| Internet-Draft | OSPFv3 Extensions for BIER | December 2022 |
| Psenak, et al. | Expires 4 June 2023 | [Page] |
Bit Index Explicit Replication (BIER) is an architecture that provides multicast forwarding through a "BIER domain" without requiring intermediate routers to maintain multicast related per-flow state. BIER architecture uses MPLS or other encapsulation to steer the multicast traffic towards the receivers.¶
This document describes the OSPFv3 protocol extensions required for BIER with MPLS encapsulation. Support for other encapsulation types is outside the scope of this document. The use of multiple encapsulation types is outside the scope of this document.¶
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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 header to the packet. The BIER header contains a bit-string 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 setting the bits that correspond to those routers in the BIER header.¶
BIER architecture requires routers participating in BIER to exchange BIER related information within a given domain. BIER architecture permits link-state routing protocols to perform distribution of such information. [RFC8444] proposes the OSPFv2 protocol extensions to distribute BIER specific information. This document describes extensions to OSPFv3 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", "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.¶
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 IPv4 or IPv6 address that is assigned to each BFR as described in more detail in [RFC8279].¶
[RFC8362] defines the encoding of OSPFv3 LSA in TLV format that allows to carry additional informations. This section defines the required Sub-TLVs to carry BIER information that is associated with the BFR-Prefix. The Sub-TLV defined in this section MAY be carried in the below OSPFv3 Extended LSA TLVs [RFC8362]:¶
A Sub-TLV of the above mentioned Prefix TLVs is defined for distributing BIER information. The Sub-TLV is called the BIER Sub-TLV. Multiple BIER Sub-TLVs may be included in any of the above mentioned 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) | +- -+ | |¶
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 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 MUST be ignored.¶
If the MT-ID value is outside of the values specified in [RFC4915], the BIER Sub-TLV 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. A router receiving BIER Sub-TLV advertisement with a value in BAR or IPA fields which does not match the locally configured value for a given BFR sub-domain, MUST report a misconfiguration for such BIER sub-domain and MUST ignore such BIER sub-TLV.¶
The use of non-zero values in either the BAR field or the IPA field is outside the scope of this document.¶
The BIER MPLS Encapsulation Sub-TLV is a Sub-TLV of the BIER Sub-TLV defined in Section 2.1. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
If the label associated with the Maximum Set Identifier exceeds the 20 bit range, the BIER MPLS Encapsulation Sub-TLV MUST be ignored.¶
If the BS length is set to a value that does not match any of the allowed values specified in [RFC8296], the BIER MPLS Encapsulation Sub-TLV MUST be ignored.¶
If same BS length is repeated in multiple BIER MPLS Encapsulation Sub-TLV inside the same BIER Sub-TLV, the BIER sub-TLV MUST be ignored.¶
Label ranges within all BIER MPLS Encapsulation Sub-TLVs advertised by the same BFR MUST NOT overlap. If the overlap is detected, the advertising router MUST be treated as if it did not advertise any BIER sub-TLVs.¶
The flooding scope of the Extended LSAs [RFC8362] that is used for advertising the BIER Sub-TLV is area-local. To allow BIER deployment in a multi-area environment, OSPFv3 must propagate BIER information between areas.¶
( ) ( ) ( )
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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 OSPFv3 Area Border Router (ABR) advertises E-Inter-Area-Prefix-LSA from an intra-area or inter-area prefix to all its attached areas, it determines whether a BIER Sub-TLV should be included in this LSA. When doing so, an OSPFv3 ABR will:¶
This document introduces new sub-TLVs for OSPFv3 Extended-LSAs. It does not introduce any new security risks to OSPFv3. Existing security concerns documented in [RFC8362] is applicable for the Sub-TLVs defined in this document.¶
It is assumed that both BIER and OSPF layer is under a single administrative domain. There can be deployments where potential attackers have access to one or more networks in the OSPFv3 routing domain. In these deployments, stronger authentication mechanisms such as those specified in [RFC4552] 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 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. That 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 of [RFC8279]), such a failure cannot be caused by an attack on the OSPF control plane.¶
Implementations MUST assure that malformed TLV and Sub-TLV defined in this document are detected and do not provide a vulnerability for attackers to crash the OSPFv3 router or routing process. Reception of malformed TLV or Sub-TLV SHOULD be counted and/or logged for further analysis. Logging of malformed TLVs and Sub-TLVs SHOULD be rate-limited to prevent a Denial of Service (DoS) attack (distributed or otherwise) from overloading the OSPFv3 control plane.¶
The document requests two new allocations from the OSPFv3 Extended-LSA Sub-TLVs registry as defined in [RFC8362].¶
The authors would like to thank Mankamana Mishra, Tony Przygienda, Huaimo Chen and Greg Shepherd for their review comments.¶