Packet Structure

Game packet structures

Game packets are composed of 3 distinct parts, the packet header, the segment header and a (sometimes) optional IPC header.

While 'IPC' is incorrect terminology given the usage, SE calls it IPC so the naming has been preserved.

Packet Header

struct FFXIVARR_PACKET_HEADER
{
  uint64_t magic[2];
  uint64_t timestamp;
  uint32_t size;
  uint16_t connectionType;
  uint16_t segmentCount;
  uint8_t unknown_20;
  uint8_t isCompressed;
  uint32_t unknown_24;
};

Field	Description
magic	A magic value that identifies a packet. This is FF14ARR if you read it both in its hexadecimal and ASCII representation at the same time.
timestamp	The number of milliseconds since the unix epoch when the packet was sent.
size	The size of the entire packet including its segments and data.
connectionType	The connection type. This will be 1 for zone channel connections and 2 for chat. This is only sent on the initial connection now, previously this was sent with every packet but that is no longer the case.
unknown_20	Alignment, most likely.
isCompressed	Whether the segments + remaining data is compressed. The header is always uncompressed. This data is compressed with zlib and there is no header - default compression settings.
unknown_24	Alignment, most likely.

Segment Header

struct FFXIVARR_PACKET_SEGMENT_HEADER
{
  uint32_t size;
  uint32_t source_actor;
  uint32_t target_actor;
  uint16_t type;
  uint16_t padding;
};

Field	Description
size	The size of this segment and its data (if any).
source_actor	The actor ID of the actor who effectively caused this packet to be sent. For example, if another player casts an action, the source_actor field will contain their actor ID.
target_actor	The actor ID of the actor who is affected by the packet. This isn't used consistently, but the same logical rules apply as source_actor.
type	The type of segment, see below for more detail. Based on the value of this field indicates what kind of data you'd expect to find after the segment header.
paddding	Alignment.

Segment Types

enum FFXIVARR_SEGMENT_TYPE
{
  SEGMENTTYPE_SESSIONINIT = 1,
  SEGMENTTYPE_IPC = 3,
  SEGMENTTYPE_KEEPALIVE = 7,
  //SEGMENTTYPE_RESPONSE = 8,
  SEGMENTTYPE_ENCRYPTIONINIT = 9,
};

Type	Description
SEGMENTTYPE_SESSIONINIT	Used to login to a world or chat server. The packet that has a segment that has a type set to this will contain a correct connectionType set in the packet header. Use this to record what kind of connection it is. Example implementation is in Sapphire.
SEGMENTTYPE_IPC	Used for segments that contain data that should be handled by the packet router for the associated channel. Chat messages, using actions and etc. will always be sent via this segment type and there will be a FFXIVARR_IPC_HEADER immediately following the segment header.
SEGMENTTYPE_KEEPALIVE	TODO: can't remember where this is actually used - lobby? As a note, world (and chat?) use IPCs to ping/pong. Because reasons.
SEGMENTTYPE_ENCRYPTIONINIT	Used to initialize Blowfish for the lobby channel. The client sends a packet to lobby with it's key phrase which is then used to """secure""" a lobby session (spoiler alert: it's not secure).

IPC Header

Only present when the parent segment type is set to SEGMENTTYPE_IPC.

struct FFXIVARR_IPC_HEADER
{
  uint16_t reserved;
  uint16_t type;
  uint16_t padding;
  uint16_t serverId;
  uint32_t timestamp;
  uint32_t padding1;
};

Field	Description
reserved
type	This will contain the opcode of the packet which identifies which hanadler the packet data following this packet should go.
padding	Potentially data here and not padding but it's probably not that important. Right?
serverId	TODO: write about retail server architecture.
timestamp	A Unix timestamp in seconds since the epoch. Not really sure why this exists here but it does and that's what it has in it.
padding1	Alignment.

Decoding Packets

Decoding packets is reasonably simple and assuming you have a buffer that you write data in to for each connection, it's something like the following:

if buf.size < sizeof(FFXIVARR_PACKET_HEADER):
    return

header = &buf[0] as FFXIVARR_PACKET_HEADER:

if buf.size < header.size:
    return

data = slice buf from sizeof(FFXIVARR_PACKET_HEADER) to end of buf

if buf.isCompressed:
    data = zlib_inflate(data)

pos = 0
while true:
    segment = &data[pos] as FFXIVARR_PACKET_SEGMENT_HEADER

    if segment.size >= buf.size
        burn them

    if segment.size >= data.size:
        also burn them

    pos = segment.size

    seg_hdr_size = sizeof(FFXIVARR_PACKET_SEGMENT_HEADER)

    if segment.type == SEGMENTTYPE_IPC:
        ipc_size = segment.size - seg_hdr_size
        ipc_data = slice segment from seg_hdr_size to ipc_size

        ipc_hdr = &ipc_data[0] as FFXIVARR_IPC_HEADER
        ipc_hdr_size = sizeof(FFXIVARR_IPC_HEADER)

        packet_data = slice ipc_data from ipc_hdr_size to remaining size
        process_channel_packet(ipc_hdr.type, packet_data)

    // other segment types depend on the type of channel, but it's more of the same

A lot of detail is omitted for brevity, but it's generally pretty straightforward.

A more comprehensive example of packet parsing can be found in Sapphire:

• Packet container (and some container-level parsing)
• Packet parsing