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Internet Engineering Task Force D. Zhong, Ed.
Internet-Draft INSA Lyon
Intended status: Standards Track D. Barthel
Expires: May 7, 2016 Orange
E. Baccelli
INRIA
November 4, 2015


DIS Modifications
draft-zhong-roll-dis-modifications-00

Abstract

This document augments [RFC6550] with DIS flags and options that
allow a RPL node to better control how neighbor RPL routers respond
to its solicitation for DIOs.

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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on May 7, 2016.

Copyright Notice

Copyright (c) 2015 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
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publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of




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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. RFC6550 refresher . . . . . . . . . . . . . . . . . . . . 2
1.2. Undesirable effects . . . . . . . . . . . . . . . . . . . 3
1.3. Desired improvments . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. DIS Base Object flags . . . . . . . . . . . . . . . . . . . . 5
4. DIS Options . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Metric Container . . . . . . . . . . . . . . . . . . . . 5
4.2. Response Spreading . . . . . . . . . . . . . . . . . . . 6
5. Full behavior illustration . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. DIS Flags . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. RPL Control Message Options . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Applications . . . . . . . . . . . . . . . . . . . . 10
A.1. A Leaf Node Joining a DAG . . . . . . . . . . . . . . . . 10
A.2. Identifying A Defunct DAG . . . . . . . . . . . . . . . . 11
Appendix B. Experimental data . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13

1. Introduction

This document augments [RFC6550], the RPL routing protocol
specification.

1.1. RFC6550 refresher

Per [RFC6550], a RPL node can send a DODAG Information Solicitation
(DIS) message to solicit DODAG Information Object (DIO) messages from
neighbor RPL routers.

A DIS can be multicast to all the routers in range or it can be
unicast to a specific neighbor router.

A DIS may carry a Solicited Information option that specifies the
predicates of the DAG(s) the soliciting node is interested in. In
the absence of such Solicited Information option, the soliciting node
is deemed interested in receiving DIOs for all the DAGs known by the
solicited router(s).



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[RFC6550] requires a router to treat the receipt of a multicast DIS
as an inconsistency and hence reset its Trickle timers for the
matching DAGs. As a result of the general Trickle timer mechanism,
future DIOs will be sent at a higher rate. See [RFC6206] for the
specification of Trickle timers and the definition of
"inconsistency".

[RFC6550] requires a router that receives a unicast DIS to respond by
unicasting a DIO for each matching DAG and to not reset the
associated Trickle timer. Such a DIO generated in response to a
unicast DIS must contain a Configuration option.

This description is summarized in Table 1.

+----------------------------+----------------------+---------------+
| | Unicast DIS | Multicast DIS |
+----------------------------+----------------------+---------------+
| no option present | unicast DIO, don't | do reset |
| | reset Trickle timer | Trickle timer |
| -------------------------- | -------------------- | ------------- |
| Solicited Information | do nothing | do nothing |
| option present, not | | |
| matching | | |
| -------------------------- | -------------------- | ------------- |
| Solicited Information | unicast DIO, don't | do reset |
| option present, matching | reset Trickle timer | Trickle timer |
+----------------------------+----------------------+---------------+

Table 1: Router behavior on receiving a DIS, as per RFC6550

More precisely, Table 1 describes the behavior of routers for each
DAG they belong to. In the general case where multiple RPL instances
co-exist in a network, routers will maintain a Trickle timer for the
one DAG of each RPL instance they belong to, and nodes may send a DIS
with multiple Solicited Information options pertaining to different
DAGs or instances. In this more general case, routers will respond
for each individual DAG/instance they belong to as per Table 1.

1.2. Undesirable effects

Now, consider a RPL leaf node that desires to join a certain DAG.
This node can either wait for its neighbor RPL routers to voluntarily
transmit DIOs or it can proactively solicit DIOs using a DIS message.
Voluntary DIO transmissions may happen after a very long time if the
network is stable and the Trickle timer intervals have reached large
values. Thus, proactively seeking DIOs using a DIS may be the only
reasonable option. Since the node does not know which neighbor
routers belong to the DAG, it must solicit the DIOs using a multicast



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DIS (with predicates of the desired DAG specified inside a Solicited
Information option). On receiving this DIS, the neighbor routers
that belong to the desired DAG will reset their Trickle timers and
quickly transmit their DIOs. The downside of resetting Trickle
timers is that the routers will keep transmitting frequent DIOs for a
considerable duration until the Trickle timers again reach long
intervals. These DIO transmissions are unnecessary, consume precious
energy and may contribute to congestion in the network.

There are other scenarios where resetting of Trickle timer following
the receipt of a multicast DIS is not appropriate. For example,
consider a RPL router that desires to free up memory by deleting
state for the defunct DAGs it belongs to. Identifying a defunct DAG
may require the node to solicit DIOs from its DAG parents using a
multicast DIS.

1.3. Desired improvments

To deal with the situations described above, there is a need in the
industry for DIS flags and options that allow a RPL node to control
how neighbor RPL routers respond to its solicitation for DIOs, for
example by expressing:

o the routing constraints that routers should meet to be allowed to
respond, thereby lowering the number of responders

o whether the responding routers should reset their Trickle timers
or not, thereby limiting the cumulated number of transmitted DIOs

o whether the responding routers should respond with a unicast DIO
instead of a multicast one, thereby lowering the overhearing cost
in the network

o the time interval over which the responding routers should
schedule their DIO transmissions, thereby lowering the occurence
of collisions.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].

Additionally, this document uses terminology from [RFC6550].
Specifically, the term RPL node refers to a RPL router or a RPL host
as defined in [RFC6550].




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3. DIS Base Object flags

This document defines two new flags inside the DIS base object:

o the "No Inconsistency" (N) flag: On receiving a multicast DIS with
the N flag set, a RPL router MUST NOT reset the Trickle timers for
the matching DAGs. In addition, it MUST take specific action,
which is to respond by explicitely sending a DIO. This DIO MUST
include a Configuration option. This behavior augments [RFC6550],
which had provision for such flag. Since this specific, one-shot
DIO is not a consequence of the general Trickle timer mechanism,
it will be sent right away if no Response Spreading option is
present or it will be scheduled according to the Response
Spreading option if one is present in the DIS (see Section 4.2).

o the "DIO Type" (T) flag: In case the N flag is set, this T flag
specifies what type of DIO is sent in response. It MUST be a
unicast DIO if this flag is set and it MUST be a multicast DIO if
this flag is reset.

When a unicast DIS is transmitted, both its N and T flags SHOULD be
0, which are the default values per [RFC6550]. On receiving a
unicast DIS, the N and T flags MUST be ignored and treated as 00.

The modified DIS base object is shown in Figure 1.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |N|T| Reserved | Option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Figure 1: Modified DIS Base Object

4. DIS Options

4.1. Metric Container

In order to lower the number of routers that will respond to a DIS,
this document allows routing constraints to be carried by a DIS.
Only the router(s) that satisfy these constraints is (are) allowed to
respond to the DIS.

These routing constraints are described using a Metric Container
option contained in the DIS. Metric Containers are defined in
[RFC6550] and [RFC6551]. Metric Containers options were previously




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only allowed in DIOs. This document augments [RFC6550] by allowing
the inclusion of a Metric Container option inside a DIS as well.

A RPL router that receives a DIS with a Metric Container option MUST
ignore any Metric object in it, and MUST evaluate the "mandatory"
Constraint objects in it by comparing the constraint value to the
value of the corresponding routing metric that the router maintains
for the matching DAG(s). These routing metric values MUST satisfy
all the mandatory constraints in order for the router to consider the
solicitation successful for the matching DAG(s). This augments the
behavior already present in [RFC6550] with the Solicited Information
option.

This option can be used in both unicast and multicast DIS.

4.2. Response Spreading

0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x0A | Length | Spread. Inter.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: The Response Spreading option

Even with the use of the Solicited Information and the Section 4.1
options, a multicast DIS may still lead to a large number of RPL
routers taking immediate action and responding with DIOs. Concurrent
transmissions by multiple routers are not desirable since they may
lead to poor channel utilization or even to packet loss. Unicast
DIOs may be able to avail of link-level retransmissions. However,
multicast DIOs usually have no such protection, since they commonly
make use of link layer broadcast. To avoid such problems, this
document specifies an optional DIO response spreading mechanism.

This document defines a new RPL control message option called
Response Spreading option, shown in Figure 2, with a recommended Type
value 0x0A (to be confirmed by IANA). A RPL router that explicitely
responds with a specific, one-shot DIO to a DIS that includes a
Response Spreading option, MUST wait for a time uniformly chosen in
the interval [O..2^SpreadingInterval], expressed in ms, before
attempting to transmit its DIO. If the DIS does not include a
Response Spreading option, the node is free to transmit the DIO as it
otherwise would.

A Response Spreading option MAY be included inside a unicast DIS
message, but there is no benefit in doing so.




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Multiple Response Spreading options SHOULD NOT be used inside a same
DIS message.

This mechanism MUST NOT affect the Trickle timer mechanism.

5. Full behavior illustration

Figure 3 and Figure 4 illustrate the normative behavior described in
Section 3 and Section 4.1.

+--------------------+----------------------
| Unicast DIS | Multicast DIS
+--------------------+--------------------+-
| | N=0 |
+--------------------+--------------------+--------------------+-
| | unicast DIO, | |
| no option present | don't | do |
| | reset Trickle timer| reset Trickle timer|
+--------------------+--------------------+--------------------+-
| Solicited Informa- | | |
| tion/Metric Contai-| do nothing | do nothing |
| ner option present,| | |
| not matching. | | |
+--------------------+--------------------+--------------------+-
| Solicited Informa- | unicast DIO, | |
| tion/Metric Contai-| don't | do |
| ner option present,| reset Trickle timer| reset Trickle timer|
| matching. | | |
+--------------------+--------------------+--------------------+-

Figure 3: Overall DIS behavior, part 1

Notice that Figure 3 is indeed identical to Table 1 when Metric
Container options are not used in DIS.

















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-------------------------------------------+
Multicast DIS |
----------------------+--------------------+
| N=1, T=0 | N=1, T=1 |
-+--------------------+--------------------+
| multicast DIO, | unicast DIO, |
| don't | don't |
| reset Trickle timer| reset Trickle timer|
-+--------------------+--------------------+
| | |
| do nothing | do nothing |
| | |
| | |
-+--------------------+--------------------+
| multicast DIO, | unicast DIO, |
| don't | don't |
| reset Trickle timer| reset Trickle timer|
| | |
-+--------------------+--------------------+

Figure 4: Overall DIS behavior, part 2

For the sake of completeness, let's remind here that a specific, one-
shot DIO generated in response to a DIS must contain a Configuration
option and that its transmission is delayed according to the Delay
Spreading option of the DIS, if one such option is present.

6. IANA Considerations

6.1. DIS Flags

IANA is requested to allocate bits 6 and 7 of the DIS Flag Field to
become the "No Inconsistency" and "DIO Type" bits, the functionality
of which is described in Section 3 of this document.

+-------+------------------+---------------+
| Value | Meaning | Reference |
+-------+------------------+---------------+
| 6 | No Inconsistency | This document |
| 7 | DIO Type | This document |
+-------+------------------+---------------+

6.2. RPL Control Message Options

IANA is requested to allocate a new code point in the "RPL Control
Message Options" registry for the "Response Spreading" option, the
behavior of which is described in Section 4.2.




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+-------+--------------------+---------------+
| Value | Meaning | Reference |
+-------+--------------------+---------------+
| 0x0A | Response Spreading | This document |
+-------+--------------------+---------------+

RPL Control Message Options

7. Security Considerations

TBA

8. Acknowledgements

A lot of text in this document originates from now-expired [I-
D.goyal-roll-dis-modifications] co-authored with M. Goyal. The
requirements and solutions also draw from now-expired [I-D.dejean-
roll-selective-dis] co-authored with N. Dejean. Their contribution
is deeply acknowledged.

We also thank (TBA) for their useful feedback and discussion.

9. References

9.1. 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,
.

[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/
RFC6550, March 2012,
.

[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
and D. Barthel, "Routing Metrics Used for Path Calculation
in Low-Power and Lossy Networks", RFC 6551, DOI 10.17487/
RFC6551, March 2012,
.








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9.2. Informative References

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
.

[RFC5184] Teraoka, F., Gogo, K., Mitsuya, K., Shibui, R., and K.
Mitani, "Unified Layer 2 (L2) Abstractions for Layer 3
(L3)-Driven Fast Handover", RFC 5184, DOI 10.17487/
RFC5184, May 2008,
.

[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, DOI
10.17487/RFC5881, June 2010,
.

[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
"The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
March 2011, .

Appendix A. Applications

This section details two example mechanisms that use the DIS flags
and options defined in this document. The first mechanism describes
how a leaf node may join a desired DAG in an energy efficient manner.
The second mechanism details how a node may identify defunct DAGs for
which it still maintains state.

A.1. A Leaf Node Joining a DAG

A new leaf node that joins an established LLN runs an iterative
algorithm in which it requests (using multicast DIS) DIOs from
routers belonging to the desired DAG.

The DIS message has the "No Inconsistency" flag set to prevent
resetting of Trickle timer in responding routers, thereby keeping the
aggregated number of transmissions low. It also has the "DIO Type"
flag set to make responding routers send unicast DIOs back, thereby
not triggering full reception in nearby nodes that have state-of-the-
art radio receivers with hardware-based address filtering.

The DIS message can include a Response Spreading option prescribing a
suitable spreading interval based on the expected density of nearby
routers and on the expected Layer 2 technology.





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The DIS will likely include a Metric Container listing the routing
constraints that the responding routers must satisfy in order to be
allowed to respond.

At each iteration, the node multicasts such a DIS and waits for
forthcoming DIOs. After a time equal to the spreading interval, the
node considers the current iteration to be unsuccessful. The node
consequently relaxes the routing constraints somewhat and proceeds to
the next iteration.

The cycle repeats until the node receives one or more DIOs or until
it has relaxed the constraints to the lowest acceptable values.

This algorithm has been proven in the field to be extremely energy-
efficient, especially when routers have a wide communication range.

A.2. Identifying A Defunct DAG

A RPL node may remove a neighbor from its parent set for a DAG for a
number of reasons:

o The neighbor is no longer reachable, as determined using a
mechanism such as Neighbor Unreachanility Detection (NUD)
[RFC4861], Bidirectional Forwarding Detection (BFD) [RFC5881] or
L2 triggers [RFC5184]; or

o The neighbor advertises an infinite rank in the DAG; or

o Keeping the neighbor as a parent would required the node to
increase its rank beyond L + DAGMaxRankIncrease, where L is the
minimum rank the node has had in this DAG; or

o The neighbor advertises membership in a different DAG within the
same RPL Instance, where a different DAG is recognised by a
different DODAGID or a different DODAGVersionNumber.

Even if the conditions listed above exist, a RPL node may fail to
remove a neighbor from its parent set because:

o The node may fail to receive the neighbor's DIOs advertising an
increased rank or the neighbor's membership in a different DAG;

o The node may not check, and hence may not detect, the neighbor's
unreachability for a long time. For example, the node may not
have any data to send to this neighbor and hence may not encounter
any event (such as failure to send data to this neighbor) that
would trigger a check for the neighbor's reachability.




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In such cases, a node would continue to consider itself attached to a
DAG even if all its parents in the DAG are unreachable or have moved
to different DAGs. Such a DAG can be characterized as being defunct
from the node's perspective. If the node maintains state about a
large number of defunct DAGs, such state may prevent a considerable
portion of the total memory in the node from being available for more
useful purposes.

To alleviate the problem described above, a RPL node may invoke the
following procedure to identify a defunct DAG and delete the state it
maintains for this DAG. Note that, given the proactive nature of RPL
protocol, the lack of data traffic using a DAG can not be considered
a reliable indication of the DAG's defunction. Further, the Trickle
timer based control of DIO transmissions means the possibility of an
indefinite delay in the receipt of a new DIO from a functional DAG
parent. Hence, the mechanism described here is based on the use of a
DIS message to solicit DIOs about a DAG suspected of defunction.
Further, a multicast DIS is used so as to avoid the need to query
each parent individually and also to discover other neighbor routers
that may serve as the node's new parents in the DAG.

When a RPL node has not received a DIO from any of its parents in a
DAG for more than a locally configured time duration:

o The node generates a multicast DIS message with:

* the "No Inconsistency" flag set so that the responding routers
do not reset their Trickle timers.

* the "DIO Type" flag not set so that the responding routers send
multicast DIOs and other nodes in the vicinity do not need to
invoke this procedure.

* a Solicited Information option to identify the DAG in question.
This option must have the I and D flags set and the
RPLInstanceID/DODAGID fields must be set to values identifying
the DAG. The V flag inside the Solicited Information option
should not be set so as to allow the neighbors to send DIOs
advertising the latest version of the DAG.

* a Response Spreading option specifying a suitable time interval
over which the DIO responses may arrive.

o After sending the DIS, the node waits for the duration specified
inside the Response Spreading option to receive the DIOs generated
by its neighbors. At the conclusion of the wait duration:





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* If the node has received one or more DIOs advertising newer
version(s) of the DAG, it joins the latest version of the DAG,
selects a new parent set among the neighbors advertising the
latest DAG version and marks the DAG status as functional.

* Otherwise, if the node has not received a DIO advertising the
current version of the DAG from a neighbor in the parent set,
it removes that neighbor from the parent set. As a result, if
the node has no parent left in the DAG, it marks the DAG as
defunct and schedule the deletion of the state it has
maintained for the DAG after a locally configured "hold"
duration. (This is because, as per RPL specification, when a
node no longer has any parents left in a DAG, it is still
required to remember the DAG's identity (RPLInstanceID,
DODAGID, DODAGVersionNumber), the lowest rank (L) it has had in
this DAG and the DAGMaxRankIncrease value for the DAG for a
certain time interval to ensure that the node does not join an
earlier version of the DAG and does not rejoin the current
version of the DAG at a rank higher than L +
DAGMaxRankIncrease.)

Appendix B. Experimental data

The effectiveness of these flags and options has been measured on
real industrial hardware.

Data to be added

Authors' Addresses

Zhong Denglin (editor)
INSA Lyon
20 Avenue Albert Einstein
Villeurbanne 69621
France

Email: denglin.zhong@insa-lyon.fr


Dominique Barthel
Orange
28 Chemin Du Vieux Chene, BP 98
Meylan 38243
France

Email: dominique.barthel@orange.com





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Emmanuel Baccelli
INRIA

Phone: +33-169-335-511
Email: Emmanuel.Baccelli@inria.fr
URI: http://www.emmanuelbaccelli.org/













































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