Application-Layer Traffic Optimization
(ALTO) Cost Calendar
Nokia Bell Labs
Route de Villejust
Nozay
91460
France
Sabine.Randriamasy@nokia-bell-labs.com
Yale University
51 Prospect St.
New Haven
CT
06520
United States of America
yry@cs.yale.edu
Huawei
101 Software Avenue
Yuhua District
Nanjing
Jiangsu
210012
China
sunseawq@huawei.com
China Mobile
China
denglingli@chinamobile.com
Thales Deutschland
Lorenzstrasse 10
Stuttgart70435
Germany
nico.schwan@thalesgroup.com
This document is an extension to the base Application-Layer Traffic
Optimization (ALTO) protocol. It extends the ALTO cost information
service so that applications decide not only 'where' to connect but
also 'when'. This is useful for applications that need to perform bulk
data transfer and would like to schedule these transfers during an
off-peak hour, for example. This extension introduces the ALTO Cost
Calendar with which an ALTO Server exposes ALTO cost values in JSON
arrays where each value corresponds to a given time interval. The time
intervals, as well as other Calendar attributes, are specified in the
Information Resources Directory and ALTO Server responses.
Introduction
The base Application-Layer Traffic Optimization (ALTO) protocol
specified in provides guidance
to overlay applications that need to select one or several hosts from a
set of candidates able to provide a desired resource. This guidance is
based on parameters that affect performance and efficiency of the data
transmission between the hosts, such as the topological distance. The
goal of ALTO is to improve the Quality of Experience (QoE) in the
application while optimizing resource usage in the underlying network
infrastructure.
The ALTO protocol in
specifies a network map that defines groupings of endpoints in
provider-defined network regions identified by Provider-defined
Identifiers (PIDs). The Cost Map Service, Endpoint Cost Service (ECS),
and Endpoint Ranking Service then provide ISP-defined costs and rankings
for connections among the specified endpoints and PIDs and thus
incentives for application clients to connect to ISP-preferred
locations, for instance, to reduce their costs. For the reasons
outlined in the ALTO problem statement and requirement AR-14 of , ALTO does not disseminate network metrics that
change frequently. In a network, the costs can fluctuate for many
reasons having to do with instantaneous traffic load or diurnal
patterns of traffic demand or planned events, such as network
maintenance, holidays, or highly publicized events. Thus, an ALTO
application wishing to use the Cost Map and Endpoint Cost Service at
some future time will have to estimate the state of the network at that
time, a process that is, at best, fragile and brittle, since the
application does not have any visibility into the state of the network.
Providing network costs for only the current time thus may not be
sufficient, in particular for applications that can schedule their
traffic in a span of time, for example, by deferring backups or other
background traffic to off- peak hours.
In case the ALTO cost value changes are predictable over a certain
period of time and the application does not require immediate data
transfer, it can save time to get the whole set of cost values over this
period in one single ALTO response. Using this set to schedule data
transfers allows optimizing the network resources usage and QoE. ALTO
Clients and Servers can also minimize their workload by reducing and
accordingly scheduling their data exchanges.
This document extends to
allow an ALTO Server to provide network costs for a given duration of
time. A sequence of network costs across a time span for a given pair
of network locations is named an "ALTO Cost Calendar". The Filtered
Cost Map Service and Endpoint Cost Service are extended to provide Cost
Calendars. In addition to this functional ALTO enhancement, we expect
to further save network and storage resources by gathering multiple cost
values for one cost type into one single ALTO Server response.
In this document, an "ALTO Cost Calendar" is specified in terms of
information resource capabilities that are applicable to time-sensitive
ALTO metrics. An ALTO Cost Calendar exposes ALTO cost values in JSON
arrays, see , where each value
corresponds to a given time interval. The time intervals, as well as
other Calendar attributes, are specified in the Information Resources
Directory (IRD) and in the Server response to allow the ALTO Client to
interpret the received ALTO values. Last, the extensions for ALTO
Calendars are applicable to any cost mode, and they ensure backwards
compatibility with legacy ALTO Clients -- those that only support .
In the rest of this document, provides the design characteristics. Sections and define the formal specifications for the IRD and the
information resources. IANA, security considerations, and operational
considerations are addressed respectively in Sections , , and .
Some Recent Known Uses
A potential use case is implementing smart network services that
allow applications to dynamically build end-to-end, virtual networks
to satisfy given demands with no manual intervention. For example,
data-transfer automation applications may need a network service to
determine the availability of bandwidth resources to decide when
to transfer their data sets. The SENSE project supports such
applications by requiring that a network provides services such as the
Time-Bandwidth-Product (TBP) service, which informs applications of
bandwidth availability during a specific time period. ALTO Calendars
can support this service if the Calendar start date and duration cover
the period of interest of the requesting application.
The need of future scheduling of large-scale traffic that can be
addressed by the ALTO protocol is also motivated by Unicorn, a unified
resource orchestration framework for multi-domain, geo-distributed
data analytics, see .
Terminology
- ALTO transaction:
- A request/response exchange between an ALTO
Client and an ALTO Server.
- Client:
- When used with a capital "C", this term refers to an ALTO
Client.
- Calendar, Cost Calendar, ALTO Calendar:
- When used with capitalized words, these terms refer to an ALTO
Cost Calendar.
- Calendared:
- This adjective qualifies information resources providing Cost
Calendars and information on costs that are provided in the form of
a Cost Calendar.
- Endpoint (EP):
- An endpoint is defined as in . 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.
- ECM:
- An abbreviation for Endpoint Cost Map.
- FCM:
- An abbreviation for Filtered Cost Map.
- Server:
- When used with a capital "S", this term refers to an ALTO Server.
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
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.
Overview of ALTO Cost Calendars and Terminology
This section gives a high-level overview of the design.
It assumes the reader is familiar with the ALTO protocol and its Multi-Cost ALTO extension
.
ALTO Cost Calendar Overview
An ALTO Cost Calendar provided by the ALTO Server provides 2
information items:
- an array of values for a given metric, where each value
specifies the metric corresponding to a time interval, where the
value array can sometimes be a cyclic pattern that repeats a certain
number of times and
- attributes describing the time scope of the Calendar, including
the size and number of the intervals and the date of the starting
point of the Calendar, allowing an ALTO Client to interpret the
values properly.
An ALTO Cost Calendar can be used like a "time table" to figure out
the best time to schedule data transfers and also to proactively
manage application traffic given predictable events, such as an expected
spike in traffic due to crowd gathering (concerts, sports, etc.),
traffic-intensive holidays, and network maintenance. A Calendar may be
viewed as a synthetic abstraction of, for example, real measurements
gathered over previous periods on which statistics have been computed.
However, like for any schedule, unexpected network incidents may
require the current ALTO Calendar to be updated and resent to the
ALTO Clients needing it. The "ALTO Incremental Updates Using
Server-Sent Events (SSE)" Service can be used to directly update the Calendar upon
value changes if supported by both the Server and the Client.
Most likely, the ALTO Cost Calendar would be used for the Endpoint
Cost Service, assuming that a limited set of feasible endpoints for a
non-real time application is already identified, and that those
endpoints do not need to be accessed immediately and that their
access can be scheduled within a given time period.
The Filtered Cost Map Service is also
applicable as long as the size of the Map allows it.
ALTO Cost Calendar Information Features
The Calendar attributes are provided in the Information Resources
Directory (IRD) and in ALTO Server responses. The IRD announces
attributes without date values
in its information resources capabilities, whereas attributes with
time-dependent values are provided in the "meta" section of Server responses.
The ALTO Cost Calendar attributes provide the following information:
-
attributes to describe the time scope of the Calendar value array:
- "time-interval-size": the applicable time interval size for
each Calendar value, defined in seconds, that can cover a wide
range of values.
- "number-of-intervals": the number of intervals provided in
the Calendar.
- "calendar-start-time": specifying when the Calendar starts; that
is, to which date the first value of the Cost Calendar is
applicable.
- "repeated": an optional attribute indicating how many iterations
of the provided Calendar will have the same values. The Server may
use it to allow the Client to schedule its next request and thus
save its own workload by reducing processing of similar
requests.
Attribute "repeated" may take a very high value if a Calendar
represents a cyclic value pattern that the Server considers valid for
a long period. In this case, the Server will only update the Calendar
values once this period has elapsed or if an unexpected event occurs
on the network. See for more
discussion.
ALTO Calendar Design Characteristics
The present document uses the notations defined in "Notation"
().
The extensions in this document encode requests and responses using
JSON .
In the base protocol , an ALTO cost is
specified as a generic JSONValue to allow extensions. However, that section () states:
An implementation of the protocol in
this document
SHOULD assume that the cost is a JSONNumber and fail to
parse if it is not, unless the implementation is using an extension to
this document that indicates when and how costs of other data types
are signaled.
The present document extends the definition of a legacy cost map
given in to allow a cost
entry to be an array of values, with one value per time interval,
instead of being just one number, when the Cost Calendar functionality
is activated on this cost. Therefore, the implementor of this extension
MUST consider that a cost entry is an array of values
if this cost has been queried as a Calendar.
Specifically, an implementation of this extension
MUST parse the "number-of-intervals" attribute of the
Calendar attributes in an IRD entry announcing a service providing a
Cost Calendar for a given cost type. The implementation then will know
that a cost entry of the service will be an array of values, and the
expected size of the array is that specified by the
"number-of-intervals" attribute. The following rules attempt to ensure
consistency between the array size announced by the Server and the
actual size of the array received by the Client:
- The size of the array of values conveyed in a Cost Calendar and
received by the Client MUST be equal to the value of
attribute "number-of-intervals" indicated in the IRD for the
requested cost type.
- When the size of the array received by the Client is different
from the expected size, the Client SHOULD ignore the
received array.
To realize an ALTO Calendar, this document extends the IRD and the
ALTO requests and responses for Cost Calendars.
This extension is designed to be lightweight and to ensure
backwards compatibility with base protocol ALTO Clients and with other
extensions. It relies on "Parsing of Unknown Fields" (), which states:
"Extensions may include additional fields within JSON objects defined
in this document. ALTO implementations MUST ignore
unknown fields when processing ALTO messages."
The Calendar-specific capabilities are integrated in the
information resources of the IRD and in the "meta" member of ALTO
responses to Cost Calendars requests. A Calendar and its capabilities
are associated with a given information resource and within this
information resource with a given cost type. This design has several
advantages:
- it does not introduce a new mode,
- it does not introduce new media types, and
- it allows an ALTO Server to offer, for a cost type, different
Calendars with attributes that are specific to the information
resources providing a Calendar for this cost type, instead of being
globally specific to the cost type.
The applicable Calendared information resources are:
- the Filtered Cost Map and
- the Endpoint Cost Map.
The ALTO Server can choose in which frequency it provides cost
Calendars to ALTO Clients. It may either provide Calendar updates
starting at the request date or carefully schedule its updates so as
to take profit from a potential repetition/periodicity of Calendar
values.
Since Calendar attributes are specific to an information resource,
a Server may adapt the granularity of the calendared information so as
to moderate the volume of exchanged data. For example, suppose a
Server provides a Calendar for cost type name "routingcost". The
Server may offer a Calendar in a Cost Map resource, which may be a
voluminous resource, as an array of 6 intervals lasting each 4
hours. It may also offer a Calendar in an Endpoint Cost Map resource,
which is potentially less voluminous, as a finer-grained array of 24
intervals lasting 1 hour each.
The ALTO Server does not support constraints on Calendars, provided
Calendars are requested for numerical values, for two main
reasons:
- Constraints on an array of values may be various. For instance,
some Clients may refuse Calendars with one single value violating a
constraint, whereas other ones may tolerate Calendars with values
violating constraints, for example, at given times. Therefore,
expressing constraints in a way that covers all possible Client
preferences is challenging.
- If constraints were to be supported, the processing overhead
would be substantial for the Server as it would have to parse all
the values of the Calendar array before returning a response.
As providing the constraint functionality in conjunction with the
Calendar functionality is not feasible for the reasons described
above, the two features are mutually exclusive. The absence of
constraints on Filtered Cost Map and Endpoint Cost Map Calendars
reflects a divergence from the non-calendared information resources
defined in and extended in
, which support optional
constraints.
ALTO Cost Calendar for All Cost Modes
An ALTO Cost Calendar is well suited for values encoded in the
"numerical" mode. Actually, a Calendar can also represent metrics
in other modes considered as compatible with time-varying
values. For example, types of cost values (such as JSONBool) can also
be calendared (as their value may be 'true' or 'false' depending on
given time periods or likewise) values represented by strings, such
as "medium", "high", "low", "blue", and "open".
Note also that a Calendar is suitable as well for time-varying
metrics provided in the "ordinal" mode if these values are
time-varying and the ALTO Server provides updates of
cost-value-based preferences.
Compatibility with Legacy ALTO Clients
The ALTO protocol extensions for Cost Calendars have been defined
so as to ensure that Calendar-capable ALTO Servers can provide
legacy ALTO Clients with legacy information resources as well. That
is, a legacy ALTO Client can request resources and receive responses
as specified in .
A Calendar-aware ALTO Server MUST implement the
base protocol specified in .
A Calendar-aware ALTO Client MUST implement the
base protocol specified in .
As a consequence, when a metric is available as a Calendar array,
it also MUST be available as a single value, as
required by . The Server,
in this case, provides the current value of the metric to either
Calendar-aware Clients not interested in future or time-based values
or Clients implementing
only.
For compatibility with legacy ALTO Clients specified in , calendared information
resources are not applicable for full cost maps for the following
reason: a legacy ALTO Client would receive a calendared cost map via
an HTTP 'GET' command. As specified in , it will ignore the Calendar
attributes indicated in the "meta" of the responses. Therefore,
lacking information on Calendar attributes, it will not be able to
correctly interpret and process the values of the received array of
Calendar cost values.
Therefore, calendared information resources MUST
be requested via the Filtered Cost Map Service or the Endpoint Cost
Service using a POST method.
ALTO Calendar Specification: IRD Extensions
The Calendar attributes in the IRD information resources capabilities
carry dateless values. A Calendar is associated with an information
resource rather than a cost type. For example, a Server can provide a
"routingcost" Calendar for the Filtered Cost Map Service at a
granularity of one day and a "routingcost" Calendar for the Endpoint
Cost Service at a finer granularity but for a limited number of
endpoints. An example IRD with Calendar-specific features is provided
in .
Calendar Attributes in the IRD Resource Capabilities
A Cost Calendar for a given cost type MUST be
indicated in the IRD by an object of type CalendarAttributes. A
CalendarAttributes object is represented by the "calendar-attributes"
member of a resource entry. Member "calendar-attributes" is an array
of CalendarAttributes objects. Each CalendarAttributes object lists a
set of one or more cost types it applies to. A cost type name
MUST NOT appear more than once in the
"calendar-attributes" member of a resource entry; multiple appearances
of a cost type name in the CalendarAttributes object of the
"calendar-attributes" member MUST cause the ALTO Client
to ignore any occurrences of this name beyond the first encountered
occurrence. The Client SHOULD consider the
CalendarAttributes object in the array containing the first
encountered occurrence of a cost type as the valid one for this cost
type. As an alternative, the Client may want to avoid the risks of
erroneous guidance associated to the use of potentially invalid
Calendar values. In this case, the Client MAY ignore
the totality of occurrences of CalendarAttributes objects containing
the cost type name and query the cost type using .
The encoding format for object CalendarAttributes using JSON is as follows:
CalendarAttributes calendar-attributes <1..*>;
;
JSONNumber time-interval-size;
JSONNumber number-of-intervals;
} CalendarAttributes;
]]>
- "cost-type-names":
- An array of one or more elements indicating the cost type
names in the IRD entry to which the values of "time-interval-size"
and "number-of-intervals" apply.
- "time-interval-size":
- The duration of an ALTO Calendar time interval in a unit of
seconds. A "time-interval-size" value contains a non-negative
JSONNumber. Example values are 300 and 7200, meaning that each
Calendar value applies on a time interval that lasts 5 minutes and
2 hours, respectively. Since an interval size (e.g., 100 ms) can
be smaller than the unit, the value specified may be a floating
point (e.g., 0.1). Both ALTO Clients and Servers should be aware
of potential precision issues caused by using floating point
numbers; for example, the floating number 0.1 cannot be
represented precisely using a finite number of binary bits. To
improve interoperability and be consistent with on the
use of floating point numbers, the Server and the Client
SHOULD use IEEE 754 double-precision floating point
to store this
value.
- "number-of-intervals":
- A strictly positive integer (greater or equal to 1) that
indicates the number of values of the Cost Calendar array.
- An ALTO Server SHOULD specify the "time-interval-size" in the IRD
as the smallest it is able to provide. A Client that needs
a longer interval can aggregate multiple cost values to obtain it.
- Attribute "cost-type-names" is associated with
"time-interval-size" and "number-of-intervals", because multiple cost
types may share the same values for attributes "time-interval-size"
and "number-of-intervals". To avoid redundancies, cost type names
sharing the same values for "time-interval-size" and
"number-of-intervals" are grouped in the "cost-type-names"
attribute. In the example IRD provided in , the information resource
"filtered-cost-map-calendar" provides a Calendar for cost type names
"num-routingcost", "num-throughputrating", and
"string-servicestatus". Cost type names "num-routingcost" and
"num-throughputrating" are grouped in the "cost-type-names" attribute
because they share the same values for "time-interval-size" and
"number-of-intervals", which are respectively 7200 and 12.
- Multiplying "time-interval-size" by "number-of-intervals" provides
the duration of the provided Calendar. For example, an ALTO Server
may provide a Calendar for ALTO values changing every
"time-interval-size" equal to 5 minutes. If "number-of-intervals" has
the value 12, then the duration of the provided Calendar is 1
hour.
Calendars in a Delegate IRD
It may be useful to distinguish IRD resources supported by the base
ALTO protocol from resources supported by its extensions. To achieve
this, one option is that a "root" ALTO Server implementing resources and running at a given
domain delegates "specialized" information resources, such as the ones
providing Cost Calendars, to another ALTO Server running in a
subdomain. The "root" ALTO Server can provide a Calendar-specific
resource entry that has a media-type of
"application/alto-directory+json" and that specifies the URI allowing
to retrieve the location of a Calendar-aware Server and discover its
resources. This option is described in "Delegation Using IRD" ().
This document provides an example where a "root" ALTO Server runs
in a domain called "alto.example.com". It delegates the announcement
of Calendars capabilities to an ALTO Server running in a subdomain
called "custom.alto.example.com". The location of the "delegate
Calendar IRD" is assumed to be indicated in the "root" IRD by the
resource entry: "custom-calendared-resources".
Another benefit of delegation is that some cost types for some resources
may be more advantageous as Cost Calendars, and it makes little sense to get them
as a single value. For example, if a cost type has predictable and frequently
changing values calendared in short time intervals, such as a minute,
it saves time and network resources to track the cost values via a focused
delegate Server rather than the more general "root" Server.
Example IRD with ALTO Cost Calendars
This section provides an example ALTO Server IRD that supports
various cost metrics and cost modes. In particular, since makes it mandatory, the Server
uses metric "routingcost" in the "numerical" mode.
For illustrative purposes, this section introduces 3 other
fictitious example metrics and modes that should be understood as
examples and should not be used or considered as normative.
The cost type names used in the example IRD are as follows:
- "num-routingcost":
- Refers to metric "routingcost" in the
numerical mode, as defined in and registered with IANA.
- "num-owdelay":
- Refers to fictitious performance metric "owdelay"
in the "numerical" mode to reflect the one-way packet transmission
delay on a path. A related performance metric is currently under
definition in .
- "num-throughputrating":
- Refers to fictitious metric
"throughputrating" in the "numerical" mode to reflect the provider
preference in terms of end-to-end throughput.
- "string-servicestatus":
- Refers to fictitious metric
"servicestatus" containing a string to reflect the availability,
defined by the provider, of, for instance, path connectivity.
The example IRD includes 2 particular URIs providing Calendars:
- "https://custom.alto.example.com/calendar/costmap/filtered":
- A Filtered Cost Map in which Calendar capabilities are indicated
for cost type names "num-routingcost", "num-throughputrating", and
"string-servicestatus" and
- "https://custom.alto.example.com/calendar/endpointcost/lookup":
- An Endpoint Cost Map in which Calendar capabilities are indicated
for cost type names "num-routingcost", "num-owdelay",
"num-throughputrating", and "string-servicestatus".
The design of the Calendar capabilities allows some Calendars with
the same cost type name to be available in several information
resources with different Calendar attributes. This is the case for
Calendars on "num-routingcost", "num-throughputrating", and
"string-servicestatus", available in both the Filtered Cost Map and
Endpoint Cost Service but with different time interval sizes for
"num-throughputrating" and "string-servicestatus".
In this example IRD, for the Filtered Cost Map Service:
- the Calendar for "num-routingcost" and "num-throughputrating" is
an array of 12 values, each provided on a time interval lasting 7200
seconds (2 hours) and
- the Calendar for "string-servicestatus" is an array of 48
values, each provided on a time interval lasting 1800 seconds (30
minutes).
For the Endpoint Cost Service:
- the Calendar for "num-routingcost" is an array of 24 values, each
provided on a time interval lasting 3600 seconds (1 hour),
- the Calendar for "num-owdelay" is an array of 12 values, each
provided on a time interval lasting 300 seconds (5 minutes),
- the Calendar for "num-throughputrating" is an array of 60
values, each provided on a time interval lasting 60 seconds (1
minute), and
- the Calendar for "string-servicestatus" is an array of 30
values, each provided on a time interval lasting 120 seconds (2
minutes).
Note that in this example IRD, member "cost-constraints" is present
with a value set to "true" in both information resources
"filtered-cost-map-calendar" and
"endpoint-cost-map-calendar". Although a Calendar-aware ALTO Server
does not support constraints for the reasons explained in
, it MUST
support constraints on cost types that are not requested as Calendars
but are requested as specified in and .
ALTO Calendar Specification: Service Information Resources
This section documents extensions to two basic ALTO information
resources (Filtered Cost Maps and Endpoint Cost Service) to provide
calendared information services for them.
Both extensions return calendar start time (calendar-start-time, a
point in time), which MUST be specified as an HTTP
"Date" header field using the IMF-fixdate format specified in
. Note that the
IMF-fixdate format uses "GMT", not "UTC", to designate the time zone,
as in this example:
Calendar Extensions for Filtered Cost Maps (FCM)
A legacy ALTO Client requests and gets Filtered Cost Map responses,
as specified in .
Calendar Extensions in Filtered Cost Map Requests
The input parameters of a "legacy" request for a Filtered Cost
Map, defined by object ReqFilteredCostMap in , are augmented with one
additional member. The same augmentation applies to object
ReqFilteredCostMap defined in .
A Calendar-aware ALTO Client requesting a Calendar on a given
cost type for a Filtered Cost Map resource having Calendar
capabilities MUST add the following field to its
input parameters:
;
]]>
This field is an array of 1 to N boolean values, where N is the
number of requested metrics. N is greater than 1 when the Client and
the Server also implement .
Each entry corresponds to the requested metric at the same array
position. Each boolean value indicates whether or not the ALTO
Server should provide the values for this cost type as a Calendar.
The array MUST contain exactly N boolean values,
otherwise, the Server returns an error.
This field MUST NOT be included if no member
"calendar-attributes" is specified in this information resource.
If a value of field "calendared" is 'true' for a cost type name
for which no Calendar attributes have been specified, an ALTO
Server, whether it implements the extensions of this document or
only implements ,
MUST ignore it and return a response with a single
cost value, as specified in .
If this field is not present, it MUST be assumed
to have only values equal to 'false'.
A Calendar-aware ALTO Client that supports requests for only one
cost type at a time and wants to request a Calendar
MUST provide an array of 1 element:
A Calendar-aware ALTO Client that supports requests for more than
one cost type at a time, as specified in , MUST provide an array of N values
set to 'true' or 'false', depending whether it wants the applicable
cost type values as a single or calendared value.
Calendar Extensions in Filtered Cost Map Responses
In a calendared ALTO Filtered Cost Map, a cost value between a source
and a destination is a JSON array of JSON values. An ALTO Calendar
values array has a number of values equal to the value of member
"number-of-intervals" of the Calendar attributes that are indicated
in the IRD. These attributes will be conveyed as metadata in the
Filtered Cost Map response. Each element of the array is valid for
the time interval that matches its array position.
The FCM response conveys metadata, among which:
- some are not specific to Calendars and ensure compatibility
with and and
- some are specific to Calendars.
The non-Calendar-specific "meta" fields of a calendared Filtered
Cost Map response MUST include at least:
-
if the ALTO Client requests cost values for one cost type at
a time, only the "meta" fields specified in for these information
service responses:
- "dependent-vtags" and
- "cost-type" field.
-
if the ALTO Client implements the Multi-Cost ALTO extension
specified in and
requests cost values for several cost types at a time, the
"meta" fields specified in for these information service responses:
- "dependent-vtags",
- "cost-type" field with value set to '{}', for backwards
compatibility with ,
and
- "multi-cost-types" field.
If the Client request does not provide member "calendared" or if
it provides it with a value equal to 'false' for all the requested
cost types, then the ALTO Server response is exactly as specified in
and .
If the value of member "calendared" is equal to 'false' for a given
requested cost type, the ALTO Server MUST return, for this cost type,
a single cost value as specified in .
If the value of member "calendared" is equal to 'true' for a given
requested cost type, the ALTO Server returns, for this cost type, a
cost value Calendar, as specified above in this section. In addition
to the above cited non-Calendar-specific "meta" members, the Server
MUST provide a Calendar-specific metadata field.
The Calendar-specific "meta" field that a calendared Filtered Cost
Map response MUST include is a member called "calendar-response-attributes",
which describes properties of the Calendar and where:
- member "calendar-response-attributes" is an array of one or
more objects of type "CalendarResponseAttributes",
- each "CalendarResponseAttributes" object in the array is specified
for one or more cost types for which the value of member
"calendared", in object ReqFilteredCostMap provided in the Client request,
is equal to 'true' and for which a Calendar is
provided for the requested information resource, and
- the "CalendarResponseAttributes" object that applies to a cost
type name has a corresponding "CalendarAttributes" object defined
for this cost type name in the IRD capabilities of the requested
information resource. This object is the entry in the
"calendar-attributes" array member of the IRD resource entry,
which includes the name of the requested cost type. This
corresponding "CalendarAttributes" object has the same values as
object "CalendarResponseAttributes" for members
"time-interval-size" and "number-of-intervals". The members of the
"CalendarResponseAttributes" object include all the members of the
corresponding "CalendarAttributes" object.
The format of member "CalendarResponseAttributes is defined as follows:
CalendarResponseAttributes calendar-response-attributes <1..*>;
;]
JSONString calendar-start-time;
JSONNumber time-interval-size;
JSONNumber number-of-intervals;
[JSONNumber repeated;]
} CalendarResponseAttributes;
Object CalendarResponseAttributes has the following attributes:
]]>
- "cost-type-names":
- An array of one or more cost type names to which the value
of the other members of CalendarResponseAttributes apply and for
which a Calendar has been requested. The value of this member is a
subset of the "cost-type-names" member of the abovementioned
corresponding "CalendarAttributes" object in the
"calendar-attributes" array member in the IRD. This member
MUST be present when Cost Calendars are provided
for more than one cost type.
- "calendar-start-time":
- Indicates the date at which the first value of the Calendar
applies. The value is a string that, as specified in , contains an HTTP "Date" header
field using the IMF-fixdate format specified in . The
value provided for attribute "calendar-start-time" SHOULD
NOT be later than the request date.
- "time-interval-size":
- As specified in and
with the same value as in the abovementioned corresponding
"CalendarAttributes" object.
- "number-of-intervals":
- As specified in and
with the same value as in the abovementioned corresponding
"CalendarAttributes" object.
- "repeated":
- An optional field provided for Calendars. It is an integer
N greater or equal to '1' that indicates how many iterations of
the Calendar value array starting at the date indicated by
"calendar-start-time" have the same values. The number N includes
the iteration provided in the returned response.
For example, suppose the "calendar-start-time" member has value
"Mon, 30 Jun 2019 00:00:00 GMT", the "time-interval-size" member has
value '3600', the "number-of-intervals" member has value '24', and
the value of member "repeated" is equal to '4'. This means that the
Calendar values are the same on Monday, Tuesday, Wednesday, and
Thursday on a period of 24 hours starting at 00:00:00 GMT. The ALTO
Client thus may use the same Calendar for the next 4 days starting
at "calendar-start-time" and will only need to request a new one for
Friday, July 4th at 00:00:00 GMT.
Attribute "repeated" may take a very high value if a Calendar
represents a cyclic value pattern that the Server considers valid
for a long period and hence will only update once this period has
elapsed or if an unexpected event occurs on the network. In the
latter case, the Client will be notified if it uses the "ALTO
Incremental Updates Using Server-Sent Events (SSE)" Service,
specified in . To this
end, it is RECOMMENDED that ALTO Servers providing
ALTO Calendars also provide the "ALTO Incremental Updates Using
Server-Sent Events (SSE)" Service, which is specified in . Likewise, ALTO Clients capable
of using ALTO Calendars SHOULD also use the SSE
Service. See also discussion in "Operational Considerations".
Use Case and Example: FCM with a Bandwidth Calendar
An example of non-real-time information that can be provisioned
in a Calendar is the expected path throughput. While the
transmission rate can be measured in real time by end systems, the
operator of a data center is in the position of formulating
preferences for given paths at given time periods to avoid traffic
peaks due to diurnal usage patterns. In this example, we assume
that an ALTO Client requests a Calendar of network-provider-defined
throughput ratings as specified in the IRD to schedule its bulk
data transfers as described in the use cases.
In the example IRD, Calendars for cost type name
"num-throughputrating" are available for the information resources
"filtered-cost-calendar-map" and "endpoint-cost-map-calendar". The
ALTO Client requests a Calendar for "num-throughputrating" via a
POST request for a Filtered Cost Map.
We suppose in the present example that the ALTO Client sends its
request on Tuesday, July 1st 2019 at 13:15. The Server returns
Calendars with arrays of 12 numbers for each source and destination
pair. The values for metric "throughputrating", in this example, are
assumed to be encoded in 2 digits.
Calendar Extensions in the Endpoint Cost Service
This document extends the Endpoint Cost Service, as defined in
, by adding new
input parameters and capabilities and by returning JSONArrays instead
of JSONNumbers as the cost values. The media type () and HTTP
method () are unchanged.
Calendar-Specific Input in Endpoint Cost Requests
The extensions to the requests for calendared Endpoint Cost Maps
are the same as for the Filtered Cost Map Service, specified in
of this
document. Likewise, the rules defined around the extensions to ECM
requests are the same as those defined in for FCM requests.
The ReqEndpointCostMap object for a calendared ECM request will have
the following format:
;]
[JSONBoolean calendared<1..*>;]
EndpointFilter endpoints;
} ReqEndpointCostMap;
object {
[TypedEndpointAddr srcs<0..*>;]
[TypedEndpointAddr dsts<0..*>;]
} EndpointFilter;
]]>
Member "cost-type" is optional because, in the ReqEndpointCostMap
object definition of this document, it is jointly present with
member "multi-cost-types" to ensure compatibility with . In , members "cost-type" and "multi-cost-types" are
both optional and have to obey the rule specified in stating that
"the Client MUST specify either "cost-type" or
"multi-cost-types" but MUST NOT specify both".
The interpretation of member "calendared" is the same as for the
ReqFilteredCostMap object defined in of this document. The interpretation of the other
members is the same as for object ReqEndpointCostMap defined in
and . The type TypedEndpointAddr is defined in .
For the reasons explained in , a Calendar-aware ALTO Server does not support
constraints. Therefore, member "[constraints]" is not present in the
ReqEndpointCostMap object, and member "constraints" MUST
NOT be present in the input parameters of a request for an
Endpoint Cost Calendar. If this member is present, the Server
MUST ignore it.
Calendar Attributes in the Endpoint Cost Response
The "meta" field of a calendared Endpoint Cost response
MUST include at least:
-
if the ALTO Client supports cost values for one cost type at
a time only, the "meta" fields specified in for the
Endpoint Cost response:
-
if the ALTO Client supports cost values for several cost
types at a time, as specified in , the "meta" fields specified in for the Endpoint Cost
response:
- "cost-type" field with value set to '{}', for backwards
compatibility with .
- "multi-cost-types" field.
If the Client request does not provide member "calendared" or if
it provides it with a value equal to 'false', for all the requested
cost types, then the ALTO Server response is exactly as specified in
and .
If the ALTO Client provides member "calendared" in the input
parameters with a value equal to 'true' for given requested cost
types, the "meta" member of a calendared Endpoint Cost response
MUST include, for these cost types, an additional
member "calendar-response-attributes", the contents of which obey
the same rules as for the Filtered Cost Map Service, specified in
. The Server response
is thus changed as follows, with respect to and :
- the "meta" member has one additional field
"CalendarResponseAttributes", as specified for the Filtered Cost
Map Service, and
- the calendared costs are JSONArrays instead of the JSONNumbers
format used by legacy ALTO implementations. All arrays have a
number of values equal to 'number-of-intervals'. Each value
corresponds to the cost in that interval.
If the value of member "calendared" is equal to 'false' for a
given requested cost type, the ALTO Server MUST
return, for this cost type, a single cost value as specified in
.
Use Case and Example: ECS with a routingcost Calendar
Let us assume an Application Client is located in an end system
with limited resources and has access to the network that is either
intermittent or provides an acceptable quality in limited but
predictable time periods. Therefore, it needs to schedule both its
resource-greedy networking activities and its ALTO transactions.
The Application Client has the choice to trade content or
resources with a set of endpoints and needs to decide with which one
it will connect and at what time. For instance, the endpoints are
spread in different time zones or have intermittent access. In
this example, the 'routingcost' is assumed to be time-varying, with
values provided as ALTO Calendars.
The ALTO Client associated with the Application Client queries an
ALTO Calendar on 'routingcost' and will get the Calendar covering
the 24-hour time period "containing" the date and time of the ALTO
Client request.
For cost type "num-routingcost", the solicited ALTO Server has
defined 3 different daily patterns, each represented by a Calendar to
cover the week of Monday, June 30th at 00:00 to Sunday, July 6th 23:59:
- C1 for Monday, Tuesday, Wednesday, and Thursday (weekdays)
- C2 for Saturday and Sunday (weekends)
- C3 for Friday (maintenance outage on July 4, 2019 from
02:00:00 GMT to 04:00:00 GMT or a big holiday that
is widely celebrated and generates a large number of connections).
In the following example, the ALTO Client sends its request on
Tuesday, July 1st 2019 at 13:15.
The "routingcost" values are assumed to be encoded in 3
digits.
When the Client gets the Calendar for "routingcost", it sees that
the "calendar-start-time" is Monday at 00h00 GMT and member
"repeated" is equal to '4'. It understands that the provided values
are valid until Thursday and will only need to get a Calendar update
on Friday.
Use Case and Example: ECS with a Multi-cost Calendar for
routingcost and owdelay
In this example, it is assumed that the ALTO Server implements
multi-cost capabilities, as specified in . That is, an ALTO Client can request and receive
values for several cost types in one single transaction. An
illustrating use case is a path selection done on the basis of 2
metrics: routingcost and owdelay.
As in the previous example, the IRD indicates that the ALTO
Server provides "routingcost" Calendars in terms of 24 time
intervals of 1 hour (3600 seconds) each.
For metric "owdelay", the IRD indicates that the ALTO Server
provides Calendars in terms of 12 time interval values lasting 5
minutes (300 seconds) each.
In the following example transaction, the ALTO Client sends its
request on Tuesday, July 1st 2019 at 13:15.
This example assumes that the values of metric "owdelay" and
"routingcost" are encoded in 3 digits.
When receiving the response, the Client sees that the Calendar
values for metric "routingcost" are repeated for 4 iterations.
Therefore, in its next requests until the "routingcost" Calendar is
expected to change, the Client will only need to request a Calendar
for "owdelay".
Without the ALTO Calendar extensions, the ALTO Client would have no
clue on the dynamicity of the metric value change and would spend
needless time requesting values at an inappropriate pace. In
addition, without the Multi-Cost ALTO capabilities, the ALTO Client
would duplicate this waste of time as it would need to send one
request per cost metric.
IANA Considerations
This document has no IANA actions.
Security Considerations
As an extension of the base ALTO protocol , this document fits into the architecture of the base
protocol and hence the security considerations () fully apply when this extension is
provided by an ALTO Server. For example, the same authenticity and
integrity considerations () still fully apply; the same considerations for the
privacy of ALTO users () also still fully apply.
The calendaring information provided by this extension requires
additional considerations on three security considerations discussed
in : potential undesirable guidance to Clients
(), confidentiality of ALTO information
(), and
availability of ALTO (). For example, by providing network information in the
future in a Calendar, this extension may improve availability of
ALTO when the ALTO Server is unavailable but related information is
already provided in the Calendar.
For confidentiality of ALTO information, an operator should be
cognizant that this extension may introduce a new risk, a malicious ALTO
Client may get information for future events that are scheduled
through Calendaring. Possessing such information, the malicious Client may use
it to generate massive connections to the network at times where its load
is expected to be high.
To mitigate this risk, the operator should address the risk of ALTO
information being leaked to malicious Clients or third parties. As
specified in "Protection Strategies" (), the ALTO Server should
authenticate ALTO Clients and use the Transport Layer Security (TLS)
protocol so that man-in-the-middle (MITM) attacks to intercept an ALTO
Calendar are not possible. "Authentication and Encryption" () ensures the availability of such a
solution. It specifies that "ALTO
Server implementations as well as ALTO Client implementations
MUST support the "https" URI scheme of and Transport Layer Security (TLS)
of ".
Section of
TLS 1.3 states: "While TLS 1.3 is not directly compatible with previous
versions, all versions of TLS incorporate a versioning mechanism which
allows Clients and Servers to interoperably negotiate a common version
if one is supported by both peers". ALTO Clients and Servers
SHOULD support both TLS 1.3 and TLS 1.2
and MAY support and use newer versions of TLS as long as
the negotiation process succeeds.
The operator should be cognizant that the preceding mechanisms do not
address all security risks. In particular, they will not help in the
case of "malicious Clients" possessing valid authentication
credentials. The threat here is that legitimate Clients have become
subverted by an attacker and are now 'bots' being asked to participate
in a DDoS attack. The Calendar information now becomes valuable in
knowing exactly when to perpetrate a DDoS attack. A mechanism, such as a
monitoring system that detects abnormal behaviors, may still be needed.
To avoid malicious or erroneous guidance from ALTO information, an
ALTO Client should be cognizant that using calendaring information can
have risks: (1) Calendar values, especially in "repeated" Calendars, may
be only statistical and (2) future events may change. Hence, a more
robust ALTO Client should adapt and extend protection strategies
specified in .
For example, to be notified immediately when a particular ALTO value
that the Client depends on changes, it is RECOMMENDED
that both the ALTO Client and ALTO Server using this extension support
"Application-Layer Traffic
Optimization (ALTO) Incremental Updates Using Server-Sent
Events (SSE)" .
Another risk of erroneous guidance appears when the Server exposes an
occurrence of a same cost type name in different elements of the
Calendar objects array associated to an information resource. In this
case, there is no way for the Client to figure out which Calendar object
in the array is valid. The specification in this document recommends, in
this case, that the Client uses the first encountered Calendar object
occurrence containing the cost type name. However, the Client may want
to avoid the risks of erroneous guidance associated to the use of
potentially invalid Calendar values. To this end, as an alternative to
the recommendation in this document, the Client MAY
ignore the totality of occurrences of CalendarAttributes objects
containing the cost type name and query this cost type using .
Operational Considerations
It is important that both the operator of the network and the
operator of the applications consider both the feedback aspect and the
prediction-based (uncertainty) aspect of using the Cost Calendar.
First, consider the feedback aspect and consider the Cost Calendar as
a traffic-aware map service (e.g., Google Maps). Using the service
without considering its own effect, a large fleet can turn a
not-congested road into a congested one; a large number of individual
cars each choosing a road with light traffic ("cheap link") can also
result in congestion or result in a less-optimal global outcome (e.g.,
the Braess' Paradox ).
Next, consider the prediction aspect. Conveying ALTO Cost Calendars
tends to reduce the on-the-wire data exchange volume compared to
multiple single-cost ALTO transactions. An application using Calendars
has a set of time-dependent values upon which it can plan its
connections in advance with no need for the ALTO Client to query
information at each time. Additionally, the Calendar response attribute
"repeated", when provided, saves additional data exchanges in that it
indicates that the ALTO Client does not need to query Calendars during a
period indicated by this attribute. The preceding is true only when
"accidents" do not happen.
Although individual network operators and application operators can
choose their own approaches to address the aforementioned issues, this
document recommends the following considerations. First, a typical
approach to reducing instability and handling uncertainty is to ensure
timely update of information. The SSE Service, as discussed in , can handle updates if supported by
both the Server and the Client. Second, when a network operator updates
the Cost Calendar and when an application reacts to the update, they
should consider the feedback effects. This is the best approach even
though there is theoretical analysis and
Internet-based evaluation showing that uncoordinated behaviors do not always
cause substantial suboptimal results.
High-resolution intervals may be needed when values change, sometimes
during very small time intervals but in a significant manner. A way
to avoid conveying too many entries is to leverage on the "repeated"
feature. A Server can smartly set the Calendar start time and number
of intervals so as to declare them "repeated" for a large number of
periods until the Calendar values change and are conveyed to
requesting Clients.
The newer JSON Data Interchange Format specification used in ALTO Calendars replaces the
older one used in the base
ALTO protocol . The newer JSON
mandates UTF-8 text encoding to improve interoperability. Therefore,
ALTO Clients and Servers implementations using UTF-{16,32} need to be
cognizant of the subsequent interoperability risks and
MUST switch to UTF-8 encoding if they want to
interoperate with Calendar-aware Servers and Clients.
References
Normative References
Application-Layer Traffic Optimization (ALTO) Incremental Updates Using
Server-Sent Events (SSE)
IEEE Standard for Floating-Point Arithmetic
IEEE
Informative References
SDN for End-to-End Networked Science at the Exascale (SENSE)
Department of Energy Office of Science Advanced
Scientific Computing Research (ASCR) Program
The Prevalence of Braess' Paradox
Transportation Science Vol. 17, No. 3
Unicorn: Unified resource orchestration for multi-domain,
geo-distributed data analytics
Future Generation Computer Systems (FGCS), Vol. 93,
Pages 188-197
Selfish Routing
Dissertation Thesis, Cornell
Selfish Routing in Internet-Like Environments
Proceedings of SIGCOMM '03
Acknowledgments
The authors would like to thank , , , , and for fruitful
discussions and feedback on earlier draft versions. , , , , , , and
provided substantial review feedback and
suggestions to the protocol design.