| Internet-Draft | ASLA Any Application Bit | August 2021 |
| Hegde, et al. | Expires 20 February 2022 | [Page] |
RFC 8919 and RFC 8920 define Application Specific Link Attributes (ASLA). Each ASLA includes an Application Identifier Bit Mask. The Application Identifier Bit Mask includes a Standard Application Bit Mask (SABM) and a User Defined Application Bit Mask (UDABM). The SABM and UDABM determine which applications can use the ASLA as an input.¶
This document introduces a new bit to the Standard Application Identifier Bit Mask. This bit is called the Any Application Bit (i.e., the A-bit). If the A-bit is set, the link attribute can be used by any application. This includes currently defined applications as well as applications to be defined in the future.¶
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[RFC8919] and [RFC8920] define Application Specific Link Attributes (ASLA). Each ASLA includes an Application Identifier Bit Mask. The Application Identifier Bit Mask includes a Standard Application Bit Mask (SABM) and a User Defined Application Bit Mask (UDABM).¶
Each bit in the SABM represents a standard application while each bit in the UDABM represents a user defined application. If a bit in the SABM or UDABM is set, the corresponding application can use the ASLA as an input. If a bit in the SABM or UDABM is not set, the corresponding application cannot use the associated ASLA as an input.¶
"If link attributes are advertised associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications, then any standard application and/or any user-defined application is permitted to use that set of link attributes so long as there is not another set of attributes advertised on that same link that is associated with a non-zero-length Application Identifier Bit Mask with a matching Application Identifier Bit set."¶
This restriction introduces complexity. For example, assume that a network runs many applications. All applications use Attribute 1 as an input. So, it would be convenient to advertise Attribute 1 with a zero-length SABM / UDABM.¶
However, Applications X and Y also use Attribute 2 as an input. Because Applications X and Y required unique values for Attribute 2, Attribute 2 cannot be advertised with a zero-length SABM. Therefore, Attribute 1 cannot be advertised with a zero-length SABM / UDABM either, because Applications X and X require it.¶
This document reduces operational complexity by introducing a new bit to the Standard Application Identifier Bit Mask. This bit is called the Any Application Bit (i.e., the A-bit). If the A-bit is set, the link attribute can be used by any application. This includes currently defined applications as well as applications to be defined in the future.¶
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.¶
A new bit is defined in the Standard Application Identifier Bit Mask. This bit is called the Any Application Bit (i.e., the A-bit). If the A-bit is set, the link attribute can be used by any application. This includes currently defined applications as well as applications to be defined in the future.¶
If a link advertises an ASLA twice, once with the A-bit set and once with a more specific Application Identifier Bit set, the indicated application MUST use the value from the ASLA with the more specific Application Indicator Bit set.¶
IS-IS uses Bit 4 of the SABM to encode the A-bit.¶
OSPF uses Bit 4 of the SABM to encode the A-bit.¶
The solution described in this document is backward compatible with [RFC8919] and [RFC8920]. An implementation that does not recognize the A-bit will process the SABM as specified in [RFC8919] and [RFC8920].¶
Implementations MAY advertise attributes under both A-bit and with SABM and UDABM length set to zero for backward compatibility reasons. When same attributes are received with A-bit set as well as in ASLA with SABM and UDABM set to zero, the attributes MUST be used from the ASLA with SABM and UDABM set to zero and procedures described in [RFC8919] sec 6.2 MUST be followed.¶
The security considerations discussed in [RFC8919] and [RFC8920] are applicable to this document. This document does not introduce any new security risks.¶
This document requests that IANA add the following entry to the registry titled "Link Attribute Application Identifiers" under the "Interior Gateway Protocol (IGP) Parameters" registry:¶
Thanks to Xuesong Geng and Ram Santhanakrishnan for contributing to this document.¶