Internet DRAFT - draft-toutain-core-time-scale
draft-toutain-core-time-scale
Network Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Informational L. Toutain
Expires: April 29, 2018 Institut Mines Telecom Atlantique
October 26, 2017
CoAP Time Scale Option
draft-toutain-core-time-scale-00
Abstract
SCHC compression mechanism for LPWAN network enables IPv6 on devices
connected to a constrained network (LPWAN). They can communicate
with a CoAP server located anywhere in the Internet. LPWAN network
characteristics limits the number of exchanges and may impose a long
RTT. The CoAP server must be aware of these properties to manage
correctly requests. The Time Scale option allows a device to inform
a CoAP server of the duration the message ID value should be kept in
memory to manage correctly message duplication.
Status of This Memo
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This Internet-Draft will expire on April 29, 2018.
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1. CoAP Message ID
Constraint Application Protocol (CoAP) [RFC7252] implements a simple
reliable transport mechanism based on ARQ. Each CoAP message
contains a 16 bit Message ID (noted afterward MID). A client selects
a MID in a CON message and expects an ACK message containing the same
MID value. A timer makes the client resend the request if no ACK is
received during a pre-defined period.
To avoid a second process of duplicated requests by the server, a
list of messages ID already acknowledged must be maintained for a
period of time. If the message ID is already in the list, the
message is just acknowledged and not processed by upper layer.
Therefore, the client cannot use this MID value in another request
during the same period of time.
client server
| CON MID = XXXX |
|---------------------------------->|=> process
EXCHANGE ^ | ACK MID = XXXX | ^
LIFETIME | | <----------------| | EXCHANGE
| | | | LIFETIME
XXXX | | CON MID = XXXX | |
cannot | |---------------------------------->| | XXXX in
be reused | | ACK MID = XXXX | | memory
| |<----------------------------------| |
| | | |
v | | v
XXXX can be . .
reused . .
. .
| CON MID = XXXX |
|---------------------------------->|=> process
^ | ACK MID = XXXX | ^
| |<----------------------------------| |
. .
Figure 1: Delayed transmission.
[RFC7252] calls the period a MID is assigned to a request the
EXCHANGE_LIFETIME. The value is based on the worst case scenario
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taking into account the propagation time, the number of
retransmissions and the processing time. The default value for
EXCHANGE_LIFETIME is set to 247 seconds for MAX_RTT of 202 seconds.
2. LPWAN networks
Low Power Wide Area Network (LPWAN) family regroups networks
dedicated to the Internet of Things. They provide a large coverage
with a limited energy consumption. They mostly use the license-free
ISM band. The [I-D.ietf-lpwan-overview] gives an overview of the
technology and the star oriented topology architecture. A Network
Gateway (NGW) is at the interconnection between the LPWAN and the
Internet network.
To ensure fairness among nodes, regulation imposes a duty cycle. In
practice, with a 1% duty cycle, a node sending a message of s seconds
must wait 99 x s seconds before sending another message. For
instance, in some technologies sending a 50 bytes message takes 2
seconds, forcing a silence of 198 seconds.
The device sleeps most of the time to preserve energy. If a device
can use the uplink channel at any time, downlink channel is generally
available during a short receiving window following the message
emission. Therefore a message sent to a device out of this receiving
window will be lost. Network Gateways are aware of this restriction
and buffers downlink messages until an uplink message is received
which opens the receiving window.
Figure 2 illustrates this. A CoAP client sends a request every hour.
Even if the server replies immediately, the answer may be buffered by
the Network GW until an new uplink message is sent. In that case,
the client will only receive the answer after one hour when the next
request is sent. The RTT is influenced by the message periodicity
and the EXCHANGE_LIFTEIME value can be computed locally by client to
dimension its timers.
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client on LPWAN NGW server
| | |
^ |---------------------------------->| CON MID = 1
| | | |
1 hour | | delayed H<----------------| ACK MID = 1
| | H |
| | H |
| | H |
v |---------------------------------->| CON MID = 2
ACK MID = 1 |<----------------| |
| | |
v v v
Figure 2: Delayed transmission.
The server should remain as generic as possible and EXCHANGE_LIFETIME
parameter has to be adapted to the client behavior. If the period is
too large, the server will have to memorize a longer list of MID for
fast responding client. On the other hand, if the EXCHANGE_LIFETIME
is too short, this leads to misbehaviors as shown in Figure 3, a
retransmission will be viewed as a new request.
client on LPWAN NGW server
| CON MID = 1 | |
Timer - |---------------------------------->| => process
| | delayed H<----------------| ^
| | H ACK MID = 1 | | EXCHANGE
| | H | v LIFETIME
| | CON MID = 2 H |
| |---------------------------------->|
| | X---------| |
| | | |
| | CON MID = 1 | |
Expire O |---------------------------------->| => process
| |<----------------|
. . ACK MID = 1 .
. . .
Figure 3: Retransmission.
The Time Scale option, added into all the CoAP requests, informs the
server of the duration a message ID should be memorized into the
server and therefore the duration during which a client should not
reuse the same message ID for a new request. This way, the server
can adapt its behavior to different environments.
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It is important to notice that this option will not contribute to an
DoS attack. This option does not increase the number of message ID
memorized by the server. In fact, the Time Scale option can be
viewed as a contract between the client and the server, which means
that the client will send a reasonable number of request during that
period. The number of memorized message ID is independent of the
duration of the exchange but linked to the number a simultaneous
request a client can send. If a client is sending a number of
request larger than expected, they can be easily discarded by the
server.
3. Timescale Option
Timescale is a new CoAP option that tells the server how many seconds
the MID should be memorized by the server. This option must be
included in all the exchanges coming from a high latency device.
+--------+---+---+---+---+-------------+--------+--------+---------+
| Number | C | U | N | R | Name | Format | Length | Default |
+--------+---+---+---+---+-------------+--------+--------+---------+
| 259 | X | | | | Time Scale | uint | 1-4 | 3600 |
+--------+---+---+---+---+-------------+--------+--------+---------+
Figure 4: Time Scale Option.
This option is critical, if a server does not recognize it, it must
inform the client that EXCHANGE_LIFETIME cannot be modified. The
option is Safe-to-forward so a proxy does not have to understand this
option, since only the server is concerned with the MID management.
The value (in seconds) contains the new EXCHANGE_LIFETIME set by the
server for this request. If the value is smaller than the default
value, this option is discarded and the client receives an error
message.
4. Normative References
[I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-07 (work in progress), October 2017.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
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Authors' Addresses
Ana Minaburo
Acklio
2bis rue de la Chataigneraie
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
Laurent Toutain
Institut Mines Telecom Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@imt-atlantique.fr
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