ROLL R. Jadhav, Ed.
Internet-Draft Huawei
Intended status: Standards Track P. Thubert
Expires: March 30, 2019 Cisco
R. Sahoo
Z. Cao
Huawei
September 26, 2018
Efficient Route Invalidation
draft-ietf-roll-efficient-npdao-06
Abstract
This document describes the problems associated with the use of NPDAO
messaging in RPL and signaling changes to improve route invalidation
efficiency.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language and Terminology . . . . . . . . . . 3
1.2. Current NPDAO messaging . . . . . . . . . . . . . . . . . 3
1.3. Why NPDAO is important? . . . . . . . . . . . . . . . . . 4
2. Problems with current NPDAO messaging . . . . . . . . 5
2.1. Lost NPDAO due to link break to the previous parent . . . 5
2.2. Invalidate routes to dependent nodes . . . . . . . . . . 5
2.3. Possible route downtime caused by async operation of
NPDAO and DAO . . . . . . . . . . . . . . . . . . . . . . 5
3. Requirements for the NPDAO Optimization . . . . . . . . . . . 5
3.1. Req#1: Tolerant to link failures to the previous
parents . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Req#2: Dependent nodes route invalidation on parent
switching . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Req#3: Route invalidation should not impact data traffic 6
4. Proposed changes to RPL signaling . . . . . . . . . . . . . . 6
4.1. Change in RPL route invalidation semantics . . . . . . . 6
4.2. Transit Information Option changes . . . . . . . . . . . 7
4.3. Destination Cleanup Object (DCO) . . . . . . . . . . . . 8
4.3.1. Secure DCO . . . . . . . . . . . . . . . . . . . . . 9
4.3.2. DCO Options . . . . . . . . . . . . . . . . . . . . . 9
4.3.3. Path Sequence number in the DCO . . . . . . . . . . . 9
4.3.4. Destination Cleanup Option Acknowledgement (DCO-ACK) 9
4.3.5. Secure DCO-ACK . . . . . . . . . . . . . . . . . . . 10
4.4. Other considerations . . . . . . . . . . . . . . . . . . 11
4.4.1. Dependent Nodes invalidation . . . . . . . . . . . . 11
4.4.2. NPDAO and DCO in the same network . . . . . . . . . . 11
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Example DCO Messaging . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
RPL [RFC6550] specifies a proactive distance-vector based routing
scheme. RPL has an optional messaging in the form of DAO messages
using which the 6LBR can learn route towards the nodes. In storing
mode, DAO messages would result in routing entries been created on
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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 NPDAO is a DAO message with route
lifetime of zero, originates at the target node and always flows
upstream towards the 6LBR. This document explains the problems
associated with the current use of NPDAO messaging and also discusses
the requirements for an optimized route invalidation messaging
scheme. Further a new pro-active route invalidation message called
as "Destination Cleanup Object (DCO)" is specified which fulfills
requirements of an optimized route invalidation messaging.
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.
DCO: Destination Cleanup Object, A new RPL control message type
defined by this draft.
Regular DAO: A DAO message with non-zero lifetime.
LLN: Low Power and Lossy Networks.
Target Node: The node switching its parent whose routing adjacencies
are updated (created/removed).
This document also uses terminology described in [RFC6550].
1.2. Current NPDAO messaging
RPL uses NPDAO messaging in the storing mode so that the node
changing it routing adjacencies can invalidate the previous route.
This is needed so that nodes along previous path can release any
resources (such as the routing entry) it maintains on behalf of
target node.
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For the rest of this document consider the following topology:
(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), RPL allows sending NPDAO to (B) and regular DAO to (C).
1.3. Why NPDAO is important?
Nodes in LLNs may be resource constrained. There is limited memory
available and routing entry records are 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. 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 stale route entries.
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2. Problems with current NPDAO messaging
2.1. Lost NPDAO due to link break to the previous parent
When a node switches its parent, the NPDAO is to be sent to its
previous parent and a regular DAO to 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.
2.2. Invalidate routes to dependent nodes
RPL does not specify how route invalidation will work for dependent
nodes rooted at switching node, resulting in stale routing entries of
the dependent nodes. 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. 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).
2.3. Possible route downtime caused by async 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 impacting downward traffic for the switching node.
In the example topology, consider Node (D) switches from parent (B)
to (C). 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.
3. Requirements for the NPDAO Optimization
3.1. Req#1: Tolerant to link failures to the previous parents
When the switching node sends the NPDAO 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 NPDAO message must be
tolerant to the link failure. The link referred here represents the
link between the node and its previous parent (from whom the node is
now disassociating).
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3.2. Req#2: Dependent nodes route invalidation on parent switching
It should be possible to do route invalidation for dependent nodes
rooted at the switching node.
3.3. Req#3: Route invalidation should not impact data traffic
While sending the NPDAO and DAO messages, it is possible that the
NPDAO successfully invalidates the previous path, while the newly
sent DAO gets lost (new path not set up successfully). This will
result in downstream unreachability to the node switching paths.
Therefore, it is desirable that the route invalidation is
synchronized with the DAO to avoid the risk of route downtime.
4. Proposed changes to RPL signaling
4.1. Change in RPL route invalidation semantics
As described in Section 1.2, 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 adds
new pro-active route invalidation message called as "Destination
Cleanup Object" (DCO) that originates at a common ancestor node
between the new and old path. The common ancestor node generates a
DCO in response to the change in the next-hop on receiving a regular
DAO for the target.
In 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 a DCO 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.
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4.2. Transit Information Option 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 Transit Information option to contain
"Invalidate previous route" (I) bit. This I-bit signals the common
ancestor node to generate a DCO on behalf of the target node. The
I-bit is carried in the transit information option which augments the
reachability information for a given set of RPL Target(s).
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 common ancestor node SHOULD generate a DCO message in response to
this I-bit when it sees that the routing adjacencies have changed for
the target. I-bit governs the ownership of the DCO message in a way
that the target node is still in control of its own route
invalidation.
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4.3. Destination Cleanup Object (DCO)
A new ICMPv6 RPL control message type is defined by this
specification called as "Destination Cleanup Object" (DCO), which is
used for proactive cleanup of state and routing information held on
behalf of the target node by 6LRs. The DCO message always traverses
downstream and cleans up route information and other state
information associated with the given target.
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| Flags | Reserved | DCOSequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID(optional) +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
Figure 3: DCO base object
RPLInstanceID: 8-bit field indicating the topology instance
associated with the DODAG, as learned from the DIO.
K: The 'K' flag indicates that the recipient is expected to send a
DCO-ACK back.
D: The 'D' flag indicates that the DODAGID field is present. This
flag MUST be set when a local RPLInstanceID is used.
Flags: The 6 bits remaining unused in the Flags field are reserved
for future use. These bits MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
Reserved: 8-bit unused field. The field MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
DCOSequence: Incremented at each unique DCO message from a node and
echoed in the DCO-ACK message.
DODAGID (optional): 128-bit unsigned integer set by a DODAG root that
uniquely identifies a DODAG. This field is only present when the 'D'
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flag is set. This field is typically only present when a local
RPLInstanceID is in use, in order to identify the DODAGID that is
associated with the RPLInstanceID. When a global RPLInstanceID is in
use, this field need not be present. Unassigned bits of the DCO Base
are reserved. They MUST be set to zero on transmission and MUST be
ignored on reception.
4.3.1. Secure DCO
A Secure DCO message follows the format in [RFC6550] figure 7, where
the base message format is the DCO message shown in Figure 3.
4.3.2. DCO Options
The DCO message MAY carry valid options. This specification allows
for the DCO message to carry the following options:
0x00 Pad1
0x01 PadN
0x05 RPL Target
0x06 Transit Information
0x09 RPL Target Descriptor
The DCO carries a Target option and an associated Transit Information
option with a lifetime of 0x00000000 to indicate a loss of
reachability to that Target.
4.3.3. Path Sequence number in the DCO
A DCO message may contain a Path Sequence in the transit information
option to identify the freshness of the DCO message. The Path
Sequence in the DCO MUST use the same Path Sequence number present in
the regular DAO message when the DCO is generated in response to DAO
message.
4.3.4. Destination Cleanup Option Acknowledgement (DCO-ACK)
The DCO-ACK message may be sent as a unicast packet by a DCO
recipient in response to a unicast DCO message.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |D| Reserved | DCOSequence | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID(optional) +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: DCO-ACK base object
RPLInstanceID: 8-bit field indicating the topology instance
associated with the DODAG, as learned from the DIO.
D: The 'D' flag indicates that the DODAGID field is present. This
flag MUST be set when a local RPLInstanceID is used.
Reserved: 7-bit unused field. The field MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
DCOSequence: Incremented at each unique DCO message from a node and
echoed in the DCO-ACK message.
Status: Indicates the completion. Status 0 is defined as unqualified
acceptance in this specification. The remaining status values are
reserved as rejection codes.
DODAGID (optional): 128-bit unsigned integer set by a DODAG root that
uniquely identifies a DODAG. This field is only present when the 'D'
flag is set. This field is typically only present when a local
RPLInstanceID is in use, in order to identify the DODAGID that is
associated with the RPLInstanceID. When a global RPLInstanceID is in
use, this field need not be present. Unassigned bits of the DCO-Ack
Base are reserved. They MUST be set to zero on transmission and MUST
be ignored on reception.
4.3.5. Secure DCO-ACK
A Secure DCO-ACK message follows the format in [RFC6550] figure 7,
where the base message format is the DCO-ACK message shown in
Figure 4.
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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 document allows the dependent
nodes invalidation. Dependent nodes will generate their respective
DAOs to update their paths, and the previous route invalidation for
those nodes should work in the similar manner described for switching
node. The dependent node may set the I-bit in the transit
information option as part of regular DAO so as to request
invalidation of previous route from the common ancestor node.
4.4.2. NPDAO and DCO in the same network
Even with the changed semantics, the current NPDAO mechanism in
[RFC6550] can still be used. There are certain scenarios where
current NPDAO signalling may still be used, for example, when the
route lifetime expiry of the target happens or when the node simply
decides to gracefully terminate the RPL session on graceful node
shutdown. Moreover a deployment can have a mix of nodes supporting
the proposed DCO and the existing NPDAO mechanism.
5. Acknowledgements
Many thanks to Cenk Gundogan, Simon Duquennoy, Georgios
Papadopoulous, Peter Van Der Stok for their review and comments.
6. IANA Considerations
IANA is requested to allocate new ICMPv6 RPL control codes in RPL
[RFC6550] for DCO and DCO-ACK messages.
+------+---------------------------------------------+--------------+
| Code | Description | Reference |
+------+---------------------------------------------+--------------+
| 0x04 | Destination Cleanup Object | This |
| | | document |
| 0x05 | Destination Cleanup Object Acknowledgement | This |
| | | document |
| 0x84 | Secure Destination Cleanup Object | This |
| | | document |
| 0x85 | Secure Destination Cleanup Object | This |
| | Acknowledgement | document |
+------+---------------------------------------------+--------------+
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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 document handles security considerations inline to base RPL.
Secure versions of DCO and DCO-ACK are added similar to other RPL
messages. For general RPL security considerations, see [RFC6550].
8. References
8.1. 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,
<https://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,
<https://www.rfc-editor.org/info/rfc6550>.
8.2. Informative 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-14 (work
in progress), April 2018.
Appendix A. Example DCO 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
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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
DCO(tgt=D,pathseq=pathseq(DAO)) 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 DCO and invalidates routing entry of target D
and forwards the (un)reachability information downstream to B.
7. Similarly, B processes the DCO by invalidating the routing entry
of target D and forwards the (un)reachability information
downstream to D.
8. D ignores the DCO since the target is itself.
9. The propagation of the DCO will stop at any node where the node
does not have an routing information associated with the target.
If the routing information is present and the pathseq associated
is not older, then still the DCO is dropped.
Authors' Addresses
Rahul Arvind Jadhav (editor)
Huawei
Kundalahalli Village, Whitefield,
Bangalore, Karnataka 560037
India
Phone: +91-080-49160700
Email: rahul.ietf@gmail.com
Pascal Thubert
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis 06254
France
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
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Rabi Narayan Sahoo
Huawei
Kundalahalli Village, Whitefield,
Bangalore, Karnataka 560037
India
Phone: +91-080-49160700
Email: rabinarayans@huawei.com
Zhen Cao
Huawei
W Chang'an Ave
Beijing
China
Email: zhencao.ietf@gmail.com
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