2/SPB - The Size-Prefixed Blob Format

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SPB is a minimalist format for framing opaque binary blobs. The blobs have no properties and no content structure, and are represented as a size field followed by a blob content.

Name: rfc.zeromq.org/spec:2/SPB
Editor: Pieter Hintjens <moc.xitami|hp#moc.xitami|hp>
State: stable

License

This Specification is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.

This Specification is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, see <http://www.gnu.org/licenses>.

Change Process

This Specification is a free and open standard[2] and is governed by the Digital Standards Organization's Consensus-Oriented Specification System (COSS)[3].

Language

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[1].

Goals

SPB is designed to be a portable optimal wire-framing format for opaque blobs of data carried over streaming protocols such as TCP/IP.

Architecture

A SPB frame consists of a frame length, extension point, and frame data. The size of the frame data MUST correspond to the frame length.

For frames of 0 to 254 octets, the length is represented by single octet.

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message size  | Extensions    |  Message body ...             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message body ...
+-+-+-+-+-+-+-...

For frames of 255 or more octets the length is represented by a single octet %xFF followed by a 64-bit unsigned integer length in network byte order.

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| 0xff          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...+-+
| Message size (8 bytes)                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...+-+
| Extensions    |  Message body ...                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message body ...
+-+-+-+-+-+-+-+...

SPB is defined by this grammar:

frame      = length extensions data
length     = OCTET | escape 8*OCTET
escape     = %xFF
extensions = %x00
data       = *OCTET

Reference implementation

A reference implementation for SPB can be found in the 0MQ project.

References

Bibliography

1. "Key words for use in RFCs to Indicate Requirement Levels" - ietf.org

2. "Definition of a Free and Open Standard" - digistan.org

3. "Consensus Oriented Specification System" - digistan.org

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