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draft-ietf-bier-te-arch-08.txt
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draft-ietf-bier-te-arch-08.txt
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Network Working Group T. Eckert, Ed.
Internet-Draft Futurewei
Intended status: Standards Track G. Cauchie
Expires: January 2, 2021 Bouygues Telecom
M. Menth
University of Tuebingen
July 1, 2020
Tree Engineering for Bit Index Explicit Replication (BIER-TE)
draft-ietf-bier-te-arch-08
Abstract
This memo introduces per-packet stateless strict and loose path
steered replication and forwarding for Bit Index Explicit Replication
packets (RFC8279). This is called BIER Tree Engineering (BIER-TE).
BIER-TE can be used as a path steering mechanism in future Traffic
Engineering solutions for BIER (BIER-TE).
BIER-TE leverages RFC8279 and extends it with a new semantic for bits
in the bitstring. BIER-TE can leverage BIER forwarding engines with
little or no changes.
In BIER, the BitPositions (BP) of the packets bitstring indicate BIER
Forwarding Egress Routers (BFER), and hop-by-hop forwarding uses a
Routing Underlay such as an IGP.
In BIER-TE, BitPositions indicate adjacencies. The BIFT of each BFR
are only populated with BPs that are adjacent to the BFR in the BIER-
TE topology. The BIER-TE topology can consist of layer 2 or remote
(routed) adjacencies. The BFR then replicates and forwards BIER
packets to those adjacencies. This results in the aforementioned
strict and loose path steering and replications.
BIER-TE can co-exist with BIER forwarding in the same domain, for
example by using separate BIER sub-domains. In the absence of routed
adjacencies, BIER-TE does not require a BIER routing underlay, and
can then be operated without requiring an Interior Gateway Routing
protocol (IGP).
BIER-TE operates without explicit in-network tree-state and carries
the multicast distribution tree in the packet header. It can
therefore be a good fit to support multicast path steering in Segment
Routing (SR) networks.
Eckert, et al. Expires January 2, 2021 [Page 1]
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Name explanation
[RFC-editor: This section to be removed before publication.]
Explanation for name change from BIER-TE to mean "Traffic
Engineering" to BIER-TE "Tree Engineering" in WG last-call (to
benefit IETF/IESG reviewers):
This document started by calling itself BIER-TE, "Traffic
Engineering" as it is a mode of BIER specifically beneficial for
Traffic Engineering. It supports per-packet bitstring based policy
steering and replication. BIER-TE technology itself does not provide
a complete traffic engineering solution for BIER but would require
combination with other technologies for a full BIER based TE
solution, such as a PCE and queuing mechanisms to provide bandwidth
and latency reservations. It is also not the only option to build a
traffic engineering solution utilizing BIER, for example BIER trees
could be steered through IGP metric engineering, such as through
Flex-Topologies. The architecure for Traffic Engineering with either
modes of BIER (BIER-TE/BIER) is intended to be defined in a separate
document, most likely in TEAs WG.
Because the name of such an overall solution is intended to be BIER-
TE, the expansion of BIER-TE was therefore changed to name this BIER
mode "Tree Engineering", so the overall solution can be distinguished
better from its tree building/engineering method without having to
change the long time well-established abbreviation BIER-TE.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 2, 2021.
Eckert, et al. Expires January 2, 2021 [Page 2]
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Basic Examples . . . . . . . . . . . . . . . . . . . . . 5
1.2. BIER-TE Topology and adjacencies . . . . . . . . . . . . 8
1.3. Comparison with BIER . . . . . . . . . . . . . . . . . . 9
1.4. Requirements Language . . . . . . . . . . . . . . . . . . 9
2. Components . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1. The Multicast Flow Overlay . . . . . . . . . . . . . . . 10
2.2. The BIER-TE Controller . . . . . . . . . . . . . . . . . 10
2.2.1. Assignment of BitPositions to adjacencies of the
network topology . . . . . . . . . . . . . . . . . . 11
2.2.2. Changes in the network topology . . . . . . . . . . . 11
2.2.3. Set up per-multicast flow BIER-TE state . . . . . . . 11
2.2.4. Link/Node Failures and Recovery . . . . . . . . . . . 12
2.3. The BIER-TE Forwarding Layer . . . . . . . . . . . . . . 12
2.4. The Routing Underlay . . . . . . . . . . . . . . . . . . 12
2.5. Traffic Engineering Considerations . . . . . . . . . . . 12
3. BIER-TE Forwarding . . . . . . . . . . . . . . . . . . . . . 13
3.1. The Bit Index Forwarding Table (BIFT) . . . . . . . . . . 13
3.2. Adjacency Types . . . . . . . . . . . . . . . . . . . . . 14
3.2.1. Forward Connected . . . . . . . . . . . . . . . . . . 15
3.2.2. Forward Routed . . . . . . . . . . . . . . . . . . . 15
3.2.3. ECMP . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.4. Local Decap . . . . . . . . . . . . . . . . . . . . . 16
3.3. Encapsulation considerations . . . . . . . . . . . . . . 16
3.4. Basic BIER-TE Forwarding Example . . . . . . . . . . . . 16
3.5. Forwarding comparison with BIER . . . . . . . . . . . . . 19
3.6. Requirements . . . . . . . . . . . . . . . . . . . . . . 19
4. BIER-TE Controller BitPosition Assignments . . . . . . . . . 20
4.1. P2P Links . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2. BFER . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.3. Leaf BFERs . . . . . . . . . . . . . . . . . . . . . . . 20
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4.4. LANs . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.5. Hub and Spoke . . . . . . . . . . . . . . . . . . . . . . 22
4.6. Rings . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.7. Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . . . 23
4.8. Routed adjacencies . . . . . . . . . . . . . . . . . . . 26
4.8.1. Reducing BitPositions . . . . . . . . . . . . . . . . 26
4.8.2. Supporting nodes without BIER-TE . . . . . . . . . . 26
4.9. Reuse of BitPositions (without DNR) . . . . . . . . . . . 26
4.10. Summary of BP optimizations . . . . . . . . . . . . . . . 28
5. Avoiding loops and duplicates . . . . . . . . . . . . . . . . 29
5.1. Loops . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.2. Duplicates . . . . . . . . . . . . . . . . . . . . . . . 29
6. BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . . . 29
7. Managing SI, subdomains and BFR-ids . . . . . . . . . . . . . 32
7.1. Why SI and sub-domains . . . . . . . . . . . . . . . . . 33
7.2. Bit assignment comparison BIER and BIER-TE . . . . . . . 34
7.3. Using BFR-id with BIER-TE . . . . . . . . . . . . . . . . 34
7.4. Assigning BFR-ids for BIER-TE . . . . . . . . . . . . . . 35
7.5. Example bit allocations . . . . . . . . . . . . . . . . . 36
7.5.1. With BIER . . . . . . . . . . . . . . . . . . . . . . 36
7.5.2. With BIER-TE . . . . . . . . . . . . . . . . . . . . 37
7.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 38
8. BIER-TE and Segment Routing . . . . . . . . . . . . . . . . . 38
9. Security Considerations . . . . . . . . . . . . . . . . . . . 39
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 40
12. Change log [RFC Editor: Please remove] . . . . . . . . . . . 40
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 45
13.1. Normative References . . . . . . . . . . . . . . . . . . 45
13.2. Informative References . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46
1. Introduction
BIER-TE shares architecture, terminology and packet formats with BIER
as described in [RFC8279] and [RFC8296]. This document describes
BIER-TE in the expectation that the reader is familiar with these two
documents.
In BIER-TE, BitPositions (BP) indicate adjacencies. The BIFT of each
BFR is only populated with BP that are adjacent to the BFR in the
BIER-TE Topology. Other BPs are left without adjacency. The BFR
replicate and forwards BIER packets to adjacent BPs that are set in
the packet. BPs are normally also reset upon forwarding to avoid
duplicates and loops. This is detailed further below.
Note that related work, [I-D.ietf-roll-ccast] uses bloom filters to
represent leaves or edges of the intended delivery tree. Bloom
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filters in general can support larger trees/topologies with fewer
addressing bits than explicit bitstrings, but they introduce the
heuristic risk of false positives and cannot reset bits in the
bitstring during forwarding to avoid loops. For these reasons, BIER-
TE uses explicit bitstrings like BIER. The explicit bitstrings of
BIER-TE can also be seen as a special type of bloom filter, and this
is how related work [ICC] describes it.
1.1. Basic Examples
BIER-TE forwarding is best introduced with simple examples.
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BIER-TE Topology:
Diagram:
p5 p6
--- BFR3 ---
p3/ p13 \p7
BFR1 ---- BFR2 BFR5 ----- BFR6
p1 p2 p4\ p14 /p10 p11 p12
--- BFR4 ---
p8 p9
(simplified) BIER-TE Bit Index Forwarding Tables (BIFT):
BFR1: p1 -> local_decap
p2 -> forward_connected to BFR2
BFR2: p1 -> forward_connected to BFR1
p5 -> forward_connected to BFR3
p8 -> forward_connected to BFR4
BFR3: p3 -> forward_connected to BFR2
p7 -> forward_connected to BFR5
p13 -> local_decap
BFR4: p4 -> forward_connected to BFR2
p10 -> forward_connected to BFR5
p14 -> local_decap
BFR5: p6 -> forward_connected to BFR3
p9 -> forward_connected to BFR4
p12 -> forward_connected to BFR6
BFR6: p11 -> forward_connected to BFR5
p12 -> local_decap
Figure 1: BIER-TE basic example
Consider the simple network in the above BIER-TE overview example
picture with 6 BFRs. p1...p14 are the BitPositions (BP) used. All
BFRs can act as ingress BFR (BFIR), BFR1, BFR3, BFR4 and BFR6 can
also be egress BFR (BFER). Forward_connected is the name for
adjacencies that are representing subnet adjacencies of the network.
Local_decap is the name of the adjacency to decapsulate BIER-TE
packets and pass their payload to higher layer processing.
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Assume a packet from BFR1 should be sent via BFR4 to BFR6. This
requires a bitstring (p2,p8,p10,p12). When this packet is examined
by BIER-TE on BFR1, the only BitPosition from the bitstring that is
also set in the BIFT is p2. This will cause BFR1 to send the only
copy of the packet to BFR2. Similarly, BFR2 will forward to BFR4
because of p8, BFR4 to BFR5 because of p10 and BFR5 to BFR6 because
of p12. p12 also makes BFR6 receive and decapsulate the packet.
To send in addition to BFR6 via BFR4 also a copy to BFR3, the
bitstring needs to be (p2,p5,p8,p10,p12,p13). When this packet is
examined by BFR2, p5 causes one copy to be sent to BFR3 and p8 one
copy to BFR4. When BFR3 receives the packet, p13 will cause it to
receive and decapsulate the packet.
If instead the bitstring was (p2,p6,p8,p10,p12,p13), the packet would
be copied by BFR5 towards BFR3 because p6 instead of BFR2 to BFR5
because of p6 in the prior case. This is showing the ability of the
shown BIER-TE Topology to make the traffic pass across any possible
path and be replicated where desired.
BIER-TE has various options to minimize BP assignments, many of which
are based on assumptions about the required multicast traffic paths
and bandwidth consumption in the network.
The following picture shows a modified example, in which Rtr2 and
Rtr5 are assumed not to support BIER-TE, so traffic has to be unicast
encapsulated across them. Unicast tunneling of BIER-TE packets can
leverage any feasible mechanism such as MPLS or IP, these
encapsulations are out of scope of this document. To emphasize non-
native forwarding of BIER-TE packets, these adjacencies are called
"forward_routed", but otherwise there is no difference in their
processing over the aforementioned "forward_connected" adjacencies.
In addition, bits are saved in the following example by assuming that
BFR1 only needs to be BFIR but not BFER or transit BFR.
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BIER-TE Topology:
Diagram:
p1 p3 p7
....> BFR3 <.... p5
........ ........>
BFR1 (Rtr2) (Rtr5) BFR6
........ ........>
....> BFR4 <.... p6
p2 p4 p8
(simplified) BIER-TE Bit Index Forwarding Tables (BIFT):
BFR1: p1 -> forward_routed to BFR3
p2 -> forward_routed to BFR4
BFR3: p3 -> local_decap
p5 -> forward_routed to BFR6
BFR4: p4 -> local_decap
p6 -> forward_routed to BFR6
BFR6: p5 -> local_decap
p6 -> local_decap
p7 -> forward_routed to BFR3
p8 -> forward_routed to BFR4
Figure 2: BIER-TE basic overlay example
To send a BIER-TE packet from BFR1 via BFR3 to BFR6, the bitstring is
(p1,p5). From BFR1 via BFR4 to BFR6 it is (p2,p6). A packet from
BFR1 to BFR3,BFR4 and BFR6 can use (p1,p2,p3,p4,p5) or
(p1,p2,p3,p4,p6), or via BFR6 (p2,p3,p4,p6,p7) or (p1.p3,p4,p5,p8).
1.2. BIER-TE Topology and adjacencies
The key new component in BIER-TE to control where replication can or
should happens and how to minimize the required BP for segments is -
as shown in these two examples - the BIER-TE topology.
The BIER-TE Topology consists of the BIFT of all the BFR and can also
be expressed in a diagram as a graph where the edges are the
adjacencies between the BFR. Adjacencies are naturally
unidirectional. BP can be reused across multiple adjacencies as long
as this does not lead to undesired duplicates or loops as explained
further down in the text.
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If the BIER-TE topology represents the underlying (layer 2) topology
of the network, this is called "native" BIER-TE as shown in the first
example. This can be freely mixed with "overlay" BIER-TE, in
"forward_routed" adjacencies are used.
1.3. Comparison with BIER
The key differences over BIER are:
o BIER-TE replaces in-network autonomous path calculation by
explicit paths calculated off-path by the BIER-TE Controller.
o In BIER-TE every BitPosition of the BitString of a BIER-TE packet
indicates one or more adjacencies - instead of a BFER as in BIER.
o BIER-TE in each BFR has no routing table but only a BIER-TE
Forwarding Table (BIFT) indexed by SI:BitPosition and populated
with only those adjacencies to which the BFR should replicate
packets to.
BIER-TE headers use the same format as BIER headers.
BIER-TE forwarding does not require/use the BFIR-ID. The BFIR-ID can
still be useful though for coordinated BFIR/BFER functions, such as
the context for upstream assigned labels for MPLS payloads in MVPN
over BIER-TE.
If the BIER-TE domain is also running BIER, then the BFIR-ID in BIER-
TE packets can be set to the same BFIR-ID as used with BIER packets.
If the BIER-TE domain is not running full BIER or does not want to
reduce the need to allocate bits in BIER bitstrings for BFIR-ID
values, then the allocation of BFIR-ID values in BIER-TE packets can
be done through other mechanisms outside the scope of this document,
as long as this is appropriately agreed upon between all BFIR/BFER.
1.4. Requirements Language
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].
2. Components
End to end BIER-TE operations consists of four mayor components: The
"Multicast Flow Overlay", the "BIER-TE control plane" consisting of
the "BIER-TE Controller" and its signaling channels to the BFR, the
"Routing Underlay" and the "BIER-TE forwarding layer". The Bier-TE
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Controller is the new architectural component in BIER-TE compared to
BIER.
Picture 2: Components of BIER-TE
<------BGP/PIM----->
|<-IGMP/PIM-> multicast flow <-PIM/IGMP->|
overlay
[BIER-TE Controller] <=> [BIER-TE Topology]
BIER-TE control plane
^ ^ ^
/ | \ BIER-TE control protocol
| | | e.g. Netconf/Restconf/Yang
v v v
Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr
|<----------------->|
BIER-TE forwarding layer
|<- BIER-TE domain->|
|<--------------------->|
Routing underlay
Figure 3: BIER-TE architecture
2.1. The Multicast Flow Overlay
The Multicast Flow Overlay operates as in BIER. See [RFC8279].
Instead of interacting with the BIER forwarding layer (as in BIER),
it interacts with the BIER-TE Controller.
2.2. The BIER-TE Controller
The BIER-TE Controller is representing the control plane of BIER-TE.
It communicates two sets of information with BFRs:
During initial provisioning or modifications of the network topology,
the BIER-TE Controller discovers the network topology and creates the
BIER-TE topology from it: determine which adjacencies are required/
desired and assign BitPositions to them. Then it signals the
resulting of BitPositions and their adjacencies to each BFR to set up
their BIER-TE BIFTs.
During day-to-day operations of the network, the BIER-TE Controller
signals to BFIRs what multicast flows are mapped to what BitStrings.
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Communications between the BIER-TE Controller and BFRs is ideally via
standardized protocols and data-models such as Netconf/Restconf/Yang.
This is currently outside the scope of this document. Vendor-
specific CLI on the BFRs is also a possible stopgap option (as in
many other SDN solutions lacking definition of standardized data
model).
For simplicity, the procedures of the BIER-TE Controller are
described in this document as if it is a single, centralized
automated entity, such as an SDN controller. It could equally be an
operator setting up CLI on the BFRs. Distribution of the functions
of the BIER-TE Controller is currently outside the scope of this
document.
2.2.1. Assignment of BitPositions to adjacencies of the network
topology
The BIER-TE Controller tracks the BFR topology of the BIER-TE domain.
It determines what adjacencies require BitPositions so that BIER-TE
explicit paths can be built through them as desired by operator
policy.
The BIER-TE Controller then pushes the BitPositions/adjacencies to
the BIFT of the BFRs, populating only those SI:BitPositions to the
BIFT of each BFR to which that BFR should be able to send packets to
- adjacencies connecting to this BFR.
2.2.2. Changes in the network topology
If the network topology changes (not failure based) so that
adjacencies that are assigned to BitPositions are no longer needed,
the BIER-TE Controller can re-use those BitPositions for new
adjacencies. First, these BitPositions need to be removed from any
BFIR flow state and BFR BIFT state, then they can be repopulated,
first into BIFT and then into the BFIR.
2.2.3. Set up per-multicast flow BIER-TE state
The BIER-TE Controller interacts with the multicast flow overlay to
determine what multicast flow needs to be sent by a BFIR to which set
of BFER. It calculates the desired distribution tree across the
BIER-TE domain based on algorithms outside the scope of this document
(e.g. CSFP, Steiner Tree, ...). It then pushes the calculated
BitString into the BFIR.
See [I-D.ietf-bier-multicast-http-response] for a solution describing
this interaction.
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2.2.4. Link/Node Failures and Recovery
When link or nodes fail or recover in the topology, BIER-TE can
quickly respond with the optional FRR procedures described in [I-
D.eckert-bier-te-frr]. It can also more slowly react by
recalculating the BitStrings of affected multicast flows. This
reaction is slower than the FRR procedure because the BIER-TE
Controller needs to receive link/node up/down indications,
recalculate the desired BitStrings and push them down into the BFIRs.
With FRR, this is all performed locally on a BFR receiving the
adjacency up/down notification.
2.3. The BIER-TE Forwarding Layer
When the BIER-TE Forwarding Layer receives a packet, it simply looks
up the BitPositions that are set in the BitString of the packet in
the Bit Index Forwarding Table (BIFT) that was populated by the BIER-
TE Controller. For every BP that is set in the BitString, and that
has one or more adjacencies in the BIFT, a copy is made according to
the type of adjacencies for that BP in the BIFT. Before sending any
copy, the BFR resets all BP in the BitString of the packet for which
the BFR has one or more adjacencies in the BIFT, except when the
adjacency indicates "DoNotReset" (DNR, see Section 3.2.1). This is
done to inhibit that packets can loop.
2.4. The Routing Underlay
BIER-TE is sending BIER packets to directly connected BIER-TE
neighbors as L2 (unicasted) BIER packets without requiring a routing
underlay. BIER-TE forwarding uses the Routing underlay for
forward_routed adjacencies which copy BIER-TE packets to not-
directly-connected BFRs (see below for adjacency definitions).
If the BFR intends to support FRR for BIER-TE, then the BIER-TE
forwarding plane needs to receive fast adjacency up/down
notifications: Link up/down or neighbor up/down, e.g. from BFD.
Providing these notifications is considered to be part of the routing
underlay in this document.
2.5. Traffic Engineering Considerations
Traffic Engineering ([I-D.dt-teas-rfc3272bis]) provides performance
optimization of operational IP networks while utilizing network
resources economically and reliably. The key elements needed to
effect TE are policy, path steering and resource management. These
elements require support at the control/controller level and within
the forwarding plane.
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Policy decisions are made within the BIER-TE control plane, i.e.,
within BIER-TE Controllers. Controllers use policy when composing
BitStrings (BFR flow state) and BFR BIFT state. The mapping of user/
IP traffic to specific BitStrings/BIER-TE flows is made based on
policy. The specifics details of BIER-TE policies and how a
controller uses such are out of scope of this document.
Path steering is supported via the definition of a BitString.
BitStrings used in BIER-TE are composed based on policy and resource
management considerations. When composing BIER-TE BitStrings, a
Controller MUST take into account the resources available at each BFR
and for each BP when it is providing congestion loss free services
such as for Rate Controlled Service Disciplines (RCSD). Resource
availability could be provided for example via routing protocol
information, but may also be obtained via a BIER-TE control protocol.
The resource usage of the BIER-TE traffic admitted by the BIER-TE
controller can be solely tracked on the BIER-TE Controller based on
local accounting as long as no forward_routed adjacencies are used
(see below for definition of forward_routed adjacencies). When
forward_routed adjacencies are used, the paths selected by the
underlying routing protocol need to be tracked as well.
Resource management has implications on the forwarding plane beyond
the BIER-TE defined steering of packets. This includes allocation of
buffers to guarantee the worst case requirements of admitted RCSD
trafic and potential policing and/or rate-shaping mechanisms,
typically done via various forms of queuing. This level of resource
control, while optional, is important in networks that wish to
support congestion management policies to control or regulate the
offered traffic to deliver different levels of service and alleviate
congestion problems, or those networks that wish to control latencies
experienced by specific traffic flows.
3. BIER-TE Forwarding
3.1. The Bit Index Forwarding Table (BIFT)
The Bit Index Forwarding Table (BIFT) exists in every BFR. For every
subdomain in use, it is a table indexed by SI:BitPosition and is
populated by the BIER-TE control plane. Each index can be empty or
contain a list of one or more adjacencies.
BIER-TE can support multiple subdomains like BIER. Each one with a
separate BIFT
In the BIER architecture, indices into the BIFT are explained to be
both BFR-id and SI:BitString (BitPosition). This is because there is
a 1:1 relationship between BFR-id and SI:BitString - every bit in
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every SI is/can be assigned to a BFIR/BFER. In BIER-TE there are
more bits used in each BitString than there are BFIR/BFER assigned to
the bitstring. This is because of the bits required to express the
engineered path through the topology. The BIER-TE forwarding
definitions do therefore not use the term BFR-id at all. Instead,
BFR-ids are only used as required by routing underlay, flow overlay
of BIER headers. Please refer to Section 7 for explanations how to
deal with SI, subdomains and BFR-id in BIER-TE.
------------------------------------------------------------------
| Index: | Adjacencies: |
| SI:BitPosition | <empty> or one or more per entry |
==================================================================
| 0:1 | forward_connected(interface,neighbor{,DNR}) |
------------------------------------------------------------------
| 0:2 | forward_connected(interface,neighbor{,DNR}) |
| | forward_connected(interface,neighbor{,DNR}) |
------------------------------------------------------------------
| 0:3 | local_decap({VRF}) |
------------------------------------------------------------------
| 0:4 | forward_routed({VRF,}l3-neighbor) |
------------------------------------------------------------------
| 0:5 | <empty> |
------------------------------------------------------------------
| 0:6 | ECMP({adjacency1,...adjacencyN}, seed) |
------------------------------------------------------------------
...
| BitStringLength | ... |
------------------------------------------------------------------
Bit Index Forwarding Table
Figure 4: BIFT adjacencies
The BIFT is programmed into the data plane of BFRs by the BIER-TE
Controller and used to forward packets, according to the rules
specified in the BIER-TE Forwarding Procedures.
Adjacencies for the same BP when populated in more than one BFR by
the BIER-TE Controller does not have to have the same adjacencies.
This is up to the BIER-TE Controller. BPs for p2p links are one case
(see below).
3.2. Adjacency Types
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3.2.1. Forward Connected
A "forward_connected" adjacency is towards a directly connected BFR
neighbor using an interface address of that BFR on the connecting
interface. A forward_connected adjacency does not route packets but
only L2 forwards them to the neighbor.
Packets sent to an adjacency with "DoNotReset" (DNR) set in the BIFT
will not have the BitPosition for that adjacency reset when the BFR
creates a copy for it. The BitPosition will still be reset for
copies of the packet made towards other adjacencies. This can be
used for example in ring topologies as explained below.
3.2.2. Forward Routed
A "forward_routed" adjacency is an adjacency towards a BFR that is
not a forward_connected adjacency: towards a loopback address of a
BFR or towards an interface address that is non-directly connected.
Forward_routed packets are forwarded via the Routing Underlay.
If the Routing Underlay has multiple paths for a forward_routed
adjacency, it will perform ECMP independent of BIER-TE for packets
forwarded across a forward_routed adjacency. This is independent of
BIER-TE ECMP described in Section 3.2.3.
If the Routing Underlay has FRR, it will perform FRR independent of
BIER-TE for packets forwarded across a forward_routed adjacency.
3.2.3. ECMP
The ECMP mechanisms in BIER are tied to the BIER BIFT and are
therefore not directly useable with BIER-TE. The following
procedures describe ECMP for BIER-TE that we consider to be
lightweight but also well manageable. It leverages the existing
entropy parameter in the BIER header to keep packets of the flows on
the same path and it introduces a "seed" parameter to allow for
traffic flows to be polarized or randomized across multiple hops.
An "Equal Cost Multipath" (ECMP) adjacency has a list of two or more
adjacencies included in it. It copies the BIER-TE to one of those
adjacencies based on the ECMP hash calculation. The BIER-TE ECMP
hash algorithm must select the same adjacency from that list for all
packets with the same "entropy" value in the BIER-TE header if the
same number of adjacencies and same seed are given as parameters.
Further use of the seed parameter is explained below.
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3.2.4. Local Decap
A "local_decap" adjacency passes a copy of the payload of the BIER-TE
packet to the packets NextProto within the BFR (IPv4/IPv6,
Ethernet,...). A local_decap adjacency turns the BFR into a BFER for
matching packets. Local_decap adjacencies require the BFER to
support routing or switching for NextProto to determine how to
further process the packet.
3.3. Encapsulation considerations
Specifications for BIER-TE encapsulation are outside the scope of
this document. This section gives explanations and guidelines.
Because a BFR needs to interpret the BitString of a BIER-TE packet
differently from a BIER packet, it is necessary to distinguish BIER
from BIER-TE packets. This is subject to definitions in BIER
encapsulation specifications.
MPLS encapsulation [RFC8296] for example assigns one label by which
BFRs recognizes BIER packets for every (SI,subdomain) combination.
If it is desirable that every subdomain can forward only BIER or
BIER-TE packets, then the label allocation could stay the same, and
only the forwarding model (BIER/BIER-TE) would have to be defined per
subdomain. If it is desirable to support both BIER and BIER-TE
forwarding in the same subdomain, then additional labels would need
to be assigned for BIER-TE forwarding.
"forward_routed" requires an encapsulation permitting to unicast
BIER-TE packets to a specific interface address on a target BFR.
With MPLS encapsulation, this can simply be done via a label stack
with that addresses label as the top label - followed by the label
assigned to (SI,subdomain) - and if necessary (see above) BIER-TE.
With non-MPLS encapsulation, some form of IP tunneling (IP in IP,
LISP, GRE) would be required.
The encapsulation used for "forward_routed" adjacencies can equally
support existing advanced adjacency information such as "loose source
routes" via e.g. MPLS label stacks or appropriate header extensions
(e.g. for IPv6).
3.4. Basic BIER-TE Forwarding Example
[RFC Editor: remove this section.]
THIS SECTION TO BE REMOVED IN RFC BECAUSE IT WAS SUPERCEEDED BY
SECTION 1.1 EXAMPLE - UNLESS REVIEWERS CHIME IN AND EXPRESS DESIRE TO
KEEP THIS ADDITIONAL EXAMPLE SECTION.
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Step by step example of basic BIER-TE forwarding. This does not use
ECMP or forward_routed adjacencies nor does it try to minimize the
number of required BitPositions for the topology.
[BIER-TE Controller]
/ | \
v v v
| p13 p1 |
+- BFIR2 --+ |
| | p2 p6 | LAN2
| +-- BFR3 --+ |
| | | p7 p11 |
Src -+ +-- BFER1 --+
| | p3 p8 | |
| +-- BFR4 --+ +-- Rcv1
| | | |
| |
| p14 p4 |
+- BFIR1 --+ |
| +-- BFR5 --+ p10 p12 |
LAN1 | p5 p9 +-- BFER2 --+
| +-- Rcv2
|
LAN3
IP |..... BIER-TE network......| IP
Figure 5: BIER-TE Forwarding Example
pXX indicate the BitPositions number assigned by the BIER-TE
Controller to adjacencies in the BIER-TE topology. For example, p9
is the adjacency towards BFR5 on the LAN connecting to BFER2.
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BIFT BFIR2:
p13: local_decap()
p2: forward_connected(BFR3)
BIFT BFR3:
p1: forward_connected(BFIR2)
p7: forward_connected(BFER1)
p8: forward_connected(BFR4)
BIFT BFER1:
p11: local_decap()
p6: forward_connected(BFR3)
p8: forward_connected(BFR4)
Figure 6: BIER-TE Forwarding Example Adjacencies
...and so on.
For example, we assume that some multicast traffic seen on LAN1 needs
to be sent via BIER-TE by BFIR2 towards Rcv1 and Rcv2. The BIER-TE
Controller determines it wants it to pass this traffic across the
following paths:
-> BFER1 ---------------> Rcv1
BFIR2 -> BFR3
-> BFR4 -> BFR5 -> BFER2 -> Rcv2
Figure 7: BIER-TE Forwarding Example Paths
These paths equal to the following BitString: p2, p5, p7, p8, p10,
p11, p12.
This BitString is assigned by BFIR2 to the example multicast traffic
received from LAN1.
Then BFIR2 forwards this multicast traffic with BIER-TE based on that
BitString. The BIFT of BFIR2 has only p2 and p13 populated. Only p2
is in the BitString and this is an adjacency towards BFR3. BFIR2
therefore resets p2 in the BitString and sends a copy towards BFR2.
BFR3 sees a BitString of p5,p7,p8,p10,p11,p12. It is only interested
in p1,p7,p8. It creates a copy of the packet to BFER1 (due to p7)
and one to BFR4 (due to p8). It resets p7, p8 before sending.