Internet DRAFT - draft-petrescu-ipwave-diaser-checksum
draft-petrescu-ipwave-diaser-checksum
IPWAVE Working Group A. Petrescu
Internet-Draft M. Sarr
Intended status: Standards Track CEA, LIST
Expires: November 21, 2019 May 20, 2019
The checksum in DIASER/UDP/IP is a XOR without STX
draft-petrescu-ipwave-diaser-checksum-01
Abstract
This document defines the manner in which to calculate the checksum
for the protocol DIASER. DIASER is a protocol for communication with
traffic lights controllers used in France. DIASER is specified at
AFNOR. The specification misses an instruction on how to compute the
'BCC' checksum ('Byte Character Control').
Status of This Memo
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This Internet-Draft will expire on November 21, 2019.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. DIASER Protocol and IETF Protocols . . . . . . . . . . . . . 2
3. Checksum . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 4
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
8. Normative References . . . . . . . . . . . . . . . . . . . . 4
Appendix A. ChangeLog . . . . . . . . . . . . . . . . . . . . . 4
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 4
1. 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 [RFC2119].
2. DIASER Protocol and IETF Protocols
The DIASER protocol does this and that with a Traffic Lights
controller - it is an Application Layer protocol. DIASER can be
transported over UDP and over IP. The DIASER specification is
available at a cost from AFNOR, on the World Wide Web. The DIASER
specification does not describe the mechanism to compute the checksum
'BCC' (Byte Control Character). There exist several verbal (oral)
versions of this mechanism: to cover or not to cover the STX (Start
of Text), or the ETX (End of Text). Studying some implementations of
a particular controller (Aximum) and packet dumps from independent
sources, lead to a possibility to reverse engineer the specification
of the checksum calculation of DIASER.
The DIASER protocol is an application layer protocol. Initially it
was designed to run on serial lines like RS-232. Later on it was put
on UDP (User Datagram Protocol). The UDP protocol is an IETF
protocol. Running DIASER over UDP over IP is a great fact; contrary
to RS-232, it allows to query a Traffic Lights Controller from a
remote 'Poste de Commande et Control', and further from any other
point in the Internet; second, it allows to query the controller over
the future generation of Internet Protocol namely IPv6; thirdly, it
allows to query the controller from nearby like an autonomous
shuttle, with lower latency communications, thus supporting higher
autonomous shuttle speeds; fourthly, it is easy to secure - rely on
numerous security tools for software (IPsec, VPN) widely available
and enjoy trust developped by Certificate Authorities and Let's
Encrypt.
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IETF has its own way to specify a checksum. UDP, ICMP and other IETF
protocols use a unique kind of checksum, which is specified in a
particular RFC. It is clear that the RFC checksum is different from
the DIASER checksum, at least because it involves also a notion of
complement (not just XOR).
3. Checksum
The DIASER checksum is independent of the checksum field in the UDP
header preceding the DIASER payload. In a DIASER/UDP packet there
are two checksums: the DIASER checksum and the UDP checksum.
The DIASER checksum is an exclusive or operation (XOR) performed
sequentially on the DIASER command; the bytes are read from left to
right; the STX character is not read; the ETX character is read.
In French, the description of the checksum can be found in a proposal
of updating the DIASER standard dated September 2018. It is written
like this: 'Le caractA¨re de contrA'le BCC [checksum] est le
rA(C)sultat du "ou exclusif" de tous les caractA¨res du message
aprA¨s le STX non inclus, et peut prendre n'importe quelle
valeur ASCII.'
A python implementation of the DIASER checksum is the following.
Note the use of '^' operator for XOR and the 'for' iteration on each
byte. Note that only ETX is covered by the checksum, not the STX.
#!/usr/bin/env python
# Mariama for CEA
packet="\x43\x6B\x30\x33\x47\x30\x30\x47\x30\x31\x47\x30\x32\x47\x30\x33\x47\x30\x34\x47\x30\x35\x47\x30\x36\x47\x30\x37\x47\x30\x38\x2A\x2A\x2A\x2A\x2A\x2A\x2A\x2A\x2A\x03"
# packet is 'Ck03G00G01G02G03G04G05G06G07G08*********|ETX'
checksum = 0
for el in packet:
checksum ^= ord(el)
print hex(checksum)
# code snippet found on 'stack overflow' on April 25th, 2019; and
# adapted for DIASER.
Figure 1: Code snippet for DIASER Checksum
4. Security Considerations
Wrongly calculating a checksum may lead to security risks.
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5. IANA Considerations
no request.
6. Contributors
Listed.
7. Acknowledgements
StA(C)phane GOEURIOT suggested an initial approximative oral
description of the checksum calculation. Christophe DAMAS detailed
the description of the checksum calculation with more precision, as
an improvement to the DIASER standard. Marco PEROTTI faced necessity
of proper checksum calculation.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Appendix A. ChangeLog
The changes are listed in reverse chronological order, most recent
changes appearing at the top of the list.
-01: added the description of checksum calculation as reported in
French.
Authors' Addresses
Alexandre Petrescu
CEA, LIST
CEA Saclay
Gif-sur-Yvette , Ile-de-France 91190
France
Phone: +33169089223
Email: Alexandre.Petrescu@cea.fr
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Mariama Sarr
CEA, LIST
CEA Saclay
Gif-sur-Yvette , Ile-de-France 91190
France
Phone: +phone
Email: Mariama.Sarr@cea.fr
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