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draft-ietf-roll-useofrplinfo-28.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: November 18, 2019 P. Thubert
Cisco
May 17, 2019
Using RPL Option Type, Routing Header for Source Routes and IPv6-in-IPv6
encapsulation in the RPL Data Plane
draft-ietf-roll-useofrplinfo-28
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 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 November 18, 2019.
Robles, et al. Expires November 18, 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 . . . . . . . . . . . . 5
3. Updates to RFC6553, RFC6550 and RFC8138 . . . . . . . . . . . 6
3.1. Updates to RFC6553: Indicating the new RPI value. . . . . 6
3.2. Updates to RFC6550: Indicating the new RPI in the
DODAG Configuration Option Flag. . . . . . . . . . . . . 10
3.3. Updates to RFC8138: Indicating the way to decompress with
the new RPI value. . . . . . . . . . . . . . . . . . . . 11
4. Sample/reference topology . . . . . . . . . . . . . . . . . . 11
5. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Storing mode . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Storing Mode: Interaction between Leaf and Root . . . . . 17
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 . . . 19
6.1.3. SM: Example of Flow from root to not-RPL-aware-leaf . 19
6.1.4. SM: Example of Flow from not-RPL-aware-leaf to root . 20
6.2. Storing Mode: Interaction between Leaf and Internet. . . 21
6.2.1. SM: Example of Flow from RPL-aware-leaf to Internet . 21
6.2.2. SM: Example of Flow from Internet to RPL-aware-leaf . 22
6.2.3. SM: Example of Flow from not-RPL-aware-leaf to
Internet . . . . . . . . . . . . . . . . . . . . . . 23
6.2.4. SM: Example of Flow from Internet to not-RPL-aware-
leaf. . . . . . . . . . . . . . . . . . . . . . . . . 24
6.3. Storing Mode: Interaction between Leaf and Leaf . . . . . 25
6.3.1. SM: Example of Flow from RPL-aware-leaf to RPL-aware-
leaf . . . . . . . . . . . . . . . . . . . . . . . . 25
6.3.2. SM: Example of Flow from RPL-aware-leaf to not-RPL-
aware-leaf . . . . . . . . . . . . . . . . . . . . . 26
6.3.3. SM: Example of Flow from not-RPL-aware-leaf to RPL-
aware-leaf . . . . . . . . . . . . . . . . . . . . . 27
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6.3.4. SM: Example of Flow from not-RPL-aware-leaf to not-
RPL-aware-leaf . . . . . . . . . . . . . . . . . . . 29
7. Non Storing mode . . . . . . . . . . . . . . . . . . . . . . 30
7.1. Non-Storing Mode: Interaction between Leaf and Root . . . 31
7.1.1. Non-SM: Example of Flow from RPL-aware-leaf to root . 32
7.1.2. Non-SM: Example of Flow from root to RPL-aware-leaf . 32
7.1.3. Non-SM: Example of Flow from root to not-RPL-aware-
leaf . . . . . . . . . . . . . . . . . . . . . . . . 33
7.1.4. Non-SM: Example of Flow from not-RPL-aware-leaf to
root . . . . . . . . . . . . . . . . . . . . . . . . 34
7.2. Non-Storing Mode: Interaction between Leaf and Internet . 35
7.2.1. Non-SM: Example of Flow from RPL-aware-leaf to
Internet . . . . . . . . . . . . . . . . . . . . . . 35
7.2.2. Non-SM: Example of Flow from Internet to RPL-aware-
leaf . . . . . . . . . . . . . . . . . . . . . . . . 36
7.2.3. Non-SM: Example of Flow from not-RPL-aware-leaf to
Internet . . . . . . . . . . . . . . . . . . . . . . 37
7.2.4. Non-SM: Example of Flow from Internet to not-RPL-
aware-leaf . . . . . . . . . . . . . . . . . . . . . 38
7.3. Non-Storing Mode: Interaction between Leafs . . . . . . . 39
7.3.1. Non-SM: Example of Flow from RPL-aware-leaf to RPL-
aware-leaf . . . . . . . . . . . . . . . . . . . . . 39
7.3.2. Non-SM: Example of Flow from RPL-aware-leaf to not-
RPL-aware-leaf . . . . . . . . . . . . . . . . . . . 41
7.3.3. Non-SM: Example of Flow from not-RPL-aware-leaf to
RPL-aware-leaf . . . . . . . . . . . . . . . . . . . 42
7.3.4. Non-SM: Example of Flow from not-RPL-aware-leaf to
not-RPL-aware-leaf . . . . . . . . . . . . . . . . . 43
8. Operational Considerations of supporting
not-RPL-aware-leaves . . . . . . . . . . . . . . . . . . . . 44
9. Operational considerations of introducing 0x23 . . . . . . . 45
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
11. Security Considerations . . . . . . . . . . . . . . . . . . . 47
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 50
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
13.1. Normative References . . . . . . . . . . . . . . . . . . 50
13.2. Informative References . . . . . . . . . . . . . . . . . 51
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54
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" (RPL Packet Information or 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
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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
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 RPI option value to make
RFC8200 routers ignore this option by default.
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].
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
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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
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: [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."
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 RPL Option Type. Thus the network does not work correctly.
Hop-by-hop re-encapsulation: The term "hop-by-hop re-encapsulation"
header refers to adding a header that originates from a node to an
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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.
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.
+--------------+
| Upper Layers |
| |
+--------------+
| RPL |
| |
+--------------+
| ICMPv6 |
| |
+--------------+
| IPv6 |
| |
+--------------+
| 6LoWPAN |
| |
+--------------+
| PHY-MAC |
| |
+--------------+
Figure 1: RPL Stack.
RPL supports two modes of Downward traffic: in storing mode (RPL-SM),
it is fully stateful; in non-storing mode (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.
3. Updates to RFC6553, RFC6550 and RFC8138
3.1. Updates to RFC6553: Indicating the new RPI value.
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.
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[RFC6553] (Section 6, Page 7) states as shown in Figure 2, 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.
Hex Value Binary Value
act chg rest Description Reference
--------- --- --- ------- ----------------- ----------
0x63 01 1 00011 RPL Option [RFC6553]
Figure 2: Option Type in RPL Option.
This document illustrates that is 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] that compresses the IP-in-IP header, this
approach yields extra bytes in a packet which 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 IP-in-IP
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". This means that while it should be avoided, the impact on
the Internet of leaking a Hop-by-Hop header is acceptable.
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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
to [RFC8200], it will drop the packet. Host systems should do the
same, irrespective of the configuration.
Thus, this document updates the Option Type field 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 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.
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 RPL-aware-leaf to Internet (see
Section 6.2.1).
This is a significant update to [RFC6553].
Hex Value Binary Value
act chg rest Description Reference
--------- --- --- ------- ----------------- ----------
0x23 00 1 00011 RPL Option [RFCXXXX]
Figure 3: Revised Option Type in RPL Option.
Without the signaling described below, this change would otherwise
create 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.
When forwarding packets, implementations SHOULD use the same value as
it was received (This is required because, RPI type code can not be
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changed by [RFC8200]). It allows to the network to be incrementally
upgraded, and for the DODAG root to know which parts of the network
are upgraded.
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.
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 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 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 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. This means that the non-storing mode case can
avoid ever using hop-by-hop re-encapsulation headers for traffic
originating from the root to the 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.
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3.2. 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 (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. This means, the flag is going to indicate the
type of RPI that the network is using. Thus, when a node join 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 RPI. A node that
forwards a packet with an RPI MUST not modify the option type of the
RPI.
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 shown in Figure 4 :
+------------+-----------------+---------------+
| 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.
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3.3. Updates to RFC8138: Indicating the way to decompress with the new
RPI value.
This modification is required to be able to decompress the RPL RPI
option with the new value (0x23).
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 flag in the DODAG
Configuration Option defined in Section 3.2. 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, for the examples of Flow from RPL-aware-leaf to not-
RPL-aware-leaf and not-RPL-aware-leaf to not-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. As exemplification of compressing the RPI,
section A.1 of [RFC8138] illustrates the case in Storing mode where
the packet is received from the Internet, then the root encapsulates
the packet to insert the RPI. The result is shown in Figure 5.
+-+ ... -+-+-...-+-+-- ... -+-+-+-+- ... -+-+ ... -+-+-+ ... -+-+-+...
|11110001| RPI- | IP-in-IP | NH=1 |11110CPP| Compressed | UDP
|Page 1 | 6LoRH | 6LoRH | LOWPAN_IPHC | UDP | UDP header | Payld
+-+ ... -+-+-...-+-+-- ... -+-+-+-+- ... -+-+ ... -+-+-+ ... -+-+-+...
Figure 5: RPI Inserted by the Root in Storing Mode
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 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
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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, thus the 6BBR is not present in
the figure.
The 6LN leaves (Raf) marked as (F, H and I) are RPL nodes with no
children hosts.
The leafs marked as ~Raf (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 6: A reference RPL Topology.
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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
describes the communication between nodes acting as leaves that do
not understand RPL (~Raf nodes), but are part of the LLN. (e.g.
Section 6.1.4 Flow from not-RPL-aware-leaf to root) This document
depicts as well the communication inside of the LLN when it has the
final destination addressed outside of the LLN e.g. with destination
to Internet. For example, 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(Raf) to root
root to RPL-aware-leaf(Raf)
not-RPL-aware-leaf(~Raf) to root
root to not-RPL-aware-leaf(~Raf)
Interaction between Leaf and Internet:
RPL-aware-leaf(Raf) to Internet
Internet to RPL-aware-leaf(Raf)
not-RPL-aware-leaf(~Raf) to Internet
Internet to not-RPL-aware-leaf(~Raf)
Interaction between Leafs:
RPL-aware-leaf(Raf) to RPL-aware-leaf(Raf) (storing and non-
storing)
RPL-aware-leaf(Raf) to not-RPL-aware-leaf(~Raf) (non-storing)
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not-RPL-aware-leaf(~Raf) to RPL-aware-leaf(Raf) (storing and non-
storing)
not-RPL-aware-leaf(~Raf) to not-RPL-aware-leaf(~Raf) (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 headers may not be added or
removed except by the sender or the receiver.
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 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 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.
RPI MUST be present in every single RPL data packet.
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].
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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 (SM) (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 indicates if the IPv6-in-IPv6
header that is added, must be addressed to the final destination (the
Raf node that is the target(tgt)), to the "root" or if a hop-by-hop
header must be added (indicated by "hop").
In cases where no IPv6-in-IPv6 header is needed, the column states as
"No". If the IPv6-in-IPv6 header is needed is a "must".
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 is the intermediate router from source to
destination. "1 <= i <= n", n is the number of routers (6LR) that
the packet goes through from source (6LN) to destination.
The leaf can be a router 6LR or a host, both indicated as 6LN. The
root refers to the 6LBR (see Figure 6).
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+---------------------+--------------+------------+------------------+
| Interaction between | Use Case |IPv6-in-IPv6| IPv6-in-IPv6 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 | Raf (tgt) |
+ +--------------+------------+------------------+
| | ~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 | Raf (tgt) |
+ +--------------+------------+------------------+
| | ~Raf to ~Raf | must | 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
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.
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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 D -->
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.
The Table 1 summarizes what headers are needed for this use case.
+-------------------+---------+-------+----------+
| Header | 6LN src | 6LR_i | 6LBR dst |
+-------------------+---------+-------+----------+
| Inserted headers | RPI | -- | -- |
| Removed headers | -- | -- | RPI |
| Re-added headers | -- | -- | -- |
| Modified headers | -- | RPI | -- |
| Untouched headers | -- | -- | -- |
+-------------------+---------+-------+----------+
Table 1: Storing mode: Summary of the use of headers from RPL-aware-
leaf to root