Application-Layer Traffic Optimization (ALTO) Performance Cost Metrics
RFC 9439
| Document | Type | RFC - Proposed Standard (August 2023) | |
|---|---|---|---|
| Authors | Qin Wu , Y. Richard Yang , Young Lee , Dhruv Dhody , Sabine Randriamasy , Luis M. Contreras | ||
| Last updated | 2023-08-11 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| IESG | Responsible AD | Martin Duke | |
| Send notices to | (None) |
RFC 9439
Internet Engineering Task Force (IETF) Q. Wu
Request for Comments: 9439 Huawei
Category: Standards Track Y. Yang
ISSN: 2070-1721 Yale University
Y. Lee
Samsung
D. Dhody
Huawei
S. Randriamasy
Nokia Networks France
L. Contreras
Telefonica
August 2023
Application-Layer Traffic Optimization (ALTO) Performance Cost Metrics
Abstract
The cost metric is a basic concept in Application-Layer Traffic
Optimization (ALTO), and different applications may use different
types of cost metrics. Since the ALTO base protocol (RFC 7285)
defines only a single cost metric (namely, the generic "routingcost"
metric), if an application wants to issue a cost map or an endpoint
cost request in order to identify a resource provider that offers
better performance metrics (e.g., lower delay or loss rate), the base
protocol does not define the cost metric to be used.
This document addresses this issue by extending the specification to
provide a variety of network performance metrics, including network
delay, delay variation (a.k.a. jitter), packet loss rate, hop count,
and bandwidth.
There are multiple sources (e.g., estimations based on measurements
or a Service Level Agreement) available for deriving a performance
metric. This document introduces an additional "cost-context" field
to the ALTO "cost-type" field to convey the source of a performance
metric.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9439.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Requirements Language
3. Performance Metric Attributes
3.1. Performance Metric Context: "cost-context"
3.2. Performance Metric Statistics
4. Packet Performance Metrics
4.1. Cost Metric: One-Way Delay (delay-ow)
4.1.1. Base Identifier
4.1.2. Value Representation
4.1.3. Intended Semantics and Use
4.1.4. Cost-Context Specification Considerations
4.2. Cost Metric: Round-Trip Delay (delay-rt)
4.2.1. Base Identifier
4.2.2. Value Representation
4.2.3. Intended Semantics and Use
4.2.4. Cost-Context Specification Considerations
4.3. Cost Metric: Delay Variation (delay-variation)
4.3.1. Base Identifier
4.3.2. Value Representation
4.3.3. Intended Semantics and Use
4.3.4. Cost-Context Specification Considerations
4.4. Cost Metric: Loss Rate (lossrate)
4.4.1. Base Identifier
4.4.2. Value Representation
4.4.3. Intended Semantics and Use
4.4.4. Cost-Context Specification Considerations
4.5. Cost Metric: Hop Count (hopcount)
4.5.1. Base Identifier
4.5.2. Value Representation
4.5.3. Intended Semantics and Use
4.5.4. Cost-Context Specification Considerations
5. Throughput/Bandwidth Performance Metrics
5.1. Cost Metric: TCP Throughput (tput)
5.1.1. Base Identifier
5.1.2. Value Representation
5.1.3. Intended Semantics and Use
5.1.4. Cost-Context Specification Considerations
5.2. Cost Metric: Residual Bandwidth (bw-residual)
5.2.1. Base Identifier
5.2.2. Value Representation
5.2.3. Intended Semantics and Use
5.2.4. Cost-Context Specification Considerations
5.3. Cost Metric: Available Bandwidth (bw-available)
5.3.1. Base Identifier
5.3.2. Value Representation
5.3.3. Intended Semantics and Use
5.3.4. Cost-Context Specification Considerations
6. Operational Considerations
6.1. Source Considerations
6.2. Metric Timestamp Considerations
6.3. Backward-Compatibility Considerations
6.4. Computation Considerations
6.4.1. Configuration Parameter Considerations
6.4.2. Aggregation Computation Considerations
7. Security Considerations
8. IANA Considerations
8.1. ALTO Cost Metrics Registry
8.2. ALTO Cost Source Types Registry
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
Application-Layer Traffic Optimization (ALTO) provides a means for
network applications to obtain network information so that the
applications can identify efficient application-layer traffic
patterns using the networks. Cost metrics are used in both the ALTO
cost map service and the ALTO endpoint cost service in the ALTO base
protocol [RFC7285].
Since different applications may use different cost metrics, the ALTO
base protocol introduced the "ALTO Cost Metrics" registry
(Section 14.2 of [RFC7285]) as a systematic mechanism to allow
different metrics to be specified. For example, a delay-sensitive
application may want to use latency-related metrics, and a bandwidth-
sensitive application may want to use bandwidth-related metrics.
However, the ALTO base protocol has registered only a single cost
metric, i.e., the generic "routingcost" metric (Section 14.2 of
[RFC7285]); no latency- or bandwidth-related metrics are defined in
the base protocol.
This document registers a set of new cost metrics (Table 1) to allow
applications to determine where to connect based on network
performance criteria, including delay- and bandwidth-related metrics.
+============+===============+=====================================+
| Metric | Definition in | Semantics Based On |
| | This Document | |
+============+===============+=====================================+
| One-Way | Section 4.1 | Base: [RFC7471] [RFC8570] [RFC8571] |
| Delay | | sum of Unidirectional Delay of |
| | | links along the path |
+------------+---------------+-------------------------------------+
| Round-Trip | Section 4.2 | Base: Sum of two directions of |
| Delay | | Unidirectional Delay |
+------------+---------------+-------------------------------------+
| Delay | Section 4.3 | Base: [RFC7471] [RFC8570] [RFC8571] |
| Variation | | Sum of Unidirectional Delay |
| | | Variation of links along the path |
+------------+---------------+-------------------------------------+
| Loss Rate | Section 4.4 | Base: [RFC7471] [RFC8570] [RFC8571] |
| | | aggr Unidirectional Link Loss |
+------------+---------------+-------------------------------------+
| Residual | Section 5.2 | Base: [RFC7471] [RFC8570] [RFC8571] |
| Bandwidth | | min Unidirectional Residual BW |
+------------+---------------+-------------------------------------+
| Available | Section 5.3 | Base: [RFC7471] [RFC8570] [RFC8571] |
| Bandwidth | | min Unidirectional Available BW |
+------------+---------------+-------------------------------------+
| TCP | Section 5.1 | [RFC9438] |
| Throughput | | |
+------------+---------------+-------------------------------------+
| Hop Count | Section 4.5 | [RFC7285] |
+------------+---------------+-------------------------------------+
Table 1: Cost Metrics Defined in This Document
The first six metrics listed in Table 1 (i.e., one-way delay, round-
trip delay, delay variation, loss rate, residual bandwidth, and
available bandwidth) are derived from the set of Traffic Engineering
(TE) performance metrics commonly defined in OSPF [RFC3630]
[RFC7471], IS-IS [RFC5305] [RFC8570], and BGP - Link State (BGP-LS)
[RFC8571]. Deriving ALTO cost performance metrics from existing
network-layer TE performance metrics, and making it exposed to ALTO,
can be a typical mechanism used by network operators to deploy ALTO
[RFC7971] [FlowDirector]. This document defines the base semantics
of these metrics by extending them from link metrics to end-to-end
metrics for ALTO. The "Semantics Based On" column specifies at a
high level how the end-to-end metrics are computed from link metrics;
details will be specified in the following sections.
The Min/Max Unidirectional Link Delay metric as defined in [RFC8570]
and [RFC8571], and Maximum (Link) Bandwidth as defined in [RFC3630]
and [RFC5305], are not listed in Table 1 because they can be handled
by applying the statistical operators defined in this document. The
metrics related to utilized bandwidth and reservable bandwidth (i.e.,
Maximum Reservable (Link) Bandwidth and Unreserved Bandwidth as
defined in [RFC3630] and [RFC5305]) are outside the scope of this
document.
The seventh metric in Table 1 (the estimated TCP-flow throughput
metric) provides an estimation of the bandwidth of a TCP flow, using
TCP throughput modeling, to support use cases of adaptive
applications [Prophet] [G2]. Note that other transport-specific
metrics can be defined in the future. For example, QUIC-related
metrics [RFC9000] can be considered when the methodology for
measuring such metrics is more mature (e.g., see
[QUIC-THROUGHPUT-TESTING]).
The eighth metric in Table 1 (the hop count metric) is mentioned, but
not defined, in the ALTO base protocol [RFC7285]; this document
provides a definition for it.
These eight performance metrics can be classified into two
categories: those derived from the performance of individual packets
(i.e., one-way delay, round-trip delay, delay variation, loss rate,
and hop count) and those related to bandwidth/throughput (residual
bandwidth, available bandwidth, and TCP throughput). These two
categories are defined in Sections 4 and 5, respectively. Note that
all metrics except round-trip delay are unidirectional. An ALTO
client will need to query both directions if needed.
The purpose of this document is to ensure proper usage of these eight
performance metrics in the context of ALTO. This document follows
the guidelines defined in Section 14.2 of [RFC7285] on registering
ALTO cost metrics. Hence, it specifies the identifier, the intended
semantics, and the security considerations of each one of the metrics
specified in Table 1.
The definitions of the intended semantics of the metrics tend to be
coarse grained and are for guidance only, and they may work well for
ALTO. On the other hand, a performance measurement framework, such
as the IP Performance Metrics (IPPM) framework, may provide more
details for defining a performance metric. This document introduces
a mechanism called "cost-context" to provide additional details, when
they are available; see Section 3.
Following the ALTO base protocol, this document uses JSON to specify
the value type of each defined metric. See [RFC8259] for JSON data
type specifications. In particular, [RFC7285] specifies that cost
values should be assumed by default to be 'JSONNumber'. When
defining the value representation of each metric in Table 1, this
document conforms to [RFC7285] but specifies additional, generic
constraints on valid JSONNumbers for each metric. For example, each
new metric in Table 1 will be specified as non-negative (>= 0); Hop
Count is specified to be an integer.
An ALTO server may provide only a subset of the metrics described in
this document. For example, those that are subject to privacy
concerns should not be provided to unauthorized ALTO clients. Hence,
all cost metrics defined in this document are optional; not all of
them need to be exposed to a given application. When an ALTO server
supports a cost metric defined in this document, it announces the
metric in its information resource directory (IRD) as defined in
Section 9.2 of [RFC7285].
An ALTO server introducing these metrics should consider related
security issues. As a generic security consideration regarding
reliability and trust in the exposed metric values, applications
SHOULD promptly stop using ALTO-based guidance if they detect that
the exposed information does not preserve their performance level or
even degrades it. Section 7 discusses security considerations in
more detail.
2. Requirements Language
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.
3. Performance Metric Attributes
The definitions of the metrics in this document are coarse grained,
based on network-layer TE performance metrics, and for guidance only.
A fine-grained framework as specified in [RFC6390] requires that the
fine-grained specification of a network performance metric include
six components: (1) Metric Name, (2) Metric Description, (3) Method
of Measurement or Calculation, (4) Units of Measurement, (5)
Measurement Points, and (6) Measurement Timing. Requiring that an
ALTO server provide precise, fine-grained values for all six
components for each metric that it exposes may not be feasible or
necessary for all ALTO use cases. For example, an ALTO server
computing its metrics from network-layer TE performance metrics may
not have information about the method of measurement or calculation
(e.g., measured traffic patterns).
To address the issue and realize ALTO use cases for the metrics
listed in Table 1, this document defines performance metric
identifiers that can be used in the ALTO Protocol with the following
well-defined items: (1) Metric Name, (2) Metric Description, (3)
Units of Measurement, and (4) Measurement Points, which are always
specified by the specific ALTO services; for example, the endpoint
cost service is between the two endpoints. Hence, the ALTO
performance metric identifiers provide basic metric attributes.
To allow the flexibility of allowing an ALTO server to provide fine-
grained information such as Method of Measurement or Calculation
according to its policy and use cases, this document introduces
context information so that the server can provide these additional
details.
3.1. Performance Metric Context: "cost-context"
The core additional details of a performance metric specify how the
metric is obtained. This is referred to as the source of the metric.
Specifically, this document defines three types of coarse-grained
metric information sources: "nominal", "sla", and "estimation".
For a given type of source, precise interpretation of a performance
metric value can depend on specific measurement and computation
parameters.
To make it possible to specify the source and the aforementioned
parameters, this document introduces an optional "cost-context" field
to the "cost-type" field defined by the ALTO base protocol
(Section 10.7 of [RFC7285]) as follows:
object {
CostMetric cost-metric;
CostMode cost-mode;
[CostContext cost-context;]
[JSONString description;]
} CostType;
object {
JSONString cost-source;
[JSONValue parameters;]
} CostContext;
"cost-context" will not be used as a key to distinguish among
performance metrics. Hence, an ALTO information resource MUST NOT
announce multiple CostType entries with the same "cost-metric",
"cost-mode", and "cost-context". They must be placed into different
information resources.
The "cost-source" field of the "cost-context" field is defined as a
string consisting of only ASCII alphanumeric characters
(U+0030-U+0039, U+0041-U+005A, and U+0061-U+007A). The "cost-source"
field is used in this document to indicate a string of this format.
As mentioned above, this document defines three values for "cost-
source": "nominal", "sla", and "estimation". The "cost-source" field
of the "cost-context" field MUST be one that is registered in the
"ALTO Cost Source Types" registry (Section 8).
The "nominal" category indicates that the metric value is statically
configured by the underlying devices. Not all metrics have
reasonable "nominal" values. For example, throughput can have a
nominal value, which indicates the configured transmission rate of
the involved devices; latency typically does not have a nominal
value.
The "sla" category indicates that the metric value is derived from
some commitment, which this document refers to as a Service Level
Agreement (SLA). Some operators also use terms such as "target" or
"committed" values. For an "sla" metric, it is RECOMMENDED that the
"parameters" field provide a link to the SLA definition.
The "estimation" category indicates that the metric value is computed
through an estimation process. An ALTO server may compute
"estimation" values by retrieving and/or aggregating information from
routing protocols (e.g., see [RFC7471], [RFC8570], and [RFC8571]),
traffic measurement management tools (e.g., the Two-Way Active
Measurement Protocol (TWAMP) [RFC5357]), and measurement frameworks
(e.g., IPPM), with corresponding operational issues. An illustration
of potential information flows used for estimating these metrics is
shown in Figure 1. Section 6 discusses in more detail the
operational issues and how a network may address them.
+--------+ +--------+ +--------+
| Client | | Client | | Client |
+----^---+ +---^----+ +---^----+
| | |
+-----------|-----------+
|ALTO Protocol
|
|
+--+-----+ retrieval +-----------+
| ALTO |<----------------| Routing |
| Server | and aggregation| Protocols |
| |<-------------+ | |
+--------+ | +-----------+
|
| +------------+
| |Performance |
---| Monitoring |
| Tools |
+------------+
Figure 1: A Framework to Compute Estimation of Performance Metrics
There can be multiple options available when choosing the "cost-
source" category; the operator of an ALTO server will make that
choice. If a metric does not include a "cost-source" value, the
application MUST assume that the value of "cost-source" is the most
generic source, i.e., "estimation".
3.2. Performance Metric Statistics
The measurement of a performance metric often yields a set of samples
from an observation distribution [Prometheus], instead of a single
value. A statistical operator is applied to the samples to obtain a
value to be reported to the client. Multiple statistical operators
(e.g., min, median, and max) are commonly being used.
Hence, this document extends the general ASCII alphanumeric cost
metric strings, formally specified as the CostMetric type defined in
Section 10.6 of [RFC7285], as follows:
A cost metric string consists of a base metric identifier (or base
identifier for short) string, followed by an optional statistical
operator string, connected by the ASCII colon character (':',
U+003A), if the statistical operator string exists. The total
length of the cost metric string MUST NOT exceed 32, as required
by [RFC7285].
The statistical operator string MUST be one of the following:
cur: The instantaneous observation value of the metric from the most
recent sample (i.e., the current value).
percentile, with the letter 'p' followed by a number: Gives the
percentile specified by the number following the letter 'p'. The
number MUST be a non-negative JSON number in the range [0, 100]
(i.e., greater than or equal to 0 and less than or equal to 100),
followed by an optional decimal part, if higher precision is
needed. The decimal part should start with the '.' separator
(U+002E) and be followed by a sequence of one or more ASCII
numbers between '0' and '9'. Assume that this number is y, and
consider the case where the samples are coming from a random
variable X. The metric then returns x, such that the probability
of X is less than or equal to x, i.e., Prob(X <= x), = y/100. For
example, delay-ow:p99 gives the 99th percentile of observed one-
way delay; delay-ow:p99.9 gives the 99.9th percentile. Note that
some systems use quantile, which is in the range [0, 1]. When
there is a more common form for a given percentile, it is
RECOMMENDED that the common form be used; that is, instead of p0,
use min; instead of p50, use median; instead of p100, use max.
min: The minimal value of the observations.
max: The maximal value of the observations.
median: The midpoint (i.e., p50) of the observations.
mean: The arithmetic mean value of the observations.
stddev: The standard deviation of the observations.
stdvar: The standard variance of the observations.
Examples of cost metric strings then include "delay-ow", "delay-
ow:min", and "delay-ow:p99", where "delay-ow" is the base metric
identifier string; "min" and "p99" are example statistical operator
strings.
If a cost metric string does not have the optional statistical
operator string, the statistical operator SHOULD be interpreted as
the default statistical operator in the definition of the base
metric. If the definition of the base metric does not provide a
definition for the default statistical operator, the metric MUST be
considered the median value.
Note that [RFC7285] limits the overall cost metric identifier to 32
characters. The cost metric variants with statistical operator
suffixes defined by this document are also subject to the same
overall 32-character limit, so certain combinations of (long) base
metric identifiers and statistical operators will not be
representable. If such a situation arises, it could be addressed by
defining a new base metric identifier that is an "alias" of the
desired base metric, with identical semantics and just a shorter
name.
4. Packet Performance Metrics
This section introduces ALTO network performance metrics on one-way
delay, round-trip delay, delay variation, packet loss rate, and hop
count. They measure the "quality of experience" of the stream of
packets sent from a resource provider to a resource consumer. The
measurements of each individual packet (pkt) can include the delay
from the time when the packet enters the network to the time when the
packet leaves the network (pkt.delay), whether the packet is dropped
before reaching the destination (pkt.dropped), and the number of
network hops that the packet traverses (pkt.hopcount). The semantics
of the performance metrics defined in this section are that they are
statistics computed from these measurements; for example, the
x-percentile of the one-way delay is the x-percentile of the set of
delays {pkt.delay} for the packets in the stream.
4.1. Cost Metric: One-Way Delay (delay-ow)
4.1.1. Base Identifier
The base identifier for this performance metric is "delay-ow".
4.1.2. Value Representation
The metric value type is a single 'JSONNumber' type value conforming
to the number specifications provided in Section 6 of [RFC8259]. The
unit is expressed in microseconds. Hence, the number can be a
floating-point number to express delay that is smaller than
microseconds. The number MUST be non-negative.
4.1.3. Intended Semantics and Use
Intended Semantics: To specify the temporal and spatial aggregated
delay of a stream of packets from the specified source to the
specified destination. The base semantics of the metric is the
Unidirectional Delay metric as defined in [RFC8571], [RFC8570],
and [RFC7471], but instead of specifying the delay for a link, it
is the (temporal) aggregation of the link delays from the source
to the destination. A non-normative reference definition of the
end-to-end one-way delay metric is provided in [RFC7679]. The
spatial aggregation level is specified in the query context, e.g.,
provider-defined identifier (PID) to PID, or endpoint to endpoint,
where the PID is as defined in Section 5.1 of [RFC7285].
Use: This metric could be used as a cost metric constraint attribute
or as a returned cost metric in the response.
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 239
Content-Type: application/alto-endpointcostparams+json
Accept:
application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "delay-ow"
},
"endpoints": {
"srcs": [
"ipv4:192.0.2.2"
],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34"
]
}
}
HTTP/1.1 200 OK
Content-Length: 247
Content-Type: application/alto-endpointcost+json
{
"meta": {
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "delay-ow"
}
},
"endpoint-cost-map": {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89": 10,
"ipv4:198.51.100.34": 20
}
}
}
Figure 2: Delay Value on Source-Destination Endpoint Pairs
(Example 1)
Note that since the "cost-type" does not include the "cost-source"
field, the values are based on "estimation". Since the identifier
does not include the statistical operator string component, the
values will represent median values.
Figure 3 shows an example that is similar to Example 1 (Figure 2),
but for IPv6.
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 252
Content-Type: application/alto-endpointcostparams+json
Accept:
application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "delay-ow"
},
"endpoints": {
"srcs": [
"ipv6:2001:db8:100::1"
],
"dsts": [
"ipv6:2001:db8:100::2",
"ipv6:2001:db8:100::3"
]
}
}
HTTP/1.1 200 OK
Content-Length: 257
Content-Type: application/alto-endpointcost+json
{
"meta": {
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "delay-ow"
}
},
"endpoint-cost-map": {
"ipv6:2001:db8:100::1": {
"ipv6:2001:db8:100::2": 10,
"ipv6:2001:db8:100::3": 20
}
}
}
Figure 3: Delay Value on Source-Destination Endpoint Pairs for
IPv6 (Example 1a)
4.1.4. Cost-Context Specification Considerations
"nominal": Typically, network one-way delay does not have a nominal
value.
"sla": Many networks provide delay-related parameters in their
application-level SLAs. It is RECOMMENDED that the "parameters"
field of an "sla" one-way delay metric include a link (i.e., a
field named "link") providing a URI for the specification of SLA
details, if available. Such a specification can be either
(1) free text for possible presentation to the user or (2) a
formal specification. The format of the specification is outside
the scope of this document.
"estimation": The exact estimation method is outside the scope of
this document. There can be multiple sources for estimating one-
way delay. For example, the ALTO server may estimate the end-to-
end delay by aggregation of routing protocol link metrics; the
server may also estimate the delay using active, end-to-end
measurements -- for example, using the IPPM framework [RFC2330].
If the estimation is computed by aggregation of routing protocol link
metrics (e.g., Unidirectional Link Delay metrics for OSPF [RFC7471],
IS-IS [RFC8570], or BGP-LS [RFC8571]), it is RECOMMENDED that the
"parameters" field of an "estimation" one-way delay metric include
the following information: (1) the RFC defining the routing protocol
metrics (e.g., see [RFC7471] for derived metrics), (2) configurations
of the routing link metrics such as configured intervals, and (3) the
aggregation method from link metrics to end-to-end metrics. During
aggregation from link metrics to end-to-end metrics, the server
should be cognizant of potential issues when computing an end-to-end
summary statistic from link statistics. The default end-to-end
average one-way delay is the sum of average link one-way delays. If
an ALTO server provides the min and max statistical operators for the
one-way delay metric, the values can be computed directly from the
routing link metrics, as [RFC7471], [RFC8570], and [RFC8571] provide
Min/Max Unidirectional Link Delay.
If the estimation is from the IPPM measurement framework, it is
RECOMMENDED that the "parameters" field of an "estimation" one-way
delay metric include the URI in the "URI" field of the IPPM metric
defined in the IPPM "Performance Metrics" registry [IANA-IPPM] (e.g.,
<https://www.iana.org/assignments/performance-metrics/
OWDelay_Active_IP-UDP-Poisson-
Payload250B_RFC8912sec7_Seconds_95Percentile>). The IPPM metric MUST
be one-way delay (i.e., IPPM OWDelay* metrics). The statistical
operator of the ALTO metric MUST be consistent with the IPPM
statistical property (e.g., 95th percentile).
4.2. Cost Metric: Round-Trip Delay (delay-rt)
4.2.1. Base Identifier
The base identifier for this performance metric is "delay-rt".
4.2.2. Value Representation
The metric value type is a single 'JSONNumber' type value conforming
to the number specifications provided in Section 6 of [RFC8259]. The
number MUST be non-negative. The unit is expressed in microseconds.
4.2.3. Intended Semantics and Use
Intended Semantics: To specify temporal and spatial aggregated
round-trip delay between the specified source and specified
destination. The base semantics is that it is the sum of the one-
way delay from the source to the destination and the one-way delay
from the destination back to the source, where the one-way delay
is as defined in Section 4.1. A non-normative reference
definition of the end-to-end round-trip delay metric is provided
in [RFC2681]. The spatial aggregation level is specified in the
query context (e.g., PID to PID, or endpoint to endpoint).
Note that it is possible for a client to query two one-way delay
(delay-ow) items and then compute the round-trip delay. The
server should be cognizant of the consistency of values.
Use: This metric could be used as a cost metric constraint attribute
or as a returned cost metric in the response.
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 238
Content-Type: application/alto-endpointcostparams+json
Accept:
application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "delay-rt"
},
"endpoints": {
"srcs": [
"ipv4:192.0.2.2"
],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34"
]
}
}
HTTP/1.1 200 OK
Content-Length: 245
Content-Type: application/alto-endpointcost+json
{
"meta": {
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "delay-rt"
}
},
"endpoint-cost-map": {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89": 4,
"ipv4:198.51.100.34": 3
}
}
}
Figure 4: Round-Trip Delay of Source-Destination Endpoint Pairs
(Example 2)
4.2.4. Cost-Context Specification Considerations
"nominal": Typically, network round-trip delay does not have a
nominal value.
"sla": See the "sla" entry in Section 4.1.4.
"estimation": See the "estimation" entry in Section 4.1.4. For
estimation by aggregation of routing protocol link metrics, the
aggregation should include all links from the source to the
destination and then back to the source; for estimation using
IPPM, the IPPM metric MUST be round-trip delay (i.e., IPPM
RTDelay* metrics). The statistical operator of the ALTO metric
MUST be consistent with the IPPM statistical property (e.g., 95th
percentile).
4.3. Cost Metric: Delay Variation (delay-variation)
4.3.1. Base Identifier
The base identifier for this performance metric is "delay-variation".
4.3.2. Value Representation
The metric value type is a single 'JSONNumber' type value conforming
to the number specifications provided in Section 6 of [RFC8259]. The
number MUST be non-negative. The unit is expressed in microseconds.
4.3.3. Intended Semantics and Use
Intended Semantics: To specify temporal and spatial aggregated delay
variation (also called delay jitter) with respect to the minimum
delay observed on the stream over the one-way delay from the
specified source and destination, where the one-way delay is as
defined in Section 4.1. A non-normative reference definition of
the end-to-end one-way delay variation metric is provided in
[RFC3393]. Note that [RFC3393] allows the specification of a
generic selection function F to unambiguously define the two
packets selected to compute delay variations. This document
defines the specific case where F selects the packet with the
smallest one-way delay as the "first" packet. The spatial
aggregation level is specified in the query context (e.g., PID to
PID, or endpoint to endpoint).
Note that in statistics, variation is typically evaluated by the
distance from samples relative to the mean. In the context of
networking, it is more commonly defined from samples relative to
the min. This definition follows the networking convention.
Use: This metric could be used as a cost metric constraint attribute
or as a returned cost metric in the response.
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 245
Content-Type: application/alto-endpointcostparams+json
Accept:
application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "delay-variation"
},
"endpoints": {
"srcs": [
"ipv4:192.0.2.2"
],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34"
]
}
}
HTTP/1.1 200 OK
Content-Length: 252
Content-Type: application/alto-endpointcost+json
{
"meta": {
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "delay-variation"
}
},
"endpoint-cost-map": {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89": 0,
"ipv4:198.51.100.34": 1
}
}
}
Figure 5: Delay Variation Value on Source-Destination Endpoint
Pairs (Example 3)
4.3.4. Cost-Context Specification Considerations
"nominal": Typically, network delay variation does not have a
nominal value.
"sla": See the "sla" entry in Section 4.1.4.
"estimation": See the "estimation" entry in Section 4.1.4. For
estimation by aggregation of routing protocol link metrics, the
default aggregation of the average of delay variations is the sum
of the link delay variations; for estimation using IPPM, the IPPM
metric MUST be delay variation (i.e., IPPM OWPDV* metrics). The
statistical operator of the ALTO metric MUST be consistent with
the IPPM statistical property (e.g., 95th percentile).
4.4. Cost Metric: Loss Rate (lossrate)
4.4.1. Base Identifier
The base identifier for this performance metric is "lossrate".
4.4.2. Value Representation
The metric value type is a single 'JSONNumber' type value conforming
to the number specifications provided in Section 6 of [RFC8259]. The
number MUST be non-negative. The value represents the percentage of
packet losses.
4.4.3. Intended Semantics and Use
Intended Semantics: To specify the temporal and spatial aggregated
one-way packet loss rate from the specified source and the
specified destination. The base semantics of the metric is the
Unidirectional Link Loss metric as defined in [RFC8571],
[RFC8570], and [RFC7471], but instead of specifying the loss for a
link, it is the aggregated loss of all links from the source to
the destination. The spatial aggregation level is specified in
the query context (e.g., PID to PID, or endpoint to endpoint).
Use: This metric could be used as a cost metric constraint attribute
or as a returned cost metric in the response.
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 238
Content-Type: application/alto-endpointcostparams+json
Accept:
application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "lossrate"
},
"endpoints": {
"srcs": [
"ipv4:192.0.2.2"
],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34"
]
}
}
HTTP/1.1 200 OK
Content-Length: 248
Content-Type: application/alto-endpointcost+json
{
"meta": {
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "lossrate"
}
},
"endpoint-cost-map": {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89": 0,
"ipv4:198.51.100.34": 0.01
}
}
}
Figure 6: Loss Rate Value on Source-Destination Endpoint Pairs
(Example 4)
4.4.4. Cost-Context Specification Considerations
"nominal": Typically, the packet loss rate does not have a nominal
value, although some networks may specify zero losses.
"sla": See the "sla" entry in Section 4.1.4.
"estimation": See the "estimation" entry in Section 4.1.4. For
estimation by aggregation of routing protocol link metrics, the
default aggregation of the average loss rate is the sum of the
link loss rates. But this default aggregation is valid only if
two conditions are met: (1) link loss rates are low and (2) one
assumes that each link's loss events are uncorrelated with every
other link's loss events. When loss rates at the links are high
but independent, the general formula for aggregating loss,
assuming that each link is independent, is to compute end-to-end
loss as one minus the product of the success rate for each link.
Aggregation when losses at links are correlated can be more
complex, and the ALTO server should be cognizant of correlated
loss rates. For estimation using IPPM, the IPPM metric MUST be
packet loss (i.e., IPPM OWLoss* metrics). The statistical
operator of the ALTO metric MUST be consistent with the IPPM
statistical property (e.g., 95th percentile).
4.5. Cost Metric: Hop Count (hopcount)
The hop count (hopcount) metric is mentioned in Section 9.2.3 of
[RFC7285] as an example. This section further clarifies its
properties.
4.5.1. Base Identifier
The base identifier for this performance metric is "hopcount".
4.5.2. Value Representation
The metric value type is a single 'JSONNumber' type value conforming
to the number specifications provided in Section 6 of [RFC8259]. The
number MUST be a non-negative integer (greater than or equal to 0).
The value represents the number of hops.
4.5.3. Intended Semantics and Use
Intended Semantics: To specify the number of hops in the path from
the specified source to the specified destination. The hop count
is a basic measurement of distance in a network and can be exposed
as the number of router hops computed from the routing protocols
originating this information. A hop, however, may represent other
units. The spatial aggregation level is specified in the query
context (e.g., PID to PID, or endpoint to endpoint).
Use: This metric could be used as a cost metric constraint attribute
or as a returned cost metric in the response.
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 238
Content-Type: application/alto-endpointcostparams+json
Accept:
application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "hopcount"
},
"endpoints": {
"srcs": [
"ipv4:192.0.2.2"
],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34"
]
}
}
HTTP/1.1 200 OK
Content-Length: 245
Content-Type: application/alto-endpointcost+json
{
"meta": {
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "hopcount"
}
},
"endpoint-cost-map": {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89": 5,
"ipv4:198.51.100.34": 3
}
}
}
Figure 7: Hop Count Value on Source-Destination Endpoint Pairs
(Example 5)
4.5.4. Cost-Context Specification Considerations
"nominal": Typically, the hop count does not have a nominal value.
"sla": Typically, the hop count does not have an SLA value.
"estimation": The exact estimation method is outside the scope of
this document. An example of estimating hop count values is by
importing from IGP routing protocols. It is RECOMMENDED that the
"parameters" field of an "estimation" hop count define the meaning
of a hop.
5. Throughput/Bandwidth Performance Metrics
This section introduces three metrics related to throughput and
bandwidth. Given a specified source and a specified destination,
these metrics reflect the volume of traffic that the network can
carry from the source to the destination.
5.1. Cost Metric: TCP Throughput (tput)
5.1.1. Base Identifier
The base identifier for this performance metric is "tput".
5.1.2. Value Representation
The metric value type is a single 'JSONNumber' type value conforming
to the number specifications provided in Section 6 of [RFC8259]. The
number MUST be non-negative. The unit is bytes per second.
5.1.3. Intended Semantics and Use
Intended Semantics: To give the throughput of a congestion control
conforming TCP flow from the specified source to the specified
destination. The throughput SHOULD be interpreted as only an
estimation, and the estimation is designed only for bulk flows.
Use: This metric could be used as a cost metric constraint attribute
or as a returned cost metric in the response.
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 234
Content-Type: application/alto-endpointcostparams+json
Accept:
application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "tput"
},
"endpoints": {
"srcs": [
"ipv4:192.0.2.2"
],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34"
]
}
}
HTTP/1.1 200 OK
Content-Length: 251
Content-Type: application/alto-endpointcost+json
{
"meta": {
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "tput"
}
},
"endpoint-cost-map": {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89": 256000,
"ipv4:198.51.100.34": 128000
}
}
}
Figure 8: TCP Throughput Value on Source-Destination Endpoint
Pairs (Example 6)
5.1.4. Cost-Context Specification Considerations
"nominal": Typically, TCP throughput does not have a nominal value
and SHOULD NOT be generated.
"sla": Typically, TCP throughput does not have an SLA value and
SHOULD NOT be generated.
"estimation": The exact estimation method is outside the scope of
this document. It is RECOMMENDED that the "parameters" field of
an "estimation" TCP throughput metric include the following
information: (1) the congestion control algorithm and (2) the
estimation methodology. To specify (1), it is RECOMMENDED that
the "parameters" field (object) include a field named "congestion-
control-algorithm", which provides a URI for the specification of
the algorithm; for example, for an ALTO server to provide
estimation of the throughput of a CUBIC congestion control flow,
its "parameters" field includes the "congestion-control-algorithm"
field, with value being set to the URI for [RFC9438]; for an
ongoing congestion control algorithm such as BBR, a link to its
specification can be added. To specify (2), the "parameters"
field includes as many details as possible; for example, for the
TCP Cubic throughout estimation, the "parameters" field specifies
that the throughput is estimated by setting _C_ to 0.4, and the
equation in [RFC9438], Section 5.1, Figure 8 is applied; as an
alternative, the methodology may be based on the NUM model
[Prophet] or the model described in [G2]. The exact specification
of the "parameters" field is outside the scope of this document.
5.2. Cost Metric: Residual Bandwidth (bw-residual)
5.2.1. Base Identifier
The base identifier for this performance metric is "bw-residual".
5.2.2. Value Representation
The metric value type is a single 'JSONNumber' type value that is
non-negative. The unit of measurement is bytes per second.
5.2.3. Intended Semantics and Use
Intended Semantics: To specify temporal and spatial residual
bandwidth from the specified source to the specified destination.
The base semantics of the metric is the Unidirectional Residual
Bandwidth metric as defined in [RFC8571], [RFC8570], and
[RFC7471], but instead of specifying the residual bandwidth for a
link, it is the residual bandwidth of the path from the source to
the destination. Hence, it is the minimal residual bandwidth
among all links from the source to the destination. When the max
statistical operator is defined for the metric, it typically
provides the minimum of the link capacities along the path, as the
default value of the residual bandwidth of a link is its link
capacity [RFC8571] [RFC8570] [RFC7471]. The spatial aggregation
unit is specified in the query context (e.g., PID to PID, or
endpoint to endpoint).
The default statistical operator for residual bandwidth is the
current instantaneous sample; that is, the default is assumed to
be "cur".
Use: This metric could be used as a cost metric constraint attribute
or as a returned cost metric in the response.
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 241
Content-Type: application/alto-endpointcostparams+json
Accept:
application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "bw-residual"
},
"endpoints": {
"srcs": [
"ipv4:192.0.2.2"
],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34"
]
}
}
HTTP/1.1 200 OK
Content-Length: 255
Content-Type: application/alto-endpointcost+json
{
"meta": {
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "bw-residual"
}
},
"endpoint-cost-map": {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89": 0,
"ipv4:198.51.100.34": 2000
}
}
}
Figure 9: Residual Bandwidth Value on Source-Destination Endpoint
Pairs (Example 7)
5.2.4. Cost-Context Specification Considerations
"nominal": Typically, residual bandwidth does not have a nominal
value.
"sla": Typically, residual bandwidth does not have an SLA value.
"estimation": See the "estimation" entry in Section 4.1.4. The
current ("cur") residual bandwidth of a path is the minimal
residual bandwidth of all links on the path.
5.3. Cost Metric: Available Bandwidth (bw-available)
5.3.1. Base Identifier
The base identifier for this performance metric is "bw-available".
5.3.2. Value Representation
The metric value type is a single 'JSONNumber' type value that is
non-negative. The unit of measurement is bytes per second.
5.3.3. Intended Semantics and Use
Intended Semantics: To specify temporal and spatial available
bandwidth from the specified source to the specified destination.
The base semantics of the metric is the Unidirectional Available
Bandwidth metric as defined in [RFC8571], [RFC8570], and
[RFC7471], but instead of specifying the available bandwidth for a
link, it is the available bandwidth of the path from the source to
the destination. Hence, it is the minimal available bandwidth
among all links from the source to the destination. The spatial
aggregation unit is specified in the query context (e.g., PID to
PID, or endpoint to endpoint).
The default statistical operator for available bandwidth is the
current instantaneous sample; that is, the default is assumed to
be "cur".
Use: This metric could be used as a cost metric constraint attribute
or as a returned cost metric in the response.
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 244
Content-Type: application/alto-endpointcostparams+json
Accept:
application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "bw-available"
},
"endpoints": {
"srcs": [
"ipv4:192.0.2.2"
],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34"
]
}
}
HTTP/1.1 200 OK
Content-Length: 255
Content-Type: application/alto-endpointcost+json
{
"meta": {
"cost-type": {
"cost-mode": "numerical",
"cost-metric": "bw-available"
}
},
"endpoint-cost-map": {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89": 0,
"ipv4:198.51.100.34": 2000
}
}
}
Figure 10: Available Bandwidth Value on Source-Destination
Endpoint Pairs (Example 8)
5.3.4. Cost-Context Specification Considerations
"nominal": Typically, available bandwidth does not have a nominal
value.
"sla": Typically, available bandwidth does not have an SLA value.
"estimation": See the "estimation" entry in Section 4.1.4. The
current ("cur") available bandwidth of a path is the minimum of
the available bandwidth of all links on the path.
6. Operational Considerations
The exact measurement infrastructure, measurement conditions, and
computation algorithms can vary between different networks and are
outside the scope of this document. Both the ALTO server and the
ALTO clients, however, need to be cognizant of the operational issues
discussed in the following subsections.
Also, the performance metrics specified in this document are similar
in that they may use similar data sources and have similar issues in
their calculation. Hence, this document specifies issues that the
performance metrics might have in common and also discusses
challenges regarding the computation of ALTO performance metrics
(Section 6.4).
6.1. Source Considerations
The addition of the "cost-source" field solves a key issue: an ALTO
server needs data sources to compute the cost metrics described in
this document, and an ALTO client needs to know the data sources to
better interpret the values.
To avoid information that is too fine grained, this document
introduces "cost-source" to indicate only the high-level types of
data sources: "estimation", "nominal", or "sla", where "estimation"
is a type of measurement data source, "nominal" is a type of static
configuration, and "sla" is a type that is based more on policy.
For example, for "estimation", the ALTO server may use log servers or
the Operations, Administration, and Maintenance (OAM) system as its
data source, as recommended by [RFC7971]. In particular, the cost
metrics defined in this document can be computed using routing
systems as the data sources.
6.2. Metric Timestamp Considerations
Despite the introduction of the additional "cost-context"
information, the metrics do not have a field to indicate the
timestamps of the data used to compute the metrics. To indicate this
attribute, the ALTO server SHOULD return an HTTP Last-Modified value
to indicate the freshness of the data used to compute the performance
metrics.
If the ALTO client obtains updates through an incremental update
mechanism [RFC8895], the client SHOULD assume that the metric is
computed using a snapshot at the time that is approximated by the
receiving time.
6.3. Backward-Compatibility Considerations
One potential issue introduced by the optional "cost-source" field is
backward compatibility. Consider the case where an IRD defines two
"cost-type" entries with the same "cost-mode" and "cost-metric", but
one with "cost-source" being "estimation" and the other being "sla".
In such a case, an ALTO client that is not aware of the extension
will not be able to distinguish between these two types. A similar
issue can arise even with a single "cost-type" whose "cost-source" is
"sla": an ALTO client that is not aware of this extension will ignore
this field and instead consider the metric estimation.
To address the backward-compatibility issue, if a "cost-metric" is
"routingcost" and the metric contains a "cost-context" field, then it
MUST be "estimation"; if it is not, the client SHOULD reject the
information as invalid.
6.4. Computation Considerations
The metric values exposed by an ALTO server may result from
additional processing of measurements from data sources to compute
exposed metrics. This may involve data processing tasks such as
aggregating the results across multiple systems, removing outliers,
and creating additional statistics. The computation of ALTO
performance metrics can present two challenges.
6.4.1. Configuration Parameter Considerations
Performance metrics often depend on configuration parameters, and
exposing such configuration parameters can help an ALTO client to
better understand the exposed metrics. In particular, an ALTO server
may be configured to compute a TE metric (e.g., packet loss rate) at
fixed intervals, say every T seconds. To expose this information,
the ALTO server may provide the client with two pieces of additional
information: (1) when the metrics were last computed and (2) when the
metrics will be updated (i.e., the validity period of the exposed
metric values). The ALTO server can expose these two pieces of
information by using the HTTP response headers Last-Modified and
Expires.
6.4.2. Aggregation Computation Considerations
An ALTO server may not be able to measure the performance metrics to
be exposed. The basic issue is that the "source" information can
often be link-level information. For example, routing protocols
often measure and report only per-link loss and not end-to-end loss;
similarly, routing protocols report link-level available bandwidth
and not end-to-end available bandwidth. The ALTO server then needs
to aggregate these data to provide an abstract and unified view that
can be more useful to applications. The server should be aware that
different metrics may use different aggregation computations. For
example, the end-to-end latency of a path is the sum of the latencies
of the links on the path; the end-to-end available bandwidth of a
path is the minimum of the available bandwidth of the links on the
path; in contrast, aggregating loss values is complicated by the
potential for correlated loss events on different links in the path.
7. Security Considerations
The properties defined in this document present no security
considerations beyond those in Section 15 of the base ALTO
specification [RFC7285].
However, concerns addressed in Sections 15.1, 15.2, and 15.3 of
[RFC7285] remain of utmost importance. Indeed, TE performance is
highly sensitive ISP information; therefore, sharing TE metric values
in numerical mode requires full mutual confidence between the
entities managing the ALTO server and the ALTO client. ALTO servers
will most likely distribute numerical TE performance to ALTO clients
under strict and formal mutual trust agreements. On the other hand,
ALTO clients must be cognizant of the risks attached to such
information that they would have acquired outside formal conditions
of mutual trust.
To mitigate confidentiality risks during information transport of TE
performance metrics, the operator should address the risk of ALTO
information being leaked to malicious clients or third parties
through such attacks as person-in-the-middle (PITM) attacks. As
specified in Section 15.3.2 ("Protection Strategies") of [RFC7285],
the ALTO server should authenticate ALTO clients when transmitting an
ALTO information resource containing sensitive TE performance
metrics. Section 8.3.5 ("Authentication and Encryption") of
[RFC7285] specifies that ALTO server implementations as well as ALTO
client implementations MUST support the "https" URI scheme [RFC9110]
and Transport Layer Security (TLS) [RFC8446].
8. IANA Considerations
8.1. ALTO Cost Metrics Registry
IANA created and now maintains the "ALTO Cost Metrics" registry, as
listed in [RFC7285], Section 14.2, Table 3. This registry is located
at <https://www.iana.org/assignments/alto-protocol/>. IANA has added
the following entries to the "ALTO Cost Metrics" registry.
+=================+====================+===========+
| Identifier | Intended Semantics | Reference |
+=================+====================+===========+
| delay-ow | See Section 4.1 | RFC 9439 |
+-----------------+--------------------+-----------+
| delay-rt | See Section 4.2 | RFC 9439 |
+-----------------+--------------------+-----------+
| delay-variation | See Section 4.3 | RFC 9439 |
+-----------------+--------------------+-----------+
| lossrate | See Section 4.4 | RFC 9439 |
+-----------------+--------------------+-----------+
| hopcount | See Section 4.5 | RFC 9439 |
+-----------------+--------------------+-----------+
| tput | See Section 5.1 | RFC 9439 |
+-----------------+--------------------+-----------+
| bw-residual | See Section 5.2 | RFC 9439 |
+-----------------+--------------------+-----------+
| bw-available | See Section 5.3 | RFC 9439 |
+-----------------+--------------------+-----------+
Table 2: ALTO Cost Metrics Registry
8.2. ALTO Cost Source Types Registry
IANA has created the "ALTO Cost Source Types" registry. This
registry serves two purposes. First, it ensures the uniqueness of
identifiers referring to ALTO cost source types. Second, it provides
references to particular semantics of allocated cost source types to
be applied by both ALTO servers and applications utilizing ALTO
clients.
A new ALTO cost source type can be added after IETF Review [RFC8126],
to ensure that proper documentation regarding the new ALTO cost
source type and its security considerations has been provided. The
RFC(s) documenting the new cost source type should be detailed enough
to provide guidance to both ALTO service providers and applications
utilizing ALTO clients as to how values of the registered ALTO cost
source type should be interpreted. Updates and deletions of ALTO
cost source types follow the same procedure.
Registered ALTO address type identifiers MUST conform to the
syntactical requirements specified in Section 3.1. Identifiers are
to be recorded and displayed as strings.
Requests to add a new value to the registry MUST include the
following information:
Identifier: The name of the desired ALTO cost source type.
Intended Semantics: ALTO cost source types carry with them semantics
to guide their usage by ALTO clients. Hence, a document defining
a new type should provide guidance to both ALTO service providers
and applications utilizing ALTO clients as to how values of the
registered ALTO endpoint property should be interpreted.
Security Considerations: ALTO cost source types expose information
to ALTO clients. ALTO service providers should be made aware of
the security ramifications related to the exposure of a cost
source type.
IANA has registered the identifiers "nominal", "sla", and
"estimation" as listed in the table below.
+============+=========================+================+===========+
| Identifier | Intended | Security | Reference |
| | Semantics | Considerations | |
+============+=========================+================+===========+
| nominal | Values in nominal | Section 7 | RFC 9439 |
| | cases | | |
| | (Section 3.1) | | |
+------------+-------------------------+----------------+-----------+
| sla | Values reflecting | Section 7 | RFC 9439 |
| | Service Level | | |
| | Agreement | | |
| | (Section 3.1) | | |
+------------+-------------------------+----------------+-----------+
| estimation | Values by | Section 7 | RFC 9439 |
| | estimation | | |
| | (Section 3.1) | | |
+------------+-------------------------+----------------+-----------+
Table 3: ALTO Cost Source Types Registry
9. References
9.1. Normative References
[IANA-IPPM]
IANA, "Performance Metrics",
<https://www.iana.org/assignments/performance-metrics/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC6390] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
DOI 10.17487/RFC6390, October 2011,
<https://www.rfc-editor.org/info/rfc6390>.
[RFC7285] Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
"Application-Layer Traffic Optimization (ALTO) Protocol",
RFC 7285, DOI 10.17487/RFC7285, September 2014,
<https://www.rfc-editor.org/info/rfc7285>.
[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
<https://www.rfc-editor.org/info/rfc7471>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
2019, <https://www.rfc-editor.org/info/rfc8570>.
[RFC8571] Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
IGP Traffic Engineering Performance Metric Extensions",
RFC 8571, DOI 10.17487/RFC8571, March 2019,
<https://www.rfc-editor.org/info/rfc8571>.
[RFC8895] Roome, W. and Y. Yang, "Application-Layer Traffic
Optimization (ALTO) Incremental Updates Using Server-Sent
Events (SSE)", RFC 8895, DOI 10.17487/RFC8895, November
2020, <https://www.rfc-editor.org/info/rfc8895>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC9438] Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed.,
"CUBIC for Fast and Long-Distance Networks", RFC 9438,
DOI 10.17487/RFC9438, August 2023,
<https://www.rfc-editor.org/info/rfc9438>.
9.2. Informative References
[FlowDirector]
Pujol, E., Poese, I., Zerwas, J., Smaragdakis, G., and A.
Feldmann, "Steering Hyper-Giants' Traffic at Scale", ACM
CoNEXT '19, December 2019.
[G2] Ros-Giralt, J., Bohara, A., Yellamraju, S., Harper
Langston, M., Lethin, R., Jiang, Y., Tassiulas, L., Li,
J., Tan, Y., and M. Veeraraghavan, "On the Bottleneck
Structure of Congestion-Controlled Networks", Proceedings
of the ACM on Measurement and Analysis of Computing
Systems, Vol. 3, No. 3, Article No. 59, pp. 1-31,
DOI 10.1145/3366707, December 2019,
<https://dl.acm.org/doi/10.1145/3366707>.
[Prometheus]
Volz, J. and B. Rabenstein, "Prometheus: A Next-Generation
Monitoring System (Talk)", SREcon15 Europe, May 2015.
[Prophet] Zhang, J., Gao, K., Yang, YR., and J. Bi, "Prophet: Toward
Fast, Error-Tolerant Model-Based Throughput Prediction for
Reactive Flows in DC Networks", IEEE/ACM Transactions on
Networking, Volume 28, Issue 601, pp. 2475-2488, December
2020, <https://dl.acm.org/doi/10.1109/TNET.2020.3016838>.
[QUIC-THROUGHPUT-TESTING]
Corre, K., "Framework for QUIC Throughput Testing", Work
in Progress, Internet-Draft, draft-corre-quic-throughput-
testing-00, 17 September 2021,
<https://datatracker.ietf.org/doc/html/draft-corre-quic-
throughput-testing-00>.
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330,
DOI 10.17487/RFC2330, May 1998,
<https://www.rfc-editor.org/info/rfc2330>.
[RFC2681] Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
Delay Metric for IPPM", RFC 2681, DOI 10.17487/RFC2681,
September 1999, <https://www.rfc-editor.org/info/rfc2681>.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
DOI 10.17487/RFC3393, November 2002,
<https://www.rfc-editor.org/info/rfc3393>.
[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, DOI 10.17487/RFC5357, October 2008,
<https://www.rfc-editor.org/info/rfc5357>.
[RFC7679] Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
Ed., "A One-Way Delay Metric for IP Performance Metrics
(IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
2016, <https://www.rfc-editor.org/info/rfc7679>.
[RFC7971] Stiemerling, M., Kiesel, S., Scharf, M., Seidel, H., and
S. Previdi, "Application-Layer Traffic Optimization (ALTO)
Deployment Considerations", RFC 7971,
DOI 10.17487/RFC7971, October 2016,
<https://www.rfc-editor.org/info/rfc7971>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
Acknowledgments
The authors of this document would like to thank Martin Duke for the
highly informative, thorough AD reviews and comments. We thank
Christian Amsüss, Elwyn Davies, Haizhou Du, Kai Gao, Geng Li, Lili
Liu, Danny Alex Lachos Perez, and Brian Trammell for their reviews
and comments. We thank Benjamin Kaduk, Erik Kline, Francesca
Palombini, Lars Eggert, Martin Vigoureux, Murray Kucherawy, Roman
Danyliw, Zaheduzzaman Sarker, and Éric Vyncke for discussions and
comments that improved this document.
Authors' Addresses
Qin Wu
Huawei
Yuhua District
101 Software Avenue
Nanjing
Jiangsu, 210012
China
Email: bill.wu@huawei.com
Y. Richard Yang
Yale University
51 Prospect St.
New Haven, CT 06520
United States of America
Email: yry@cs.yale.edu
Young Lee
Samsung
Email: younglee.tx@gmail.com
Dhruv Dhody
Huawei
India
Email: dhruv.ietf@gmail.com
Sabine Randriamasy
Nokia Networks France
France
Email: sabine.randriamasy@nokia-bell-labs.com
Luis Miguel Contreras Murillo
Telefonica
Madrid
Spain
Email: luismiguel.contrerasmurillo@telefonica.com