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draft-ietf-roll-aodv-rpl-15.xml
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draft-ietf-roll-aodv-rpl-15.xml
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<?xml version="1.0" encoding="US-ASCII"?>
<!-- This template is for creating an Internet Draft using xml2rfc,
which is available here: http://xml.resource.org.
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [ ]> -->
<!-- For a complete list and description of processing instructions (PIs),
please see http://xml.resource.org/authoring/README.html. -->
<!-- Below are generally applicable Processing Instructions (PIs) that most
I-Ds might want to use.
(Here they are set differently than their defaults in xml2rfc v1.32) -->
<?rfc strict="yes" ?>
<!-- give errors regarding ID-nits and DTD validation -->
<!-- control the table of contents (ToC) -->
<?rfc toc="yes"?>
<!-- generate a ToC -->
<?rfc tocdepth="4"?>
<!-- the number of levels of subsections in ToC. default: 3 -->
<!-- control references -->
<?rfc symrefs="yes"?>
<!-- use symbolic references tags, i.e, [RFC2119] instead of [1] -->
<?rfc sortrefs="yes" ?>
<!-- sort the reference entries alphabetically -->
<!-- control vertical white space
(using these PIs as follows is recommended by the RFC Editor) -->
<?rfc compact="yes" ?>
<!-- do not start each main section on a new page -->
<?rfc subcompact="no" ?>
<!-- keep one blank line between list items -->
<!-- end of list of popular I-D processing instructions -->
<rfc category="std" docName="draft-ietf-roll-aodv-rpl-15" ipr="trust200902"
submissionType="IETF" consensus="true">
<!-- category values: std, bcp, info, exp, and historic
http://umeeting.huawei.com/Portal/business.action?BMECID=1474233&BMETimestamp=1426658395147
ipr values: full3667, noModification3667, noDerivatives3667
you can add the attributes updates="NNNN" and obsoletes="NNNN"
they will automatically be output with "(if approved)" -->
<!-- ***** FRONT MATTER ***** -->
<!-- TODO:
-->
<front>
<!-- The abbreviated title is used in the page header - it is only
necessary if the full title is longer than 39 characters -->
<title abbrev="AODV-RPL">
Supporting Asymmetric Links in Low Power Networks: AODV-RPL
</title>
<!-- add 'role="editor"' below for the editors if appropriate -->
<!-- Another author who claims to be an editor -->
<author fullname="Charles E. Perkins" initials="C.E." surname="Perkins">
<organization>Lupin Lodge</organization>
<address>
<postal>
<street/>
<city>Los Gatos</city>
<region/>
<code>95033</code>
<country>United States</country>
</postal>
<phone/>
<email>[email protected]</email>
<!-- uri and facsimile elements may also be added -->
</address>
</author>
<author fullname="S.V.R Anand" initials="" surname="S.V.R.Anand">
<organization>Indian Institute of Science</organization>
<address>
<postal>
<street></street>
<!-- Reorder these if your country does things differently -->
<city>Bangalore</city>
<region/>
<code>560012</code>
<country>India</country>
</postal>
<phone/>
<email>[email protected]</email>
<!-- uri and facsimile elements may also be added -->
</address>
</author>
<author fullname="Satish Anamalamudi" initials="S." surname="Anamalamudi">
<organization>SRM University-AP</organization>
<address>
<postal>
<street>Amaravati Campus</street>
<!-- Reorder these if your country does things differently -->
<city>Amaravati, Andhra Pradesh</city>
<region/>
<code>522 502</code>
<country>India</country>
</postal>
<phone/>
<email>[email protected]</email>
<!-- uri and facsimile elements may also be added -->
</address>
</author>
<author fullname="Bing Liu" initials="B." surname="Liu">
<organization>Huawei Technologies</organization>
<address>
<postal>
<street>No. 156 Beiqing Rd. Haidian District</street>
<!-- Reorder these if your country does things differently -->
<city>Beijing</city>
<region/>
<code>100095</code>
<country>China</country>
</postal>
<phone/>
<email>[email protected]</email>
</address>
</author>
<date year="2022"/>
<!-- If the month and year are both specified and are the current ones,
xml2rfc will fill in the current day for you. If only the current
year is specified, xml2rfc will fill in the current day and month for
you. If the year is not the current one, it is necessary to specify
at least a month (xml2rfc assumes day="1" if not specified for the
purpose of calculating the expiry date). With drafts it is normally
sufficient to specify just the year. -->
<!-- Meta-data Declarations -->
<area>Internet</area>
<workgroup>ROLL</workgroup>
<!-- WG name at the upperleft corner of the doc;
IETF is fine for individual submissions. If this element is not
present, the default is "Network Working Group", which is used by
the RFC Editor as a nod to the history of the IETF. -->
<keyword>AODV, Peer-to-Peer Route Discovery, Asymmetric</keyword>
<!-- Keywords will be incorporated into HTML output
files in a meta tag but they have no effect on text or nroff
output. If you submit your draft to the RFC Editor, the
keywords will be used for the search engine. -->
<abstract>
<t> 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.
</t>
</abstract>
</front>
<middle>
<section anchor="Introduction" title="Introduction">
<t>
Routing Protocol for Low-Power and Lossy Networks (RPL)
<xref target="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 <xref target="RFC6687"/>, <xref target="RFC6997"/>,
<xref target="RFC6998"/>, <xref target="RFC9010"/>).
The network environment that is considered in this document
is assumed to be the same as described in Section 1 of
<xref target="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.
</t>
<t>
The route discovery process in AODV-RPL is modeled on the analogous
peer-to-peer procedure specified in AODV <xref target="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.
</t>
<t>
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
(<xref target="RFC3561"/>), multihoming, and handling unnumbered
interfaces.
</t>
<t>
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 <xref target="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.
<!-- The P2P routes do not have to go through the tree root. I don't remember
what are the point-to-multipoint routes under discussion here. -->
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 <xref target="iana"/>), there is no conflict
with P2P-RPL <xref target="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.
<!--
However, there does not seem to be much value in
maintaining two routing protocols even if they are compatible.
-->
</t>
</section> <!-- End of section "Introduction" -->
<section anchor="terms" title="Terminology">
<t> 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 <xref target="RFC2119"/> <xref target="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.
</t>
<t>
AODV-RPL reuses names for messages and data structures, including
Rank, DODAG and DODAGID, as defined in RPL <xref target="RFC6550"/>.
</t>
<t><list style="hanging">
<t hangText="AODV"><vspace />
Ad Hoc On-demand Distance Vector Routing <xref target="RFC3561"/>.</t>
<!-- /* Murray Kucherawy: does not appear anywhere else in the document. */
<t hangText="AODV-RPL Instance"><vspace />
Either the RREQ-Instance or RREP-Instance</t>
-->
<t hangText="ART option"><vspace />
AODV-RPL Target option: a target option defined in this document.</t>
<t hangText="Asymmetric Route"><vspace />
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.
<!-- CEP: Need to check this!!
But the RREQ *still* has to store the reverse route...
If the OrigNode doesn't require an upward route towards
itself, the route is also considered as asymmetric. --> </t>
<t hangText="Bi-directional Asymmetric Link"><vspace />
A link that can be used in both directions but with different link
characteristics. </t>
<t hangText="DIO"><vspace />
DODAG Information Object (as defined in <xref target="RFC6550"/>) </t>
<t hangText="DODAG RREQ-Instance (or simply RREQ-Instance)"><vspace />
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. </t>
<t hangText="DODAG RREP-Instance (or simply RREP-Instance)"><vspace />
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. </t>
<t hangText="Downward Direction"><vspace />
The direction from the OrigNode to the TargNode.</t>
<t hangText="Downward Route"><vspace />
A route in the downward direction. </t>
<t hangText="hop-by-hop route"><vspace />
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".</t>
<t hangText="on-demand routing"><vspace />
Routing in which a route is established only when needed. </t>
<t hangText="OrigNode"><vspace />
The IPv6 router (Originating Node) initiating the AODV-RPL
route discovery to obtain a route to TargNode. </t>
<t hangText="Paired DODAGs"><vspace />
Two DODAGs for a single route discovery process between OrigNode
and TargNode.</t>
<t hangText="P2P"><vspace />
Peer-to-Peer -- in other words, not constrained a priori to
traverse a common ancestor. </t>
<t hangText="reactive routing"><vspace />
Same as "on-demand" routing. </t>
<t hangText="REJOIN_REENABLE"><vspace />
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.</t>
<t hangText="RREQ-DIO message"><vspace />
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 <xref
target="RFC6550"/>. </t>
<t hangText="RREQ-InstanceID"><vspace />
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. </t>
<t hangText="RREP-DIO message"><vspace />
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 <xref target="asymmetricrrep"/>. The RREP-DIO message has a secure
variant as noted in <xref target="RFC6550"/>. </t>
<t hangText="RREP-InstanceID"><vspace />
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. </t>
<t hangText="Source routing"><vspace />
A mechanism by which the source supplies a vector of addresses
towards the destination node along with each data packet
<xref target="RFC6550"/>. </t>
<t hangText="Symmetric route"><vspace />
The upstream and downstream routes traverse the same routers and over
the same links. </t>
<!-- CEP: pagination :-( -->
<t hangText="TargNode"><vspace />
The IPv6 router (Target Node) for which OrigNode requires a
route and initiates Route Discovery within the LLN network. </t>
<t hangText="Upward Direction"><vspace />
The direction from the TargNode to the OrigNode.</t>
<t hangText="Upward Route"><vspace />
A route in the upward direction. </t>
</list></t>
</section> <!-- End of section "Terminology" -->
<section title="Overview of AODV-RPL">
<t> 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.
</t>
<t>
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 <xref target="channel"/>).
</t>
<t>
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.
</t>
<t>
Intermediate routers join the DODAGs
based on the Rank <xref target="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 <xref target="RREQmsg"/>. 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.
</t>
<t>
Similarly, "RREP-DIO message"
means the DIO message from TargNode toward OrigNode, containing the
RREP option as specified in <xref target="RREPmsg"/>.
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.
</t>
</section> <!-- End of section "Overview of AODV-RPL" -->
<section anchor="Options" title="AODV-RPL DIO Options">
<section anchor="RREQmsg" title="AODV-RPL RREQ Option">
<t>
OrigNode selects one of its IPv6 addresses and sets it in the DODAGID
<!-- CEP: SHOULD changed to MUST by request of Alvaro Retana. -->
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.
<figure anchor="figRREQ" title="Format for AODV-RPL RREQ Option">
<artwork align="center"><![CDATA[
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) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
OrigNode supplies the following information in the RREQ option: </t>
<t><list style="hanging">
<t hangText="Option Type"><vspace />
TBD2</t>
<t hangText="Option Length"><vspace />
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.</t>
<t hangText="S"><vspace />
Symmetric bit indicating a symmetric route from the OrigNode to the
router transmitting this RREQ-DIO. See <xref target="channel"/>.</t>
<t hangText="H"><vspace />
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. </t>
<t hangText="X"><vspace />
Reserved; MUST be initialized to zero and
ignored upon reception.</t>
<t hangText="Compr"><vspace />
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.</t>
<!-- CEP: Shouldn't we allow address compression for the Target Option? -->
<t hangText="L"><vspace />
<?rfc subcompact="yes" ?>
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.
<list style="symbols">
<t>0x00: No time limit imposed. </t>
<t>0x01: 16 seconds </t>
<t>0x02: 64 seconds </t>
<t>0x03: 256 seconds </t>
</list>
<?rfc subcompact="no" ?>
L is independent from the route lifetime, which is defined in the
DODAG configuration option.
<!-- The route entries in hop-by-hop routing
and states of source routing can still be maintained
even after the node no longer maintains DAG connectivity or
messaging. -->
<!-- according to email to the list, 12/27/2020 -->
</t>
<t hangText="RankLimit"><vspace />
This field indicates the upper limit on the integer portion of the
Rank (calculated using the DAGRank() macro defined in
<xref target="RFC6550"/>). A value of 0 in this field
indicates the limit is infinity. </t>
<t hangText="Orig SeqNo"><vspace />
Sequence Number of OrigNode. See <xref target="rreq"/>. </t>
<t hangText="Address Vector"><vspace />
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.</t>
</list>
</t>
<t> 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. </t>
<t> </t>
</section> <!-- End of section "RREQ Message" -->
<section anchor="RREPmsg" title="AODV-RPL RREP Option">
<t>
TargNode sets one of its IPv6 addresses in the DODAGID
<!-- CEP: SHOULD changed to MUST, by request of Alvaro Retana. -->
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:
<figure anchor="figRREP" title="Format for AODV-RPL RREP option">
<artwork align="center"><![CDATA[
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) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork> </figure>
<list style="hanging">
<t hangText="Option Type"><vspace />
TBD3 </t>
<t hangText="Option Length"><vspace />
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.</t>
<t hangText="G"><vspace />
Gratuitous RREP (see <xref target="GRREP"/>).</t>
<t hangText="H"><vspace />
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.</t>
<t hangText="X"><vspace />
Reserved; MUST be initialized to zero and
ignored upon reception.</t>
<t hangText="Compr"><vspace />
4-bit unsigned integer. Same definition as in RREQ option. </t>
<t hangText="L"><vspace />
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.</t>
<t hangText="RankLimit"><vspace />
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. </t>
<!-- CEP: is 7 bits O.K. for RankLimit? -->
<t hangText="Delta"><vspace />
6-bit unsigned integer. This field is used to recover the
RREQ-InstanceID (see <xref target="instancepairing"/>);
0 indicates that the RREQ-InstanceID has the same value
as the RPLInstanceID of the RREP message. </t>
<t hangText="X X"><vspace />
Reserved; MUST be initialized to zero and
ignored upon reception.</t>
<t hangText="Address Vector"><vspace />
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. </t>
</list>
</t>
<!--
/* Make the following into an XML comment */
[A] It is technically feasible to have partially active DODAG pair.
Having this condition lets graceful shutdown of the current route discovery
instance initiated by OrigNode. It marks the end of DODAG pairing as RREQ
and RREP Instances can be treated as belonging to the same route discovery.
The resources held by the intermediate nodes is released, and OrigNode can
start reusing the same RPLInstanceID in the RREQ for its new
route discovery. Having RREQ-Instance lifetime thus enables this.
-->
</section> <!-- End of section "AODV-RPL RREP Option" -->
<section anchor="artop" title="AODV-RPL Target Option">
<t> The AODV-RPL Target (ART) Option is based on the Target Option
in core RPL <xref target="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. </t>
<t>
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.
<!-- CEP: Is it needed for RREPs with symmetric routes? -->
</t>
<t>
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.
</t>
<t>
<figure anchor="figTarg" title="ART Option format for AODV-RPL">
<artwork align="center"><![CDATA[
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) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<list style="hanging">
<t hangText="Option Type"> <vspace />
TBD4
</t>
<t hangText="Option Length"> <vspace />
Length of the option in octets excluding the
Type and Length fields.
</t>
<t hangText="Dest SeqNo"> <vspace /></t>
<t> 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.
</t>
<t hangText="X"> <vspace />
A one-bit reserved field. This field MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
</t>
<t hangText="Prefix Length"> <vspace />
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.
</t>
<t hangText="Target Prefix / Address"> <vspace />
(variable-length field) An IPv6 destination address or prefix.
The Prefix Length field contains the number of valid leading bits
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.
<!-- CEP: Previous wording for last sentence:
The remaining bits in the Target Prefix / Address field
after the prefix length (if any) MUST be set to zero on
transmission and MUST be ignored on receipt.
-->
</t>
</list>
</t>
</section> <!-- End of section "AODV-RPL Target Option" -->
</section> <!-- End of section "AODV-RPL Options" -->
<section anchor="channel" title="Symmetric and Asymmetric Routes">
<t>
Links are considered symmetric until indication to the contrary is
received. In <xref target="figSymm-a"/> and
<xref target="figSymm-b"/>, 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 <xref target="figSymm-a"/>. 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.
</t>
<t><figure anchor="figSymm-a"
title="AODV-RPL with Symmetric Instances">
<artwork align="center"><![CDATA[
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) -------< ]]></artwork>
</figure></t>
<t>
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 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 (<xref target="figSymm-b"/>). 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.
</t>
<t>
<!-- CEP: Change to ...used .... are -->
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 <xref target="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 <xref target="RFC7548"/>,
<xref target="RFC7276"/>, <xref target="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 <xref target="Link_Asymmetry"/>,
<xref target="low-power-wireless"/>,
and <xref target="empirical-study"/>.
</t>
<t>
<xref target="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.
<figure anchor="figSymm-b"
title="AODV-RPL with Asymmetric Paired Instances">
<artwork align="center"><![CDATA[
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) -------<]]></artwork>
</figure>
As illustrated in <xref target="figSymm-b"/>, 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.
</t>
</section> <!-- End of section "Symmetric and Asymmetric Routes" -->
<section anchor="aodvrplop" title="AODV-RPL Operation">
<section anchor="rreq" title="Route Request Generation">
<t>
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 <xref target="RREQmsg"/>) to multicast group all-AODV-RPL-nodes.
The RREQ-DIO MUST contain at least one ART Option
(see <xref target="artop"/>), which indicates the TargNode.
<!-- CEP: or network prefix containing the TargNode. -->
The S bit in RREQ-DIO sent out by the OrigNode is set to 1.
</t>
<t>
Each node maintains a sequence number; the operation is specified in
section 7.2 of <xref target="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.
</t>
<t> 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.
</t>
<t> OrigNode can maintain different RPLInstances to discover routes with
different requirements to the same targets. Using the RPLInstanceID
pairing mechanism (see <xref target="instancepairing"/>), route replies
(RREP-DIOs) for different RPLInstances can be generated.
</t>
<t> The transmission of RREQ-DIO obeys the Trickle timer
<xref target="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.
</t>
</section>
<!-- CEP: The Trickle timer eliminates the need for RREQ_WAIT_TIME? -->
<section anchor="process_rreq"
title="Receiving and Forwarding RREQ messages">
<section anchor="rreq_step1"
title="Step 1: RREQ reception and evaluation">
<!-- CEP: descriptive text, might decide to include it somewhere.
An intermediate router X receives a RREQ message a neighbor Y. If X can
use the incoming link to transmit a packet to OrigNode by way of Y, X will
propagate the RREQ message in hopes of eventually providing Targnode with
a route towards OrigNode. In that case, X could use Y as the first hop
of its own route towards OrigNode, but very likely X does not otherwise
need a route to OrigNode. X determines whether it can use the incoming
link to transmit a packet to OrigNode by determining whether or not the
upstream direction of the incoming link satisfies the OF.
When TargNode receives a RREQ, and the upstream direction of the incoming
link satisfies the OF, TargNode has a route to OrigNode via the neighbor Y
that transmitted the RREQ. If in addition the S bit is set in the
OrigNode, and if the downstream direction of the incoming link is suitable
for TargNode to receive packets from that neighbor Y, then the entire
path traversed by the RREQ is symmetric and OrigNode can use that path
to send packets to TargNode. In order to provide that routing information
(about a viable path to TargNode) to OrigNode, TargNode unicasts a RREP
back to Y.
-->
<t> 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.
</t>
<t>
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 (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.
</t>
</section><!--End of section "Step 1: RREQ reception and evaluation"-->
<section anchor="rreq_step2"
title="Step 2: TargNode and Intermediate Router determination">
<t> <!-- Kaduk comment 16 -->
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
<xref target="gen-rrep"/>. If S=0, the determination of TargNode
status and determination of a usable route to OrigNode is the same.
</t>
<t>
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.
</t>
<t>
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
<xref target="rreq_step3"/>.
</t>
<t>
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.
</t>
</section><!--End of section
"Step 2: TargNode and Intermediate Router determination"-->
<section anchor="rreq_step3"
title="Step 3: Intermediate Router RREQ processing">
<t>
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.
<!-- CEP TODO: What is the Destination Address, if not OrigNode? -->
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.
<!-- CEP TODO: Need to specify that the information from the existing
RREQ updates the route entry? What happens if the existing
route entry has a newer SeqNo than the RREQ? Proposal:
intermediate router updates the RREQ with its newer SeqNo. -->
</t>
</section>
<!--End of section "Step 3: Intermediate Router RREQ processing"-->
<section anchor="rreq_step4"
title="Step 4: Symmetric Route Processing at an Intermediate Router">
<t>
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.
</t>
<t>
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 <xref target="asymmetricrrep"/>. <!-- for RPLInstance
pairing as described in <xref target="forwardRREP"/>.
CEP TODO: check language about pairing. -->
</t>
<t>
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