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IETF RFC 8401
Bit Index Explicit Replication (BIER) Support via IS-IS
Last modified on Thursday, June 7th, 2018
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Internet Engineering Task Force (IETF) L. Ginsberg, Ed.
Request for Comments: 8401 Cisco Systems
Category: Standards Track A. Przygienda
ISSN: 2070-1721 Juniper Networks
S. Aldrin
Google
J. Zhang
Juniper Networks, Inc.
June 2018
Bit Index Explicit Replication (BIER) Support via IS-IS
 Abstract
This document defines IS-IS extensions to support multicast
forwarding using the Bit Index Explicit Replication (BIER)
architecture.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 8401.
Copyright Notice
Copyright (c) 2018 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.
Ginsberg, et al. Standards Track PAGE 1 
RFC 8401 BIER Support via IS-IS June 2018
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. BIER Domains and Subdomains . . . . . . . . . . . . . . . 5
4.2. Advertising BIER Information . . . . . . . . . . . . . . 5
5. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Multi-Topology and Subdomain . . . . . . . . . . . . . . 5
5.2. BFR-id Advertisements . . . . . . . . . . . . . . . . . . 6
5.3. Logging Misconfiguration . . . . . . . . . . . . . . . . 6
5.4. Flooding Reduction . . . . . . . . . . . . . . . . . . . 6
6. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. BIER Info Sub-TLV . . . . . . . . . . . . . . . . . . . . 7
6.2. BIER MPLS Encapsulation Sub-sub-TLV . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Bit Index Explicit Replication (BIER) [RFC 8279] defines an
architecture where all intended multicast receivers are encoded as a
bitmask in the multicast packet header within different
encapsulations such as described in [RFC 8296]. A router that
receives such a packet will forward the packet based on the bit
position in the packet header towards the receiver(s) following a
precomputed tree for each of the bits in the packet. Each receiver
is represented by a unique bit in the bitmask.
This document presents necessary extensions to the currently deployed
IS-IS for IP [RFC 1195] to support distribution of information
necessary for operation of BIER domains and subdomains. This
document defines a new TLV to be advertised by every router
participating in BIER signaling.
This document defines support for MPLS encapsulation as specified in
[RFC 8296]. Support for other encapsulation types and the use of
multiple encapsulation types are outside the scope of this document.
Ginsberg, et al. Standards Track PAGE 2
RFC 8401 BIER Support via IS-IS June 2018
2. Terminology
Some of the terminology specified in [RFC 8279] is replicated here and
extended by necessary definitions:
BIER: Bit Index Explicit Replication. The overall architecture of
forwarding multicast using a bit position.
BIER-OL: BIER Overlay Signaling. The method for the BFIR to learn
about BFERs.
BFR: Bit Forwarding Router. A router that participates in Bit Index
Multipoint Forwarding. A BFR is identified by a unique BFR-prefix
in a BIER domain.
BFIR: Bit Forwarding Ingress Router. The ingress border router that
inserts the BitString into the packet. Each BFIR must have a
valid BFR-id assigned.
BFER: Bit Forwarding Egress Router. A router that participates in
Bit Index Forwarding as a leaf. Each BFER must be a BFR. Each
BFER must have a valid BFR-id assigned.
BFT: Bit Forwarding Tree used to reach all BFERs in a domain.
BIER subdomain: A further distinction within a BIER domain
identified by its unique subdomain identifier. A BIER subdomain
can support multiple BitString Lengths.
BFR-id: An optional, unique identifier for a BFR within a BIER
subdomain.
Invalid BFR-id: Unassigned BFR-id. The special value 0 is reserved
for this purpose.
BAR: BIER Algorithm. Used to calculate underlay next hops.
IPA: IGP Algorithm. May be used to modify, enhance, or replace the
calculation of underlay paths as defined by the BAR value.
SPF: Shortest Path First routing calculation based on the IGP link
metric.
Ginsberg, et al. Standards Track PAGE 3
RFC 8401 BIER Support via IS-IS June 2018
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
3. IANA Considerations
This document adds the following entry to the "Sub-TLVs for TLVs 135,
235, 236, and 237" registry.
Value: 32
Name: BIER Info
This document also introduces a new registry for sub-sub-TLVs for the
BIER Info sub-TLV. The registration policy is Expert Review as
defined in [RFC 8126]. The "Sub-sub-TLVs for BIER Info Sub-TLV" has
been created within the "IS-IS TLV Codepoints" registry. The defined
value is as follows:
Type Name
---- ----
1 BIER MPLS Encapsulation
IANA has created the "BIER Algorithms" registry within the "Bit Index
Explicit Replication (BIER)" registry. The registration policies
[RFC 8126] for this registry are:
"Standards Action" for values 0-127
"Specification Required" for values 128-239
"Experimental Use" for values 240-254
The initial values in the "BIER Algorithms" registry are:
0: No BIER-specific algorithm is used
255: Reserved
Ginsberg, et al. Standards Track PAGE 4
RFC 8401 BIER Support via IS-IS June 2018
4. Concepts
4.1. BIER Domains and Subdomains
An IS-IS-signaled BIER domain is aligned with the scope of
distribution of BFR-prefixes that identify the BFRs within IS-IS. In
such a case, IS-IS acts as the supporting BIER underlay.
Within such a domain, the extensions defined in this document
advertise BIER information for one or more BIER subdomains. Each
subdomain is uniquely identified by a subdomain-id (SD). Each
subdomain is associated with a single IS-IS topology (MT) [RFC 5120],
which may be any of the topologies supported by IS-IS. Local
configuration controls which <MT,SD> pairs are supported by a router.
The mapping of subdomains to topologies MUST be consistent within the
IS-IS flooding domain used to advertise BIER information.
Each BIER subdomain has as its unique attributes the encapsulation
used and the type of tree it uses to forward BIER frames (currently
always SPF). Additionally, per supported BitString length in the
subdomain, each router will advertise the necessary label ranges to
support it.
4.2. Advertising BIER Information
BIER information advertisements are associated with a new sub-TLV in
the extended reachability TLVs. BIER information is always
associated with a host prefix, which MUST be a node address for the
advertising node. If this is not the case, the advertisement MUST be
ignored. Therefore, the following restrictions apply:
o Prefix length MUST be 32 for an IPv4 prefix or 128 for an IPv6
prefix.
o When the Prefix Attributes Flags sub-TLV [RFC 7794] is present, the
N flag MUST be set and the R flag MUST NOT be set.
o BIER sub-TLVs MUST be included when a prefix reachability
advertisement is leaked between levels.
5. Procedures
5.1. Multi-Topology and Subdomain
A given subdomain is supported within one and only one topology. All
routers in the flooding scope of the BIER sub-TLVs MUST advertise the
same subdomain within the same multi-topology. A router receiving an
<MT,SD> advertisement that does not match the locally configured pair
Ginsberg, et al. Standards Track PAGE 5
RFC 8401 BIER Support via IS-IS June 2018
MUST report a misconfiguration of the received <MT,SD> pair. All
received BIER advertisements associated with the conflicting <MT,SD>
pair MUST be ignored. Note that in the presence of such a
misconfiguration, this will lead to partitioning of the subdomain.
Example:
The following combination of advertisements are valid: <0,0> <0,1>,
and <2,2>.
The following combination of advertisements are invalid: <0,0> <0,1>,
and <2,0>. Advertisements associated with <0,0> and <2,0> must be
ignored.
5.2. BFR-id Advertisements
If a BFER/BFIR is configured with a BFR-id, then it advertises this
value in its BIER advertisements. If no BFR-id is configured, then
the value "Invalid BFR-id" is advertised. A valid BFR-id MUST be
unique within the flooding scope of the BIER advertisements. All
BFERs/BFIRs MUST detect advertisement of duplicate valid BFR-IDs for
a given <MT,SD>. When such duplication is detected, all of the
routers advertising duplicates MUST be treated as if they did not
advertise a valid BFR-id. This implies they cannot act as BFER or
BFIR in that <MT,SD>.
5.3. Logging Misconfiguration
Whenever an advertisement is received that violates any of the
constraints defined in this document, the receiving router MUST
support logging this occurrence. Logging SHOULD be dampened to avoid
excessive output.
5.4. Flooding Reduction
It is expected that changes in the BIER domain information that is
advertised by IS-IS occur infrequently. If this expectation is not
met for an extended period of time (more than a few seconds of
burstiness), changes will increase the number of Link State PDU (LSP)
updates and negatively impact performance in the network.
Implementations SHOULD protect against this possibility by, for
example, dampening updates if they occur over an extended period of
time.
Ginsberg, et al. Standards Track PAGE 6
RFC 8401 BIER Support via IS-IS June 2018
6. Packet Formats
All IS-IS BIER information is carried within the TLVs 235, 237,
[RFC 5120], 135 [RFC 5305], or 236 [RFC 5308].
6.1. BIER Info Sub-TLV
This sub-TLV carries the information for the BIER subdomains that the
router participates in as a BFR. This sub-TLV MAY appear multiple
times in a given prefix-reachability TLV -- once for each subdomain
supported in the associated topology.
The sub-TLV advertises a single <MT,SD> combination followed by
optional sub-sub-TLVs as described in the following sections.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BAR | IPA | subdomain-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BFR-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-sub-TLVs (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: As indicated in the IANA section.
Length: Variable
BAR: BIER Algorithm. Specifies a BIER-specific algorithm used to
calculate underlay paths to reach BFERs. Values are allocated
from the "BIER Algorithms" registry. 1 octet.
IPA: IGP Algorithm. Specifies an IGP Algorithm to either modify,
enhance, or replace the calculation of underlay paths to reach
BFERs as defined by the BAR value. Values are from the IGP
Algorithm registry. 1 octet.
subdomain-id: Unique value identifying the BIER subdomain. 1 octet.
BFR-id: A 2-octet field encoding the BFR-id, as documented in
[RFC 8279]. If no BFR-id has been assigned, the value of this
field is set to "Invalid BFR-id", which is defined as illegal in
[RFC 8279].
Ginsberg, et al. Standards Track PAGE 7
RFC 8401 BIER Support via IS-IS June 2018
The use of non-zero values in either the BAR field or the IPA field
is outside the scope of this document. If an implementation does not
support the use of non-zero values in these fields but receives a
BIER Info sub-TLV containing non-zero values in these fields, it
SHOULD treat the advertising router as incapable of supporting BIER
(one way of handling incapable routers is documented in Section 6.9
of [RFC 8279] and additional methods may be defined in the future).
6.2. BIER MPLS Encapsulation Sub-sub-TLV
This sub-sub-TLV carries the information for the BIER MPLS
encapsulation including the label range for a specific BitString
length for a certain <MT,SD>. It is advertised within the BIER Info
sub-TLV (Section 6.1). This sub-sub-TLV MAY appear multiple times
within a single BIER Info sub-TLV.
If the same BitString length is repeated in multiple sub-sub-TLVs
inside the same BIER Info sub-TLV, the BIER Info sub-TLV MUST be
ignored.
Label ranges within all BIER MPLS Encapsulation sub-sub-TLVs across
all BIER Info sub-TLVs advertised by the same BFR MUST NOT overlap.
If overlap is detected, the advertising router MUST be treated as if
it did not advertise any BIER sub-TLVs.
Label values MUST NOT match any of the reserved values defined in
[RFC 3032].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max SI |BS Len | Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: Value of 1 indicating MPLS encapsulation.
Length: 4
Max SI: Maximum Set Identifier (Section 1 of [RFC 8279]) used in the
encapsulation for this BIER subdomain for this BitString length, 1
octet. Each SI maps to a single label in the label range. The
first label is for SI=0, the second label is for SI=1, etc. If
the label associated with the Maximum Set Identifier exceeds the
20-bit range, the sub-sub-TLV MUST be ignored.
Ginsberg, et al. Standards Track PAGE 8
RFC 8401 BIER Support via IS-IS June 2018
Local BitString Length (BS Len): Encoded BitString length as per
[RFC 8296]. 4 bits.
Label: First label of the range, 20 bits. The labels are as defined
in [RFC 8296].
7. Security Considerations
Security concerns for IS-IS are addressed in [RFC 5304] and [RFC 5310].
The Security Considerations section of [RFC 8279] discusses the
possibility of performing a Denial-of-Service (DoS) attack by setting
too many bits in the BitString of a BIER-encapsulated packet.
However, this sort of DoS attack cannot be initiated by modifying the
IS-IS BIER advertisements specified in this document. A BFIR decides
which systems are to receive a BIER-encapsulated packet. In making
this decision, it is not influenced by the IS-IS control messages.
When creating the encapsulation, the BFIR sets one bit in the
encapsulation for each destination system. The information in the
IS-IS BIER advertisements is used to construct the forwarding tables
that map each bit in the encapsulation into a set of next hops for
the host that is identified by that bit, but it is not used by the
BFIR to decide which bits to set. Hence, an attack on the IS-IS
control plane cannot be used to cause this sort of DoS attack.
While a BIER-encapsulated packet is traversing the network, a BFR
that receives a BIER-encapsulated packet with n bits set in its
BitString may have to replicate the packet and forward multiple
copies. However, a given bit will only be set in one copy of the
packet. This means that each transmitted replica of a received
packet has fewer bits set (i.e., is targeted to fewer destinations)
than the received packet. This is an essential property of the BIER-
forwarding process as defined in [RFC 8279]. While a failure of this
process might cause a DoS attack (as discussed in the Security
Considerations of [RFC 8279]), such a failure cannot be caused by an
attack on the IS-IS control plane.
Further discussion of BIER-specific security considerations can be
found in [RFC 8279].
Ginsberg, et al. Standards Track PAGE 9
RFC 8401 BIER Support via IS-IS June 2018
8. References
8.1. Normative References
[RFC 1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC 1195,
December 1990, <https://www.rfc-editor.org/info/RFC 1195>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC 3032, January 2001,
<https://www.rfc-editor.org/info/RFC 3032>.
[RFC 5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC 5120, February 2008,
<https://www.rfc-editor.org/info/RFC 5120>.
[RFC 5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC 5304, October
2008, <https://www.rfc-editor.org/info/RFC 5304>.
[RFC 5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC 5305, October
2008, <https://www.rfc-editor.org/info/RFC 5305>.
[RFC 5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC 5308, October 2008,
<https://www.rfc-editor.org/info/RFC 5308>.
[RFC 5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC 5310, February
2009, <https://www.rfc-editor.org/info/RFC 5310>.
[RFC 7794] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC 7794,
March 2016, <https://www.rfc-editor.org/info/RFC 7794>.
Ginsberg, et al. Standards Track PAGE 10
RFC 8401 BIER Support via IS-IS June 2018
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC 8279, November 2017,
<https://www.rfc-editor.org/info/RFC 8279>.
[RFC 8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in MPLS and Non-
MPLS Networks", RFC 8296, DOI 10.17487/RFC 8296, January
2018, <https://www.rfc-editor.org/info/RFC 8296>.
8.2. Informative References
[OPSFv2BIER]
Psenak, P., Kumar, N., Wijnands, I., Dolganow, A.,
Przygienda, T., Zhang, Z., and S. Aldrin, "OSPFv2
Extensions for BIER", Work in Progress, draft-ietf-bier-
ospf-bier-extensions-18, June 2018.
[RFC 8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC 8126, June 2017,
<https://www.rfc-editor.org/info/RFC 8126>.
Acknowledgements
This RFC is aligned with "OSPFv2 Extensions for BIER" [OPSFv2BIER]
document as far as the protocol mechanisms overlap.
Many thanks for comments from (in no particular order) Hannes
Gredler, IJsbrand Wijnands, Peter Psenak, and Chris Bowers.
Special thanks to Eric Rosen.
Ginsberg, et al. Standards Track PAGE 11
RFC 8401 BIER Support via IS-IS June 2018
Authors' Addresses
Les Ginsberg (editor)
Cisco Systems
510 McCarthy Blvd.
Milpitas, CA 95035
United States of America
Email: ginsberg@cisco.com
Tony Przygienda
Juniper Networks
Email: prz@juniper.net
Sam Aldrin
Google
1600 Amphitheatre Parkway
Mountain View, CA
United States of America
Email: aldrin.ietf@gmail.com
Jeffrey (Zhaohui) Zhang
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
United States of America
Email: zzhang@juniper.net
Ginsberg, et al. Standards Track PAGE 12
Bit Index Explicit Replication (BIER) Support via IS-IS
RFC TOTAL SIZE: 24287 bytes
PUBLICATION DATE: Thursday, June 7th, 2018
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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