Network Working Group O. Bergmann
Internet-Draft C. Bormann
Intended status: Standards Track S. Gerdes
Expires: May 13, 2015 Universitaet Bremen TZI
November 09, 2014
Constrained-Cast: Source-Routed Multicast for RPL
draft-bergmann-bier-ccast-00
Abstract
This specification defines a protocol for forwarding multicast
traffic in a constrained node network employing the RPL routing
protocol in non-storing mode.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. The BIER Approach . . . . . . . . . . . . . . . . . . . . . . 3
3. The Constrained-Cast Approach . . . . . . . . . . . . . . . . 3
4. False Positives . . . . . . . . . . . . . . . . . . . . . . . 3
5. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 3
6. Implementation . . . . . . . . . . . . . . . . . . . . . . . 4
7. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 4
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
As defined in [RFC6550], RPL Multicast assumes that the RPL network
operates in Storing Mode. Multicast DAOs are used to indicate
subscription to multicast address to a parent; these DAOs percolate
up and create bread-crumbs. This specification, although part of RFC
6550, appears to be incomplete and untested. More importantly,
Storing Mode is not in use in constrained node networks outside
research operating environments.
The present specification addresses multicast forwarding for RPL
networks in the much more common Non-Storing Mode. Non-Storing is
based on the root node adding source-routing information to downward
packets. Evidently, to make this work, RPL multicast needs to
source-route multicast packets. A source route here is a list of
outgoing interfaces, which subsets the whole set of potential
forwarders available in the RPL DODAG to those that need to forward
in order to reach known multicast listeners.
Including an actual list of outgoing interfaces is rarely applicable,
as this is likely to be a large list of 16-byte IPv6 addresses. Even
with [RFC6554] style compression, the size of the list becomes
prohibitively quickly.
1.1. 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 RFC 2119 [RFC2119].
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2. The BIER Approach
Bit-Indexed Explicit Replication [I-D.wijnands-bier-architecture]
lists all egress routers in a bitmap included in each multicast
packet. This requires creating a mostly contiguous numbering of all
egress routers; more importantly, BIER requires the presence of a
network map in each forwarders to be able to interpret the bitmap and
map it to a set of local outgoing interfaces.
3. The Constrained-Cast Approach
Constrained-Cast employs Bloom Filters [BLOOM] as a compact
representation of a match or non-match for elements in a large set:
Each element to be included is hashed with multiple hash functions;
the result is used to index a bitmap and set the corresponding bit.
To check for the presence of an element, the same hash functions are
applied to obtain bit positions; if all corresponding bits are set,
this is used to indicate a match. (Multiple hash functions are most
easily obtained by adding a varying seed value to a single hash
algorithm.)
By including a bloom filter in each packet that matches all outgoing
interfaces that need to forward the packet, each forwarder can
efficiently decide whether (and on which interfaces) to forward the
packet.
4. False Positives
Bloom filters are probabilistic. A false positive might be
indicating a match where the bits are set by aliasing of the hash
values. In case of Constrained-Cast, this causes spurious
transmission and wastes some energy and radio bandwidth. However,
there is no semantic damage (hosts still filter out unneeded
multicasts). The total waste in energy and spectrum can be
visualized as the false-positive-rate multiplied by the density of
the RPL network. A network can easily live with a significant
percentage of false positives. By changing the set of hash functions
(i.e., seed) over time, the root can avoid a single node with a false
positive to become an unnecessary hotspot for that multicast group.
5. Protocol
The protocol uses DAO-like "MLAO" messages to announce membership to
the root. (To do: write up the format, which should be pretty much
obvious anyway.)
For downward messages, the root adds a new routing header that
includes a hash function identifier and a seed value; another one of
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its fields gives the number of hash functions (k) to ask for k
instances of application of the hash function, with increasing seed.
Typical sizes of the bloom filter bitmap that the root inserts into
the packet can be 64, 128, or 256 bit, with acceptable false positive
rates for total numbers of forwarders in the 10s and 100s. (To do:
write more about the math here. Note that this number tallies
forwarding routers, not end hosts.)
A potential forwarder that receives a multicast packet adorned with a
constrained-cast routing header first checks that the packet is
marked with a RPL rank smaller than its own (loop prevention). If
yes, it then forwards the packet to all outgoing interfaces that
match the bloom filter in the packet.
6. Implementation
In 2013, Constrained-Cast was implemented in Contiki. It turns out
that forwarders can compute the hash functions once for their
outgoing interfaces and then cache them, simply bit-matching their
outgoing interface hash bits against the bloom filter in the packet
(a match is indicated when all bits in the outgoing interface hash
are set in the bloom filter).
The Root computes the tree for each multicast group, computes the
bloom filter for it, caches these values, and then simply adds the
bloom filter routing header to each downward packet. For adding a
new member, the relevant outgoing interfaces are simply added to the
bloom filter. For removing a leaving member, however, the bloom
filter needs to be recomputed (which can be sped up logarithmically
if desired).
7. Benefits
Constrained-Cast:
o operates in Non-Storing Mode, with the simple addition of a
membership information service;
o performs all routing decisions at the root.
Further optimizations might include using a similar kind of bloom
filter routing header for unicast forwarding as well (representing,
instead of the outgoing interface list, a list of children that
forwarding parents need to forward to).
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8. Acknowledgments
This work has been supported by Siemens Corporate Technology.
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.
[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.
9.2. Informative References
[BLOOM] Bloom, B., "Space/time trade-offs in hash coding with
allowable errors", ISSN 0001-0782, ACM Press
Communications of the ACM vol 13 no 7 pp 422-426, 1970,
<http://doi.acm.org/10.1145/362686.362692>.
[I-D.wijnands-bier-architecture]
Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
S. Aldrin, "Multicast using Bit Index Explicit
Replication", draft-wijnands-bier-architecture-01 (work in
progress), October 2014.
[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, March
2012.
Authors' Addresses
Olaf Bergmann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63904
Email: bergmann@tzi.org
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Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Stefanie Gerdes
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63906
Email: gerdes@tzi.org
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