# Banana Protocol Specifications

## Introduction

Banana is an efficient, extendable protocol for sending and receiving s-expressions. A s-expression in this context is a list composed of byte strings, integers, large integers, floats and/or s-expressions.

## Banana Encodings

The banana protocol is a stream of data composed of elements. Each element has the following general structure - first, the length of element encoded in base-128, least signficant bit first. For example length 4674 will be sent as `0x42 0x24`. For certain element types the length will be omitted (e.g. float) or have a different meaning (it is the actual value of integer elements).

Following the length is a delimiter byte, which tells us what kind of element this is. Depending on the element type, there will then follow the number of bytes specified in the length. The byte's high-bit will always be set, so that we can differentiate between it and the length (since the length bytes use 128-base, their high bit will never be set).

## Element Types

Given a series of bytes that gave us length N, these are the different delimiter bytes:

List -- 0x80
  The following N bytes are a list of elements.
Integer -- 0x81
  The value of this element is the positive integer N. Following bytes are not part of this element. Integers can have values of 0 <= N <= 2147483647.
String -- 0x82
  The following N bytes are a string element.
Negative Integer -- 0x83
  The value of this element is the integer N * -1, i.e. -N. Following bytes are not part of this element. Negative integers can have values of 0 >= -N >= -2147483648.
Float - 0x84
  The next 8 bytes are the float encoded in IEEE 754 floating-point "double format" bit layout. No length bytes should have been defined.
Large Integer -- 0x81
  The value of this element is the positive large integer N. Following bytes are not part of this element. Large integers have no size limitation.
Large Negative Integer -- 0x83
  The value of this element is the negative large integer -N. Following bytes are not part of this element. Large integers have no size limitation.

Large integers are intended for arbitary length integers. Regular integers types (positive and negative) are limited to 32-bit values.

### Examples

Here are some examples of elements and their encodings:

1
  `0x01 0x81`
-1
  `0x01 0x83`
1.5
  `0x84 0x3f 0xf8 0x00 0x00 0x00 0x00 0x00 0x00`
"hello"
  `0x05 0x82 0x68 0x65 0x6c 0x6c 0x6f`
[]
  `0x00 0x80`
[1, 23]
  `0x02 0x80 0x01 0x81 0x17 0x81`
123456789123456789
  `0x15 0x3e 0x41 0x66 0x3a 0x69 0x26 0x5b 0x01 0x85`
[1, ["hello"]]
  `0x02 0x80 0x01 0x81 0x01 0x80 0x05 0x82 0x68 0x65 0x6c 0x6c 0x6f`

## Profiles

The Banana protocol is extendable. Therefore, it supports the concept of profiles. Profiles allow developers to extend the banana protocol, adding new element types, while still keeping backwards compatability with implementations that don't support the extensions. The profile used in each session is determined at the handshake stage (see below.)

A profile is specified by a unique string. This specification defines two profiles - `"none"` and `"pb"`. The `"none"` profile is the standard profile that should be supported by all Banana implementations. Additional profiles may be added in the future.

### The `"none"` Profile

The `"none"` profile is identical to the delimiter types listed above. It is highly recommended that all Banana clients and servers support the `"none"` profile.

### The `"pb"` Profile

The `"pb"` profile is intended for use with the Perspective Broker protocol, that runs on top of Banana. Basically, it converts commonly used PB strings into shorter versions, thus minimizing bandwidth usage. It does this by adding an additional delimiter byte, 0x87. This byte should not be prefixed by a length. It should be followed by a single byte, which tells us to which string element to convert it:

0x01
  'None'
0x02
  'class'
0x03
  'dereference'
0x04
  'reference'
0x05
  'dictionary'
0x06
  'function'
0x07
  'instance'
0x08
  'list'
0x09
  'module'
0x0a
  'persistent'
0x0b
  'tuple'
0x0c
  'unpersistable'
0x0d
  'copy'
0x0e
  'cache'
0x0f
  'cached'
0x10
  'remote'
0x11
  'local'
0x12
  'lcache'
0x13
  'version'
0x14
  'login'
0x15
  'password'
0x16
  'challenge'
0x17
  'logged_in'
0x18
  'not_logged_in'
0x19
  'cachemessage'
0x1a
  'message'
0x1b
  'answer'
0x1c
  'error'
0x1d
  'decref'
0x1e
  'decache'
0x1f
  'uncache'

## Protocol Handshake and Behaviour

The initiating side of the connection will be referred to as "client", and the other side as "server".

Upon connection, the server will send the client a list of string elements, signifying the profiles it supports. It is recommended that `"none"` be included in this list. The client then sends the server a string from this list, telling the server which profile it wants to use. At this point the whole session will use this profile.

Once a profile has been established, the two sides may start exchanging elements. There is no limitation on order or dependencies of messages. Any such limitation (e.g. "server can only send an element to client in response to a request from client") is application specific.

Upon receiving illegal messages, failed handshakes, etc., a Banana client or server should close its connection.