BIER OSPFv2: OSPFv2 Extensions for Bit Index Explicit Replication (BIER)
How OSPFv2 routers tell each other the information BIER multicast forwarding needs, in MPLS networks.
Psenak, P., Ed., Kumar, N., Wijnands, IJ., Dolganow, A., Przygienda, T., Zhang, J., and S. Aldrin, "OSPFv2 Extensions for Bit Index Explicit Replication (BIER)", RFC 8444, DOI 10.17487/RFC8444, November 2018, <https://www.rfc-editor.org/info/rfc8444>.
@misc{rfc8444,
series = {Request for Comments},
number = 8444,
howpublished = {RFC 8444},
publisher = {RFC Editor},
doi = {10.17487/RFC8444},
url = {https://www.rfc-editor.org/info/rfc8444},
author = {P. Psenak and N. Kumar and IJ. Wijnands and A. Dolganow and T. Przygienda and J. Zhang and S. Aldrin},
title = {{OSPFv2 Extensions for Bit Index Explicit Replication (BIER)}},
pagetotal = 12,
year = 2018,
month = nov,
} RFC text
Contents
Internet Engineering Task Force (IETF) P. Psenak, Ed.
Request for Comments: 8444 N. Kumar
Category: Standards Track IJ. Wijnands
ISSN: 2070-1721 Cisco
A. Dolganow
Nokia
T. Przygienda
J. Zhang
Juniper Networks, Inc.
S. Aldrin
Google, Inc.
November 2018
OSPFv2 Extensions for Bit Index Explicit Replication (BIER)
Abstract
Bit Index Explicit Replication (BIER) is an architecture that
provides optimal multicast forwarding through a "BIER domain" without
requiring intermediate routers to maintain multicast-related, per-
flow state. BIER also does not require an explicit tree-building
protocol for its operation. A multicast data packet enters a BIER
domain at a Bit-Forwarding Ingress Router (BFIR) and leaves the BIER
domain at one or more Bit-Forwarding Egress Routers (BFERs). The
BFIR adds a BIER packet header to the packet. The BIER packet header
contains a BitString in which each bit represents exactly one BFER to
forward the packet to. The set of BFERs to which the multicast
packet needs to be forwarded is expressed by the set of bits in the
BIER packet header.
This document describes the OSPF protocol extension (from RFC 2328)
that is required for BIER with MPLS encapsulation (which is defined
in RFC 8296). Support for other encapsulation types and the use of
multiple encapsulation types are outside the scope of this document.
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/rfc8444.
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.
Table of Contents
1. Introduction ....................................................3
2. Flooding of the BIER Information in OSPF ........................4
2.1. BIER Sub-TLV ...............................................4
2.2. BIER MPLS Encapsulation Sub-TLV ............................5
2.3. Flooding Scope of BIER Information .........................7
3. Security Considerations .........................................8
4. IANA Considerations .............................................9
5. References ......................................................9
5.1. Normative References .......................................9
5.2. Informative References ....................................10
Acknowledgments ...................................................11
Authors' Addresses ................................................11
1. Introduction
Bit Index Explicit Replication (BIER) is an architecture that
provides optimal multicast forwarding through a "BIER domain" without
requiring intermediate routers to maintain any multicast-related,
per-flow state. Neither does BIER explicitly require a tree-building
protocol for its operation. A multicast data packet enters a BIER
domain at a Bit-Forwarding Ingress Router (BFIR) and leaves the BIER
domain at one or more Bit-Forwarding Egress Routers (BFERs). The
BFIR router adds a BIER packet header to the packet. The BIER packet
header contains a BitString in which each bit represents exactly one
BFER to forward the packet to. The set of BFERs to which the
multicast packet needs to be forwarded is expressed by the set of
bits in the BIER packet header.
The BIER architecture requires routers participating in BIER to
exchange BIER-related information within a given domain and permits
link-state routing protocols to perform distribution of such
information. This document describes extensions to OSPF necessary to
advertise BIER-specific information in the case where BIER uses MPLS
encapsulation as described in [RFC8296].
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Flooding of the BIER Information in OSPF
All BIER-specific information that a Bit-Forwarding Router (BFR)
needs to advertise to other BFRs is associated with a BFR-prefix. A
BFR-prefix is a unique (within a given BIER domain) routable IP
address that is assigned to each BFR as described in detail in
Section 2 of [RFC8279].
Given that BIER information must be associated with a BFR-prefix, the
OSPFv2 Extended Prefix Opaque LSA [RFC7684] has been chosen for
advertisement.
2.1. BIER Sub-TLV
A sub-TLV of the OSPFv2 Extended Prefix TLV (defined in [RFC7684]) is
defined for distributing BIER information. The sub-TLV is called the
BIER Sub-TLV. Multiple BIER Sub-TLVs may be included in the OSPFv2
Extended Prefix TLV.
The BIER Sub-TLV has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-domain-id | MT-ID | BFR-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BAR | IPA | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs (variable) |
+- -+
| |
Type: 9
Length: Variable, dependent on sub-TLVs.
sub-domain-id: Unique value identifying the BIER sub-domain within
the BIER domain, as described in Section 1 of [RFC8279].
MT-ID: Multi-Topology ID (as defined in [RFC4915]) that identifies
the topology that is associated with the BIER sub-domain.
BFR-id: A 2-octet field encoding the BFR-id, as documented in
Section 2 of [RFC8279]. If the BFR is not locally configured with
a valid BFR-id, the value of this field is set to 0, which is
defined as illegal in [RFC8279].
BAR: Single-octet BIER Algorithm used to calculate underlay paths to
reach other BFRs. Values are allocated from the "BIER Algorithm"
registry defined in [RFC8401].
IPA: Single-octet IGP Algorithm used to either modify, enhance, or
replace the calculation of underlay paths to reach other BFRs as
defined by the BAR value. Values are defined in the "IGP
Algorithm Types" registry [IANA-IGP].
Each BFR sub-domain MUST be associated with one and only one OSPF
topology that is identified by the MT-ID. If the association between
the BIER sub-domain and OSPF topology advertised in the BIER Sub-TLV
by other BFRs is in conflict with the association locally configured
on the receiving router, the BIER Sub-TLV for such conflicting sub-
domains MUST be ignored.
If the MT-ID contains an invalid value as specified in [RFC4915], the
BIER Sub-TLV for such subdomains with conflict MUST be ignored.
If a BFR advertises the same sub-domain-id in multiple BIER Sub-TLVs,
the BFR MUST be treated as if it did not advertise a BIER Sub-TLV for
such sub-domain.
All BFRs MUST detect advertisement of duplicate valid BFR-ids for a
given MT-ID and sub-domain-id. When such duplication is detected by
the BFR, it MUST behave as described in Section 5 of [RFC8279].
The supported BAR and IPA algorithms MUST be consistent for all
routers supporting a given BFR sub-domain. If a router receives a
BIER Sub-TLV advertisement with a value in the BAR or IPA fields that
does not match the locally configured value for a given BFR sub-
domain, the router MUST report a misconfiguration for such BIER sub-
domain and MUST ignore the BIER Sub-TLV containing the error.
The use of non-zero values in either the BAR field or the IPA field
is outside the scope of this document.
2.2. BIER MPLS Encapsulation Sub-TLV
The BIER MPLS Encapsulation Sub-TLV is a sub-TLV of the BIER Sub-TLV.
The BIER MPLS Encapsulation Sub-TLV is used in order to advertise
MPLS-specific information used for BIER. It MAY appear multiple
times in the BIER Sub-TLV.
The BIER MPLS Encapsulation Sub-TLV has the following format:
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 | Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|BS Len | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 10
Length: 8 octets
Max SI: A 1-octet field encoding the maximum Set Identifier (SI)
(see Section 1 of [RFC8279]) used in the encapsulation for this
BIER sub-domain for this BitString length.
Label: A 3-octet field, where the 20 rightmost bits represent the
first label in the label range. The 4 leftmost bits MUST be
ignored.
BS Len (BitString Length): A 4-bit field encoding the supported
BitString length associated with this BFR-prefix. The values
allowed in this field are specified in Section 2 of [RFC8296].
Reserved: SHOULD be set to 0 on transmission and MUST be ignored on
reception.
The "label range" is the set of labels beginning with the Label and
ending with (Label + (Max SI)). A unique label range is allocated
for each BitString length and sub-domain-id. These labels are used
for BIER forwarding as described in [RFC8279] and [RFC8296].
The size of the label range is determined by the number of SIs
(Section 1 of [RFC8279]) that are used in the network. 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 BIER MPLS Encapsulation Sub-TLV containing the
error MUST be ignored.
If the BitString length is set to a value that does not match any of
the allowed values specified in [RFC8296], the BIER MPLS
Encapsulation Sub-TLV containing the error MUST be ignored.
If the same BitString length is repeated in multiple BIER MPLS
Encapsulation Sub-TLVs inside the same BIER Sub-TLV, the whole BIER
Sub-TLV containing the conflicts MUST be ignored.
Label ranges within all BIER MPLS Encapsulation Sub-TLVs advertised
by the same BFR MUST NOT overlap. If an overlap is detected, all
BIER sub-TLVs advertised by such a router MUST be ignored.
2.3. Flooding Scope of BIER Information
The flooding scope of the OSPFv2 Extended Prefix Opaque LSA [RFC7684]
that is used for advertising the BIER Sub-TLV is set to area-local.
To allow BIER deployment in a multi-area environment, OSPF must
propagate BIER information between areas.
( ) ( ) ( )
( ) ( ) ( )
R1 Area 1 R2 Area 0 R3 Area 2 R4
( ) ( ) ( )
( ) ( ) ( )
Figure 1: BIER Propagation between Areas
The following procedure is used in order to propagate BIER-related
information between areas:
When an OSPF Area Border Router (ABR) advertises a Type-3 Summary
LSA from an intra-area or inter-area prefix to all its attached
areas, it will also originate an OSPFv2 Extended Prefix Opaque
LSA, as described in [RFC7684]. The flooding scope of the OSPFv2
Extended Prefix Opaque LSA type will be set to area-local. The
route-type in the OSPFv2 Extended Prefix TLV is set to inter-area.
When determining whether a BIER Sub-TLV should be included in this
LSA, an OSPF ABR will:
* Examine its best path to the prefix in the source area and find
the advertising router associated with the best path to that
prefix.
* Determine if the advertising router advertised a BIER Sub-TLV
for the prefix. If yes, the ABR will copy the information from
that BIER Sub-TLV when advertising the BIER Sub-TLV to each
attached area.
In Figure 1, R1 advertises a prefix 192.0.2.1/32 in Area 1. It
also advertises an OSPFv2 Extended Prefix Opaque LSA for prefix
192.0.2.1/32 and includes a BIER Sub-TLV in it. ABR R2 calculates
the reachability for prefix 192.0.2.1/32 inside Area 1 and
propagates it to Area 0. When doing so, it copies the entire BIER
Sub-TLV (including all of its Sub-TLVs) that it received from R1
in Area 1 and includes it in the OSPFv2 Extended Prefix Opaque LSA
it generates for 192.0.2.1/32 in Area 0. ABR R3 calculates the
reachability for prefix 192.0.2.1/32 inside Area 0 and propagates
it to Area 2. When doing so, it copies the entire BIER Sub-TLV
(including all of its sub-TLVs) that it received from R2 in Area 0
and includes it in the OSPFv2 Extended Prefix Opaque LSA it
generates for 192.0.2.1/32 in Area 2.
3. Security Considerations
This document introduces new sub-TLVs for the existing OSPFv2
Extended Prefix TLV. It does not introduce any new security risks to
OSPF. Existing security extensions as described in [RFC2328] and
[RFC7684] apply.
It is assumed that both the BIER and OSPF layers are under a single
administrative domain. There can be deployments where potential
attackers have access to one or more networks in the OSPF routing
domain. In these deployments, stronger authentication mechanisms
such as those specified in [RFC7474] SHOULD be used.
The Security Considerations section of [RFC8279] 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
OSPF 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 OSPF control messages.
When creating the encapsulation, the BFIR sets one bit in the
encapsulation for each destination system. The information in the
OSPF 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 the information is not
used by the BFIR to decide which bits to set. Hence, an attack on
the OSPF 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 [RFC8279]. While a failure of this
process might cause a DoS attack (as discussed in the Security
Considerations section of [RFC8279]), such a failure cannot be caused
by an attack on the OSPF control plane.
Implementations MUST ensure that malformed BIER and BIER MPLS
Encapsulation Sub-TLVs as defined in this document are detected and
that they do not provide a vulnerability for attackers to crash the
OSPF router or routing process. Reception of malformed TLVs or sub-
TLVs SHOULD be counted and/or logged for further analysis. Logging
of malformed TLVs and sub-TLVs SHOULD be rate-limited to prevent a
DoS attack (distributed or otherwise) from overloading the OSPF
control plane.
4. IANA Considerations
IANA has allocated the following from the "OSPFv2 Extended Prefix TLV
Sub-TLVs" registry defined in [RFC7684].
BIER Sub-TLV: 9
BIER MPLS Encapsulation Sub-TLV: 10
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8279] 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/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
[RFC8296] 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/RFC8296, January
2018, <https://www.rfc-editor.org/info/rfc8296>.
[RFC8401] Ginsberg, L., Ed., Przygienda, T., Aldrin, S., and Z.
Zhang, "Bit Index Explicit Replication (BIER) Support via
IS-IS", RFC 8401, DOI 10.17487/RFC8401, June 2018,
<https://www.rfc-editor.org/info/rfc8401>.
5.2. Informative References
[IANA-IGP] IANA, "IGP Algorithm Types",
<https://www.iana.org/assignments/igp-parameters/>.
Acknowledgments
The authors would like to thank Rajiv Asati, Christian Martin, Greg
Shepherd, and Eric Rosen for their contributions.
Authors' Addresses
Peter Psenak (editor)
Cisco
Apollo Business Center
Mlynske nivy 43
Bratislava 821 09
Slovakia
Email: ppsenak@cisco.com
Nagendra Kumar
Cisco
7200 Kit Creek Road
Research Triangle Park, NC 27709
United States of America
Email: naikumar@cisco.com
IJsbrand Wijnands
Cisco
De Kleetlaan 6a
Diegem 1831
Belgium
Email: ice@cisco.com
Andrew Dolganow
Nokia
750 Chai Chee Rd
06-06 Viva Business Park
Singapore 469004
Singapore
Email: andrew.dolganow@nokia.com
Tony Przygienda
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
United States of America
Email: prz@juniper.net
Jeffrey Zhang
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
United States of America
Email: zzhang@juniper.net
Sam Aldrin
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA
United States of America
Email: aldrin.ietf@gmail.com
AI ELI5
On this page
§1Why BIER Needs OSPF
Bit Index Explicit Replication (BIER) is an architecture for multicast forwarding through a "BIER domain" that does not require intermediate routers to keep multicast-related, per-flow state. It also does not require an explicit tree-building protocol.
A multicast packet enters the domain at a Bit-Forwarding Ingress Router (BFIR) and leaves at one or more Bit-Forwarding Egress Routers (BFERs). The BFIR adds a BIER header that contains a BitString, in which each bit stands for exactly one BFER. The set of bits that are set is the set of egress routers that must receive the packet.
The BIER architecture requires participating routers to exchange BIER-related information within a domain, and it permits link-state routing protocols to distribute it. RFC 8444 describes the OSPF side of that, for BIER with MPLS encapsulation as defined in RFC 8296. Other encapsulation types, and using several encapsulation types together, are outside its scope.
Each message carries a checklist of which receivers need a copy. For the routers to read the checklist correctly, they first have to agree on which box on the list means which receiver. This RFC is how they agree.
§2Advertising BIER Information in OSPF
All BIER information that a Bit-Forwarding Router (BFR) advertises is tied to its BFR-prefix: a routable IP address, unique within the BIER domain, assigned to each BFR as described in RFC 8279.
Because the information must be associated with a prefix, the authors chose the OSPFv2 Extended Prefix Opaque LSA from RFC 7684 to carry it. Inside the OSPFv2 Extended Prefix TLV, the RFC defines a new sub-TLV for BIER, and that sub-TLV can in turn contain a sub-TLV for MPLS encapsulation.
Every router has one unique address that acts like its name tag. All the BIER details are stuck onto that name tag, so other routers know whose details they are reading.
§2.1The BIER Sub-TLV
The BIER Sub-TLV is type 9. Its fixed fields are the sub-domain-id, the MT-ID, the 2-octet BFR-id, BAR and IPA, followed by a Reserved field and optional nested sub-TLVs. Several BIER Sub-TLVs may appear in one Extended Prefix TLV.
sub-domain-ididentifies the BIER sub-domain within the domain.MT-IDis the Multi-Topology ID, defined in RFC 4915, of the topology associated with the sub-domain.BFR-idis the router's BFR-id. If the BFR has no valid BFR-id configured, it is set to0, which RFC 8279 defines as illegal.BARis the BIER Algorithm used to compute underlay paths, taken from the registry in RFC 8401.IPAis the IGP Algorithm that can modify, enhance or replace the path calculation defined by BAR.
The RFC then lists rules for bad input. A sub-domain MUST be tied to exactly one OSPF topology, and an advertisement that conflicts with the local configuration MUST be ignored. If a BFR advertises the same sub-domain-id in several BIER Sub-TLVs, it is treated as if it advertised none for that sub-domain. BFRs MUST detect duplicate valid BFR-ids and then act as RFC 8279 describes. A BAR or IPA value that does not match the local configuration MUST be reported as a misconfiguration and the Sub-TLV MUST be ignored. Non-zero BAR or IPA values are outside the scope of this document.
This is the main entry in the phone book: which group the router belongs to, its personal number, and which route-finding recipe it uses. If two entries clash, routers ignore the confusing ones instead of guessing.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 2 bytes | 9 | Sub-TLV type. |
LENGTH | Length | 2 bytes | Variable, depends on the nested sub-TLVs. | |
SUB-DOMAIN-ID | sub-domain-id | 1 byte | Identifies the BIER sub-domain within the domain. | |
MT-ID | MT-ID | 1 byte | Multi-Topology ID of the topology tied to the sub-domain. | |
BFR-ID | BFR-id | 2 bytes | The router's BFR-id; 0 if none is configured. | |
BAR | BAR | 1 byte | BIER Algorithm used to compute underlay paths. | |
IPA | IPA | 1 byte | IGP Algorithm that modifies or replaces the path calculation. | |
RESERVED | Reserved | 2 bytes | Reserved field. Nested sub-TLVs follow. |
§2.2The BIER MPLS Encapsulation Sub-TLV
This is a sub-TLV of the BIER Sub-TLV, type 10, with a length of 8 octets. It MAY appear several times inside one BIER Sub-TLV and carries the MPLS-specific details.
Max SIis a 1-octet maximum Set Identifier used in the encapsulation for this sub-domain and BitString length.Labelis a 3-octet field whose 20 rightmost bits give the first label of a range. The 4 leftmost bits MUST be ignored.BS Lenis a 4-bit BitString length, using the values allowed in RFC 8296.
The label range runs from Label to Label + Max SI. Each Set Identifier maps to one label: the first label is for SI=0, the second for SI=1, and so on. A unique range is allocated for each BitString length and sub-domain-id.
Errors are handled by ignoring. A sub-TLV whose maximum label exceeds the 20-bit range, or whose BitString length is not an allowed value, MUST be ignored. If the same BitString length repeats inside one BIER Sub-TLV, the whole BIER Sub-TLV is ignored. Label ranges advertised by one BFR MUST NOT overlap; if they do, all BIER sub-TLVs from that router MUST be ignored.
BIER on MPLS uses numbered tickets called labels. This entry says which block of tickets a router uses for each list size, so no two blocks from the same router ever overlap.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 2 bytes | 10 | Sub-TLV type. |
LENGTH | Length | 2 bytes | 8 | Length of the sub-TLV. |
MAX-SI | Max SI | 1 byte | Maximum Set Identifier used for this sub-domain and BitString length. | |
LABEL | Label | 3 bytes | Rightmost 20 bits are the first label of the range; the 4 leftmost bits are ignored. | |
BS-LEN | BS Len | 4 bits | Supported BitString length. | |
RESERVED | Reserved | 28 bits | SHOULD be 0 on transmission, MUST be ignored on reception. |
§2.3Flooding Scope and Crossing Areas
The Extended Prefix Opaque LSA that carries the BIER Sub-TLV has an area-local flooding scope. To make BIER work across several OSPF areas, the information has to be passed between areas by the Area Border Routers (ABRs).
When an ABR advertises a Type-3 Summary LSA for a prefix into its attached areas, it also originates an area-local Extended Prefix Opaque LSA with the route-type set to inter-area. To decide whether to add a BIER Sub-TLV, the ABR finds its best path to the prefix in the source area and the router that advertised that path. If that router advertised a BIER Sub-TLV for the prefix, the ABR copies the information into the BIER Sub-TLV for each attached area.
In the RFC's example, R1 advertises 192.0.2.1/32 in Area 1 with a BIER Sub-TLV. ABR R2 copies the entire BIER Sub-TLV, including its sub-TLVs, into the LSA it generates for Area 0, and ABR R3 does the same for Area 2.
A note pinned up in one town stays in that town. So the border officers photocopy the note and pin it up in the next town, and the next, so everyone gets the same note.
- Extended Prefix Opaque LSA for 192.0.2.1/32 with BIER Sub-TLVAdvertised in Area 1.
- Same prefix, entire BIER Sub-TLV copiedGenerated by R2 in Area 0.
- Copies it again for Area 2R3 includes the entire BIER Sub-TLV in the LSA it generates in Area 2.
§3Security Considerations
The RFC says the new sub-TLVs add no new security risks to OSPF, and the existing OSPF security extensions from RFC 2328 and RFC 7684 apply. It assumes BIER and OSPF sit in a single administrative domain. Where attackers might reach the OSPF routing domain, stronger authentication such as RFC 7474 SHOULD be used.
It also argues that the denial-of-service concern discussed in RFC 8279, setting too many bits in a BitString, cannot be triggered through these advertisements. The BFIR decides which bits to set without being influenced by OSPF control messages, and the OSPF data only builds the forwarding tables that map each bit to next hops.
Implementations MUST detect malformed BIER and BIER MPLS Encapsulation Sub-TLVs so that they cannot crash the OSPF router or process. Malformed TLVs SHOULD be counted or logged, and logging SHOULD be rate-limited.
Tampering with the phone book cannot make the sender stamp extra names on a message, because the sender picks the names itself. Routers must also stay calm when they read a garbled entry, and not keep shouting about it.
§4IANA Allocations
IANA allocated two values from the "OSPFv2 Extended Prefix TLV Sub-TLVs" registry defined in RFC 7684: the BIER Sub-TLV is 9 and the BIER MPLS Encapsulation Sub-TLV is 10.
The official number keeper gave the two new entries their numbers, 9 and 10, so everyone uses the same ones.