IS-IS Traffic Engineering (TE)
Metric ExtensionsCisco Systems, Inc.ginsberg@cisco.comstefano@previdi.netMicrosoftspencer.giacalone@gmail.comCisco Systems, Inc.wardd@cisco.comJuniper Networks1194 N. Matilda Ace.SunnyvaleC94089United Statesldrake@juniper.netHuawei101 Software Avenue, Yuhua DistrictNanjingJiangsu210012Chinasunseawq@huawei.com
Routing
Link State RoutingIGPIS-ISIn certain networks, such as, but not limited to, financial
information networks (e.g., stock market data providers), network-
performance criteria (e.g., latency) are becoming as critical to
data-path selection as other metrics.This document describes extensions to IS-IS Traffic Engineering
Extensions (RFC 5305) such that network-performance information can be
distributed and collected in a scalable fashion. The information
distributed using IS-IS TE Metric Extensions can then be used to make
path-selection decisions based on network performance.Note that this document only covers the mechanisms with which
network-performance information is distributed. The mechanisms for
measuring network performance or acting on that information, once
distributed, are outside the scope of this document.This document obsoletes RFC 7810.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.In certain networks, such as, but not limited to, financial
information networks (e.g., stock market data providers), network-
performance information (e.g., latency) is becoming as critical to
data-path selection as other metrics.In these networks, extremely large amounts of money rest on the
ability to access market data in "real time" and to predictably make
trades faster than the competition. Because of this, using metrics such
as hop count or cost as routing metrics is becoming only tangentially
important. Rather, it would be beneficial to be able to make
path-selection decisions based on performance data (such as latency) in
a cost-effective and scalable way.This document describes extensions (hereafter called "IS-IS TE Metric
Extensions") to the IS-IS Extended Reachability TLV defined in
[RFC5305], that can be used to distribute network-performance
information (such as link delay, delay variation, packet loss, residual
bandwidth, and available bandwidth).The data distributed by the IS-IS TE Metric Extensions proposed in
this document is meant to be used as part of the operation of the
routing protocol (e.g., by replacing cost with latency or considering
bandwidth as well as cost), to enhance Constrained-SPF (CSPF), or for
other uses such as supplementing the data used by an ALTO server
[RFC7285]. With respect to CSPF, the data distributed by IS-IS TE Metric
Extensions can be used to set up, fail over, and fail back data paths
using protocols such as RSVP-TE [RFC3209].Note that the mechanisms described in this document only disseminate
performance information. The methods for initially gathering that
performance information, such as described in [RFC6375], or acting on it
once it is distributed are outside the scope of this document. Example
mechanisms to measure latency, delay variation, and loss in an MPLS
network are given in [RFC6374]. While this document does not specify how
the performance information should be obtained, the measurement of delay
SHOULD NOT vary significantly based upon the offered traffic load. Thus,
queuing delays SHOULD NOT be included in the delay measurement. For
links such as Forwarding Adjacencies, care must be taken that
measurement of the associated delay avoids significant queuing delay;
that could be accomplished in a variety of ways, including either by
measuring with a traffic class that experiences minimal queuing or by
summing the measured link delays of the components of the link's
path.This document registers new IS-IS TE sub-TLVs that can be announced
in the "Sub-TLVs for TLVs 22, 23, 141, 222, and 223" registry in order
to distribute network-performance information. The extensions in this
document build on the ones provided in IS-IS TE [RFC5305] and GMPLS
[RFC4203].IS-IS Extended Reachability TLV 22 (defined in [RFC5305]), Inter-AS
Reachability Information TLV 141 (defined in [RFC5316]), and MT-ISIS TLV
222 (defined in [RFC5120]) have nested sub-TLVs that permit the TLVs to
be readily extended. This document registers several sub- TLVs:As can be seen in the list above, the sub-TLVs described in
this document carry different types of network-performance information.
The new sub-TLVs include a bit called the Anomalous (or "A") bit. When
the A bit is clear (or when the sub-TLV does not include an A bit), the
sub-TLV describes steady-state link performance. This information could
conceivably be used to construct a steady-state performance topology for
initial tunnel-path computation, or to verify alternative failover
paths.When network performance violates configurable link-local thresholds,
a sub-TLV with the A bit set is advertised. These sub-TLVs could be used
by the receiving node to determine whether to fail traffic to a backup
path or whether to calculate an entirely new path. From an MPLS
perspective, the intent of the A bit is to permit label switched path
ingress nodes to determine whether the link referenced in the sub-TLV
affects any of the label switched paths for which it is ingress. If they
are affected, then they can determine whether those label switched paths
still meet end-to-end performance objectives. If not, then the node
could conceivably move affected traffic to a pre-established protection
label switched path or establish a new label switched path and place the
traffic in it.If link performance then improves beyond a configurable minimum value
(reuse threshold), that sub-TLV can be re-advertised with the A bit
cleared. In this case, a receiving node can conceivably do whatever
re-optimization (or failback) it wishes to do (including nothing).Note that when a sub-TLV does not include the A bit, that sub-TLV
cannot be used for failover purposes. The A bit was intentionally
omitted from some sub-TLVs to help mitigate oscillations. See Section 5
for more information.Consistent with existing IS-IS TE specification [RFC5305], the
bandwidth advertisements defined in this document MUST be encoded as
IEEE floating-point values. The delay and delay-variation advertisements
defined in this document MUST be encoded as integer values. Delay values
MUST be quantified in units of microseconds, packet loss MUST be
quantified as a percentage of packets sent, and bandwidth MUST be sent
as bytes per second. All values (except residual bandwidth) MUST be
calculated as rolling averages where the averaging period MUST be a
configurable period of time. See Section 5 for more information.The use of IS-IS TE Metric Extensions sub-TLVs is not confined to the
TE context. In other words, IS-IS TE Metric Extensions sub-TLVs defined
in this document can also be used for computing paths in the absence of
a TE subsystem.However, as for the TE case, Interface Address and Neighbor Address
sub-TLVs (IPv4 or IPv6) MUST be present. The encoding is defined in
[RFC5305] for IPv4 and in [RFC6119] for IPv6.This sub-TLV advertises the average link delay between two directly
connected IS-IS neighbors. The delay advertised by this sub-TLV MUST
be the delay from the local neighbor to the remote one (i.e., the
forward-path latency). The format of this sub-TLV is shown in the
following diagram:where:Type: 33Length: 4A bit: The A bit represents the Anomalous (A) bit. The A bit is set
when the measured value of this parameter exceeds its configured
maximum threshold. The A bit is cleared when the measured value falls
below its configured reuse threshold. If the A bit is clear, the
sub-TLV represents steady-state link performance.RESERVED: This field is reserved for future use. It MUST be set to
0 when sent and MUST be ignored when received.Delay: This 24-bit field carries the average link delay over a
configurable interval in microseconds, encoded as an integer value.
When set to the maximum value 16,777,215 (16.777215 sec), then the
delay is at least that value and may be larger.This sub-TLV advertises the minimum and maximum delay values
between two directly connected IS-IS neighbors. The delay advertised
by this sub-TLV MUST be the delay from the local neighbor to the
remote one (i.e., the forward-path latency). The format of this
sub-TLV is shown in the following diagram:where:Type: 34Length: 8A bit: This field represents the Anomalous (A) bit. The A bit is
set when one or more measured values exceed a configured maximum
threshold. The A bit is cleared when the measured value falls below
its configured reuse threshold. If the A bit is clear, the sub-TLV
represents steady-state link performance.RESERVED: This field is reserved for future use. It MUST be set to
0 when sent and MUST be ignored when received.Min Delay: This 24-bit field carries the minimum measured link
delay value (in microseconds) over a configurable interval, encoded as
an integer value.Max Delay: This 24-bit field carries the maximum measured link
delay value (in microseconds) over a configurable interval, encoded as
an integer value.Implementations MAY also permit the configuration of an offset
value (in microseconds) to be added to the measured delay value, to
facilitate the communication of operator-specific delay
constraints.It is possible for the Min and Max delay to be the same value.When the delay value (Min or Max) is set to the maximum value
16,777,215 (16.777215 sec), then the delay is at least that value and
may be larger.This sub-TLV advertises the average link delay variation between
two directly connected IS-IS neighbors. The delay variation advertised
by this sub-TLV MUST be the delay from the local neighbor to the
remote one (i.e., the forward-path latency). The format of this sub-
TLV is shown in the following diagram:whereType: 35Length: 4RESERVED: This field is reserved for future use. It MUST be set to
0 when sent and MUST be ignored when received.Delay Variation: This 24-bit field carries the average link delay
variation over a configurable interval in microseconds, encoded as an
integer value. When set to 0, it has not been measured. When set to
the maximum value 16,777,215 (16.777215 sec), then the delay is at
least that value and may be larger.This sub-TLV advertises the loss (as a packet percentage) between
two directly connected IS-IS neighbors. The link loss advertised by
this sub-TLV MUST be the packet loss from the local neighbor to the
remote one (i.e., the forward-path loss). The format of this sub-TLV
is shown in the following diagram:where:Type: 36Length: 4A bit: The A bit represents the Anomalous (A) bit. The A bit is set
when the measured value of this parameter exceeds its configured
maximum threshold. The A bit is cleared when the measured value falls
below its configured reuse threshold. If the A bit is clear, the
sub-TLV represents steady-state link performance.RESERVED: This field is reserved for future use. It MUST be set to
0 when sent and MUST be ignored when received.Link Loss: This 24-bit field carries link packet loss as a
percentage of the total traffic sent over a configurable interval. The
basic unit is 0.000003%, where (2^24 - 2) is 50.331642%. This value is
the highest packet-loss percentage that can be expressed (the
assumption being that precision is more important on high-speed links
than the ability to advertise loss rates greater than this, and that
high- speed links with over 50% loss are unusable). Therefore,
measured values that are larger than the field maximum SHOULD be
encoded as the maximum value.This sub-TLV advertises the residual bandwidth between two directly
connected IS-IS neighbors. The residual bandwidth advertised by this
sub-TLV MUST be the residual bandwidth from the system originating the
Link State Advertisement (LSA) to its neighbor.where:Type: 37Length: 4Residual Bandwidth: This field carries the residual bandwidth on a
link, forwarding adjacency [RFC4206], or bundled link in IEEE
floating-point format with units of bytes per second. For a link or
forwarding adjacency, residual bandwidth is defined to be the Maximum
Bandwidth [RFC5305] minus the bandwidth currently allocated to RSVP-
TE label switched paths. For a bundled link, residual bandwidth is
defined to be the sum of the component link residual bandwidths.The calculation of residual bandwidth is different than that of
unreserved bandwidth [RFC5305]. Residual bandwidth subtracts tunnel
reservations from maximum bandwidth (i.e., the link capacity)
[RFC5305] and provides an aggregated remainder across priorities.
Unreserved bandwidth, on the other hand, is subtracted from the
maximum reservable bandwidth (the bandwidth that can theoretically be
reserved) and provides per-priority remainders. Residual bandwidth and
unreserved bandwidth [RFC5305] can be used concurrently and each has a
separate use case (e.g., the former can be used for applications like
Weighted ECMP while the latter can be used for call admission
control).This sub-TLV advertises the available bandwidth between two
directly connected IS-IS neighbors. The available bandwidth advertised
by this sub-TLV MUST be the available bandwidth from the system
originating this sub-TLV. The format of this sub-TLV is shown in the
following diagram:where:Type: 38Length: 4Available Bandwidth: This field carries the available bandwidth on
a link, forwarding adjacency, or bundled link in IEEE floating-point
format with units of bytes per second. For a link or forwarding
adjacency, available bandwidth is defined to be residual bandwidth
(see Section 4.5) minus the measured bandwidth used for the actual
forwarding of non-RSVP-TE label switched path packets. For a bundled
link, available bandwidth is defined to be the sum of the component
link available bandwidths minus the measured bandwidth used for the
actual forwarding of non-RSVP-TE label switched path packets. For a
bundled link, available bandwidth is defined to be the sum of the
component link available bandwidths.This sub-TLV advertises the bandwidth utilization between two
directly connected IS-IS neighbors. The bandwidth utilization
advertised by this sub-TLV MUST be the bandwidth from the system
originating this sub-TLV. The format of this sub-TLV is shown in the
following diagram:where:Type: 39Length: 4Utilized Bandwidth: This field carries the bandwidth utilization on
a link, forwarding adjacency, or bundled link in IEEE floating-point
format with units of bytes per second. For a link or forwarding
adjacency, bandwidth utilization represents the actual utilization of
the link (i.e., as measured by the advertising node). For a bundled
link, bandwidth utilization is defined to be the sum of the component
link bandwidth utilizations.The values advertised in all sub-TLVs (except min/max delay and
residual bandwidth) MUST represent an average over a period or be
obtained by a filter that is reasonably representative of an average.
For example, a rolling average is one such filter.Min and max delay MUST each be derived in one of the following ways:
by taking the lowest and/or highest measured value over a measurement
interval or by making use of a filter or other technique to obtain a
reasonable representation of a min and max value representative of the
interval, with compensation for outliers.The measurement interval, any filter coefficients, and any
advertisement intervals MUST be configurable per sub-TLV.In addition to the measurement intervals governing re-advertisement,
implementations SHOULD provide configurable accelerated advertisement
thresholds per sub-TLV, such that:If the measured parameter falls outside a configured upper bound
for all but the minimum delay metric (or lower bound for minimum
delay metric only) and the advertised sub-TLV is not already outside
that bound or,If the difference between the last advertised value and current
measured value exceeds a configured threshold then,The advertisement is made immediately.For sub-TLVs that include an A bit, an additional threshold
SHOULD be included corresponding to the threshold for which the
performance is considered anomalous (and sub-TLVs with the A bit are
sent). The A bit is cleared when the sub-TLV's performance has been
below (or re-crosses) this threshold for an advertisement
interval(s) to permit fail back.To prevent oscillations, only the high threshold or the low threshold
(but not both) may be used to trigger any given sub-TLV that supports
both.Additionally, once outside the bounds of the threshold, any
re-advertisement of a measurement within the bounds would remain
governed solely by the measurement interval for that sub-TLV.When link-performance values change by small amounts that fall under
thresholds that would cause the announcement of a sub-TLV,
implementations SHOULD suppress sub-TLV re-advertisement and/or lengthen
the period within which they are refreshed.Only the accelerated advertisement threshold mechanism described in
Section 5 may shorten the re-advertisement interval. All suppression and
re-advertisement interval backoff timer features SHOULD be
configurable.Sections 5 and 6 provide configurable mechanisms to bound the number
of re-advertisements. Instability might occur in very large networks if
measurement intervals are set low enough to overwhelm the processing of
flooded information at some of the routers in the topology. Therefore,
care should be taken in setting these values.Additionally, the default measurement interval for all sub-TLVs
SHOULD be 30 seconds.Announcements MUST also be able to be throttled using configurable
inter-update throttle timers. The minimum announcement periodicity is 1
announcement per second. The default value SHOULD be set to 120
seconds.Implementations SHOULD NOT permit the inter-update timer to be lower
than the measurement interval.Furthermore, it is RECOMMENDED that any underlying performance-
measurement mechanisms not include any significant buffer delay, any
significant buffer-induced delay variation, or any significant loss due
to buffer overflow or due to active queue management.Implementations MUST make it possible to individually enable or
disable each sub-TLV based on configuration.Implementations SHOULD permit static configuration and/or manual
override of dynamic measurements for each sub-TLV in order to simplify
migration and to mitigate scenarios where dynamic measurements are not
possible.As per [RFC5305], unrecognized sub-TLVs should be silently
ignored.The sub-TLVs introduced in this document allow an operator to
advertise state information of links (bandwidth, delay) that could be
sensitive and that an operator may not want to disclose.Section 7 describes a mechanism to ensure network stability when the
new sub-TLVs defined in this document are advertised. Implementation
SHOULD follow the described guidelines to mitigate the instability
risk.[RFC5304] describes an authentication method for IS-IS Link State
PDUs that allows cryptographic authentication of IS-IS Link State
PDUs.It is anticipated that in most deployments, the IS-IS protocol is
used within an infrastructure entirely under control of the same
operator. However, it is worth considering that the effect of sending
IS-IS Traffic Engineering sub-TLVs over insecure links could result in a
man-in-the-middle attacker delaying real-time data to a given site or
destination, which could negatively affect the value of the data for
that site or destination. The use of Link State PDU cryptographic
authentication allows mitigation the risk of man-in- the-middle
attack.IANA maintains the registry for the sub-TLVs. IANA has registered the
following sub-TLVs in the "Sub-TLVs for TLVs 22, 23, 141, 222, and 223"
registry:For the original version of this document the authors recognized
Ayman Soliman, Nabil Bitar, David McDysan, Edward Crabbe, Don Fedyk,
Hannes Gredler, Uma Chunduri, Alvaro Retana, Brian Weis, and Barry Leiba
for their contribution and review of this document.The authors also recognized Curtis Villamizar for significant
comments and direct content collaboration.For the second version of this document the authors thank Jeff Haas
for identifying and reporting the incorrect encoding of the bandwidth
related sub-TLVs.The following people contributed substantially to the content of this
document and should be considered co-authors:Errata ID: 5293
(https://www.rfc-editor.org/errata_search.php?rfc=7810) correctly
identified that in [RFC7810] the length associated with the following
sub-TLVs did not match the figures associated with each:The length specified was 4 which did not include the RESERVED
field shown in the figures. Subsequent investigation revealed that some
implementations had used the specified length (4) and omitted the
RESERVED field while other implementations included the specified
RESERVED field and used a length of 5.Because these different implementation choices are not interoperable,
it was decided that a bis version should be generated which resolved the
ambiguity.The choice made here is to omit the unused RESERVED field from these
sub-TLVs and use the length of 4. This matches the corresponding
advertisements specified in the equivalent OSPF specification [RFC7471]
and the corresponding BGP-LS specification .Some minor editorial corrections have also been made.