Difference between revisions of "Packet Layout"

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==Header==
==Header (Packet ID)==
* Byte 0, 4 most significant bits: Packet Information
* Byte 0, 4 most significant bits: Packet Information
** LL_ZERO_CODE_FLAG 0x80 -- Packet data is run length encoded, such that series of 1 to 255 zero bytes are encoded to take 2 bytes.
** LL_ZERO_CODE_FLAG 0x80 -- Packet data is run length encoded, such that series of 1 to 255 zero bytes are encoded to take 2 bytes.
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** LL_ACK_FLAG 0x10 -- This packet contains appended acks.
** LL_ACK_FLAG 0x10 -- This packet contains appended acks.
* Byte 0, 4 least significant bits: currently unused
* Byte 0, 4 least significant bits: currently unused
* Bytes 1 - 3: (for a total of 24 bits) Packet ID, assigned by circuit
* Header and Body are potentially zerocoded for compression
* Everything up to ACKs is potentially zerocoded for compression
* Bytes n thru n+(frequency size)
** 1/2/4 byte ID of the [[message]] the packet contains


For further explanation of the bits in the header, see [[Messages]] and [[Packet Accounting]]
For further explanation of the bits in the header, see [[Messages]] and [[Packet Accounting]]

Revision as of 18:15, 15 January 2007

Packet Layout Visualization

 +-+-+-+-+-------+---------------+---------------+---------------+
 |Z|R|R|A|                                                       |
 |E|E|E|C|                   Packet ID (28 bits)                 |
 |R|L|S|K|                                                       |
 +-+-+-+-+-------+---------------+---------------+---------------+
 |                                                               |
 :                                                               :

Header (Packet ID)

  • Byte 0, 4 most significant bits: Packet Information
    • LL_ZERO_CODE_FLAG 0x80 -- Packet data is 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
  • Header and Body are potentially zerocoded for compression

For further explanation 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