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IETF RFC 8189
Last modified on Wednesday, October 25th, 2017
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Internet Engineering Task Force (IETF) S. Randriamasy
Request for Comments: 8189 W. Roome
Category: Standards Track Nokia Bell Labs
ISSN: 2070-1721 N. Schwan
Thales Deutschland
October 2017
Multi-Cost Application-Layer Traffic Optimization (ALTO)
 Abstract
The Application-Layer Traffic Optimization (ALTO) protocol, specified
in RFC 7285, defines several services that return various metrics
describing the costs between network endpoints.
This document defines a new service that allows an ALTO Client to
retrieve several cost metrics in a single request for an ALTO
filtered cost map and endpoint cost map. In addition, it extends the
constraints to further filter those maps by allowing an ALTO Client
to specify a logical combination of tests on several cost metrics.
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/RFC 8189.
Randriamasy, et al. Standards Track PAGE 1 
RFC 8189 Multi-Cost ALTO October 2017
Copyright Notice
Copyright (c) 2017 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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Randriamasy, et al. Standards Track PAGE 2
RFC 8189 Multi-Cost ALTO October 2017
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview Of Approach . . . . . . . . . . . . . . . . . . . . 6
3.1. Multi-Cost Data Format . . . . . . . . . . . . . . . . . 6
3.2. Compatibility with Legacy ALTO Clients . . . . . . . . . 7
3.3. Filtered Multi-Cost Map Resources . . . . . . . . . . . . 7
3.4. Endpoint Cost Service Resources . . . . . . . . . . . . . 8
3.5. Full Cost Map Resources . . . . . . . . . . . . . . . . . 8
3.6. Extended Constraint Tests . . . . . . . . . . . . . . . . 8
3.6.1. Extended Constraint Predicates . . . . . . . . . . . 9
3.6.2. Extended Logical Combination of Predicates . . . . . 9
3.6.3. Testable Cost Types in Constraints . . . . . . . . . 9
3.6.4. Testable Cost Type Names in IRD Capabilities . . . . 10
3.6.5. Legacy ALTO Client Issues . . . . . . . . . . . . . . 10
4. Protocol Extensions for Multi-Cost ALTO Transactions . . . . 12
4.1. Filtered Cost Map Extensions . . . . . . . . . . . . . . 12
4.1.1. Capabilities . . . . . . . . . . . . . . . . . . . . 13
4.1.2. Accept Input Parameters . . . . . . . . . . . . . . . 14
4.1.3. Response . . . . . . . . . . . . . . . . . . . . . . 17
4.2. Endpoint Cost Service Extensions . . . . . . . . . . . . 17
4.2.1. Capabilities . . . . . . . . . . . . . . . . . . . . 17
4.2.2. Accept Input Parameters . . . . . . . . . . . . . . . 18
4.2.3. Response . . . . . . . . . . . . . . . . . . . . . . 19
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.1. Information Resource Directory . . . . . . . . . . . . . 19
5.2. Multi-Cost Filtered Cost Map: Example #1 . . . . . . . . 21
5.3. Multi-Cost Filtered Cost Map: Example #2 . . . . . . . . 23
5.4. Multi-Cost Filtered Cost Map: Example #3 . . . . . . . . 24
5.5. Multi-Cost Filtered Cost Map: Example #4 . . . . . . . . 25
5.6. Endpoint Cost Service . . . . . . . . . . . . . . . . . . 26
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
7. Privacy and Security Considerations . . . . . . . . . . . . . 28
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1. Normative References . . . . . . . . . . . . . . . . . . 28
8.2. Informative References . . . . . . . . . . . . . . . . . 28
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
Randriamasy, et al. Standards Track PAGE 3
RFC 8189 Multi-Cost ALTO October 2017
1. Introduction
IETF has defined ALTO services in [RFC 7285] to provide guidance to
overlay applications, which have to select one or several hosts from
a set of candidates that are able to provide a desired resource.
This guidance is based on parameters such as the topological distance
that affect performance of the data transmission between the hosts.
The purpose of ALTO is to improve Quality of Experience (QoE) in the
application while reducing resource consumption in the underlying
network infrastructure. The ALTO protocol conveys a view of the
Internet called a Network Map, which is composed of provider-defined
locations spanning from subnets to several Autonomous Systems (ASes).
ALTO may also convey the provider-determined costs between Network
Map locations or between groups of individual endpoints.
Current ALTO cost types provide values such as "hopcount" and
administrative "routingcost" to reflect ISP routing preferences.
Recently, new use cases have extended the usage scope of ALTO to
Content Delivery Networks (CDNs), data centers, and applications that
need additional information to select their endpoints or network
locations. Thus, a multitude of new cost types that better reflect
the requirements of these applications are expected to be specified.
The ALTO protocol [RFC 7285], which this document refers to as the
base protocol, restricts ALTO cost maps and Endpoint Cost Services to
only one cost type per ALTO request. To retrieve information for
several cost types, an ALTO Client must send several separate
requests to the Server.
It is far more efficient, in terms of Round-Trip Time (RTT), traffic,
and processing load on the ALTO Client and Server, to get all costs
with a single query/response transaction. One cost map reporting on
N cost types is less bulky than N cost maps containing one cost type
each. This is valuable for both the storage of these maps and their
transmission. Additionally, for many emerging applications that need
information on several cost types, having them gathered in one map
will save time. Another advantage is consistency: providing values
for several cost types in one single batch is useful for ALTO Clients
needing synchronized ALTO information updates. This document defines
how to retrieve multiple cost metrics in a single request for ALTO
filtered cost maps and endpoint cost maps. To ensure compatibility
with legacy ALTO Clients, only the Filtered Cost Map and Endpoint
Cost Map Services are extended to return multi-cost values.
Along with multi-cost values queries, the filtering capabilities need
to be extended to allow constraints on multiple metrics. The base
protocol allows an ALTO Client to provide optional constraint tests
for a Filtered Cost Map Service or the Endpoint Cost Service, where
Randriamasy, et al. Standards Track PAGE 4
RFC 8189 Multi-Cost ALTO October 2017
the constraint tests are limited to the AND combination of comparison
tests on the value of the (single) requested cost type. However,
applications that are sensitive to several metrics and struggle with
complicated network conditions may need to arbitrate between
conflicting objectives such as routing cost and network performance.
To this end, this document extends the base protocol with constraints
that may test multiple metrics and may be combined with logical 'ORs'
as well as logical 'ANDs'. This allows an application to make
requests such as: "select solutions with either (moderate "hopcount"
AND high "routingcost") OR (higher "hopcount" AND moderate
"routingcost")".
This document is organized as follows. Section 2 defines terminology
used in this document. Section 3 gives a non-normative overview of
the multi-cost extensions, and Section 4 gives the formal
definitions. Section 5 gives several complete examples. The
remaining sections describe the IANA, privacy, and security
considerations.
1.1. 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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
When the words appear in lower case, they are to be interpreted with
their natural language meanings.
2. Terminology
o ALTO transaction: A request/response exchange between an ALTO
Client and an ALTO Server.
o Client: When used with a capital "C", this term refers to an ALTO
Client.
o Endpoint (EP): An endpoint is defined as in Section 2.1 of
[RFC 7285]. It can be, for example, a peer, a CDN storage
location, a physical server involved in a virtual server-supported
application, a party in a resource-sharing swarm such as a
computation grid, or an online multi-party game.
o Server: When used with a capital "S", this term refers to an ALTO
Server.
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RFC 8189 Multi-Cost ALTO October 2017
3. Overview Of Approach
The following is a non-normative overview of the multi-cost ALTO
extensions defined in this document. It assumes the reader is
familiar with cost map resources in the ALTO protocol [RFC 7285].
3.1. Multi-Cost Data Format
Formally, the cost entries in an ALTO cost map can be any type of
JSON value [RFC 7159] (see the DstCosts object in Section 11.2.3.6 of
[RFC 7285]). However, that section also says that an implementation
may assume costs are JSON numbers, unless the implementation is using
an extension that signals a different data type.
Therefore, this document extends the definition of a cost map to
allow a cost to be an array of costs, one per metric, instead of just
one number. For example, here is a cost map with the "routingcost"
and "hopcount" metrics. Note that this is identical to a regular
ALTO cost map, except that the values are arrays instead of numbers.
The multiple metrics are listed in member "multi-cost-types",
indicating to the Client how to map values in the array to cost
metrics.
{
"meta" : {
"dependent-vtags" : [ ... ],
"cost-type" : {},
"multi-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "hopcount"}
]
}
"cost-map" : {
"PID1": { "PID1":[1,0], "PID2":[5,23], "PID3":[10,5] },
...
}
}
Note also the presence of member '"cost-type" : {}' to maintain
backwards compatibility with [RFC 7285].
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3.2. Compatibility with Legacy ALTO Clients
This document does not define any new media types. Instead, as
described below, it extends the specifications in the ALTO Server's
Information Resource Directory (IRD) so that legacy Clients will not
request array-valued multi-cost map resources. This relies on the
requirement that ALTO Clients MUST ignore unknown fields
(Section 8.3.7 of [RFC 7285]).
3.3. Filtered Multi-Cost Map Resources
This document extends the Filtered Cost Map Service to allow the same
resource to return either a single-valued cost map, as defined in
[RFC 7285], or an array-valued multi-cost map, as defined in this
document. An extended Filtered Cost Map resource has a new
capability, "max-cost-types". The value is the maximum number of
cost types this resource can return for one request. The existence
of this capability means the resource understands the extensions in
this document.
For example, the following fragment from an IRD defines an extended
Filtered Cost Map resource:
"filtered-multicost-map" : {
"uri" : "http://alto.example.com/multi/costmap/filtered",
"media-type" : "application/alto-costmap+json",
"accepts" : "application/alto-costmapfilter+json",
"uses" : [ "my-default-network-map" ],
"capabilities" : {
"max-cost-types" : 2,
"cost-type-names" : [ "num-routingcost",
"num-hopcount" ],
...
}
A legacy ALTO Client will ignore the "max-cost-types" capability and
will send a request with the input parameter "cost-type" describing
the desired cost metric, as defined in [RFC 7285]. The ALTO Server
will return a single-valued legacy cost map.
However, a multi-cost-aware ALTO Client will realize that this
resource supports the multi-cost extensions and can send a POST
request with the new input parameter "multi-cost-types", whose value
is an array of cost types. Because the request has the "multi-cost-
types" parameter (rather than the "cost-type" parameter defined in
the base protocol), the Server realizes that the ALTO Client also
Randriamasy, et al. Standards Track PAGE 7
RFC 8189 Multi-Cost ALTO October 2017
supports the extensions in this document and hence responds with a
multi-cost map with the costs in the order listed in "multi-cost-
types".
3.4. Endpoint Cost Service Resources
Section 4.1.4 of [RFC 7285] specifies that "The Endpoint Cost Service
allows an ALTO server to return costs directly amongst endpoints",
whereas the Filtered Cost Map Service returns costs amongst Provider-
defined Identifiers (PIDs). This document uses the technique
described in Section 3.3 to extend the Endpoint Cost Service to
return array-valued costs to ALTO Clients who also are aware of these
extensions.
3.5. Full Cost Map Resources
Section 11.3.2.3 of [RFC 7285] requires a filtered cost map to return
the entire cost map if the ALTO Client omits the source and
destination PIDs. Hence, a multi-cost-aware ALTO Client can use an
extended Filtered Cost Map resource to get a full multi-cost map.
Full cost map resources are GET-mode requests. The response for a
full cost map conveying multiple cost types would include a "meta"
field that would itself include a "cost-type" field that would list
several values corresponding to the cost types of the cost map. A
legacy ALTO Client would not be able to understand this list.
Neither would it be able to interpret the cost values array provided
by a full multi-cost map.
3.6. Extended Constraint Tests
[RFC 7285] defines a simple constraint test capability for Filtered
Cost Map and Endpoint Cost Services. If a resource supports
constraints, the Server restricts the response to costs that satisfy
a list of simple predicates provided by the ALTO Client. For
example, if the ALTO Client gives the following constraints:
"constraints": ["ge 10", "le 20"]
then the Server only returns costs in the range [10,20].
To be useful with multi-cost requests, the constraint tests require
several extensions.
Randriamasy, et al. Standards Track PAGE 8
RFC 8189 Multi-Cost ALTO October 2017
3.6.1. Extended Constraint Predicates
First, because a multi-cost request involves more than one cost
metric, the simple predicates must be extended to specify the metric
to test. Therefore, we extend the predicate syntax to "[##] op
value", where "##" is the index of a cost metric in this multi-cost
request.
3.6.2. Extended Logical Combination of Predicates
Second, once multiple cost metrics are involved, the "AND" of simple
predicates is no longer sufficient. To be useful, Clients must be
able to express "OR" tests. Hence, we add a new field,
"or-constraints", to the Client request. The value is an array of
arrays of simple predicates and represents the OR of ANDs of those
predicates.
Thus, the following request tells the Server to limit its response to
cost points with "routingcost" <= 100 AND "hopcount" <= 2, OR else
"routingcost" <= 10 AND "hopcount" <= 6:
{
"multi-cost-types": [
{"cost-metric": "routingcost", "cost-mode": "numerical"},
{"cost-metric": "hopcount", "cost-mode": "numerical"}
],
"or-constraints": [
["[0] le 100", "[1] le 2"],
["[0] le 10", "[1] le 6"]
],
"pids": {...}
}
Note that a "constraints" parameter with the array of predicates [P1,
P2, ...] is equivalent to an "or-constraints" parameter with one
array of value [[P1, P2, ...]]. A Client is therefore allowed to
express either "constraints" or "or-constraints" but not both.
3.6.3. Testable Cost Types in Constraints
Finally, a Client may want to test a cost type whose actual value is
irrelevant, as long as it satisfies the tests. For example, a Client
may want the value of the cost metric "routingcost" for all PID pairs
that satisfy constraints on the metric "hopcount", without needing
the actual value of "hopcount".
Randriamasy, et al. Standards Track PAGE 9
RFC 8189 Multi-Cost ALTO October 2017
To this end, we add a specific parameter named "testable-cost-types"
that does not contain the same cost types as parameter "multi-cost-
types". The Client can express constraints only on cost types listed
in "testable-cost-types".
For example, the following request tells the Server to return just
"routingcost" for those source and destination pairs for which
"hopcount" is <= 6:
{
"multi-cost-types": [
{"cost-metric": "routingcost", "cost-mode": "numerical"},
],
"testable-cost-types": [
{"cost-metric": "hopcount", "cost-mode": "numerical"},
],
"constraints": ["[0] le 6"],
"pids": {...}
}
3.6.4. Testable Cost Type Names in IRD Capabilities
In [RFC 7285], when a resource's capability "constraints" is true, the
Server accepts constraints on all the cost types listed in the "cost-
type-names" capability. However, some ALTO Servers may not be
willing to allow constraint tests on all available cost metrics.
Therefore, the multi-cost ALTO protocol extension defines the
capability field "testable-cost-type-names". Like "cost-type-names",
it is an array of cost type names. If present, that resource only
allows constraint tests on the cost types in that list. "testable-
cost-type-names" must be a subset of "cost-type-names".
3.6.5. Legacy ALTO Client Issues
While a multi-cost-aware Client will recognize the "testable-cost-
type-names" field and will honor those restrictions, a legacy Client
will not. Hence, when "constraints" has the value 'true', a legacy
Client may send a request with a constraint test on any of the cost
types listed in "cost-type-names".
To avoid that problem, the "testable-cost-type-names" and "cost-
constraints" fields are mutually exclusive: a resource may define one
or the other capability but MUST NOT define both. Thus, a resource
that does not allow constraint tests on all cost metrics will set
"testable-cost-type-names" to the testable metrics and will set
"cost-constraints" to 'false'. A multi-cost-aware Client will
recognize the "testable-cost-type-names" field and will realize that
its existence means the resource does allow (limited) constraint
Randriamasy, et al. Standards Track PAGE 10
RFC 8189 Multi-Cost ALTO October 2017
tests, while a legacy Client will think that resource does not allow
constraint tests at all. To allow legacy Clients to use constraint
tests, the ALTO Server can define an additional resource with "cost-
constraints" set to 'true' and "cost-type-names" set to the metrics
that can be tested.
In the IRD example below, the resource "filtered-cost-map-extended"
provides values for three metrics: "num-routingcost", "num-hopcount",
and "num-bwscore". The capability "testable-cost-type-names"
indicates that the Server only allows constraints on "routingcost"
and "hopcount". A multi-cost-capable Client will see this capability
and will limit its constraint tests to those metrics. Because
capability "cost-constraints" is false (by default), a legacy Client
will not use constraint tests on this resource at all.
The second resource, "filtered-multicost-map", is similar to the
first, except that all the metrics it returns are testable.
Therefore, it sets "cost-constraints" to 'true' and does not set the
"testable-cost-type-names" field. A legacy Client that needs a
constraint test will use this resource rather than the first. A
multi-cost-aware Client that does not need to retrieve the
"num-bwscore" metric may use either resource.
Note that if a multi-cost Server specifies a "filtered-cost-map-
extended", it will most likely not specify an "filtered-multicost-
map" if the capabilities of the latter are covered by the
capabilities of the former or unless the "filtered-multicost-map"
resource is also intended for legacy Clients.
Randriamasy, et al. Standards Track PAGE 11
RFC 8189 Multi-Cost ALTO October 2017
"filtered-cost-map-extended" : {
"uri" : "http://alto.example.com/multi/extn/costmap/filtered",
"media-type" : "application/alto-costmap+json",
"accepts" : "application/alto-costmapfilter+json",
"uses" : [ "my-default-network-map" ],
"capabilities" : {
"max-cost-types" : 3,
"cost-type-names" : [ "num-routingcost",
"num-hopcount",
"num-bwscore"],
"testable-cost-type-names" : [ "num-routingcost",
"num-hopcount" ]
}
},
"filtered-multicost-map" : {
"uri" : "http://alto.example.com/multi/costmap/filtered",
"media-type" : "application/alto-costmap+json",
"accepts" : "application/alto-costmapfilter+json",
"uses" : [ "my-default-network-map" ],
"capabilities" : {
"cost-constraints" : true,
"max-cost-types" : 2,
"cost-type-names" : [ "num-routingcost",
"num-hopcount"],
}
}
4. Protocol Extensions for Multi-Cost ALTO Transactions
This section formally specifies the extensions to [RFC 7285] to
support multi-cost ALTO transactions.
This document uses the notation rules specified in Section 8.2 of
[RFC 7285]. In particular, an optional field is enclosed by [ ]. In
the definitions, the JSON names of the fields are case sensitive. An
array is indicated by two numbers in angle brackets, <m..n>, where m
indicates the minimal number of values and n is the maximum. When
this document uses * for n, it means no upper bound.
4.1. Filtered Cost Map Extensions
This document extends Filtered Cost Maps, as defined in
Section 11.3.2 of [RFC 7285], by adding new input parameters and
capabilities and by returning JSONArrays instead of JSONNumbers as
the cost values.
Randriamasy, et al. Standards Track PAGE 12
RFC 8189 Multi-Cost ALTO October 2017
The media type, HTTP method, and "uses" specifications (described in
Sections 11.3.2.1, 11.3.2.2, and 11.3.2.5 of [RFC 7285], respectively)
are unchanged.
4.1.1. Capabilities
The filtered cost map capabilities are extended with two new members:
o max-cost-types
o testable-cost-type-names
The capability "max-cost-types" indicates whether this resource
supports the multi-cost ALTO extensions, and the capability
"testable-cost-type-names" allows the resource to restrict constraint
tests to a subset of the available cost types. With these two
additional members, the FilteredCostMapCapabilities object in
Section 11.3.2.4 of [RFC 7285] is structured as follows:
object {
JSONString cost-type-names<1..*>;
[JSONBool cost-constraints;]
[JSONNumber max-cost-types;]
[JSONString testable-cost-type-names<1..*>;]
} FilteredCostMapCapabilities;
cost-type-names: As defined in Section 11.3.2.4 of [RFC 7285].
cost-constraints: As defined in Section 11.3.2.4 of [RFC 7285].
Thus, if "cost-constraints" is true, the resource MUST accept
constraint tests on any cost type in "cost-type-names". In
addition, note that if "cost-constraints" is true, the "testable-
cost-type-names" capability MUST NOT be present.
max-cost-types: If present with value N greater than 0, this
resource understands the multi-cost extensions in this document
and can return a multi-cost map with any combination of N or fewer
cost types in the "cost-type-names" list. If omitted, the default
value is 0.
testable-cost-type-names: If present, the resource allows constraint
tests, but only on the cost type names in this array. Each name
in "testable-cost-type-names" MUST also be in "cost-type-names".
If "testable-cost-type-names" is present, the "cost-constraints"
capability MUST NOT be true.
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RFC 8189 Multi-Cost ALTO October 2017
As discussed in Section 3.6.4, this capability is useful when a
Server is unable or unwilling to implement constraint tests on all
cost types. As discussed in Section 3.6.5, "testable-cost-type-
names" and "cost-constraints" are mutually exclusive to prevent
legacy Clients from issuing constraint tests on untestable cost
types.
4.1.2. Accept Input Parameters
The ReqFilteredCostMap object in Section 11.3.2.3 of [RFC 7285] is
extended as follows:
object {
[CostType cost-type;]
[CostType multi-cost-types<1..*>;]
[CostType testable-cost-types<1..*>;]
[JSONString constraints<0..*>;]
[JSONString or-constraints<1..*><1..*>;]
[PIDFilter pids];
} ReqFilteredCostMap;
cost-type: As defined in Section 11.3.2.3 of [RFC 7285], with the
additional requirement that the Client MUST specify either "cost-
type" or "multi-cost-types" but MUST NOT specify both. Therefore,
this field is made optional. When placing a single cost request
as specified in [RFC 7285], a Client MUST use "cost-type".
multi-cost-types: If present, the ALTO Server MUST return array-
valued costs for the cost types in this list. For each entry, the
"cost-metric" and "cost-mode" fields MUST match one of the
supported cost types indicated in member "cost-type-names" of this
resource's "capabilities" field (Section 4.1.1). The Client MUST
NOT use this field unless this resource's "max-cost-types"
capability exists and has a value greater than 0. This field MUST
NOT have more than "max-cost-types" cost types. The Client MUST
specify either "cost-type" or "multi-cost-types" but MUST NOT
specify both.
Note that if "multi-cost-types" has one cost type, the values in
the cost map will be arrays with one value.
testable-cost-types: A list of cost types used for extended
constraint tests, as described for the "constraints" and
"or-constraints" parameters. These cost types must either be a
subset of the cost types in the resource's
"testable-cost-type-names" capability (Section 4.1.1), or else, if
the resource's capability "cost-constraints" is true, a subset of
the cost types in the resource's "cost-type-names" capability.
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If "testable-cost-types" is omitted, it is assumed to have the
cost types in "multi-cost-types" or "cost-type".
This feature is useful when a Client wants to test a cost type
whose actual value is irrelevant, as long as it satisfies the
tests. For example, a Client may want the cost metric
"routingcost" for those PID pairs whose "hopcount" is less than
10. The exact hop count does not matter.
constraints: If this resource's "max-cost-types" capability
(Section 4.1.1) has the value 0 (or is not defined), this
parameter is as defined in Section 11.3.2.3 of [RFC 7285]: an array
of constraint tests related to each other by a logical AND. In
this case, it MUST NOT be specified unless the resource's "cost-
constraints" capability is true.
If this resource's "max-cost-types" capability has a value greater
than 0, then this parameter is an array of extended constraint
predicates as defined below and related to each other by a logical
AND. In this case, it MAY be specified if the resource allows
constraint tests (the resource's "cost-constraints" capability is
true, or its "testable-cost-type-names" capability is not empty).
This parameter MUST NOT be specified if the "or-constraints"
parameter is specified.
An extended constraint predicate consists of two or three entities
separated by white space: (1) an optional cost type index of the
form "[#]" with default value "[0]", (2) a required operator, and
(3) a required target value. The operator and target value are as
defined in Section 11.3.2.3 of [RFC 7285]. The cost type index, i,
specifies the cost type to test. If the "testable-cost-type"
parameter is present, the test applies to the i'th cost type in
"testable-cost-types", starting with index 0. Otherwise, if the
"multi-cost-types" parameter is present, the test applies to the
i'th cost type in that array. If neither parameter is present,
the test applies to the cost type in the "cost-type" parameter, in
which case the index MUST be 0. Regardless of how the tested cost
type is selected, it MUST be in the resource's "testable-cost-
type-names" capability or, if not present, in the "cost-type-
names" capability.
As an example, suppose "multi-cost-types" has the single element
"routingcost", "testable-cost-types" has the single element
"hopcount", and "constraints" has the single element "[0] le 5".
This is equivalent to the database query "SELECT and provide
routingcost WHERE hopcount <= 5".
Randriamasy, et al. Standards Track PAGE 15
RFC 8189 Multi-Cost ALTO October 2017
Note that the index is optional, so a constraint test as defined
in Section 11.3.2.3 of [RFC 7285], such as "le 10", is equivalent
to "[0] le 10". Thus, legacy constraint tests are also legal
extended constraint tests.
Note that a "constraints" parameter with the array of extended
predicates [P1, P2, ...] is equivalent to an "or-constraints"
parameter as defined below with the value [[P1, P2, ...]].
or-constraints: A JSONArray of JSONArrays of JSONStrings, where each
string is an extended constraint predicate as defined above. The
"or-constraint" tests are interpreted as the logical OR of ANDs of
predicates. That is, the ALTO Server should return a cost point
only if it satisfies all constraints in any one of the sub-arrays.
This parameter MAY be specified if this resource's "max-cost-
types" capability is defined with a value greater than 0
(Section 4.1.1) and if the resource allows constraint tests (the
resource's "cost-constraints" capability is true, or its
"testable-cost-type-names" capability is not empty). Otherwise,
this parameter MUST NOT be specified.
This parameter MUST NOT be specified if the "constraints"
parameter is specified.
This parameter MUST NOT contain any empty array of AND predicates.
An empty array would be equivalent to a constraint that is always
true. An OR combination including such a constraint would be
always true and thus useless.
As an example, suppose "multi-cost-types" has the two elements
"routingcost" and "bandwidthscore", "testable-cost-types" has the
two elements "routingcost" and "hopcount", and "or-constraints"
has the two elements ["[0] le 100", "[1] le 2"] and ["[0] le 10",
"[1] le 6"]. This is equivalent to the words: "SELECT and provide
routingcost and bandwidthscore WHERE ("routingcost" <= 100 AND
"hopcount" <= 2) OR ("routingcost" <= 10 AND "hopcount" <= 6)".
Note that if the "max-cost-types" capability has a value greater
than 0, a Client MAY use the "or-constraints" parameter together
with the "cost-type" parameter. That is, if the Client and Server
are both aware of the extensions in this document, a Client MAY
use an "OR" test for a single-valued cost request.
pids: As defined in Section 11.3.2.3 of [RFC 7285].
Randriamasy, et al. Standards Track PAGE 16
RFC 8189 Multi-Cost ALTO October 2017
4.1.3. Response
If the Client specifies the "cost-type" input parameter, the response
is exactly as defined in Section 11.2.3.6 of [RFC 7285]. If the
Client provides the "multi-cost-types" instead, then the response is
changed as follows:
o In "meta", the value of field "cost-type" will be ignored by the
receiver and set to {}. Instead, the field "multi-cost-types" is
added with the same value as the "multi-cost-types" input
parameter.
o The costs are JSONArrays instead of JSONNumbers. All arrays have
the same cardinality as the "multi-cost-types" input parameter and
contain the cost type values in that order. If a cost type is not
available for a particular source and destination, the ALTO Server
MUST use the JSON "null" value for that array element. If none of
the cost types are available for a particular source and
destination, the ALTO Server MAY omit the entry for that source
and destination.
4.2. Endpoint Cost Service Extensions
This document extends the Endpoint Cost Service, as defined in
Section 11.5.1 of [RFC 7285], by adding new input parameters and
capabilities and by returning JSONArrays instead of JSONNumbers as
the cost values.
The media type, HTTP method, and "uses" specifications (described in
Sections 11.5.1.1, 11.5.1.2, and 11.5.1.5 of [RFC 7285], respectively)
are unchanged.
4.2.1. Capabilities
The extensions to the Endpoint Cost Service capabilities are
identical to the extensions to the Filtered Cost Map (see
Section 4.1.1).
Randriamasy, et al. Standards Track PAGE 17
RFC 8189 Multi-Cost ALTO October 2017
4.2.2. Accept Input Parameters
The ReqEndpointCostMap object in Section 11.5.1.3 of [RFC 7285] is
extended as follows:
object {
[CostType cost-type;]
[CostType multi-cost-types<1..*>;]
[CostType testable-cost-types<1..*>;]
[JSONString constraints<0..*>;]
[JSONString or-constraints<1..*><1..*>;]
EndpointFilter endpoints;
} ReqEndpointCostMap;
cost-type: As defined in Section 11.5.1.3 of [RFC 7285], with the
additional requirement that the Client MUST specify either "cost-
type" or "multi-cost-types" but MUST NOT specify both.
multi-cost-types: If present, the ALTO Server MUST return array-
valued costs for the cost types in this list. For each entry, the
"cost-metric" and "cost-mode" fields MUST match one of the
supported cost types indicated in this resource's "capabilities"
field (Section 4.2.1). The Client MUST NOT use this field unless
this resource's "max-cost-types" capability exists and has a value
greater than 0. This field MUST NOT have more than "max-cost-
types" cost types. The Client MUST specify either "cost-type" or
"multi-cost-types" but MUST NOT specify both.
Note that if "multi-cost-types" has one cost type, the values in
the cost map will be arrays with one value.
testable-cost-types, constraints, or-constraints: Defined
equivalently to the corresponding input parameters for an extended
filtered cost map (Section 4.1.2).
endpoints: As defined in Section 11.5.1.3 of [RFC 7285].
Randriamasy, et al. Standards Track PAGE 18
RFC 8189 Multi-Cost ALTO October 2017
4.2.3. Response
The extensions to the Endpoint Cost Service response are similar to
the extensions to the Filtered Cost Map response (Section 4.1.3).
Specifically, if the Client specifies the "cost-type" input
parameter, the response is exactly as defined in Section 11.5.1.6 of
[RFC 7285]. If the Client provides the "multi-cost-types" instead,
then the response is changed as follows:
o In "meta", the value of field "cost-type" will be ignored by the
receiver and set to {}. Instead, the field "multi-cost-types" is
added with the same value as the "multi-cost-types" input
parameter.
o The costs are JSONArrays instead of JSONNumbers. All arrays have
the same cardinality as the "multi-cost-types" input parameter and
contain the cost type values in that order. If a cost type is not
available for a particular source and destination, the ALTO Server
MUST use the JSON "null" value for that array element. If none of
the cost types are available for a particular source and
destination, the ALTO Server MAY omit the entry for that source
and destination.
5. Examples
This section provides examples of multi-cost ALTO transactions. It
uses cost metrics, in addition to the mandatory legacy "routingcost",
that are deliberately irrelevant and not registered with IANA.
5.1. Information Resource Directory
The following is an example of an ALTO Server's Information Resource
Directory. In addition to network and cost map resources, it defines
two Filtered Cost Maps and an Endpoint Cost Service, which all
understand the multi-cost extensions.
GET /directory HTTP/1.1
Host: alto.example.com
Accept: application/alto-directory+json,application/alto-error+json
HTTP/1.1 200 OK
Content-Length: 2704
Content-Type: application/alto-directory+json
Randriamasy, et al. Standards Track PAGE 19
RFC 8189 Multi-Cost ALTO October 2017
{
"meta" : {
"default-alto-network-map" : "my-default-network-map",
"cost-types" : {
"num-routing" : {
"cost-mode" : "numerical",
"cost-metric" : "routingcost"
},
"num-shoesize" : {
"cost-mode" : "numerical",
"cost-metric" : "shoesize"
},
"num-scenery" : {
"cost-mode" : "numerical",
"cost-metric" : "sceneryrate"
}
}
},
"resources" : {
"my-default-network-map" : {
"uri" : "http://alto.example.com/networkmap",
"media-type" : "application/alto-networkmap+json"
},
"numerical-routing-cost-map" : {
"uri" : "http://alto.example.com/costmap/num-routing",
"media-type" : "application/alto-costmap+json",
"uses" : [ "my-default-network-map" ],
"capabilities" : {
"cost-type-names" : [ "num-routing" ]
}
},
"numerical-shoesize-cost-map" : {
"uri" : "http://alto.example.com/costmap/num-shoesize",
"media-type" : "application/alto-costmap+json",
"uses" : [ "my-default-network-map" ],
"capabilities" : {
"cost-type-names" : [ "num-shoesize" ]
}
},
"filtered-multicost-map" : {
"uri" : "http://alto.example.com/multi/costmap/filtered",
"media-type" : "application/alto-costmap+json",
"accepts" : "application/alto-costmapfilter+json",
"uses" : [ "my-default-network-map" ],
"capabilities" : {
"cost-constraints" : true,
"max-cost-types" : 2,
"cost-type-names" : [ "num-routingcost",
Randriamasy, et al. Standards Track PAGE 20
RFC 8189 Multi-Cost ALTO October 2017
"num-shoesize" ]
}
},
"filtered-cost-map-extended" : {
"uri" : "http://alto.example.com/multi/extn/costmap/filtered",
"media-type" : "application/alto-costmap+json",
"accepts" : "application/alto-costmapfilter+json",
"uses" : [ "my-default-network-map" ],
"capabilities" : {
"max-cost-types" : 3,
"cost-type-names" : [ "num-routingcost",
"num-shoesize",
"num-scenery"],
"testable-cost-type-names" : [ "num-routingcost",
"num-shoesize" ]
}
},
"endpoint-multicost-map" : {
"uri" : "http://alto.example.com/multi/endpointcost/lookup",
"media-type" : "application/alto-endpointcost+json",
"accepts" : "application/alto-endpointcostparams+json",
"uses" : [ "my-default-network-map" ],
"capabilities" : {
"cost-constraints" : true,
"max-cost-types" : 2,
"cost-type-names" : [ "num-routingcost",
"num-shoesize" ]
}
}
}
}
5.2. Multi-Cost Filtered Cost Map: Example #1
This example illustrates a simple multi-cost ALTO transaction. The
ALTO Server provides two cost types, "routingcost" and "shoesize",
both in "numerical" mode. The Client wants the entire multi-cost
map. The Server does not know the value of "routingcost" between
PID2 and PID3 and hence returns the value 'null' for "routingcost"
between PID2 and PID3.
Randriamasy, et al. Standards Track PAGE 21
RFC 8189 Multi-Cost ALTO October 2017
POST /multi/costmap/filtered" HTTP/1.1
Host: alto.example.com
Accept: application/alto-costmap+json,application/alto-error+json
Content-Type: application/alto-costmapfilter+json
Content-Length: 206
{
"multi-cost-types": [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "shoesize"}
],
"pids" : {
"srcs" : [ ],
"dsts" : [ ]
}
}
HTTP/1.1 200 OK
Content-Type: application/alto-costmap+json
Content-Length: 549
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-default-network-map",
"tag": "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
}
],
"cost-type" : {},
"multi-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "shoesize"}
]
}
"cost-map" : {
"PID1": { "PID1":[1,0], "PID2":[4,3], "PID3":[10,2] },
"PID2": { "PID1":[15,5], "PID2":[1,0], "PID3":[null,9] },
"PID3": { "PID1":[20,12], "PID2":[null,1], "PID3":[1,0] }
}
}
Randriamasy, et al. Standards Track PAGE 22
RFC 8189 Multi-Cost ALTO October 2017
5.3. Multi-Cost Filtered Cost Map: Example #2
This example uses constraints to restrict the returned source/
destination PID pairs to those with "routingcost" between 5 and 10 or
"shoesize" equal to 0.
POST /multi/costmap/filtered HTTP/1.1
Host: alto.example.com
Accept: application/alto-costmap+json,application/alto-error+json
Content-Type: application/alto-costmapfilter+json
Content-Length: 333
{
"multi-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "shoesize"}
],
"or-constraints" : [ ["[0] ge 5", "[0] le 10"],
["[1] eq 0"] ]
"pids" : {
"srcs" : [ "PID1", "PID2" ],
"dsts" : [ "PID1", "PID2", "PID3" ]
}
}
HTTP/1.1 200 OK
Content-Type: application/alto-costmap+json
Content-Length: 461
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-default-network-map",
"tag": "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
}
],
"cost-type" : {},
"multi-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "shoesize"}
]
}
"cost-map" : {
"PID1": { "PID1": [1,0], "PID3": [10,5] },
"PID2": { "PID2": [1,0] }
}
}
Randriamasy, et al. Standards Track PAGE 23
RFC 8189 Multi-Cost ALTO October 2017
5.4. Multi-Cost Filtered Cost Map: Example #3
This example uses extended constraints to limit the response to cost
points with ("routingcost" <= 10 AND "shoesize" <= 2), OR else
("routingcost" <= 3 AND "shoesize" <= 6). Unlike the previous
example, the Client is only interested in the "routingcost" cost type
and uses the "cost-type" parameter instead of "multi-cost-types" to
tell the Server to return scalar costs instead of array costs.
In this example, "[0]" means the constraint applies to "routingcost"
because that is the first cost type in the "testable-cost-types"
parameter. (If "testable-cost-types" is omitted, it is assumed to be
the same as "multi-cost-types".) The choice of using an index to
refer to cost types aims at minimizing the length of the expression
of constraints, especially for those combining several OR and AND
expressions. It was also the shortest path from the constraints
design in [RFC 7285].
POST /multi/multicostmap/filtered HTTP/1.1
Host: alto.example.com
Accept: application/alto-costmap+json,application/alto-error+json
Content-Type: application/alto-costmapfilter+json
Content-Length: 390
{
"cost-type" : {
"cost-mode": "numerical", "cost-metric": "routingcost"
},
"testable-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "shoesize"}
],
"or-constraints": [
["[0] le 10", "[1] le 2"],
["[0] le 3", "[1] le 6"]
],
"pids" : {
"srcs" : [ ],
"dsts" : [ ]
}
}
Randriamasy, et al. Standards Track PAGE 24
RFC 8189 Multi-Cost ALTO October 2017
HTTP/1.1 200 OK
Content-Type: application/alto-costmap+json
Content-Length: 368
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-default-network-map",
"tag": "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
}
],
"cost-type" : {
"cost-mode": "numerical", "cost-metric": "routingcost"
}
}
"cost-map" : {
"PID1": { "PID1": 1, "PID3": 10 },
"PID2": { "PID2": 1 },
"PID3": { "PID3": 1 }
}
}
5.5. Multi-Cost Filtered Cost Map: Example #4
This example uses extended constraints to limit the response to cost
points with ("routingcost" <= 10 AND "shoesize" <= 2), OR else
("routingcost" <= 3 AND "shoesize" <= 6). In this example, the
Client is interested in the "routingcost" and "sceneryrate" cost
metrics but not in the "shoesize" metric:
POST /multi/extn/costmap/filtered HTTP/1.1
Host: alto.example.com
Accept: application/alto-costmap+json,application/alto-error+json
Content-Type: application/alto-costmapfilter+json
Content-Length: 461
{
"multi-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "sceneryrate"}
],
"testable-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "shoesize"}
],
Randriamasy, et al. Standards Track PAGE 25
RFC 8189 Multi-Cost ALTO October 2017
"or-constraints": [
["[0] le 10", "[1] le 2"],
["[0] le 3", "[1] le 6"]
],
"pids" : {
"srcs" : [ ],
"dsts" : [ ]
}
}
HTTP/1.1 200 OK
Content-Type: application/alto-costmap+json
Content-Length: 481
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-default-network-map",
"tag": "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
}
],
"cost-type" : {},
"multi-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "sceneryrate"}
]
}
"cost-map" : {
"PID1": { "PID1": [1,16] "PID3": [10,19] },
"PID2": { "PID2": [1,8] },
"PID3": { "PID3": [1,19] }
}
}
5.6. Endpoint Cost Service
This example uses the Endpoint Cost Service to retrieve the
"routingcost" and "shoesize" for selected endpoints, limiting the
response to costs with either low "shoesize" and reasonable
"routingcost" ("shoesize" <= 2 AND "routingcost" <= 10), OR else low
"routingcost" and reasonable "shoesize" ("routingcost" <= 3 AND
"shoesize" <= 6).
POST /multi/endpointcost/lookup HTTP/1.1
Host: alto.example.com
Accept: application/alto-endpointcost+json,
application/alto-error+json
Randriamasy, et al. Standards Track PAGE 26
RFC 8189 Multi-Cost ALTO October 2017
Content-Type: application/alto-endpoincostparams+json
Content-Length: 455
{
"multi-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "shoesize"}
],
"or-constraints": [
["[0] le 10", "[1] le 2"],
["[0] le 3", "[1] le 6"]
],
"endpoints" : {
"srcs": [ "ipv4:192.0.2.2", "ipv6:2001:db8::1:0 ],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34",
"ipv4:203.0.113.45",
"ipv6:2001:db8::10"
]
}
}
HTTP/1.1 200 OK
Content-Length: 419
Content-Type: application/alto-endpointcost+json
{
"meta" : {
"multi-cost-types" : [
{"cost-mode": "numerical", "cost-metric": "routingcost"},
{"cost-mode": "numerical", "cost-metric": "shoesize"}
]
}
"endpoint-cost-map" : {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89": [15, 5],
"ipv4:203.0.113.45": [4, 23]
}
"ipv6:2001:db8::1:0": {
"ipv4:198.51.100.34": [16, 5],
"ipv6:2001:db8::10": [10, 2]
}
}
}
Randriamasy, et al. Standards Track PAGE 27
RFC 8189 Multi-Cost ALTO October 2017
6. IANA Considerations
This document does not define any new media types or introduce any
new IANA considerations.
7. Privacy and Security Considerations
This document does not introduce any privacy or security issues not
already present in the ALTO protocol.
The multi-cost optimization even tends to reduce the on-the-wire data
exchange volume compared to multiple single cost ALTO transactions.
Likewise, the risk related to massive multi-cost requests is
moderated by the fact that multi-cost constraints additionally filter
ALTO Server responses and thus reduce their volume.
Note that, because queries for multiple metrics represent a stronger
fingerprinting signal than queries for a single metric,
implementations of this protocol may leak more information about the
ALTO Client than would occur with a succession of individual queries.
Though, in many cases, it would already be possible to link those
queries by using the source IP address or other existing information.
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 7285] 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/RFC 7285, September 2014,
<https://www.rfc-editor.org/info/RFC 7285>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
8.2. Informative References
[RFC 7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC 7159, March
2014, <https://www.rfc-editor.org/info/RFC 7159>.
Randriamasy, et al. Standards Track PAGE 28
RFC 8189 Multi-Cost ALTO October 2017
Acknowledgements
The authors would like to thank Richard Alimi, Fred Baker, Dhruv
Dhodi, Vijay Gurbani, Dave Mac Dysan, Young Lee, and Richard Yang for
fruitful discussions and feedback on this document and earlier draft
versions. Gao Kai, Hans Seidel, Richard Yang, Qiao Xiang, and Wang
Xin provided substantial review feedback and suggestions to the
protocol design.
Authors' Addresses
Sabine Randriamasy
Nokia Bell Labs
Route de Villejust
Nozay 91460
France
Email: Sabine.Randriamasy@nokia-bell-labs.com
Wendy Roome
Nokia Bell Labs
124 Burlington Rd
Murray Hill, NJ 07974
United States of America
Email: ietf@wdroome.com
Nico Schwan
Thales Deutschland
Lorenzstrasse 10
Stuttgart 70435
Germany
Email: nico.schwan@thalesgroup.com
Randriamasy, et al. Standards Track PAGE 29
RFC TOTAL SIZE: 57010 bytes
PUBLICATION DATE: Wednesday, October 25th, 2017
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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