| Network Working Group | T. Eckert |
| Internet-Draft | Cisco Systems, Inc. |
| Intended status: Standards Track | G. Cauchie |
| Expires: April 20, 2016 | Bouygues Telecom |
| October 18, 2015 |
Traffic Enginering for Bit Index Explicit Replication BIER-TE
draft-eckert-bier-te-arch-02
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.ietf-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.
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 April 20, 2016.
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 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.
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.ietf-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:
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 bierstrings 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 appropraitely agreed upon between all BFIR/BFER.
Currently, this specification has no considerations for BIER sub-domains.
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].
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
The Multicast Flow Overlay operates as in BIER. See [I-D.ietf-bier-architecture]. Instead of interacting with the BIER layer, it interacts with the BIER-TE Controller Host
The BIER-TE controller host is representing the control plane of BIER-TE. It communicates two sets of informations with BFRs:
During bring-up or modifications of the network topology, the controller discovers the network topology, assigns BitPositions to adjacencies and signals the resulting mapping of BitPositions to adjacencies to each BFR connecting to the adjacency.
During day-to-day operations of the network, the controller signals to BFIRs what multicast flows are mapped to what BitStrings.
Communications between the BIER-TE controller host to BFRs is ideally via standardized protocols and data-models such as Netconf/Retconf/Yang. This is currently outside the scope of this document. Vendor-specific CLI on the BFRs is also a posible stopgap option (as in many other SDN solutions lacking definition of standardized data model).
For simplicity, the procedures of the BIER-TE controller host 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 host is currently outside the scope of this document.
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 SI:BitPositions to the BIFT of each BFR to which that BFR should be able to send packets to - adjacencies connecting to this BFR.
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.
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.
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.
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.
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.
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 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 (traffic 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 8 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
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).
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. The can be used for example in ring topologies as explained below.
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.
The ECMP mechanisms in BIER are tied to the BIER BIFT and are 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 anbd it introduces a "seed" parameter to allow engineering traffic 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.
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.
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 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 id desirable to support both BIER and BIER-TE forwarding in the same subdomain, then additional label 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 eg: MPLS label stacks or appropriate header extensions (eg: for IPv6).
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.
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:
-> 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.
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).
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.
Every BFER is given a unique BitPosition with a local_decap adjacency.
Leaf BFERs are BFERs where incoming BIER-TE packets never need to be forwarded to another BFR but are only sent to the BFER 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-BFER in a BIER-TE domain can share a single BitPosition. This is possible because the BitPosition for the adjacency to reach the BFER can be used to distinguish whether or not packets should reach the BFER.
This optimization will not work if an upstream interface of the BFER is using a BitPosition optimized as described in the following two sections (LAN, Hub and Spoke).
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.
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.
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 the ring BitPosition set, therefore minimizing the chance to create loops.
v v
| |
L1 | L2 | L3
/-------- BFRa ---- BFRb --------------------\
| |
\- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
| | L4 | |
p33| p15|
BFRd BFRc
Note that this example only permits for packets to enter the ring at BFRa and BFRb, and that packets will always travel clockwise. If packets should be allowed to enter the ring at any ring BFR, then one would have to use two ring BitPositions. One for clockwise, one for counterlockwise.
Both would be set up to stop rotating on the same link, eg: L1. When the ingres ring BFR creates the clockwise copy, it will reset the counterlockwise BitPosition because the DNR bit only applies to the bit for which the replication is done. Likewise for the clockwise BitPosition for the counterlockwise copy. In result, the ring ingres BFR will send a copy in both directions, serving BFRs on either side of the ring up to L1.
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
--L1-----
BFR1 --L2----- BFR2
--L3-----
BIFT entry in BFR1:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | ECMP({L1-to-BFR2,L2-to-BFR2,L3-to-BFR2}, seed) |
------------------------------------------------------------------
BIFT entry in BFR2:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | 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.
BFR1
/ \
/L11 \L12
BFR2 BFR3
/ \ / \
/L21 \L22 /L31 \L32
BFR4 BFR5 BFR6 BFR7
\ / \ /
\ / \ /
BFR8 BFR9
\ /
\ /
BFR10
BIFT entry in BFR1:
------------------------------------------------------------------
| 0:6 | ECMP({L11-to-BFR2,L12-to-BFR3}, seed) |
------------------------------------------------------------------
BIFT entry in BFR2:
------------------------------------------------------------------
| 0:6 | ECMP({L21-to-BFR4,L22-to-BFR5}, seed) |
------------------------------------------------------------------
BIFT entry in BFR3:
------------------------------------------------------------------
| 0:6 | 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.
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 the 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).
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.
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 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.
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.
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.
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 | | | | =========================================================== | 0:1 | 5 | ..0010000 | ..11000000 | ----------------------------------------------------------- ...
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
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.
The basic rules how the BIER-TE controller host would calculate ResetBitMask and AddBitmask are as follows:
Compared to other FRR solutions, such as RSVP-TE/P2MP FRR, BIER-TE FRR has two key distinctions
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:
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:
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)
}
When the number of bits required to represent the necessary hops in the topology and BFER exceeds the supported bitstring length, multiple SI and/or subdomains must be used. This section discusses how.
BIER-TE forwarding does not require the concept of BFR-id, but routing underlay, flow overlay and BIER headers may. This section also discusses how BFR-id can be assigned to BFIR/BFER for BIER-TE.
For BIER and BIER-TE forwarding, the most important result of using multiple SI and/or subdomains is the same: Packets that need to be sent to BFER in different SI or subdomains require different BIER packets: each one with a bitstring for a different (SI,subdomain) bitstring. Each such bitstring uses one bitstring legth sized SI block in the BIFT of the subdomain. We call this a BIFT:SI (block).
For BIER and BIER-TE forwarding itself there is also no difference whether different SI and/or sub-domains are choosen, but SI and subdomain have different purposes in the BIER architecture shared by BIER-TE. This impacts how operators are managing them and how especially flow overlays will likely use them.
By default, every possible BFIR/BFER in a BIER network would likey be given a BFR-id in subdomain 0 (unless there are > 64k BFIR/BFER).
If there are different flow services (or service instances) requiring replication to different subsets of BFER, then it will likely not be possible to achieve the best replication efficieny for all of these service instances via subdomain 0. Ideal replication efficiency for N BFER exists in a subdomain if they are split over not more than ceiling(N/bitstring-length) SI.
If service instances justify additional BIER:SI state in the network, additional subdomains will be used: BFIR/BFER are assigned BFIR-id in those subdomains and each service instance is configured to use the most appropriate subdomain. This results in improved replication efficiency for different services.
Even if creation of subdomains and assignment of BFR-id to BFIR/BFER in those subdomains is automated, it is not expected that individual service instances can deal with BFER in different subdomains. A service instance may only support configuration of a single subdomain it should rely on.
To be able to easily reuse (and modify as little as possible) existing BIER procedures including flow-overlay and routing underlay, when BIER-TE forwarding is added, we therefore reuse SI and subdomain logically in the same way as they are used in BIER: All necessary BFIR/BFER for a service use a single BIER-TE BIFT and are split across as many SI as necessary (see below). Different services may use different subdomains that primarily exist to provide more efficient replication (and for BIER-TE desirable traffic engineering) for different subsets of BFIR/BFER.
In BIER, bitstrings only need to carry bits for BFER, which lead to the model that BFR-ids map 1:1 to each bit in a bitstring.
In BIER-TE, bitstrings need to carry bits to indicate not only the receiving BFER but also the intermediate hops/links across which the packet must be sent. The maximum number of BFER that can be supported in a single bitstring or BIFT:SI depends on the number of bits necessary to represent the desired topology between them.
"Desired" topology because it depends on the physical topology, and on the desire of the operator to allow for explicit traffic engineering across every single hop (which requires more bits), or reducing the number of required bits by exploiting optimizations such as unicast (forward_route), ECMP or flood (DNR) over "uninteresting" sub-parts of the topology - eg: parts where different trees do not need to take different paths due to traffic-engineering reasons.
The total number of bits to describe the topology in a BIFT:SI can therefore easily be as low as 20% or as high as 80%. The higher the percentage, the higher the likelyhood, that those topology bits are not just BIER-TE overhead without additional benefit, but instead they will allow to express the desired traffic-engineering alternatives.
Because there is no 1:1 mapping between bits in the bitstring and BFER, BIER-TE can not simply rely on the BIER 1:1 mapping between bits in a bitstring and BFR-id.
In BIER, automatic schemes could assign all possible BFR-ids sequentially to BFERs. This will not work in BIER-TE. In BIER-TE, the operator or BIER-TE controller host has to determine a BFR-id for each BFER in each required subdomain. The BFR-id may or may not have a relationship with a bit in the bitstring. Suggestions are detailled below. Once determined, the BFR-id can then be configured on the BFER and used by flow overlay, routing underlay and the BIER header almost the same as the BFR-id in BIER.
The one exception are application/flow-overlays that automatically calculate the bitstring(s) of BIER packets by converting BFR-id to bits. In BIER-TE, this operation can be done in two ways:
"Independent branches": For a given application or (set of) trees, the branches from a BFIR to every BFER are independent of the branches to any other BFER. For example, shortest part trees have independent branches.
"Interdependent braches": When a BFER is added or deleted from a particular distribution tree, branches to other BFER still in the tree may need to change. Steiner tree are examples of dependent branch trees.
If "independent branches" are sufficient, the BIER-TE controller host can provide to such applications for every BFR-id a SI:bitstring with the BIER-TE bits for the branch towards that BFER. The application can then independently calculate the SI:bitstring for all desired BFER by OR'ing their bitstrings.
If "interdependent branches" are required, the application could call a BIER-TE controller host API with the list of required BFER-id and get the required bitstring back. Whenever the set of BFER-id changes, this is repeated.
Note that in either case (unlike in BIER), the bits in BIER-TE may need to change upon link/node failure/recovery, network expansion and network load by other traffic (as part of traffic engineering goals). Interactions between such BFIR applications and the BIER-TE controller host do therefore need to support dynamic updates to the bitstrings.
For non-leaf BFER, there is usually a single bit k for that BFER with a local_decap() adjacency on the BFER. The BFR-id for such a BFER is therefore most easily the one it would have in BIER: SI * bitstring-length + k.
As explained earlier in the document, leaf BFER do not need such a separate bit because the fact alone that the BIER-TE packet is forwarded to the leaf BFER indicates that the BFER should decapsulate it. Such a BFER will have one or more bits for the links leading only to it. The BFR-id could therefore most easily be the BFR-id derived from the lowest bit for those links.
These two rules are only recommendations for the operator or BIER-TE controller assigning the BFR-ids. Any allocation scheme can be used, the BFR-ids just need to be unique across BFRs in each subdomain.
It is not currently determined if a single subdomain could or should be allowed to forward both BIER and BIER-TE packets. If this should be supported, there are two options:
A. BIER and BIER-TE have different BFR-id in the same subdomain. This allows higher replication efficiency for BIER because their BFR-id can be assigned sequentially, while the bitstrings for BIER-TE will have also the additional bits for the topology. There is no relationship between a BFR BIER BFR-id and BIER-TE BFR-id.
B. BIER and BIER-TE share the same BFR-id. The BFR-id are assigned as explained above for BIER-TE and simply reused for BIER. The replication efficiency for BIER will be as low as that for BIER-TE in this approach. Depending on topology, only the same 20%..80% of bits as possible for BIER-TE can be used for BIER.
Consider a network setup with a bitstring length of 256 for a network topology as shown in the picture below. The network has 6 areas, each with ca. 180 BFR, connecting via a core with some larger (core) BFR. To address all BFER with BIER, 4 SI are required. To send a BIER packet to all BFER in the network, 4 copies need to be sent by the BFIR. On the BFIR it does not make a difference how the BFR-id are allocated to BFER in the network, but for efficiency further down in the network it does make a difference.
area1 area2 area3
BFR1a BFR1b BFR2a BFR2b BFR3a BFR3b
| \ / \ / |
................................
. Core .
................................
| / \ / \ |
BFR4a BFR4b BFR5a BFR5b BFR6a BFR6b
area4 area5 area6
With randomn allocation of BFR-id to BFER, each receiving area would (most likely) have to receive all 4 copies of the BIER packet because there would be BFR-id for each of the 4 SI in each of the areas. Only further towards each BFER would this duplication subside - when each of the 4 trees runs out of branches.
If BFR-id are allocated intelligently, then all the BFER in an area would be given BFR-id with as few as possible different SI. Each area would only have to forward one or two packets instead of 4.
Given how networks can grow over time, replication efficiency in an area will also easily go down over time when BFR-id are network wide allocated sequentially over time. An area that initially only has BFR-id in one SI might end up with many SI over a longer period of growth. Allocating SIs to areas with initially sufficienctly many spare bits for growths can help to aleviate this issue. Or renumber BFR-id after network expansion. In this example one may consider to use 6 SI and assign one to each area.
This example shows that intelligent BFR-id allocation within at least subdomain 0 can even be helpfull or even necessary in BIER.
In BIER-TE one needs to determine a subset of the physical topology and attached BFER so that the "desired" representation of this topology and the BFER fit into a single bitstring. This process needs to be repeated intil the whole topology is covered.
Once bits/SIs are assigned to topology and BFER, BFR-id is just a derived set of identifiers from the operator/BIER-TE controller as explained above.
Every time that different sub-topologies have overlap, bits need to be repeated across the bitstrings, increasing the overall amount of bits required across all bitstring/SIs. In the worst case, randomn subsets of BFER are assigned to different SI. This is much worse than in BIER because it not only reduces replication efficiency with the same number of overall bits, but even further - because more bits are required due to duplication of bits for topology across multiple SI. Intelligent BFER to SI assignment and selecting specific "desired" subtopologies can minimize this problem.
To set up BIER-TE efficiently for above topology, the following bit allocation methods can be used. This method can easily be expanded to other, similarily structured larger topologies.
Each area is allocated one or more SI depending on the number of future expected BFER and number of bits required for the topology in the area. In this example, 6 SI, one per area.
In addition, we use 4 bits in each SI: bia, bib, bea, beb: bit ingres a, bit ingres b, bit egres a, bit egres b. These bits will be used to pass BIER packets from any BFIR via any combination of ingres area a/b BFR and egres area a/b BFR into a specific target area. These bits are then set up with the right forward_routed adjacencies on the BFIR and area edge BFR:
On all BFIR in an area j, bia in each BIFT:SI is populated with the same forward_routed(BFRja), and bib with forward_routed(BFRjb). On all area edge BFR, bea in BIFT:SI=k is populated with forward_routed(BFRka) and beb in BIFT:SI=k with forward_routed(BFRkb).
For BIER-TE forwarding of a packet to some subset of BFER across all areas, a BFIR would create at most 6 copies, with SI=1...SI=6, In each packet, the bits indicate bits for topology and BFER in that topology plus the four bits to indicate whether to pass this packet via the ingres area a or b border BFR and the egres area a or b border BFR, therefore allowing path engineering for those two "unicast" legs: 1) BFIR to ingres are edge and 2) core to egres area edge. Replication only happens inside the egres areas. For BFER in the same area as in the BFIR, these four bits are not used.
BIER-TE can like BIER support multiple SI within a sub-domain to allow re-using the concept of BFR-id and therefore minimize BIER-TE specific functions in underlay routing, flow overlay methods and BIER headers.
The number of BFIR/BFER possible in a subdomain is smaller than in BIER because BIER-TE uses additional bits for topology.
Subdomains can in BIER-TE be used like in BIER to create more efficient replication to known subsets of BFER.
Assigning bits for BFER intelligently into the right SI is more important in BIER-TE than in BIER because of replication efficiency and overall amount of bits required.
BIER-TE as described above is an advanced method for mode-protection where the replication in a failed node is on the fly replaced by another replication tree through bit operations on the BitString.
If BIER-TE is not feasible or necessary, it is also possible for BIER-TE to leverage any existing form of "link" protection. For example: instead of dorectly setting up a forward_connected adjacency to a next-hop neighbor, this can be a "protected" adjacency that is maintained by RSVP-TE (or another FRR mechanism) and passes via a backup path if the link fails.
Segment Routing aims to achieve lightweight path engineering via loose source routing. Compared for example to RSVP-TE, it does not require per-path signaling to each of these hops.
BIER-TE is supports the same design philosophy for multicast. Like in SR, it relies on source-routing - via the definition of a BitString. Like SR, it only requires to consider the "hops" on which either replication has to happen, or across which the traffic should be steered (even without replication). Any other hops can be skipped via the use of routed adjacencies.
Instead of defining BitPositions for non-replicating hops, it is equally possible to use segment routing encapsulations (eg: MPLS label stacks) for "forward_routed" adjacencies.
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.
This document requests no action by IANA.
The authors would like to thank Greg Shepherd, Ijsbrand Wijnands and Neale Ranns for their extensive review and suggestions.
| [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-02, July 2015. |
| [I-D.ietf-bier-mpls-encapsulation] | Wijnands, I., Rosen, E., Dolganow, A., Tantsura, J. and S. Aldrin, "Encapsulation for Bit Index Explicit Replication in MPLS Networks", Internet-Draft draft-ietf-bier-mpls-encapsulation-02, August 2015. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |