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draft-ietf-roll-useofrplinfo-25.txt
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ROLL Working Group M. Robles
Internet-Draft Aalto
Updates: 6553, 6550, 8138 (if approved) M. Richardson
Intended status: Standards Track SSW
Expires: September 12, 2019 P. Thubert
Cisco
March 11, 2019
Using RPL Option Type, Routing Header for Source Routes and IPv6-in-IPv6
encapsulation in the RPL Data Plane
draft-ietf-roll-useofrplinfo-25
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 RFC 6553 (RPL Option Type), RFC 6554
(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
RFC 6553 adding a change to the RPL Option Type. Additionally, this
document updates RFC 6550 to indicate about this change and updates
RFC8138 as well to consider the new Option Type when 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 12, 2019.
Robles, et al. Expires September 12, 2019 [Page 1]
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Copyright Notice
Copyright (c) 2019 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 . . . . . . . . . . . . 4
3. Updates to RFC6553, RFC6550 and RFC 8138 . . . . . . . . . . 5
3.1. Updates to RFC 6553 . . . . . . . . . . . . . . . . . . . 5
3.2. Updates to RFC 8138 . . . . . . . . . . . . . . . . . . . 8
3.3. Updates to RFC 6550: Indicating the new RPI in the
DODAG Configuration Option Flag. . . . . . . . . . . . . 8
4. Sample/reference topology . . . . . . . . . . . . . . . . . . 9
5. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Storing mode . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Storing Mode: Interaction between Leaf and Root . . . . . 16
6.1.1. SM: Example of Flow from RPL-aware-leaf to root . . . 17
6.1.2. SM: Example of Flow from root to RPL-aware-leaf . . . 18
6.1.3. SM: Example of Flow from root to not-RPL-aware-leaf . 18
6.1.4. SM: Example of Flow from not-RPL-aware-leaf to root . 19
6.2. Storing Mode: Interaction between Leaf and Internet. . . 20
6.2.1. SM: Example of Flow from RPL-aware-leaf to Internet . 20
6.2.2. SM: Example of Flow from Internet to RPL-aware-leaf . 21
6.2.3. SM: Example of Flow from not-RPL-aware-leaf to
Internet . . . . . . . . . . . . . . . . . . . . . . 22
6.2.4. SM: Example of Flow from Internet to non-RPL-aware-
leaf. . . . . . . . . . . . . . . . . . . . . . . . . 23
6.3. Storing Mode: Interaction between Leaf and Leaf . . . . . 24
6.3.1. SM: Example of Flow from RPL-aware-leaf to RPL-aware-
leaf . . . . . . . . . . . . . . . . . . . . . . . . 24
6.3.2. SM: Example of Flow from RPL-aware-leaf to non-RPL-
aware-leaf . . . . . . . . . . . . . . . . . . . . . 26
6.3.3. SM: Example of Flow from not-RPL-aware-leaf to RPL-
aware-leaf . . . . . . . . . . . . . . . . . . . . . 26
6.3.4. SM: Example of Flow from not-RPL-aware-leaf to not-
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RPL-aware-leaf . . . . . . . . . . . . . . . . . . . 28
7. Non Storing mode . . . . . . . . . . . . . . . . . . . . . . 29
7.1. Non-Storing Mode: Interaction between Leaf and Root . . . 30
7.1.1. Non-SM: Example of Flow from RPL-aware-leaf to root . 31
7.1.2. Non-SM: Example of Flow from root to RPL-aware-leaf . 31
7.1.3. Non-SM: Example of Flow from root to not-RPL-aware-
leaf . . . . . . . . . . . . . . . . . . . . . . . . 32
7.1.4. Non-SM: Example of Flow from not-RPL-aware-leaf to
root . . . . . . . . . . . . . . . . . . . . . . . . 33
7.2. Non-Storing Mode: Interaction between Leaf and Internet . 34
7.2.1. Non-SM: Example of Flow from RPL-aware-leaf to
Internet . . . . . . . . . . . . . . . . . . . . . . 34
7.2.2. Non-SM: Example of Flow from Internet to RPL-aware-
leaf . . . . . . . . . . . . . . . . . . . . . . . . 35
7.2.3. Non-SM: Example of Flow from not-RPL-aware-leaf to
Internet . . . . . . . . . . . . . . . . . . . . . . 36
7.2.4. Non-SM: Example of Flow from Internet to not-RPL-
aware-leaf . . . . . . . . . . . . . . . . . . . . . 37
7.3. Non-Storing Mode: Interaction between Leafs . . . . . . . 38
7.3.1. Non-SM: Example of Flow from RPL-aware-leaf to RPL-
aware-leaf . . . . . . . . . . . . . . . . . . . . . 38
7.3.2. Non-SM: Example of Flow from RPL-aware-leaf to not-
RPL-aware-leaf . . . . . . . . . . . . . . . . . . . 40
7.3.3. Non-SM: Example of Flow from not-RPL-aware-leaf to
RPL-aware-leaf . . . . . . . . . . . . . . . . . . . 41
7.3.4. Non-SM: Example of Flow from not-RPL-aware-leaf to
not-RPL-aware-leaf . . . . . . . . . . . . . . . . . 42
8. Operational Considerations of supporting
not-RPL-aware-leaves . . . . . . . . . . . . . . . . . . . . 42
9. Operational considerations of introducing 0x23 . . . . . . . 43
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
11. Security Considerations . . . . . . . . . . . . . . . . . . . 45
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 48
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 48
13.1. Normative References . . . . . . . . . . . . . . . . . . 48
13.2. Informative References . . . . . . . . . . . . . . . . . 49
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 51
1. Introduction
RPL (IPv6 Routing Protocol for Low-Power and Lossy Networks)
[RFC6550] is a routing protocol for constrained networks. RFC 6553
[RFC6553] defines the "RPL option" (RPI), carried within the IPv6
Hop-by-Hop header to quickly identify inconsistencies (loops) in the
routing topology. RFC 6554 [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.
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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
artifacts as possible that not all implementors agree when artifacts
are necessary, or when they can be safely omitted, or removed.
An interim meeting went through the 24 cases defined here to discover
if there were any shortcuts, and this document is the result of that
discussion. This document clarifies examples that intend to
illustrate the result of the normative language in RFC8200 and
RFC6553. In other words, the examples are intended to be normative
explanation of the results of executing that language.
A Routing Header Dispatch for 6LoWPAN (6LoRH)([RFC8138]) defines a
mechanism for compressing RPL Option information and Routing Header
type 3 [RFC6554], as well as an efficient IPv6-in-IPv6 technique.
1.1. Overview
The rest of the document is organized as follows: Section 2 describes
the used terminology. Section 3 describes the updates to RFC6553,
RFC6550 and RFC 8138. Section 4 provides the reference topology used
for the uses cases. Section 5 describes the uses cases included.
Section 6 describes the storing mode cases and section 7 the non-
storing mode cases. Section 8 describes the operational
considerations of supporting not-RPL-aware-leaves. Section 9 depicts
operational considerations for the proposed change on RPL Option
type, section 10 the IANA considerations and then section 11
describes the security aspects.
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: LBR, LLN,
RPL, RPL Domain and ROLL.
RPL-node: A device which implements RPL, thus the device is RPL-
aware. Please note that the device can be found inside the LLN or
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outside LLN. In this document a RPL-node which is a leaf of a
(Destination Oriented Directed Acyclic Graph) DODAG is called RPL-
aware-leaf (Raf).
RPL-not-capable: 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. In this document a not-RPL-aware node which is a
leaf of a DODAG is called not-RPL-aware-leaf (~Raf).
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.
6LR, 6LBR are defined in [RFC6775].
Flag Day: A transition that involves having a network with different
values of RPL Option Type. Thus the network does not work correctly.
Hop-by-hop IPv6-in-IPv6 headers: The term "hop-by-hop IPv6-in-IPv6"
header refers to: adding a header that originates from a node to an
adjacent node, using the addresses (usually the GUA or ULA, but could
use the link-local addresses) of each node. If the packet must
traverse multiple hops, then it must be decapsulated at each hop, and
then re-encapsulated again in a similar fashion.
3. Updates to RFC6553, RFC6550 and RFC 8138
3.1. Updates to RFC 6553
This modification is required to be able to send, for example, IPv6
packets from a RPL-aware-leaf to a not-RPL-aware node through
Internet (see Section 6.2.1), without requiring IPv6-in-IPv6
encapsulation.
[RFC6553] states as shown below, that in the Option Type field of the
RPL Option header, 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 Value Binary Value
act chg rest Description Reference
--------- --- --- ------- ----------------- ----------
0x63 01 1 00011 RPL Option [RFC6553]
Figure 1: Option Type in RPL Option.
Recent changes in [RFC8200] (section 4, page 8), states: "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". Processing of the Hop-by-Hop Options header (by IPv6
intermediate nodes) is now optional, but if they are configured to
process the header, and if such nodes encounter an option with the
first two bits set to 01, they will drop the packet (if they conform
to [RFC8200]). Host systems should do the same, irrespective of the
configuration.
Based on that, if an IPv6 (intermediate) node (RPL-not-capable)
receives a packet with an RPL Option, it should ignore the HBH 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 RPL-aware-leaf to Internet (see Section 6.2.1).
Thus, this document updates the Option Type field to: 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
remaining bits serve as the option type and remain as 0x3. 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
[RFC6553] RPL Hop-by-Hop option known as RPI.
This is a significant update to [RFC6553]. [RFCXXXX] represents this
document.
Hex Value Binary Value
act chg rest Description Reference
--------- --- --- ------- ----------------- ----------
0x23 00 1 00011 RPL Option [RFCXXXX]
Figure 2: Revised Option Type in RPL Option.
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This change creates a flag day for existing networks which are
currently using 0x63 as the RPI value. A move to 0x23 will not be
understood by those networks. It is suggested that implementations
accept both 0x63 and 0x23 when processing.
In the cases where a forwarding node is forwarding traffic that is
not addressed directly to it (such as when the outer IPv6-in-IPv6
header is not a Link-Local address), then RFC8200 forbids changing
the RPI type code when forwarding.
When forwarding traffic that is wrapped in Link-Local IPv6-in-IPv6
headers, the forwarding node is in effect originating new packets,
and it MAY make a choice as to whether to use the old (0x63) RPI Type
code, or the new (0x23) RPI Type code. In that situation,
implementations SHOULD use the same value as was received. This
allows to the network to be incrementally upgraded, and in some cases
may allow the DODAG root to know which parts of the network are
upgraded.
A network which is switching from straight 6lowpan compression
mechanism to those described in [RFC8138] will experience a flag day
in the data compression anyway, and if possible this change can be
deployed at the same time.
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 an IPv6-
in-IPv6 header, and how to address them: The Hop-by-Hop Options
Header containing the RPI option SHOULD always be added when 6LRs
originate packets (without IPv6-in-IPv6 headers), and IPv6-in-IPv6
headers SHOULD always be added when a 6LR find that it needs to
insert a Hop-by-Hop Options Header containing the RPI 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.
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].
In the non-storing case, dealing with non-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 non-RPL aware leaf nodes because it will
receive a DAO about that node from the 6LR immediately above that
non-RPL aware node. This means that the non-storing mode case can
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avoid ever using hop-by-hop IPv6-in-IPv6 headers for traffic
originating from the root to leafs.
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.
3.2. Updates to RFC 8138
RPI-6LoRH header provides a compressed form for the RPL RPI [RFC8138]
in section 6. A node that is decompressing this header MUST
decompress using the RPL 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 DODAG Configuration
Option described in the next section. 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 options.
In Storing Mode, for the examples of Flow from RPL-aware-leaf to non-
RPL-aware-leaf and non-RPL-aware-leaf to non-RPL-aware-leaf comprise
an IPv6-in-IPv6 and RPI compression headers. The use of the IPv6-in-
IPv6 header is MANDATORY in this case, and it SHOULD be compressed
with [RFC8138] section 7.
+--+-----+---+--------------+-----------+-----------+-----------+
|1 | 0|0 |TSE| 6LoRH Type 6 | Hop Limit | RPI-6LoRH |LOWPAN IPHC|
+--+-----+---+--------------+-----------+-----------+-----------+
Figure 3: IPv6-in-IPv6 (RPI).
3.3. Updates to RFC 6550: 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 (0x23) and old RPI (0x63) nodes, this section defines a flag
in the DIO Configuration Option, to indicate when then new RPI value
can be safely used. Without this, there could be a mix of new nodes
(which understand 0x23 and 0x63), and old nodes (which understand
0x63 only). A new node would not know if it was safe to use 0x23.
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This is done via a DODAG Configuration Option flag which will
propagate through the network. 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 (0x63) value.
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.
The DODAG Configuration Option has a Flag field which is modified by
this document. 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".
Bit number three of the flag field in the DODAG Configuration option
is to be used as follows:
+------------+-----------------+---------------+
| Bit number | Description | Reference |
+------------+-----------------+---------------+
| 3 | RPI 0x23 enable | This document |
+------------+-----------------+---------------+
Figure 4: DODAG Configuration Option Flag to indicate the RPI-flag-
day.
In case of rebooting, the node (6LN or 6LR) does not remember if the
flag is set, so DIO messages would be set with the flag unset until a
DIO is received with the flag set.
4. Sample/reference topology
A RPL network in general is composed of a 6LBR (6LoWPAN Border
Router), Backbone Router (6BBR), 6LR (6LoWPAN Router) and 6LN
(6LoWPAN Node) as leaf logically organized in a DODAG structure.
Figure 4 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
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6LBR has direct access to Internet, thus the 6BBR is not present in
the figure.
The 6LN leaves (Raf - "RPL aware leaf"-) marked as (F, H and I) are
RPL nodes with no children hosts.
The leafs marked as ~Raf "not-RPL aware leaf" (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 leafs (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 | |
+-----+-+ +-|-----+ +---|--+ +---|---+ +---|---+
| Raf | | ~Raf | | Raf | | Raf | | ~Raf |
| 6LN | | 6LN | | 6LN | | 6LN | | 6LN |
+-------+ +-------+ +------+ +-------+ +-------+
Figure 5: A reference RPL Topology.
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
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Advertisement Object) messages are all RPL Control messages but with
different Code values. A RPL Stack is shown in Figure 5.
+--------------+
| Upper Layers |
| |
+--------------+
| RPL |
| |
+--------------+
| ICMPv6 |
| |
+--------------+
| IPv6 |
| |
+--------------+
| 6LoWPAN |
| |
+--------------+
| PHY-MAC |
| |
+--------------+
Figure 6: RPL Stack.
RPL supports two modes of Downward traffic: in storing mode (RPL-SM),
it is fully stateful; in non-storing (RPL-NSM), 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.
5. 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
(0x23).
The uses cases describe the communication between RPL-aware-nodes,
with the root (6LBR), and with Internet. This document also describe
the communication between nodes acting as leaves that do not
understand RPL, but are part of the LLN. these nodes are named as
not-RPL-aware-leaf, mentioned previously. (e.g. Section 6.1.4 Flow
from not-RPL-aware-leaf to root) This document describes also how is
the communication inside of the LLN when it has the final destination
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addressed outside of the LLN e.g. with destination to Internet.
(e.g. Section 6.2.3 Flow from not-RPL-aware-leaf to Internet)
The uses cases comprise as follow:
Interaction between Leaf and Root:
RPL-aware-leaf to root
root to RPL-aware-leaf
not-RPL-aware-leaf to root
root to not-RPL-aware-leaf
Interaction between Leaf and Internet:
RPL-aware-leaf to Internet
Internet to RPL-aware-leaf
not-RPL-aware-leaf to Internet
Internet to not-RPL-aware-leaf
Interaction between Leafs:
RPL-aware-leaf to RPL-aware-leaf (storing and non-storing)
RPL-aware-leaf to not-RPL-aware-leaf (non-storing)
not-RPL-aware-leaf to RPL-aware-leaf (storing and non-storing)
not-RPL-aware-leaf to not-RPL-aware-leaf (non-storing)
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]. Extensions may not be added or removed except
by the sender or the receiver.
However, unlike [RFC6553], the Hop-by-Hop Option Header used for the
RPI artifact has the first two bits set to '00'. This means that the
RPI artifact will be ignored when received by a host or router that
does not understand that option ( Section 4.2 [RFC8200]).
This means that when the no-drop RPI option code 0x23 is used, a
packet that leaves the RPL domain of an LLN (or that leaves the LLN
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entirely) will not be discarded when it contains the [RFC6553] RPL
Hop-by-Hop option known as RPI. Thus, the RPI Hop-by-Hop option is
left in place even if the end host does not understand it.
NOTE: There is some possible security risk when the RPI information
is released to the Internet. At this point this is a theoretical
situation; no clear attack has been described. At worst, it is clear
that the RPI option would waste some network bandwidth when it
escapes. This is traded off against the savings in the LLN by not
having to encapsulate the packet in order to remove the artifact.
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 SHOULD 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 (RH3 or RPI
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 RPI 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 RPI and RH3 headers may be modified in very specific ways by
routers on the path of the packet without the need to add to 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.
RPI MUST be present in every single RPL data packet. There is one
exception in non-storing mode: when a packet is going down from the
root the RPI MAY be omitted. The rational is that in a downward non-
storing mode, the entire route is written, so there can be no loops
by construction, nor any confusion about which forwarding table to
use (as the root has already made all routing decisions). However,
there are still cases, such as in 6tisch, where the instanceID
portion of the RPI header may still be needed [RFC8180] to pick an
appropriate priority or channel at each hop.
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Prior to [RFC8138], there was significant interest in removing the
RPI for downward flows in non-storing mode. The exception covered a
very small number of cases, and causes significant interoperability
challenges, yet costed significant code and testing complexity. 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.
6. Storing mode
In storing mode (fully stateful), the sender can determine if the
destination is inside the LLN by looking if the destination address
is matched by the DIO's Prefix Information Option (PIO) option.
The following table (Figure 7) itemizes which headers are needed in
each of the following scenarios. It indicate if an IPv6-in-IPv6
header must be inserted, and whether the destination address of the
IPv6-in-IPv6 header is the next hop, or the final target address.
There are these possible situations: hop-by-hop necessary (indicated
by "hop"), or final target address possible (indicated by "tgt"). In
all cases hop by hop may be used rather than the final target
address.
In cases where no IPv6-in-IPv6 header is needed, the column states as
"No".
In all cases the RPI headers are needed, since it identifies
inconsistencies (loops) in the routing topology. In all cases the
RH3 is not needed because it is not used in storing mode.
In each case, 6LR_i are the intermediate routers from source to
destination. "1 <= i <= n", n is the number of routers (6LR) that
the packet go through from source (6LN) to destination.
The leaf can be a router 6LR or a host, both indicated as 6LN (see
Figure 5).
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+---------------------+--------------+------------+--------------+
| Interaction between | Use Case |IPv6-in-IPv6| v6-in-v6 dst |
+---------------------+--------------+------------+--------------+
| | Raf to root | No | No |
+ +--------------+------------+--------------+
| Leaf - Root | root to Raf | No | No |
+ +--------------+------------+--------------+
| | root to ~Raf | No | No |
+ +--------------+------------+--------------+
| | ~Raf to root | must | root |
+---------------------+--------------+------------+--------------+
| | Raf to Int | No | No |
+ +--------------+------------+--------------+
| Leaf - Internet | Int to Raf | must | tgt (Raf) |
+ +--------------+------------+--------------+
| | ~Raf to Int | must | root |
+ +--------------+------------+--------------+
| | Int to ~Raf | must | hop |
+---------------------+--------------+------------+--------------+
| | Raf to Raf | No | No |
+ +--------------+------------+--------------+
| | Raf to ~Raf | No | No |
+ Leaf - Leaf +--------------+------------+--------------+
| | ~Raf to Raf | must | tgt (Raf) |
+ +--------------+------------+--------------+
| | ~Raf to ~Raf | Yes | hop |
+---------------------+--------------+------------+--------------+
Figure 7: Table of IPv6-in-IPv6 encapsulation in Storing mode.
6.1. Storing Mode: Interaction between Leaf and Root
In this section is described the communication flow in storing mode
(SM) between,
RPL-aware-leaf to root
root to RPL-aware-leaf
not-RPL-aware-leaf to root
root to not-RPL-aware-leaf
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6.1.1. SM: Example of Flow from RPL-aware-leaf to root
In storing mode, RFC 6553 (RPI) is used to send RPL Information
instanceID and rank information.
As stated in Section 16.2 of [RFC6550] a RPL-aware-leaf node does not
generally issue DIO messages; a leaf node accepts DIO messages from
upstream. (When the inconsistency in routing occurs, a leaf node
will generate a DIO with an infinite rank, to fix it). It may issue
DAO and DIS messages though it generally ignores DAO and DIS
messages.
In this case the flow comprises:
RPL-aware-leaf (6LN) --> 6LR_i --> root(6LBR)
For example, a communication flow could be: Node F --> Node E -->
Node B --> Node A root(6LBR)
As it was mentioned in this document 6LRs, 6LBR are always full-
fledged RPL routers.
The 6LN (Node F) inserts the RPI header, and sends the packet to 6LR
(Node E) which decrements the rank in RPI and sends the packet up.
When the packet arrives at 6LBR (Node A), the RPI is removed and the
packet is processed.
No IPv6-in-IPv6 header is required.
The RPI header can be removed by the 6LBR because the packet is
addressed to the 6LBR. The 6LN must know that it is communicating
with the 6LBR to make use of this scenario. The 6LN can know the
address of the 6LBR because it knows the address of the root via the
DODAGID in the DIO messages.
+-------------------+-----+-------+------+
| Header | 6LN | 6LR_i | 6LBR |
+-------------------+-----+-------+------+
| Inserted headers | RPI | -- | -- |
| Removed headers | -- | -- | RPI |
| Re-added headers | -- | -- | -- |
| Modified headers | -- | RPI | -- |
| Untouched headers | -- | -- | -- |
+-------------------+-----+-------+------+
Table 1: Storing: Summary of the use of headers from RPL-aware-leaf
to root
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6.1.2. SM: Example of Flow from root to RPL-aware-leaf
In this case the flow comprises:
root (6LBR) --> 6LR_i --> RPL-aware-leaf (6LN)
For example, a communication flow could be: Node A root(6LBR) -->
Node B --> Node D --> Node F
In this case the 6LBR inserts RPI header and sends the packet down,
the 6LR is going to increment the rank in RPI (it examines the
instanceID to identify the right forwarding table), the packet is
processed in the 6LN and the RPI removed.
No IPv6-in-IPv6 header is required.
+-------------------+------+-------+------+
| Header | 6LBR | 6LR_i | 6LN |
+-------------------+------+-------+------+
| Inserted headers | RPI | -- | -- |
| Removed headers | -- | -- | RPI |
| Re-added headers | -- | -- | -- |
| Modified headers | -- | RPI | -- |
| Untouched headers | -- | -- | -- |
+-------------------+------+-------+------+
Table 2: Storing: Summary of the use of headers from root to RPL-
aware-leaf
6.1.3. SM: Example of Flow from root to not-RPL-aware-leaf
In this case the flow comprises:
root (6LBR) --> 6LR_i --> not-RPL-aware-leaf (IPv6)
For example, a communication flow could be: Node A root(6LBR) -->
Node B --> Node E --> Node G
As the RPI extension can be ignored by the not-RPL-aware leaf, this
situation is identical to the previous scenario.