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Description

Stupid-Simple Messaging Protocol.

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Stupid-Simple Messaging Protocol

This document describes the Stupid-Simple Messaging Protocol, an application-level protocol for 1:1 and 1:many messaging which aims to be a lightweight alternative to open messaging protocols such as XMPP or STOMP.

Key design goals: - Text-based, for easy debugging - Interleave request/responses and server events on a single connection - Simple enough that a complete and efficient client or server can be written in pretty much any programming language within a few hours

Copyright Notice

Copyright (c) 2015, Hugues Bruant
All Rights Reserved.

This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.

The limited permissions granted above are perpetual and will not be revoked by Hugues Bruant or its successors or assigns.

This document and the information contained herein is provided on an "AS IS" basis and HUGUES BRUANT DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

History

  • 1.1: Binary payloads and updated length bounds
  • 1.0: Initial version

Introduction

SSMP supports both 1:1 (aka unicast) and 1:many (aka multicast) messaging.

Unicast messages can be addressed to any peer using the identifier supplied upon login.

Multicast messaging uses a publish/subscribe approach, where messages are sent to a "topic" and forwarded to every client that subscribed to the topic in question.

A limited form of broadcasting is also allowed, wherein a peer can send a message to all peers sharing at least one topic subscription.

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.

Assumptions

SSMP is designed to run atop a reliable 2-way streaming transport such as TCP.

Grammar

Using the augmented BNF format specified in Section 2.1 of RFC 2616

message     = ( request | response | event ) LF

request = "LOGIN" SP id SP id [ SP payload ] | "CLOSE" | "PING" | "PONG" | forwardable

response = code [ SP payload ]

event = "000" SP id SP ( forwardable | "PING" | "PONG" )

forwardable = "SUBSCRIBE" SP id [ SP "PRESENCE" ] | "UNSUBSCRIBE" SP id | "UCAST" SP id SP payload | "MCAST" SP id SP payload | "BCAST" SP payload | compat

compat = verb [ SP id ] [ SP payload ]

code = 3DIGIT verb = 116UPALPHA id = 164ID payload = ( NUL | SOH | STX | ETX ) 2*1025ANY | TEXT0 *1023TEXT

ID = UPALPHA | LOALPHA | DIGIT | "." | ":" | "@" | "/" | "_" | "-" | "+" | "=" | "~" TEXT0 = TEXT = ANY = UPALPHA = LOALPHA = DIGIT = SP = LF = NUL = SOH = STX = ETX =

Binary payloads

SSMP is primarily designed with text payloads in mind but it can also accommodate binary payloads.

A binary payload can be distinguished from a text payload from the first byte:

  • for a binary payload, it MUST be one of: NUL, SOH, STX, or ETX

  • for a text payload it MUST NOT be any of these values

The first two bytes of a binary payload describe the length of the rest of the payload. The length is computed by interpreting these two bytes as a big-endian integer and adding one to it. This ensures that the length bounds for binary payloads match those of text payloads.

For instance, "Hello" can be encoded as the following binary payload:

00 04 48 65 6C 6C 6F

Response codes

Response code values are borrowed from HTTP where appropriate.

  • 200
    OK
  • 400
    Bad Request
  • 401
    Unauthorized
  • 404
    Not Found
  • 405
    Not Allowed
  • 501
    Not Implemented

The special

000
code is used to distinguish server events from request/responses, thereby allowing events to be freely interleaved with regular responses on the same connection.

Invalid messages

Upon receiving data that does not respect the protocol grammar, a server MUST send a

400
response and immediately close the connection.

Upon receiving data that does not respect the protocol grammar, a client MUST immediately close the connection.

Login

The first client request in any connection MUST be a

LOGIN
.

The format of a

LOGIN
request is:
LOGIN   [  ]

If the authentication is successful, the server MUST send a

200
response with no payload.

If no request is received after a reasonable period of time, typically a few seconds, the server MUST close the connection without sending any response.

If the first request is not a

LOGIN
the server MUST send a
400
response and immediately close the connection.

If the

scheme
value is not supported or if the authentication fails for any other reason, the server MUST send a
401
response with with a space- separated list of supported authentication schemes as payload and immediately close the connection.

After a successful

LOGIN
request, servers MUST reject any subsequent
LOGIN
request on the same connection with code
405
.

Named connections

The identifier supplied upon login can be used by other peers to send unicast messages, as described later in this document.

Upon successful login the server MUST close any previous connection that used the same identifier.

Anonymous connections

Servers MAY allow login with the reserved

.
user identifier.

Servers which allow anonymous login SHOULD allow multiple such connections simultaneously.

Anonymous connections are intended for publishers. They MAY NOT subscribe to any topic and cannot receive unicast messages but they can publish messages to existing topics.

Authentication schemes

Client certificate

All servers that accept connection over SSL/TLS MUST allow authentication through client certificates.

The

scheme
value for certificate authentication is
cert
.

When a connection is made with a client certificate,

LOGIN
MUST succeed for any
identifier
matching either the Common Name or one of the Subject Alternative Names specified in the client certificate.

To accommodate multiple connections being opened using the same certificate, servers MAY accept identifiers consisting of a valid Common Name or Subject Alternative Name followed by a forward slash (

/
) and a sequence of one or more
ID
characters.

Shared secret

Servers MAY allow authentication through a pre-shared secret.

The

scheme
value for shared secret authentication is
secret
.

Open login

Servers MAY allow unauthenticated

LOGIN
.

The

scheme
value for open login is
open
.

The client SHOULD NOT include a

credential
field in an
open
login request and the server MUST ignore its content if it is present.

Open login is subject to easy abuse and SHOULD therefore only be enabled for debugging purposes.

Other

Servers MAY support other authentication schemes. The

scheme
value MUST be a sequence of one or more
ID
characters.

Ping

Periodic ping messages are used to test connection liveness and prevent closure by aggressive firewalls.

Client-initiated

Clients SHOULD send a

PING
request after an implementation-defined period where no server event is received, typically about 30s.

Upon reception of a

PING
request, servers MUST send a
PONG
event using the anonymous identifier as its provenance.
Client          Server

PING ---->

Clients SHOULD close the connection if no

PONG
event is received during an implementation-defined period, typically 30s, after a
PING
request was sent.

Server-initiated

Servers SHOULD send a

PING
event after an implementation-defined period where no client request is received, typically about 30s. The provenance MUST be the anonymous identifier.

Upon reception of a

PING
event, clients MUST send a
PONG
message.

Servers MUST NOT send any message in response to a

PONG
message.
Client          Server


Servers SHOULD close the connection if no

PONG
message is received during an implementation-defined period, typically 30s, after a
PING
event was sent.

Topic subscriptions

Subscribe to multicast topic

Opt in to receiving events for messages sent to a multicast topic.

SUBSCRIBE  [ PRESENCE ]

Any

SUBSCRIBE
request from an anonymous user MUST be rejected with code
405
.

If the caller was already subscribed to the given topic, the server MUST respond with code

409
.

The optional

PRESENCE
flag can be used to subscribe to presence notifications, as described later in the following section.

Unsubscribe from multicast topic

Opt out of receiving events for messages sent to a multicast topic.

UNSUBSCRIBE 

Any

UNSUBSCRIBE
request from an anonymous user MUST be rejected with code
405
.

If the caller was not subscribed to the given topic, the server MUST respond with code

404
.

Presence notifications

When the

PRESENCE
flag is provided, the caller will receive an initial batch of
SUBSCRIBE
events for all current subscribers and subsequently,
SUBSCRIBE
and
UNSUBSCRIBE
events as topic membership changes.

The server MUST ensure that presence notifications are delivered in a safe order. Crucially, an

UNSUBSCRIBE
event MUST NOT be re-ordered before the corresponding
SUBSCRIBE
event.

Upon successful subscription, the server MUST forward the message to every client who specified the

PRESENCE
flag when subscribing to the topic.
000  SUBSCRIBE  [ PRESENCE ]

Upon successful unsubscription, the server MUST forward the message to every client who specified the

PRESENCE
flag when subscribing to the topic.
000  UNSUBSCRIBE 

Messages

Message delivery is: - in-order: two messages from the same sender to the same recipient MUST arrive in-order at the recipient - at most once: recipients MUST NOT receive duplicate messages - best effort with no acknowledgment: a successful response from the server indicates that the message was received by the server but not necessarily by the final recipient

Unicast

Send message to a single peer.

UCAST  

If no peer with the requested identifier is currently connected, the server MUST send a

404
response.

Otherwise it MUST forward the message to the given peer:

000  UCAST  

Multicast

Send message to all peers subscribed to a given topic.

MCAST  

The server MUST NOT send a 404 response, even if no peer has subscribed to the given topic.

The server MUST forward the message to to every peer having subscribed to the topic, except the sender.

000  MCAST  

A client does not need to be subscribed to a topic to send messages to it.

Broadcast

Broadcast to all peers sharing at least one topic.

BCAST 

Any

BCAST
request from an anonymous user MUST be rejected with code
405
.

The server MUST send forward the message to every peer sharing at least one topic with the sender of the

BCAST
request.
000  BCAST 

Peers that share multiple topics with the sender MUST NOT receive multiple identical

BCAST
events.

Closing connections

A client may cleanly close a connection by sending a

CLOSE
request.
CLOSE

Upon receiving a

CLOSE
request, servers MUST reply with code
200
and immediately close the connection.

Servers MUST send appropriate

UNSUBSCRIBE
events for all topics to which the client was subscribed.

Similarly, if the server closes a connection for any reason, either mandated by this specification or due to underlying network issues, it MUST send appropriate

UNSUBSCRIBE
events.

Forward compatibility

Upon reception of a request with an unrecognized

verb
, servers MUST send a
501
response and keep the connection open.

This is intended to allow client to safely detect whether servers support any new or optional requests that may be added in future versions of this specification.

Upon reception of an event with an unrecognized

verb
, clients MUST immediately close the connection.

Payload encoding considerations

Servers are oblivious to the encoding of text payloads. They MUST NOT make any assumption about character set or encoding and MUST NOT alter the contents of the payload in any way before forwarding them to recipients.

Clients SHOULD encode text payloads as UTF-8.

Security considerations

Clients and servers SHOULD secure communications by connecting over TLS, especially if a pre-shared secret is used for authentication purposes.

Known implementations

  • lipwig : reference implementation (server and client)
  • jssmp : Java implementation (server and client)

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