Traffic Enginering for Bit Index Explicit Replication BIER-TE
draft-eckert-bier-te-arch-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Author | Toerless Eckert | ||
| Last updated | 2015-03-06 | ||
| Replaced by | draft-ietf-bier-te-arch, draft-ietf-bier-te-arch, draft-ietf-bier-te-arch, RFC 9262 | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
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| Send notices to | (None) |
draft-eckert-bier-te-arch-00
Network Working Group T. Eckert
Internet-Draft Cisco
Intended status: Standards Track March 5, 2015
Expires: September 6, 2015
Traffic Enginering for Bit Index Explicit Replication BIER-TE
draft-eckert-bier-te-arch-00
Abstract
This document proposes an architecture for BIER-TE: Traffic
Engineering for Bit Index Explicit Replication (BIER).
BIER-TE shares part of its architecture with BIER as described in
[I-D.wijnands-bier-architecture]. It also proposes to share the
packet format with BIER.
BIER-TE forwards and replicates packets like BIER based on a
BitString in the packet header but it does not require an IGP. It
does support traffic engineering by explicit hop-by-hop forwarding
and loose hop forwarding of packets. It does support Fast ReRoute
(FRR) for link and node protection and incremental deployment.
Because BIER-TE like BIER operates without explicit in-network tree-
building but also supports traffic engineering, it is more similar to
SR than RSVP-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 http://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 September 6, 2015.
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Copyright Notice
Copyright (c) 2015 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Layering . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The Multicast Flow Overlay . . . . . . . . . . . . . . . 4
2.2. The BIER-TE Controller Host . . . . . . . . . . . . . . . 4
2.2.1. Assignment of BitPositions to adjacencies of the
network topology . . . . . . . . . . . . . . . . . . 5
2.2.2. Changes in the network topology . . . . . . . . . . . 5
2.2.3. Set up per-multicast flow BIER-TE state . . . . . . . 5
2.2.4. Link/Node Failures and Recovery . . . . . . . . . . . 6
2.3. The BIER-TE Forwarding Layer . . . . . . . . . . . . . . 6
2.4. The Routing Underlay . . . . . . . . . . . . . . . . . . 6
3. BIER-TE Forwarding . . . . . . . . . . . . . . . . . . . . . 6
3.1. The Bit Index Forwarding Table (BIFT) . . . . . . . . . . 7
3.2. Adjacency Types . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. Forward Connected . . . . . . . . . . . . . . . . . . 7
3.2.2. Forward Routed . . . . . . . . . . . . . . . . . . . 8
3.2.3. ECMP . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.4. Local Decap . . . . . . . . . . . . . . . . . . . . . 8
3.3. Basic BIER-TE Forwarding Example . . . . . . . . . . . . 8
4. BIER-TE Controller Host BitPosition Assignments . . . . . . . 10
4.1. P2P Links . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. BFER . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Leaf BFIRs . . . . . . . . . . . . . . . . . . . . . . . 11
4.4. LANs . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5. Hub and Spoke . . . . . . . . . . . . . . . . . . . . . . 12
4.6. Rings . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.7. Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . . . 12
4.8. Routed adjacencies . . . . . . . . . . . . . . . . . . . 15
4.8.1. Supporting nodes without BIER-TE . . . . . . . . . . 15
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5. Avoiding loops and duplicates . . . . . . . . . . . . . . . . 15
5.1. Loops . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2. Duplicates . . . . . . . . . . . . . . . . . . . . . . . 16
6. FRR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. The BIER-TE Adjacency FRR Table (BTAFT) . . . . . . . . . 16
6.2. FRR in BIER-TE forwarding . . . . . . . . . . . . . . . . 17
6.3. FRR in the BIER-TE Controller Host . . . . . . . . . . . 17
6.4. BIER-TE FRR Benefits . . . . . . . . . . . . . . . . . . 18
7. BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
11. Change log [RFC Editor: Please remove] . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
1.1. Overview
This document specifies the architecture for BIER-TE: traffic
engineering for Bit Index Explicit Replication BIER.
BIER-TE shares architecture and packet formats with BIER as described
in [I-D.wijnands-bier-architecture].
BIER-TE forwards and replicates packets like BIER based on a
BitString in the packet header but it does not require an IGP. It
does support traffic engineering by explicit hop-by-hop forwarding
and loose hop forwarding of packets. It does support Fast ReRoute
(FRR) for link and node protection and incremental deployment.
Because BIER-TE like BIER operates without explicit in-network tree-
building but also supports traffic engineering, it is more similar to
SR than RSVP-TE.
The key differences over BIER are:
o BIER-TE replaces in-network autonomous path calculation by
explicit paths calculated offpath by the BIER-TE controller host.
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 BitPosition and populated with
only those adjacencies to which the BFR should replicate packets
to.
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Currently, BIER-TE does not support BIER-sub-domains and it does not
not use BFR-id or "Set Identifiers" (SI) in BIER-TE headers that
share the same format as BIER headers.
1.2. 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. Layering
End to end BIER-TE operations consists of four components: The
"Multicast Flow Overlay", the "BIER-TE Controller Host", the "Routing
Underlay" and the "BIER-TE forwarding layer".
Picture 2: Layers of BIER-TE
<------BGP/PIM----->
|<-IGMP/PIM-> multicast flow <-PIM/IGMP->|
overlay
[Bier-TE Controller Host]
^ ^ ^
/ | \ BIER-TE control protocol
| | | eg.: Netconf/Restconf/Yang
v v v
Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr
|--------------------->|
BIER-TE forwarding layer
|<- BIER-TE domain-->|
|<--------------------->|
Routing underlay
2.1. The Multicast Flow Overlay
The Multicast Flow Overlay operates as in BIER. See
[I-D.wijnands-bier-architecture]. Instead of interacting with the
BIER layer, it interacts with the BIER-TE Controller Host
2.2. The BIER-TE Controller Host
The BIER-TE controller host is an offpath central host. It
communicates via protocols such as Netconf/Restconf/Yang with BFRs.
The protocols used between BFRs and the controller are outside the
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scope of this document. This document is only concerned about the
logic how a controller can assign BitPositions to the topology and
BitStrings to BIER-TE packets:
During bring-up or modifications of the network topology, the
controller needs to talk to all BFRs to assign BitPositions to
adjacencies of the network topology. During day-to-day operations of
the network it only needs to talks to BFIRs to install BitStrings for
multicast flows.
These two tasks have the following steps:
2.2.1. Assignment of BitPositions to adjacencies of the network
topology
The BIER-TE controller host 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 controller then pushes the BitPositions/adjacencies to the BIFT
of the BFRs, populating only those 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 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 (and BTAFT if FRR is supported, see below), then
they can be repopulated, first into BIFT (and if FRR is supported
BTAFT), then into BFIR.
2.2.3. Set up per-multicast flow BIER-TE state
The BIER-TE controller host tracks 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
(eg.: CSFP, Steiner Tree,...). It then pushes the calculated
BitString into the BFIR.
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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 below. It
can also more slowly react by recalculating the BitStrings of
affected multicast flows. This reaction is slower than the FR
procedure because the 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 host. 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 BitPositions in the BitString of
the packet to which it can create a copy. This is done to inhibit
that packets can loop.
If the BFR support BIER-TE FRR operations, then the BIER-TE
forwarding layer will receive fast adjacency up/down notification
uses the BIER-TE FRR Adjacency Table to modify the BitString of the
packet before it performs BIER-TE forwarding. This is detailed in
the FRR section.
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, eg.: from BFD.
Providing these notifications is considered to be part of the routing
underlay in this document.
3. BIER-TE Forwarding
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3.1. The Bit Index Forwarding Table (BIFT)
The Bit Index Forwarding Table (BIFT) exists in every BFR. It is a
table indexed by BitPosition and is populated by the BIER-TE control
plane. Each index can be empty or contain a list of one or more
adjacencies.
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 1 | forward_connected(interface,neighbor,DNR) |
------------------------------------------------------------------
| 2 | forward_connected(interface,neighbor,DNR) |
| | forward_connected(interface,neighbor,DNR) |
------------------------------------------------------------------
| 3 | local_decap([VRF]) |
------------------------------------------------------------------
| 4 | forward_routed([VRF,]l3-neighbor) |
------------------------------------------------------------------
| 5 | <empty> |
------------------------------------------------------------------
| 6 | ECMP({adjacency1,...adjacencyN}, seed) |
------------------------------------------------------------------
...
| BitStringLength | ... |
------------------------------------------------------------------
Bit Index Forwarding Table
The BIFT is programmed into the data plane of BFRs by the BIER-TE
controller host 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 controller do not have to have the same adjacencies. This is up
to the controller. BPs for p2p links are one case (see below).
3.2. Adjacency Types
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
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creates a copy for it. The BitPosition will still be reset for
copies of the packet made towards other adjacencies. The 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.
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
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.
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. Basic BIER-TE Forwarding Example
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.
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Picture 1: Forwarding Example
[Bier-Te Controller Host]
/ | \
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
pXX indicate the BitPositions number assigned by the BIER-TE
controller host to adjacencies in the BIER-TE topology. For example,
p9 is the adjacency towards BFR9 on the LAN connecting to BFER2.
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)
...and so on.
Traffic needs to flow from BFIR2 towards Rcv1, Rcv2. The controller
determines it wants it to pass across the following paths:
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-> BFER1 ---------------> Rcv1
BFIR2 -> BFR3
-> BFR4 -> BFR5 -> BFER2 -> Rcv2
These paths equal to the following BitString: p2, p5, p7, p8, p10,
p11, p12
This BitString is set up in BFIR2. Multicast packets arriving at
BFIR2 from Src are assigned this BitString.
BFIR2 forwards based on that BitString. It has p2 and p13 populated.
Only p13 is in BitString which has an adjacency towards BFR3. BFIR2
resets p2 in 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.
BFER1 sees a BitString of p5,p10,p11,p12. It is only interested in
p6,p7,p8,p11 and therefore considers only p11. p11 is a "local_decap"
adjacency installed by the BIER-TE controller host because BFER1
should pass packets to IP multicast. The local_decap adjacency
instructs BFER1 to create a copy, decapsulate it from the BIER header
and pass it on to the NextProtocol, in this example IP multicast. IP
multicast will then forward the packet out to LAN2 because it did
receive PIM or IGMP joins on LAN2 for the traffic.
Further processing of the packet in BFR4, BFR5 and BFER2 accordingly.
4. BIER-TE Controller Host BitPosition Assignments
This section describes how the BIER-TE controller host can use the
different BIER-TE adjacency types to define the BitPositions of a
BIER-TE domain.
Because the size of the BitString is limiting the size of the BIER-TE
domain, many of the options described exist to support larger
topologies with fewer BitPositions (4.1, 4.3, 4.4, 4.5, 4.6, 4.7,
4.8).
4.1. P2P Links
Each P2p link in the BIER-TE domain is assigned one unique
BitPosition with a forward_connected adjacency pointing to the
neighbor on the p2p link.
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4.2. BFER
Every BFER is given a unique BitPosition with a local_decap
adjacency.
4.3. Leaf BFIRs
Leaf BFIRs are BFIRs where incoming BIER-TE packets never need to be
forwarded to another BFR but are only sent to the BFIR to exit the
BIER-TE domain. For example, in networks where PEs are spokes
connected to P routers, those PEs are Leaf BFIRs unless there is a
U-turn between two PEs.
All leaf-BFIR in a BIER-TE domain can share a single BitPosition.
This is possible because the BitPosition for the adjacency to reach
the BFIR can be used to distinguish whether or not packets should
reach the BFIR.
This optimization will not work if an upstream interface of the BFIR
is using a BitPosition optimized as described in the following two
sections (LAN, Hub and Spoke).
4.4. LANs
In a LAN, the adjacency to each neighboring BFR on the LAN is given a
unique BitPosition. The adjacency of this BitPosition is a
forward_connected adjacency towards the BFR and this BitPosition is
populated into the BIFT of all the other BFRs on that LAN.
BFR1
|p1
LAN1-+-+---+-----+
p3| p4| p2|
BFR3 BFR4 BFR7
If Bandwidth on the LAN is not an issue and most BIER-TE traffic
should be copied to all neighbors on a LAN, then BitPositions can be
saved by assigning just a single BitPosition to the LAN and
populating the BitPosition of the BIFTs of each BFRs on the LAN with
a list of forward_connected adjacencies to all other neighbors on the
LAN.
This optimization does not work in the face of BFRs redundantly
connected to more than one LANs with this optimization because these
BFRs would receive duplicates and forward those duplicates into the
opposite LANs. Adjacencies of such BFRs into their LANs still need a
separate BitPosition.
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4.5. Hub and Spoke
In a setup with a hub and multiple spokes connected via separate p2p
links to the hub, all p2p links can share the same BitPosition. The
BitPosition on the hubs BIFT is set up with a list of
forward_connected adjacencies, one for each Spoke.
This option is similar to the BitPosition optimization in LANs:
Redundantly connected spokes need their own BitPositions.
4.6. Rings
In L3 rings, instead of assigning a single BitPosition for every p2p
link in the ring, it is possible to save BitPositions by setting the
"Do Not Reset" (DNR) flag on forward_connected adjacencies.
For the rings shown in the following picture, a single BitPosition
will suffice to forward traffic entering the ring at BFRa or BFRb all
the way up to BFR1:
On BFRa, BFRb, BFR30,... BFR3, the BitPosition is populated with a
forward_connected adjacency pointing to the clockwise neighbor on the
ring and with DNR set. On BFR2, the adjacency also points to the
clockwise neighbor BFR1, but without DNR set. Handling DNR this way
ensures that copies forwarded from any BFR in the ring to a BFR
outside the ring will not have this BitPosition, therefore minimizing
the chance to create loops.
v v
| |
L1 | L2 | L3
/-------- BFRa ---- BFRb --------------------\
| |
\- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
| | L4 | |
p33| p15|
BFRd BFRc
4.7. Equal Cost MultiPath (ECMP)
The ECMP adjacency allows to use just one BP per link bundle between
two BFRs instead of one BP for each p2p member link of that link
bundle. In the following picture, one BP is used across L1,L2,L3 and
BFR1/BFR2 have for the BP
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--L1-----
BFR1 --L2----- BFR2
--L3-----
BIFT entry in BFR1:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 6 | ECMP({L1-to-BFR2,L2-to-BFR2,L3-to-BFR2}, seed) |
------------------------------------------------------------------
BIFT entry in BFR2:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 6 | ECMP({L1-to-BFR1,L2-to-BFR1,L3-to-BFR1}, seed) |
------------------------------------------------------------------
In the following example, all traffic from BFR1 towards BFR10 is
intended to be ECMP load split equally across the topology. This
example is not mean as a likely setup, but to illustrate that ECMP
can be used to share BPs not only across link bundles, and it
explains the use of the seed parameter.
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BFR1
/ \
/L11 \L12
BFR2 BFR3
/ \ / \
/L21 \L22 /L31 \L32
BFR4 BFR5 BFR6 BFR7
\ / \ /
\ / \ /
BFR8 BFR9
\ /
\ /
BFR10
BIFT entry in BFR1:
------------------------------------------------------------------
| 6 | ECMP({L11-to-BFR2,L12-to-BFR3}, seed) |
------------------------------------------------------------------
BIFT entry in BFR2:
------------------------------------------------------------------
| 6 | ECMP({L21-to-BFR4,L22-to-BFR5}, seed) |
------------------------------------------------------------------
BIFT entry in BFR3:
------------------------------------------------------------------
| 6 | ECMP({L31-to-BFR6,L32-to-BFR7}, seed) |
------------------------------------------------------------------
With the setup of ECMP in above topology, traffic would not be
equally load-split. Instead, links L22 and L31 would see no traffic
at all: BFR2 will only see traffic from BFR1 for which the ECMP hash
in BFR1 selected the first adjacency in a list of 2 adjacencies: link
L11-to-BFR2. When forwarding in BFR2 performs again an ECMP with two
adjacencies on that subset of traffic, then it will again select the
first of its two adjacencies to it: L21-to-BFR4. And therefore L22
and BFR5 sees no traffic.
To resolve this issue, the ECMP adjaceny on BFR1 simply needs to be
set up with a different seed than the ECMP adjacncies on BFR2/BFR3
This issue is called polarization. It depends on the ECMP hash. It
is possible to build ECMP that does not have polarization, for
example by taking entropy from the actual adjacency members into
account, but that can make it harder to achieve evenly balanced load-
splitting on all BFR without making the ECMP hash algorithm
potentially too complex for fast forwarding in the BFRs.
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4.8. Routed adjacencies
Routed adjacencies can reduce the number of BitPositions required
when the traffic engineering requirement is not hop-by-hop explicit
path selection, but loose-hop selection.
............... ...............
BFR1--... Redundant ...--L1-- BFR2... Redundant ...---
\--... Network ...--L2--/ ... Network ...---
BFR4--... Segment 1 ...--L3-- BFR3... Segment 2 ...---
............... ...............
Assume he requirement in above network is to explicitly engineer
paths such that specific traffic flows are passed from segment 1 to
segment 2 via link L1 (or via L2 or via L3).
To achieve this, BFR1 and BFR4 are set up with a forward_routed
adjacency BitPosition towards an address of BFR2 on link L1 (or link
L2 BFR3 via L3).
For paths to be engineered through a specific node BFR2 (or BFR3),
BFR1 and BFR4 are set up up with a forward_routed adjacency
BitPosition towards a loopback address of BFR2 (or BFR3).
4.8.1. Supporting nodes without BIER-TE
Routed adjacencies also enable incremental deployment of BIER-TE.
Only the nodes through which BIER-TE traffic needs to be steered -
with or without replication - need to support BIER-TE. Where they
are not directly connected to each other, forward_routed adjacencies
are used to pass over non BIER-TE enabled nodes.
5. Avoiding loops and duplicates
5.1. Loops
Whenever BIER-TE creates a copy of a packet, the BitString of that
copy will have all BitPositions cleared that are associated with
adjacencies in the BFR. This inhibits looping of packets. The only
exception are adjacencies with DNR set.
With DNR set, looping can happen. Consider in the ring picture that
link L4 from BFR3 is plugged into the L1 interface of BFRa. This
creates a loop where the rings clockwise BitPosition is never reset
for copies of the packets traveling clockwise around the ring.
To inhibit looping in the face of such physical misconfiguration,
only forward_connected adjacencies are permitted to have DNR set, and
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the link layer destination address of the adjacency (eg.: MAC
address) protects against closing the loop. Link layers without port
unique link layer addresses should not used with the DNR flag set.
5.2. Duplicates
Duplicates happen when the topology of the BitString is not a tree
but redundantly connecting BFRs with each other. The controller must
therefore ensure to only create BitStrings that are trees in the
topology.
When links are incorrectly physically re-connected before the
controller updates BitStrings in BFIRs, duplicates can happen. Like
loops, these can be inhibited by link layer addressing in
forward_connected adjacencies.
If interface or loopback addresses used in forward_routed adjacencies
are moved from one BFR to another, duplicates can equally happen.
Such re-addressing operations must be coordinated with the
controller.
6. FRR
FRR is an optional procedure. To leverage it, the BIER-TE controller
host and BFRs need to support it. It does not have to be supported
on all BFRs, but only those that are attached to a link/adjacency for
which FRR support is required.
If BIER-TE FRR is supported by the BIER-TE controller host, then it
needs to calculate the desired backup paths for link and/or node
failures in the BIER-TE domain and download this information into the
BIER-TE Adjacency FRR Table (BTAFT) of the BFRs. The BTAFT then
drives FRR operations in the BIER-TE forwarding plane of that BFR.
6.1. The BIER-TE Adjacency FRR Table (BTAFT)
The BIER-TE IF FRR Table exists in every BFR that is supporting BIER-
TE FRR procedures. It is indexed by FRR Adjacency Index. Associated
with each FRR Adjacency Index is a ResetBitmask, AddBitmask and
BitPosition.
-----------------------------------------------------------
| FRR Adjacency | BitPosition | ResetBitmask | AddBitmask |
| Index | | | |
===========================================================
| 1 | 5 | ..0010000 | ..11000000 |
-----------------------------------------------------------
...
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An FRR Adjacency is an adjacency that is used in the BIFT of the BFR.
The BFR has to be able to determine whether the adjacency is up or
down in less than 50msec. An FRR adjacency can be a
forward_connected adjacency with fast L2 link state Up/Down state
notifications or a forward_connected or forward_routed adjacency with
a fast aliveness mechanism such as BFD. Details of those mechanism
are outside the scope of this architecture.
The FRR Adjacency Index is the index that would be indicated on the
fast Up/Down notifications to the BIER-TE forwarding plane
The BitPosition is the BP in the BIFT in which the FRR Adjacency is
used
6.2. FRR in BIER-TE forwarding
The BIER-TE forwarding plane receives fast Up/Down notifications with
the FRR Adjacency Index. From the BitPosition in the BTAFT entry, it
remembers which BPs are currently affected (have a down adjacency).
When a packet is received, BIER-TE forwarding checks if it has
affected BPs to which it would forward. If it does, it will remove
the ResetBitmask bits from the packets BitString and add the
AddBitmask bits to the packets BitString.
Afterwards, normal BIER-TE forwarding occurs, taking the modified
BitString into account.
6.3. FRR in the BIER-TE Controller Host
The basic rules how the BIER-TE controller host would calculate
ResetBitMask and AddBitmask are as follows:
1. The BIER-TE controller host has to determine whether a failure of
the adjacency should be taken to indicate link or node failure.
This is a policy decision.
2. The ResetBitmask has the BitPosition of the failed adjacency.
3. In the case of link protection, the AddBitmask are the segments
forming a path from the BFR over to the BFR on the other end of
the failed link.
4. In the case of node protection, the AddBitmask are the segments
forming a tree from the BFR over to all necessary BFR downstream
of the (assumed to be failed) BFR across the failed adjacency.
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5. The ResetBitmask is extended with those segments that could lead
to duplicate packets if the AddBitmask is added to possible
BitStrings of packets using the failing BitPosition.
6.4. BIER-TE FRR Benefits
Compared to other FRR solutions, such as RSVP-TE/P2MP FRR, BIER-TE
FRR has two key distinctions
o It maintains the goal of BIER-TE not to establish in-network per
multicast traffic flow state. For that reason, the backup path/
trees are only tied to the topology but not to individual
distribution trees.
o For the case of node failure, it allows to build a path engineered
backup tree (4.) as opposed to only a set of p2p backup tunnels.
7. BIER-TE Forwarding Pseudocode
The following sections of Pseudocode are meant to illustrate the
BIER-TE forwarding plane. This code is not meant to be normative but
to serve both as a potentially easier to read and more precise
representation of the forwarding functionality and to illustrate how
simple BIER-TE forwarding is and that it can be efficiently be
implemented.
The following procedure is executed on a BFR whenever the BIFT is
changed by the BIER-TE controller host:
global MyBitsOfInterest
void BIFTChanged()
{
for (Index = 0; Index++ ; Index <= BitStringLength)
if(BIFT[Index] != <empty>)
MyBitsOfInterest != 2<<(Index-1)
}
The following procedure is executed whenever an adjacency used for
BIER-TE FRR changes state:
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global ResetBitMaskByBT[BitStringLength]
global AddtBitMaskByBT[BitStringLength]
global FRRaffectedBP
void FrrUpDown(FrrAdjacencyIndex, UpDown)
{
global FRRAdjacenciesDown
local Idx = FrrAdjacencyIndex
if (UpDown == Up)
FRRAdjacenciesDown &= ~ 2<<(FrrAdjacencyIndex-1)
else
FRRAdjacenciesDown |= 2<<(FrrAdjacencyIndex-1)
for (Index = GetFirstBitPosition(FRRAdjacenciesDown); Index ;
Index = GetNextBitPosition(FRRAdjacenciesDown, Index))
local BP = BTAFT[Index].BitPosition
FRRaffectedBP |= 2<<(Index)
ResetBitMaskByBT[BP] |= BTAFT[Index].ResetBitMask
AddBitMaskByBT[BP] |= BTAFT[Index].AddBitMask
}
The following procedure is executed whenever a BIER-TE packet is to
be forwarded:
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void ForwardBierTePacket (Packet)
{
// We calculate in BitMask the subset of BPs of the BitString
// for which we have adjacencies. This is purely an
// optimization to avoid to replicate for every BP
// set in BitString only to discover that for most of them,
// the BIFT has no adjacency.
local BitMask = Packet->BitString
Packet->BitString &= ~MyBitsOfInterest
BitMask &= MyBitsOfInterest
// FRR Operations
// Note: this algorithm is not optimal yet for ECMP cases
// it performs FRR replacement for all candidate ECMP paths
local MyFRRBP = BitMask & FRRaffectedBP
for (BP = GetFirstBitPosition(MyFRRNP); BP ;
BP = GetNextBitPosition(MyFRRNP, BP))
BitMask &= ~ResetBitMaskByBT[BP]
BitMask |= ResetBitMaskByBT[BT]
// Replication
for (Index = GetFirstBitPosition(BitMask); Index ;
Index = GetNextBitPosition(BitMask, Index))
foreach adjacency BIFT[Index]
if(adjacency == ECMP(ListOfAdjacencies, seed) )
I = ECMP_hash(sizeof(ListOfAdjacencies),
Packet->Entropy, seed)
adjacency = ListOfAdjacencies[I]
PacketCopy = Copy(Packet)
switch(adjacency)
case forward_connected(interface,neighbor,DNR):
if(DNR)
PacketCopy->BitString |= 2<<(Index-1)
SendToL2Unicast(PacketCopy,interface,neighbor)
case forward_routed([VRF],neighbor):
SendToL3(PacketCopy,[VRF,]l3-neighbor)
case local_decap([VRF],neighbor):
DecapBierHeader(PacketCopy)
PassTo(PacketCopy,[VRF,]Packet->NextProto)
}
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8. Security Considerations
The security considerations are the same as for BIER with the
following differences:
BFR-ids and BFR-prefixes are not used in BIER-TE, nor are procedures
for their distribution, so these are not attack vectors against BIER-
TE.
9. IANA Considerations
This document requests no action by IANA.
10. Acknowledgements
The author would like to thank Ijsbrand Wijnands and Neale Ranns for
their extensive review and suggestions.
11. Change log [RFC Editor: Please remove]
00: Initial version.
12. References
[I-D.wijnands-bier-architecture]
Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
S. Aldrin, "Multicast using Bit Index Explicit
Replication", draft-wijnands-bier-architecture-04 (work in
progress), February 2015.
[I-D.wijnands-mpls-bier-encapsulation]
Wijnands, I., Rosen, E., Dolganow, A., Tantsura, J., and
S. Aldrin, "Encapsulation for Bit Index Explicit
Replication in MPLS Networks", draft-wijnands-mpls-bier-
encapsulation-02 (work in progress), December 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Toerless Eckert
Cisco
Email: eckert@cisco.com
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