WG Review: Bit Indexed Explicit Replication (bier)

The IESG <iesg-secretary@ietf.org> Fri, 20 February 2015 18:47 UTC

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From: The IESG <iesg-secretary@ietf.org>
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Subject: WG Review: Bit Indexed Explicit Replication (bier)
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A new IETF working group has been proposed in the Routing Area. The IESG
has not made any determination yet. The following draft charter was
submitted, and is provided for informational purposes only. Please send
your comments to the IESG mailing list (iesg at ietf.org) by 2015-03-02.

Bit Indexed Explicit Replication (bier)
Current Status: Proposed WG

  Greg Shepherd <gjshep@gmail.com>
  Tony Przygienda <tonysietf@gmail.com>

Assigned Area Director:
  Alia Atlas <akatlas@gmail.com>

Mailing list
  Address: bier@ietf.org
  To Subscribe: https://www.ietf.org/mailman/listinfo/bier


In conventional IP multicast forwarding, the packets of a given
multicast "flow" are forwarded along a tree that has been constructed
for the specific purpose of carrying that flow.  This requires transit
nodes to maintain state on a per-flow basis, and requires the transit
nodes to participate in multicast-specific tree building protocols.
The flow to which a packet belongs is determined by its IP source and
destination address fields.

BIER (Bit Index Explicit Replication) is an alternative method of
multicast forwarding.  It does not require any multicast-specific
trees, and hence does not require any multicast-specific tree building
protocols.  Within a given "BIER domain", an ingress node encapsulates
a multicast data packet in a "BIER header".  The BIER header
identifies the packet's egress nodes in that domain.  Each possible
egress node is represented by a a single bit within a bitstring; to
send a packet to a particular set of egress nodes, the ingress node
sets the bits for each of those egress nodes, and clears the other
bits in the bistring.  Each packet can then be forwarded along the
unicast shortest path tree from the ingress node to the egress nodes.
Thus there are no per-flow forwarding entries.

Due to the particular sensitivity of adding new significant
functionality into the data-plane at high link speeds, the BIER work
will progress as Experimental.  As described in item (9) below, the
work may become Standards Track once there is sufficient experience
with the benefits and downsides of the technology.

BIER is initially chartered to do experimental work on this new
multicast forwarding mechanism as follows:

   1) BIER architecture: The WG will publish an architecture, based
   upon draft-wijnands-bier-architecture-04.  It will include the
   normative algorithm for how BIER packet forwarding is done.  It
   will specify the information that is required by a BIER header to
   support BIER forwarding.

   2) BIER encapsulation: The working group should assume that the
   technology will need to be embedded in the data plane and operate
   at the highest packet line speeds.  The WG will publish a document
   defining an MPLS-based encapsulation based upon
   draft-wijnands-mpls-bier-encapsulation-02. Due to the critical need
   to have a high-quality and stable RFC for a new data-plane
   encapsulation, the MPLS-based encapsulation draft shall wait after
   WGLC and not progress to IETF Last Call until there are two
   independent interoperable implementations.

   As a secondary focus, the WG may also work on one non-MPLS
   data-plane encapsulation.  This draft also shall wait after WGLC
   and not progress to IETF Last Call until there are two independent
   interoperable implementations.  This draft must focus on and
   include the following details:

       a) What is the applicability of the encapsulation and for which
       use-cases is this encapsulation required?

       b) Does this proposed encapsulation imply any changes to the
       MPLS-based encapsulation?

       c) What design choices have been made for the encapsulation
       type and the included fields.

       d) The proposed encapsulation with considerations given to at
       least OAM, Class of Service, security, fragmentation, TTL.

   3) Transition Mechanisms: The WG will describe how BIER can be
   partially deployed and still provide useful functionality.  A
   minimum of the necessary mechanisms to support incremental
   deployment and/or managing different BIER mask-length compatibility
   may be defined.  Each such mechanism must include an applicability
   statement to differentiate its necessity from other proposed

   4) Applicability Statements: The WG will work on a document
   describing how BIER can be applied to multicast L3VPN and to EVPN.
   This draft will describe what mechanism is used to communicate the
   group membership between the ingress router and the egress routers,
   what scalability considerations may arise, and any deployment

   5) Use Case: The WG may produce one use-case document that clearly
   articulates the potential benefits of BIER for different use-cases.
   This would be based upon draft-kumar-bier-use-cases-01.

   6) OAM: The WG will describe how OAM will work in a BIER domain and
   what simplifications BIER offers for managing the multicast
   traffic.  A strong preference will be given to extensions to
   existing protocols.

   7) Management models: The WG may work on YANG models and, if needed,
   MIB modules to support common manageability.

   8) IGP extensions.  When a BIER domain falls within a "link state
   IGP"" network, the information needed to set up the BIER forwarding 
   tables (e.g., the mapping between a given bit position and a given 
   egress router) may be carried in the link state advertisements of the 
   IGP. The link state advertisments may also carry other information 
   related to forwarding (e.g., the IGP may support multiple topologies, 
   in which case it may be necessary to advertise which topologies are 
   to be used for BIER forwarding).  Any necessary extensions to the IGP 
   will be specified by the WG, in cooperation with the ISIS and OSPF 

   9) Deployment Experience: Once there is deployment experience, the
   WG will produce a document describing the benefits, problems, and
   trade-offs for using BIER instead of traditional multicast
   forwarding mechanisms.  Ideally, this should also contain an
   analysis of the impact and benefit of the new BIER data-plane to
   the overall Internet architecture.  This document is intended to be
   used to evaluate whether to recharter BIER to produce Standards
   Track RFCs.

The BIER working group will coordinate with several different working
groups and must include the relevant other working groups during
working group last call on the relevant drafts.  BIER will coordinate
with MPLS on the MPLS-based encapsulation and associated MPLS-based
OAM mechanisms.  BIER will coordinate with ISIS and OSPF on extensions
to flood BIER-related information.  BIER will coordinate with BESS and
IDR on the applicability of existing BGP-based mechanisms for
providing multicast group membership information.