Core format

This is a description of the transit-independent, extensible PearDrop protocol.

Packet types

All offsets are in big endian, and offsets and lengths are in bytes.

In bitfields, offsets are in little endian, and offsets and lengths are in bits.

Advertisement packet

Maximum length: 128 bytes

From: sender

To: advertise

Offset	Length	Type	Name	Description
0x0	0x1	u8	dname_len	Length of the name of this device. Maximum 30 bytes.
0x1	dname_len	&str	dname	The name of this device. Maximum length of 30 bytes.
0x1f	0x20	SHA-256 hash ([u8; 8])	ssid	The SHA-256 hash of the currently connected SSID. Empty if the device is not connected to a WiFi network.
0x3f	0x1	u8	name_len	Length of the name of the advertised file. Maximum 32 bytes.
0x40	name_len	&str	name	Name of the advertised file. Maximum length of 32 bytes.
0x60	0x1	u8	mt_len	Length of the mimetype of the advertised file. Maximum 30 bytes.
0x61	mt_len	&str	mimetype	MIME type of the advertised file. Maximum length of 30 bytes.
0x7f	0x1	AdFlags	flags	Extra flags indicating the capabilities of this device. See below for the definition of the AdFlags bitfield.

AdFlags bitfield

Reserved for extensions.

Sender packet

Maximum length: 128 bytes

From: sender

To: single receiver

Offset	Length	Type	Name	Description
0x0	0x1	u8	ext_len	Length of the extensions information structure. Maximum of 32 entries.
0x1	max 0x77	[Extension; ext_len]	exts	The extensions in this packet. Maximum 119 bytes total.
0x78	0x8	`u64	len	Length of the data.

Extension trait

Reserved for extensions. All sub-structs must have these fields at the start:

Offset	Length	Type	Name	Description
0x0	4 bits	ExtType	type	Type of this extension.
4 bits	4 bits	u4	len	Length of this entire structure.

ExtType enum

Must fit in a u4. Reserved for extensions.

Receiver acknowledge packet

Maximum length: 32 bytes

From: receiver

To: sender

Offset	Length	Type	Name	Description
0x0	4 bits	ReceiverAcknowledgeType	type	Type of the packet being acknowledged.
4 bits	4 bits	u4	ext_len	Length of the extensions. Maximum of 32 entries.
0x1	max 0x1f	[Extension; ext_len]	exts	The extensions in this packet. Maximum 31 bytes total.

Data packet

No maximum length. This should be the length specified in the sender packet.

This should be sent over a reliable stream protocol (like TCP), or properly chunked otherwise.

This packet simply consists of the data and nothing else.

Handshake

Sender     Receiver(s)     Receiver
   ------------->
   advertisement

   <----------------------------
   acknowledge: advertisement

   ---------------------------->
   sender packet

   <----------------------------
   acknowledge: sender

   <--------------------------->
   (optional) extension handshakes

   ---------------------------->
   data packet

   <----------------------------
   acknowledge: data

   transfer complete :)