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draft-ietf-roll-useofrplinfo-38.txt
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ROLL Working Group M. Robles
Internet-Draft UTN-FRM/Aalto
Updates: 6553, 6550, 8138 (if approved) M. Richardson
Intended status: Standards Track SSW
Expires: September 24, 2020 P. Thubert
Cisco
March 23, 2020
Using RPI Option Type, Routing Header for Source Routes and IPv6-in-IPv6
encapsulation in the RPL Data Plane
draft-ietf-roll-useofrplinfo-38
Abstract
This document looks at different data flows through LLN (Low-Power
and Lossy Networks) where RPL (IPv6 Routing Protocol for Low-Power
and Lossy Networks) is used to establish routing. The document
enumerates the cases where RFC6553 (RPI Option Type), RFC6554
(Routing Header for Source Routes) and IPv6-in-IPv6 encapsulation is
required in data plane. This analysis provides the basis on which to
design efficient compression of these headers. This document updates
RFC6553 adding a change to the RPI Option Type. Additionally, this
document updates RFC6550 defining a flag in the DIO Configuration
option to indicate about this change and updates [RFC8138] as well to
consider the new Option Type when the RPL Option is decompressed.
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 September 24, 2020.
Robles, et al. Expires September 24, 2020 [Page 1]
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology and Requirements Language . . . . . . . . . . . . 5
3. RPL Overview . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Updates to RFC6553, RFC6550 and RFC8138 . . . . . . . . . . . 7
4.1. Updates to RFC6550: Advertising External Routes with Non-
Storing Mode Signaling. . . . . . . . . . . . . . . . . . 7
4.2. Updates to RFC6553: Indicating the new RPI Option Type. . 8
4.3. Updates to RFC6550: Indicating the new RPI in the
DODAG Configuration option Flag. . . . . . . . . . . . . 11
4.4. Updates to RFC8138: Indicating the way to decompress with
the new RPI Option Type. . . . . . . . . . . . . . . . . 12
5. Sample/reference topology . . . . . . . . . . . . . . . . . . 14
6. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Storing mode . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1. Storing Mode: Interaction between Leaf and Root . . . . . 20
7.1.1. SM: Example of Flow from RAL to Root . . . . . . . . 21
7.1.2. SM: Example of Flow from Root to RAL . . . . . . . . 22
7.1.3. SM: Example of Flow from Root to RUL . . . . . . . . 22
7.1.4. SM: Example of Flow from RUL to Root . . . . . . . . 24
7.2. SM: Interaction between Leaf and Internet. . . . . . . . 25
7.2.1. SM: Example of Flow from RAL to Internet . . . . . . 25
7.2.2. SM: Example of Flow from Internet to RAL . . . . . . 27
7.2.3. SM: Example of Flow from RUL to Internet . . . . . . 28
7.2.4. SM: Example of Flow from Internet to RUL. . . . . . . 29
7.3. SM: Interaction between Leaf and Leaf . . . . . . . . . . 30
7.3.1. SM: Example of Flow from RAL to RAL . . . . . . . . . 30
7.3.2. SM: Example of Flow from RAL to RUL . . . . . . . . . 31
7.3.3. SM: Example of Flow from RUL to RAL . . . . . . . . . 33
7.3.4. SM: Example of Flow from RUL to RUL . . . . . . . . . 34
8. Non Storing mode . . . . . . . . . . . . . . . . . . . . . . 35
Robles, et al. Expires September 24, 2020 [Page 2]
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8.1. Non-Storing Mode: Interaction between Leaf and Root . . . 37
8.1.1. Non-SM: Example of Flow from RAL to root . . . . . . 38
8.1.2. Non-SM: Example of Flow from root to RAL . . . . . . 39
8.1.3. Non-SM: Example of Flow from root to RUL . . . . . . 39
8.1.4. Non-SM: Example of Flow from RUL to root . . . . . . 40
8.2. Non-Storing Mode: Interaction between Leaf and Internet . 41
8.2.1. Non-SM: Example of Flow from RAL to Internet . . . . 42
8.2.2. Non-SM: Example of Flow from Internet to RAL . . . . 43
8.2.3. Non-SM: Example of Flow from RUL to Internet . . . . 44
8.2.4. Non-SM: Example of Flow from Internet to RUL . . . . 45
8.3. Non-SM: Interaction between leaves . . . . . . . . . . . 46
8.3.1. Non-SM: Example of Flow from RAL to RAL . . . . . . . 46
8.3.2. Non-SM: Example of Flow from RAL to RUL . . . . . . . 49
8.3.3. Non-SM: Example of Flow from RUL to RAL . . . . . . . 51
8.3.4. Non-SM: Example of Flow from RUL to RUL . . . . . . . 52
9. Operational Considerations of supporting
RUL-leaves . . . . . . . . . . . . . . . . . . . . . . . . . 53
10. Operational considerations of introducing 0x23 . . . . . . . 54
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 54
12. Security Considerations . . . . . . . . . . . . . . . . . . . 55
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 58
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 59
14.1. Normative References . . . . . . . . . . . . . . . . . . 59
14.2. Informative References . . . . . . . . . . . . . . . . . 60
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62
1. Introduction
RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks)
[RFC6550] is a routing protocol for constrained networks. [RFC6553]
defines the RPL Option carried within the IPv6 Hop-by-Hop Header to
carry the RPLInstanceID and quickly identify inconsistencies (loops)
in the routing topology. The RPL Option is commonly referred to as
the RPL Packet Information (RPI) though the RPI is really the
abstract information that is defined in [RFC6550] and transported in
the RPL Option. RFC6554 [RFC6554] defines the "RPL Source Route
Header" (RH3), an IPv6 Extension Header to deliver datagrams within a
RPL routing domain, particularly in non-storing mode.
These various items are referred to as RPL artifacts, and they are
seen on all of the data-plane traffic that occurs in RPL routed
networks; they do not in general appear on the RPL control plane
traffic at all which is mostly Hop-by-Hop traffic (one exception
being DAO messages in non-storing mode).
It has become clear from attempts to do multi-vendor
interoperability, and from a desire to compress as many of the above
Robles, et al. Expires September 24, 2020 [Page 3]
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artifacts as possible that not all implementers agree when artifacts
are necessary, or when they can be safely omitted, or removed.
The ROLL WG analysized how [RFC2460] rules apply to storing and non-
storing use of RPL. The result was 24 data plane use cases. They
are exhaustively outlined here in order to be completely unambiguous.
During the processing of this document, new rules were published as
[RFC8200], and this document was updated to reflect the normative
changes in that document.
This document updates [RFC6553], changing the value of the Option
Type of the RPL Option to make [RFC8200] routers ignore this option
when not recognized.
A Routing Header Dispatch for 6LoWPAN (6LoRH)([RFC8138]) defines a
mechanism for compressing RPL Option information and Routing Header
type 3 (RH3) [RFC6554], as well as an efficient IPv6-in-IPv6
technique.
Since some of the uses cases here described, use IPv6-in-IPv6
encapsulation. It MUST take in consideration, when encapsulation is
applied, the RFC6040 [RFC6040], which defines how the explicit
congestion notification (ECN) field of the IP header should be
constructed on entry to and exit from any IPV6-in-IPV6 tunnel.
Additionally, it is recommended the reading of
[I-D.ietf-intarea-tunnels] that explains the relationship of IP
tunnels to existing protocol layers and the challenges in supporting
IP tunneling.
Non-constrained uses of RPL are not in scope of this document, and
applicability statements for those uses may provide different advice,
E.g. [I-D.ietf-anima-autonomic-control-plane].
1.1. Overview
The rest of the document is organized as follows: Section 2 describes
the used terminology. Section 3 provides a RPL Overview. Section 4
describes the updates to RFC6553, RFC6550 and RFC 8138. Section 5
provides the reference topology used for the uses cases. Section 6
describes the uses cases included. Section 7 describes the storing
mode cases and section 8 the non-storing mode cases. Section 9
describes the operational considerations of supporting RPL-unaware-
leaves. Section 10 depicts operational considerations for the
proposed change on RPI Option Type, section 11 the IANA
considerations and then section 12 describes the security aspects.
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2. Terminology and Requirements Language
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.
Terminology defined in [RFC7102] applies to this document: LLN, RPL,
RPL domain and ROLL.
RPL Leaf: An IPv6 host that is attached to a RPL router and obtains
connectivity through a RPL Destination Oriented Directed Acyclic
Graph (DODAG). As an IPv6 node, a RPL Leaf is expected to ignore a
consumed Routing Header and as an IPv6 host, it is expected to ignore
a Hop-by-Hop header. It results that a RPL Leaf can correctly
receive a packet with RPL artifacts. On the other hand, a RPL Leaf
is not expected to generate RPL artifacts or to support IP-in-IP
encapsulation. For simplification, this document uses the standalone
term leaf to mean a RPL leaf.
RPL Packet Information (RPI): The abstract information that [RFC6550]
places in IP packets. The term is commonly used, including in this
document, to refer to the RPL Option [RFC6553] that transports that
abstract information in an IPv6 Hob-by-Hop Header.
RPL-aware-node (RAN): A device which implements RPL. Please note
that the device can be found inside the LLN or outside LLN.
RPL-Aware-Leaf(RAL): A RPL-aware-node that is also a RPL Leaf.
RPL-unaware-node: A device which does not implement RPL, thus the
device is not-RPL-aware. Please note that the device can be found
inside the LLN.
RPL-Unaware-Leaf(RUL): A RPL-unaware-node that is also a RPL Leaf.
6LoWPAN Node (6LN): [RFC6775] defines it as: "A 6LoWPAN node is any
host or router participating in a LoWPAN. This term is used when
referring to situations in which either a host or router can play the
role described.". In this document, a 6LN acts as a leaf.
6LoWPAN Router (6LR): [RFC6775] defines it as:" An intermediate
router in the LoWPAN that is able to send and receive Router
Advertisements (RAs) and Router Solicitations (RSs) as well as
forward and route IPv6 packets. 6LoWPAN routers are present only in
route-over topologies."
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6LoWPAN Border Router (6LBR): [RFC6775] defines it as:"A border
router located at the junction of separate 6LoWPAN networks or
between a 6LoWPAN network and another IP network. There may be one
or more 6LBRs at the 6LoWPAN network boundary. A 6LBR is the
responsible authority for IPv6 prefix propagation for the 6LoWPAN
network it is serving. An isolated LoWPAN also contains a 6LBR in
the network, which provides the prefix(es) for the isolated network."
Flag Day: A transition that involves having a network with different
values of RPI Option Type. Thus the network does not work correctly
(Lack of interoperation).
Non-Storing Mode (Non-SM): RPL mode of operation in which the RPL-
aware-nodes send information to the root about their parents. Thus,
the root knows the topology. Because the root knows the topology,
the intermediate 6LRs do not maintain routing state and source
routing is needed.
Storing Mode (SM): RPL mode of operation in which RPL-aware-nodes
(6LRs) maintain routing state (of the children) so that source
routing is not needed.
Note: Due to lack of space in some figures (tables) we refer to IPv6-
in-IPv6 as IP6-IP6.
3. RPL Overview
RPL defines the RPL Control messages (control plane), a new ICMPv6
[RFC4443] message with Type 155. DIS (DODAG Information
Solicitation), DIO (DODAG Information Object) and DAO (Destination
Advertisement Object) messages are all RPL Control messages but with
different Code values. A RPL Stack is shown in Figure 1.
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+--------------+
| Upper Layers |
| |
+--------------+
| RPL |
| |
+--------------+
| ICMPv6 |
| |
+--------------+
| IPv6 |
| |
+--------------+
| 6LoWPAN |
| |
+--------------+
| PHY-MAC |
| |
+--------------+
Figure 1: RPL Stack.
RPL supports two modes of Downward traffic: in storing mode (SM), it
is fully stateful; in non-storing mode (Non-SM), it is fully source
routed. A RPL Instance is either fully storing or fully non-storing,
i.e. a RPL Instance with a combination of storing and non-storing
nodes is not supported with the current specifications at the time of
writing this document.
4. Updates to RFC6553, RFC6550 and RFC8138
4.1. Updates to RFC6550: Advertising External Routes with Non-Storing
Mode Signaling.
Section 6.7.8. of [RFC6550] introduces the 'E' flag that is set to
indicate that the 6LR that generates the DAO redistributes external
targets into the RPL network. An external Target is a Target that
has been learned through an alternate protocol, for instance a route
to a prefix that is outside the RPL domain but reachable via a 6LR.
Being outside of the RPL domain, a node that is reached via an
external target cannot be guaranteed to ignore the RPL artifacts and
cannot be expected to process the [RFC8138] compression correctly.
This means that the RPL artifacts should be contained in an IP-in-IP
encapsulation that is removed by the 6LR, and that any remaining
compression should be expanded by the 6LR before it forwards a packet
outside the RPL domain.
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This specification updates [RFC6550] to RECOMMEND that external
targets are advertised using Non-Storing Mode DAO messaging even in a
Storing-Mode network. This way, external routes are not advertised
within the DODAG and all packets to an external target reach the Root
like normal Non-Storing Mode traffic. The Non-Storing Mode DAO
informs the Root of the address of the 6LR that injects the external
route, and the root uses IP-in-IP encapsulation to that 6LR, which
terminates the IP-in-IP tunnel and forwards the original packet
outside the RPL domain free of RPL artifacts. In the other
direction, for traffic coming from an external target into the LLN,
the parent (6LR) that injects the traffic always encapsulates to the
root. This whole operation is transparent to intermediate routers
that only see traffic between the 6LR and the Root, and only the Root
and the 6LRs that inject external routes in the network need to be
upgraded to add this function to the network.
A RUL is a special case of external target when the target is
actually a host and it is known to support a consumed Routing Header
and to ignore a HbH header as prescribed by [RFC8200]. The target
may have been learned through as a host route or may have been
registered to the 6LR using [RFC8505].
In order to enable IP-in-IP all the way to a 6LN, it is beneficial
that the 6LN supports decapsulating IP-in-IP, but that is not assumed
by [RFC8504]. If the 6LN is a RUL, the Root that encapsulates a
packet SHOULD terminate the tunnel at a parent 6LR unless it is aware
that the RUL supports IP-in-IP decapsulation.
A node that is reachable over an external route is not expected to
support [RFC8138]. Whether a decapsulation took place or not and
even when the 6LR is delivering the packet to a RUL, the 6LR that
injected an external route MUST uncompress the packet before
forwarding over that external route.
4.2. Updates to RFC6553: Indicating the new RPI Option Type.
This modification is required in order to be able to send, for
example, IPv6 packets from a RPL-Aware-Leaf to a RPL-unaware node
through Internet (see Section 7.2.1), without requiring IPv6-in-IPv6
encapsulation.
[RFC6553] (Section 6, Page 7) states as shown in Figure 2, that in
the Option Type field of the RPL Option, the two high order bits must
be set to '01' and the third bit is equal to '1'. The first two bits
indicate that the IPv6 node must discard the packet if it doesn't
recognize the Option Type, and the third bit indicates that the
Option Data may change in route. The remaining bits serve as the
Option Type.
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+-------+-------------------+----------------+-----------+
| Hex | Binary Value | Description | Reference |
+ Value +-------------------+ + +
| | act | chg | rest | | |
+-------+-----+-----+-------+----------------+-----------+
| 0x63 | 01 | 1 | 00011 | RPL Option | [RFC6553] |
+-------+-----+-----+-------+----------------+-----------+
Figure 2: Option Type in RPL Option.
This document illustrates that it is not always possible to know for
sure at the source that a packet will only travel within the RPL
domain or may leave it.
At the time [RFC6553] was published, leaking a Hop-by-Hop header in
the outer IPv6 header chain could potentially impact core routers in
the internet. So at that time, it was decided to encapsulate any
packet with a RPL Option using IPv6-in-IPv6 in all cases where it was
unclear whether the packet would remain within the RPL domain. In
the exception case where a packet would still leak, the Option Type
would ensure that the first router in the Internet that does not
recognize the option would drop the packet and protect the rest of
the network.
Even with [RFC8138], where the IPv6-in-IPv6 header is compressed,
this approach yields extra bytes in a packet; this means consuming
more energy, more bandwidth, incurring higher chances of loss and
possibly causing a fragmentation at the 6LoWPAN level. This impacts
the daily operation of constrained devices for a case that generally
does not happen and would not heavily impact the core anyway.
While intention was and remains that the Hop-by-Hop header with a RPL
Option should be confined within the RPL domain, this specification
modifies this behavior in order to reduce the dependency on IPv6-in-
IPv6 and protect the constrained devices. Section 4 of [RFC8200]
clarifies the behaviour of routers in the Internet as follows: "it is
now expected that nodes along a packet's delivery path only examine
and process the Hop-by-Hop Options header if explicitly configured to
do so".
When unclear about the travel of a packet, it becomes preferable for
a source not to encapsulate, accepting the fact that the packet may
leave the RPL domain on its way to its destination. In that event,
the packet should reach its destination and should not be discarded
by the first node that does not recognize the RPL Option. But with
the current value of the Option Type, if a node in the Internet is
configured to process the Hop-by-Hop header, and if such node
encounters an option with the first two bits set to 01 and conforms
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to [RFC8200], it will drop the packet. Host systems should do the
same, irrespective of the configuration.
Thus, this document updates the Option Type of the RPL Option
[RFC6553], abusively naming it RPI Option Type for simplicity, to
(Figure 3): the two high order bits MUST be set to '00' and the third
bit is equal to '1'. The first two bits indicate that the IPv6 node
MUST skip over this option and continue processing the header
([RFC8200] Section 4.2) if it doesn't recognize the Option Type, and
the third bit continues to be set to indicate that the Option Data
may change en route. The rightmost five bits remain at 0x3(00011).
This ensures that a packet that leaves the RPL domain of an LLN (or
that leaves the LLN entirely) will not be discarded when it contains
the RPL Option.
With the new Option Type, if an IPv6 (intermediate) node (RPL-not-
capable) receives a packet with an RPL Option, it should ignore the
Hop-by-Hop RPL Option (skip over this option and continue processing
the header). This is relevant, as it was mentioned previously, in
the case that there is a flow from RAL to Internet (see
Section 7.2.1).
This is a significant update to [RFC6553].
+-------+-------------------+-------------+------------+
| Hex | Binary Value | Description | Reference |
+ Value +-------------------+ + +
| | act | chg | rest | | |
+-------+-----+-----+-------+-------------+------------+
| 0x23 | 00 | 1 | 00011 | RPL Option |[RFCXXXX](*)|
+-------+-----+-----+-------+-------------+------------+
Figure 3: Revised Option Type in RPL Option. (*)represents this
document
Without the signaling described below, this change would otherwise
create a lack of interoperation (flag day) for existing networks
which are currently using 0x63 as the RPI Option Type value. A move
to 0x23 will not be understood by those networks. It is suggested
that RPL implementations accept both 0x63 and 0x23 when processing
the header.
When forwarding packets, implementations SHOULD use the same value of
RPI Type as was received. This is required because the RPI Option
Type does not change en route ([RFC8200] - Section 4.2). It allows
the network to be incrementally upgraded and allows the DODAG root to
know which parts of the network have been upgraded.
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When originating new packets, implementations SHOULD have an option
to determine which value to originate with, this option is controlled
by the DIO option described below.
The change of RPI Option Type from 0x63 to 0x23, makes all [RFC8200]
Section 4.2 compliant nodes tolerant of the RPL artifacts. There is
therefore no longer a necessity to remove the artifacts when sending
traffic to the Internet. This change clarifies when to use IPv6-in-
IPv6 headers, and how to address them: The Hop-by-Hop Options header
containing the RPI MUST always be added when 6LRs originate packets
(without IPv6-in-IPv6 headers), and IPv6-in-IPv6 headers MUST always
be added when a 6LR finds that it needs to insert a Hop-by-Hop
Options header containing the RPL Option. The IPv6-in-IPv6 header is
to be addressed to the RPL root when on the way up, and to the end-
host when on the way down.
In the non-storing case, dealing with not-RPL aware leaf nodes is
much easier as the 6LBR (DODAG root) has complete knowledge about the
connectivity of all DODAG nodes, and all traffic flows through the
root node.
The 6LBR can recognize not-RPL aware leaf nodes because it will
receive a DAO about that node from the 6LR immediately above that
not-RPL aware node.
The non-storing mode case does not require the type change from 0x63
to 0x23, as the root can always create the right packet. The type
change does not adversely affect the non-storing case.
4.3. Updates to RFC6550: Indicating the new RPI in the DODAG
Configuration option Flag.
In order to avoid a Flag Day caused by lack of interoperation between
new RPI Option Type (0x23) and old RPI Option Type (0x63) nodes, this
section defines a flag in the DIO Configuration option, to indicate
when the new RPI Option Type can be safely used. This means, the
flag is going to indicate the value of Option Type that the network
will be using for the RPL Option. Thus, when a node joins to a
network will know which value to use. With this, RPL-capable nodes
know if it is safe to use 0x23 when creating a new RPL Option. A
node that forwards a packet with an RPI MUST NOT modify the Option
Type of the RPL Option.
This is done using a DODAG Configuration option flag which will
signal "RPI 0x23 enable" and propagate through the network.
Section 6.3.1. of [RFC6550] defines a 3-bit Mode of Operation (MOP)
in the DIO Base Object. The flag is defined only for MOP value
between 0 to 6. For a MOP value of 7 or above, the flag MAY indicate
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something different and MUST NOT be interpreted as "RPI 0x23 enable"
unless the specification of the MOP indicates to do so.
As stated in [RFC6550] the DODAG Configuration option is present in
DIO messages. The DODAG Configuration option distributes
configuration information. It is generally static, and does not
change within the DODAG. This information is configured at the DODAG
root and distributed throughout the DODAG with the DODAG
Configuration option. Nodes other than the DODAG root do not modify
this information when propagating the DODAG Configuration option.
Currently, the DODAG Configuration option in [RFC6550] states: "the
unused bits MUST be initialize to zero by the sender and MUST be
ignored by the receiver". If the flag is received with a value zero
(which is the default), then new nodes will remain in RFC6553
Compatible Mode; originating traffic with the old-RPI Option Type
(0x63) value. If the flag is received with a value of 1, then the
value for the RPL Option MUST be set to 0x23.
Bit number three of the flag field in the DODAG Configuration option
is to be used as shown in Figure 4 (which is the same as Figure 39 in
Section 11 and is shown here for convenience):
+------------+-----------------+---------------+
| Bit number | Description | Reference |
+------------+-----------------+---------------+
| 3 | RPI 0x23 enable | This document |
+------------+-----------------+---------------+
Figure 4: DODAG Configuration option Flag to indicate the RPI-flag-
day.
In the case of reboot, the node (6LN or 6LR) does not remember the
RPI Option Type (i.e., whether or not the flag is set), so the node
will not trigger DIO messages until a DIO message is received
indicating the RPI value to be used. The node will use the value
0x23 if the network supports this feature
4.4. Updates to RFC8138: Indicating the way to decompress with the new
RPI Option Type.
This modification is required in order to be able to decompress the
RPL Option with the new Option Type of 0x23.
RPI-6LoRH header provides a compressed form for the RPL RPI; see
[RFC8138], Section 6. A node that is decompressing this header MUST
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decompress using the RPI Option Type that is currently active: that
is, a choice between 0x23 (new) and 0x63 (old). The node will know
which to use based upon the presence of the flag in the DODAG
Configuration option defined in Section 4.3. E.g. If the network is
in 0x23 mode (by DIO option), then it should be decompressed to 0x23.
[RFC8138] section 7 documents how to compress the IPv6-in-IPv6
header.
There are potential significant advantages to having a single code
path that always processes IPv6-in-IPv6 headers with no conditional
branches.
In Storing Mode, the scenarios where the flow goes from RAL to RUL
and RUL to RUL include compression of the IPv6-in-IPv6 and RPI
headers. The use of the IPv6-in-IPv6 header is MANDATORY in this
case, and it SHOULD be compressed with [RFC8138] section 7. Figure 5
illustrates the case in Storing mode where the packet is received
from the Internet, then the root encapsulates the packet to insert
the RPI. In that example, the leaf is not known to support RFC 8138,
and the packet is encapsulated to the 6LR that is the parent and last
hop to the final destination.
+-+ ... -+-+ ... +-+- ... -+-+- +-+-+-+ ... +-+-+ ... -+++ ... +-...
|11110001|SRH-6LoRH| RPI- |IP-in-IP| NH=1 |11110CPP| UDP | UDP
|Page 1 |Type1 S=0| 6LoRH |6LoRH |LOWPAN_IPHC| UDP | hdr |Payld
+-+ ... -+-+ ... +-+- ... -+-+-.+-+-+-+-+ ... +-+-+ ... -+ ... +-...
<-4bytes-> <- RFC 6282 ->
No RPL artifact
Figure 5: RPI Inserted by the Root in Storing Mode
In Figure 5, the source of the IPv6-in-IPv6 encapsulation is the
Root, so it is elided in the IP-in-IP 6LoRH. The destination is the
parent 6LR of the destination of the inner packet so it cannot be
elided. It is placed as the single entry in an SRH-6LoRH as the
first 6LoRH. There is a single entry so the SRH-6LoRH Size is 0. In
that example, the type is 1 so the 6LR address is compressed to 2
bytes. It results that the total length of the SRH-6LoRH is 4 bytes.
Follows the RPI-6LoRH and then the IP-in-IP 6LoRH. When the IP-in-IP
6LoRH is removed, all the router headers that precede it are also
removed. The Paging Dispatch [RFC8025] may also be removed if there
was no previous Page change to a Page other than 0 or 1, since the
LOWPAN_IPHC is encoded in the same fashion in the default Page 0 and
in Page 1. The resulting packet to the destination is the inner
packet compressed with [RFC6282].
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5. Sample/reference topology
A RPL network in general is composed of a 6LBR, a Backbone Router
(6BBR), a 6LR and a 6LN as a leaf logically organized in a DODAG
structure.
Figure 6 shows the reference RPL Topology for this document. The
letters above the nodes are there so that they may be referenced in
subsequent sections. In the figure, 6LR represents a full router
node. The 6LN is a RPL aware router, or host (as a leaf).
Additionally, for simplification purposes, it is supposed that the
6LBR has direct access to Internet and is the root of the DODAG, thus
the 6BBR is not present in the figure.
The 6LN leaves (RAL) marked as (F, H and I) are RPL nodes with no
children hosts.
The leaves marked as RUL (G and J) are devices which do not speak RPL
at all (not-RPL-aware), but uses Router-Advertisements, 6LowPAN DAR/
DAC and efficient-ND only to participate in the network [RFC6775].
In the document these leaves (G and J) are also referred to as an
IPv6 node.
The 6LBR ("A") in the figure is the root of the Global DODAG.
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+------------+
| INTERNET ----------+
| | |
+------------+ |
|
|
|
A |
+-------+
|6LBR |
+-----------|(root) |-------+
| +-------+ |
| |
| |
| |
| |
| B |C
+---|---+ +---|---+
| 6LR | | 6LR |
+---------| |--+ +--- ---+
| +-------+ | | +-------+ |
| | | |
| | | |
| | | |
| | | |
| D | E | |
+-|-----+ +---|---+ | |
| 6LR | | 6LR | | |
| | +------ | | |
+---|---+ | +---|---+ | |
| | | | |
| | +--+ | |
| | | | |
| | | | |
| | | I | J |
F | | G | H | |
+-----+-+ +-|-----+ +---|--+ +---|---+ +---|---+
| RAL | | RUL | | RAL | | RAL | | RUL |
| 6LN | | 6LN | | 6LN | | 6LN | | 6LN |
+-------+ +-------+ +------+ +-------+ +-------+
Figure 6: A reference RPL Topology.
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6. Use cases
In the data plane a combination of RFC6553, RFC6554 and IPv6-in-IPv6
encapsulation are going to be analyzed for a number of representative
traffic flows.
This document assumes that the LLN is using the no-drop RPI Option
Type of 0x23.
The use cases describe the communication in the following cases: -
Between RPL-aware-nodes with the root (6LBR) - Between RPL-aware-
nodes with the Internet - Between RUL nodes within the LLN (e.g. see
Section 7.1.4) - Inside of the LLN when the final destination address
resides outside of the LLN (e.g. see Section 7.2.3).
The uses cases are as follows:
Interaction between Leaf and Root:
RAL to root
root to RAL
RUL to root
root to RUL
Interaction between Leaf and Internet:
RAL to Internet
Internet to RAL
RUL to Internet
Internet to RUL
Interaction between leaves:
RAL to RAL
RAL to RUL
RUL to RAL
RUL to RUL
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This document is consistent with the rule that a Header cannot be
inserted or removed on the fly inside an IPv6 packet that is being
routed. This is a fundamental precept of the IPv6 architecture as
outlined in [RFC8200].
As the rank information in the RPI artifact is changed at each hop,
it will typically be zero when it arrives at the DODAG root. The
DODAG root MUST force it to zero when passing the packet out to the
Internet. The Internet will therefore not see any SenderRank
information.
Despite being legal to leave the RPI artifact in place, an
intermediate router that needs to add an extension header (e.g. RH3
or RPL Option) MUST still encapsulate the packet in an (additional)
outer IP header. The new header is placed after this new outer IP
header.
A corollary is that an RH3 or RPL Option can only be removed by an
intermediate router if it is placed in an encapsulating IPv6 Header,
which is addressed TO the intermediate router. When it does so, the
whole encapsulating header must be removed. (A replacement may be
added). This sometimes can result in outer IP headers being
addressed to the next hop router using link-local address.
Both the RPL Option and the RH3 headers may be modified in very
specific ways by routers on the path of the packet without the need
to add and remove an encapsulating header. Both headers were
designed with this modification in mind, and both the RPL RH3 and the
RPL Option are marked mutable but recoverable: so an IPsec AH
security header can be applied across these headers, but it can not
secure the values which mutate.
The RPI MUST be present in every single RPL data packet.
Prior to [RFC8138], there was significant interest in creating an
exception to this rule and removing the RPI for downward flows in
non-storing mode. This exception covered a very small number of
cases, and caused significant interoperability challenges while
adding significant in the code and tests. The ability to compress
the RPI down to three bytes or less removes much of the pressure to
optimize this any further [I-D.ietf-anima-autonomic-control-plane].
The earlier examples are more extensive to make sure that the process
is clear, while later examples are more concise.
The uses cases are delineated based on the following requirements:
The RPI has to be in every packet that traverses the LLN.
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- Because of the above requirement, packets from the Internet have
to be encapsulated.
- A Header cannot be inserted or removed on the fly inside an IPv6
packet that is being routed.
- Extension headers may not be added or removed except by the
sender or the receiver.
- RPI and RH3 headers may be modified by routers on the path of
the packet without the need to add and remove an encapsulating
header.
- an RH3 or RPL Option can only be removed by an intermediate
router if it is placed in an encapsulating IPv6 Header, which is
addressed to the intermediate router.
- Non-storing mode requires downstream encapsulation by root for
RH3.
The uses cases are delineated based on the following assumptions:
This document assumes that the LLN is using the no-drop RPI Option
Type (0x23).
- Each IPv6 node (including Internet routers) obeys [RFC8200], so
that 0x23 RPI Option Type can be safely inserted.
- All 6LRs obey [RFC8200].
- The RPI is ignored at the IPv6 dst node (RUL).
- In the uses cases, we assume that the RAL supports IP-in-IP
encapsulation.
- In the uses cases, we dont assume that the RUL supports IP-in-IP
encapsulation.
- For traffic leaving a RUL, if the RUL adds an opaque RPI then
the description of the RAL applies.
- The description for RALs applies to RAN in general.
- Non-constrained uses of RPL are not in scope of this document.