IGP Reverse Metric AlgorithmCisco SystemsApollo Business CenterMlynske nivy 43Bratislava82109Slovakiappsenak@cisco.comCisco SystemsMilpitas95035CAUSAjakuhorn@cisco.comCisco SystemsMilpitas95035CAUSAginsberg@cisco.com
Routing Area
LSR Working GroupIGPDraftIANA has set up a subregistry called "IGP Algorithm Type" under the
"Interior Gateway Protocol (IGP) Parameters" registry. This draft introduces
a new algorithm type which utilizes the cost in the reverse direction
on each link.This document also discusses using this new algorithm type in combination
with IGP Flexible Algorithm to compute constraint-based paths.Existing IGP Algorithm types as defined in the IANA IGP Algorithm Types
registry created by utilize metrics in the forward direction
on each link. But there are use cases (discussed later in this document)
where utilizing the metric in the reverse direction on each link is desirable.This document defines a new IGP Algorithm type which uses link metrics in
the reverse direction.An IGP Flexible Algorithm as specified in
computes a constraint-based path. It supports various metric types,
including native Interior Gateway Protocol (IGP) metric, Min Unidirectional
Link Delay, Traffic Engineering Default Metric, all defined in .
In addition, Bandwidth Metric and User defined metrics are defined in
.Existing IGP Flexible Algorithm use cases specify an IGP Algorithm Type where
all these link metric types are used by Flexible Algorithm in the forward direction,
e.g., the direction in which the link is added to the topology during the
computation.This document extends IGP Flexible Algorithm to be able to use the IGP Algorithm
Type which specifies using the link metric in a reverse direction.Usage of the Reverse Shortest Path First (RSPF) algorithm outside of the IGP
Flexible Algorithm is outside of the scope of this draft.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 multicast tree is typically established from the receivers (R) towards
the multicast source (S). When determining the path for traffic originating
from S towards R, a shortest path from R to S is often used. When the metric on all
links is symmetric, e.g. equal in both directions, this provides the correct
outcome.With the introduction of IGP Flexible Algorithms, additional metrics become
available for calculating the shortest path. One commonly employed metric is
latency, which often exhibits asymmetry. Multicast traffic is frequently required
to take the path with the minimal latency. In the presence of the asymmetric
link latency, the usage of the reverse link latency is required for multicast
tree calculation. Failure to use the reverse link latency can result in
multicast traffic not taking the least latency path across the network.Flexible Algorithm Definition Sub-TLV defines the
Calc-Type field, that carries the calculation-type used by the Flexible Algorithm.
Values are defined in IANA "IGP Algorithm Types" registry defined under the
"Interior Gateway Protocol (IGP) Parameters" registry.We define a new IGP Algorithm Type - Reverse Shortest Path First (SPF)
algorithm based on link metric. It is equivalent to the IGP Algorithm Type 0,
but the link metric is taken from the reverse direction of the link.If the winning Flexible Algorithm Definition signals the Reverse SPF algorithm
calculation-type, defines the metric-type other than IGP metric and such metric
is not advertised for the link in the reverse direction in a topology for which
the computation is done, such link MUST be pruned from the computation. A metric
of value 0 MUST NOT be assumed in such a case. This is the application of
the rule (5), as specified in section 13 of ,
in a reverse direction.This document defines a new algorithm type in the IANA "IGP Algorithm Types"
registry defined under the "Interior Gateway Protocol (IGP) Parameters"
registry. The "value" is suggested - to be assigned by IANA.This document inherits security considerations from the