3. Deployment Considerations by ISPs
3.1. Objectives for the Guidance to Applications
3.1.1. General Objectives for Traffic Optimization
The Internet consists of many networks. The networks are owned and
managed by different network operators, such as commercial ISPs,
enterprise IT departments, universities, and other organizations.
These network operators provide network connectivity, e.g., by access
networks, such as cable networks, xDSL networks, 3G/4G mobile
networks, etc. Network operators need to manage, control, and audit
the traffic. Therefore, it is important to understand how to deploy
an ALTO service and what its expected impact might be.
The general objective of ALTO is to give guidance to applications on
what endpoints (e.g., IP addresses or IP prefixes) are to be
preferred according to the operator of the ALTO server. The ALTO
protocol gives means to let the ALTO server operator express its
preference, whatever this preference is.
ALTO enables network operators to support application-level traffic
engineering by influencing application resource provider selection.
This traffic engineering can have different objectives:
1. Inter-network traffic localization: ALTO can help to reduce
inter-domain traffic. The networks of different network
operators are interconnected through peering points. From a
business view, the inter-network settlement is needed for
exchanging traffic between these networks. These peering
agreements can be costly. To reduce these costs, a simple
objective is to decrease the traffic exchange across the peering
points and thus keep the traffic in the own network or Autonomous
System (AS) as far as possible.
2. Intra-network traffic localization: In case of large network
operators, the network may be grouped into several networks,
domains, or ASes. The core network includes one or several
backbone networks, which are connected to multiple aggregation,
metro, and access networks. If traffic can be limited to certain
areas such as access networks, this decreases the usage of
backbone and thus helps to save resources and costs.
3. Network offloading: Compared to fixed networks, mobile networks
have some special characteristics, including lower link
bandwidth, high cost, limited radio frequency resource, and
limited terminal battery. In mobile networks, wireless links
should be used efficiently. For example, in the case of a P2P
service, it is likely that hosts should prefer retrieving data
from hosts in fixed networks, and avoid retrieving data from
4. Application tuning: ALTO is also a tool to optimize the
performance of applications that depend on the network and
perform resource provider selection decisions among network
endpoints; an example is the network-aware selection of CDN
In the following, these objectives are explained in more detail with
3.1.2. Inter-Network Traffic Localization
ALTO guidance can be used to keep traffic local in a network, for
instance, in order to reduce peering costs. An ALTO server can let
applications prefer other hosts within the same network operator's
network instead of randomly connecting to other hosts that are
located in another operator's network. Here, a network operator
would always express its preference for hosts in its own network,
while hosts located outside its own network are to be avoided (i.e.,
they are undesired to be considered by the applications). Figure 5
shows such a scenario where hosts prefer hosts in the same network
(e.g., Host 1 and Host 2 in ISP1 and Host 3 and Host 4 in ISP2).
,---. ,-' `-. | Host 1 |
,-' `-. / ISP 1 ########|ALTO Client|
/ \ / # \ +-----------+
/ ISP X \ | # | +-----------+
/ \ \ ########| Host 2 |
; +----------------------------|ALTO Client|
| | | `-. ,-' +-----------+
| | | `-------'
| Inter- | | ,-------. +-----------+
: network | ; ,-' `########| Host 3 |
\ traffic | / / ISP 2 # \ |ALTO Client|
\ | / / # \ +-----------+
\ |/ | # | +-----------+
`-. ,-| \ ########| Host 4 |
`---' +----------------------------|ALTO Client|
`-. ,-' +-----------+
### preferred "connections"
--- non-preferred "connections"
Figure 5: Inter-Network Traffic Localization
Examples for corresponding ALTO maps can be found in Section 3.5.
Depending on the application characteristics, it may not be possible
or even desirable to completely localize all traffic.
3.1.3. Intra-Network Traffic Localization
The previous section describes the results of the ALTO guidance on an
inter-network level. In the same way, ALTO can also be used for
intra-network localization. In this case, ALTO provides guidance on
which internal hosts are to be preferred inside a single network
(e.g., one AS). This application-level traffic engineering can
reduce the capacity requirements in the core network of an ISP.
Figure 6 shows such a scenario where Host 1 and Host 2 are located in
an access net 1 of ISP 1 and connect via a low capacity link to the
core of the same ISP 1. If Host 1 and Host 2 exchange their data
with remote hosts, they would probably congest the bottleneck link.
Bottleneck ,-------. +-----------+
,---. | ,-' `-. | Host 1 |
,-' `-. | / ISP 1 ########|ALTO Client|
/ \ | / (Access # \ +-----------+
/ ISP 1 \| | net 1) # | +-----------+
/ (Core V \ ########| Host 2 |
; network) +--X~~~X---------------------|ALTO Client|
| | | `-. ,-' +-----------+
| | | `-------'
| | | ,-------. +-----------+
: | ; ,-' `########| Host 3 |
\ | / / ISP 1 # \ |ALTO Client|
\ | / / (Access # \ +-----------+
\ |/ | net 2) # | +-----------+
`-. ,-X \ ########| Host 4 |
`---' ~~~~~~~X---------------------|ALTO Client|
^ `-. ,-' +-----------+
### preferred "connections"
--- non-preferred "connections"
Figure 6: Intra-Network Traffic Localization
In such a situation, the operator can guide the hosts to try local
hosts in the same network islands first, avoiding or at least
lowering the effect on the bottleneck link, as shown in Figure 6.
The objective is to avoid bottlenecks by optimized endpoint selection
at the application level. That said, it must be understood that ALTO
is not a general-purpose method to deal with the congestion at the
3.1.4. Network Offloading
Another scenario is offloading traffic from networks. This use of
ALTO can be beneficial in particular in mobile networks. A network
operator may have the desire to guide hosts in its mobile network to
use hosts outside this mobile network. One reason could be that the
wireless network or the mobile hosts were not designed for direct
peer-to-peer communications between mobile hosts, and therefore, it
makes sense for peers to fetch content from remote peers in other
parts of the Internet.
,---. ,-' `-. | Host 1 |
,-' `-. / ISP 1 +-------|ALTO Client|
/ \ / (Mobile | \ +-----------+
/ ISP X \ | network) | | +-----------+
/ \ \ +-------| Host 2 |
; #############################|ALTO Client|
| # | `-. ,-' +-----------+
| # | `-------'
| # | ,-------.
: # ; ,-' `-.
\ # / / ISP 2 \
\ # / / (Fixed \
\ #/ | network) | +-----------+
`-. ,-# \ / | Host 3 |
`---' #############################|ALTO Client|
`-. ,-' +-----------+
### preferred "connections"
--- non-preferred "connections"
Figure 7: ALTO Traffic Network De-localizationFigure 7 shows the result of such a guidance process where Host 2
prefers a connection with Host 3 instead of Host 1, as shown in
A realization of this scenario may have certain limitations and may
not be possible in all cases. For instance, it may require the ALTO
server to distinguish mobile and non-mobile hosts based on their IP
address. This may depend on mobility solutions and may not be
possible or accurate. In general, ALTO is not intended as a fine-
grained traffic engineering solution for individual hosts. Instead,
it typically works on aggregates (e.g., if it is known that certain
IP prefixes are often assigned to mobile users).
3.1.5. Application Tuning
ALTO can also provide guidance to optimize the application-level
topology of networked applications, e.g., by exposing network
performance information. Applications can often run their own
measurements to determine network performance, e.g., by active delay
measurements or bandwidth probing, but such measurements result in
overhead and complexity. Accessing an ALTO server can be a simpler
alternative. In addition, an ALTO server may also expose network
information that applications cannot easily measure or reverse-
3.2. Provisioning of ALTO Topology Data
3.2.1. High-Level Process and Requirements
A process to generate ALTO topology information typically comprises
several steps. The first step is to gather information, which is
described in the following section. The subsequent sections describe
how the gathered data can be processed and which methods can be
applied to generate the information exposed by ALTO, such as network
and cost maps.
Providing ALTO guidance can result in a win-win situation for network
providers and users of the ALTO information. Applications possibly
get a better performance, while the network provider has means to
optimize the traffic engineering and thus its costs. Yet, there can
be security concerns with exposing topology data. Corresponding
limitations are discussed in Section 7.2.
ISPs may have important privacy requirements when deploying ALTO,
which have to be taken into account when processing ALTO topology
data. In particular, an ISP may not be willing to expose sensitive
operational details of its network. The topology abstraction of ALTO
enables an ISP to expose the network topology at a desired
granularity only, determined by security policies.
With the ECS, the ALTO client does not have to implement any specific
algorithm or mechanism in order to retrieve, maintain and process
network topology information (of any kind). The complexity of the
network topology (computation, maintenance and distribution) is kept
in the ALTO server and ECS is delivered on demand. This allows the
ALTO server to enhance and modify the way the topology information
sources are used and combined. This simplifies the enforcement of
privacy policies of the ISP.
The ALTO Network and Cost Map Service expose an abstract view on the
ISP network topology. Therefore, care is needed when constructing
those maps in order to take privacy policies into account, as further
discussed in Section 3.2.3. The ALTO protocol also supports further
features such as endpoint properties, which could also be used to
expose topology guidance. The privacy considerations for ALTO maps
also apply to such ALTO extensions.
3.2.2. Data Collection from Data Sources
The first step in the process of generating ALTO information is to
gather the required information from the network. An ALTO server can
collect topological information from a variety of sources in the
network and provides a cohesive, abstract view of the network
topology to applications using an ALTO client. Topology data sources
may include routing protocols, network policies, state and
performance information, geolocation, etc. An ALTO server requires
at least some topology and/or routing information, i.e., information
about existing endpoints and their interconnection. With this
information, it is in principle possible to compute paths between all
known endpoints. Based on such basic data, the ALTO server builds an
ALTO-specific network topology that represents the network as it
should be understood and utilized by applications (resource
consumers) at endpoints using ALTO services (e.g., Network and Cost
Map Service or ECS). A basic dataset can be extended by many other
information obtainable from the network.
The ALTO protocol does not assume a specific network technology or
topology. In principle, ALTO can be used with various types of
addresses (Endpoint Addresses). [RFC7285] defines the use of IPv4/
IPv6 addresses or prefixes in ALTO, but further address types could
be added by extensions. In this document, only the use of IPv4/IPv6
addresses is considered.
The exposure of network topology information is controlled and
managed by the ALTO server. ALTO abstract network topologies can be
automatically generated from the physical or logical topology of the
network, e.g., using "live" network data. The generation would
typically be based on policies and rules set by the network operator.
The maps and the guidance can significantly differ depending on the
use case, the network architecture, and the trust relationship
between ALTO server and ALTO client, etc. Besides the security
requirements that consist of not delivering any confidential or
critical information about the infrastructure, there are efficiency
requirements in terms of what aspects of the network are visible and
required by the given use case and/or application.
The ALTO server operator has to ensure that the ALTO topology does
not reveal any details that would endanger the network integrity and
security. For instance, ALTO is not intended to leak raw Interior
Gateway Protocol (IGP) or Border Gateway Protocol (BGP) databases to
| ALTO | | ALTO |
| Client | | Client |
|| || ALTO protocol
| ALTO |
| Server |
: : : :
: : : :
+..........+ : : +..........+ Provisioning
: : : : protocol
: : : :
+---------+ +---------+ +---------+ +---------+
| BGP | | I2RS | | PCE | | NMS | Potential
| Speaker | | Client | | | | OSS | data sources
+---------+ +---------+ +---------+ +---------+
^ ^ ^ ^
| | | |
Link-State I2RS TED Topology and traffic-related
NLRI for data data data from SNMP, NETCONF,
IGP/BGP RESTCONF, REST, IPFIX, etc.
Figure 8: Potential Data Sources for ALTO
As illustrated in Figure 8, the topology data used by an ALTO server
can originate from different data sources:
o Relevant information sources are IGPs or BGP. An ALTO server
could get network routing information by listening to IGPs and/or
peering with BGP speakers. For data collection, link-state
protocols are more suitable since every router propagates its
information throughout the whole network. Hence, it is possible
to obtain information about all routers and their neighbors from
one single router in the network. In contrast, distance-vector
protocols are less suitable since routing information is only
shared among neighbors. To obtain the whole topology with
distance-vector routing protocols it is necessary to retrieve
routing information from every router in the network.
o [RFC7752] describes a mechanism by which link-state and Traffic
Engineering (TE) information can be collected from networks and
shared with external components using the BGP routing protocol.
This is achieved using a new BGP Network Layer Reachability
Information (NLRI) encoding format. The mechanism is applicable
to physical and virtual IGP links and can also include TE data.
For instance, prefix data can be carried and originated in BGP,
while TE data is originated and carried in an IGP. The mechanism
described is subject to policy control.
o The Interface to the Routing System (I2RS) is a solution for state
transfer in and out of the Internet's routing system [RFC7921].
An ALTO server could use an I2RS client to observe routing-related
information. With the rise of Software-Defined Networking (SDN)
and a decoupling of network data and control plane, topology
information could also be fetched from an SDN controller. If I2RS
is used, [RFC7922] provides traceability for these interactions.
This scenario is not further discussed in the remainder of this
o Another potential source of topology information could be a Path
Computation Element (PCE) [RFC4655]. Topology and traffic-related
information can be retrieved from the Traffic Engineering Database
(TED) and Label Switched Path Database (LSP-DB). This scenario is
not further discussed in the remainder of this document.
o An ALTO server can also leverage a Network Management System (NMS)
or an Operations Support System (OSS) as data sources. NMS or OSS
solutions are used to control, operate, and manage a network,
e.g., using the Simple Network Management Protocol (SNMP) or
Network Configuration Protocol (NETCONF). As explained for
instance in [RFC7491], the NMS and OSS can be consumers of network
events reported and can act on these reports as well as displaying
them to users and raising alarms. In addition, NMS and OSS
systems may have access to routing information and network
inventory data (e.g., links, nodes, or link properties not visible
to routing protocols, such as Shared Risk Link Groups).
Furthermore, Operations, Administration, and Maintenance (OAM)
information can be leveraged, including traffic utilization
obtained from IP Flow Information Export (IPFIX), event
notifications (e.g., via syslog), liveness detection (e.g.,
bidirectional forwarding detection, BFD). NMS or OSS systems also
may have functions to correlate and orchestrate information
originating from other data sources. For instance, it could be
required to correlate IP prefixes with routers (Provider, Provider
Edge, Customer Edge, etc.), IGP areas, VLAN IDs, or policies.
In the context of the provisioning protocol, topology information
could be modeled in a YANG data model [NETWORK-TOPO].
The data sources mentioned so far are only a subset of potential
topology sources and protocols. Depending on the network type,
(e.g., mobile, satellite network) different hardware and protocols
are in operation to form and maintain the network.
In general, it is challenging to gather detailed information about
the whole Internet, since the network consists of multiple domains
and in many cases it is not possible to collect information across
network borders. Hence, potential information sources may be limited
to a certain domain.
3.2.3. Partitioning and Grouping of IP Address Ranges
ALTO introduces provider-defined network location identifiers called
Provider-defined Identifiers (PIDs) to aggregate network endpoints in
the Map Services. Endpoints within one PID may be treated as single
entity, assuming proximity based on network topology or other
similarity. A key use case of PIDs is to specify network preferences
(costs) between PIDs instead of individual endpoints. It is up to
the operator of the ALTO server how to group endpoints and how to
assign PIDs. For example, a PID may denote a subnet, a set of
subnets, a metropolitan area, a POP, an autonomous system, or a set
of autonomous systems.
This document only considers deployment scenarios in which PIDs
expand to a set of IP address ranges (CIDR). A PID is characterized
by a string identifier and its associated set of endpoint addresses
[RFC7285]. If an ALTO server offers the Map Service, corresponding
identifiers have to be configured.
An automated ALTO implementation may use dynamic algorithms to
aggregate network topology. However, it is often desirable to have a
mechanism through which the network operator can control the level
and details of network aggregation based on a set of requirements and
constraints. This will typically be governed by policies that
enforce a certain level of abstraction and prevent leakage of
sensitive operational data.
For instance, an ALTO server may leverage BGP information that is
available in a network's service provider network layer and compute
the group of prefix. An example being BGP communities, which are
used in MPLS/IP networks as a common mechanism to aggregate and group
prefixes. A BGP community is an attribute used to tag a prefix to
group prefixes based on mostly any criteria (as an example, most ISP
networks originate BGP prefixes with communities identifying the
Point of Presence (PoP) where the prefix has been originated). These
BGP communities could be used to map IP address ranges to PIDs. By
an additional policy, the ALTO server operator may decide an
arbitrary cost defined between groups. Alternatively, there are
algorithms that allow the dynamic computation of costs between
groups. The ALTO protocol itself is independent of such algorithms
3.2.4. Rating Criteria and/or Cost Calculation
An ALTO server indicates preferences amongst network locations in the
form of abstract costs. These costs are generic costs and can be
internally computed by the operator of the ALTO server according to
its own policy. For a given ALTO network map, an ALTO cost map
defines directional costs pairwise amongst the set of source and
destination network locations defined by the PIDs.
The ALTO protocol permits the use of different cost types. An ALTO
cost type is defined by the combination of a cost metric and a cost
mode. The cost metric identifies what the costs represent. The cost
mode identifies how the costs should be interpreted, i.e., whether
returned costs should be interpreted as numerical values or ordinal
rankings. The ALTO protocol also allows the definition of additional
constraints defining which elements of a cost map shall be returned.
The ALTO protocol specification [RFC7285] defines the "routingcost"
cost metric as the basic set of rating criteria, which has to be
supported by all implementations. This cost metric conveys a generic
measure for the cost of routing traffic from a source to a
destination. A lower value indicates a higher preference for traffic
to be sent from a source to a destination. How that metric is
calculated is up to the ALTO server.
It is possible to calculate the "routingcost" cost metric based on
actual routing protocol information. Typically, IGPs provide details
about endpoints and links within a given network, while the BGP is
used to provide details about links to endpoints in other networks.
Besides topology and routing information, networks have a multitude
of other attributes about their state, condition, and operation that
comprises but is not limited to attributes like link utilization,
bandwidth and delay, ingress/egress points of data flows from/towards
endpoints outside of the network up to the location of nodes and
In order to enable use of extended information, there is a protocol
extension procedure to add new ALTO cost types. The following list
gives an overview on further rating criteria that have been proposed
or that are in use by ALTO-related prototype implementations. This
list is not intended as normative text. Instead, its only purpose is
to document and discuss rating criteria that have been proposed so
far. Whether such rating criteria are useful and whether the
corresponding information would actually be made available by ISPs
can also depend on the use case of ALTO. A list of rating criteria
for which normative specifications exist and which have successfully
passed the IETF review process can be found at IANA's "ALTO Cost
Metric Registry", available from [ALTO-REG].
Distance-related rating criteria:
o Relative topological distance: The term relative means that a
larger numerical value means greater distance, but it is up to the
ALTO service how to compute the values, and the ALTO client will
not be informed about the nature of the computation. One way to
determine relative topological distance may be counting AS hops,
but when querying this parameter, the ALTO client must not assume
that the numbers actually are AS hops. In addition to the AS
path, a relative cost value could also be calculated taking into
account other routing protocol parameters, such as BGP local
preference or Multi-Exit Discriminator (MED) attributes.
o Absolute topological distance, expressed in the number of
traversed autonomous systems.
o Absolute topological distance, expressed in the number of router
hops (i.e., how much the TTL value of an IP packet will be
decreased during transit).
o Absolute physical distance, based on knowledge of the approximate
geolocation (e.g., continent, country) of an IP address.
Performance-related rating criteria:
o The minimum achievable throughput between the resource consumer
and the candidate resource provider, which is considered useful by
the application (only in ALTO queries).
o An arbitrary upper bound for the throughput from/to the candidate
resource provider (only in ALTO responses). This may be, but is
not necessarily, the provisioned access bandwidth of the candidate
o The maximum Round-Trip Time (RTT) between resource consumer and
the candidate resource provider, which is acceptable for the
application for useful communication with the candidate resource
provider (only in ALTO queries).
o An arbitrary lower bound for the RTT between resource consumer and
the candidate resource provider (only in ALTO responses). This
may be, for example, based on measurements of the propagation
delay in a completely unloaded network.
Charging-related rating criteria:
o Metrics representing an abstract cost, e.g., determined by
policies that distinguish "cheap" from "expensive" IP subnet
ranges without detailing the cost function. According to
[RFC7285], the abstract metric "routingcost" is an example for a
metric for which the cost function does not have to be disclosed.
o Traffic volume caps, in case the Internet access of the resource
consumer is not charged with a "flat rate". For each candidate
resource location, the ALTO service could indicate the amount of
data or the bitrate that may be transferred from/to this resource
location until a given point in time, and how much of this amount
has already been consumed. Furthermore, an ALTO server may have
to indicate how excess traffic would be handled (e.g., blocked,
throttled, or charged separately at an indicated price), e.g., by
a new endpoint property. This is outside the scope of this
document. Also, it is left for further study how several
applications would interact if only some of them use this
criterion. Also left for further study is the use of such a
criterion in resource directories that issue ALTO queries on
behalf of other endpoints.
All the above-listed rating criteria are subject to the remarks
The ALTO client must be aware that with high probability the actual
performance values will differ from whatever an ALTO server exposes.
In particular, an ALTO client must not consider a throughput
parameter as a permission to send data at the indicated rate without
using congestion control mechanisms.
The discrepancies are due to various reasons, including, but not
limited to the following facts:
o The ALTO service is not an admission control system.
o The ALTO service may not know the instantaneous congestion status
of the network.
o The ALTO service may not know all link bandwidths, i.e., where the
bottleneck really is, and there may be shared bottlenecks.
o The ALTO service may not have all information about the actual
o The ALTO service may not know whether the candidate endpoint
itself is overloaded.
o The ALTO service may not know whether the candidate endpoint
throttles the bandwidth it devotes for the considered application.
o The ALTO service may not know whether the candidate endpoint will
throttle the data it sends to the client (e.g., because of some
fairness algorithm, such as tit for tat).
Because of these inaccuracies and the lack of complete, instantaneous
state information, which are inherent to the ALTO service, the
application must use other mechanisms (such as passive measurements
on actual data transmissions) to assess the currently achievable
throughput, and it must use appropriate congestion control mechanisms
in order to avoid a congestion collapse. Nevertheless, the rating
criteria may provide a useful shortcut for quickly excluding
candidate resource providers from such probing, if it is known in
advance that connectivity is in any case worse than what is
considered the minimum useful value by the respective application.
Rating criteria that should not be defined for and used by the ALTO
o Performance metrics that are closely related to the instantaneous
congestion status. The definition of alternate approaches for
congestion control is explicitly out of the scope of ALTO.
Instead, other appropriate means, such as using TCP-based
transport, have to be used to avoid congestion. In other words,
ALTO is a service to provide network and policy information, with
update intervals that are possibly several orders of magnitude
slower than congestion-control loops (e.g., in TCP) can react on
changes in network congestion state. This clear separation of
responsibilities avoids traffic oscillations and can help for
network stability and cost optimization.
o Performance metrics that raise privacy concerns. For instance, it
has been questioned whether an ALTO service should publicly expose
the provisioned access bandwidth of cable/DSL customers, as this
could enable identification of "premium customers" of an ISP.
3.3. ALTO Focus and Scope
The purpose of this section is ensure that administrators and users
of ALTO services are aware of the objectives of the ALTO protocol
design. Using ALTO beyond this scope may limit its efficiency.
Likewise, Map-based and Endpoint-based ALTO Services may face certain
issues during deployment. This section explains these limitations
and also outlines potential solutions.
3.3.1. Limitations of Using ALTO beyond Design Assumptions
ALTO is designed as a protocol between clients integrated in
applications and servers that provide network information and
guidance (e.g., basic network location structure and preferences of
network paths). The objective is to modify network resource
consumption patterns at application level while maintaining or
improving application performance. This design focus results in a
number of characteristics of ALTO:
o Endpoint focus: In typical ALTO use cases, neither the consumer of
the topology information (i.e., the ALTO client) nor the
considered resources (e.g., files at endpoints) are part of the
network. The ALTO server presents an abstract network topology
containing only information relevant to an application overlay for
better-than-random resource provider selection among its
endpoints. The ALTO protocol specification [RFC7285] is not
designed to expose network internals such as routing tables or
configuration data that are not relevant for application-level
resource provider selection decisions in network endpoints.
o Abstraction: The ALTO services such as the Network and Cost Map
Service or the ECS provide an abstract view of the network only.
The operator of the ALTO server has full control over the
granularity (e.g., by defining policies how to aggregate subnets
into PIDs) and the level of detail of the abstract network
representation (e.g., by deciding what cost types to support).
o Multiple administrative domains: The ALTO protocol is designed for
use cases where the ALTO server and client can be located in
different organizations or trust domains. ALTO assumes a loose
coupling between server and client. In addition, ALTO does not
assume that an ALTO client has any a priori knowledge about the
ALTO server and its supported features. An ALTO server can be
o Read-only: ALTO is a query/response protocol to retrieve guidance
information. Neither network/cost map queries nor queries to the
ECS are designed to affect state in the network.
If ALTO shall be deployed for use cases beyond the scope defined by
these assumptions, the protocol design may result in limitations.
For instance, in an Application-Based Network Operations (ABNO)
environment, the application could issue an explicit service request
to the network [RFC7491]. In this case, the application would
require detailed knowledge about the internal network topology and
the actual state. A network configuration would also require a
corresponding security solution for authentication and authorization.
ALTO is not designed for operations to control, operate, and manage a
Such deployments could be addressed by network management solutions,
e.g., based on SNMP [RFC3411] or NETCONF [RFC6241] and YANG
[RFC6020], that are typically designed to manipulate configuration
state. [RFC7491] contains a more detailed discussion of interfaces
between components such as Element Management System (EMS), Network
Management System (NMS), Operational Support System (OSS), Traffic
Engineering Database (TED), Label Switched Path Database (LSP-DB),
Path Computation Element (PCE), and other Operations, Administration,
and Maintenance (OAM) components.
3.3.2. Limitations of Map-Based Services and Potential Solutions
The specification of the Map Service in the ALTO protocol [RFC7285]
is based on the concept of network maps. A network map partitions
the network into PIDs that group one or more endpoints (e.g.,
subnetworks) to a single aggregate. The "costs" between the various
PIDs are stored in a cost map. Map-based approaches such as the ALTO
Network and Cost Map Service lower the signaling load on the server
as maps have to be retrieved only if they change.
One main assumption for map-based approaches is that the information
provided in these maps is static for a long period of time. This
assumption is fine as long as the network operator does not change
any parameter, e.g., routing within the network and to the upstream
peers, and IP address assignment stays stable (and thus the mapping
to the partitions). However, there are several cases where this
assumption is not valid:
1. ISPs reallocate IP subnets from time to time.
2. ISPs reallocate IP subnets on short notice.
3. IP prefix blocks may be assigned to a router that serves a
variety of access networks.
4. Network costs between IP prefixes may change depending on the
ISP's routing and traffic engineering.
These effects can be explained as follows:
Case 1: ISPs may reallocate IP subnets within their infrastructure
from time to time, partly to ensure the efficient usage of IPv4
addresses (a scarce resource), and partly to enable efficient route
tables within their network routers. The frequency of these
"renumbering events" depends on the growth in number of subscribers
and the availability of address space within the ISP. As a result, a
subscriber's household device could retain an IP address for as short
as a few minutes or for months at a time or even longer.
It has been suggested that ISPs providing ALTO services could
subdivide their subscribers' devices into different IP subnets (or
certain IP address ranges) based on the purchased service tier, as
well as based on the location in the network topology. The problem
is that this sub-allocation of IP subnets tends to decrease the
efficiency of IP address allocation, in particular for IPv4. A
growing ISP that needs to maintain high efficiency of IP address
utilization may be reluctant to jeopardize their future acquisition
of IP address space.
However, this is not an issue for map-based approaches if changes are
applied in the order of days.
Case 2: ISPs can use techniques that allow the reallocation of IP
prefixes on very short notice, i.e., within minutes. An IP prefix
that has no IP address assignment to a host anymore can be
reallocated to areas where there is currently a high demand for IP
Case 3: In residential access networks (e.g., DSL, cable), IP
prefixes are assigned to broadband gateways, which are the first IP-
hop in the access-network between the Customer Premises Equipment
(CPE) and the Internet. The access-network between CPE and broadband
gateway (called aggregation network) can have varying characteristics
(and thus associated costs), but still using the same IP prefix. For
instance, one IP address IP1 out of a given CIDR prefix can be
assigned to a VDSL access line (e.g., 2 Mbit/s uplink) while another
IP address IP2 within the same given CIDR prefix is assigned to a
slow ADSL line (e.g., 128 kbit/s uplink). These IP addresses may be
assigned on a first come first served basis, i.e., a single IP
address out of the same CIDR prefix can change its associated costs
quite fast. This may not be an issue with respect to the used
upstream provider (thus the cross ISP traffic), but, depending on the
capacity of the aggregation network, this may raise to an issue.
Case 4: The routing and traffic engineering inside an ISP network, as
well as the peering with other autonomous systems, can change
dynamically and affect the information exposed by an ALTO server. As
a result, cost maps and possibly also network maps can change.
One solution to deal with map changes is to use incremental ALTO
3.3.3. Limitations of Non-Map-Based Services and Potential Solutions
The specification of the ALTO protocol [RFC7285] also includes the
ECS mechanism. ALTO clients can ask the ALTO server for guidance for
specific IP addresses, thereby avoiding the need of processing maps.
This can mitigate some of the problems mentioned in the previous
However, frequent requests, particularly with long lists of IP
addresses, may overload the ALTO server. The server has to rank each
received IP address, which causes load at the server. This may be
amplified when a large number of ALTO clients are asking for
guidance. The results of the ECS are also more difficult to cache
than ALTO maps. Therefore, the ALTO client may have to await the
server response before starting a communication, which results in an
Caching of IP addresses at the ALTO client or the use of the H12
approach [ALTO-H12] in conjunction with caching may lower the query
load on the ALTO server.
When an ALTO server receives an ECS request, it may not have the most
appropriate topology information in order to accurately determine the
ranking. [RFC7285] generally assumes that a server can always offer
some guidance. In such a case, the ALTO server could adopt one of
the following strategies:
o Reply with available information (best effort).
o Query another ALTO server presumed to have better topology
information and return that response (cascaded servers).
o Redirect the request to another ALTO server presumed to have
better topology information (redirection).
The protocol mechanisms and decision processes that would be used to
determine if redirection is necessary and which mode to use is out of
the scope of this document, since protocol extensions could be
3.4. Monitoring ALTO
3.4.1. Impact and Observation on Network Operation
ALTO presents a new opportunity for managing network traffic by
providing additional information to clients. In particular, the
deployment of an ALTO server may shift network traffic patterns, and
the potential impact to network operation can be large. An ISP
providing ALTO may want to assess the benefits of ALTO as part of the
management and operations (cf. [RFC7285]). For instance, the ISP
might be interested in understanding whether the provided ALTO maps
are effective in order to decide whether an adjustment of the ALTO
configuration would be useful. Such insight can be obtained from a
monitoring infrastructure. An ISP offering ALTO could consider the
impact on (or integration with) traffic engineering and the
deployment of a monitoring service to observe the effects of ALTO
operations. The measurement of impacts can be challenging because
ALTO-enabled applications may not provide related information back to
the ALTO service provider.
To construct an effective monitoring infrastructure, the ALTO service
provider should decide how to monitor the performance of ALTO and
identify and deploy data sources to collect data to compute the
performance metrics. In certain trusted deployment environments, it
may be possible to collect information directly from ALTO clients.
It may also be possible to vary or selectively disable ALTO guidance
for a portion of ALTO clients either by time, geographical region, or
some other criteria to compare the network traffic characteristics
with and without ALTO. Monitoring an ALTO service could also be
realized by third parties. In this case, insight into ALTO data may
require a trust relationship between the monitoring system operator
and the network service provider offering an ALTO service.
The required monitoring depends on the network infrastructure and the
use of ALTO, and an exhaustive description is outside the scope of
3.4.2. Measurement of the Impact
ALTO realizes an interface between the network and applications.
This implies that an effective monitoring infrastructure may have to
deal with both network and application performance metrics. This
document does not comprehensively list all performance metrics that
could be relevant, nor does it formally specify metrics.
The impact of ALTO can be classified regarding a number of different
o Total amount and distribution of traffic: ALTO enables ISPs to
influence and localize traffic of applications that use the ALTO
service. Therefore, an ISP may be interested in analyzing the
impact on the traffic, i.e., whether network traffic patterns are
shifted. For instance, if ALTO shall be used to reduce the inter-
domain P2P traffic, it makes sense to evaluate the total amount of
inter-domain traffic of an ISP. Then, one possibility is to study
how the introduction of ALTO reduces the total inter-domain
traffic (inbound and/our outbound). If the ISP's intention is to
localize the traffic inside his network, the network-internal
traffic distribution will be of interest. Effectiveness of
localization can be quantified in different ways, e.g., by the
load on core routers and backbone links or by considering more-
advanced effects, such as the average number of hops that traffic
traverses inside a domain.
o Application performance: The objective of ALTO is to improve
application performance. ALTO can be used by very different types
of applications, with different communication characteristics and
requirements. For instance, if ALTO guidance achieves traffic
localization, one would expect that applications achieve a higher
throughput and/or smaller delays to retrieve data. If
application-specific performance characteristics (e.g., video or
audio quality) can be monitored, such metrics related to user
experience could also help to analyze the benefit of an ALTO
deployment. If available, selected statistics from the TCP/IP
stack in hosts could be leveraged, too.
Of potential interest can also be the share of applications or
customers that actually use an offered ALTO service, i.e., the
adoption of the service.
Monitoring statistics can be aggregated, averaged, and normalized in
different ways. This document does not mandate specific ways how to
3.4.3. System and Service Performance
A number of interesting parameters can be measured at the ALTO
server. [RFC7285] suggests certain ALTO-specific metrics to be
o Requests and responses for each service listed in an Information
Directory (total counts and size in bytes).
o CPU and memory utilization
o ALTO map updates
o Number of PIDs
o ALTO map sizes (in-memory size, encoded size, number of entries)
This data characterizes the workload, the system performance as well
as the map data. Obviously, such data will depend on the
implementation and the actual deployment of the ALTO service.
Logging is also recommended in [RFC7285].
3.4.4. Monitoring Infrastructures
Understanding the impact of ALTO may require interaction between
different systems operating at different layers. Some information
discussed in the preceding sections is only visible to an ISP, while
application-level performance can hardly be measured inside the
network. It is possible that not all information of potential
interest can directly be measured, either because no corresponding
monitoring infrastructure or measurement method exists or because it
is not easily accessible.
One way to quantify the benefit of deploying ALTO is to measure
before and after enabling the ALTO service. In addition to passive
monitoring, some data could also be obtained by active measurements,
but due to the resulting overhead, the latter should be used with
care. Yet, in all monitoring activities, an ALTO service provider
has to take into account that ALTO clients are not bound to ALTO
server guidance as ALTO is only one source of information, and any
measurement result may thus be biased.
Potential sources for monitoring the use of ALTO include:
o Network monitoring and performance management systems: Many ISPs
deploy systems to monitor the network traffic, which may have
insight into traffic volumes, network topology, bandwidth
information inside the management area. Data can be obtained by
SNMP, NETCONF, IP Flow Information Export (IPFIX), syslog, etc.
On-demand OAM tests (such as Ping or BDF) could also be used.
o Applications/clients: Relevant data could be obtained by
instrumentation of applications.
o ALTO server: If available, log files or other statistics data
could be analyzed.
o Other application entities: In several use cases, there are other
application entities that could provide data as well. For
instance, there may be centralized log servers that collect data.
In many ALTO use cases, some data sources are located within an ISP
network while some other data is gathered at the application level.
Correlation of data could require a collaboration agreement between
the ISP and an application owner, including agreements of data
interchange formats, methods of delivery, etc. In practice, such a
collaboration may not be possible in all use cases of ALTO, because
the monitoring data can be sensitive and because the interacting
entities may have different priorities. Details of how to build an
overarching monitoring system for evaluating the benefits of ALTO are
outside the scope of this memo.