Introduction

HSP is a simple bidirectional stream based protocol to transmit messages between two connected peers. Messages carry a message type and received message can be acknowledged or rejected with an error.

Protocols for actual applications can be based on HSP by defining those message types and the corresponding syntax of the payload, message flows, authentication, etc.

Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174.

Data Types

HSP defines two basic data types, Integers and Byte Arrays:

Integers

Three integer types are defined:

• 1-Byte: 8 bit unsigned integer
• 2-Byte: 16 bit unsigned integer
• 4-Byte: 32 bit unsigned integer

Signedness

All key-fields MUST be represented unsigned.

Byte Order

The byte-order of all key-fields MUST be Big-Endian. Byte-order of payload-fields are defined by the application.

Examples

• 1-Byte: 194 (decimal) is encoded as "c4" (hex).
• 2-Byte: 45678 (decimal) is encoded as "b2 6e" (hex).
• 4-Byte: 13500844 (decimal) is encoded as "00 ce 01 ac" (hex).

ByteArray

A ByteArray is just an array of bytes. The meaning of those bytes is defined by the application. The application MAY also restrict the maximum length to avoid utilizing too much memory while receiving but MUST NOT reduce the defined size of the Length-field.

+--------+------+
| Length | Data |
+--------+------+

• Length: 4-Byte.
• Data: Arbitrary bytes.

Examples

• "Hello" is be encoded as "00 00 00 05 48 65 6c 6c 6f" (hex).

Protocol

Each peer sends a sequence of messages to the other peer. The general structure of each message is:

+---------+-------------------
| Command | Variable Part
+---------+-------------------

The command is represented by 1-Byte. The variable part depends on the command.

Commands

Value	Command	Description
0	DATA	Send data, don't expect acknowledgement
1	DATA_ACK	Send data, expect ACK or ERROR in return
2	ACK	Acknowledge a previous DATA_ACK
3	PING	Expect PONG in return
4	PONG	Response to PING
5	ERROR	Previous DATA_ACK could not be processed; with error msg
6	ERROR_UNDEF	Previous DATA_ACK could not be processed; unknown error

DATA

Send data to the peer. This has fire-and-forget semantics. If the peer doesn't receive the message (e.g. due to connection loss) or cannot process it for whatever reason, the sender will never know.

The sender SHOULD NOT try to guess if the recipient got a message by looking for example at TCP ACKs as those cannot be trusted even if TLS is used.

+---+------+---------+
| 0 | Type | Payload |
+---+------+---------+

• Type (2-Byte): Valid values are specified by the application. The Type defines the format of the Payload.
• Payload (ByteArray): Arbitrary data.

DATA_ACK

Send data to the peer. Each such message MUST eventually be acknowledged by the recipient, either with an ACK, an ERROR or an ERROR_UNDEF. If multiple DATA_ACK messages are sent, the responses MAY arrive in different order. If the connection is lost before a response is received, the sender SHOULD assume that the recipient did not receive it and MAY try to send it again after it reconnected.

+---+-----------+------+---------+
| 1 | MessageID | Type | Payload |
+---+-----------+------+---------+

• MessageID (4-Byte): The MessageID is used to correlate messages to their Acknowledges or Errors. They are defined by the sender of the message and can be arbitrary numbers. The same MessageID MUST only be reused once a response was received for it.
• Type (2-Byte): Valid values are specified by the application. The Type defines the format of the Payload.
• Payload (ByteArray): Arbitrary data.

ACK

Acknowledge that a DATA_ACK was received and processed successfully.

+---+-----------+
| 2 | MessageID |
+---+-----------+

• MessageID (4-Byte): The MessageID of a previously received DATA_ACK.

PING

Request a PONG from the recipient. For each PING received, exactly one PONG MUST be sent.

+---+
| 3 |
+---+

PONG

Acknowledge a PING.

+---+
| 4 |
+---+

ERROR

Acknowledge that a DATA_ACK was received but could not be processed.

+---+-----------+------+---------+
| 5 | MessageID | Type | Payload |
+---+-----------+------+---------+

• MessageID (4-Byte): The MessageID of a previously received DATA_ACK.
• Type (2-Byte): Application defined error code, meant for automatic processing by machines. The Type defines the format of the Payload.
• Payload (ByteArray): Error details. Can be arbitrary data.

ERROR_UNDEF

Acknowledge that a DATA_ACK was received but could not be processed. The reason for the error is not defined.

+---+-----------+
| 6 | MessageID |
+---+-----------+

• MessageID (4-Byte): The MessageID of a previously received DATA_ACK.

Security Considerations

It is strongly recommended to use a secure transport layer such as TLS.

Authentication

Authentication is optional and defined by the application. For example, Client Certificates on a TLS connection may be used or the application can define message types to implement authentication.