sdw333 / cobs-c

Consistent Overhead Byte Stuffing — C implementation

Consistent Overhead Byte Stuffing (COBS)

Author:	Craig McQueen
Contact:	http://craig.mcqueen.id.au/
Copyright:	2017 Craig McQueen

C functions for encoding and decoding COBS.

Intro

A library is provided, which contains functions for encoding and decoding according to COBS methods.

What Is COBS?

COBS is a method of encoding a packet of bytes into a form that contains no bytes with value zero (0x00). The input packet of bytes can contain bytes in the full range of 0x00 to 0xFF. The COBS encoded packet is guaranteed to generate packets with bytes only in the range 0x01 to 0xFF. Thus, in a communication protocol, packet boundaries can be reliably delimited with 0x00 bytes.

The COBS encoding does have to increase the packet size to achieve this encoding. However, compared to other byte-stuffing methods, the packet size increase is reasonable and predictable. COBS always adds 1 byte to the message length. Additionally, for longer packets of length n, it may add n/254 (rounded down) additional bytes to the encoded packet size.

For example, compare to the PPP protocol, which uses 0x7E bytes to delimit PPP packets. The PPP protocol uses an "escape" style of byte stuffing, replacing all occurences of 0x7E bytes in the packet with 0x7D 0x5E. But that byte-stuffing method can potentially double the size of the packet in the worst case. COBS uses a different method for byte-stuffing, which has a much more reasonable worst-case overhead.

For more details about COBS, see the references [1] [2].

I have included a variant on COBS, COBS/R, which slightly modifies COBS to often avoid the +1 byte overhead of COBS. So in many cases, especially for smaller packets, the size of a COBS/R encoded packet is the same size as the original packet. See below for more details about COBS/R.

References

[1]	(1, 2, 3, 4) Consistent Overhead Byte Stuffing Stuart Cheshire and Mary Baker IEEE/ACM Transations on Networking, Vol. 7, No. 2, April 1999

[2]	(1, 2) PPP Consistent Overhead Byte Stuffing (COBS) PPP Working Group Internet Draft James Carlson, IronBridge Networks Stuart Cheshire and Mary Baker, Stanford University November 1997

Files and Functions Provided

File	Short Name	Long Name
cobs.c	COBS	Consistent Overhead Byte Stuffing (basic method) [1]
cobsr.c	COBS/R	Consistent Overhead Byte Stuffing--Reduced

"Consistent Overhead Byte Stuffing--Reduced" (COBS/R) is my own invention, a modification of basic COBS encoding, and is described in more detail below.

The following are not implemented:

Short Name	Long Name
COBS/ZPE	Consistent Overhead Byte Stuffing--Zero Pair Elimination [1]
COBS/ZRE	Consistent Overhead Byte Stuffing--Zero Run Elimination [2]

License

The code is released under the MIT license. See LICENSE.txt for details.

Consistent Overhead Byte Stuffing--Reduced (COBS/R)

A modification of COBS, which I'm calling "Consistent Overhead Byte Stuffing--Reduced" (COBS/R), is provided in the cobsr.c file. Its purpose is to save one byte from the encoded form in some cases. Plain COBS encoding always has a +1 byte encoding overhead. See the references for details [1]. COBS/R can often avoid the +1 byte, which can be a useful savings if it is mostly small messages that are being encoded.

In plain COBS, the last length code byte in the message has some inherent redundancy: if it is greater than the number of remaining bytes, this is detected as an error.

In COBS/R, instead we opportunistically replace the final length code byte with the final data byte, whenever the value of the final data byte is greater than or equal to what the final length value would normally be. This variation can be unambiguously decoded: the decoder notices that the length code is greater than the number of remaining bytes.

Examples

The byte values in the examples are in hex.

First example:

Input:

2F

A2

00

92

73

02

This example is encoded the same in COBS and COBS/R. Encoded (length code bytes are bold):

03

2F

A2

04

92

73

02

Second example:

The second example is almost the same, except the final data byte value is greater than what the length byte would be.

Input:

2F

A2

00

92

73

26

Encoded in plain COBS (length code bytes are bold):

03

2F

A2

04

92

73

26

Encoded in COBS/R:

03

2F

A2

26

92

73

Because the last data byte (26) is greater than the usual length code (04), the last data byte can be inserted in place of the length code, and removed from the end of the sequence. This avoids the usual +1 byte overhead of the COBS encoding.

The decoder detects this variation on the encoding simply by detecting that the length code is greater than the number of remaining bytes. That situation would be a decoding error in regular COBS, but in COBS/R it is used to save one byte in the encoded message.