Network Working Group O. Bergmann
Internet-Draft C. Bormann
Intended status: Standards Track S. Gerdes
Expires: April 8, 2017 Universität Bremen TZI
H. Chen
Huawei Technologies
October 05, 2016

Constrained-Cast: Source-Routed Multicast for RPL


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

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.)

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.

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.

        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.

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
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.

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 filter needs to be recomputed (which can be sped up logarithmically if desired).

7. Benefits


  • operates in Non-Storing Mode, with the simple addition of a membership information service;
  • 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

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]

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.
[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, DOI 10.17487/RFC6550, March 2012.

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.
[I-D.ietf-bier-architecture] Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T. and S. Aldrin, "Multicast using Bit Index Explicit Replication", Internet-Draft draft-ietf-bier-architecture-04, 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.
[RFC6838] Freed, N., Klensin, J. and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013.

Authors' Addresses

Olaf Bergmann Universität Bremen TZI Postfach 330440 Bremen, D-28359 Germany Phone: +49-421-218-63904 EMail:
Carsten Bormann Universität Bremen TZI Postfach 330440 Bremen, D-28359 Germany Phone: +49-421-218-63921 EMail:
Stefanie Gerdes Universität Bremen TZI Postfach 330440 Bremen, D-28359 Germany Phone: +49-421-218-63906 EMail:
Hao Chen Huawei Technologies 12, E. Mozhou Rd Nanjing, 211111 China Phone: +86-25-5662-7052 EMail: