Internet DRAFT - draft-goyal-roll-dis-modifications
draft-goyal-roll-dis-modifications
Internet Engineering Task Force M. Goyal
Internet-Draft University of Wisconsin
Intended status: Experimental Milwaukee
Expires: April 15, 2013 D. Barthel
France Telecom Orange
E. Baccelli
INRIA
October 12, 2012
DIS Modifications
draft-goyal-roll-dis-modifications-01
Abstract
This document specifies the DIS flags and options that allow an RPL
node to control how neighbor RPL routers respond to its solicitiation
for DIOs.
Status of this Memo
This Internet-Draft is submitted to IETF 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 April 15, 2013.
Copyright Notice
Copyright (c) 2012 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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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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. New Flags in the DIS Base Object . . . . . . . . . . . . . . . 4
4. DIS Options . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Metric Container . . . . . . . . . . . . . . . . . . . . . 5
4.2. Response Spreading Option . . . . . . . . . . . . . . . . 5
5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. A Leaf Node Joining a DAG . . . . . . . . . . . . . . . . 6
5.2. Identifying A Defunct DAG . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. DIS Flags . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. RPL Control Message Options . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
An RPL node can use a DODAG Information Solicitation (DIS) message to
solicit DODAG Information Object (DIO) messages from its neighbor RPL
routers. A DIS may carry a Solicited Information option that
specifies the predicates of the DAG(s) the node is interested in. In
the absence of a Solicited Information option, it is assumed that the
node generating the DIS is interested in receiving DIOs for all the
DAGs. A DIS can be multicast to all the in-range routers or it can
be unicast to a specific neighbor router. RPL requires a router to
consider the receipt of a multicast DIS as an inconsistency and hence
reset its Trickle timers [RFC6206] for the matching DAGs. The
receipt of a unicast DIS causes an RPL router to generate the DIOs
for all the matching DAGs without resetting the Trickle timers.
Consider an 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
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 would continue to transmit the DIOs
frequently for a considerable time interval. 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 an 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.
To deal with the situations described above, this document specifies
the DIS flags and options that allow an RPL node to control how
neighbor RPL routers respond to its solicitiation for DIOs:
o Using routing constraints to limit the number of responding
routers;
o Whether the responding routers should reset their Trickle timers;
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o Whether the responding routers should send a unicast DIO or a
multicast one;
o The time interval over which the responding routers must schedule
their DIO transmissions.
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 an RPL router or an RPL
host as defined in [RFC6550].
3. New Flags in the DIS Base Object
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
This document defines two new flags inside the DIS base object:
o "No Inconsistency" (N) flag: On receiving a unicast/multicast DIS
with N flag set, an RPL router MUST NOT reset the Trickle timers
for the matching DAGs. Also, a DIO generated in response to a DIS
with N flag set MUST always contain a Configuration option.
o "DIO Type" (T) flag: This flag specifies whether the responding
routers should transmit a multicast DIO or a unicast one. The
responding router MUST transmit a multicast DIO if this flag is
set.
The modified DIS base object is shown in Figure 1.
4. DIS Options
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4.1. Metric Container
In order to limit the number of routers that will respond to a
multicast DIS, this document allows the specification of routing
constraints inside a DIS that a router must satisfy in order to
respond to the DIS. These routing constraints are specified inside a
Metric Container option contained in the DIS. Thus, this document
allows the inclusion of a Metric Container option inside a DIS. An
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
aggregated value of the corresponding routing metric that the router
maintains for the matching DAG(s). The aggregated routing metric
values MUST satisfy all the mandatory constraints in order for the
router to generate DIOs for the matching DAG(s).
4.2. Response Spreading Option
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
A multicast DIS may lead to a large number of RPL routers responding
with DIOs. Concurrent transmissions by multiple routers are not
desirable. Such transmissions may end up in collisions. Unicast
DIOs may be able to avail of link-level retransmissions. However,
multicast DIOs have no such protection. These transmissions and
retransmissions may also cause congestion in the network. 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", shown in Figure 2, with a recommended Type
value 0x0A (to be confirmed by IANA). A Response Spreading option
may be included only inside a multicast DIS message. An RPL router
that responds to a multicast 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.
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5. 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. The second mechanism details
how a node may identify defunct DAGs for which it still maintains
state.
5.1. 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) and the "DIO Type" flag reset (to make responding
routers send unicast DIOs back). The DIS message can include a
Response Spreading option listing a suitable spreading interval and a
Metric Container listing the routing constraints that the responding
routers must satisfy. In each iteration, the node multicasts such a
DIS and waits for the DIOs. Once the spreading interval has expired,
the node considers the current iteration to be unsuccessful. Now the
node relaxes the routing constraints somewhat and proceeds to the
next iteration. The cycle repeats until the node receives one or
more DIOs in a particular iteration or if maximum number of
iterations have been reached.
5.2. Identifying A Defunct DAG
An 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, an RPL node may fail to
remove a neighbor from its parent set because:
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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.
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, an 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 next 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 an 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:
* "No Inconsistency" flag set so that the responding routers do
not reset their Trickle timers.
* "DIO Type" flag 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.
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* 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:
* 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.)
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 |
+-------+------------------+---------------+
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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.
+-------+--------------------+---------------+
| Value | Meaning | Reference |
+-------+--------------------+---------------+
| 0x0A | Response Spreading | This document |
+-------+--------------------+---------------+
RPL Control Message Options
7. Security Considerations
TBA
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6550] Winter, T., Thubert, P., 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, March 2012.
8.2. Informative References
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
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, May 2008.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
June 2010.
[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
"The Trickle Algorithm", RFC 6206, March 2011.
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Authors' Addresses
Mukul Goyal
University of Wisconsin Milwaukee
3200 N Cramer St
Milwaukee, WI 53201
USA
Phone: +1 414 2295001
Email: mukul@uwm.edu
Dominique Barthel
France Telecom Orange
28 Chemin Du Vieux Chene, BP 98
Meylan, 38243
France
Email: dominique.barthel@orange-ftgroup.com
Emmanuel Baccelli
INRIA
Phone: +33-169-335-511
Email: Emmanuel.Baccelli@inria.fr
URI: http://www.emmanuelbaccelli.org/
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