Packet Layout

Header (Packet ID)

 +-+-+-+-+-------+---------------+---------------+---------------+
 |Z|R|R|A|       |                                               |
 |E|E|E|C|       |           Sequence number (24 bits)           |
 |R|L|S|K|       |                                               |
 +-+-+-+-+-------+---------------+---------------+---------------+
 |                                                               |
 :                                                               :

• Byte 0, 4 most significant bits: Flags
  • LL_ZERO_CODE_FLAG 0x80 -- 0's in packet body are run length encoded, such that series of 1 to 255 zero bytes are encoded to take 2 bytes.
  • LL_RELIABLE_FLAG 0x40 -- This packet was sent reliably (implies please ack this packet)
  • LL_RESENT_FLAG 0x20 -- This packet is a resend from the source.
  • LL_ACK_FLAG 0x10 -- This packet contains appended acks.
• Byte 0, 4 least significant bits: currently unused
• Bytes 1-3: Sequence number

The term "Packet ID" is sometimes used to refer to all 4 bytes and sometimes just the sequence number.

For further discussion of the bits in the header, see Messages and Packet Accounting

Body

• Bytes 4 to 4 + (data length)
  • Message number. This is a numeric encoding of the message types defined in the message_template.msg file. It may be 1, 2 or 4 bytes in length, depending on the message frequency (High, Medium, Low or Fixed).
    • High frequency messages are assigned (at run time) numbers 0x00 - 0xFE.
    • Medium frequency messages are assigned (at run time) numbers 0xFF00 - 0xFFFE.
    • Low frequency messages are assigned (at run time) numbers 0xFFFF0000 and up.
    • Messages with "Fixed" frequency are really those with fixed message numbers, i.e. the numbers are assigned in the message_template.msg file itself. There are currently 6 of these, 0xFFFFFFFA - 0xFFFFFFFF.
  • Message data. This is different for each message type, and is defined in the message_template.msg file.

Appended Acks

• Bytes n+(data length) thru acks
  • Rest of packet is filled with as many acks from previous reliable messages as will fit.

See also: libsecondlife documentation