Internet DRAFT - draft-bergmann-bier-ccast
draft-bergmann-bier-ccast
Network Working Group O. Bergmann
Internet-Draft C. Bormann
Intended status: Standards Track S. Gerdes
Expires: April 8, 2017 Universitaet Bremen TZI
H. Chen
Huawei Technologies
October 05, 2016
Constrained-Cast: Source-Routed Multicast for RPL
draft-bergmann-bier-ccast-02
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 . . . . . . . . . . . . . . . . . . . . . . . 3
2. The BIER Approach . . . . . . . . . . . . . . . . . . . . . . 3
3. The Constrained-Cast Approach . . . . . . . . . . . . . . . . 3
4. False Positives . . . . . . . . . . . . . . . . . . . . . . . 4
5. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Multicast Listener Advertisement Object (MLAO) . . . . . 4
5.2. Routing Header . . . . . . . . . . . . . . . . . . . . . 5
6. Implementation . . . . . . . . . . . . . . . . . . . . . . . 6
7. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8.1. ICMPv6 Parameter Registration . . . . . . . . . . . . . . 7
8.2. IPv6 Routing Type Registration . . . . . . . . . . . . . 7
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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
identifiers to instruct forwarders to relay the respective IP
datagram.
As every forwarder in an IP-based constrained node network has at
least one network interface, it is straight-forward to use the
address of an outgoing interface as identifiers in this source-route.
(Typically, this is a globally unique public address of the node's
only network adapter.)
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The source-route 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].
In this specification, the term "byte" is used in its now customary
sense as a synonym for "octet".
All multi-byte integers in this protocol are interpreted in network
byte order.
2. The BIER Approach
Bit-Indexed Explicit Replication [I-D.ietf-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.
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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 as specified in Section 5.1.
For downward messages, the root adds a new routing header that
includes a hash function identifier and a seed value; another one of
its fields gives the number of hash functions (k) to ask for k
instances of application of the hash function, with increasing seed.
The format of the new routing header is specified in Section 5.2.
Typical sizes of the bloom filter bitmap that the root inserts into
the packet can be 64, 128, or 256 bit, which may lead to acceptable
false positive rates if the total number 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.
5.1. Multicast Listener Advertisement Object (MLAO)
The header format of the MLAO is depicted in Figure 1. The basic
structure of the MLAO message is similar to the RPL Destination
Advertisement Object (DAO). In particular, it starts with RPL ICMP
base header with a type value of 155 and the code {IANA TBD1} (MLAO),
followed by the Checksum, RPLInstanceID, parameters and flags as in a
DAO.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x05 | Option Length | Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ +
| Group Address |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RPL Target Option for MLAO
The group address field indicates the group that the sender of the
MLAO is interested in. This field usually contains a 128 bit IPv6
multicast group address. Shorter group identifiers could be used
together with a protocol for explicit creation of groups. The MLAO
message must have at least one RPL target option to specify the
address of the listener that has generated the MLAO. The message is
directed to the global unicast address of the DODAG root and travels
upwards the routing tree.
Note: It has been suggested to use the RPL Transit Option (Type
0x06) instead as it is used in Non-Storing mode to inform the
DODAG root of path attributes. Specifically, this option can be
used to limit the subscription by providing a proper Path
Lifetime.
5.2. Routing Header
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number | Func set | Modulus |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Filter data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Routing header
Routing Type: {IANA TBD2} 253
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Segments Left: This value is always 0, so network nodes that do not
support this routing header do not generate ICMP6 error messages.
Sequence Number: 16 bits sequence number. The number space is
unique for a sequence of multicast datagrams for a specific group
that arrive at the DAG root on their way up. The DAG root
increments the number for each datagram it sends down the
respective DODAG.
Func set: The set of hash functions used to generate the Filter data
value.
Note: As the function set contains a combination of several distinct
hash functions, it is currently unclear if 8 bits number space is
large enough.
Modulus: The modulus that is used by the hash functions, minus 64
(the minimum filter data size that can be used). The DAG root
chooses the modulus (and thus the filter data size) to achieve its
objectives for false positive rates (Section 4).
Filter data: A bit field that indicates which nodes should relay
this multicast datagram. The length of this field is a multiple
of 8 bytes. The actual length is derived from the contents of the
field Header Ext Length.
Note: The modulus could be derived from the length of the filter data
which is known from the extension header size. On the other hand,
keeping a separate record of the modulus means that the DAG root
could leave out 8-byte multiples of trailing zero bits if they happen
to occur. But then, a modulus that leaves 8-byte sequences of zero
bits in the filter is probably too large.
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
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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).
8. IANA Considerations
The following registrations are done following the procedure
specified in [RFC6838].
Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
with the RFC number of this specification and "IANA TBD1" with the
code selected for TBD1 below and "IANA TBD2" with the value selected
for TBD2 below.
8.1. ICMPv6 Parameter Registration
IANA is requested to add the following entry to the Code fields of
the RPL Control Codes registry:
+------+------+------------+
| Code | Name | Reference |
+------+------+------------+
| TBD1 | MLAO | [RFC-XXXX] |
+------+------+------------+
8.2. IPv6 Routing Type Registration
IANA is requested to add the following entries to the IPv6 Routing
Types registry:
+-------+----------------------+------------+
| Value | Name | Reference |
+-------+----------------------+------------+
| TBD2 | CCast Routing Header | [RFC-XXXX] |
+-------+----------------------+------------+
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9. Acknowledgments
Thanks to Yasuyuki Tanaka for valuable comments.
This work has been supported by Siemens Corporate Technology.
10. References
10.1. Normative References
[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>.
[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>.
10.2. 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.ietf-bier-architecture]
Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
S. Aldrin, "Multicast using Bit Index Explicit
Replication", draft-ietf-bier-architecture-04 (work in
progress), July 2016.
[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>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.
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Authors' Addresses
Olaf Bergmann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63904
Email: bergmann@tzi.org
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
Hao Chen
Huawei Technologies
12, E. Mozhou Rd
Nanjing 211111
China
Phone: +86-25-5662-7052
Email: philips.chenhao@huawei.com
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