Internet DRAFT - draft-jadhav-roll-efficient-npdao
draft-jadhav-roll-efficient-npdao
ROLL R. Jadhav, Ed.
Internet-Draft R. Sahoo
Intended status: Standards Track Z. Cao
Expires: November 13, 2017 Huawei Tech
May 12, 2017
No-Path DAO modifications
draft-jadhav-roll-efficient-npdao-01
Abstract
This document describes the problems associated with the use of No-
Path DAO messaging in RPL and a signaling changes to improve route
invalidation efficiency.
Status of This Memo
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This Internet-Draft will expire on November 13, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language and Terminology . . . . . . . . . . 3
1.2. Current No-Path DAO messaging . . . . . . . . . . . . . . 3
1.3. Cases when No-Path DAO may be used . . . . . . . . . . . 4
1.4. Why No-Path DAO is important? . . . . . . . . . . . . . . 5
2. Problems with current No-Path DAO messaging . . . . . . . . 5
2.1. Lost NP-DAO due to link break to the previous parent . . 5
2.2. Invalidate routes to dependent nodes of the switching
node . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Route downtime caused by asynchronous operation of
NPDAO and DAO . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirements for the No-Path DAO Optimization . . . . . . . . 6
3.1. Req#1: Tolerant to the link failures to the previous
parents . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Req#2: Dependent nodes route invalidation on parent
switching . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Req#3: No impact on traffic while NP-DAO operation in
progress . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Proposed changes to NPDAO signaling . . . . . . . . . . . . . 7
4.1. Change in NPDAO semantics . . . . . . . . . . . . . . . . 7
4.2. DAO message format changes . . . . . . . . . . . . . . . 7
4.2.1. Path Sequence number in the reverse NPDAO . . . . . . 9
4.3. Example messaging . . . . . . . . . . . . . . . . . . . . 9
4.4. Other considerations . . . . . . . . . . . . . . . . . . 10
4.4.1. Dependent Nodes invalidation . . . . . . . . . . . . 10
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
RPL [RFC6550] specifies a proactive distance-vector based routing
scheme. The specification has an optional messaging in the form of
DAO messages using which the 6LBR can learn route towards any of the
nodes. In storing mode, DAO messages would result in routing entries
been created on all intermediate hops from the node's parent all the
way towards the 6LBR.
RPL allows use of No-Path DAO (NPDAO) messaging to invalidate a
routing path corresponding to the given target, thus releasing
resources utilized on that path. A No-Path DAO is a DAO message with
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route lifetime of zero, signaling route invalidation for the given
target. This document explains the problems associated with the
current use of NPDAO messaging and also discusses the requirements
for an optimized No-Path DAO messaging scheme. The signalling change
specified fulfills all mentioned requirements of an optimized NPDAO
messaging.
6TiSCH architecture [I-D.ietf-6tisch-architecture] leverages RPL and
specifies use of non-storing and storing MOP for its routing
operation. Thus an improvement in NPDAO messaging will help optimize
6TiSCH based networks.
1.1. Requirements Language and 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].
The document only caters to the RPL's storing mode of operation
(MOP). The non-storing MOP does not require use of NPDAO for route
invalidation since routing entries are not maintained on 6LRs.
Common Ancestor node: 6LR node which is the first common node on the
old and new path for the child node.
NPDAO: No-Path DAO. A DAO message which has target with lifetime 0.
Reverse NPDAO: A No-Path DAO message which traverses downstream in
the network.
Regular DAO: A DAO message with non-zero lifetime.
This document also uses terminology described in [RFC6550].
1.2. Current No-Path DAO messaging
RPL introduced No-Path DAO messaging in the storing mode so that the
node switching its current parent can inform its parents and
ancestors to invalidate the existing route. Subsequently parents or
ancestors would release any resources (such as the routing entry) it
maintains on behalf of target node. The NPDAO message always
traverses the RPL tree in upward direction, originating at the target
node itself.
For the rest of this document consider the following topology:
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(6LBR)
|
|
|
(A)
/ \
/ \
/ \
(G) (H)
| |
| |
| |
(B) (C)
\ ;
\ ;
\ ;
(D)
/ \
/ \
/ \
(E) (F)
Figure 1: Sample topology
Node (D) is connected via preferred parent (B). (D) has an alternate
path via (C) towards the BR. Node (A) is the common ancestor for (D)
for paths through (B)-(G) and (C)-(H). When (D) switches from (B) to
(C), [RFC6550] suggests sending No-Path DAO to (B) and regular DAO to
(C).
1.3. Cases when No-Path DAO may be used
There are following cases in which a node switches its parent and may
employ No-Path DAO messaging:
Case I: Current parent becomes unavailable because of transient or
permanent link or parent node failure.
Case II: The node finds a better parent node i.e. the metrics of
another parent is better than its current parent.
Case III: The node switches to a new parent whom it "thinks" has a
better metric but does not in reality.
The usual steps of operation when the node switches the parent is
that the node sends a No-Path DAO message via its current parent to
invalidate its current route and subsequently it tries to establish a
new routing path by sending a new DAO via its new parent.
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1.4. Why No-Path DAO is important?
Nodes in LLNs may be resource constrained. There is limited memory
available and routing entry records are the one of the primary
elements occupying dynamic memory in the nodes. Route invalidation
helps 6LR nodes to decide which entries could be discarded to better
achieve resource utilization in case of contention. Thus it becomes
necessary to have efficient route invalidation mechanism. Also note
that a single parent switch may result in a "sub-tree" switching from
one parent to another. Thus the route invalidation needs to be done
on behalf of the sub-tree and not the switching node alone. In the
above example, when Node (D) switches parent, the route invalidation
needs to be done for (D), (E) and (F). Thus without efficient route
invalidation, a 6LR may have to hold a lot of unwanted route entries.
2. Problems with current No-Path DAO messaging
2.1. Lost NP-DAO due to link break to the previous parent
When a node switches its parent, the NPDAO is to be sent via its
previous parent and a regular DAO via its new parent. In cases where
the node switches its parent because of transient or permanent parent
link/node failure then the NPDAO message is bound to fail. RPL
assumes communication link with the previous parent for No-Path DAO
messaging.
RPL allows use of route lifetime to remove unwanted routes in case
the routes could not be refreshed. But route lifetimes in case of
LLNs could be substantially high and thus the route entries would be
stuck for long.
2.2. Invalidate routes to dependent nodes of the switching node
No-path DAO is sent by the node who has switched the parent but it
does not work for the dependent child nodes below it. The
specification does not specify how route invalidation will work for
sub-childs, resulting in stale routing entries on behalf of the sub-
childs on the previous route. The only way for 6LR to invalidate the
route entries for dependent nodes would be to use route lifetime
expiry which could be substantially high for LLNs.
In the example topology, when Node (D) switches its parent, Node (D)
generates an NPDAO on its behalf. Post switching, Node (D) transmits
a DIO with incremented DTSN so that child nodes, node (E) and (F),
generate DAOs to trigger route update on the new path for themselves.
There is no NPDAO generated by these child nodes through the previous
path resulting in stale entries on nodes (B) and (G) for nodes (E)
and (F).
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2.3. Route downtime caused by asynchronous operation of NPDAO and DAO
A switching node may generate both an NPDAO and DAO via two different
paths at almost the same time. There is a possibility that an NPDAO
generated may invalidate the previous route and the regular DAO sent
via the new path gets lost on the way. This may result in route
downtime thus impacting downward traffic for the switching node. In
the example topology, consider Node (D) switches from parent (B) to
(C) because the metrics of the path via (C) are better. Note that
the previous path via (B) may still be available (albeit at
relatively bad metrics). An NPDAO sent from previous route may
invalidate the existing route whereas there is no way to determine
whether the new DAO has successfully updated the route entries on the
new path.
An implementation technique to avoid this problem is to further delay
the route invalidation by a fixed time interval after receiving an
NPDAO, considering the time taken for the new path to be established.
Coming up with such a time interval is tricky since the new route may
also not be available and it may subsequently require more parent
switches to establish a new path.
3. Requirements for the No-Path DAO Optimization
3.1. Req#1: Tolerant to the link failures to the previous parents
When the switching node send the NP-DAO message to the previous
parent, it is normal that the link to the previous parent is prone to
failure. Therefore, it is required that the NP-DAO message MUST be
tolerant to the link failure during the switching.
3.2. Req#2: Dependent nodes route invalidation on parent switching
While switching the parent node and sending NP-DAO message, it is
required that the routing entries to the dependent nodes of the
switching node will be updated accordingly on the previous parents
and other relevant upstream nodes.
3.3. Req#3: No impact on traffic while NP-DAO operation in progress
While sending the NP-DAO and DAO messages, it is possible that the
NP-DAO successfully invalidates the previous path, while the newly
sent DAO gets lost (new path not set up successfully). This will
result into downstream unreachability to the current switching node.
Therefore, it is desirable that the NP-DAO is synchronized with the
DAO to avoid the risk of route downtime.
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4. Proposed changes to NPDAO signaling
4.1. Change in NPDAO semantics
As described in Section 1.2, currently the NPDAO originates at the
node switching the parent and traverses upstream towards the root.
In order to solve the problems as mentioned in Section 2, the draft
proposes to change the way NPDAO originates and traverses the
network. The proposed NPDAO originates at a common ancestor node
between the new and old path. The trigger for the common ancestor
node to generate this NPDAO is the change in the next hop for the
node on reception of an update message in the form of regular DAO for
the target.
In the Figure 1, when node D decides to switch the path from B to C,
it sends a regular DAO to node C with reachability information
containing target as address of D and a incremented path sequence
number. Node C will update the routing table based on the
reachability information in DAO and in turn generate another DAO with
the same reachability information and forward it to H. Node H also
follows the same procedure as Node C and forwards it to node A. When
node A receives the regular DAO, it finds that it already has a
routing table entry on behalf of the target address of node D. It
finds however that the next hop information for reaching node D has
changed i.e. the node D has decided to change the paths. In this
case, Node A which is the common ancestor node for node D along the
two paths (previous and new), may generate an NPDAO which traverses
downwards in the network. The document in the subsequent section
will explain the message format changes to handle this downward flow
of NPDAO.
4.2. DAO message format changes
Every RPL message is divided into base message fields and additional
Options. The base fields apply to the message as a whole and options
are appended to add message/use-case specific attributes. As an
example, a DAO message may be attributed by one or more "RPL Target"
options which specifies the reachability information for the given
targets. Similarly, a Transit Information option may be associated
with a set of RPL Target options.
The draft proposes a change in DAO message to contain "Invalidate
previous route" (I) bit. This I-bit which is carried in regular DAO
message, signals the common ancestor node to generate a downstream
NPDAO on behalf of the target node. The I-bit is carried in the
transit container option which augments the reachability information
for a given set of RPL Target(s).
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x06 | Option Length |E|I| Flags | Path Control |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Sequence | Path Lifetime | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ +
| |
+ Parent Address* +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Updated Transit Information Option (New I flag added)
I (Invalidate previous route) bit: 1 bit flag. The 'I' flag is set
by the target node to indicate that it wishes to invalidate the
previous route by a common ancestor node between the two paths.
The NPDAO thus generated by the common ancestor node needs to
traverse downstream. An additional flag called as "Reverse NPDAO"
(R) is added in the base DAO object to signal this change.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |K|D|R| Flags | Reserved | DAOSequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID* +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
Figure 3: Updated DAO base object (New R flag added)
R (Reverse DAO) bit: 1 bit flag. The 'R' flag is used to signal that
the DAO traverses downwards.
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4.2.1. Path Sequence number in the reverse NPDAO
Every DAO message may contain a Path Sequence in the transit
information option to identify the freshness of the DAO message. The
Path Sequence in the downward NPDAO generated by common ancestor
should use the same Path Sequence number present in the regular DAO
message.
4.3. Example messaging
In Figure 1, node (D) switches its parent from (B) to (C). The
sequence of actions is as follows:
1. Node D switches its parent from node B to node C
2. D sends a regular DAO(tgt=D,pathseq=x+1,I_flag=1) in the updated
path to C
3. C checks for routing entry on behalf of D, since it cannot find
an entry on behalf of D it creates a new routing entry and
forwards the reachability information of the target D to H in a
DAO.
4. Similar to C, node H checks for routing entry on behalf of D,
cannot find an entry and hence creates a new routing entry and
forwards the reachability information of the target D to H in a
DAO.
5. Node A receives the DAO, and checks for routing entry on behalf
of D. It finds a routing entry but checks that the next hop for
target D is now changed. Node A checks the I_flag and generates
downstream NPDAO(tgt=D,pathseq=x+1,R_flag=1) to previous next hop
for target D which is G. Subsequently, A updates the routing
entry and forwards the reachability information of target D
upstream DAO(tgt=D,pathseq=x+1,I_flag=x) (the I_flag carries no
significance henceforth).
6. Node G receives the downstream NPDAO and invalidates routing
entry of target D and then checks the reverse (R) flag and
forwards the (un)reachability information downstream to B.
7. Similarly, B processes the downstream NPDAO by invalidating the
routing entry of target D and then checks the reverse (R) flag
and forwards the (un)reachability information downstream to D.
8. D ignores the downstream NPDAO since the target is itself.
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4.4. Other considerations
4.4.1. Dependent Nodes invalidation
Current RPL [RFC6550] does not provide a mechanism for route
invalidation for dependent nodes.
This section describes approaches for invalidating routes of
dependent nodes if the implementation chooses to solve this problem.
The common ancestor node realizes that the paths for dependent nodes
have changed (based on next hop change) when it receives a regular
DAO on behalf of the dependent nodes. Thus dependent nodes route
invalidation can be handled in the same way as the switching node.
Note that there is no way that dependent nodes can set the I_flag in
the DAO message selectively since they are unaware that their parent/
grand parent node is switching paths. There are two ways to handle
dependent node route invalidation:
1. One way to resolve is that the common ancestor does not depend
upon the I_flag to generate the reverse NPDAO. The only factor
it makes the decision will be based on next_hop change for an
existing target to generate the NPDAO. Thus when the switching
nodes and all the below dependent nodes advertise a regular DAO,
the common ancestor node will detect a change in next hop and
generate NPDAO for the same target as in the regular DAO.
2. Another way is that the nodes always set the I_flag whenever they
send regular DAO. Thus common ancestor will first check whether
I_flag is set and then check whether the next_hop has changed and
subsequently trigger NPDAO if required.
This document recommends the approach in point 2. The advantage with
I_flag is that the generation of downstream NPDAO is still controlled
by the target node and thus is still in control of its own routing
state.
5. Acknowledgements
We would like to thank Cenk Gundogan, Simon Duquennoy and Pascal
Thubert for their review and insightful comments.
6. IANA Considerations
IANA is requested to allocate bit 11 in the DAO base object defined
in RPL [RFC6550] section 6.4 for reverse 'R' NPDAO flag.
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IANA is requested to allocate bit 18 in the Transit Information
Option defined in RPL [RFC6550] section 6.7.8 for Invalidate route
'I' flag.
7. Security Considerations
This draft does not add any new messages but extends existing
messaging. The seucrity considerations applicable to DAO messaging
in RPL is also applicable here.
8. References
8.1. Normative References
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-11 (work
in progress), January 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc6550>.
8.2. Informative References
[CONTIKI] Thingsquare, "Contiki: The Open Source OS for IoT", 2012,
<http://www.contiki-os.org>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<http://www.rfc-editor.org/info/rfc3552>.
Appendix A. Additional Stuff
This becomes an Appendix.
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Authors' Addresses
Rahul Arvind Jadhav (editor)
Huawei Tech
Kundalahalli Village, Whitefield,
Bangalore, Karnataka 560037
India
Phone: +91-080-49160700
Email: rahul.ietf@gmail.com
Rabi Narayan Sahoo
Huawei Tech
Kundalahalli Village, Whitefield,
Bangalore, Karnataka 560037
India
Phone: +91-080-49160700
Email: rabinarayans@huawei.com
Zhen Cao
Huawei Tech
W Chang'an Ave
Beijing 560037
China
Email: zhencao.ietf@gmail.com
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