Internet DRAFT - draft-thubert-roll-flow-label
draft-thubert-roll-flow-label
ROLL P. Thubert, Ed.
Internet-Draft Cisco
Intended status: Standards Track November 6, 2012
Expires: May 10, 2013
Use of the IPv6 Flow Label within an LLN
draft-thubert-roll-flow-label-02
Abstract
This document present how the Flow Label can be used inside a LLN as
a replacement to the RPL option and provides rules for the root to
set and reset the Flow Label when forwarding between the inside of
RPL domain and the larger Internet, in both direction. This new
operation aims at saving an IPv6 in IPv6 encapsulation within the RPL
domain that is required with the RPL option for all packets that
reach outside of the RPL domain.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 10, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Flow Label Format Within the RPL Domain . . . . . . . . . . . . 5
4. Root Operation . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Incoming Packets . . . . . . . . . . . . . . . . . . . . . 6
4.2. Outgoing Packets . . . . . . . . . . . . . . . . . . . . . 7
5. RPL node Operation . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
In some Low Power and Lossy Network (LLN) applications such as
control systems [RFC5673], a packet loss is usually acceptable but
jitter and latency must be strictly controlled as they can play a
critical role in the interpretation of the measured information.
Sensory systems are often distributed, and the control information
can in fact be originated from multiple sources and aggregated. As a
result, it can be a requirement for related measurements from
multiple sources to be treated as a single flow following a same path
over the Internet in order to experience similar jitter and latency.
The traditional tuple of source, destination and ports might then not
be the proper indication to isolate a meaningful flow.
In a typical LLN application, the bulk of the traffic consists of
small chunks of data (in the order few bytes to a few tens of bytes)
at a time. In the industrial case, a typical frequency is 4Hz but it
can be a lot slower than that for, say, environmental monitoring.
The granularity of traffic from a single source is too small to make
a lot of sense in load balancing application.
In such cases, related packets from multiple sources should not be
load-balanced along their path in the Internet; load-balancing can be
discouraged by tagging those packets with a same Flow Label in the
IPv6 [RFC2460] header. This can be achieved if the Flow Label in
packets outgoing a RPL domain are set by the root of the RPL
structure as opposed to the actual source. It derives that the Flow
Label could be reused inside the RPL domain.
In a LLN, each transmitted bit represents energy and every saving
counts dearly. Considering that the value for which the Flow Label
is used in the IPv6 Flow Label Specification [RFC6437] is to serve
load balancing in the core, it is unlikely that LLN devices will
consume energy to generate and then transmit a Flow Label to serve
interests in some other place. On the other hand, it makes sense to
recommend the computation of a stateless Flow Label at the root of
the LLN towards the Internet.
Reciprocally, [RFC6437] requires that once set, a non-zero flow label
value is left unchanged. The value for that setting is consumed once
the packet has traversed the core and reaches the LLN. Then again,
there is little value but a high cost for the LLN in spending 20 bits
to transport a Flow Label from the Internet over the constrained
network to the destination node. It results that the MUST in
[RFC6437] should be alleviated for packets coming from the outside on
the LLN, and that it should be acceptable that the compression over
the LLN erases the original flow label. It should also be acceptable
that the Flow Label field is reused in the LLN as proposed in this
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draft.
The Routing Protocol for Low Power and Lossy Networks (RPL) [RFC6550]
specification defines a generic Distance Vector protocol that is
adapted to a variety of LLNs. RPL forms Destination Oriented
Directed Acyclic Graphs (DODAGs) which root often acts as the Border
Router to connect the RPL domain to the Internet. The root is
responsible to select the RPL Instance that is used to forward a
packet coming from the Internet into the RPL domain.
A classical RPL implementation will use the RPL Option for Carrying
RPL Information in Data-Plane Datagrams [RFC6553] to tag a packet
with the Instance ID and other information that RPL requires for its
operation within the RPL domain. Sadly, the Option must be placed in
a Hop-by-Hop header that must be added to or removed from packets
that cross the border of the RPL domain. For reasons such as the
capability to send ICMP errors, back, this operation involves an
extra 6in6 encapsulation within the RPL domain that is detrimental to
the LLN operation, in particular with regards to bandwidth and
battery constraints. The extra encapsulation may cause a containing
frame to grow above maximum frame size, leading to Layer 2 or 6LoWPAN
[RFC4944] fragmentation, which in turn cause even more energy
spending and issues discussed in the LLN Fragment Forwarding and
Recovery [I-D.thubert-roll-forwarding-frags].
------+---------
| Internet |
| | Native IPv6
+-----+ |
| | Border Router (RPL Root) |
| | || | ||
+-----+ || | || IPv6 +
| || | || HbH
o o o o || | || headers
o o o o o o o o o || | ||
o o o o o o o o o o || | ||
o o o o o o o o o || | ||
o o o o o o o o
o o o o o o
o o o o
LLN
Figure 1: 6in6 Encapsulation within the LLN
Additionally, Compression Format for IPv6 Datagrams over IEEE
802.15.4-Based Networks [RFC6282] and its variants for other types of
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LLNs do not provide an efficient compression for the RPL option so
the cost in current implementations can not be alleviated in any
fashion. So even for packets that are confined within the RPL domain
and do not need the 6in6 encapsulation, the use of the flow label
instead of the RPL option is a valuable saving.
All the packets that are leaving a DODAG of a RPL domain towards the
Internet will transit via a same root. The root is an ideal place to
set the IPv6 Flow Label to a same value across multiple sources of a
same flow when that operation is needed, ensuring complience with the
rules defined by the IPv6 Flow Label Specification [RFC6437] within
the Internet. At the same time, the root segragates the Internet and
the RPL domain, allowing to reuse the Flow Label within the RPL
domain.
It can be noted that [RFC6282] provides an efficient header
compression for packets that do have the Flow Label set in the IPv6
header. It results that the same information as transported in the
RPL option itself represents actually less bits in the air when the
Flow Label is used instead. This document specifies how the Flow
Label can be reused within the RPL domain as a replacement to the RPL
option. The use of the Flow Label within a RPL domain is an instance
of the stateful scenarios as discussed in [RFC6437] where the states
include the rank of a node and the RPLInstanceID that identifies the
routing topology.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD 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' [I-D.ietf-roll-terminology] and [RFC6550].
3. Flow Label Format Within the RPL Domain
[RFC6550] section 11.2 specifies the fields that are to be placed
into the packets for the purpose of Instance Identification, as well
as Loop Avoidance and Detection. Those fields include an 'O', and
'R' and an 'F' bits, the 8-bit RPLInstanceID, and the 16-bit
SenderRank. SenderRank is the result of the DAGRank operation on the
rank of the sender, where the DAGRank operation is defined in section
3.5.1 as:
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DAGRank(rank) = floor(rank/MinHopRankIncrease)
If MinHopRankIncrease is set to a multiple of 256, it appears that
the most significant 8 bits of the SenderRank will be all zeroes and
could be ommitted. In that case, the Flow Label MAY be used as a
replacement to the [RFC6553] RPL option. To achive this, the
SenderRank is expressed with 8 least significant bits, and the
information carried within the Flow Label in a packet is constructed
follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |O|R|F| SenderRank | RPLInstanceID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: The RPL Flow Label
The first (leftmost) bit of the Flow Label is reserved and should be
set to zero.
4. Root Operation
[RFC6437] section 3 intentionally does not consider flow label values
in which any of the bits have semantic significance. However, the
present specification assigns semantics to various bits in the flow
label, destroying within the edge network that is the RPL domaina
property of belonging to a statistically uniform distribution that is
desirable in the rest of the Internet. This property MUST be
restored by the root for outgoing packets.
It can be noted that the rationale for the statistically uniform
distribution does not necessarily bring a lot of value within the RPL
domain. In a specific use case where it would, that value must be
compared with that of the battery savings in order to decide which
technique the deployment will use to transport the RPL information.
4.1. Incoming Packets
When routing a packet towards the RPL domain, the root applies a
policy to determine whether the Flow Label is to be used to carry the
RPL information. If so, the root MUST reset the Flow Label and then
it MUST set all the fields in the Flow Label as prescribed by
[RFC6553] using the format specified in Figure 1. In particular, the
root selects the Instance that will be used to forward the packet
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within the RPL domain.
4.2. Outgoing Packets
When routing a packet outside the RPL domain, the root applies a
policy to determine whether the Flow Label was used to carry the RPL
information. If so, the root MUST reset the Flow Label. The root
SHOULD recompute a Flow Label following the rules prescribed by
[RFC6553]. In particular, the root MAY ignore the source address but
it SHOULD use the RPLInstanceID for the computation.
5. RPL node Operation
Depending on the policy in place, the source of a packet will decide
whether to use this specification to transport the RPL information in
the IPv6 packets. If it does, the source in the LLN SHOULD set the
Flow Label to zero and MUST NOT expect that the flow label will be
conserved end-to-end".
6. Security Considerations
The process of using the Flow Label as opposed to the RPL option does
not appear to create any opening for new threat compared to
[RFC6553].
7. IANA Considerations
No IANA action is required for this specification.
8. Acknowledgments
The author wishes to thank Brian Carpenter for his in-depth review
and constructive approach to the problem and its resolution.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
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[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011.
[RFC6550] Winter, T., Thubert, P., 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, March 2012.
[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,
March 2012.
9.2. Informative References
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-06 (work in
progress), September 2011.
[I-D.thubert-roll-forwarding-frags]
Thubert, P. and J. Hui, "LLN Fragment Forwarding and
Recovery", draft-thubert-roll-forwarding-frags-00 (work in
progress), March 2012.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
"Industrial Routing Requirements in Low-Power and Lossy
Networks", RFC 5673, October 2009.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437, November 2011.
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Author's Address
Pascal Thubert (editor)
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
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