]>
IGP Flexible Algorithm
with Link LossHuaweiHuawei Bld., No. 156 Beiqing Rd.Beijing100095Chinawangyifan82@huawei.comHuaweixuguoqi@huawei.comHuaweigengxuesong@huawei.comHuaweijie.dong@huawei.comCisco Systemsppsenak@cisco.comIGP Flexible Algorithms allow IGPs to compute constraint-based paths.
Since link packet loss rate plays an important role in network
evaluation, links with high packet loss rate should be bypassed during
forwarding. This draft proposes a path computation method based on a
maximum link loss constraint to prune unsatisfied links in Flexible
Algorithms.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.Link packet loss rate (link loss) is a measure of the percentage of
data packets that are lost during transmission over a network. It is an
important performance metric that directly impacts the quality of
service, network congestion, security, and overall network efficiency.
Ensuring a low packet loss rate is essential for maintaining efficient
and secure network operations. Consequently, It is necessary to avoid
passing through links with a high packet loss rate during
forwarding.The link loss is advertised by the Unidirectional Link Loss Sub-TLV
defined in by IS-IS and by OSPF, which describes the loss (as a packet
percentage) between two directly connected IS-IS neighbors. This Sub-TLV
is carried in the Application-Specific Link Attributes Sub-TLV
advertised by IS-IS or OSPF . The link packet loss rate can be measured by methods
such as TWAMP and STAMP , which is beyond the scope of this document.
The link-loss measurement should be consistent in the IGP routing domain.IGP Flexible Algorithms allow IGPs to compute constraint-based paths
. Current path computation methods are based on
calculating the minimum cost of the path from the source to the
destination. Flex-Algorithm has already supported path computation with
the IGP cost, the minimum link delay and the traffic-engineering metric.
defines a family of
generic metrics (e.g. bandwidth based metric type) and bandwidth related
constraints to support path computation based on bandwidth. However,
current calculation types and metric types cannot support path
computation based on link loss, since the cost of the path should be
defined as the maximum/minimum value among all passing links.To overcome the above issue, there are two solutions. First, new
operators like maximum value operator can be defined, which works as a
step function. When the link loss exceeds a threshold, the cost of the
link is set to the maximum. Second, new Flexible Algorithm Definition
(FAD) constraints can be defined to exclude links that do not meet the
link loss requirements during path calculation. The second method is
specifically demonstrated in this document. The general ideas are as
below.1. The link loss is used as a link constraint for path
computation. That is, the link whose loss rate is greater than the
specified value is excluded.2. Metric-type remains unchanged: igp, te, and delay.With a new FAD constraint Sub-TLV advertised by IGP, links with low
packet loss rate will be selected for path computation. The new Exclude
Maximum Link Loss Sub-TLVs are defined in Section 2. The Flex-Algorithm
calculation method based on link loss is presented in Section 3. Link
packet loss rate is obtained from the existing Unidirectional Link Loss
Sub-TLV defined in RFC9479 and RFC9492.A new Exclude Maximum Link Loss Sub-TLV is defined as a sub-TLV of
the FAD TLV. To guarantee loop free forwarding, all routers that
participate in a Flex-Algorithm MUST agree on the FAD. Selected nodes
within the IGP domain MUST advertise FADs as described in Sections 5, 6,
and 7 of .The Exclude Maximum Link Loss Sub-TLV is proposed to specify the
upper limit of the link loss. When this Sub-TLV is carried in a FAD TLV,
all links with packet loss rate larger than the defined maximum link
loss value will be excluded from the Flex-Algorithm topology.IS-IS Flex-Algorithm Exclude Maximum Link Loss Sub-TLV (FAEML) is a
sub-TLV of the IS-IS FAD sub-TLV. It has the following format: The FAEML sub-TLV MUST appear at most once in the FAD Sub-TLV. If
it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by
the receiver.The maximum link loss advertised in FAEML Sub-TLV MUST be compared
with the link loss advertised in Sub-Sub-TLV 36 of ASLA Sub- TLV . If
L-Flag is set in the ASLA sub-TLV, the maximum link loss advertised in
FAEML sub-TLV MUST be compared with the link loss advertised by the
sub-TLV 36 of the TLV 22/222/23/223/141 as
defined in Section 4.2.If the link loss is larger than the maximum link loss advertised in
FAEML sub-TLV, the link MUST be excluded from the Flex-Algorithm
topology. If a link does not have the link loss advertised but the FAD
contains the FAEML sub-TLV, then it MUST NOT be excluded from the
Flex-Algorithm topology.OSPF Flex-Algorithm Exclude Maximum Link Loss Sub-TLV (FAEML) is a
sub-TLV of the OSPF FAD sub-TLV. It has the following format: The FAEML sub-TLV MUST appear at most once in the FAD Sub-TLV. If
it appears more than once, the IS-IS FAD Sub-TLV MUST be ignored by
the receiver.The maximum link loss advertised in FAEML Sub-TLV MUST be compared
with the link loss advertised in Sub-Sub-TLV 30 of ASLA Sub- TLV . The ASLA
Sub-TLV is advertised in Extended Link Opaque LSAs for OSPFv2 and E-Router-LSAs for OSPFv3.If the link loss is larger than the maximum link loss advertised in
FAEML sub-TLV, the link MUST be excluded from the Flex-Algorithm
topology. If a link does not have the link loss advertised but the FAD
contains the FAEML sub-TLV, then it MUST NOT be excluded from the
Flex-Algorithm topology.A new rule is added to the rules used to prune links from the
topology during the Flex-Algorithm computation in Section 13 of .1. Check if any exclude FAEML rule is part of the Flex-Algorithm
definition. If such exclude rule exists and the link has link loss
advertised, check if the link satisfies the FAEML rule. If not, the
link MUST be pruned from the computation.In some scenarios, the link status can be frequently changed between available and unavailable
since the link packet loss rate may fluctuate around the threshold value.
Consequently, flex-algo calculation may be triggered frequently.
There are a few mechanisms to solve this problem.
1. Delayed collection: IGP advertised loss could be calculated on an interval which is long enough, such as 10 minutes.2. Averaging and normalization: IGP advertised loss should be some form of average (e.g. exponential weighted average) from the collected loss values and
the advertised loss can be normalized to avoid advertising non-significant changes in loss.3. Flapping suppression: When the IGP advertised loss is detected on a frequent change, a timer can be set to delay the update process.Type: 252(TBA)Description: IS-IS Exclude Maximum Link Loss Sub-TLVReference: This document Section 2.1Type: 252(TBA)Description: OSPF Exclude Maximum Link Loss Sub-TLVReference: This document Section 2.2
&RFC2119;
&RFC8174;
&RFC7684;
&RFC9479;
&RFC9492;
&RFC8362;
&RFC5305;
&RFC9350;
&RFC8570;
&RFC7471;
&RFC5357;
&RFC8762;