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draft-ietf-roll-aodv-rpl-15.txt
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ROLL C.E. Perkins
Internet-Draft Lupin Lodge
Intended status: Standards Track S.V.R.Anand
Expires: 3 April 2023 Indian Institute of Science
S. Anamalamudi
SRM University-AP
B. Liu
Huawei Technologies
30 September 2022
Supporting Asymmetric Links in Low Power Networks: AODV-RPL
draft-ietf-roll-aodv-rpl-15
Abstract
Route discovery for symmetric and asymmetric Peer-to-Peer (P2P)
traffic flows is a desirable feature in Low power and Lossy Networks
(LLNs). For that purpose, this document specifies a reactive P2P
route discovery mechanism for both hop-by-hop routes and source
routing: Ad Hoc On-demand Distance Vector Routing (AODV) based RPL
protocol (AODV-RPL). Paired Instances are used to construct
directional paths, for cases where there are asymmetric links between
source and target nodes.
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 https://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 3 April 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
Perkins, et al. Expires 3 April 2023 [Page 1]
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of AODV-RPL . . . . . . . . . . . . . . . . . . . . 7
4. AODV-RPL DIO Options . . . . . . . . . . . . . . . . . . . . 8
4.1. AODV-RPL RREQ Option . . . . . . . . . . . . . . . . . . 8
4.2. AODV-RPL RREP Option . . . . . . . . . . . . . . . . . . 10
4.3. AODV-RPL Target Option . . . . . . . . . . . . . . . . . 11
5. Symmetric and Asymmetric Routes . . . . . . . . . . . . . . . 13
6. AODV-RPL Operation . . . . . . . . . . . . . . . . . . . . . 15
6.1. Route Request Generation . . . . . . . . . . . . . . . . 15
6.2. Receiving and Forwarding RREQ messages . . . . . . . . . 16
6.2.1. Step 1: RREQ reception and evaluation . . . . . . . . 16
6.2.2. Step 2: TargNode and Intermediate Router
determination . . . . . . . . . . . . . . . . . . . . 17
6.2.3. Step 3: Intermediate Router RREQ processing . . . . . 18
6.2.4. Step 4: Symmetric Route Processing at an Intermediate
Router . . . . . . . . . . . . . . . . . . . . . . . 18
6.2.5. Step 5: RREQ propagation at an Intermediate Router . 19
6.2.6. Step 6: RREQ reception at TargNode . . . . . . . . . 19
6.3. Generating Route Reply (RREP) at TargNode . . . . . . . . 19
6.3.1. RREP-DIO for Symmetric route . . . . . . . . . . . . 19
6.3.2. RREP-DIO for Asymmetric Route . . . . . . . . . . . . 20
6.3.3. RPLInstanceID Pairing . . . . . . . . . . . . . . . . 20
6.4. Receiving and Forwarding Route Reply . . . . . . . . . . 21
6.4.1. Step 1: Receiving and Evaluation . . . . . . . . . . 21
6.4.2. Step 2: OrigNode or Intermediate Router . . . . . . . 21
6.4.3. Step 3: Build Route to TargNode . . . . . . . . . . . 21
6.4.4. Step 4: RREP Propagation . . . . . . . . . . . . . . 22
7. Gratuitous RREP . . . . . . . . . . . . . . . . . . . . . . . 22
8. Operation of Trickle Timer . . . . . . . . . . . . . . . . . 23
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10. Security Considerations . . . . . . . . . . . . . . . . . . . 24
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . 26
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Appendix A. Example: Using ETX/RSSI Values to determine value of S
bit . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Appendix B. Changelog . . . . . . . . . . . . . . . . . . . . . 29
B.1. Changes from version 13 to version 14 . . . . . . . . . . 30
B.2. Changes from version 12 to version 13 . . . . . . . . . . 30
B.3. Changes from version 11 to version 12 . . . . . . . . . . 31
B.4. Changes from version 10 to version 11 . . . . . . . . . . 32
B.5. Changes from version 09 to version 10 . . . . . . . . . . 33
B.6. Changes from version 08 to version 09 . . . . . . . . . . 33
B.7. Changes from version 07 to version 08 . . . . . . . . . . 33
B.8. Changes from version 06 to version 07 . . . . . . . . . . 34
B.9. Changes from version 05 to version 06 . . . . . . . . . . 34
B.10. Changes from version 04 to version 05 . . . . . . . . . . 35
B.11. Changes from version 03 to version 04 . . . . . . . . . . 35
B.12. Changes from version 02 to version 03 . . . . . . . . . . 35
Appendix C. Contributors . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
Routing Protocol for Low-Power and Lossy Networks (RPL) [RFC6550] is
an IPv6 distance vector routing protocol designed to support multiple
traffic flows through a root-based Destination-Oriented Directed
Acyclic Graph (DODAG). Typically, a router does not have routing
information for most other routers. Consequently, for traffic
between routers within the DODAG (i.e., Peer-to-Peer (P2P) traffic)
data packets either have to traverse the root in non-storing mode, or
traverse a common ancestor in storing mode. Such P2P traffic is
thereby likely to traverse longer routes and may suffer severe
congestion near the root (for more information see [RFC6687],
[RFC6997], [RFC6998], [RFC9010]). The network environment that is
considered in this document is assumed to be the same as described in
Section 1 of [RFC6550]. Each radio interface/link and the associated
address should be treated as an independent intermediate router.
Such routers have different links and the rules for the link symmetry
apply independently for each of these.
The route discovery process in AODV-RPL is modeled on the analogous
peer-to-peer procedure specified in AODV [RFC3561]. The on-demand
nature of AODV route discovery is natural for the needs of routing in
RPL-based LLNs when routes are needed but aren't yet established.
Peer-to-peer routing is desirable to discover shorter routes, and
especially when it is desired to avoid directing additional traffic
through a root or gateway node of the network. It may happen that
some routes need to be established proactively when known beforehand
and when AODV-RPL's route discovery process introduces unwanted delay
at the time when the application is launched.
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AODV terminology has been adapted for use with AODV-RPL messages,
namely RREQ for Route Request, and RREP for Route Reply. AODV-RPL
currently omits some features compared to AODV -- in particular,
flagging Route Errors, "blacklisting" unidirectional links
([RFC3561]), multihoming, and handling unnumbered interfaces.
AODV-RPL reuses and extends the core RPL functionality to support
routes with bidirectional asymmetric links. It retains RPL's DODAG
formation, RPL Instance and the associated Objective Function
(defined in [RFC6551]), trickle timers, and support for storing and
non-storing modes. AODV-RPL adds basic messages RREQ and RREP as
part of RPL DODAG Information Object (DIO) control message, which go
in separate (paired) RPL instances. AODV-RPL does not utilize the
Destination Advertisement Object (DAO) control message of RPL. AODV-
RPL uses the "P2P Route Discovery Mode of Operation" (MOP == 4) with
three new Options for the DIO message, dedicated to discover P2P
routes. These P2P routes may differ from routes discoverable by
native RPL. Since AODV-RPL uses newly defined Options and a newly
allocated multicast group (see Section 9), there is no conflict with
P2P-RPL [RFC6997], a previous document using the same MOP. AODV-RPL
can be operated whether or not P2P-RPL or native RPL is running
otherwise. For many networks AODV-RPL could be a replacement for
RPL, since it can find better routes at very moderate cost.
Consequently, it seems unlikely that RPL would be needed in a network
that is running AODV-RPL, even though it would be possible to run
both protocols at the same time.
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
AODV-RPL reuses names for messages and data structures, including
Rank, DODAG and DODAGID, as defined in RPL [RFC6550].
AODV
Ad Hoc On-demand Distance Vector Routing [RFC3561].
ART option
AODV-RPL Target option: a target option defined in this document.
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Asymmetric Route
The route from the OrigNode to the TargNode can traverse different
nodes than the route from the TargNode to the OrigNode. An
asymmetric route may result from the asymmetry of links, such that
only one direction of the series of links satisfies the Objective
Function during route discovery.
Bi-directional Asymmetric Link
A link that can be used in both directions but with different link
characteristics.
DIO
DODAG Information Object (as defined in [RFC6550])
DODAG RREQ-Instance (or simply RREQ-Instance)
RPL Instance built using the DIO with RREQ option; used for
transmission of control messages from OrigNode to TargNode, thus
enabling data transmission from TargNode to OrigNode.
DODAG RREP-Instance (or simply RREP-Instance)
RPL Instance built using the DIO with RREP option; used for
transmission of control messages from TargNode to OrigNode thus
enabling data transmission from OrigNode to TargNode.
Downward Direction
The direction from the OrigNode to the TargNode.
Downward Route
A route in the downward direction.
hop-by-hop route
A route for which each router along the routing path stores
routing information about the next hop. A hop-by-hop route is
created using RPL's "storing mode".
on-demand routing
Routing in which a route is established only when needed.
OrigNode
The IPv6 router (Originating Node) initiating the AODV-RPL route
discovery to obtain a route to TargNode.
Paired DODAGs
Two DODAGs for a single route discovery process between OrigNode
and TargNode.
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P2P
Peer-to-Peer -- in other words, not constrained a priori to
traverse a common ancestor.
reactive routing
Same as "on-demand" routing.
REJOIN_REENABLE
The duration during which a node is prohibited from joining a
DODAG with a particular RREQ-InstanceID, after it has left a DODAG
with the same RREQ-InstanceID. The default value of
REJOIN_REENQBLE is 15 minutes.
RREQ-DIO message
A DIO message containing the RREQ option. The RPLInstanceID in
RREQ-DIO is assigned locally by the OrigNode. The RREQ-DIO
message has a secure variant as noted in [RFC6550].
RREQ-InstanceID
The RPLInstanceID for the RREQ-Instance. The RREQ-InstanceID is
formed as the ordered pair (Orig_RPLInstanceID, OrigNode-IPaddr),
where Orig_RPLInstanceID is the local RPLInstanceID allocated by
OrigNode, and OrigNode-IPaddr is an IP address of OrigNode. The
RREQ-InstanceID uniquely identifies the RREQ-Instance.
RREP-DIO message
A DIO message containing the RREP option. OrigNode pairs the
RPLInstanceID in RREP-DIO to the one in the associated RREQ-DIO
message (i.e., the RREQ-InstanceID) as described in Section 6.3.2.
The RREP-DIO message has a secure variant as noted in [RFC6550].
RREP-InstanceID
The RPLInstanceID for the RREP-Instance. The RREP-InstanceID is
formed as the ordered pair (Targ_RPLInstanceID, TargNode-IPaddr),
where Targ_RPLInstanceID is the local RPLInstanceID allocated by
TargNode, and TargNode-IPaddr is an IP address of TargNode. The
RREP-InstanceID uniquely identifies the RREP-Instance. The
RPLInstanceID in the RREP message along with the Delta value
indicates the associated RREQ-InstanceID.
Source routing
A mechanism by which the source supplies a vector of addresses
towards the destination node along with each data packet
[RFC6550].
Symmetric route
The upstream and downstream routes traverse the same routers and
over the same links.
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TargNode
The IPv6 router (Target Node) for which OrigNode requires a route
and initiates Route Discovery within the LLN network.
Upward Direction
The direction from the TargNode to the OrigNode.
Upward Route
A route in the upward direction.
3. Overview of AODV-RPL
With AODV-RPL, routes from OrigNode to TargNode within the LLN
network are established "on-demand". In other words, the route
discovery mechanism in AODV-RPL is invoked reactively when OrigNode
has data for delivery to the TargNode but existing routes do not
satisfy the application's requirements. AODV-RPL works without
requiring the use of RPL or any other routing protocol.
The routes discovered by AODV-RPL are not constrained to traverse a
common ancestor. AODV-RPL can enable asymmetric communication paths
in networks with bidirectional asymmetric links. For this purpose,
AODV-RPL enables discovery of two routes: namely, one from OrigNode
to TargNode, and another from TargNode to OrigNode. AODV-RPL also
enables discovery of symmetric routes along Paired DODAGs, when
symmetric routes are possible (see Section 5).
In AODV-RPL, routes are discovered by first forming a temporary DAG
rooted at the OrigNode. Paired DODAGs (Instances) are constructed
during route formation between the OrigNode and TargNode. The RREQ-
Instance is formed by route control messages from OrigNode to
TargNode whereas the RREP-Instance is formed by route control
messages from TargNode to OrigNode. The route discovered in the
RREQ-Instance is used for transmitting data from TargNode to
OrigNode, and the route discovered in RREP-Instance is used for
transmitting data from OrigNode to TargNode.
Intermediate routers join the DODAGs based on the Rank [RFC6550] as
calculated from the DIO message. Henceforth in this document, "RREQ-
DIO message" means the DIO message from OrigNode toward TargNode,
containing the RREQ option as specified in Section 4.1. The RREQ-
InstanceID is formed as the ordered pair (Orig_RPLInstanceID,
OrigNode-IPaddr), where Orig_RPLInstanceID is the local RPLInstanceID
allocated by OrigNode, and OrigNode-IPaddr is the IP address of
OrigNode. A node receiving the RREQ-DIO can use the RREQ-InstanceID
to identify the proper OF whenever that node receives a data packet
with Source Address == OrigNode-IPaddr and IPv6 RPL Option having the
RPLInstanceID == Orig_RPLInstanceID along with 'D' == 0.
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Similarly, "RREP-DIO message" means the DIO message from TargNode
toward OrigNode, containing the RREP option as specified in
Section 4.2. The RREP-InstanceID is formed as the ordered pair
(Targ_RPLInstanceID, TargNode-IPaddr), where Targ_RPLInstanceID is
the local RPLInstanceID allocated by TargNode, and TargNode-IPaddr is
the IP address of TargNode. A node receiving the RREP-DIO can use
the RREP-InstanceID to identify the proper OF whenever that node
receives a data packet with Source Address == TargNode-IPaddr and
IPv6 RPL Option having the RPLInstanceID == Targ_RPLInstanceID along
with 'D' == 0.
4. AODV-RPL DIO Options
4.1. AODV-RPL RREQ Option
OrigNode selects one of its IPv6 addresses and sets it in the DODAGID
field of the RREQ-DIO message. The address scope of the selected
address must encompass the domain where the route is built (e.g, not
link-local). Exactly one RREQ option MUST be present in a RREQ-DIO
message, otherwise the message MUST be dropped.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |S|H|X| Compr | L | RankLimit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Orig SeqNo | |
+-+-+-+-+-+-+-+-+ |
| |
| |
| Address Vector (Optional, Variable Length) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format for AODV-RPL RREQ Option
OrigNode supplies the following information in the RREQ option:
Option Type
TBD2
Option Length
The length of the option in octets, excluding the Type and Length
fields. Variable due to the presence of the address vector and
the number of octets elided according to the Compr value.
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S
Symmetric bit indicating a symmetric route from the OrigNode to
the router transmitting this RREQ-DIO. See Section 5.
H
Set to one for a hop-by-hop route. Set to zero for a source
route. This flag controls both the downstream route and upstream
route.
X
Reserved; MUST be initialized to zero and ignored upon reception.
Compr
4-bit unsigned integer. When Compr is nonzero, exactly that
number of prefix octets MUST be elided from each address before
storing it in the Address Vector. The octets elided are shared
with the IPv6 address in the DODAGID. This field is only used in
source routing mode (H=0). In hop-by-hop mode (H=1), this field
MUST be set to zero and ignored upon reception.
L
2-bit unsigned integer determining the time duration that a node
is able to belong to the RREQ-Instance (a temporary DAG including
the OrigNode and the TargNode). Once the time is reached, a node
MUST leave the RREQ-Instance and stop sending or receiving any
more DIOs for the RREQ-Instance. This naturally depends on the
node's ability to keep track of time. Once a node leaves an RREQ-
Instance, it MUST NOT rejoin the same RREQ-Instance for at least
the time interval specified by the configuration variable
REJOIN_REENABLE. L is independent from the route lifetime, which
is defined in the DODAG configuration option.
* 0x00: No time limit imposed.
* 0x01: 16 seconds
* 0x02: 64 seconds
* 0x03: 256 seconds
RankLimit
This field indicates the upper limit on the integer portion of the
Rank (calculated using the DAGRank() macro defined in [RFC6550]).
A value of 0 in this field indicates the limit is infinity.
Orig SeqNo
Sequence Number of OrigNode. See Section 6.1.
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Address Vector
A vector of IPv6 addresses representing the route that the RREQ-
DIO has passed. It is only present when the H bit is set to 0.
The prefix of each address is elided according to the Compr field.
TargNode can join the RREQ instance at a Rank whose integer portion
is less than or equal to the RankLimit. Any other node MUST NOT join
a RREQ instance if its own Rank would be equal to or higher than
RankLimit. A router MUST discard a received RREQ if the integer part
of the advertised Rank equals or exceeds the RankLimit.
4.2. AODV-RPL RREP Option
TargNode sets one of its IPv6 addresses in the DODAGID field of the
RREP-DIO message. The address scope of the selected address must
encompass the domain where the route is built (e.g, not link-local).
Exactly one RREP option MUST be present in a RREP-DIO message,
otherwise the message MUST be dropped. TargNode supplies the
following information in the RREP option:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |G|H|X| Compr | L | RankLimit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delta |X X| |
+-+-+-+-+-+-+-+-+ |
| |
| |
| Address Vector (Optional, Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format for AODV-RPL RREP option
Option Type
TBD3
Option Length
The length of the option in octets, excluding the Type and Length
fields. Variable due to the presence of the address vector and
the number of octets elided according to the Compr value.
G
Gratuitous RREP (see Section 7).
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H
The H bit in the RREP option MUST be set to be the same as the H
bit in RREQ option. It requests either source routing (H=0) or
hop-by-hop (H=1) for the downstream route.
X
Reserved; MUST be initialized to zero and ignored upon reception.
Compr
4-bit unsigned integer. Same definition as in RREQ option.
L
2-bit unsigned integer defined as in RREQ option. The lifetime of
the RREP-Instance MUST be no greater than the lifetime of the
RREQ-Instance to which it is paired.
RankLimit
Similarly to RankLimit in the RREQ message, this field indicates
the upper limit on the integer portion of the Rank. A value of 0
in this field indicates the limit is infinity.
Delta
6-bit unsigned integer. This field is used to recover the RREQ-
InstanceID (see Section 6.3.3); 0 indicates that the RREQ-
InstanceID has the same value as the RPLInstanceID of the RREP
message.
X X
Reserved; MUST be initialized to zero and ignored upon reception.
Address Vector
Only present when the H bit is set to 0. For an asymmetric route,
the Address Vector represents the IPv6 addresses of the path
through the network the RREP-DIO has passed. For a symmetric
route, it is the Address Vector when the RREQ-DIO arrives at the
TargNode, unchanged during the transmission to the OrigNode.
4.3. AODV-RPL Target Option
The AODV-RPL Target (ART) Option is based on the Target Option in
core RPL [RFC6550]. The Flags field is replaced by the Destination
Sequence Number of the TargNode and the Prefix Length field is
reduced to 7 bits so that the value is limited to be no greater than
127.
A RREQ-DIO message MUST carry at least one ART Option. A RREP-DIO
message MUST carry exactly one ART Option. Otherwise, the message
MUST be dropped.
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OrigNode can include multiple TargNode addresses via multiple AODV-
RPL Target Options in the RREQ-DIO, for routes that share the same
requirement on metrics. This reduces the cost to building only one
DODAG.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Dest SeqNo |X|Prefix Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ |
| Target Prefix / Address (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ART Option format for AODV-RPL
Option Type
TBD4
Option Length
Length of the option in octets excluding the Type and Length
fields.
Dest SeqNo
In RREQ-DIO, if nonzero, it is the Sequence Number for the last
route that OrigNode stored to the TargNode for which a route is
desired. In RREP-DIO, it is the destination sequence number
associated to the route. Zero is used if there is no known
information about the sequence number of TargNode, and not used
otherwise.
X
A one-bit reserved field. This field MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
Prefix Length
7-bit unsigned integer. Number of valid leading bits in the IPv6
Prefix. If Prefix Length is 0, then the value in the Target
Prefix / Address field represents an IPv6 address, not a prefix.
Target Prefix / Address
(variable-length field) An IPv6 destination address or prefix.
The Prefix Length field contains the number of valid leading bits
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in the prefix. The Target Prefix / Address field contains the
least number of octets that can represent all of the bits of the
Prefix, in other words Ceil(Prefix Length/8) octets. The initial
bits in the Target Prefix / Address field preceding the prefix
length (if any) MUST be set to zero on transmission and MUST be
ignored on receipt. If Prefix Length is zero, the Address field
is 128 bits for IPv6 addresses.
5. Symmetric and Asymmetric Routes
Links are considered symmetric until indication to the contrary is
received. In Figure 4 and Figure 5, BR is the Border Router, O is
the OrigNode, each R is an intermediate router, and T is the
TargNode. In this example, the use of BR is only for illustrative
purposes; AODV does not depend on the use of border routers for its
operation. If the RREQ-DIO arrives over an interface that is known
to be symmetric, and the S bit is set to 1, then it remains as 1, as
illustrated in Figure 4. If an intermediate router sends out RREQ-
DIO with the S bit set to 1, then each link en route from the
OrigNode O to this router has met the requirements of route
discovery, and the route can be used symmetrically.
BR
/----+----\
/ | \
/ | \
R R R
_/ \ | / \
/ \ | / \
/ \ | / \
R -------- R --- R ----- R -------- R
/ \ <--S=1--> / \ <--S=1--> / \
<--S=1--> \ / \ / <--S=1-->
/ \ / \ / \
O ---------- R ------ R------ R ----- R ----------- T
/ \ / \ / \ / \
/ \ / \ / \ / \
/ \ / \ / \ / \
R ----- R ----------- R ----- R ----- R ----- R ---- R----- R
>---- RREQ-Instance (Control: O-->T; Data: T-->O) ------->
<---- RREP-Instance (Control: T-->O; Data: O-->T) -------<
Figure 4: AODV-RPL with Symmetric Instances
Upon receiving a RREQ-DIO with the S bit set to 1, a node determines
whether this link can be used symmetrically, i.e., both directions
meet the requirements of data transmission. If the RREQ-DIO arrives
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over an interface that is not known to be symmetric, or is known to
be asymmetric, the S bit is set to 0. If the S bit arrives already
set to be '0', it is set to be '0' when the RREQ-DIO is propagated
(Figure 5). For an asymmetric route, there is at least one hop which
doesn't satisfy the Objective Function. Based on the S bit received
in RREQ-DIO, TargNode T determines whether or not the route is
symmetric before transmitting the RREP-DIO message upstream towards
the OrigNode O.
It is beyond the scope of this document to specify the criteria used
when determining whether or not each link is symmetric. As an
example, intermediate routers can use local information (e.g., bit
rate, bandwidth, number of cells used in 6tisch [RFC9030]), a priori
knowledge (e.g., link quality according to previous communication) or
use averaging techniques as appropriate to the application. Other
link metric information can be acquired before AODV-RPL operation, by
executing evaluation procedures; for instance test traffic can be
generated between nodes of the deployed network. During AODV-RPL
operation, OAM techniques for evaluating link state (see [RFC7548],
[RFC7276], [co-ioam]) MAY be used (at regular intervals appropriate
for the LLN). The evaluation procedures are out of scope for AODV-
RPL. For further information on this topic, see [Link_Asymmetry],
[low-power-wireless], and [empirical-study].
Appendix A describes an example method using the upstream Expected
Number of Transmissions (ETX) and downstream Received Signal Strength
Indicator (RSSI) to estimate whether the link is symmetric in terms
of link quality using an averaging technique.
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BR
/----+----\
/ | \
/ | \
R R R
/ \ | / \
/ \ | / \
/ \ | / \
R --------- R --- R ---- R --------- R
/ \ --S=1--> / \ --S=0--> / \
--S=1--> \ / \ / --S=0-->
/ \ / \ / \
O ---------- R ------ R------ R ----- R ----------- T
/ \ / \ / \ / \
/ <--S=0-- / \ / \ / <--S=0--
/ \ / \ / \ / \
R ----- R ----------- R ----- R ----- R ----- R ---- R----- R
<--S=0-- <--S=0-- <--S=0-- <--S=0-- <--S=0--
>---- RREQ-Instance (Control: O-->T; Data: T-->O) ------->
<---- RREP-Instance (Control: T-->O; Data: O-->T) -------<
Figure 5: AODV-RPL with Asymmetric Paired Instances
As illustrated in Figure 5, an intermediate router determines the S
bit value that the RREQ-DIO should carry using link asymmetry
detection methods as discussed earlier in this section. In many
cases the intermediate router has already made the link asymmetry
decision by the time RREQ-DIO arrives.
6. AODV-RPL Operation
6.1. Route Request Generation
The route discovery process is initiated when an application at the
OrigNode has data to be transmitted to the TargNode, but does not
have a route that satisfies the Objective Function for the target of
the application's data. In this case, the OrigNode builds a local
RPLInstance and a DODAG rooted at itself. Then it transmits a DIO
message containing exactly one RREQ option (see Section 4.1) to
multicast group all-AODV-RPL-nodes. The RREQ-DIO MUST contain at
least one ART Option (see Section 4.3), which indicates the TargNode.
The S bit in RREQ-DIO sent out by the OrigNode is set to 1.
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Each node maintains a sequence number; the operation is specified in
section 7.2 of [RFC6550]. When the OrigNode initiates a route
discovery process, it MUST increase its own sequence number to avoid
conflicts with previously established routes. The sequence number is
carried in the Orig SeqNo field of the RREQ option.
The Target Prefix / Address in the ART Option can be a unicast IPv6
address or a prefix. The OrigNode can initiate the route discovery
process for multiple targets simultaneously by including multiple ART
Options. Within a RREQ-DIO the Objective Function for the routes to
different TargNodes MUST be the same.
OrigNode can maintain different RPLInstances to discover routes with
different requirements to the same targets. Using the RPLInstanceID
pairing mechanism (see Section 6.3.3), route replies (RREP-DIOs) for
different RPLInstances can be generated.
The transmission of RREQ-DIO obeys the Trickle timer [RFC6206]. If
the duration specified by the L field has elapsed, the OrigNode MUST
leave the DODAG and stop sending RREQ-DIOs in the related
RPLInstance. OrigNode needs to set L field such that the DODAG will
not prematurely timeout during data transfer with the TargNode. For
setting this value, it has to consider factors such as Trickle timer,
TargNode hop distance, network size, link behavior, expected data
usage time, and so on.
6.2. Receiving and Forwarding RREQ messages
6.2.1. Step 1: RREQ reception and evaluation
When a router X receives a RREQ message over a link from a neighbor
Y, X first determines whether or not the RREQ is valid. If so, X
then determines whether or not it has sufficient resources available
to maintain the state needed to process an eventual RREP if the RREP
were to be received. If not, then X MUST drop the packet and
discontinue processing of the RREQ. Otherwise, X next determines
whether the RREQ advertises a usable route to OrigNode, by checking
whether the link to Y can be used to tramsmit packets to OrigNode.
When H=0 in the incoming RREQ, the router MUST drop the RREQ-DIO if
one of its addresses is present in the Address Vector. When H=1 in
the incoming RREQ, the router MUST drop the RREQ message if Orig
SeqNo field of the RREQ is older than the SeqNo value that X has
stored for a route to OrigNode. Otherwise, the router determines
whether to propagate the RREQ-DIO. It does this by determining
whether or not a route to OrigNode using the upstream direction of
the incoming link satisfies the Objective Function (OF). In order to
evaluate the OF, the router first determines the maximum useful rank
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(MaxUsefulRank). If the router has previously joined the RREQ-
Instance associated with the RREQ-DIO, then MaxUsefulRank is set to
be the Rank value that was stored when the router processed the best
previous RREQ for the DODAG with the given RREQ-Instance. Otherwise,
MaxUsefulRank is set to be RankLimit. If OF cannot be satisfied
(i.e., the Rank evaluates to a value greater than MaxUsefulRank) the
RREQ-DIO MUST be dropped, and the following steps are not processed.
Otherwise, the router MUST join the RREQ-Instance and prepare to
propagate the RREQ-DIO, as follows. The upstream neighbor router
that transmitted the received RREQ-DIO is selected as the preferred
parent.
6.2.2. Step 2: TargNode and Intermediate Router determination
After determining that a received RREQ provides a usable route to
OrigNode, a router determines whether it is a TargNode, or a possible
intermediate router between OrigNode and a TargNode, or both. The
router is a TargNode if it finds one of its own addresses in a Target
Option in the RREQ. After possibly propagating the RREQ according to
the procedures in Steps 3, 4, and 5, the TargNode generates a RREP as
specified in Section 6.3. If S=0, the determination of TargNode
status and determination of a usable route to OrigNode is the same.
If the OrigNode tries to reach multiple TargNodes in a single RREQ-
Instance, one of the TargNodes can be an intermediate router to other
TargNodes. In this case, before transmitting the RREQ-DIO to
multicast group all-AODV-RPL-nodes, a TargNode MUST delete the Target
Option encapsulating its own address, so that downstream routers with
higher Rank values do not try to create a route to this TargNode.
An intermediate router could receive several RREQ-DIOs from routers
with lower Rank values in the same RREQ-Instance with different lists
of Target Options. For the purposes of determining the intersection
with previous incoming RREQ-DIOs, the intermediate router maintains a
record of the targets that have been requested for a given RREQ-
Instance. An incoming RREQ-DIO message having multiple ART Options
coming from a router with higher Rank than the Rank of the stored
targets is ignored. When transmitting the RREQ-DIO, the intersection
of all received lists MUST be included if it is nonempty after
TargNode has deleted the Target Option encapsulating its own address.
If the intersection is empty, it means that all the targets have been
reached, and the router MUST NOT transmit any RREQ-DIO. Otherwise it
proceeds to Section 6.2.3.
For example, suppose two RREQ-DIOs are received with the same
RPLInstance and OrigNode. Suppose further that the first RREQ has
(T1, T2) as the targets, and the second one has (T2, T4) as targets.
Then only T2 needs to be included in the generated RREQ-DIO.
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6.2.3. Step 3: Intermediate Router RREQ processing
The intermediate router establishes itself as a viable node for a
route to OrigNode as follows. If the H bit is set to 1, for a hop-
by-hop route, then the router MUST build or update its upward route
entry towards OrigNode, which includes at least the following items:
Source Address, RPLInstanceID, Destination Address, Next Hop,
Lifetime, and Sequence Number. The Destination Address and the
RPLInstanceID respectively can be learned from the DODAGID and the
RPLInstanceID of the RREQ-DIO. The Source Address is the address
used by the router to send data to the Next Hop, i.e., the preferred
parent. The lifetime is set according to DODAG configuration (not
the L field) and can be extended when the route is actually used.
The sequence number represents the freshness of the route entry; it
is copied from the Orig SeqNo field of the RREQ option. A route
entry with the same source and destination address, same
RPLInstanceID, but stale sequence number, MUST be deleted.
6.2.4. Step 4: Symmetric Route Processing at an Intermediate Router
If the S bit of the incoming RREQ-DIO is 0, then the route cannot be
symmetric, and the S bit of the RREQ-DIO to be transmitted is set to
0. Otherwise, the router MUST determine whether the downward (i.e.,
towards the TargNode) direction of the incoming link satisfies the
OF. If so, the S bit of the RREQ-DIO to be transmitted is set to 1.
Otherwise the S bit of the RREQ-DIO to be transmitted is set to 0.
When a router joins the RREQ-Instance, it also associates within its
data structure for the RREQ-Instance the information about whether or
not the RREQ-DIO to be transmitted has the S-bit set to 1. This
information associated to RREQ-Instance is known as the S-bit of the
RREQ-Instance. It will be used later during the RREP-DIO message
processing Section 6.3.2.
Suppose a router has joined the RREQ-Instance, and H=0, and the S-bit
of the RREQ-Instance is set to 1. In this case, the router MAY
optionally associate to the RREQ-Instance, the Address Vector of the
symmetric route back to OrigNode. This is useful if the router later
receives an RREP-DIO that is paired with the RREQ-Instance. If the
router does NOT associate the Address Vector, then it has to rely on
multicast for the RREP. This can impose a substantial performance
penalty.