Multiple [U8]|Variable
- Determines how many blocks are contained in the message. "Multiple" means a static number of blocks in every message, and is followed by an 8 bit number to tell how many times the block is repeated, "Variable" lets the sender specify the number of blocks on sending (also an 8 bit number).

Variables

Variables contain the data sent with the message

Name
- Values: [string]
- Name of the variable. Follows C variable naming restrictions.
Type
- Values: Null|Fixed [U32]|Variable [U32]|U8|U16|U32|U64|S8|S16|S32|S64|F32|F64
- Values Cont: LLVector3|LLVector3d|LLVector4|LLQuaternion|LLUUID|BOOL|IPADDR|IPPORT|U16Vec3|U16Quat|S16Array
- The type of the variable. "Fixed"/"Variable" are used to denote sizes for things like blobs.

Data Types

Null - no data, 0 bytes wide
Fixed - byte array, interpreted depending on packet type, width determined in message definition
Variable 1 - first byte determines number of bytes that follow (U8)
Variable 2 - first two bytes determine number of bytes that follow (U16)
U8 - unsigned byte, 1 byte wide
U16 - unsigned short, 2 bytes wide
U32 - unsigned int, 4 bytes wide
U64 - unsigned long, 8 bytes wide
S8 - signed byte, 1 byte wide
S16 - signed short, 2 bytes wide
S32 - signed int, 4 bytes wide
S64 - signed long, 8 bytes wide
F32 - float, 4 bytes wide
F64 - double, 8 bytes wide
LLVector3 - triplet of floats, 12 bytes wide
LLVector3d - triplet of doubles, 24 bytes wide
LLVector4 - quad of floats, 16 bytes wide
LLQuaternion - because it's always a unit quaternion, transmitted in messages as a triplet of floats, 12 bytes wide (represented in memory as a quad of floats, 16 bytes wide)
LLUUID - Universal ID, 16 bytes wide
BOOL - 0 or 1, 1 byte wide
IPADDR - IP Address, one place per byte, 4 bytes wide
IPPORT - IP Port, two bytes wide
U16Vec3 - not used
U16Quat - not used
S16Array - not used

Message

Contents