| Rfc | 7971 |
|---|
| Title | Application-Layer Traffic Optimization (ALTO) Deployment
Considerations |
|---|
| Author | M. Stiemerling, S. Kiesel, M. Scharf, H. Seidel, S.
Previdi |
|---|
| Date | October 2016 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Internet Engineering Task Force (IETF) M. Stiemerling
Request for Comments: 7971 Hochschule Darmstadt
Category: Informational S. Kiesel
ISSN: 2070-1721 University of Stuttgart
M. Scharf
Nokia
H. Seidel
BENOCS
S. Previdi
Cisco
October 2016
Application-Layer Traffic Optimization (ALTO) Deployment Considerations
Abstract
Many Internet applications are used to access resources such as
pieces of information or server processes that are available in
several equivalent replicas on different hosts. This includes, but
is not limited to, peer-to-peer file sharing applications. The goal
of Application-Layer Traffic Optimization (ALTO) is to provide
guidance to applications that have to select one or several hosts
from a set of candidates capable of providing a desired resource.
This memo discusses deployment-related issues of ALTO. It addresses
different use cases of ALTO such as peer-to-peer file sharing and
Content Delivery Networks (CDNs) and presents corresponding examples.
The document also includes recommendations for network administrators
and application designers planning to deploy ALTO, such as
recommendations on how to generate ALTO map information.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7971.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4
2. General Considerations ..........................................4
2.1. ALTO Entities ..............................................4
2.1.1. Baseline Scenario ...................................4
2.1.2. Placement of ALTO Entities ..........................6
2.2. Classification of Deployment Scenarios .....................8
2.2.1. Roles in ALTO Deployments ...........................8
2.2.2. Information Exposure ...............................11
2.2.3. More-Advanced Deployments ..........................12
3. Deployment Considerations by ISPs ..............................15
3.1. Objectives for the Guidance to Applications ...............15
3.1.1. General Objectives for Traffic Optimization ........15
3.1.2. Inter-Network Traffic Localization .................16
3.1.3. Intra-Network Traffic Localization .................17
3.1.4. Network Offloading .................................18
3.1.5. Application Tuning .................................19
3.2. Provisioning of ALTO Topology Data ........................20
3.2.1. High-Level Process and Requirements ................20
3.2.2. Data Collection from Data Sources ..................21
3.2.3. Partitioning and Grouping of IP Address Ranges .....24
3.2.4. Rating Criteria and/or Cost Calculation ............25
3.3. ALTO Focus and Scope ......................................29
3.3.1. Limitations of Using ALTO beyond Design
Assumptions ........................................29
3.3.2. Limitations of Map-Based Services and
Potential Solutions ................................30
3.3.3. Limitations of Non-Map-Based Services and
Potential Solutions ................................32
3.4. Monitoring ALTO ...........................................33
3.4.1. Impact and Observation on Network Operation ........33
3.4.2. Measurement of the Impact ..........................33
3.4.3. System and Service Performance .....................34
3.4.4. Monitoring Infrastructures .........................35
3.5. Abstract Map Examples for Different Types of ISPs .........36
3.5.1. Small ISP with Single Internet Uplink ..............36
3.5.2. ISP with Several Fixed-Access Networks .............39
3.5.3. ISP with Fixed and Mobile Network ..................40
3.6. Comprehensive Example for Map Calculation .................42
3.6.1. Example Network ....................................42
3.6.2. Potential Input Data Processing and Storage ........44
3.6.3. Calculation of Network Map from the Input Data .....47
3.6.4. Calculation of Cost Map ............................49
3.7. Deployment Experiences ....................................50
4. Using ALTO for P2P Traffic Optimization ........................52
4.1. Overview ..................................................52
4.1.1. Usage Scenario .....................................52
4.1.2. Applicability of ALTO ..............................53
4.2. Deployment Recommendations ................................55
4.2.1. ALTO Services ......................................55
4.2.2. Guidance Considerations ............................56
5. Using ALTO for CDNs ............................................58
5.1. Overview ..................................................58
5.1.1. Usage Scenario .....................................58
5.1.2. Applicability of ALTO ..............................60
5.2. Deployment Recommendations ................................62
5.2.1. ALTO Services ......................................62
5.2.2. Guidance Considerations ............................63
6. Other Use Cases ................................................64
6.1. Application Guidance in Virtual Private Networks (VPNs) ...64
6.2. In-Network Caching ........................................66
6.3. Other Application-Based Network Operations ................68
7. Security Considerations ........................................68
7.1. ALTO as a Protocol Crossing Trust Boundaries ..............68
7.2. Information Leakage from the ALTO Server ..................69
7.3. ALTO Server Access ........................................70
7.4. Faking ALTO Guidance ......................................71
8. References .....................................................72
8.1. Normative References ......................................72
8.2. Informative References ....................................73
Acknowledgments ...................................................76
Authors' Addresses ................................................77
1. Introduction
Many Internet applications are used to access resources such as
pieces of information or server processes that are available in
several equivalent replicas on different hosts. This includes, but
is not limited to, peer-to-peer (P2P) file sharing applications and
Content Delivery Networks (CDNs). The goal of Application-Layer
Traffic Optimization (ALTO) is to provide guidance to applications
that have to select one or several hosts from a set of candidates
capable of providing a desired resource. The basic ideas and problem
space of ALTO is described in [RFC5693] and the set of requirements
is discussed in [RFC6708]. The ALTO protocol is specified in
[RFC7285]. An ALTO server discovery procedure is defined in
[RFC7286].
This document discusses use cases and operational issues that can be
expected when ALTO gets deployed. This includes, but is not limited
to, location of the ALTO server, imposed load to the ALTO server, and
who initiates the queries. This document provides guidance on which
ALTO services to use, and it summarizes known challenges as well as
deployment experiences, including potential processes to generate
ALTO network and cost maps. It thereby complements the management
considerations in the protocol specification [RFC7285], which are
independent of any specific use of ALTO.
2. General Considerations
2.1. ALTO Entities
2.1.1. Baseline Scenario
The ALTO protocol [RFC7285] is a client/server protocol, operating
between a number of ALTO clients and an ALTO server, as sketched in
Figure 1. Below, the baseline deployment scenario for ALTO entities
is first reviewed independently of the actual use case. Specific
examples are then discussed in the remainder of this document.
+----------+
| ALTO |
| Server |
+----------+
^
_.-----|------.
,-'' | `--.
,' | `.
( Network | )
`. | ,'
`--. | _.-'
`------|-----''
v
+----------+ +----------+ +----------+
| ALTO | | ALTO |...| ALTO |
| Client | | Client | | Client |
+----------+ +----------+ +----------+
Figure 1: Baseline Deployment Scenario of the ALTO Protocol
This document uses the terminology introduced in [RFC5693]. In
particular, the following terms are defined by [RFC5693]:
o ALTO Service: Several resource providers may be able to provide
the same resource. The ALTO service gives guidance to a resource
consumer and/or resource directory about which resource
provider(s) to select in order to optimize the client's
performance or quality of experience, while improving resource
consumption in the underlying network infrastructure.
o ALTO Server: A logical entity that provides interfaces to satisfy
the queries about a particular ALTO service.
o ALTO Client: The logical entity that sends ALTO queries.
Depending on the architecture of the application, one may embed it
in the resource consumer and/or in the resource directory.
o Resource Consumer: For P2P applications, a resource consumer is a
specific peer that needs to access resources. For client-server
or hybrid applications, a consumer is a client that needs to
access resources.
o Resource Directory: An entity that is logically separate from the
resource consumer and that assists the resource consumer to
identify a set of resource providers. Some P2P applications refer
to the resource directory as a P2P tracker.
We differentiate between an ALTO Client and a Resource Consumer as
follows: the resource consumer is specific instance of a software
("process") running on a specific host. It is a client instance of a
client/server application or a peer of a peer-to-peer application.
It is the given (constant) endpoint of the data transmissions to be
optimized using ALTO. The optimization is done by wisely choosing
the other ends of these data flows (i.e., the server(s) in a client/
server application or the peers in a peer-to-peer application), using
guidance from ALTO and possibly other information. An ALTO client is
a piece of software (e.g., a software library) that implements the
client entity of the ALTO protocol as specified in [RFC7285]. It
consists of data structures that are suitable for representing ALTO
queries, replies, network and cost maps, etc. Furthermore, it has to
implement an HTTP client and a JSON encoder/decoder, or it has to
include other software libraries that provide these building blocks.
In the simplest case, this ALTO client library can be linked (or
otherwise incorporated) into the resource consumer, in order to
retrieve information from an ALTO server and thereby satisfy the
resource consumer's need for guidance. However, other configurations
are possible as well, as discussed in Section 2.1.2 and other
sections of this document.
According to these definitions, both an ALTO server and an ALTO
client are logical entities. A particular ALTO service may be
offered by more than one ALTO server. In ALTO deployments, the
functionality of an ALTO server can therefore be realized by several
server instances, e.g., by using load balancing between different
physical servers. The term ALTO server should not be confused with
use of a single physical server.
This document uses the term "Resource Directory" as defined in
[RFC5693]. This term and its meaning is not to be confused with the
"Information Resource Directory (IRD)" defined as a part of the ALTO
protocol [RFC7285], i.e., a list of available information resources
offered by a specific ALTO service and the URIs at which each can be
accessed.
2.1.2. Placement of ALTO Entities
The ALTO server and ALTO clients may be situated at various places in
a network topology. An important differentiation is whether the ALTO
client is located on the host that is the endpoint of the data
transmissions to be optimized with ALTO (see Figure 2) or whether the
ALTO client is located on a resource directory, which assists peers
or clients in finding other peers or servers, respectively, but does
not directly take part in the data transmission (see Figure 3).
+--------------+
| App |
+-----------+ |
===>|ALTO Client| |****
=== +-----------+--+ *
=== * *
=== * *
+-------+ +-------+<=== +--------------+ *
| | | | | App | *
| |.....| |<======== +-----------+ | *
| | | | ========>|ALTO Client| | *
+-------+ +-------+<=== +-----------+--+ *
Source of ALTO == * *
topological Server == * *
information == +--------------+ *
== | App | *
== +-----------+ |****
==>|ALTO Client| |
+-----------+--+
Application
Legend:
=== ALTO protocol
*** Application protocol
... Provisioning protocol
Figure 2: Overview of Protocol Interaction between ALTO Elements
without a Resource Directory
Figure 2 shows the operational model for an ALTO client running at
endpoints. An example would be a peer-to-peer file sharing
application that does not use a tracker, such as edonkey. In
addition, ALTO clients at peers could also be used in a similar way
even if there is a tracker, as further discussed in Section 4.1.2.
+-----+
**| App |****
** +-----+ *
** * *
** * *
+-------+ +-------+ +--------------+ * *
| | | | | | +-----+ *
| |.....| | +-----------+ |*****| App | *
| | | |<===>|ALTO Client| | +-----+ *
+-------+ +-------+ +-----------+--+ * *
Source of ALTO Resource ** * *
topological Server directory ** * *
information ** +-----+ *
**| App |****
+-----+
Application
Legend:
=== ALTO protocol
*** Application protocol
... Provisioning protocol
Figure 3: Overview of Protocol Interaction between
ALTO Elements with a Resource Directory
In Figure 3, a use case with a resource directory is illustrated,
e.g., a tracker in a peer-to-peer file-sharing application such as
BitTorrent. Both deployment scenarios may differ in the number of
ALTO clients that access an ALTO service. If an ALTO client is
implemented in a resource directory, an ALTO server may be accessed
by a limited and less dynamic set of clients, whereas in the general
case any host could be an ALTO client. This use case is further
detailed in Section 4.
Using ALTO in CDNs may be similar to a resource directory [CDN-USE].
The ALTO server can also be queried by CDN entities to get guidance
about where a particular client accessing data in the CDN is located
in the Internet Service Provider's network, as discussed in
Section 5.
2.2. Classification of Deployment Scenarios
2.2.1. Roles in ALTO Deployments
ALTO is a general-purpose protocol and it is intended to be used by a
wide range of applications. In different use cases, applications,
resource directories, etc., can correspond to different
functionality. The use cases listed in this document are not meant
to be comprehensive. This also implies that there are different
possibilities where the ALTO entities are actually located, i.e., if
the ALTO clients and the ALTO server are in the same Internet Service
Provider (ISP) domain, or if the clients and the ALTO server are
managed/owned/located in different domains.
An ALTO deployment involves four kinds of entities:
1. Source of topological information
2. ALTO server
3. ALTO client
4. Resource consumer
Each of these entities corresponds to a certain role, which results
in requirements and constraints on the interaction between the
entities.
A key design objective of the ALTO service is that each of these four
roles can be separated, i.e., they can be realized by different
organizations or disjoint system components. ALTO is inherently
designed for use in multi-domain environments. Most importantly,
ALTO is designed to enable deployments in which the ALTO server and
the ALTO client are not located within the same administrative
domain.
As explained in [RFC5693], from this follows that at least three
different kinds of entities can operate an ALTO server:
1. Network operators. Network Service Providers (NSPs) such as ISPs
may have detailed knowledge of their network topology and
policies. In this case, the source of the topology information
and the provider of the ALTO server may be part of the same
organization.
2. Third parties. Topology information could also be collected by
companies or organizations that are distinct from the network
operators, yet have arranged certain legal agreements with one or
more network operators, regarding access to their topology
information and/or doing measurements in their networks.
Examples of such entities could be CDN operators or companies
specialized in offering ALTO services on behalf of ISPs.
3. User communities. User communities could run distributed
measurements for estimating the topology of the Internet. In
this case, the topology information may not originate from ISP
data.
Regarding the interaction between ALTO server and client, ALTO
deployments can be differentiated according to the following aspects:
1. Applicable trust model: The deployment of ALTO can differ
depending on whether or not the ALTO client and ALTO server are
operated within the same organization and/or network. This
affects a number of constraints because the trust model is very
different. For instance, as discussed later in this memo, the
level of detail of maps can depend on who the involved parties
actually are.
2. Composition of the user group: The main use case of ALTO is to
provide guidance to any Internet application. However, an
operator of an ALTO server could also decide to offer guidance
only to a set of well-known ALTO clients, e.g., after
authentication and authorization. In the peer-to-peer
application use case, this could imply that only selected
trackers are allowed to access the ALTO server. The security
implications of using ALTO in closed groups differ from the
public Internet.
3. Covered destinations: In general, an ALTO server has to be able
to provide guidance for all potential destinations. Yet, in
practice, a given ALTO client may only be interested in a subset
of destinations, e.g., only in the network cost between a limited
set of resource providers. For instance, CDN optimization may
not need the full ALTO cost maps because traffic between
individual residential users is not in scope. This may imply
that an ALTO server only has to provide the costs that matter for
a given user, e.g., by customized maps.
The following sections enumerate different classes of use cases for
ALTO, and they discuss deployment implications of each of them. In
principle, an ALTO server can be operated by any organization, and
there is no requirement that an ALTO server be deployed and operated
by an ISP. Yet, since the ALTO solution is designed for ISPs, most
examples in this document assume that the operator of an ALTO server
is a network operator (e.g., an ISP or the network department in a
large enterprise) that offers ALTO guidance in particular to users of
this network.
It must be emphasized that any application using ALTO must also work
if no ALTO servers can be found or if no responses to ALTO queries
are received, e.g., due to connectivity problems or overload
situations (see also [RFC6708]).
2.2.2. Information Exposure
There are basically two different approaches to how an ALTO server
can provide network information and guidance:
1. The ALTO server provides maps that contain provider-defined cost
values between network location groupings (e.g., sets of IP
prefixes). These maps can be retrieved by clients via the ALTO
protocol, and the actual processing of the map data is done
inside the client. Since the maps contain (aggregated) cost
information for all endpoints, the client does not have to reveal
any internal operational data, such as the IP addresses of
candidate resource providers. The ALTO protocol supports this
mode of operation by the Network and Cost Map Service.
2. The ALTO server provides a query interface that returns costs or
rankings for explicitly specified endpoints. This means that the
query of the ALTO client has to include additional information
(e.g., a list of IP addresses). The server then calculates and
returns costs or rankings for the endpoints specified in the
request (e.g., a sorted list of the IP addresses). In ALTO, this
approach can be realized by the Endpoint Cost Service (ECS) and
other related services.
Both approaches have different privacy implications for the server
and client:
For the client, approach 1 has the advantage that all operational
information stays within the client and is not revealed to the
provider of the server. However, this service implies that a network
operator providing an ALTO server has to expose a certain amount of
information about its network structure (e.g., IP prefixes or
topology information in general).
For the operator of a server, approach 2 has the advantage that the
query responses reveal less topology information to ALTO clients.
However, it should be noted that collaborating ALTO clients could
gather more information than expected by aggregating and correlating
responses to multiple queries sent to the ALTO server (see
Section 5.2.1, item (3) of [RFC6708]). Furthermore, this method
requires that clients send internal operational information to the
server, such as the IP addresses of hosts also running the
application. For clients, such data can be sensitive.
As a result, both approaches have their pros and cons, as further
detailed in Section 3.3.
2.2.3. More-Advanced Deployments
From an ALTO client's perspective, there are different ways to use
ALTO:
1. Single-service instance with single-metric guidance: An ALTO
client only obtains guidance regarding a single metric (e.g.,
"routingcost") from a single ALTO service, e.g., an ALTO server
that is offered by the network service provider of the
corresponding access network. Corresponding ALTO server
instances can be discovered, e.g., by ALTO server discovery
[RFC7286] [XDOM-DISC]. Since the ALTO protocol is an HTTP-based,
REST-ful (Representational State Transfer) protocol, the operator
of an ALTO may use well-known techniques for serving large web
sites, such as load balancers, in order to serve a large number
of ALTO queries. The ALTO protocol also supports the use of
different URIs for different ALTO features and thereby the
distribution of the service onto several servers.
2. Single service instance with multiple metric guidance: An ALTO
client could also query an ALTO service for different kinds of
information, e.g., cost maps with different metrics. The ALTO
protocol is extensible and permits such operation. However, ALTO
does not define how a client shall deal with different forms of
guidance, and it is up to the client to interpret the received
information accordingly.
3. Multiple service instances: An ALTO client can also decide to
access multiple ALTO servers providing guidance, possibly from
different operators or organizations. Each of these services may
only offer partial guidance, e.g., for a certain network
partition. In that case, it may be difficult for an ALTO client
to compare the guidance from different services. Different
organization may use different methods to determine maps, and
they may also have different (possibly even contradicting or
competing) guidance objectives. How to discover multiple ALTO
servers and how to deal with conflicting guidance is an open
issue.
There are also different options regarding the synchronization of
guidance offered by an ALTO service:
1. Authoritative servers: An ALTO server instance can provide
guidance for all destinations for all kinds of ALTO clients.
2. Cascaded servers: An ALTO server may itself include an ALTO
client and query other ALTO servers, e.g., for certain
destinations. This results is a cascaded deployment of ALTO
servers, as further explained below.
3. Inter-server synchronization: Different ALTO servers may
communicate by other means. This approach is not further
discussed in this document.
An assumption of the ALTO design is that ISPs operate ALTO servers
independently, irrespective of other ISPs. This may be true for most
envisioned deployments of ALTO, but there may be certain deployments
that may have different settings. Figure 4 shows such a setting with
a university network that is connected to two upstream providers.
NREN is a National Research and Education Network, which provides
cheap high-speed connectivity to specific destinations, e.g., other
universities. ISP is a commercial upstream provider from which the
university buys connectivity to all destinations that cannot be
reached via the NREN. The university, as well as ISP, are operating
their own ALTO server. The ALTO clients, located on the peers in the
university network will contact the ALTO server located at the
university.
+-----------+
| ISP |
| ALTO |<==========================++
| Server | ||
+-----------+ ||
,-------. ,------. ||
,-' `-. ,-' `-. ||
/ Commercial \ / \ ||
( Upstream ) ( NREN ) ||
\ ISP / \ / ||
`-. ,-' `-. ,-' ||
`---+---' `+------' ||
| | ||
| | ||
|,-------------. | \/
,-+ `-+ +-----------+
,' University `. |University |
( Network ) | ALTO |
`. / | Server |
`-. +--' +-----------+
`+------------'| /\ /\
| | || ||
+--------+-+ +-+--------+ || ||
| Peer1 | | PeerN |<====++ ||
+----------+ +----------+ ||
/\ ||
|| ||
++======================================++
Legend:
=== ALTO protocol
Figure 4: Example of a Cascaded ALTO Server
In this setting, all destinations that can be reached via the NREN
are preferred in the rating of the university's ALTO server. In
contrast, all traffic that is not routed via the NREN will be handled
by the commercial upstream ISP and is in general less preferred due
to the associated costs. Yet, there may be significant differences
between various destinations reached via the ISP. Therefore, the
ALTO server at the university may also include the guidance given by
the ISP ALTO server in its replies to the ALTO clients. This is an
example for cascaded ALTO servers.
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
mobile hosts.
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
caches.
In the following, these objectives are explained in more detail with
examples.
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|
`-. ,-' +-----------+
`-------'
Legend:
### 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|
^ `-. ,-' +-----------+
| `-------'
Bottleneck
Legend:
### 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
bottleneck.
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|
`-. ,-' +-----------+
`-------'
Legend:
### preferred "connections"
--- non-preferred "connections"
Figure 7: ALTO Traffic Network De-localization
Figure 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
Figure 5.
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-
engineer.
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 clients.
+--------+ +--------+
| 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
document.
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
and policies.
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
endpoints.
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
resource provider.
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
below:
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
routing.
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
service include:
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
discovered automatically.
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
network.
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
addresses.
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
updates [UPDATE-SSE].
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
section.
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
additional delay.
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
required.
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
this document.
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
criteria:
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
calculate metrics.
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
monitored:
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.
3.5. Abstract Map Examples for Different Types of ISPs
3.5.1. Small ISP with Single Internet Uplink
The ALTO protocol does not mandate how to determine costs between
endpoints and/or determine map data. In complex usage scenarios,
this can be a non-trivial problem. In order to show the basic
principle, this and the following sections explain for different
deployment scenarios how ALTO maps could be structured.
For a small ISP, the inter-domain traffic optimizing problem is how
to decrease the traffic exchanged with other ISPs, because of high
settlement costs. By using the ALTO service to optimize traffic, a
small ISP can define two "optimization areas": one is its own network
and the other one consists of all other network destinations. The
cost map can be defined as follows: the cost of a link between
clients of the inner ISP's network is lower than between clients of
the outer ISP's network and clients of inner ISP's network. As a
result, a host with an ALTO client inside the network of this ISP
will prefer retrieving data from hosts connected to the same ISP.
An example is given in Figure 9. It is assumed that ISP A is a small
ISP only having one access network. As operator of the ALTO service,
ISP A can define its network to be one optimization area, named as
PID1, and define other networks to be the other optimization area,
named as PID2. C1 is denoted as the cost inside the network of ISP
A. C2 is denoted as the cost from PID2 to PID1, and C3 from PID1 to
PID2. In the following, C2=C3 is assumed for the sake of simplicity.
In order to keep traffic local inside ISP A, it makes sense to define
C1<C2.
-----------
//// \\\\
// \\
// \\ /-----------\
| +---------+ | //// \\\\
| | ALTO | ISP A | C2 | Other Networks |
| | Service | PID 1 <----------- PID 2
| +---------+ C1 |----------->| |
| | C3 (=C2) \\\\ ////
\\ // \-----------/
\\ //
\\\\ ////
-----------
Figure 9: Example ALTO Deployment for a Small ISP
A simplified extract of the corresponding ALTO network and cost maps
is listed in Figures 10 and 11, assuming that the network of ISP A
has the IPv4 address ranges 192.0.2.0/24 and 198.51.100.0/25, as well
as the IPv6 address range 2001:db8:100::/48. In this example, the
cost values C1 and C2 can be set to any number C1<C2.
HTTP/1.1 200 OK
...
Content-Type: application/alto-networkmap+json
{
...
"network-map" : {
"PID1" : {
"ipv4" : [
"192.0.2.0/24",
"198.51.100.0/25"
],
"ipv6" : [
"2001:db8:100::/48"
]
},
"PID2" : {
"ipv4" : [
"0.0.0.0/0"
],
"ipv6" : [
"::/0"
]
}
}
}
Figure 10: Example ALTO Network Map
HTTP/1.1 200 OK
...
Content-Type: application/alto-costmap+json
{
...
"cost-type" : {"cost-mode" : "numerical",
"cost-metric": "routingcost"
}
},
"cost-map" : {
"PID1": { "PID1": C1, "PID2": C2 },
"PID2": { "PID1": C2, "PID2": 0 },
}
}
Figure 11: Example ALTO Cost Map
3.5.2. ISP with Several Fixed-Access Networks
This example discusses a P2P application traffic optimization use
case for a larger ISP with a fixed network comprising several access
networks and a core network. The traffic optimizing objectives
include (1) using the backbone network efficiently, (2) adjusting the
traffic balance in different access networks according to traffic
conditions and management policies, and (3) achieving a reduction of
settlement costs with other ISPs.
Such a large ISP deploying an ALTO service may want to optimize its
traffic according to the network topology of its access networks.
For example, each access network could be defined to be one
optimization area, i.e., traffic should be kept local withing that
area if possible. This can be achieved by mapping each area to a
PID. Then, the costs between those access networks can be defined
according to a corresponding traffic optimizing requirement by this
ISP. One example setup is further described below and also shown in
Figure 12.
In this example, ISP A has one backbone network and three access
networks, named as AN A, AN B, and AN C. A P2P application is used
in this example. For a reasonable application-level traffic
optimization, the first requirement could be a decrease of the P2P
traffic on the backbone network inside the AS of ISP A and the second
requirement could be a decrease of the P2P traffic to other ISPs,
i.e., other ASes. The second requirement can be assumed to have
priority over the first one. Also, we assume that the settlement
rate with ISP B is lower than with other ISPs. ISP A can deploy an
ALTO service to meet these traffic distribution requirements. In the
following, we will give an example of an ALTO setting and
configuration according to these requirements.
In the network of ISP A, the operator of the ALTO server can define
each access network to be one optimization area, and assign one PID
to each access network, such as PID 1, PID 2, and PID 3. Because of
different peerings with different outer ISPs, one can define ISP B to
be one additional optimization area and assign PID 4 to it. All
other networks can be added to a PID to be one further optimization
area (PID 5).
In the setup, costs (C1, C2, C3, C4, C5, C6, C7, C8) can be assigned
as shown in Figure 12. Cost C1 is denoted as the link cost in inner
AN A (PID 1), and C2 and C3 are defined accordingly. C4 is denoted
as the link cost from PID 1 to PID 2, and C5 is the corresponding
cost from PID 3, which is assumed to have a similar value. C6 is the
cost between PID 1 and PID 3. For simplicity, this scenario assumes
symmetrical costs between the AN this example. C7 is denoted as the
link cost from the ISP B to ISP A. C8 is the link cost from other
networks to ISP A.
According to previous discussion of the first requirement and the
second requirement, the relationship of these costs will be defined
as: (C1, C2, C3) < (C4, C5, C6) < (C7) < (C8)
+------------------------------------+ +----------------+
| ISP A +---------------+ | | |
| | Backbone | | C7 | ISP B |
| +---+ Network +----+ |<--------+ PID 4 |
| | +-------+-------+ | | | |
| | | | | | |
| | | | | +----------------+
| +---+--+ +--+---+ +--+---+ |
| |AN A | C4 |AN B | C5 |AN C | |
| |PID 1 +<--->|PID 2 |<--->+PID 3 | |
| |C1 | |C2 | |C3 | | +----------------+
| +---+--+ +------+ +--+---+ | | |
| ^ ^ | C8 | Other Networks |
| | | |<--------+ PID 5 |
| +------------------------+ | | |
| C6 | | |
+------------------------------------+ +----------------+
Figure 12: ALTO Deployment in Large ISPs with Layered
Fixed-Network Structures
3.5.3. ISP with Fixed and Mobile Network
An ISP with both mobile network and fixed network may focus on
optimizing the mobile traffic by keeping traffic in the fixed network
as much as possible, because wireless bandwidth is a scarce resource
and traffic is costly in mobile network. In such a case, the main
requirement of traffic optimization could be decreasing the usage of
radio resources in the mobile network. An ALTO service can be
deployed to meet these needs.
Figure 13 shows an example: ISP A operates one mobile network, which
is connected to a backbone network. The ISP also runs two fixed-
access networks AN A and AN B, which are also connected to the
backbone network. In this network structure, the mobile network can
be defined as one optimization area, and PID 1 can be assigned to it.
Access networks AN A and B can also be defined as optimization areas,
and PID 2 and PID 3 can be assigned, respectively. The cost values
are then defined as shown in Figure 13.
To decrease the usage of wireless link, the relationship of these
costs can be defined as follows:
From view of mobile network: C4 < C1 and C4 = C8. This means that
clients in mobile network requiring data resources from other clients
will prefer clients in AN A or B to clients in the mobile network.
This policy can decrease the usage of wireless link and power
consumption in terminals.
From view of AN A: C2 < C6, C5 = maximum cost. This means that
clients in other optimization area will avoid retrieving data from
the mobile network.
From view of AN B: Analog to the view of AN A, C3 < C8 and C9 =
maximum cost.
+-----------------------------------------------------------------+
| |
| ISP A +-------------+ |
| +--------+ ALTO +---------+ |
| | | Service | | |
| | +------+------+ | |
| | | | |
| | | | |
| | | | |
| +-------+-------+ | C6 +--------+------+ |
| | AN A |<--------------| AN B | |
| | PID 2 | C7 | | PID 3 | |
| | C2 |-------------->| C3 | |
| +---------------+ | +---------------+ |
| ^ | | | ^ |
| | | | | | |
| | | C4 | C8 | | |
| C5 | | | | | C9 |
| | | +--------+---------+ | | |
| | +-->| Mobile Network |<---+ | |
| | | PID 1 | | |
| +------- | C1 |----------+ |
| +------------------+ |
+-----------------------------------------------------------------+
Figure 13: ALTO Deployment in ISPs with Mobile Network
These examples show that for ALTO in particular the relationships
between different costs matter; the operator of the server has
several degrees of freedom how to set the absolute values.
3.6. Comprehensive Example for Map Calculation
In addition to the previous, abstract examples, this section presents
a more detailed scenario with a realistic IGP and BGP routing
protocol configuration. This example was first described in
[MAP-CALC].
3.6.1. Example Network
Figure 14 depicts a network that is used to explain the steps carried
out in the course of this example. The network consists of nine
routers (R1 to R9). Two of them are border routers (R1 + R8)
connected to neighbored networks (AS 2 to AS 4). Furthermore, AS 4
is not directly connected to the local network, but has AS 3 as
transit network. The links between the routers are point-to-point
connections. These connections also form the core network with the
2001:db8:1:0::/56 prefix. This prefix is large enough to provide
addresses for all router interconnections. In addition to the core
network, the local network also has five client networks attached to
five different routers (R2, R5, R6, R7 and R9). Each client network
has a /56 prefix with 2001:db8:1:x00:: (x = [1..5]) as network
address.
+-------------------+ +-----+ +-----+ +-------------------+
|2001:db8:1:200::/56+----+ R6 | | R7 +----+2001:db8:1:300::/56|
+-------------------+ +--+--+ +--+--+ +-------------------+
| |
+---------------+ | |
| AS 2 | | |
|2001:db8:2::/48| | 10 | 10
+------------+--+ | |
| | |
| | |
+--+--+ 15 +--+--+ +--+--+ +-------------------+
| R1 +--------+ R3 +----+ R5 |----+2001:db8:1:400::/56|
+--+--+ +--+--+ 5 +--+--+ +-------------------+
| \ / | |
| \ / 15 | |
| \ / | | +---------------+
| \/ | | | AS 4 |
| 20 /\ | 5 | 10 |2001:db8:4::/48|
| / \ | | +-------+-------+
| / \ 20 | | |
| / \ | | |
+--+--+ +--+--+ +--+--+ +-------+-------+
| R2 | | R4 | | R8 +--------+ AS 3 |
+--+--+ +--+--+ +--+--+ |2001:db8:3::/48|
| | | +---------------+
| | | 10
| | 20 |
+------------+------+ | +--+--+ +-------------------+
|2001:db8:1:100::/56| +-------+ R9 +----+2001:db8:1:500::/56|
+-------------------+ +-----+ +-------------------+
Figure 14: Example Network
The example network utilizes two different routing protocols, one for
IGP and another for EGP routing. The used IGP is a link-state
protocol such as IS-IS. The applied link weights are annotated in
the graph and additionally shown in Figure 15. All links are
bidirectional and their weights are symmetric. To obtain the
topology and routing information from the network, the topology data
source must be connected directly to one of the routers (R1...R9).
Furthermore, the topology data source must be enabled to communicate
with the router and vice versa.
The BGP is used in this scenario to route between autonomous systems.
External BGP is running on the two border routers R1 and R8.
Furthermore, internal BGP is used to propagate external as well as
internal prefixes within the network boundaries; it is running on
every router with an attached client network (R2, R5, R6, R7 and R9).
Since no route reflector is present it is necessary to fetch routes
from each BGP router separately.
R1 R2 R3 R4 R5 R6 R7 R8 R9
R1 0 15 15 20 - - - - -
R2 15 0 20 - - - - - -
R3 15 20 0 5 5 10 - - -
R4 20 - 5 0 5 - - - 20
R5 - - 5 5 0 - 10 10 -
R6 - - 10 - - 0 - - -
R7 - - - - 10 - 0 - -
R8 - - - - 10 - - 0 10
R9 - - - 20 - - - 10 0
Figure 15: Example Network Link Weights
For monitoring purposes, it is possible to enable, e.g., SNMP or
NETCONF on the routers within the network. This way an ALTO server
may obtain several additional information about the state of the
network. For example, utilization, latency, and bandwidth
information could be retrieved periodically from the network
components to get and keep an up-to-date view on the network
situation.
In the following, it is assumed that the listed attributes are
collected from the network:
o IS-IS: topology, link weights
o BGP: prefixes, AS numbers, AS distances, or other BGP metrics
o SNMP: latency, utilization, bandwidth
3.6.2. Potential Input Data Processing and Storage
Due to the variety of data sources available in a network, it may be
necessary to aggregate the information and define a suitable data
model that can hold the information efficiently and easily
accessible. One potential model is an annotated directed graph that
represents the topology. The attributes can be annotated at the
corresponding positions in the graph. The following shows how such a
topology graph could describe the example topology.
In the topology graph, a node represents a router in the network,
while the edges stand for the links that connect the routers. Both
routers and links have a set of attributes that store information
gathered from the network.
Each router could be associated with a basic set of information, such
as:
o ID: Unique ID within the network to identify the router.
o Neighbor IDs: List of directly connected routers.
o Endpoints: List of connected endpoints. The endpoints may also
have further attributes themselves depending on the network and
address type. Such potential attributes are costs for reaching
the endpoint from the router, AS numbers, or AS distances.
In addition to the basic set, many more attributes may be assigned to
router nodes. This mainly depends on the utilized data sources.
Examples for such additional attributes are geographic location, host
name and/or interface types, just to name a few.
The example network shown in Figure 14 represents such an internal
network graph where the routers R1 to R9 represent the nodes and the
connections between them are the links. For instance, R2 has one
directly attached IPv6 endpoint that belongs to its own AS, as shown
in Figure 16.
ID: 2
Neighbor IDs: 1,3 (R1, R3)
Endpoints:
Endpoint: 2001:db8:1:100::/56
Weight: 10 (e.g., the default IGP metric value)
ASNumber: 1 (our own AS)
ASDistance: 0
Host Name: R2
Figure 16: Example Router R2
Router R8 has two attached IPv6 endpoints, as explained in Figure 17.
The first one belongs to a directly neighbored AS with AS number 3.
The AS distance from our network to AS3 is 1. The second endpoint
belongs to an AS (AS4) that is no direct neighbor but directly
connected to AS3. To reach endpoints in AS4, it is necessary to
cross AS3, which increases the AS distance by one.
ID: 8
Neighbor IDs: 5,9 (R5, R9)
Endpoints:
Endpoint: 2001:db8:3::/48
Weight: 100
ASNumber: 3
ASDistance: 1
Endpoint: 2001:db8:4::/48
Weight: 200
ASNumber: 4
ASDistance: 2
Host Name: R8
Figure 17: Example Router R8
A potential set of attributes for a link is described in the
following list:
o Source ID: ID of the source router of the link.
o Destination ID: ID of the destination router of the link.
o Weight: The cost to cross the link, e.g., defined by the used IGP.
Additional attributes that provide technical details and state
information can be assigned to links as well. The availability of
such additional attributes depends on the utilized data sources.
Such attributes can be characteristics like maximum bandwidth,
utilization, or latency on the link as well as the link type.
In the example, the link attributes are equal for all links and only
their values differ. It is assumed that the attributes utilization,
bandwidth, and latency are collected, e.g., via SNMP or NETCONF. In
the topology of Figure 14, the links between R1 and R2 would then
have the following link attributes explained in Figure 18:
R1->R2:
Source ID: 1
Destination ID: 2
Weight: 15
Bandwidth: 10 Gbit/s
Utilization: 0.1
Latency: 2 ms
R2->R1:
Source ID: 2
Destination ID: 1
Weight: 15
Bandwidth: 10 Gbit/s
Utilization: 0.55
Latency: 5 ms
Figure 18: Link Attributes
It has to be emphasized that values for utilization and latency can
be very volatile.
3.6.3. Calculation of Network Map from the Input Data
The goal of the ALTO map calculation process is to get from the graph
representation of the network to a coarser-grained and abstract
matrix representation. The first step is to generate the network
map. Only after the network map has been generated is it possible to
compute the cost map since it relies on the network map.
To generate an ALTO network map, a grouping function is required. A
grouping function processes information from the network graph to
group endpoints into PIDs. The way of grouping is manifold and
algorithms can utilize any information provided by the network graph
to perform the grouping. The functions may omit certain endpoints in
order to simplify the map or in order to hide details about the
network that are not intended to be published in the resulting ALTO
network map.
For IP endpoints, which are either an IP (version 4 or version 6)
address or prefix, [RFC7285] requires the use of a longest-prefix
matching algorithm to map IPs to PIDs. This requirement results in
the constraints that every IP must be mapped to a PID and the same
prefix or address not be mapped to more than one PID. To meet the
first constraint, every calculated map must provide a default PID
that contains the prefixes 0.0.0.0/0 for IPv4 and ::/0 for IPv6.
Both prefixes cover their entire address space, and if no other PID
matches an IP endpoint, the default PID will. The second constraint
must be met by the grouping function that assigns endpoints to PIDs.
In case of collision, the grouping function must decide to which PID
an endpoint is assigned. These or other constraints may apply to
other endpoint types depending on the used matching algorithm.
A simple example for such grouping is to compose PIDs by host names.
For instance, each router's host name is selected as the name for a
PID and the attached endpoints are the member endpoints of the
corresponding PID. Additionally, backbone prefixes should not appear
in the map so they are filtered out. The following table in
Figure 19 shows the resulting ALTO network map, using the network in
Figure 14 as example:
PID | Endpoints
---------+-----------------------------------
R1 | 2001:db8:2::/48
R2 | 2001:db8:1:100::/56
R5 | 2001:db8:1:400::/56
R6 | 2001:db8:1:200::/56
R7 | 2001:db8:1:300::/56
R8 | 2001:db8:3::/48, 2001:db8:4::/48
R9 | 2001:db8:1:500::/56
default | 0.0.0.0/0, ::/0
Figure 19: Example ALTO Network Map
Since router R3 and R4 have no endpoints assigned, they are not
represented in the network map. Furthermore, as previously
mentioned, the "default" PID was added to represent all endpoints
that are not part of the example network.
3.6.4. Calculation of Cost Map
After successfully creating the network map, the typical next step is
to calculate the costs between the generated PIDs, which form the
cost map. Those costs are calculated by cost functions. A cost
function may calculate unidirectional values, which means it is
necessary to compute the costs from every PID to every PID. In
general, it is possible to use all available information in the
network graph to compute the costs. In case a PID contains more than
one IP address or prefix, the cost function may first calculate a set
of cost values for each source/destination IP pair. In that case, a
tiebreaker function is required to decide the resulting cost value,
as [RFC7285] allows one cost value only between two PIDs. Such a
tiebreaker can be a simple function such as minimum, maximum, or
average value.
No matter what metric the cost function uses, the path from source to
destination is usually defined by the path with minimum weight. When
the link weight is represented by an additive metric, the path weight
is the sum of link weights of all traversed links. The path may be
determined, for instance, with the Bellman-Ford or Dijkstra
algorithms. The latter progressively builds the shortest path in
terms of cumulated link lengths. In our example, the link lengths
are link weights with values illustrated in Figure 15. Hence, the
cost function generally extracts the optimal path with respect to a
chosen metric, such as the IGP link weight. It is also possible that
more than one path with the same minimum weight exists, which means
it is not entirely clear which path is going to be selected by the
network. Hence, a tiebreaker similar to the one used to resolve
costs for PIDs with multiple endpoints is necessary.
An important note is that [RFC7285] does not require cost maps to
provide costs for every PID pair, so if no path cost can be
calculated for a certain pair, the corresponding field in the cost
map is left out. Administrators may also not want to provide cost
values for some PID pairs due to various reasons. Such pairs may be
defined before the cost calculation is performed.
Based on the network map example shown in the previous section, it is
possible to calculate the cost maps. Figure 20 provides an example
where the selected metric for the cost map is the minimum number of
hops necessary to get from the endpoints in the source PID to
endpoints in the destination PID. Our chosen tiebreaker selects the
minimum hop count when more than one value is returned by the cost
function.
PID | default | R1 | R2 | R5 | R6 | R7 | R8 | R9 |
--------+---------+-----+-----+-----+-----+-----+-----+-----|
default | x | x | x | x | x | x | x | x |
R1 | x | 0 | 2 | 3 | 3 | 4 | 4 | 3 |
R2 | x | 2 | 0 | 3 | 3 | 4 | 4 | 4 |
R5 | x | 3 | 3 | 0 | 3 | 2 | 2 | 3 |
R6 | x | 3 | 3 | 3 | 0 | 4 | 4 | 4 |
R7 | x | 4 | 4 | 2 | 4 | 0 | 3 | 4 |
R8 | x | 4 | 4 | 2 | 4 | 3 | 0 | 2 |
R9 | x | 3 | 4 | 3 | 4 | 4 | 2 | 0 |
Figure 20: Example ALTO Hop Count Cost Map
It should be mentioned that R1->R9 has several paths with equal path
weights. The paths R1->R3->R5->R8->R9, R1->R3->R4->R9, and
R1->R4->R9 all have a path weight of 40. Due to the minimum hop
count value tiebreaker, 3 hops is chosen as value for the path
R1->R4->R9. Furthermore, since the "default" PID is, in a sense, a
virtual PID with no endpoints that are part of the example network,
no cost values are calculated for other PIDs from or towards it.
3.7. Deployment Experiences
There are multiple interoperable implementations of the ALTO
protocol. Some experiences in implementing and using ALTO for large-
scale networks have been documented in [MAP-CALC] and are here
summarized:
o Data collection: Retrieving topology information typically
requires implementing several protocols other than ALTO for data
collection. For such other protocols, ALTO deployments faced
protocol behaviors that were different from what would be expected
from the specification of the corresponding protocol. This
includes behavior caused by older versions of the protocol
specification, a lax interpretation on the remote side or simply
incompatibility with the corresponding standard. This sort of
problems in collecting data can make an ALTO deployment more
complicated, even if it is unrelated to ALTO protocol itself.
o Data processing: Processing network information can be very
complex and quite resource demanding. Gathering information from
an autonomous system connected to the Internet may imply that a
server must store and process hundreds of thousands of prefixes,
several hundreds of megabytes of IPFIX/Netflow information per
minute, and information from hundreds of routers and attributes of
thousands of links. A lot of disk memory, RAM, and CPU cycles as
well as efficient algorithms are required to process the
information. Operators of an ALTO server have to be aware that
significant compute resources are not only required for the ALTO
server, but also for the corresponding data collection.
o Network map calculation: Large IP-based networks consist of
hundreds of thousands of prefixes that have to be mapped to PIDs
in the process of network map calculation. As a result, network
maps get very large (up to tens of megabytes). However, depending
on the design of the network and the chosen grouping function the
calculated network maps contains redundancy that can be removed.
There are at least two ways to reduce the size by removing
redundancy. First, adjacent IP prefixes can be merged. When a
PID has two adjacent prefix entries it can merge them together to
one larger prefix. It is mandatory that both prefixes be in the
same PID. However, the large prefix being assigned to another PID
cannot be ruled out. This must be checked, and it is up to the
grouping function whether or not to merge the prefixes and remove
the larger prefix from the other PID. A simple example, when a
PID comprises the prefixes 2001:db8:0:0::/64 and 2001:db8:0:1::/64
it can easily merge them to 2001:db8:0:0::/63. Second, a prefix
and its next-longest-prefix match may be in the same PID. In this
case, the smaller prefix can simply be removed since it is
redundant for obvious reasons. A simple example, a PID comprises
the prefixes 2001:db8:0:0::/62 and 2001:db8:0:1::/64 and the /62
is the next-longer prefix match of the /64, the /64 prefix can
simply be removed. In contrast, if another PID contains the
2001:db8:0:0::/63 prefix, the entry 2001:db8:0:1::/64 cannot be
removed since the next-longest prefix is not in the same PID
anymore. Operators of an ALTO server thus have to analyze whether
their address assignment schemes allows such tuning.
o Cost map calculation: One known implementation challenge with cost
map calculations is the vast amount of CPU cycles that may be
required to calculate the costs in large networks. This is
particular problematic if costs are calculated between the
endpoints of each source-destination PID pair. Very often several
to many endpoints of a PID are attached to the same node, so the
same path cost is calculated several times. This is clearly
inefficient. A remedy could be more sophisticated algorithms,
such as looking up the routers the endpoints of each PID are
connected to in our network graph and calculated cost map based on
the costs between the routers. When deploying and configuring
ALTO servers, administrators should consider the impact of huge
cost maps and possibly ensure that map sizes do not get too large.
In addition, further deployment experiences have been documented.
One real example is described in greater detail in reference
[CHINA-TRIAL].
Also, experiments have been conducted with ALTO-like deployments in
ISP networks. For instance, NTT performed tests with their HINT
server implementation and dummy nodes to gain insight on how an ALTO-
like service can influence peer-to-peer systems [RFC6875]. The
results of an early experiment conducted in the Comcast network are
documented in [RFC5632].
4. Using ALTO for P2P Traffic Optimization
4.1. Overview
4.1.1. Usage Scenario
Originally, P2P applications were the main driver for the development
of ALTO. In this use case, it is assumed that one party (usually the
operator of a "managed" IP network domain) will disclose information
about the network through ALTO. The application overlay will query
this information and optimize its behavior in order to improve
performance or Quality of Experience in the application while
reducing the utilization of the underlying network infrastructure.
The resulting win-win situation is assumed to be the incentive for
both parties to provide or consume the ALTO information,
respectively.
P2P systems can be built with or without use of a centralized
resource directory ("tracker"). The scope of this section is the
interaction of P2P applications with the ALTO service. In this
scenario, the resource consumer ("peer") asks the resource directory
for a list of candidates that can provide the desired resource.
There are different options for how ALTO can be deployed in such use
cases with a centralized resource directory.
For efficiency reasons (i.e., message size), only a subset of all
resource providers known to the resource directory will be returned
to the resource consumer. Some or all of these resource providers,
plus further resource providers learned by other means such as direct
communication between peers, will be contacted by the resource
consumer for accessing the resource. The purpose of ALTO is giving
guidance on this peer selection, which should yield better-than-
random results. The tracker response as well as the ALTO guidance
are most beneficial in the initial phase after the resource consumer
has decided to access a resource, as long as only few resource
providers are known. Later, when the resource consumer has already
exchanged some data with other peers and measured the transmission
speed, the relative importance of ALTO may dwindle.
4.1.2. Applicability of ALTO
A tracker-based P2P application can leverage ALTO in different ways.
In the following, the different alternatives and their pros and cons
are discussed.
,-------. +-----------+
,---. ,-' ========>| Peer 1 |********
,-' `-. / ISP 1 V \ |ALTO Client| *
/ \ / +-------------+ \ +-----------+ *
/ ISP X \ | + ALTO Server | | +-----------+ *
/ \ \ +-------------+<====>| Peer 2 | *
; +---------+ : \ / |ALTO Client|****** *
| | Global | | `-. ,-' +-----------+ * *
| | Tracker | | `-------' * *
| +---------+ | ,-------. +-----------+ * *
: * ; ,-' ========>| Peer 3 | * *
\ * / / ISP 2 V \ |ALTO Client|**** * *
\ * / / +-------------+ \ +-----------+ * * *
\ * / | | ALTO Server | | +-----------+ * * *
`-. * ,-' \ +-------------+<====>| Peer 4 |** * * *
`-*-' \ / |ALTO Client| * * * *
* `-. ,-' +-----------+ * * * *
* `-------' * * * *
* * * * *
*******************************************************
Legend:
=== ALTO protocol
*** Application protocol
Figure 21: Global Tracker and Local ALTO Servers
Figure 21 depicts a tracker-based P2P system with several peers. The
peers (i.e., resource consumers) embed an ALTO client to improve the
resource provider selection. The tracker (i.e., resource directory)
itself may be hosted and operated by another entity. A tracker
external to the ISPs of the peers may be a typical use case. For
instance, a tracker like Pirate Bay can serve BitTorrent peers
worldwide. The figure only shows one tracker instance, but
deployments with several trackers could be possible, too.
The scenario depicted in Figure 21 lets the peers directly
communicate with their ISP's ALTO server (i.e., ALTO client embedded
in the peers), thus giving the peers the most control on which
information they query for, as they can integrate information
received from one tracker or several trackers and through direct
peer-to-peer knowledge exchange. For instance, the latter approach
is called peer exchange (PEX) in BitTorrent. In this deployment
scenarios, the peers have to discover a suitable ALTO server (e.g.,
offered by their ISP, as described in [RFC7286]).
There are also tracker-less P2P system architectures that do not rely
on centralized resource directories, e.g., unstructured P2P networks.
Regarding the use of ALTO, their deployment would be similar to
Figure 21, since the ALTO client would be embedded in the peers as
well. This option is not further considered in this memo.
,-------.
,---. ,-' `-. +-----------+
,-' `-. / ISP 1 \ | Peer 1 |********
/ \ / +-------------+ \ | | *
/ ISP X \ ++====>| ALTO Server | )+-----------+ *
/ \ || \ +-------------+ / +-----------+ *
; +-----------+ : || \ / | Peer 2 | *
| | Tracker |<====++ `-. ,-' | |****** *
| |ALTO Client| | `-------' +-----------+ * *
| +-----------+<====++ ,-------. * *
: * ; || ,-' `-. +-----------+ * *
\ * / || / ISP 2 \ | Peer 3 | * *
\ * / || / +-------------+ \ | |**** * *
\ * / ++====>| ALTO Server | )+-----------+ * * *
`-. * ,-' \ +-------------+ / +-----------+ * * *
`-*-' \ / | Peer 4 |** * * *
* `-. ,-' | | * * * *
* `-------' +-----------+ * * * *
* * * * *
* * * * *
*********************************************************
Legend:
=== ALTO protocol
*** Application protocol
Figure 22: Global Tracker Accessing ALTO Server at Various ISPs
An alternative deployment scenario for a tracker-based system is
depicted in Figure 22. Here, the tracker embeds the ALTO client.
When the tracker receives a request from a querying peer, it first
discovers the ALTO server responsible for the querying peer. This
discovery can be done by using various ALTO server discovery
mechanisms [RFC7286] [XDOM-DISC]. The ALTO client subsequently sends
to the querying peer only those peers that are preferred by the ALTO
server responsible for the querying peer. The peers do not query the
ALTO servers themselves. This gives the peers a better initial
selection of candidates, but does not consider peers learned through
direct peer-to-peer knowledge exchange.
ISP 1 ,-------. +-----------+
,---. +-------------+******| Peer 1 |
,-' `-. /| Tracker |\ | |
/ \ / +-------------+**** +-----------+
/ ISP X \ | === | * +-----------+
/ \ \ +-------------+ / * | Peer 2 |
; +---------+ : \| ALTO Server |/ ***| |
| | Global | | +-------------+ +-----------+
| | Tracker | | `-------'
| +---------+ | +-----------+
: * ; ,-------. | Peer 3 |
\ * / +-------------+ ****| |
\ * / /| Tracker |*** +-----------+
\ * / / +-------------+ \ +-----------+
`-. * ,-' | === | | Peer 4 |**
`-*-' \ +-------------+ / | | *
* \| ALTO Server |/ +-----------+ *
* +-------------+ *
* ISP 2 `-------' *
*************************************************
Legend:
=== ALTO protocol
*** Application protocol
Figure 23: Local Trackers and Local ALTO Servers (P4P Approach)
There are some attempts to let ISPs deploy their own trackers, as
shown in Figure 23. In this case, the client cannot get guidance
from the ALTO server other than by talking to the ISP's tracker,
which in turn communicates with the ALTO server using the ALTO
protocol. It should be noted that the peers are still allowed to
contact other trackers operated by entities other than the peer's
ISP, but in this case they cannot benefit from ALTO guidance.
4.2. Deployment Recommendations
4.2.1. ALTO Services
The ALTO protocol specification [RFC7285] details how an ALTO client
can query an ALTO server for guiding information and receive the
corresponding replies. In case of peer-to-peer networks, two
different ALTO services can be used: the cost map service is often
preferred as solution by peer-to-peer software implementors and
users, since it avoids disclosing peer IP addresses to a centralized
entity. Alternatively, network operators may have a preference for
the ECS, since it does not require exposure of the network topology.
For actual use of ALTO in P2P applications, both software vendors and
network operators have to agree which ALTO services to use. The ALTO
protocol is flexible and supports both services. Note that for other
use cases of ALTO, in particular in more controlled environments,
both the cost map service and the ECS might be feasible; it is more
of an engineering trade-off whether to use a map-based or query-based
ALTO service.
4.2.2. Guidance Considerations
As explained in Section 4.1.2, for a tracker-based P2P application,
there are two fundamentally different possibilities where to place
the ALTO client:
1. ALTO client in the resource consumer ("peer")
2. ALTO client in the resource directory ("tracker")
Both approaches have advantages and drawbacks that have to be
considered. If the ALTO client is in the resource consumer
(Figure 21), a potentially very large number of clients has to be
deployed. Instead, when using an ALTO client in the resource
directory (Figures 22 and 23), ostensibly peers do not have to
directly query the ALTO server. In this case, an ALTO server could
even not permit access to peers.
However, it seems to be beneficial for all participants to let the
peers directly query the ALTO server. Considering the plethora of
different applications that could use ALTO, e.g., multiple-tracker-
based or non-tracker-based P2P systems or other applications
searching for relays, this renders the ALTO service more useful. The
peers are also the single point having all operational knowledge to
decide whether to use the ALTO guidance and how to use the ALTO
guidance. For a given peer, one can also expect that an ALTO server
of the corresponding ISP provides useful guidance and can be
discovered.
Yet, ALTO clients in the resource consumer also have drawbacks
compared to use in the resource directory. In the following, both
scenarios are compared more in detail in order to explain the impact
on ALTO guidance and the need for third-party ALTO queries.
In the first scenario (see Figure 24), the peer (resource consumer)
queries the tracker (resource directory) for the desired resource
(F1). The resource directory returns a list of potential resource
providers without considering ALTO (F2). It is then the duty of the
resource consumer to invoke ALTO (F3/F4), in order to solicit
guidance regarding this list.
Peer w. ALTO cli. Tracker ALTO Server
--------+-------- --------+-------- --------+--------
| F1 Tracker query | |
|======================>| |
| F2 Tracker reply | |
|<======================| |
| F3 ALTO protocol query |
|---------------------------------------------->|
| F4 ALTO protocol reply |
|<----------------------------------------------|
| | |
==== Application protocol (i.e., tracker-based P2P app protocol)
---- ALTO protocol
Figure 24: Basic Message Sequence Chart for
Resource-Consumer-Initiated ALTO Query
In the second scenario (see Figure 25), the resource directory has an
embedded ALTO client, which we will refer to as Resource Directory
ALTO Client (RDAC) in this document. After receiving a query for a
given resource (F1), the resource directory invokes the RDAC to
evaluate all resource providers it knows (F2/F3). Then, it returns
a, possibly shortened, list containing the "best" resource providers
to the resource consumer (F4).
Peer Tracker w. RDAC ALTO Server
--------+-------- --------+-------- --------+--------
| F1 Tracker query | |
|======================>| |
| | F2 ALTO cli. p. query |
| |---------------------->|
| | F3 ALTO cli. p. reply |
| |<----------------------|
| F4 Tracker reply | |
|<======================| |
| | |
==== Application protocol (i.e., tracker-based P2P app protocol)
---- ALTO protocol
Figure 25: Basic Message Sequence Chart for Third-Party ALTO Query
Note: The message sequences depicted in Figures 24 and 25 may occur
both in the target-aware and the target-independent query mode (cf.
[RFC6708]). In the target-independent query mode, no message
exchange with the ALTO server might be needed after the tracker
query, because the candidate resource providers could be evaluated
using a locally cached "map", which has been retrieved from the ALTO
server some time ago.
The first approach has the following problem: While the resource
directory might know thousands of peers taking part in a swarm, the
list returned to the resource consumer is usually shortened for
efficiency reasons. Therefore, the "best" (in the sense of ALTO)
potential resource providers might not be contained in that list
anymore, even before ALTO can consider them.
Much better traffic optimization could be achieved if the tracker
would evaluate all known peers using ALTO. This list would then
include a significantly higher fraction of "good" peers. If the
tracker returned "good" peers only, there might be a risk that the
swarm might disconnect and split into several disjunct partitions.
However, finding the right mix of ALTO-biased and random peer
selection is out of the scope of this document.
Therefore, from an overall optimization perspective, the second
scenario with the ALTO client embedded in the resource directory is
advantageous, because it is ensured that the addresses of the "best"
resource providers are actually delivered to the resource consumer.
An architectural implication of this insight is that the ALTO server
discovery procedures must support third-party discovery. That is, as
the tracker issues ALTO queries on behalf of the peer that contacted
the tracker, the tracker must be able to discover an ALTO server that
can give guidance suitable for that respective peer (see
[XDOM-DISC]).
In principle, a combined approach could also be possible. For
instance, a tracker could use a coarse-grained "global" ALTO server
to find the peers in the general vicinity of the requesting peer,
while peers could use "local" ALTO servers for a more fine-grained
guidance. Yet, there is no known deployment experience for such a
combined approach.
5. Using ALTO for CDNs
5.1. Overview
5.1.1. Usage Scenario
This section briefly introduces the usage of ALTO for CDNs, as
explained in [CDN-USE]. CDNs are used in the delivery of some
Internet services (e.g., delivery of websites, software updates, and
video delivery) from a location closer to the location of the user.
A CDN typically consists of a network of servers often attached to
ISP networks. The point of attachment is often as close to content
consumers and peering points as economically or operationally
feasible in order to decrease traffic load on the ISP backbone and to
provide better user experience measured by reduced latency and higher
throughput.
CDNs use several techniques to redirect a client to a server
(surrogate). A request-routing function within a CDN is responsible
for receiving content requests from user agents, obtaining and
maintaining necessary information about a set of candidate
surrogates, and selecting and redirecting the user agent to the
appropriate surrogate. One common way is relying on the DNS system,
but there are many other ways, see [RFC3568].
+--------------------+
| CDN Request Router |
| with ALTO Client |
+--------------------+
/\
|| ALTO protocol
||
\/
+---------+
| ALTO |
| Server |
+---------+
:
: Provisioning protocol
:
,-----------.
,-' Source of `-.
( topological )
`-. information ,-'
`-----------'
Figure 26: Use of ALTO Information for CDN Request Routing
In order to derive the optimal benefit from a CDN, it is preferable
to deliver content from the servers (caches) that are "closest" to
the end user requesting the content. The definition of "closest" may
be as simple as geographical or IP topology distance, but it may also
consider other combinations of metrics and CDN or ISP policies. As
illustrated in Figure 26, ALTO could provide this information.
User Agent Request Router Surrogate
| | |
| F1 Initial Request | |
+---------------------------->| |
| +--+ |
| | | F2 Surrogate Selection |
| |<-+ (using ALTO) |
| F3 Redirection Response | |
|<----------------------------+ |
| | |
| F4 Content Request | |
+-------------------------------------------------------->|
| | |
| | F5 Content |
|<--------------------------------------------------------+
| | |
Figure 27: Example of CDN Surrogate Selection
Figure 27 illustrates the interaction between a user agent, a request
router, and a surrogate for the delivery of content in a single CDN.
As explained in [CDN-USE], the user agent makes an initial request to
the CDN (F1). This may be an application-level request (e.g., HTTP)
or a DNS request. In the second step (F2), the request router
selects an appropriate surrogate (or set of surrogates) based on the
user agent's (or its proxy's) IP address, the request router's
knowledge of the network topology (which can be obtained by ALTO) and
reachability cost between CDN caches and end users, and any
additional CDN policies. Then, the request router responds to the
initial request with an appropriate response containing a redirection
to the selected cache (F3), for example, by returning an appropriate
DNS A/AAAA record or an HTTP 302 redirect, etc. The user agent uses
this information to connect directly to the surrogate and request the
desired content (F4), which is then delivered (F5).
5.1.2. Applicability of ALTO
The most simple use case for ALTO in a CDN context is to improve the
selection of a CDN surrogate or origin. In this case, the CDN makes
use of an ALTO server to choose a better CDN surrogate or origin than
would otherwise be the case. Although it is possible to obtain raw
network map and cost information in other ways, for example,
passively listening to the ISP's routing protocols or use of active
probing, the use of an ALTO service to expose that information may
provide additional control to the ISP over how their network map/cost
is exposed. Additionally, it may enable the ISP to maintain a
functional separation between their routing plane and network map
computation functions. This may be attractive for a number of
reasons, for example:
o The ALTO service could provide a filtered view of the network and/
or cost map that relates to CDN locations and their proximity to
end users, for example, to allow the ISP to control the level of
topology detail they are willing to share with the CDN.
o The ALTO service could apply additional policies to the network
map and cost information to provide a CDN-specific view of the
network map/cost, for example, to allow the ISP to encourage the
CDN to use network links that would not ordinarily be preferred by
a Shortest Path First routing calculation.
o The routing plane may be operated and controlled by a different
operational entity (even within a single ISP) than the CDN.
Therefore, the CDN may not be able to passively listen to routing
protocols, nor may it have access to other network topology data
(e.g., inventory databases).
When CDN servers are deployed outside of an ISP's network or in a
small number of central locations within an ISP's network, a
simplified view of the ISP's topology or an approximation of
proximity is typically sufficient to enable the CDN to serve end
users from the optimal server/location. As CDN servers are deployed
deeper within ISP networks, it becomes necessary for the CDN to have
more detailed knowledge of the underlying network topology and costs
between network locations in order to enable the CDN to serve end
users from the optimal servers for the ISP.
The request router in a CDN will typically also take into account
criteria and constraints that are not related to network topology,
such as the current load of CDN surrogates, content owner policies,
end user subscriptions, etc. This document only discusses use of
ALTO for network information.
A general issue for CDNs is that the CDN logic has to match the
client's IP address with the closest CDN surrogate, for approaches
that are both DNS or HTTP redirect based (see, for instance,
[ALTO-CDN]). This matching is not trivial, for example, in DNS-based
approaches, where the IP address of the DNS original requester is
unknown (see [RFC7871] for a discussion of this and a solution
approach).
In addition to use by a single CDN, ALTO can also be used in
scenarios that interconnect several CDNs. This use case is detailed
in [CDNI-FCI].
5.2. Deployment Recommendations
5.2.1. ALTO Services
In its simplest form, an ALTO server would provide an ISP with the
capability to offer a service to a CDN that provides network map and
cost information. The CDN can use that data to enhance its surrogate
and/or origin selection. If an ISP offers an ALTO Network and Cost
Map Service to expose a cost mapping/ranking between end user IP
subnets (within that ISP's network) and CDN surrogate IP subnets/
locations, periodic updates of the maps may be needed. As introduced
in Section 3.3), it is common for broadband subscribers to obtain
their IP addresses dynamically, and in many deployments, the IP
subnets allocated to a particular network region can change
relatively frequently, even if the network topology itself is
reasonably static.
An alternative would be to use the ALTO ECS: when an end user
requests a given content, the CDN request router issues an ECS
request with the endpoint address (IPv4/IPv6) of the end user
(content requester) and the set of endpoint addresses of the
surrogate (content targets). The ALTO server receives the request
and ranks the addresses based on their distance from the content
requester. Once the request router obtained from the ALTO server the
ranked list of locations (for the specific user), it can incorporate
this information into its selection mechanisms in order to point the
user to the most appropriate surrogate.
Since CDNs operate in a controlled environment, the ALTO Network and
Cost Map Service and ECS have a similar level of security and
confidentiality of network-internal information. However, the
Network and Cost Map Service and ECS differ in the way the ALTO
service is delivered and address a different set of requirements in
terms of topology information and network operations.
If a CDN already has means to model connectivity policies, the map-
based approaches could possibly be integrated into that. If the ECS
service is preferred, a request router that uses ECS could cache the
results of ECS queries for later usage in order to address the
scalability limitations of ECS and to reduce the number of
transactions between the CDN and ALTO server. The ALTO server may
indicate in the reply message how long the content of the message is
to be considered reliable and insert a lifetime value that will be
used by the CDN in order to cache (and then flush or refresh) the
entry.
5.2.2. Guidance Considerations
The following discusses how a CDN could make use of ALTO services.
In one deployment scenario, ALTO could expose ISP end-user
reachability to a CDN. The request router needs to have information
about which end-user IP subnets are reachable via which networks or
network locations. The network map services offered by ALTO could be
used to expose this topology information while avoiding routing-plane
peering between the ISP and the CDN. For example, if CDN surrogates
are deployed within the access or aggregation network, the ISP is
likely to want to utilize the surrogates deployed in the same access/
aggregation region in preference to surrogates deployed elsewhere, in
order to alleviate the cost and/or improve the user experience.
In addition, CDN surrogates could also use ALTO guidance, e.g., if
there is more than one upstream source of content or several origins.
In this case, ALTO could help a surrogate with the decision about
which upstream source to use. This specific variant of using ALTO is
not further detailed in this document.
If content can be provided by several CDNs, there may be a need to
interconnect these CDNs. In this case, ALTO can be used as an
interface [CDNI-FCI], in particular, for footprint and capabilities
advertisement.
Other, and more advanced, scenarios of deploying ALTO are also listed
in [CDN-USE] and [ALTO-CDN].
The granularity of ALTO information required depends on the specific
deployment of the CDN. For example, an "over-the-top" CDN whose
surrogates are deployed only within the Internet backbone may only
require knowledge of which end-user IP subnets are reachable via
which ISP's networks, whereas a CDN deployed within a particular
ISP's network requires a finer granularity of knowledge.
An ALTO server ranks addresses based on topology information it
acquires from the network. By default, according to [RFC7285],
distance in ALTO represents an abstract "routingcost" that can be
computed, for instance, from routing protocol information. But an
ALTO server may also take into consideration other criteria or other
information sources for policy, state, and performance information
(e.g., geolocation), as explained in Section 3.2.2.
The different methods and algorithms through which the ALTO server
computes topology information and rankings is out of the scope of
this document. If rankings are based on routing protocol
information, it is obvious that network events may impact the ranking
computation. Due to internal redundancy and resilience mechanisms
inside current networks, most of the network events happening in the
infrastructure will be handled internally in the network, and they
should have limited impact on a CDN. However, catastrophic events
such as main trunks failures or backbone partitioning will have to be
taken into account by the ALTO server to redirect traffic away from
the impacted area.
An ALTO server implementation may want to keep state about ALTO
clients in order to inform and signal to these clients when a major
network event happened, e.g., by a notification mechanism. In a CDN/
ALTO interworking architecture with few CDN components interacting
with the ALTO server, there are less scalability issues in
maintaining state about clients in the ALTO server, compared to ALTO
guidance to any Internet user.
6. Other Use Cases
This section briefly surveys and references other use cases that have
been tested or suggested for ALTO deployments.
6.1. Application Guidance in Virtual Private Networks (VPNs)
Virtual Private Network (VPN) technology is widely used in public and
private networks to create groups of users that are separated from
other users of the network and allows these users to communicate
among themselves as if they are on a private network. Network
Service Providers (NSPs) offer different types of VPNs. [RFC4026]
distinguishes between Layer 2 VPN (L2VPN) and Layer 3 VPN (L3VPN)
using different sub-types. In the following, the term "VPN" is used
to refer to provider supplied virtual private networking.
From the perspective of an application at an endpoint, a VPN may not
be very different from any other IP connectivity solution, but there
are a number of specific applications that could benefit from ALTO
topology exposure and guidance in VPNs. As, in the general Internet,
one advantage is that applications do not have to perform excessive
measurements on their own. For instance, potential use cases for
ALTO application guidance in VPN environments are:
o Enterprise application optimization: Enterprise customers often
run distributed applications that exchange large amounts of data,
e.g., for synchronization of replicated data bases. Network
topology information could be useful for placement of replicas as
well as for the scheduling of transfers.
o Private cloud computing solution: An enterprise customer could run
its own data centers at several sites. The cloud management
system could want to understand the network costs between
different sites for intelligent routing and placement decisions of
Virtual Machines (VMs) among the VPN sites.
o Cloud-bursting: One or more VPN endpoints could be located in a
public cloud. If an enterprise customer needs additional
resources, they could be provided by a public cloud, which is
accessed through the VPN. Network topology awareness would help
to decide in which data center of the public cloud those resources
should be allocated.
These examples focus on enterprises, which are typical users of VPNs.
VPN customers typically have no insight into the network topology
that transports the VPN. Similar to other ALTO use cases, better-
than-random application-level decisions would be enabled by an ALTO
server offered by the NSP, as illustrated in Figure 28.
+---------------+
| Customer's |
| management |
| application |.
| (ALTO client) | .
+---------------+ . VPN provisioning
/\ . (out-of-scope)
|| ALTO .
\/ .
+---------------------+ +----------------+
| ALTO server | | VPN portal/OSS |
| provided by NSP | | (out-of-scope) |
+---------------------+ +----------------+
: VPN network
: and cost maps
:
/---------:---------\ Network service provider
| : |
+-------+ _______________________ +-------+
| App a | ()_____. .________. .____() | App d |
+-------+ | | | | | | +-------+
\---| |--------| |--/
| | | |
|^| |^| Customer VPN
V V
+-------+ +-------+
| App b | | App c |
+-------+ +-------+
Figure 28: Using ALTO in VPNs
A common characteristic of these use cases is that applications will
not necessarily run in the public Internet, and that the relationship
between the provider and customer of the VPN is rather well defined.
Since VPNs often run in a managed environment, an ALTO server may
have access to topology information (e.g., traffic engineering data)
that would not be available for the public Internet, and it may
expose it to the customer of the VPN only.
Also, a VPN will not necessarily be static. The customer could
possibly modify the VPN and add new VPN sites by a Web portal,
network management systems, or other OSS solutions. Prior to adding
a new VPN site, an application will not have connectivity to that
site, i.e., an ALTO server could offer access to information that an
application cannot measure on its own (e.g., expected delay to a new
VPN site).
The VPN use cases, requirements, and solutions are further detailed
in [VPN-SERVICE].
6.2. In-Network Caching
Deployment of intra-domain P2P caches has been proposed for
cooperation between the network operator and the P2P service
providers, e.g., to reduce the bandwidth consumption in access
networks [ALTO-P2PCACHE].
+--------------+ +------+
| ISP 1 network+----------------+Peer 1|
+-----+--------+ +------+
|
+--------+------------------------------------------------------+
| | ISP 2 network |
| +---------+ |
| |L1 Cache | |
| +-----+---+ |
| +--------------------+----------------------+ |
| | | | |
| +------+------+ +------+-------+ +------+-------+ |
| | AN1 | | AN2 | | AN3 | |
| | +---------+ | | +----------+ | | | |
| | |L2 Cache | | | |L2 Cache | | | | |
| | +---------+ | | +----------+ | | | |
| +------+------+ +------+-------+ +------+-------+ |
| | | |
| +--------------------+ | |
| | | | |
| +------+------+ +------+-------+ +------+-------+ |
| | SUB-AN11 | | SUB-AN12 | | SUB-AN31 | |
| | +---------+ | | | | | |
| | |L3 Cache | | | | | | |
| | +---------+ | | | | | |
| +------+------+ +------+-------+ +------+-------+ |
| | | | |
+--------+--------------------+----------------------+----------+
| | |
+---+---+ +---+---+ |
| | | | |
+--+--+ +--+--+ +--+--+ +--+--+ +--+--+
|Peer2| |Peer3| |Peer4| |Peer5| |Peer6|
+-----+ +-----+ +-----+ +-----+ +-----+
Figure 29: General Architecture of Intra-ISP Caches
Figure 29 depicts the overall architecture of potential P2P cache
deployments inside an ISP 2 with various access network types. As
shown in the figure, P2P caches may be deployed at various levels,
including the interworking gateway linking with other ISPs, internal
access network gateways linking with different types of accessing
networks (e.g., WLAN, cellular, and wired), and even within an
accessing network at the entries of individual WLAN subnetworks.
Moreover, depending on the network context and the operator's policy,
each cache can be a Forwarding Cache or a Bidirectional Cache
[ALTO-P2PCACHE].
In such a cache architecture, the locations of caches could be used
as dividers of different PIDs to guide intra-ISP network abstraction
and mark costs among them according to the location and type of
relevant caches.
Further details and deployment considerations can be found in
[ALTO-P2PCACHE].
6.3. Other Application-Based Network Operations
An ALTO server can be part of an overall framework for Application-
Based Network Operations (ABNO) [RFC7491] that brings together
different technologies. Such an architecture may include additional
components such as a PCE for on-demand and application-specific
reservation of network connectivity, reliability, and resources (such
as bandwidth). Some use cases how to leverage ALTO for joint network
and application-layer optimization are explained in [RFC7491].
7. Security Considerations
Security concerns were extensively discussed from the very beginning
of the development of the ALTO protocol, and they have been
considered in detail in the ALTO requirements document [RFC6708] as
well as in the ALTO protocol specification document [RFC7285]. The
two main security concerns are related to the unwanted disclosure of
information through ALTO and the negative impact of specially
crafted, wrong ("faked") guidance presented to an ALTO client. In
addition to this, the usual concerns related to the operation of any
networked application apply.
This section focuses on the peer-to-peer use case, which is -- from a
security perspective -- probably the most difficult ALTO use case
that has been considered. Special attention is given to the two main
security concerns.
7.1. ALTO as a Protocol Crossing Trust Boundaries
The optimization of peer-to-peer applications was the first use case
and the impetus for the development of the ALTO protocol, in
particular, file sharing applications such as BitTorrent [RFC5594].
As explained in Section 4.1.1, for the publisher of the ALTO
information (i.e., the ALTO server operator), it may not be apparent
who is in charge of the P2P application overlay. Some P2P
applications do not have any central control entity and the whole
overlay consists only of the peers, which are under control of the
individual users. Other P2P applications may have some control
entities such as super peers or trackers, but these may be located in
foreign countries and under the control of unknown organizations. As
outlined in Section 4.2.2, in some scenarios, it may be very
beneficial to forward ALTO information to such trackers, super peers,
etc., located in remote networks. This situation is aggravated by
the vast number of different P2P applications that are evolving
quickly and often without any coordination with the network
operators.
In summary, it can be said that in many instances of the P2P use
case, the ALTO protocol bridges the border between the "managed" IP
network infrastructure under strict administrative control and one or
more "unmanaged" application overlays, i.e., overlays for which it is
hard to tell who is in charge of them. This differs from more-
controlled environments (e.g., in the CDN use case), in which
bilateral agreements between the producer and consumer of guidance
are possible.
7.2. Information Leakage from the ALTO Server
An ALTO server will be provisioned with information about the ISP's
network and possibly also with information about neighboring ISPs.
This information (e.g., network topology, business relations, etc.)
is often considered to be confidential to the ISP and can include
very sensitive information. ALTO does not require any particular
level of details of information disclosure; hence, the provider
should evaluate how much information is revealed and the associated
risks.
Furthermore, if the ALTO information is very fine grained, it may
also be considered sensitive with respect to user privacy. For
example, consider a hypothetical endpoint property "provisioned
access link bandwidth" or "access technology (ADSL, VDSL, FTTH,
etc.)" and an ALTO service that publishes this property for
individual IP addresses. This information could not only be used for
traffic optimization but, for example, also for targeted advertising
to residential users with exceptionally good (or bad) connectivity,
such as special banner ads. For an advertisement system, it would be
more complex to obtain such information otherwise, e.g., by bandwidth
probing.
Different scenarios related to the unwanted disclosure of an ALTO
server's information have been itemized and categorized in RFC 6708,
Section 5.2.1., cases (1)-(3) [RFC6708].
In some use cases, it is not possible to use access control (see
Section 7.3) to limit the distribution of ALTO knowledge to a small
set of trusted clients. In these scenarios, it seems tempting not to
use network maps and cost maps at all, and instead completely rely on
ECS and endpoint ranking in the ALTO server. While this practice may
indeed reduce the amount of information that is disclosed to an
individual ALTO client, some issues should be considered: first, when
using the map-based approach, it is trivial to analyze the maximum
amount of information that could be disclosed to a client -- the full
maps. In contrast, when providing endpoint-cost service only, the
ALTO server operator could be prone to a false feeling of security,
while clients use repeated queries and/or collaboration to gather
more information than they are expected to get (see Section 5.2.1.,
case (3) in [RFC6708]). Second, the ECS reveals more information
about the user or application behavior to the ALTO server, e.g.,
which other hosts are considered as peers for the exchange of a
significant amount of data (see Section 5.2.1., cases (4)-(6) in
[RFC6708]).
Consequently, users may be more reluctant to use the ALTO service at
all if it is based on the ECS instead of providing network and cost
maps. Given that some popular P2P applications are sometimes used
for purposes such as distribution of files without the explicit
permission from the copyright owner, it may also be in the interest
of the ALTO server operator that an ALTO server cannot infer the
behavior of the application to be optimized. One possible conclusion
could be to publish network and cost maps through ALTO that are so
coarse grained that they do not violate the network operator's or the
user's interests.
In other use cases, in more-controlled environments (e.g., in the CDN
use case) bilateral agreements, access control (see Section 7.3), and
encryption could be used to reduce the risk of information leakage.
7.3. ALTO Server Access
Depending on the use case of ALTO, it may be desired to apply access
restrictions to an ALTO server, i.e., by requiring client
authentication. According to [RFC7285], ALTO requires that HTTP
Digest Authentication be supported, in order to achieve client
authentication and possibly to limit the number of parties with whom
ALTO information is directly shared. TLS Client Authentication may
also be supported.
In general, well-known security management techniques and best
current practices [RFC4778] for operational ISP infrastructure also
apply to an ALTO service, including functions to protect the system
from unauthorized access, key management, reporting security-relevant
events, and authorizing user access and privileges.
For peer-to-peer applications, a potential deployment scenario is
that an ALTO server is solely accessible by peers from the ISP
network (as shown in Figure 21). For instance, the source IP address
can be used to grant only access from that ISP network to the server.
This will "limit" the number of peers able to attack the server to
the user's of the ISP (however, including compromised computers that
are part of a botnet).
If the ALTO server has to be accessible by parties not located in the
ISP's network (see Figure 22), e.g., by a third-party tracker or by a
CDN system outside the ISP's network, the access restrictions have to
be looser. In the extreme case, i.e., no access restrictions, each
and every host in the Internet can access the ALTO server. This
might not be the intention of the ISP, as the server is not only
subject to more possible attacks, but also the server load could
increase, since possibly more ALTO clients have to be served.
There are also use cases where the access to the ALTO server has to
be much more strictly controlled, i.e., where an authentication and
authorization of the ALTO client to the server may be needed. For
instance, in case of CDN optimization, the provider of an ALTO
service as well as potential users are possibly well-known. Only CDN
entities may need ALTO access; access to the ALTO servers by
residential users may neither be necessary nor be desired.
Access control can also help to prevent Denial-of-Service (DoS)
attacks by arbitrary hosts from the Internet. DoS can both affect an
ALTO server and an ALTO client. A server can get overloaded if too
many requests hit the server, or if the query load of the server
surpasses the maximum computing capacity. An ALTO client can get
overloaded if the responses from the sever are, either intentionally
or due to an implementation mistake, too large to be handled by that
particular client.
7.4. Faking ALTO Guidance
The ALTO services enables an ALTO service provider to influence the
behavior of network applications. An attacker who is able to
generate false replies, or e.g. an attacker who can intercept the
ALTO server discovery procedure, can provide faked ALTO guidance.
Here is a list of examples of how the ALTO guidance could be faked
and what possible consequences may arise:
Sorting: An attacker could change the sorting order of the ALTO
guidance (given that the order is of importance; otherwise, the
ranking mechanism is of interest), i.e., declaring peers located
outside the ISP as peers to be preferred. This will not pose a
big risk to the network or peers, as it would mimic the "regular"
peer operation without traffic localization, apart from the
communication/processing overhead for ALTO. However, it could
mean that ALTO is reaching the opposite goal of shuffling more
data across ISP boundaries, incurring more costs for the ISP. In
another example, fake guidance could give unrealistically low
costs to devices in an ISP's mobile network, thus encouraging
other devices to contact them, thereby degrading the ISP's mobile
network and causing customer dissatisfaction.
Preference of a single peer: A single IP address (thus a peer) could
be marked as to be preferred over all other peers. This peer can
be located within the local ISP or also in other parts of the
Internet (e.g., a web server). This could lead to the case that
quite a number of peers to trying to contact this IP address,
possibly causing a DoS attack.
The ALTO protocol protects the authenticity and integrity of ALTO
information while in transit by leveraging the authenticity and
integrity protection mechanisms in TLS (see Section 8.3.5 of
[RFC7285]). It has not yet been investigated how wrong ALTO guidance
given by an authenticated ALTO server can impact the operation of the
network and the applications.
8. References
8.1. Normative References
[ALTO-REG]
IANA, "Application-Layer Traffic Optimization (ALTO)
Protocol",
<http://www.iana.org/assignments/alto-protocol>.
[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
DOI 10.17487/RFC5693, October 2009,
<http://www.rfc-editor.org/info/rfc5693>.
[RFC6708] Kiesel, S., Ed., Previdi, S., Stiemerling, M., Woundy, R.,
and Y. Yang, "Application-Layer Traffic Optimization
(ALTO) Requirements", RFC 6708, DOI 10.17487/RFC6708,
September 2012, <http://www.rfc-editor.org/info/rfc6708>.
[RFC7285] Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
"Application-Layer Traffic Optimization (ALTO) Protocol",
RFC 7285, DOI 10.17487/RFC7285, September 2014,
<http://www.rfc-editor.org/info/rfc7285>.
[RFC7286] Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
H. Song, "Application-Layer Traffic Optimization (ALTO)
Server Discovery", RFC 7286, DOI 10.17487/RFC7286,
November 2014, <http://www.rfc-editor.org/info/rfc7286>.
8.2. Informative References
[ALTO-CDN]
Penno, R., Medved, J., Alimi, R., Yang, R., and S.
Previdi, "ALTO and Content Delivery Networks", Work in
Progress, draft-penno-alto-cdn-03, March 2011.
[ALTO-H12]
Kiesel, S. and M. Stiemerling, "ALTO H12", Work in
Progress, draft-kiesel-alto-h12-02, March 2010.
[ALTO-P2PCACHE]
Lingli, D., Chen, W., Yi, Q., and Y. Zhang,
"Considerations for ALTO with network-deployed P2P
caches", Work in Progress, draft-deng-alto-p2pcache-03,
February 2014.
[CDN-USE] Niven-Jenkins, B., Watson, G., Bitar, N., Medved, J., and
S. Previdi, "Use Cases for ALTO within CDNs", Work in
Progress, draft-jenkins-alto-cdn-use-cases-03, June 2012.
[CDNI-FCI]
Seedorf, J., Yang, Y., and J. Peterson, "CDNI Footprint
and Capabilities Advertisement using ALTO", Work in
Progress, draft-seedorf-cdni-request-routing-alto-08,
March 2015.
[CHINA-TRIAL]
Li, K. and G. Jian, "ALTO and DECADE service trial within
China Telecom", Work in Progress,
draft-lee-alto-chinatelecom-trial-04, March 2012.
[MAP-CALC]
Seidel, H., "ALTO map calculation from live network data",
Work in Progress, draft-seidel-alto-map-calculation-00,
October 2015.
[NETWORK-TOPO]
Clemm, A., Medved, J., Varga, R., Tkacik, T., Bahadur, N.,
Ananthakrishnan, H., and X. Liu, "A Data Model for Network
Topologies", Work in Progress,
draft-ietf-i2rs-yang-network-topo-06, September 2016.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
DOI 10.17487/RFC3411, December 2002,
<http://www.rfc-editor.org/info/rfc3411>.
[RFC3568] Barbir, A., Cain, B., Nair, R., and O. Spatscheck, "Known
Content Network (CN) Request-Routing Mechanisms",
RFC 3568, DOI 10.17487/RFC3568, July 2003,
<http://www.rfc-editor.org/info/rfc3568>.
[RFC4026] Andersson, L. and T. Madsen, "Provider Provisioned Virtual
Private Network (VPN) Terminology", RFC 4026,
DOI 10.17487/RFC4026, March 2005,
<http://www.rfc-editor.org/info/rfc4026>.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<http://www.rfc-editor.org/info/rfc4655>.
[RFC4778] Kaeo, M., "Operational Security Current Practices in
Internet Service Provider Environments", RFC 4778,
DOI 10.17487/RFC4778, January 2007,
<http://www.rfc-editor.org/info/rfc4778>.
[RFC5594] Peterson, J. and A. Cooper, "Report from the IETF Workshop
on Peer-to-Peer (P2P) Infrastructure, May 28, 2008",
RFC 5594, DOI 10.17487/RFC5594, July 2009,
<http://www.rfc-editor.org/info/rfc5594>.
[RFC5632] Griffiths, C., Livingood, J., Popkin, L., Woundy, R., and
Y. Yang, "Comcast's ISP Experiences in a Proactive Network
Provider Participation for P2P (P4P) Technical Trial",
RFC 5632, DOI 10.17487/RFC5632, September 2009,
<http://www.rfc-editor.org/info/rfc5632>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<http://www.rfc-editor.org/info/rfc6020>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<http://www.rfc-editor.org/info/rfc6241>.
[RFC6875] Kamei, S., Momose, T., Inoue, T., and T. Nishitani, "The
P2P Network Experiment Council's Activities and
Experiments with Application-Layer Traffic Optimization
(ALTO) in Japan", RFC 6875, DOI 10.17487/RFC6875, February
2013, <http://www.rfc-editor.org/info/rfc6875>.
[RFC7491] King, D. and A. Farrel, "A PCE-Based Architecture for
Application-Based Network Operations", RFC 7491,
DOI 10.17487/RFC7491, March 2015,
<http://www.rfc-editor.org/info/rfc7491>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<http://www.rfc-editor.org/info/rfc7752>.
[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<http://www.rfc-editor.org/info/rfc7871>.
[RFC7921] Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
Nadeau, "An Architecture for the Interface to the Routing
System", RFC 7921, DOI 10.17487/RFC7921, June 2016,
<http://www.rfc-editor.org/info/rfc7921>.
[RFC7922] Clarke, J., Salgueiro, G., and C. Pignataro, "Interface to
the Routing System (I2RS) Traceability: Framework and
Information Model", RFC 7922, DOI 10.17487/RFC7922, June
2016, <http://www.rfc-editor.org/info/rfc7922>.
[UPDATE-SSE]
Roome, W. and Y. Yang, "ALTO Incremental Updates Using
Server-Sent Events (SSE)", Work in Progress,
draft-ietf-alto-incr-update-sse-03, September 2016.
[VPN-SERVICE]
Scharf, M., Gurbani, V., Soprovich, G., and V. Hilt, "The
Virtual Private Network (VPN) Service in ALTO: Use Cases,
Requirements and Extensions", Work in Progress,
draft-scharf-alto-vpn-service-02, February 2014.
[XDOM-DISC]
Kiesel, S. and M. Stiemerling, "Application Layer Traffic
Optimization (ALTO) Cross-Domain Server Discovery", Work
in Progress, draft-kiesel-alto-xdom-disc-02, July 2016.
Acknowledgments
This memo is the result of contributions made by several people:
o Xianghue Sun, Lee Kai, and Richard Yang contributed text on ISP
deployment requirements and monitoring.
o Rich Woundy contributed text to Section 3.3.
o Lingli Deng, Wei Chen, Qiuchao Yi, and Yan Zhang contributed
Section 6.2.
Thomas-Rolf Banniza, Vinayak Hegde, Qin Wu, Wendy Roome, and Sabine
Randriamasy provided very useful comments and reviewed the document.
Authors' Addresses
Martin Stiemerling
Hochschule Darmstadt
Email: mls.ietf@gmail.com
URI: http://ietf.stiemerling.org
Sebastian Kiesel
University of Stuttgart Information Center
Networks and Communication Systems Department
Allmandring 30
Stuttgart 70550
Germany
Email: ietf-alto@skiesel.de
Michael Scharf
Nokia
Lorenzstrasse 10
Stuttgart 70435
Germany
Email: michael.scharf@nokia.com
Hans Seidel
BENOCS GmbH
Winterfeldtstrasse 21
Berlin 10781
Germany
Email: hseidel@benocs.com
Stefano Previdi
Cisco Systems, Inc.
Via Del Serafico 200
Rome 00191
Italy
Email: sprevidi@cisco.com