The Application Specific Link
Attribute (ASLA) Any Application Bit
Juniper Networks
Exora Business Park
Bangalore
KA
560103
India
shraddha@juniper.net
Juniper Networks
2251 Corporate Park Drive
Herndon
20171
Virginia
USA
rbonica@juniper.net
Juniper Networks
cbowers@juniper.net
NTT Network Innovations
robert@raszuk.net
Huawei Technologies
People's Republic of China
lizhenbin@huawei.com
Routing
LSR
IS-IS
OSPF
IGP
ASLA
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.
and 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.
According to :
"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 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
and . An implementation
that does not recognize the A-bit will process the SABM as specified in
and .
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 sec 6.2 MUST be followed.
The security considerations discussed in and
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:
Bit: 4
Name: Any Application (A-bit)
Reference: This document
Thanks to Xuesong Geng and Ram Santhanakrishnan for contributing to
this document.