CoRE M. Becker, Ed.
Internet-Draft ComNets, TZI, University Bremen
Intended status: Standards Track K. Li
Expires: February 9, 2014 Huawei Technologies
T. Poetsch
K. Kuladinithi
ComNets, TZI, University Bremen
August 8, 2013
Transport of CoAP over SMS
draft-becker-core-coap-sms-gprs-04
Abstract
Short Message Service (SMS) of mobile cellular networks is frequently
used in Machine-To-Machine (M2M) communications, such as for
telematic devices. The service offers small packet sizes and high
delays just as other typical low-power and lossy networks (LLNs),
i.e. 6LoWPANs. The design of the Constrained Application Protocol
(CoAP), that took the limitations of LLNs into account, is thus also
applicable to other transports. The adaptation of CoAP to SMS
transport mechanisms is described in this document.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on February 9, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
4. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. MO-MT Scenarios . . . . . . . . . . . . . . . . . . . . . 4
4.2. MT Scenarios . . . . . . . . . . . . . . . . . . . . . . . 4
4.3. MO Scenarios . . . . . . . . . . . . . . . . . . . . . . . 5
5. Message Exchanges . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Message Exchange for SMS in a Cellular-To-Cellular
Mobile-Originated and Mobile-Terminated Scenario . . . . . 6
6. Encoding Schemes of CoAP for SMS transport . . . . . . . . . . 7
7. Message Size Implementation Considerations . . . . . . . . . . 7
8. Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Options for mixed IP operation . . . . . . . . . . . . . . . . 8
10. URI Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. Transmission Parameters . . . . . . . . . . . . . . . . . . . 9
12. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 9
13. Security Considerations . . . . . . . . . . . . . . . . . . . 9
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
14.1. CoAP Option Number . . . . . . . . . . . . . . . . . . . . 10
14.2. URI Scheme Registration . . . . . . . . . . . . . . . . . 10
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
16.1. Normative References . . . . . . . . . . . . . . . . . . . 10
16.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
This specification details the usage of the Constrained Application
Protocol on the Short Message Service (SMS) of mobile cellular
networks.
1.1. Motivation
In some M2M environments, internet connectivity is not supported by
the constrained end-points, but a cellular network connection is
supported instead. Internet connectivity might also be switched off
for power saving reasons or the cellular coverage does not allow for
Internet connectivity. In these situations, SMS will be supported,
instead of UDP/IP over General Packet Radio Service (GPRS), High
Speed Packet Access (HSPA) or Long Term Evolution (LTE) networks.
In 3GPP, SMS is identified as the transport protocol for small data
transmissions (See [3gpp_ts23_888] for Key Issue on Machine Type
Communication (MTC) Device Trigger and the proposed solutions in
Sections 6.2, 6.42, 6.44, 6.48, 6.52, 6.60, and 6.61). In
[3gpp_ts23_682] 'Architecture Enhancements to facilitate
communications with Packet Data Networks and Applications' SMS is at
the moment the only Trigger Delivery (Trigger Delivery using T4).
M2M protocols using SMS, e.g. for telematics, are using mostly
various diverse proprietary and closed binary protocols with limited
publicly available documentation at the moment.
In Open Mobile Alliance (OMA) LightweightM2M technical specification
[oma_lightweightm2m_ts], SMS is identified as an alternative
transport for CoAP messages.
2. Terminology
This document uses the following terminology:
CoAP Server and Client
The terms CoAP Server and CoAP Client are used synonymously to
Server and Client as specified in the terminology section of
[I-D.ietf-core-coap].
Mobile Station (MS)
A Mobile Station includes all required user equipment and software
that is needed for communication with a mobile network. As
defined in [etsi_ts101_748].
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3. Requirements 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 [RFC2119].
4. Scenarios
Several scenarios are presented first for M2M communications with
CoAP over SMS. First Mobile-Originating Mobile-Terminating (MO-MT)
scenarios are presented, where both CoAP endpoints are in devices in
a cellular network. Next, Mobile-Terminating (MT) scenarios are
detailed, where only the CoAP server is in a cellular network.
Finally, Mobile-Originating (MO) scenarios where the CoAP client is
in the cellular network.
4.1. MO-MT Scenarios
Two mobile cellular terminals communicate by exchanging a CoAP
Request and Response embedded into short message protocol data units
(PDUs) (depicted in Figure 1). Both terminals are connected via a
Short Message Service Centre (SMS-C).
CoAP-REQ CoAP-REQ
+------+ (SMS) +-------+ (SMS) +------+
| A | --------> | SMS-C | -------> | B |
|(cell)| <-------- | | <------- |(cell)|
+------+ CoAP-RES +-------+ CoAP-RES +------+
(SMS) (SMS)
Figure 1: Cellular and Cellular Communication (only SMS-based)
4.2. MT Scenarios
An IP host and a mobile cellular terminal communicate by exchanging
CoAP Request and Response. The IP host uses protocols offered by the
SMS-C (e.g. Short Message Peer-to-Peer (SMPP [smpp]), Computer
Interface to Message Distribution (CIMD [cimd]), Universal Computer
Protocol/External Machine Interface (UCP/EMI [ucp])) to submit a
short message for delivery, which contains the CoAP Request (depicted
in Figure 2).
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CIMD CoAP-REQ
+------+ SMPP +-------+ (SMS) +------+
| A | --------> | SMS-C | ---------> | B |
| (IP) | <-------- | | <--------- |(cell)|
+------+ +-------+ CoAP-RES +------+
(SMS)
Figure 2: IP and Cellular Communication
There are service providers that offer SMS delivery and notification
using an HTTP/REST interface (depicted in Figure 3).
HTTP-REQ CIMD CoAP-REQ
+------+ (CoAP-DATA) +----------+ SMPP +-----+ (SMS) +------+
| | | SMS | SS7 |SMS-C| | |
| A | ----------> | Service | ------> | / | ---------> | B |
| (IP) | <---------- | Provider | <------ | HLR | <--------- |(cell)|
+------+ HTTP-RES +----------+ +-----+ CoAP-RES +------+
(CoAP-DATA) (SMS)
Figure 3: IP and Cellular Communication (using an SMS Service
Provider)
4.3. MO Scenarios
A mobile cellular terminal and an IP host communicate by exchanging
CoAP Request and Response. The mobile cellular terminal sends a CoAP
Request in a short message, which is in turn forwarded by the SMS-C
(e.g. with Short Message Peer-to-Peer (SMPP [smpp]), Computer
Interface to Message Distribution (CIMD [cimd]), Universal Computer
Protocol/External Machine Interface (UCP/EMI [ucp])) as depicted in
Figure 4). This scenario can be a fall-back for mobile-originating
communication, when IP connectivity cannot be setup (e.g. due to
missing coverage).
CoAP-REQ CIMD
+------+ (SMS) +-------+ SMPP +------+
| A | --------> | SMS-C | ---------> | B |
|(cell)| <-------- | | <--------- | (IP) |
+------+ CoAP-RES +-------+ +------+
(SMS)
Figure 4: Cellular and IP Communication
There are service providers offering SMS delivery and notification
using an HTTP/REST interface (depicted in Figure 5).
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CoAP-REQ CIMD HTTP-REQ
+------+ (SMS) +-------+ SMPP +----------+ (CoAP-DATA) +----+
| | | SMS-C | SS7 | SMS | | |
| A | ---------> | / | -----> | Service | ----------> | B |
|(cell)| <--------- | HLR | <----- | Provider | <---------- |(IP)|
+------+ CoAP-RES +-------+ +----------+ HTTP-RES +----+
(SMS) (CoAP-DATA)
Figure 5: IP and Cellular Communication (using an SMS Service
Provider)
5. Message Exchanges
5.1. Message Exchange for SMS in a Cellular-To-Cellular Mobile-
Originated and Mobile-Terminated Scenario
The CoAP Client works as a Mobile Station to send the short message,
and the CoAP Server works as another Mobile Station to receive the
short message. All short messages are stored and forwarded by the
Service Center. The message exchange between the CoAP Client and the
CoAP Server is depicted in the figure below:
MS/CoAP CLIENT Service Center MS/CoAP SERVER
| | |
| ---SMS-SUBMIT---> | |
| <-SMS-SUBMIT-REPORT-- | |
| | |
| | --SMS-DELIVER---> |
| | <-SMS-DELIVER-REPORT-- |
| | |
| <-SMS-STATUS-REPORT-- | |
| | |
Figure 6: CoAP Messages over SMS
Note that the message exchange is just for one request message from
CoAP Client and CoAP Server. It includes the following steps:
Step 1: The CoAP Client sends a CoAP request in a SMS-SUBMIT message
to the Service Center. The CoAP Server address is specified as TP-
Destination-Address (see [3gpp_ts_23_040]).
Step 2: The Service Center returns a SMS-SUBMIT-REPORT message to the
CoAP Client.
Step 3: The Service Center stores the received SMS message and
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forwards it to the CoAP Server, using an SMS-DELIVER message. The
CoAP Client address is specified as a TP Originating Address (see
[3gpp_ts_23_040]).
Step 4: The CoAP Server returns an SMS-DELIVER-REPORT message to the
Service Center.
Step 5: The Service Center returns the SMS-STATUS-REPORT message to
the CoAP Client to indicate the SMS delivery status, if required by
the CoAP Client.
Note that the SMS-STATUS-REPORT message just indicates the transport
layer SMS delivery status and has no relationship with the
confirmable message or non-confirmable message. If the CoAP Client
has sent a confirmable message, the CoAP Server MUST use a separate
SMS message to transmit the ACK.
6. Encoding Schemes of CoAP for SMS transport
Short messages can be encoded by using various alphabets: GSM 7 bit
default alphabet ([3gpp_ts23_038]), 8 bit data alphabet, and 16 bit
USC2 data alphabet ([iso_ucs2]). These encodings lead to message
sizes of 160, 140, and 70 characters, respectively. The support of 7
bit encoding is mandatory on a MS. The 7 bit and 16 bit encodings
are dependent on the language that needs to be encoded, e.g. USC2
for Arabic, Chinese, Japanese, etc. 8 bit encoding can be used for
user specific encoding. Furthermore, the supported encoding scheme
highly depends on the implementations of the MS itself.
According to [3gpp_ts23_038], GSM 7 bit encoding shall be supported
by all MSs offering SMS services. Since not all MSs support 8 bit
short message encoding, the prefered encoding scheme for CoAP
messages over SMS is therefore 7 bit, e.g. together with Base64
([RFC4648]) or SMS encoding in [I-D.bormann-coap-misc].
More considerations about SMS encoding can be found in
[I-D.bormann-coap-misc].
7. Message Size Implementation Considerations
By using 7 bit encoding, a maximum length of 160 characters is
allowed in one short message [3gpp_ts23_038]. Consequently, the
maximum length for a CoAP message results in 140 bytes. 160
characters = (140 bytes * 8)/7.
Possible options for larger CoAP messages are:
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Concatenated short messages
Most MSs are able to send concatenation short messages in order to
tranmsit longer messages. The total length of a concatenated
short message can consist of up to 255 single messages and result
in total length of 39015 7 bit characters or 34170 bytes.
Resulting from this, the maximum length of each individual message
reduces to 153 (160 - 7) characters (133 bytes).
CoAP blockwise transfer
According to [I-D.ietf-core-block], the Block Size (SZX) of
blockwise transfer in CoAP is represented as a three-bit unsigned
integer. Thus, the possible block sizes are to the power of two.
(Block size = 2**(SZX + 4)). Due to the limitations of 160
characters (140 bytes) for one short message, the maximum value of
SZX is 3 (Block size = 128 byte).
However, it is RECOMMENDED that SMS is not used to transfer very
large resource data using blockwise transfer.
8. Addressing
For SMS in cellular networks, the CoAP endpoints have to work with a
SIM (Subscriber Identity Module) card and have to be addressed by the
MSISDN (Mobile Station ISDN (MSISDN) number).
To allow the CoAP client to detect that the short message contains a
CoAP message, the TP-DATA-Coding-Scheme SHOULD be included.
9. Options for mixed IP operation
In case a CoAP Server has more than one network interface, e.g. SMS
and IP, the CoAP Client might want the server to send the response
via an alternative transport, i.e. to it's alternative address.
However, that implies that the initiating CoAP Client is aware of the
presence of the alternative interface. For this reason the new
options Response-To-Uri-Host and Response-To-Uri-Port are proposed.
+----+---+---+---+---+-------------------+-------+--------+---------+
| No | C | U | N | R | Name | Forma | Length | Default |
| . | | | | | | t | | |
+----+---+---+---+---+-------------------+-------+--------+---------+
| 34 | | | | | Response-To-Uri-H | strin | 1-270 | (none) |
| | | | | | ost | g | B | |
| 38 | | | | | Response-To-Uri-P | uint | 0-2 B | 5683 |
| | | | | | ort | | | |
+----+---+---+---+---+-------------------+-------+--------+---------+
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Table 1: New CoAP Option Numbers
If the Response-To-Uri-Host is present in the request, server MUST
send the response to the indicated URI address, instead of the
client's original request URI.
The options SHOULD NOT be used in the response.
The options MUST NOT occur more than once.
10. URI Scheme
The coap:// scheme defines that a CoAP server is reachable over
UDP/IP. Hence, a new URI scheme is needed for CoAP servers which are
reachable over SMS.
TODO: Update whenever it has been clarified in
[I-D.silverajan-core-coap-alternative-transports]
11. Transmission Parameters
It is RECOMMENDED to configure the RESPONSE_TIMEOUT variable for a
higher duration than specified in [I-D.ietf-core-coap] for the
applications described here. The actual value SHOULD be chosen based
on experience with SMS .
12. Multicast
Multicast is not possible with SMS transports.
13. Security Considerations
It is possible that a malicious CoAP Client sends repeated requests,
and it may cost money for the CoAP Server to use SMS to send back
associated responses. To avoid this situation, the CoAP Server
implementation can authenticate the CoAP Client before responding to
the requests. For example, the CoAP Server can maintain an MSISDN
white list. Only the MSISDN specified in the white list will be
allowed to send requests. The requests from others will be ignored
or rejected.
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14. IANA Considerations
14.1. CoAP Option Number
The IANA is requested to add the following option number entries to
the CoAP Option Number Registry:
+--------+----------------------+----------------------------+
| Number | Name | Reference |
+--------+----------------------+----------------------------+
| 34 | Response-To-Uri-Host | Section 9 of this document |
+--------+----------------------+----------------------------+
| 38 | Response-To-Uri-Port | Section 9 of this document |
+--------+----------------------+----------------------------+
14.2. URI Scheme Registration
According to [I-D.silverajan-core-coap-alternative-transports] this
document requests the registration of the Uniform Resource Identifier
(URI) scheme "coap+alt". The registration request complies with
[RFC4395].
15. Acknowledgements
This document is partly based on research for the research project
'The Intelligent Container' which is supported by the Federal
Ministry of Education and Research, Germany, under reference number
01IA10001.
The authors of this draft would like to thank Bert Greevenbosch,
Marcus Goetting, Nils Schulte and Klaus Hartke for the discussions on
the topic and the reviews of this document.
16. References
16.1. Normative References
[3gpp_ts23_038]
ETSI 3GPP, "Technical Specification: Alphabets and
language-specific information (3GPP TS 23.038 version
11.0.0 Release 11)", 2012.
[I-D.bormann-coap-misc]
Bormann, C. and K. Hartke, "Miscellaneous additions to
CoAP", draft-bormann-coap-misc-25 (work in progress),
May 2013.
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[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-12 (work in progress), June 2013.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
[I-D.silverajan-core-coap-alternative-transports]
Silverajan, B. and T. Savolainen, "CoAP Communication with
Alternative Transports",
draft-silverajan-core-coap-alternative-transports-02 (work
in progress), July 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[etsi_ts101_748]
ETSI, "Technical Report: Digital cellular
telecommunications system; Abbreviations and acronyms (GSM
01.04 version 8.0.0 release 1999)", 2000.
[iso_ucs2]
ISO, "ISO/IEC10646: "Universal Multiple-Octet Coded
Character Set (UCS)"; UCS2, 16 bit coding.", 2000.
16.2. Informative References
[3gpp_ts23_682]
ETSI 3GPP, "Technical Specification Group Services and
System Aspects; Architecture Enhancements to facilitate
communications with Packet Data Networks and Applications;
(Release 11)", 2012.
[3gpp_ts23_888]
ETSI 3GPP, "Technical Specification Group Services and
System Aspects; System Improvements for Machine-Type
Communications; (3GPP TR 23.888 version 1.6.0, Release
11)", 2011.
[3gpp_ts_23_040]
3GPP, "Technical realization of the Short Message Service
(SMS)", 3GPP-23.040 a00, Mar 2011.
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[cimd] Nokia, "CIMD Interface Specification (SMSCDOC8000.00,
Nokia SMS Center 8.0)", 2005.
[oma_lightweightm2m_ts]
OMA, "Lightweight Machine to Machine Technical
Specification", 2013.
[smpp] SMPP Developers Forum, "Short Message Peer to Peer
Protocol Specification v3.4 Issue 1.2", 1999.
[ucp] Vodafone, "Short Message Service Centre (SMSC) External
Machine Interface (EMI) Description Version 4.3d", 2011.
Appendix A. Changelog
Changed from draft-03 to draft-04:
o Various editorial changes.
o Removed USSD and GPRS related parts.
o Removed section 5: Examples
o Removed section 14: Proxying Considerations
o Added more block size considerations.
o Added more concatenated SMS considerations.
o Rewrote encoding scheme section; 7 bit encoding only.
Changed from draft-02 to draft-03:
o Added reference to OMA LightweightM2M Technical Specification in
"Motivation" section.
o Chose CoAP option numbers and updated the option number table to
meet draft-ietf-core-coap-13.
Changed from draft-01 to draft-02:
o Added security considerations: Transport and Object Security.
Section 13
o Reply-To-* changed to Response-To-*. Section 14
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o Added URI scheme.
o Added possible CON/NON/ACK interactions.
o Added possible M2M proxy scenarios.
o Added reference to bormann-coap-misc for other SMS encoding.
Section 6
o Updated requirements on Uri-Host and Uri-Port for coap+tel://.
o Chose CoAP option numbers and updated the option number table to
meet draft-ietf-core-coap-10. />
o Added an IANA registration for the URI scheme. Section 14.2
Authors' Addresses
Markus Becker (editor)
ComNets, TZI, University Bremen
Bibliothekstrasse 1
Bremen 28359
Germany
Phone: +49 421 218 62379
Email: mab@comnets.uni-bremen.de
Kepeng Li
Huawei Technologies
Huawei Base, Bantian, Longgang District
Shenzhen, Guangdong 518129
P. R. China
Phone: +86-755-28974289
Email: likepeng@huawei.com
Thomas Poetsch
ComNets, TZI, University Bremen
Bibliothekstrasse 1
Bremen 28359
Germany
Phone: +49 421 218 62379
Email: thp@comnets.uni-bremen.de
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Koojana Kuladinithi
ComNets, TZI, University Bremen
Bibliothekstrasse 1
Bremen 28359
Germany
Phone: +49 421 218 62382
Email: koo@comnets.uni-bremen.de
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