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Documentation: IPv6 multicast

Simon Duquennoy edited this page Nov 9, 2018 · 8 revisions

Contiki-NG's IPv6 stack supports multicast. We currently support three modes:

  • 'Stateless Multicast RPL Forwarding' (SMRF)

    SMRF is a very simple, lightweight multicast forwarding engine for RPL networks. RPL in MOP 3 handles group management as per the RPL docs, whereas the SMRF engine handles datagram forwarding. SMRF can only forward multicast traffic downwards in a DODAG.

    SMRF requires RPL storing mode operation, which is only supported in RPL-classic.

    SMRF is presented and evaluated in:

  • 'Enhanced Stateless Multicast RPL Forwarding' (ESMRF)

    ESMRF is an enhanced version of the SMRF engine with the aim of resolving the sending limitation of SMRF to allow any node within the DODAG to send multicast traffic up, as well as down the RPL DODAG. ESMRF requires RPL storing mode operation, which is only supported in RPL-classic.

    ESMRF is presented and evaluated in: K. Qorany Abdel Fadeel and K. El Sayed, "ESMRF: Enhanced Stateless Multicast RPL Forwarding For IPv6-based Low-Power and Lossy Networks", in Proc. 2015 Workshop on IoT challenges in Mobile and Industrial Systems (IoT-Sys '15), ACM, New York, NY, USA, pp. 19-24, 2015.

  • 'Multicast Forwarding with Trickle'

    This is an implementation of the algorithm described in the Trickle Multicast internet draft. The version of this draft that's currently implementated is documented in roll-tm.h

    Trickle multicast is routing-independent and therefore works with both storing and non-storing modes, both RPL-classic and RPL-lite.

More engines can (and hopefully will) be added in the future. The first addition is most likely going to be an updated implementation of MPL (RFC 7731).

The Big Gotcha

Currently we only support traffic originating and destined inside a single 6LoWPAN. To be able to send multicast traffic from the internet to 6LoWPAN nodes or the other way round, we need border routers or other gateway devices to be able to achieve the following:

  • Add/Remove Trickle Multicast, RPL or other HBHO headers as necessary for datagrams entering / exiting the 6LoWPAN
  • Advertise multicast group membership to the internet (e.g. with MLD)

These are currently not implemented and are in the ToDo list. Contributions welcome.

Where to Start

The best place in examples/multicast.

There is a cooja example demonstrating basic functionality.

How to Use

Look in os/net/ipv6/multicast/uip-mcast6-engines.h for a list of supported multicast engines.

To turn on multicast support, add this line in your project- or contiki-conf.h

#define UIP_MCAST6_CONF_ENGINE xyz

where xyz is a value from uip-mcast6-engines.h.

To disable:

#define UIP_MCAST6_CONF_ENGINE 0

You also need to make sure the multicast code gets built. Your example's (or platform's) Makefile should include this:

MODULES += os/net/ipv6/multicast

How to extend

Let's assume you want to write an engine called foo. The multicast API defines a multicast engine driver in a fashion similar to the various NETSTACK layer drivers. This API defines functions for basic multicast operations (init, in, out). In order to extend multicast with a new engine, perform the following steps:

  • Open uip-mcast6-engines.h and assign a unique integer code to your engine
#define UIP_MCAST6_ENGINE_FOO        xyz
  • Include your engine's foo.h

  • In foo.c, implement:

    • init()
    • in()
    • out()
    • Define your driver like so:
const struct uip_mcast6_driver foo_driver = { ... }
  • If you want to maintain stats:

    • Standard multicast stats are maintained in uip_mcast6_stats. Don't access this struct directly, use the macros provided in uip-mcast6-stats.h instead
    • You can add your own stats extensions. To do so, declare your own stats struct in your engine's module, e.g struct foo_stats
    • When you initialise the stats module with UIP_MCAST6_STATS_INIT, pass a pointer to your stats variable as the macro's argument. An example of how to extend multicast stats, look at the ROLL TM engine
  • Open uip-mcast6.h and add a section in the #if spree. This aims to configure the uIPv6 core. More specifically, you need to:

    • Specify if you want to put RPL in MOP3 by defining RPL_WITH_MULTICAST: 1: MOP 3, 0: non-multicast MOP
    • Define your engine details
#define UIP_MCAST6             foo_driver
#define UIP_MCAST6_STATS       foo_stats
typedef struct foo_stats uip_mcast6_stats_t;
  • Optionally, add a configuration check block to stop builds when the configuration is not sane.

If you need your engine to perform operations not supported by the generic UIP_MCAST6 API, you will have to hook those in the uip core manually. As an example, see how the core is modified so that it can deliver ICMPv6 datagrams to the ROLL TM engine.

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