Internet DRAFT - draft-ietf-6lo-routing-dispatch
draft-ietf-6lo-routing-dispatch
6lo P. Thubert, Ed.
Internet-Draft Cisco
Intended status: Standards Track C. Bormann
Expires: August 15, 2016 Uni Bremen TZI
L. Toutain
IMT-TELECOM Bretagne
R. Cragie
ARM
February 12, 2016
6LoWPAN Routing Header
draft-ietf-6lo-routing-dispatch-05
Abstract
This specification introduces a new 6LoWPAN dispatch type for use in
6LoWPAN Route-Over topologies, that initially covers the needs of RPL
(RFC6550) data packets compression. Using this dispatch type, this
specification defines a method to compress RPL Option (RFC6553)
information and Routing Header type 3 (RFC6554), an efficient IP-in-
IP technique and is extensible for more applications.
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
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Drafts is at http://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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 15, 2016.
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Using the Page Dispatch . . . . . . . . . . . . . . . . . . . 6
3.1. New Routing Header Dispatch (6LoRH) . . . . . . . . . . . 6
3.2. Placement Of 6LoRH headers . . . . . . . . . . . . . . . 6
3.2.1. Relative To Non-6LoRH Headers . . . . . . . . . . . . 7
3.2.2. Relative To Other 6LoRH Headers . . . . . . . . . . . 7
4. 6LoWPAN Routing Header General Format . . . . . . . . . . . . 8
4.1. Elective Format . . . . . . . . . . . . . . . . . . . . . 8
4.2. Critical Format . . . . . . . . . . . . . . . . . . . . . 9
4.3. Compressing Addresses . . . . . . . . . . . . . . . . . . 9
4.3.1. Coalescence . . . . . . . . . . . . . . . . . . . . . 10
4.3.2. DODAG Root Address Determination . . . . . . . . . . 10
5. The SRH 6LoRH Header . . . . . . . . . . . . . . . . . . . . 11
5.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. SRH-6LoRH General Operation . . . . . . . . . . . . . . . 13
5.2.1. Uncompressed SRH Operation . . . . . . . . . . . . . 13
5.2.2. 6LoRH-Compressed SRH Operation . . . . . . . . . . . 13
5.2.3. Inner LOWPAN_IPHC Compression . . . . . . . . . . . . 14
5.3. The Design Point of Popping Entries . . . . . . . . . . . 14
5.4. Compression Reference for SRH-6LoRH header entries . . . 15
5.5. Popping Headers . . . . . . . . . . . . . . . . . . . . . 16
5.6. Forwarding . . . . . . . . . . . . . . . . . . . . . . . 17
6. The RPL Packet Information 6LoRH . . . . . . . . . . . . . . 17
6.1. Compressing the RPLInstanceID . . . . . . . . . . . . . . 19
6.2. Compressing the SenderRank . . . . . . . . . . . . . . . 19
6.3. The Overall RPI-6LoRH encoding . . . . . . . . . . . . . 19
7. The IP-in-IP 6LoRH Header . . . . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
9.1. Reserving Space in 6LoWPAN Dispatch Page 1 . . . . . . . 23
9.2. New 6LoWPAN Routing Header Type Registry . . . . . . . . 24
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . 24
11.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 26
A.1. Examples Compressing The RPI . . . . . . . . . . . . . . 26
A.2. Example Of Downward Packet In Non-Storing Mode . . . . . 28
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A.3. Example of SRH-6LoRH life-cycle . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
The design of Low Power and Lossy Networks (LLNs) is generally
focused on saving energy, a very constrained resource in most cases.
The other constraints, such as the memory capacity and the duty
cycling of the LLN devices, derive from that primary concern. Energy
is often available from primary batteries that are expected to last
for years, or is scavenged from the environment in very limited
quantities. Any protocol that is intended for use in LLNs must be
designed with the primary concern of saving energy as a strict
requirement.
Controlling the amount of data transmission is one possible venue to
save energy. In a number of LLN standards, the frame size is limited
to much smaller values than the IPv6 maximum transmission unit (MTU)
of 1280 bytes. In particular, an LLN that relies on the classical
Physical Layer (PHY) of IEEE 802.15.4 [IEEE802154] is limited to 127
bytes per frame. The need to compress IPv6 packets over IEEE
802.15.4 led to the 6LoWPAN Header Compression [RFC6282] work
(6LoWPAN-HC).
Innovative Route-over techniques have been and are still being
developed for routing inside a LLN. In a general fashion, such
techniques require additional information in the packet to provide
loop prevention and to indicate information such as flow
identification, source routing information, etc.
For reasons such as security and the capability to send ICMP errors
back to the source, an original packet must not be tampered with, and
any information that must be inserted in or removed from an IPv6
packet must be placed in an extra IP-in-IP encapsulation. This is
the case when the additional routing information is inserted by a
router on the path of a packet, for instance a mesh root, as opposed
to the source node. This is also the case when some routing
information must be removed from a packet that flows outside the LLN.
When to use RFC 6553, 6554 and IPv6-in-IPv6
[I-D.robles-roll-useofrplinfo] details different cases where RFC
6553, RFC 6554 and IPv6-in-IPv6 encapsulation is required to set the
bases to help defining the compression of RPL routing information in
LLN environments.
When using [RFC6282] the outer IP header of an IP-in-IP encapsulation
may be compressed down to 2 octets in stateless compression and down
to 3 octets in stateful compression when context information must be
added.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 0 | 1 | 1 | TF |NH | HLIM |CID|SAC| SAM | M |DAC| DAM |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 1: LOWPAN_IPHC base Encoding (RFC6282).
The Stateless Compression of an IPv6 addresses can only happen if the
IPv6 address can de deduced from the MAC addresses, meaning that the
IP end point is also the MAC-layer endpoint. This is generally not
the case in a RPL network which is generally a multi-hop route-over
(i.e., operated at Layer-3) network. A better compression, which
does not involve variable compressions depending on the hop in the
mesh, can be achieved based on the fact that the outer encapsulation
is usually between the source (or destination) of the inner packet
and the root. Also, the inner IP header can only be compressed by
[RFC6282] if all the fields preceding it are also compressed. This
specification makes the inner IP header the first header to be
compressed by [RFC6282], and keeps the inner packet encoded the same
way whether it is encapsulated or not, thus preserving existing
implementations.
As an example, the Routing Protocol for Low Power and Lossy Networks
[RFC6550] (RPL) is designed to optimize the routing operations in
constrained LLNs. As part of this optimization, RPL requires the
addition of RPL Packet Information (RPI) in every packet, as defined
in Section 11.2 of [RFC6550].
The RPL Option for Carrying RPL Information in Data-Plane Datagrams
[RFC6553] specification indicates how the RPI can be placed in a RPL
Option (RPL-OPT) that is placed in an IPv6 Hop-by-Hop header.
This representation demands a total of 8 bytes, while in most cases
the actual RPI payload requires only 19 bits. Since the Hop-by-Hop
header must not flow outside of the RPL domain, it must be inserted
in packets entering the domain and be removed from packets that leave
the domain. In both cases, this operation implies an IP-in-IP
encapsulation.
Additionally, in the case of the Non-Storing Mode of Operation (MOP),
RPL requires a Source Routing Header (SRH) in all packets that are
routed down a RPL graph. for that purpose, the [IPv6 Routing Header
for Source Routes with RPL] (#RFC6554) specification defines the type
3 Routing Header for IPv6 (RH3).
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------+--------- ^
| Internet |
| | Native IPv6
+-----+ |
| | Border Router (RPL Root) ^ | ^
| | | | |
+-----+ | | | IPv6 in
| | | | IPv6
o o o o | | | plus
o o o o o o o o o | | |
o o o o o o o o o o | | | RPL SRH
o o o o o o o o o | | |
o o o o o o o o v v v
o o o o
LLN
Figure 2: IP-in-IP Encapsulation within the LLN.
With Non-Storing RPL, even if the source is a node in the same LLN,
the packet must first reach up the graph to the root so that the root
can insert the SRH to go down the graph. In any fashion, whether the
packet was originated in a node in the LLN or outside the LLN, and
regardless of whether the packet stays within the LLN or not, as long
as the source of the packet is not the root itself, the source-
routing operation also implies an IP-in-IP encapsulation at the root
in order to insert the SRH.
6TiSCH [I-D.ietf-6tisch-architecture] specifies the operation of IPv6
over the TimeSlotted Channel Hopping [RFC7554] (TSCH) mode of
operation of IEEE 802.15.4. The architecture requires the use of
both RPL and the 6lo adaptation layer over IEEE 802.15.4. Because it
inherits the constraints on frame size from the MAC layer, 6TiSCH
cannot afford to allocate 8 bytes per packet on the RPI. Hence the
requirement for 6LoWPAN header compression of the RPI.
An extensible compression technique is required that simplifies IP-
in-IP encapsulation when it is needed, and optimally compresses
existing routing artifacts found in RPL LLNs.
This specification extends the 6lo adaptation layer framework
([RFC4944],[RFC6282]) so as to carry routing information for route-
over networks based on RPL. The specification includes the formats
necessary for RPL and is extensible for additional formats.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
The Terminology used in this document is consistent with and
incorporates that described in `Terminology in Low power And Lossy
Networks' [RFC7102] and [RFC6550].
The terms Route-over and Mesh-under are defined in [RFC6775].
Other terms in use in LLNs are found in [RFC7228].
The term "byte" is used in its now customary sense as a synonym for
"octet".
3. Using the Page Dispatch
The 6LoWPAN Paging Dispatch [I-D.ietf-6lo-paging-dispatch]
specification extends the 6lo adaptation layer framework ([RFC4944],
[RFC6282]) by introducing a concept of "context" in the 6LoWPAN
parser, a context being identified by a Page number. The
specification defines 16 Pages.
This draft operates within Page 1, which is indicated by a Dispatch
Value of binary 11110001.
3.1. New Routing Header Dispatch (6LoRH)
This specification introduces a new 6LoWPAN Routing Header (6LoRH) to
carry IPv6 routing information. The 6LoRH may contain source routing
information such as a compressed form of SRH, as well as other sorts
of routing information such as the RPI and IP-in-IP encapsulation.
The 6LoRH is expressed in a 6loWPAN packet as a Type-Length-Value
(TLV) field, which is extensible for future use.
This specification uses the bit pattern 10xxxxxx in Page 1 for the
new 6LoRH Dispatch. Section 4 describes how RPL artifacts in data
packets can be compressed as 6LoRH headers.
3.2. Placement Of 6LoRH headers
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3.2.1. Relative To Non-6LoRH Headers
In a zone of a packet where Page 1 is active (i.e., once a Page 1
Paging Dispatch is parsed and no subsequent Paging Dispatch has been
parsed, the parsing of the packet MUST follow this specification if
the 6LoRH Bit Pattern Section 3.1 is found.
With this specification, the 6LoRH Dispatch is only defined in Page
1, so it MUST be placed in the packet in a zone where the Page 1
context is active.
Because a 6LoRH header requires a Page 1 context, it MUST always be
placed after any Fragmentation Header and/or Mesh Header [RFC4944].
A 6LoRH header MUST always be placed before the LOWPAN_IPHC as
defined in 6LoWPAN Header Compression [RFC6282]. It is designed in
such a fashion that placing or removing a header that is encoded with
6LoRH does not modify the part of the packet that is encoded with
LoWPAN_IPHC, whether there is an IP-in-IP encapsulation or not. For
instance, the final destination of the packet is always the one in
the LOWPAN_IPHC whether there is a Routing Header or not.
3.2.2. Relative To Other 6LoRH Headers
IPv6 [RFC2460] defines chains of headers that are introduced by an
IPv6 header and terminated by either another IPv6 header (IP-in-IP)
or an Upper Layer Protocol ULP) header. When an outer header is
stripped from the packet, the whole chain goes with it. When one or
more header(s) are inserted by an intermediate router, that router
normally chains the headers and encapsulates the result in IP-in-IP.
With this specification, the chains of headers MUST be compressed in
the same order as they appear in the uncompressed form of the packet.
This means that if there is more than one nested IP-in-IP
encapsulations, the first IP-in-IP encapsulation, with all its chain
of headers, is encoded first in the compressed form.
In the compressed form of a packet that has SRH or HbH headers after
the inner IPv6 header (e.g. if there is no IP-in-IP encapsulation),
these headers are placed in the 6LoRH form before the 6LOWPAN-IPHC
that represents the IPv6 header Section 3.2.1. If this packet gets
encapsulated and some other SRH or HbH headers are added as part of
the encapsulation, placing the 6LoRH headers next to one another may
present an ambiguity on which header belong to which chain in the
uncompressed form.
In order to disambiguate the headers that follow the inner IPv6
header in the uncompressed form from the headers that follow the
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outer IP-in-IP header, it is REQUIRED that the compressed IP-in-IP
header is placed last in the encoded chain. This means that the
6LoRH headers that are found after the last compressed IP-in-IP
header are to be inserted after the IPv6 header that is encoded with
the 6LOWPAN-IPHC when decompressing the packet.
With regards to the relative placement of the SRH and the RPI in the
compressed form, it is a design point for this specification that the
SRH entries are consumed as the packet progresses down the LLN
Section 5.3. In order to make this operation simpler in the
compressed form, it is REQUIRED that the in the compressed form, the
addresses along the source route path are encoded in the order of the
path, and that the compressed SRH are placed before the compressed
RPI.
4. 6LoWPAN Routing Header General Format
The 6LoRH usesthe Dispatch Value Bit Pattern of 10xxxxxx in Page 1.
The Dispatch Value Bit Pattern is split in two forms of 6LoRH:
Elective (6LoRHE) that may skipped if not understood
Critical (6LoRHC) that may not be ignored
4.1. Elective Format
The 6LoRHE uses the Dispatch Value Bit Pattern of 101xxxxx. A 6LoRHE
may be ignored and skipped in parsing. If it is ignored, the 6LoRHE
is forwarded with no change inside the LLN.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+
|1|0|1| Length | Type | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+
<-- Length -->
Figure 3: Elective 6LoWPAN Routing Header.
Length:
Length of the 6LoRHE expressed in bytes, excluding the first 2
bytes. This enables a node to skip a 6LoRHE header that it does
not support and/or cannot parse, for instance if the Type is not
recognized.
Type:
Type of the 6LoRHE
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4.2. Critical Format
The 6LoRHC uses the Dispatch Value Bit Pattern of 100xxxxx.
A node which does not support the 6LoRHC Type MUST silently discard
the packet.
Note: The situation where a node receives a message with a Critical
6LoWPAN Routing Header that it does not understand is a critical
administrative error whereby the wrong device is placed in a network.
It makes no sense to overburden the constrained device with code that
would send an ICMP error to the source. Rather, it is expected that
the device will raise some management alert indicating that it cannot
operate in this network for that reason. As a result, there is no
provision for the exchange of error messages for this situation, so
it should be avoided by judicious use of administrative control and/
or capability indications by the device manufacturer.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+
|1|0|0| TSE | Type | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+
<-- Length implied by Type/TSE -->
Figure 4: Critical 6LoWPAN Routing Header.
TSE:
Type Specific Extension. The meaning depends on the Type, which
must be known in all of the nodes. The interpretation of the TSE
depends on the Type field that follows. For instance, it may be
used to transport control bits, the number of elements in an
array, or the length of the remainder of the 6LoRHC expressed in a
unit other than bytes.
Type:
Type of the 6LoRHC
4.3. Compressing Addresses
The general technique used in this draft to compress an address is
first to determine a reference that as a long prefix match with this
address, and then elide that matching piece. In order to reconstruct
the compress address, the receiving node will perform the process of
coalescence described in section Section 4.3.1.
One possible reference is the root of the RPL DODAG that is being
traversed. It is used by 6LoRH as the reference to compress an outer
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IP header, in case of an IP-in-IP encapsulation. If the root is the
source of the packet, this technique allows to fully elide the source
address in the compressed form of the IP header. If the root is not
the encapsulator, then the encapsulator address may still be
compressed using the root as reference. How the address of the root
is determined is discussed in Section 4.3.2.
Once the address of the source of the packet is determined, it
becomes the reference for the compression of the addresses that are
located in compressed SRH headers that are present inside the IP-in-
IP encapsulation in the uncompressed form.
4.3.1. Coalescence
An IPv6 compressed address is coalesced with a reference address by
overriding the N rightmost bytes of the reference address with the
compressed address, where N is the length of the compressed address,
as indicated by the Type of the SRH-6LoRH header in Figure 7.
The reference address MAY be a compressed address as well, in which
case it MUST be compressed in a form that is of an equal or greater
length than the address that is being coalesced.
A compressed address is expanded by coalescing it with a reference
address. In the particular case of a Type 4 SRH-6LoRH, the address
is expressed in full and the coalescence is a complete override as
illustrated in Figure 5.
RRRRRRRRRRRRRRRRRRRR reference address, may be compressed or not
CCCCCCC compressed address, shorter or same as reference
RRRRRRRRRRRRRCCCCCCC Coalesced address, same compression as reference
Figure 5: Coalescing addresses.
4.3.2. DODAG Root Address Determination
Stateful Address compression requires that some state is installed in
the devices to store the compression information that is elided from
the packet. That state is stored in an abstract context table and
some form of index is found in the packet to obtain the compression
information from the context table.
With [RFC6282], the state is provided to the stack by the 6LoWPAN
Neighbor Discovery Protocol (NDP) [RFC6775]. NDP exchanges the
context through 6LoWPAN Context Option in Router Advertisement (RA)
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messages. In the compressed form of the packet, the context can be
signaled in a Context Identifier Extension.
With this specification, the compression information is provided to
the stack by RPL, and RPL exchanges it through the DODAGID field in
the DAG Information Object (DIO) messages, as described in more
details below. In the compressed form of the packet, the context can
be signaled in by the RPLInstanceID in the RPI.
With RPL [RFC6550], the address of the DODAG root is known from the
DODAGID field of the DIO messages. For a Global Instance, the
RPLInstanceID that is present in the RPI is enough information to
identify the DODAG that this node participates to and its associated
root. But for a Local Instance, the address of the root MUST be
explicit, either in some device configuration or signaled in the
packet, as the source or the destination address, respectively.
When implicit, the address of the DODAG root MUST be determined as
follows:
If the whole network is a single DODAG then the root can be well-
known and does not need to be signaled in the packets. But since RPL
does not expose that property, it can only be known by a
configuration applied to all nodes.
Else, the router that encapsulates the packet and compresses it with
this specification MUST also place an RPI in the packet as prescribed
by [RFC6550] to enable the identification of the DODAG. The RPI must
be present even in the case when the router also places an SRH header
in the packet.
It is expected that the RPL implementation maintains an abstract
context table, indexed by Global RPLInstanceID, that provides the
address of the root of the DODAG that this nodes participates to for
that particular RPL Instance.
5. The SRH 6LoRH Header
5.1. Encoding
The Source Routing Header 6LoRH (SRH-6LoRH) header is a Critical
6LoWPAN Routing Header that provides a compressed form for the SRH,
as defined in [RFC6554] for use by RPL routers. Routers that need to
forward a packet with a SRH-6LoRH are expected to be RPL routers and
are expected to support this specification. If a non-RPL router
receives a packet with a SRH-6LoRH, this means that there was a
routing error and the packet should be dropped so the Type cannot be
ignored.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- -+- -+ ... +- -+
|1|0|0| Size |6LoRH Type 0..4| Hop1 | Hop2 | | HopN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- -+- -+ ... +- -+
Size indicates the number of compressed addresses
Figure 6: The SRH-6LoRH.
The 6LoRH Type indicates the compression level used in a given SRH-
6LoRH header.
One or more 6LoRH header(s) MAY be placed in a 6LoWPAN packet.
It results that all addresses in a given SRH-6LoRH header MUST be
compressed in an identical fashion, down to using the identical
number of bytes per address. In order to get different degrees of
compression, multiple consecutive SRH-6LoRH headers MUST be used.
Type 0 means that the address is compressed down to one byte, whereas
Type 4 means that the address is provided in full in the SRH-6LoRH
with no compression. The complete list of Types of SRH-6LoRH and the
corresponding compression level are provided in Figure 7:
+-----------+----------------------+
| 6LoRH | Length of compressed |
| Type | IPv6 address (bytes) |
+-----------+----------------------+
| 0 | 1 |
| 1 | 2 |
| 2 | 4 |
| 3 | 8 |
| 4 | 16 |
+-----------+----------------------+
Figure 7: The SRH-6LoRH Types.
In the case of a SRH-6LoRH header, the TSE field is used as a Size,
which encodes the number of hops minus 1; so a Size of 0 means one
hop, and the maximum that can be encoded is 32 hops. (If more than
32 hops need to be expressed, a sequence of SRH-6LoRH elements can be
employed.) It results that the Length in bytes of a SRH-6LoRH header
is:
2 + Length_of_compressed_IPv6_address * (Size + 1)
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5.2. SRH-6LoRH General Operation
5.2.1. Uncompressed SRH Operation
In the non-compressed form, when the root generates or forwards a
packet in non-Storing Mode, it needs to include a Source Routing
Header [RFC6554] to signal a strict source-route path to a final
destination down the DODAG.
All the hops along the path, but the first one, are encoded in order
in the SRH. The last entry in the SRH is the final destination and
the destination in the IPv6 header is the first hop along the source-
route path. The intermediate hops perform a swap and the Segment-
Left field indicates the active entry in the Routing Header
[RFC2460].
The current destination of the packet, which is the termination of
the current segment, is indicated at all times by the destination
address of the IPv6 header.
5.2.2. 6LoRH-Compressed SRH Operation
The handling of the SRH-6LoRH is different: there is no swap, and a
forwarding router that corresponds to the first entry in the first
SRH-6LoRH upon reception of a packet effectively consumes that entry
when forwarding. This means that the size of a compressed source-
routed packet decreases as the packet progresses along its path and
that the routing information is lost along the way. This also means
that an SRH encoded with 6LoRH is not recoverable and cannot be
protected.
When compressed with this specification, all the remaining hops MUST
be encoded in order in one or more consecutive SRH-6LoRH headers.
Whether or not there is a SRH-6LoRH header present, the address of
the final destination is indicated in the LoWPAN_IPHC at all times
along the path. Examples of this are provided in Appendix A.
The current destination (termination of the current segment) for a
compressed source-routed packet is indicated in the first entry of
the first SRH-6LoRH. In strict source-routing, that entry MUST match
an address of the router that receives the packet.
The last entry in the last SRH-6LoRH is the last router on the way to
the final destination in the LLN. This router can be the final
destination if it is found desirable to carry a whole IP-in-IP
encapsulation all the way. Else, it is the RPL parent of the final
destination, or a router acting at 6LR [RFC6775] for the destination
host, and advertising the host as an external route to RPL.
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If the SRH-6LoRH header is contained in an IP-in-IP encapsulation,
the last router removes the whole chain of headers. Otherwise, it
removes the SRH-6LoRH header only.
5.2.3. Inner LOWPAN_IPHC Compression
6LoWPAN ND [RFC6282] is designed to support more than one IPv6
address per node and per Interface Identifier (IID), an IID being
typically derived from a MAC address to optimize the LOWPAN-IPHC
compression.
Link local addresses are compressed with stateless address
compression (S/DAC=0). The other addresses are derived from
different prefixes and they can be compressed with stateful address
compression based on a context (S/DAC=1).
But stateless compression is only defined for the specific link-local
prefix as opposed to the prefix in an encapsulating header. And with
stateful compression, the compression reference is found in a
context, as opposed to an encapsulating header.
It results that in the case of an IP-in-IP encapsulation, it is
possible to compress an inner source (respectively destination) IP
address in a LOWPAN_IPHC based on the encapsulating IP header only if
stateful (context-based) compression is used. The compression will
operate only if the IID in the source (respectively the destination)
IP address in the outer and inner headers match, which usually means
that they refer to the same node . This is encoded as S/DAC = 1 and
S/AM=11. It must be noted that the outer destination address that is
used to compress the inner destination address is the last entry in
the last SRH-6LoRH header.
5.3. The Design Point of Popping Entries
In order to save energy and to optimize the chances of transmission
success on lossy media, it is a design point for this specification
that the entries in the SRH that have been used are removed from the
packet. This creates a discrepancy from the art of IPv6 where
Routing Header are mutable but recoverable.
With this specification, the packet can be expanded at any hop into a
valid IPv6 packet, including a SRH, and compressed back. But the
packet as decompressed along the way will not carry all the consumed
addresses that packet would have if it had been forwarded in the
uncompressed form.
It is noted that:
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The value of keeping the whole RH in an IPv6 header is for the
receiver to reverse it to use the symmetrical path on the way
back.
It is generally not a good idea to reverse a routing header. The
RH may have been used to stay away from the shortest path for some
reason that is only valid on the way in (segment routing).
There is no use of reversing a RH in the present RPL
specifications.
P2P RPL reverses a path that was learned reactively, as a part of
the protocol operation, which is probably a cleaner way than a
reversed echo on the data path.
Reversing a header is discouraged by [RFC2460] for RH0 unless it
is authenticated, which requires an Authentication Header (AH).
There is no definition of an AH operation for SRH, and there is no
indication that the need exists in LLNs.
It is noted that AH does not protect the RH on the way. AH is a
validation at the receiver with the sole value of enabling the
receiver to reversing it.
A RPL domain is usually protected by L2 security and that secures
both RPL itself and the RH in the packets, at every hop. This is
a better security than that provided by AH.
In summary, the benefit of saving energy and lowering the chances of
loss by sending smaller frames over the LLN are seen as overwhelming
compared to the value of possibly reversing the header.
5.4. Compression Reference for SRH-6LoRH header entries
In order to optimize the compression of IP addresses present in the
SRH headers, this specification requires that the 6LoWPAN layer
identifies an address that is used as reference for the compression.
With this specification, the Compression Reference for the first
address found in an SRH header is the source of the IPv6 packet, and
then the reference for each subsequent entry is the address of its
predecessor once it is uncompressed.
With RPL [RFC6550], an SRH header may only be present in Non-Storing
mode, and it may only be placed in the packet by the root of the
DODAG, which must be the source of the resulting IPv6 packet
[RFC2460]. In this case, the address used as Compression Reference
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is that the address of the root, and it can be implicit when the
address of the root is.
The Compression Reference MUST be determined as follows:
The reference address may be obtained by configuration. The
configuration may indicate either the address in full, or the
identifier of a 6LoWPAN Context that carries the address [RFC6775],
for instance one of the 16 Context Identifiers used in LOWPAN-IPHC
[RFC6282].
Else, and if there is no IP-in-IP encapsulation, the source address
in the IPv6 header that is compressed with LOWPAN-IPHC is the
reference for the compression.
Else, and if the IP-in-IP compression specified in this document is
used and the Encapsulator Address is provided, then the Encapsulator
Address is the reference.
5.5. Popping Headers
Upon reception, the router checks whether the address in the first
entry of the first SRH-6LoRH one of its own addresses. In that case,
router MUST consume that entry before forwarding, which is an action
of popping from a stack, where the stack is effectively the sequence
of entries in consecutive SRH-6LoRH headers.
Popping an entry of an SRH-6LoRH header is a recursive action
performed as follows:
If the Size of the SRH-6LoRH header is 1 or more, indicating that
there are at least 2 entries in the header, the router removes the
first entry and decrements the Size (by 1).
Else (meaning that this is the last entry in the SRH-6LoRH header),
and if there is no next SRH-6LoRH header after this then the SRH-
6LoRH is removed.
Else, if there is a next SRH-6LoRH of a Type with a larger or equal
value, meaning a same or lesser compression yielding same or larger
compressed forms, then the SRH-6LoRH is removed.
Else, the first entry of the next SRH-6LoRH is popped from the next
SRH-6LoRH and coalesced with the first entry of this SRH-6LoRH.
At the end of the process, if there is no more SRH-6LoRH in the
packet, then the processing node is the last router along the source
route path.
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5.6. Forwarding
When receiving a packet with a SRH-6LoRH, a router determines the
IPv6 address of the current segment endpoint.
If strict source routing is enforced and thus router is not the
segment endpoint for the packet then this router MUST drop the
packet.
If this router is the current segment endpoint, then the router pops
its address as described in Section 5.5 and continues processing the
packet.
If there is still a SRH-6LoRH, then the router determines the new
segment endpoint and routes the packet towards that endpoint.
Otherwise the router uses the destination in the inner IP header to
forward or accept the packet.
The segment endpoint of a packet MUST be determined as follows:
The router first determines the Compression Reference as discussed in
Section 4.3.1.
The router then coalesces the Compression Reference with the first
entry of the first SRH-6LoRH header as discussed in Section 5.4. If
the type of the SRH-6LoRH header is type 4 then the coalescence is a
full override.
Since the Compression Reference is an uncompressed address, the
coalesced IPv6 address is also expressed in the full 128bits.
An example of this operation is provided in Appendix A.3.
6. The RPL Packet Information 6LoRH
[RFC6550], Section 11.2, specifies the RPL Packet Information (RPI)
as a set of fields that are placed by RPL routers in IP packets to
identify the RPL Instance, detect anomalies and trigger corrective
actions.
In particular, the SenderRank, which is the scalar metric computed by
a specialized Objective Function such as [RFC6552], indicates the
Rank of the sender and is modified at each hop. The SenderRank field
is used to validate that the packet progresses in the expected
direction, either upwards or downwards, along the DODAG.
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RPL defines the RPL Option for Carrying RPL Information in Data-Plane
Datagrams [RFC6553] to transport the RPI, which is carried in an IPv6
Hop-by-Hop Options Header [RFC2460], typically consuming eight bytes
per packet.
With [RFC6553], the RPL option is encoded as six octets, which must
be placed in a Hop-by-Hop header that consumes two additional octets
for a total of eight octets. To limit the header's range to just the
RPL domain, the Hop-by-Hop header must be added to (or removed from)
packets that cross the border of the RPL domain.
The 8-byte overhead is detrimental to LLN operation, in particular
with regards to bandwidth and battery constraints. These bytes may
cause a containing frame to grow above maximum frame size, leading to
Layer 2 or 6LoWPAN [RFC4944] fragmentation, which in turn leads to
even more energy expenditure and issues discussed in LLN Fragment
Forwarding and Recovery [I-D.thubert-6lo-forwarding-fragments].
An additional overhead comes from the need, in certain cases, to add
an IP-in-IP encapsulation to carry the Hop-by-Hop header. This is
needed when the router that inserts the Hop-by-Hop header is not the
source of the packet, so that an error can be returned to the router.
This is also the case when a packet originated by a RPL node must be
stripped from the Hop-by-Hop header to be routed outside the RPL
domain.
For that reason, this specification defines an IP-in-IP-6LoRH header
in Section 7, but it must be noted that removal of a 6LoRH header
does not require manipulation of the packet in the LOWPAN_IPHC, and
thus, if the source address in the LOWPAN_IPHC is the node that
inserted the IP-in-IP-6LoRH header then this situation alone does not
mandate an IP-in-IP-6LoRH header.
Note: A typical packet in RPL non-storing mode going down the RPL
graph requires an IP-in-IP encapsulation of the SRH, whereas the RPI
is usually (and quite illegally) omitted, unless it is important to
indicate the RPLInstanceID. To match this structure, an optimized
IP-in-IP 6LoRH header is defined in Section 7.
As a result, a RPL packet may bear only an RPI-6LoRH header and no
IP-in-IP-6LoRH header. In that case, the source and destination of
the packet are specified by the LOWPAN_IPHC.
As with [RFC6553], the fields in the RPI include an 'O', an 'R', and
an 'F' bit, an 8-bit RPLInstanceID (with some internal structure),
and a 16-bit SenderRank.
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The remainder of this section defines the RPI-6LoRH header, which is
a Critical 6LoWPAN Routing Header that is designed to transport the
RPI in 6LoWPAN LLNs.
6.1. Compressing the RPLInstanceID
RPL Instances are discussed in [RFC6550], Section 5. A number of
simple use cases do not require more than one RPL Instance, and in
such cases, the RPL Instance is expected to be the Global Instance 0.
A global RPLInstanceID is encoded in a RPLInstanceID field as
follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0| ID | Global RPLInstanceID in 0..127
+-+-+-+-+-+-+-+-+
Figure 8: RPLInstanceID Field Format for Global Instances.
For the particular case of the Global Instance 0, the RPLInstanceID
field is all zeros. This specification allows to elide a
RPLInstanceID field that is all zeros, and defines a I flag that,
when set, signals that the field is elided.
6.2. Compressing the SenderRank
The SenderRank is the result of the DAGRank operation on the rank of
the sender; here the DAGRank operation is defined in [RFC6550],
Section 3.5.1, as:
DAGRank(rank) = floor(rank/MinHopRankIncrease)
If MinHopRankIncrease is set to a multiple of 256, the least
significant 8 bits of the SenderRank will be all zeroes; by eliding
those, the SenderRank can be compressed into a single byte. This
idea is used in [RFC6550] by defining DEFAULT_MIN_HOP_RANK_INCREASE
as 256 and in [RFC6552] that defaults MinHopRankIncrease to
DEFAULT_MIN_HOP_RANK_INCREASE.
This specification allows to encode the SenderRank as either one or
two bytes, and defines a K flag that, when set, signals that a single
byte is used.
6.3. The Overall RPI-6LoRH encoding
The RPI-6LoRH header provides a compressed form for the RPL RPI.
Routers that need to forward a packet with a RPI-6LoRH header are
expected to be RPL routers that support this specification. If a
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non-RPL router receives a packet with a RPI-6LoRH header, there was a
routing error and the packet should be dropped. Thus the Type field
MUST NOT be ignored.
Since the I flag is not set, the TSE field does not need to be a
length expressed in bytes. In that case the field is fully reused
for control bits that encode the O, R and F flags from the RPI, as
well as the I and K flags that indicate the compression format.
The Type for the RPI-6LoRH is 5.
The RPI-6LoRH header is immediately followed by the RPLInstanceID
field, unless that field is fully elided, and then the SenderRank,
which is either compressed into one byte or fully in-lined as two
bytes. The I and K flags in the RPI-6LoRH header indicate whether
the RPLInstanceID is elided and/or the SenderRank is compressed.
Depending on these bits, the Length of the RPI-6LoRH may vary as
described hereafter.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... -+-+-+
|1|0|0|O|R|F|I|K| 6LoRH Type=5 | Compressed fields |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... -+-+-+
Figure 9: The Generic RPI-6LoRH Format.
O, R, and F bits: The O, R, and F bits are defined in [RFC6550],
section 11.2.
I bit: If it is set, the RPLInstanceID is elided and the
RPLInstanceID is the Global RPLInstanceID 0. If it is not set,
the octet immediately following the type field contains the
RPLInstanceID as specified in [RFC6550], section 5.1.
K bit: If it is set, the SenderRank is compressed into one octet,
with the least significant octet elided. If it is not set, the
SenderRank, is fully inlined as two octets.
In Figure 10, the RPLInstanceID is the Global RPLInstanceID 0, and
the MinHopRankIncrease is a multiple of 256 so the least significant
byte is all zeros and can be elided:
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0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|O|R|F|1|1| 6LoRH Type=5 | SenderRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
I=1, K=1
Figure 10: The most compressed RPI-6LoRH.
In Figure 11, the RPLInstanceID is the Global RPLInstanceID 0, but
both bytes of the SenderRank are significant so it can not be
compressed:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|O|R|F|1|0| 6LoRH Type=5 | SenderRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
I=1, K=0
Figure 11: Eliding the RPLInstanceID.
In Figure 12, the RPLInstanceID is not the Global RPLInstanceID 0,
and the MinHopRankIncrease is a multiple of 256:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|O|R|F|0|1| 6LoRH Type=5 | RPLInstanceID | SenderRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
I=0, K=1
Figure 12: Compressing SenderRank.
In Figure 13, the RPLInstanceID is not the Global RPLInstanceID 0,
and both bytes of the SenderRank are significant:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|O|R|F|0|0| 6LoRH Type=5 | RPLInstanceID | Sender-...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...-Rank |
+-+-+-+-+-+-+-+-+
I=0, K=0
Figure 13: Least compressed form of RPI-6LoRH.
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7. The IP-in-IP 6LoRH Header
The IP-in-IP 6LoRH (IP-in-IP-6LoRH) header is an Elective 6LoWPAN
Routing Header that provides a compressed form for the encapsulating
IPv6 Header in the case of an IP-in-IP encapsulation.
An IP-in-IP encapsulation is used to insert a field such as a Routing
Header or an RPI at a router that is not the source of the packet.
In order to send an error back regarding the inserted field, the
address of the router that performs the insertion must be provided.
The encapsulation can also enable the last router prior to
Destination to remove a field such as the RPI, but this can be done
in the compressed form by removing the RPI-6LoRH, so an IP-in-IP-
6LoRH encapsulation is not required for that sole purpose.
This field is not critical for routing so the Type can be ignored,
and the TSE field contains the Length in bytes.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+
|1|0|1| Length | 6LoRH Type 6 | Hop Limit | Encaps. Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+
Figure 14: The IP-in-IP-6LoRH.
The Length of an IP-in-IP-6LoRH header is expressed in bytes and MUST
be at least 1, to indicate a Hop Limit (HL), that is decremented at
each hop. When the HL reaches 0, the packet is dropped per
[RFC2460].
If the Length of an IP-in-IP-6LoRH header is exactly 1, then the
Encapsulator Address is elided, which means that the Encapsulator is
a well-known router, for instance the root in a RPL graph.
The most efficient compression of an IP-in-IP encapsulation that can
be achieved with this specification is obtained when an endpoint of
the packet is the root of the RPL DODAG associated to the RPL
Instance that is used to forward the packet, and the root address is
known implicitly as opposed to signaled explicitly in the data
packets.
If the Length of an IP-in-IP-6LoRH header is greater than 1, then an
Encapsulator Address is placed in a compressed form after the Hop
Limit field. The value of the Length indicates which compression is
performed on the Encapsulator Address. For instance, a Size of 3
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indicates that the Encapsulator Address is compressed to 2 bytes.
The reference for the compression is the address of the root of the
DODAG. The way the address of the root is determined is discussed in
Section 4.3.2.
With RPL, the destination address in the IP-in-IP header is
implicitly the root in the RPL graph for packets going upwards, and,
in storing mode, it is the destination address in the IPHC for
packets going downwards. In non-storing mode, there is no implicit
value for packets going downwards.
If the implicit value is correct, the destination IP address of the
IP-in-IP encapsulation can be elided. Else, the destination IP
address of the IP-in-IP header is transported in a SRH-6LoRH header
as the first entry of the first of these headers.
If the final destination of the packet is a leaf that does not
support this specification, then the chain of 6LoRH headers must be
stripped by the RPL/6LR router to which the leaf is attached. In
that example, the destination IP address of the IP-in-IP header
cannot be elided.
In the special case where a 6LoRH header is used to route 6LoWPAN
fragments, the destination address is not accessible in the IPHC on
all fragments and can be elided only for the first fragment and for
packets going upwards.
8. Security Considerations
The security considerations of [RFC4944], [RFC6282], and [RFC6553]
apply.
Using a compressed format as opposed to the full in-line format is
logically equivalent and is believed to not create an opening for a
new threat when compared to [RFC6550], [RFC6553] and [RFC6554].
9. IANA Considerations
9.1. Reserving Space in 6LoWPAN Dispatch Page 1
This specification reserves Dispatch Value Bit Patterns within the
6LoWPAN Dispatch Page 1 as follows:
101xxxxx: for Elective 6LoWPAN Routing Headers
100xxxxx: for Critical 6LoWPAN Routing Headers.
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9.2. New 6LoWPAN Routing Header Type Registry
This document creates an IANA registry for the 6LoWPAN Routing Header
Type, and assigns the following values:
0..4: SRH-6LoRH [RFCthis]
5: RPI-6LoRH [RFCthis]
6: IP-in-IP-6LoRH [RFCthis]
10. Acknowledgments
The authors wish to thank Tom Phinney, Thomas Watteyne, Tengfei
Chang, Martin Turon, James Woodyatt, Samita Chakrabarti, Jonathan
Hui, Gabriel Montenegro and Ralph Droms for constructive reviews to
the design in the 6lo Working Group. The overall discussion involved
participants to the 6MAN, 6TiSCH and ROLL WGs, thank you all.
Special thanks to the chairs of the ROLL WG, Michael Richardson and
Ines Robles, and Brian Haberman, Internet Area A-D, and Adrian
Farrel, Routing Area A-D, for driving this complex effort across
Working Groups and Areas.
11. References
11.1. Normative References
[I-D.ietf-6lo-paging-dispatch]
Thubert, P., "6LoWPAN Paging Dispatch", draft-ietf-6lo-
paging-dispatch-01 (work in progress), January 2016.
[IEEE802154]
IEEE standard for Information Technology, "IEEE std.
802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
and Physical Layer (PHY) Specifications for Low-Rate
Wireless Personal Area Networks", 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
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[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<http://www.rfc-editor.org/info/rfc4944>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<http://www.rfc-editor.org/info/rfc6282>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/
RFC6550, March 2012,
<http://www.rfc-editor.org/info/rfc6550>.
[RFC6552] Thubert, P., Ed., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)", RFC
6552, DOI 10.17487/RFC6552, March 2012,
<http://www.rfc-editor.org/info/rfc6552>.
[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
Power and Lossy Networks (RPL) Option for Carrying RPL
Information in Data-Plane Datagrams", RFC 6553, DOI
10.17487/RFC6553, March 2012,
<http://www.rfc-editor.org/info/rfc6553>.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554, DOI
10.17487/RFC6554, March 2012,
<http://www.rfc-editor.org/info/rfc6554>.
[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and
Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
2014, <http://www.rfc-editor.org/info/rfc7102>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, DOI 10.17487/
RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
11.2. Informative References
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[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-09 (work
in progress), November 2015.
[I-D.robles-roll-useofrplinfo]
Robles, I., Richardson, M., and P. Thubert, "When to use
RFC 6553, 6554 and IPv6-in-IPv6", draft-robles-roll-
useofrplinfo-02 (work in progress), October 2015.
[I-D.thubert-6lo-forwarding-fragments]
Thubert, P. and J. Hui, "LLN Fragment Forwarding and
Recovery", draft-thubert-6lo-forwarding-fragments-02 (work
in progress), November 2014.
[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC
6775, DOI 10.17487/RFC6775, November 2012,
<http://www.rfc-editor.org/info/rfc6775>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<http://www.rfc-editor.org/info/rfc7554>.
Appendix A. Examples
A.1. Examples Compressing The RPI
The example in Figure 15 illustrates the 6LoRH compression of a
classical packet in Storing Mode in all directions, as well as in
non-Storing mode for a packet going up the DODAG following the
default route to the root. In this particular example, a
fragmentation process takes place per [RFC4944], and the fragment
headers must be placed in Page 0 before switching to Page 1:
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+- ... -+- ... -+-+ ... -+- ... +-+-+ ... -+-+-+-+-+-+-+-+-+-+...
|Frag type|Frag hdr |11110001| RPI- |IP-in-IP| LOWPAN-IPHC | ...
|RFC 4944 |RFC 4944 | Page 1 | 6LoRH | 6LoRH | |
+- ... -+- ... -+-+ ... -+- ... +-+-+ ... -+-+-+-+-+-+-+-+-+-+...
<- RFC 6282 ->
No RPL artifact
+- ... -+- ... -+-+ ... -+-+ ... -+- ... +-+-+-+-+-+-+-+-+-+-+...
|Frag type|Frag hdr |
|RFC 4944 |RFC 4944 | Payload (cont)
+- ... -+- ... -+-+ ... -+-+ ... -+- ... +-+-+-+-+-+-+-+-+-+-+...
+- ... -+- ... -+-+ ... -+-+ ... -+- ... +-+-+-+-+-+-+-+-+-+-+...
|Frag type|Frag hdr |
|RFC 4944 |RFC 4944 | Payload (cont)
+- ... -+- ... -+-+ ... -+-+ ... -+- ... +-+-+-+-+-+-+-+-+-+-+...
Figure 15: Example Compressed Packet with RPI.
In Storing Mode, if the packet stays within the RPL domain, then it
is possible to save the IP-in-IP encapsulation, in which case only
the RPI is compressed with a 6LoRH, as illustrated in Figure 16 in
the case of a non-fragmented ICMP packet:
+- ... -+-+- ... -+-+-+-+ ... -+-+-+-+ ... -+-+-+-+-+-+-+-+-+-+-+...
|11110001| RPI-6LoRH | NH = 0 | NH = 58 | ICMP message ...
|Page 1 | type 5 | 6LOWPAN-IPHC | (ICMP) | (no compression)
+- ... -+-+- ... -+-+-+-+ ... -+-+-+-+ ... -+-+-+-+-+-+-+-+-+-+-+...
<- RFC 6282 ->
No RPL artifact
Figure 16: Example ICMP Packet with RPI in Storing Mode.
The format in Figure 16 is logically equivalent to the non-compressed
format illustrated in Figure 17:
+-+-+-+- ... -+-+-+-+ ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
| IPv6 Header | Hop-by-Hop | RPI in | ICMP message ...
| NH = 58 | Header | RPL Option |
+-+-+-+- ... -+-+-+-+ ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Figure 17: Uncompressed ICMP Packet with RPI.
For a UDP packet, the transport header can be compressed with 6LoWPAN
HC [RFC6282] as illustrated in Figure 18:
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+- ... -+- ... -+-+-+-+- ... +-+-+-+-+-+-+-+-+-+- ... +-+-+-+-+-+...
|11110001| RPI-6LoRH | NH = 1 |11110|C| P | Compressed |UDP ...
|Page 1 | type 5 | 6LOWPAN-IPHC | UDP | | | UDP header |Payload
+- ... -+- ... -+-+-+-+- ... +-+-+-+-+-+-+-+-+-+- ... +-+-+-+-+-+...
<- RFC 6282 ->
No RPL artifact
Figure 18: Uncompressed ICMP Packet with RPI.
If the packet is received from the Internet in Storing Mode, then the
root is supposed to encapsulate the packet to insert the RPI. The
resulting format would be as represented in Figure 19:
+-+-+-+-+-+-+- ... -+-+-- ... -+-+- ... -+-+-+-+-+-+-+ ... -+-+-+-+...
|11110001 | RPI-6LoRH | IP-in-IP | NH=1 |11110CPP| Compressed | UDP
|Page 1 | | 6LoRH | IPHC | UDP | UDP header | Payload
+-+-+-+-+-+-+- ... -+-+-- ... -+-+- ... -+-+-+-+-+-+-+ ... -+-+-+-+...
<- RFC 6282 ->
No RPL artifact
Figure 19: RPI inserted by the root in Storing Mode.
A.2. Example Of Downward Packet In Non-Storing Mode
The example illustrated in Figure 20 is a classical packet in non-
Storing mode for a packet going down the DODAG following a source
routed path from the root. Say that we have 4 forwarding hops to
reach a destination. In the non-compressed form, when the root
generates the packet, the last 3 hops are encoded in a Routing Header
type 3 (SRH) and the first hop is the destination of the packet. The
intermediate hops perform a swap and the hop count indicates the
current active hop [RFC2460], [RFC6554].
When compressed with this specification, the 4 hops are encoded in
SRH-6LoRH when the root generates the packet, and the final
destination is left in the LOWPAN-IPHC. There is no swap, and the
forwarding node that corresponds to the first entry effectively
consumes it when forwarding, which means that the size of the encoded
packet decreases and that the hop information is lost.
If the last hop in a SRH-6LoRH is not the final destination then it
removes the SRH-6LoRH before forwarding.
In the particular example illustrated in Figure 20, all addresses in
the DODAG are assigned from a same /112 prefix and the last 2 octets
encoding an identifier such as a IEEE 802.15.4 short address. In
that case, all addresses can be compressed to 2 octets, using the
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root address as reference. There will be one SRH_6LoRH header, with,
in this example, 3 compressed addresses:
+-+-+-+-+-+-+- ... +-+-+- ... -+-+-- ... -+-+- ... -+-+-+-+-+ ... +-...
|11110001 |SRH-6LoRH | RPI-6LoRH | IP-in-IP | NH=1 |11110CPP| UDP | UDP
|Page 1 |Type1 S=2 | | 6LoRH | IPHC | UDP | hdr |load
+-+-+-+-+-+-+- ... +-+-+- ... -+-+-- ... -+-+- ... -+-+-+-+-+ ... +-...
<-8bytes-> <- RFC 6282 ->
No RPL artifact
Figure 20: Example Compressed Packet with SRH.
One may note that the RPI is provided. This is because the address
of the root that is the source of the IP-in-IP header is elided and
inferred from the RPLInstanceID in the RPI. Once found from a local
context, that address is used as Compression Reference to expand
addresses in the SRH-6LoRH.
With the RPL specifications available at the time of writing this
draft, the root is the only node that may incorporate a SRH in an IP
packet. When the root forwards a packet that it did not generate, it
has to encapsulate the packet with IP-in-IP.
But if the root generates the packet towards a node in its DODAG,
then it should avoid the extra IP-in-IP as illustrated in Figure 21:
+- ... -+-+-+ ... +-+-+-+ ... -+-+-+-+-+-+-+-++-+- ... -+-+-+-+-+...
|11110001| SRH-6LoRH | NH=1 | 11110CPP | Compressed | UDP
|Page 1 | Type1 S=3 | LOWPAN-IPHC| LOWPAN-NHC| UDP header | Payload
+- ... -+-+-+ ... +-+-+-+ ... -+-+-+-+-+-+-+-++-+- ... -+-+-+-+-+...
<- RFC 6282 ->
Figure 21: compressed SRH 4*2bytes entries sourced by root.
Note: the RPI is not represented though RPL [RFC6550] generally
expects it. In this particular case, since the Compression Reference
for the SRH-6LoRH is the source address in the LOWPAN-IPHC, and the
routing is strict along the source route path, the RPI does not
appear to be absolutely necessary.
In Figure 21, all the nodes along the source route path share a same
/112 prefix. This is typical of IPv6 addresses derived from an
IEEE802.15.4 short address, as long as all the nodes share a same
PAN-ID. In that case, a type-1 SRH-6LoRH header can be used for
encoding. The IPv6 address of the root is taken as reference, and
only the last 2 octets of the address of the intermediate hops is
encoded. The Size of 3 indicates 4 hops, resulting in a SRH-6LoRH of
10 bytes.
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A.3. Example of SRH-6LoRH life-cycle
This section illustrates the operation specified in Section 5.6 of
forwarding a packet with a compressed SRH along an A->B->C->D source
route path. The operation of popping addresses is exemplified at
each hop.
Packet as received by node A
----------------------------
Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA AAAA
Type 1 SRH-6LoRH Size = 0 BBBB
Type 2 SRH-6LoRH Size = 1 CCCC CCCC
DDDD DDDD
Step 1 popping BBBB the first entry of the next SRH-6LoRH
Step 2 next is if larger value (2 vs. 1) the SRH-6LoRH is removed
Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA AAAA
Type 2 SRH-6LoRH Size = 1 CCCC CCCC
DDDD DDDD
Step 3: recursion ended, coalescing BBBB with the first entry
Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA BBBB
Step 4: routing based on next segment endpoint to B
Figure 22: Processing at Node A.
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Packet as received by node B
----------------------------
Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA BBBB
Type 2 SRH-6LoRH Size = 1 CCCC CCCC
DDDD DDDD
Step 1 popping CCCC CCCC, the first entry of the next SRH-6LoRH
Step 2 removing the first entry and decrementing the Size (by 1)
Type 3 SRH-6LoRH Size = 0 AAAA AAAA AAAA BBBB
Type 2 SRH-6LoRH Size = 0 DDDD DDDD
Step 3: recursion ended, coalescing CCCC CCCC with the first entry
Type 3 SRH-6LoRH Size = 0 AAAA AAAA CCCC CCCC
Step 4: routing based on next segment endpoint to C
Figure 23: Processing at Node B.
Packet as received by node C
----------------------------
Type 3 SRH-6LoRH Size = 0 AAAA AAAA CCCC CCCC
Type 2 SRH-6LoRH Size = 0 DDDD DDDD
Step 1 popping DDDD DDDD, the first entry of the next SRH-6LoRH
Step 2 the SRH-6LoRH is removed
Type 3 SRH-6LoRH Size = 0 AAAA AAAA CCCC CCCC
Step 3: recursion ended, coalescing DDDD DDDDD with the first entry
Type 3 SRH-6LoRH Size = 0 AAAA AAAA DDDD DDDD
Step 4: routing based on next segment endpoint to D
Figure 24: Processing at Node C.
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Packet as received by node D
----------------------------
Type 3 SRH-6LoRH Size = 0 AAAA AAAA DDDD DDDD
Step 1 the SRH-6LoRH is removed.
Step 2 no more header, routing based on inner IP header.
Figure 25: Processing at Node D.
Authors' Addresses
Pascal Thubert (editor)
Cisco Systems
Building D - Regus
45 Allee des Ormes
BP1200
MOUGINS - Sophia Antipolis 06254
FRANCE
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Laurent Toutain
Institut MINES TELECOM; TELECOM Bretagne
2 rue de la Chataigneraie
CS 17607
Cesson-Sevigne Cedex 35576
France
Email: Laurent.Toutain@telecom-bretagne.eu
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Robert Cragie
ARM Ltd.
110 Fulbourn Road
Cambridge CB1 9NJ
UK
Email: robert.cragie@gridmerge.com
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