Internet DRAFT - draft-ietf-alto-protocol
draft-ietf-alto-protocol
ALTO WG R. Alimi, Ed.
Internet-Draft Google
Intended status: Standards Track R. Penno, Ed.
Expires: September 6, 2014 Cisco Systems
Y. Yang, Ed.
Yale University
March 5, 2014
ALTO Protocol
draft-ietf-alto-protocol-27.txt
Abstract
Applications using the Internet already have access to some topology
information of Internet Service Provider (ISP) networks. For
example, views to Internet routing tables at looking glass servers
are available and can be practically downloaded to many network
application clients. What is missing is knowledge of the underlying
network topologies from the point of view of ISPs. In other words,
what an ISP prefers in terms of traffic optimization -- and a way to
distribute it.
The Application-Layer Traffic Optimization (ALTO) Service provides
network information (e.g., basic network location structure and
preferences of network paths) with the goal of modifying network
resource consumption patterns while maintaining or improving
application performance. The basic information of ALTO is based on
abstract maps of a network. These maps provide a simplified view,
yet enough information about a network for applications to
effectively utilize them. Additional services are built on top of
the maps.
This document describes a protocol implementing the ALTO Service.
Although the ALTO Service would primarily be provided by ISPs, other
entities such as content service providers could also operate an ALTO
Service. Applications that could use this service are those that
have a choice to which end points to connect. Examples of such
applications are peer-to-peer (P2P) and content delivery networks.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
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This Internet-Draft will expire on September 6, 2014.
Copyright Notice
Copyright (c) 2014 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 7
1.2. Design Overview . . . . . . . . . . . . . . . . . . . . . 8
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Endpoint . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Endpoint Address . . . . . . . . . . . . . . . . . . . . . 8
2.3. Network Location . . . . . . . . . . . . . . . . . . . . . 9
2.4. ALTO Information . . . . . . . . . . . . . . . . . . . . . 9
2.5. ALTO Information Base . . . . . . . . . . . . . . . . . . 9
2.6. ALTO Service . . . . . . . . . . . . . . . . . . . . . . . 9
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. ALTO Service and Protocol Scope . . . . . . . . . . . . . 9
3.2. ALTO Information Reuse and Redistribution . . . . . . . . 12
4. ALTO Information Service Framework . . . . . . . . . . . . . . 12
4.1. ALTO Information Services . . . . . . . . . . . . . . . . 13
4.1.1. Map Service . . . . . . . . . . . . . . . . . . . . . 13
4.1.2. Map Filtering Service . . . . . . . . . . . . . . . . 13
4.1.3. Endpoint Property Service . . . . . . . . . . . . . . 13
4.1.4. Endpoint Cost Service . . . . . . . . . . . . . . . . 14
5. Network Map . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Provider-defined Identifier (PID) . . . . . . . . . . . . 14
5.2. Endpoint Addresses . . . . . . . . . . . . . . . . . . . . 15
5.3. Example Network Map . . . . . . . . . . . . . . . . . . . 15
6. Cost Map . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Cost Types . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1.1. Cost Metric . . . . . . . . . . . . . . . . . . . . . 17
6.1.2. Cost Mode . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Cost Map Structure . . . . . . . . . . . . . . . . . . . . 18
6.3. Network Map and Cost Map Dependency . . . . . . . . . . . 19
6.4. Cost Map Update . . . . . . . . . . . . . . . . . . . . . 19
7. Endpoint Properties . . . . . . . . . . . . . . . . . . . . . 20
7.1. Endpoint Property Type . . . . . . . . . . . . . . . . . . 20
7.1.1. Endpoint Property Type: pid . . . . . . . . . . . . . 20
8. Protocol Specification: General Processing . . . . . . . . . . 20
8.1. Overall Design . . . . . . . . . . . . . . . . . . . . . . 20
8.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.3. Basic Operations . . . . . . . . . . . . . . . . . . . . . 21
8.3.1. Client Discovering Information Resources . . . . . . . 22
8.3.2. Client Requesting Information Resources . . . . . . . 22
8.3.3. Server Responding to IR Request . . . . . . . . . . . 23
8.3.4. Client Handling Server Response . . . . . . . . . . . 23
8.3.5. Authentication and Encryption . . . . . . . . . . . . 24
8.3.6. Information Refreshing . . . . . . . . . . . . . . . . 24
8.3.7. Parsing of Unknown Fields . . . . . . . . . . . . . . 24
8.4. Server Response Encoding . . . . . . . . . . . . . . . . . 25
8.4.1. Meta Information . . . . . . . . . . . . . . . . . . . 25
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8.4.2. Data Information . . . . . . . . . . . . . . . . . . . 25
8.5. Protocol Errors . . . . . . . . . . . . . . . . . . . . . 25
8.5.1. Media Type . . . . . . . . . . . . . . . . . . . . . . 26
8.5.2. Response Format and Error Codes . . . . . . . . . . . 26
8.5.3. Overload Conditions and Server Unavailability . . . . 28
9. Protocol Specification: Information Resource Directory . . . . 29
9.1. Information Resource Attributes . . . . . . . . . . . . . 29
9.1.1. Resource ID . . . . . . . . . . . . . . . . . . . . . 29
9.1.2. Media Type . . . . . . . . . . . . . . . . . . . . . . 29
9.1.3. Capabilities . . . . . . . . . . . . . . . . . . . . . 29
9.1.4. Accepts Input Parameters . . . . . . . . . . . . . . . 29
9.1.5. Dependent Resources . . . . . . . . . . . . . . . . . 30
9.2. Information Resource Directory (IRD) . . . . . . . . . . . 30
9.2.1. Media Type . . . . . . . . . . . . . . . . . . . . . . 30
9.2.2. Encoding . . . . . . . . . . . . . . . . . . . . . . . 30
9.2.3. Example . . . . . . . . . . . . . . . . . . . . . . . 32
9.2.4. Delegation using IRD . . . . . . . . . . . . . . . . . 35
9.2.5. Considerations of Using IRD . . . . . . . . . . . . . 37
10. Protocol Specification: Basic Data Types . . . . . . . . . . . 37
10.1. PID Name . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.2. Resource ID . . . . . . . . . . . . . . . . . . . . . . . 38
10.3. Version Tag . . . . . . . . . . . . . . . . . . . . . . . 38
10.4. Endpoints . . . . . . . . . . . . . . . . . . . . . . . . 39
10.4.1. Typed Endpoint Addresses . . . . . . . . . . . . . . . 39
10.4.2. Address Type . . . . . . . . . . . . . . . . . . . . . 39
10.4.3. Endpoint Address . . . . . . . . . . . . . . . . . . . 39
10.4.4. Endpoint Prefixes . . . . . . . . . . . . . . . . . . 40
10.4.5. Endpoint Address Group . . . . . . . . . . . . . . . . 40
10.5. Cost Mode . . . . . . . . . . . . . . . . . . . . . . . . 41
10.6. Cost Metric . . . . . . . . . . . . . . . . . . . . . . . 41
10.7. Cost Type . . . . . . . . . . . . . . . . . . . . . . . . 42
10.8. Endpoint Property . . . . . . . . . . . . . . . . . . . . 42
10.8.1. Resource Specific Endpoint Properties . . . . . . . . 42
10.8.2. Global Endpoint Properties . . . . . . . . . . . . . . 42
11. Protocol Specification: Service Information Resources . . . . 43
11.1. Meta Information . . . . . . . . . . . . . . . . . . . . . 43
11.2. Map Service . . . . . . . . . . . . . . . . . . . . . . . 43
11.2.1. Network Map . . . . . . . . . . . . . . . . . . . . . 43
11.2.2. Mapping IP Addresses to PIDs for 'ipv4'/'ipv6'
Network Maps . . . . . . . . . . . . . . . . . . . . . 46
11.2.3. Cost Map . . . . . . . . . . . . . . . . . . . . . . . 47
11.3. Map Filtering Service . . . . . . . . . . . . . . . . . . 50
11.3.1. Filtered Network Map . . . . . . . . . . . . . . . . . 50
11.3.2. Filtered Cost Map . . . . . . . . . . . . . . . . . . 52
11.4. Endpoint Property Service . . . . . . . . . . . . . . . . 56
11.4.1. Endpoint Property . . . . . . . . . . . . . . . . . . 57
11.5. Endpoint Cost Service . . . . . . . . . . . . . . . . . . 60
11.5.1. Endpoint Cost . . . . . . . . . . . . . . . . . . . . 60
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12. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 63
12.1. ALTO Client Embedded in P2P Tracker . . . . . . . . . . . 64
12.2. ALTO Client Embedded in P2P Client: Numerical Costs . . . 65
12.3. ALTO Client Embedded in P2P Client: Ranking . . . . . . . 66
13. Discussions . . . . . . . . . . . . . . . . . . . . . . . . . 67
13.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 67
13.2. Hosts with Multiple Endpoint Addresses . . . . . . . . . . 68
13.3. Network Address Translation Considerations . . . . . . . . 68
13.4. Endpoint and Path Properties . . . . . . . . . . . . . . . 69
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 69
14.1. application/alto-* Media Types . . . . . . . . . . . . . . 69
14.2. ALTO Cost Metric Registry . . . . . . . . . . . . . . . . 71
14.3. ALTO Endpoint Property Type Registry . . . . . . . . . . . 72
14.4. ALTO Address Type Registry . . . . . . . . . . . . . . . . 74
14.5. ALTO Error Code Registry . . . . . . . . . . . . . . . . . 75
15. Security Considerations . . . . . . . . . . . . . . . . . . . 75
15.1. Authenticity and Integrity of ALTO Information . . . . . . 76
15.1.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 76
15.1.2. Protection Strategies . . . . . . . . . . . . . . . . 76
15.1.3. Limitations . . . . . . . . . . . . . . . . . . . . . 76
15.2. Potential Undesirable Guidance from Authenticated ALTO
Information . . . . . . . . . . . . . . . . . . . . . . . 77
15.2.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 77
15.2.2. Protection Strategies . . . . . . . . . . . . . . . . 77
15.3. Confidentiality of ALTO Information . . . . . . . . . . . 78
15.3.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 78
15.3.2. Protection Strategies . . . . . . . . . . . . . . . . 78
15.3.3. Limitations . . . . . . . . . . . . . . . . . . . . . 79
15.4. Privacy for ALTO Users . . . . . . . . . . . . . . . . . . 79
15.4.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 79
15.4.2. Protection Strategies . . . . . . . . . . . . . . . . 79
15.5. Availability of ALTO Service . . . . . . . . . . . . . . . 80
15.5.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 80
15.5.2. Protection Strategies . . . . . . . . . . . . . . . . 80
16. Manageability Considerations . . . . . . . . . . . . . . . . . 80
16.1. Operations . . . . . . . . . . . . . . . . . . . . . . . . 81
16.1.1. Installation and Initial Setup . . . . . . . . . . . . 81
16.1.2. Migration Path . . . . . . . . . . . . . . . . . . . . 81
16.1.3. Dependencies on Other Protocols and Functional
Components . . . . . . . . . . . . . . . . . . . . . . 82
16.1.4. Impact and Observation on Network Operation . . . . . 82
16.2. Management . . . . . . . . . . . . . . . . . . . . . . . . 83
16.2.1. Management Interoperability . . . . . . . . . . . . . 83
16.2.2. Management Information . . . . . . . . . . . . . . . . 83
16.2.3. Fault Management . . . . . . . . . . . . . . . . . . . 83
16.2.4. Configuration Management . . . . . . . . . . . . . . . 83
16.2.5. Performance Management . . . . . . . . . . . . . . . . 84
16.2.6. Security Management . . . . . . . . . . . . . . . . . 84
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17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 84
17.1. Normative References . . . . . . . . . . . . . . . . . . . 84
17.2. Informative References . . . . . . . . . . . . . . . . . . 85
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 88
Appendix B. Design History and Merged Proposals . . . . . . . . . 89
Appendix C. Authors . . . . . . . . . . . . . . . . . . . . . . . 89
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 90
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1. Introduction
1.1. Problem Statement
This document defines the ALTO Protocol, which provides a solution
for the problem stated in [RFC5693]. Specifically, in today's
networks, network information such as network topologies, link
availability, routing policies, and path costs are hidden from the
application layer, and many applications benefited from such hiding
of network complexity. However, new applications, such as
application-layer overlays, can benefit from information about the
underlying network infrastructure. In particular, these new network
applications can be adaptive, and hence become more network-efficient
(e.g., reduce network resource consumption) and achieve better
application performance (e.g., accelerated download rate), by
leveraging network-provided information.
At a high level, the ALTO Protocol specified in this document is an
information publishing interface that allows a network to publish its
network information such as network locations, costs between them at
configurable granularities, and endhost properties to network
applications. The information published by the ALTO Protocol should
benefit both the network and the applications (i.e., the consumers of
the information). Either the operator of the network or a third-
party (e.g., an information aggregator) can retrieve or derive
related information of the network and publish it using the ALTO
Protocol. When a network provides information through the ALTO
Protocol, the network is said to provide the ALTO Service.
To allow better understanding of the goal of the ALTO Protocol, this
document provides a short, non-normative overview of the benefits of
ALTO to both networks and applications:
o A network that provides an ALTO Service can achieve better
utilization of its networking infrastructure. For example, by
using ALTO as a tool to interact with applications, a network is
able to provide network information to applications so that the
applications can better manage traffic on more expensive or
difficult-to-provision links such as long distance, transit or
backup links. During the interaction, the network can choose to
protect its sensitive and confidential network state information,
by abstracting real metric values into non-real numerical scores
or ordinal ranking.
o An application that uses an ALTO Service can benefit from better
knowledge of the network to avoid network bottlenecks. For
example, an overlay application can use information provided by
the ALTO Service to avoid selecting peers connected via high-delay
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links (e.g., some intercontinental links). Using ALTO to
initialize each node with promising ("better-than-random") peers,
an adaptive peer-to-peer overlay may achieve faster, better
convergence.
1.2. Design Overview
The ALTO Protocol specified in this document meets the ALTO
requirements specified in [RFC5693], and unifies multiple protocols
previously designed with similar intentions. See Appendix A for a
list of people and Appendix B for a list of proposals that have made
significant contributions to this effort.
The ALTO Protocol uses a REST-ful design [Fielding-Thesis], and
encodes its requests and responses using JSON [RFC4627]. These
designs are chosen because of their flexibility and extensibility.
In addition, these designs make it possible for ALTO to be deployed
at scale by leveraging existing HTTP [RFC2616] implementations,
infrastructures and deployment experience.
2. Terminology
This document uses the following terms defined in [RFC5693]:
Application, Overlay Network, Peer, Resource, Resource Identifier,
Resource Provider, Resource Consumer, Resource Directory, Transport
Address, Host Location Attribute, ALTO Service, ALTO Server, ALTO
Client, ALTO Query, ALTO Reply, ALTO Transaction, Local Traffic,
Peering Traffic, Transit Traffic.
This document also uses the following additional terms: Endpoint
Address, Network Location, ALTO Information, ALTO Information Base,
and ALTO Service.
2.1. Endpoint
An Endpoint is an application or host that is capable of
communicating (sending and/or receiving messages) on a network.
An Endpoint is typically either a Resource Provider or a Resource
Consumer.
2.2. Endpoint Address
An Endpoint Address represents the communication address of an
endpoint. Common forms of Endpoint Addresses include IP address, MAC
address and overlay ID. An Endpoint Address can be network-
attachment based (e.g., IP address) or network-attachment agnostic
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(e.g., MAC address).
Each Endpoint Address has an associated Address Type, which indicates
both its syntax and semantics.
2.3. Network Location
Network Location is a generic term denoting a single Endpoint or a
group of Endpoints. For instance, it can be a single IPv4 or IPv6
address, an IPv4 or IPv6 prefix, or a set of prefixes.
2.4. ALTO Information
ALTO Information is a generic term referring to the network
information sent by an ALTO Server.
2.5. ALTO Information Base
This document uses the term ALTO Information Base to refer to the
internal representation of ALTO Information maintained by an ALTO
Server. Note that the structure of this internal representation is
not defined by this document.
2.6. ALTO Service
A network that provides ALTO Information through the ALTO Protocol is
said to provide the ALTO Service.
3. Architecture
This section defines the ALTO architecture and the ALTO Protocol's
place in the overall architecture.
3.1. ALTO Service and Protocol Scope
Each network region in the global Internet can provide its ALTO
Service, which conveys network information from the perspective of
that network region. A network region in this context can be an
Autonomous System (AS), an ISP, a region smaller than an AS or ISP,
or a set of ISPs. The specific network region that an ALTO Service
represents will depend on the ALTO deployment scenario and ALTO
service discovery mechanism.
The ALTO Service specified in this document defines network Endpoints
(and aggregations thereof) and generic costs amongst them from the
region's perspective. The network Endpoints may include all
Endpoints in the global Internet. We say that the network
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information provided by the ALTO Service of a network region
represents the "my-Internet view" of the network region.
The "my-Internet view" defined in this document does not specify the
internal topology of a network, and hence, it is said to provide a
"single-node" abstract topology. Extensions to this document may
provide topology details in "my-Internet view".
Figure 1 provides an overall picture of ALTO's system architecture,
so that one can better understand the ALTO Service and the role of
the ALTO Protocol. In this architecture, an ALTO Server prepares
ALTO Information; an ALTO Client uses ALTO Service Discovery to
identify an appropriate ALTO Server; and the ALTO Client requests
available ALTO Information from the ALTO Server using the ALTO
Protocol.
The ALTO Information provided by the ALTO Server can be updated
dynamically based on network conditions, or can be seen as a policy
which is updated at a larger time-scale.
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+-------------------------------------------------------------------+
| Network Region |
| |
| +-----------+ |
| | Routing | |
| +--------------+ | Protocols | |
| | Provisioning | +-----------+ |
| | Policy | | |
| +--------------+\ | |
| \ | |
| \ | |
| +-----------+ \+---------+ +--------+ |
| |Dynamic | | ALTO | ALTO Protocol | ALTO | |
| |Network |.......| Server | ==================== | Client | |
| |Information| +---------+ +--------+ |
| +-----------+ / / |
| / ALTO SD Query/Response / |
| / / |
| +----------+ +----------------+ |
| | External | | ALTO Service | |
| | Interface| | Discovery (SD) | |
| +----------+ +----------------+ |
| | |
+-------------------------------------------------------------------+
|
+------------------+
| Third Parties |
| |
| Content Providers|
+------------------+
Figure 1: Basic ALTO Architecture.
Figure 1 illustrates that the ALTO Information provided by an ALTO
Server may be influenced (at the service provider's discretion) by
other systems. In particular, the ALTO Server can aggregate
information from multiple systems to provide an abstract and unified
view that can be more useful to applications. Examples of other
systems include (but are not limited to) static network configuration
databases, dynamic network information, routing protocols,
provisioning policies, and interfaces to outside parties. These
components are shown in the figure for completeness but are outside
the scope of this specification. Recall that while the ALTO Protocol
may convey dynamic network information, it is not intended to replace
near-real-time congestion control protocols.
It may also be possible for an ALTO Server to exchange network
information with other ALTO Servers (either within the same
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administrative domain or another administrative domain with the
consent of both parties) in order to adjust exported ALTO
Information. Such a protocol is also outside the scope of this
specification.
3.2. ALTO Information Reuse and Redistribution
ALTO Information may be useful to a large number of applications and
users. At the same time, distributing ALTO Information must be
efficient and not become a bottleneck.
The design of the ALTO Protocol allows integration with the existing
HTTP caching infrastructure to redistribute ALTO Information. If
caching or redistribution is used, the response message to an ALTO
Client may be returned from a third-party.
Application-dependent mechanisms, such as P2P DHTs or P2P file-
sharing, may be used to cache and redistribute ALTO Information.
This document does not define particular mechanisms for such
redistribution.
Additional protocol mechanisms (e.g., expiration times and digital
signatures for returned ALTO information) are left for future
investigation.
4. ALTO Information Service Framework
The ALTO Protocol conveys network information through services, where
each service defines a set of related functionalities. An ALTO
Client can request each service individually. All of the services
defined in ALTO are said to form the ALTO service framework and are
provided through a common transport protocol, messaging structure and
encoding, and transaction model. Functionalities offered in
different services can overlap.
The goals of the services defined in this document are to convey (1)
Network Locations, which denote the locations of Endpoints at a
network, (2) provider-defined costs for paths between pairs of
Network Locations, and (3) network related properties of endhosts.
The aforementioned goals are achieved by defining the Map Service,
which provides the core ALTO information to clients, and three
additional services: the Map Filtering Service, Endpoint Property
Service, and Endpoint Cost Service. Additional services can be
defined in companion documents. Figure 2 gives an overview of the
services. Details of the services are presented in subsequent
sections.
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.-----------------------------------------.
| ALTO Information Services |
| .-----------. .----------. .----------. |
| | Map | | Endpoint | | Endpoint | |
| | Filtering | | Property | | Cost | |
| | Service | | Service | | Service | |
| `-----------' `----------' `----------' |
| .-------------------------------------. |
| | Map Service | |
| | .-------------. .--------------. | |
| | | Network Map | | Cost Map | | |
| | `-------------' `--------------' | |
| `-------------------------------------' |
`-----------------------------------------'
Figure 2: ALTO Service Framework.
4.1. ALTO Information Services
4.1.1. Map Service
The Map Service provides batch information to ALTO Clients in the
form of Network Map and Cost Map. A Network Map (See Section 5)
provides a full set of Network Location groupings defined by the ALTO
Server and the Endpoints contained within each grouping. A Cost Map
(see Section 6) provides costs between a defined grouping.
These two maps can be thought of (and implemented as) as simple files
with appropriate encoding provided by the ALTO Server.
4.1.2. Map Filtering Service
Resource constrained ALTO Clients may benefit from filtering of query
results at the ALTO Server. This avoids that an ALTO Client first
spends network bandwidth and CPU cycles to collect results and then
performs client-side filtering. The Map Filtering Service allows
ALTO Clients to query an ALTO Server on Network Map and Cost Map
based on additional parameters.
4.1.3. Endpoint Property Service
This service allows ALTO Clients to look up properties for individual
Endpoints. An example property of an Endpoint is its Network
Location (i.e., its grouping defined by the ALTO Server). Another
example property is its connectivity type such as ADSL (Asymmetric
Digital Subscriber Line), Cable, or FTTH (Fiber To The Home).
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4.1.4. Endpoint Cost Service
Some ALTO Clients may also benefit from querying for costs and
rankings based on Endpoints. The Endpoint Cost Service allows an
ALTO Server to return either numerical costs or ordinal costs
(rankings) directly amongst Endpoints.
5. Network Map
An ALTO Network Map defines a grouping of network endpoints. This
document uses Network Map to refer to the syntax and semantics of how
an ALTO Server distributes the grouping. This document does not
discuss the internal representation of this data structure within the
ALTO Server.
The definition of Network Map is based on the observation that in
reality, many endpoints are close by to one another in terms of
network connectivity. By treating a group of close-by endpoints
together as a single entity, an ALTO Server indicates aggregation of
these endpoints due to their proximity. This aggregation can also
lead to greater scalability without losing critical information when
conveying other network information (e.g., when defining Cost Map).
5.1. Provider-defined Identifier (PID)
One issue is that proximity varies depending on the granularity of
the ALTO information configured by the provider. In one deployment,
endpoints on the same subnet may be considered close; while in
another deployment, endpoints connected to the same Point of Presence
(PoP) may be considered close.
ALTO introduces provider-defined Network Location identifiers called
Provider-defined Identifiers (PIDs) to provide an indirect and
network-agnostic way to specify an aggregation of network endpoints
that may be treated similarly, based on network topology, type, or
other properties. Specifically, a PID is a string of type PIDName
(see Section 10.1) and its associated set of Endpoint Addresses. As
discussed above, there can be many different ways of grouping the
endpoints and assigning 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. Interpreting the PIDs
defined in a Network Map using the "single-node" abstraction, one can
consider that each PID represents an abstract port (PoP) that
connects a set of endpoints.
A key use case of PIDs is to specify network preferences (costs)
between PIDs instead of individual endpoints. This allows cost
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information to be more compactly represented and updated at a faster
time scale than the network aggregations themselves. For example, an
ISP may prefer that endpoints associated with the same PoP (Point-of-
Presence) in a P2P application communicate locally instead of
communicating with endpoints in other PoPs. The ISP may aggregate
endhosts within a PoP into a single PID in the Network Map. The cost
may be encoded to indicate that Network Locations within the same PID
are preferred; for example, cost(PID_i, PID_i) == c and cost(PID_i,
PID_j) > c for i != j. Section 6 provides further details on using
PIDs to represent costs in an ALTO Cost Map.
5.2. Endpoint Addresses
The endpoints aggregated into a PID are denoted by endpoint
addresses. There are many types of addresses, such as IP addresses,
MAC addresses, or overlay IDs. This document specifies in Section
10.4 how to specify IPv4/IPv6 addresses or prefixes. Extension
documents may define further address types; Section 14.4 of this
document provides an IANA registry for endpoint address types.
5.3. Example Network Map
This document uses the Network Map shown in Figure 3 in most
examples.
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.------------------------------------------------------------.
| An ALTO Network Map |
| |
| .-----------------------------------. .----------------. |
| | NetLoc: PID-1 | | NetLoc: PID-3 | |
| | .------------------------------. | | | |
| | | 192.0.2.0/24 | | | .-----------. | |
| | | .--------------------------. | | | | 0.0.0.0/0 | | |
| | | | Endpoint: 192.0.2.34 | | | | `-----------` | |
| | | `--------------------------` | | | | |
| | `------------------------------` | | | |
| | .------------------------------. | | | |
| | | 198.51.100.0/25 | | | | |
| | | .--------------------------. | | | | |
| | | | Endpoint: 198.51.100.100 | | | | | |
| | | `--------------------------` | | | | |
| | `------------------------------` | | | |
| `-----------------------------------` | | |
| | | |
| .-----------------------------------. | | |
| | NetLoc: PID-2 | | | |
| | .------------------------------. | | | |
| | | 198.51.100.128/25 | | | | |
| | `------------------------------` | | | |
| `-----------------------------------` `----------------` |
`------------------------------------------------------------`
Figure 3: Example Network Map.
6. Cost Map
An ALTO Server indicates preferences amongst network locations in the
form of Path Costs. Path Costs are generic costs and can be
internally computed by a network provider according to its own
policy.
For a given Network Map, an ALTO Cost Map defines Path Costs pairwise
amongst sets of source and destination Network Locations defined by
PIDs defined in the Network Map. Each Path Cost is the end-to-end
cost when a unit of traffic goes from the source to the destination.
Since cost is directional from the source to the destination, an
application, when using ALTO Information, may independently determine
how the Resource Consumer and Resource Provider are designated as the
source or destination in an ALTO query, and hence how to utilize the
Path Cost provided by ALTO Information. For example, if the cost is
expected to be correlated with throughput, a typical application
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concerned with bulk data retrieval may use the Resource Provider as
the source, and Resource Consumer as the destination.
One advantage of separating ALTO information into a Network Map and a
Cost Map is that the two components can be updated at different time
scales. For example, Network Maps may be stable for a longer time
while Cost Maps may be updated to reflect dynamic network conditions.
As used in this document, a Cost Map refers to the syntax and
semantics of the information distributed by the ALTO Server. This
document does not discuss the internal representation of this data
structure within the ALTO Server.
6.1. Cost Types
Path Costs have attributes:
o Metric: identifies what the costs represent;
o Mode: identifies how the costs should be interpreted.
The combination of a metric and a mode defines a Cost Type. Certain
queries for Cost Maps allow the ALTO Client to indicate the desired
Cost Type. For a given ALTO Server, the combination of Cost Type and
Network Map defines a key. In other words, an ALTO Server MUST NOT
define two Cost Maps with the same Cost Type, Network Map pair.
6.1.1. Cost Metric
The Metric attribute indicates what the cost represents. For
example, an ALTO Server could define costs representing air-miles,
hop-counts, or generic routing costs.
Cost metrics are indicated in protocol messages as strings.
6.1.1.1. Cost Metric: routingcost
An ALTO Server MUST offer the 'routingcost' Cost Metric.
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.
Note that an ISP may internally compute routing cost using any method
that it chooses (e.g., air-miles or hop-count) as long as it conforms
to these semantics.
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6.1.2. Cost Mode
The Mode attribute indicates how costs should be interpreted.
Specifically, the Mode attribute indicates whether returned costs
should be interpreted as numerical values or ordinal rankings.
It is important to communicate such information to ALTO Clients, as
certain operations may not be valid on certain costs returned by an
ALTO Server. For example, it is possible for an ALTO Server to
return a set of IP addresses with costs indicating a ranking of the
IP addresses. Arithmetic operations that would make sense for
numerical values, do not make sense for ordinal rankings. ALTO
Clients may handle such costs differently.
Cost Modes are indicated in protocol messages as strings.
An ALTO Server MUST support at least one of 'numerical' and 'ordinal'
modes. An ALTO Client needs to be cognizant of operations when its
desired Cost Mode is not supported. Specifically, an ALTO Client
desiring numerical costs MAY adjust its behaviors if only the ordinal
Cost Mode is available. Alternatively, an ALTO Client desiring
ordinal costs MAY construct ordinal costs from retrieved numerical
values, if only the numerical Cost Mode is available.
6.1.2.1. Cost Mode: numerical
This Cost Mode is indicated by the string 'numerical'. This mode
indicates that it is safe to perform numerical operations (e.g.
normalization or computing ratios for weighted load-balancing) on the
returned costs. The values are floating-point numbers.
6.1.2.2. Cost Mode: ordinal
This Cost Mode is indicated by the string 'ordinal'. This mode
indicates that the costs values in a Cost Map represent a ranking
(relative to all other values in a Cost Map), not actual costs. The
values are non-negative integers, with a lower value indicating a
higher preference. Ordinal cost values in a Cost Map need not be
unique nor contiguous. In particular, it is possible that two
entries in a map have an identical rank (ordinal cost value). This
document does not specify any behavior by an ALTO Client in this
case; an ALTO Client may decide to break ties by random selection,
other application knowledge, or some other means.
6.2. Cost Map Structure
A request for a Cost Map either explicitly or implicitly includes a
list of Source Network Locations and a list of Destination Network
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Locations. (Recall that a Network Location can be an endpoint
address or a PID.)
Specifically, assume that a request specifies a list of multiple
Source Network Locations, say [Src_1, Src_2, ..., Src_m], and a list
of multiple Destination Network Locations, say [Dst_1, Dst_2, ...,
Dst_n].
The ALTO Server will return the Path Cost for each of the m*n
communicating pairs (i.e., Src_1 -> Dst_1, ..., Src_1 -> Dst_n, ...,
Src_m -> Dst_1, ..., Src_m -> Dst_n). If the ALTO Server does not
define a Path Cost for a particular pair, it may be omitted. This
document refers to this structure as a Cost Map.
If the Cost Mode is 'ordinal', the Path Cost of each communicating
pair is relative to the m*n entries.
6.3. Network Map and Cost Map Dependency
A Cost Map gives Path Costs between the PIDs defined in a Network
Map. An ALTO Server may modify a Network Map at any time, say by
adding or deleting PIDs, or even redefining them. Hence to
effectively use an instance of a Cost Map, an ALTO Client must know
which version of the Network Map defined the PIDs in that Cost Map.
Version Tags allow ALTO Clients to correlate Cost Map instances with
the corresponding versions of the Network Map.
A Version Tag is a tuple of (1) an ID for the resource (e.g., a
Network Map), and (2) a tag (an opaque string) associated with the
version of that resource. A Network Map distributed by an ALTO
Server includes its Version Tag. A Cost Map referring to PIDs also
includes Version Tag for the Network Map on which it is based.
Two Network Maps are the same if they have the same Version Tag.
Whenever the content of the Network Map maintained by an ALTO Server
changes, tag MUST also be changed. Possibilities of setting the tag
component include the last-modified timestamp for the Network Map, or
a hash of its contents, where the collision probability is considered
zero in practical deployment scenarios.
6.4. Cost Map Update
An ALTO Server can update a Cost Map at any time. Hence, the same
Cost Map retrieved from the same ALTO Server but from different
requests can be inconsistent.
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7. Endpoint Properties
An endpoint property defines a network-aware property of an endpoint.
7.1. Endpoint Property Type
For each endpoint and an endpoint property type, there can be a value
for the property. The type of an Endpoint property is indicated in
protocol messages as a string. The value depends on the specific
property. For example, for a property such as whether an endpoint is
metered, the value is a true or false value.
7.1.1. Endpoint Property Type: pid
An ALTO Server MUST define the 'pid' Endpoint Property Type for each
Network Map that it provides. Specifically, each Network Map defines
multiple PIDs. For an 'ipv4'/'ipv6' Network Map, given an endpoint's
IP address, the ALTO Server uses the algorithm specified in
Section 11.2.2 to lookup the PID of the endpoint. This PID is the
'pid' property of the endpoint for the Network Map. See
Section 11.4.1.7 for an example.
8. Protocol Specification: General Processing
This section first specifies general client and server processing.
The details of specific services will be covered in the following
sections.
8.1. Overall Design
The ALTO Protocol uses a REST-ful design. There are two primary
components to this design:
o Information Resources: An ALTO Server provides a set of network
information resources. Each information resource has a media type
[RFC2046]. An ALTO Client may construct an HTTP request for a
particular information resource (including any parameters, if
necessary), and the ALTO Server returns the requested information
resource in an HTTP response.
o Information Resource Directory (IRD): An ALTO Server provides to
ALTO Clients a list of available information resources and the URI
at which each is provided. This document refers to this list as
the Information Resource Directory. ALTO Clients consult the
directory to determine the services provided by an ALTO Server.
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8.2. Notation
This document uses 'JSONString', 'JSONNumber', 'JSONBool' to indicate
the JSON string, number, and boolean types, respectively. The type
'JSONValue' indicates a JSON value, as specified in Section 2.1 of
[RFC4627].
This document uses an adaptation of the C-style struct notation to
define the fields (names/values) of JSON objects. An optional field
is enclosed by [ ], and an array is indicated by two numbers in angle
brackets, <m..n>, where m indicates the minimal number of values, and
n is the maximum. When this document writes * for n, it means no
upper bound. In the definitions, the JSON names of the fields are
case sensitive.
For example, the definition below defines a new type Type4, with
three field members (or fields for short) named "name1", "name2", and
"name3" respectively. The field named "name3" is optional, and the
field named "name2" is an array of at least one value.
object { Type1 name1; Type2 name2<1..*>; [Type3 name3;]
} Type4;
This document also defines dictionary maps (or maps for short) from
strings to JSON values. For example, the definition below defines a
Type3 object as a map. Type1 must be defined as string, and Type2
can be defined as any type.
object-map { Type1 -> Type2; } Type3;
This document uses subtyping to denote that one type is derived from
another type. The example below denotes that TypeDerived is derived
from TypeBase. TypeDerived includes all fields defined in TypeBase.
If TypeBase does not have a field named "name1", TypeDerived will
have a new field named "name1". If TypeBase already has a field
named "name1" but with a different type, TypeDerived will have a
field named "name1" with the type defined in TypeDerived (i.e., Type1
in the example).
object { Type1 name1; } TypeDerived : TypeBase;
Note that despite the notation, no standard, machine-readable
interface definition or schema is provided in this document.
Extension documents may document these as necessary.
8.3. Basic Operations
The ALTO Protocol employs standard HTTP [RFC2616]. It is used for
discovering available Information Resources at an ALTO Server and
retrieving Information Resources. ALTO Clients and ALTO Servers use
HTTP requests and responses carrying ALTO-specific content with
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encoding as specified in this document, and MUST be compliant with
[RFC2616].
Instead of specifying the generic application/json Media Type for all
ALTO request parameters (if any) and responses, ALTO Clients and
Servers use multiple, specific JSON-based Media Types (e.g.,
application/alto-networkmap+json, application/alto-costmap+json) to
indicate content types; see Table 2 for a list of Media Types defined
in this document. This allows easy extensibility while maintaining
clear semantics and versioning. For example, a new version of a
component of the ALTO Protocol (e.g., a new version of the Network
Map) can be defined by simply introducing a new Media Type (e.g.,
application/alto-networkmap-v2+json).
8.3.1. Client Discovering Information Resources
To discover available Information Resources, an ALTO Client requests
Information Resource Directories. Informally, an Information
Resource Directory enumerates URIs at which an ALTO Server offers
Information Resources.
Specifically, using the ALTO Discovery protocol, an ALTO Client
obtains a URI through which it can request an Information Resource
Directory (IRD). This document refers to this IRD as the Root IRD of
the ALTO Client. Each entry in an IRD indicates a URI at which an
ALTO Server accepts requests, and returns either an Information
Resource or an Information Resource Directory that references
additional Information Resources. Beginning with its Root IRD and
following links to IRDs recursively, an ALTO Client can discover all
Information Resources available to it. This set of Information
Resources is referred to as the Information Resource Closure of the
ALTO Client. By inspecting its Information Resource Closure, an ALTO
Client can determine whether an ALTO Server supports the desired
Information Resource, and if it is supported, the URI at which it is
available.
See Section 9.2 for a detailed specification on IRDs.
8.3.2. Client Requesting Information Resources
Where possible, the ALTO Protocol uses the HTTP GET method to request
resources. However, some ALTO services provide Information Resources
that are the function of one or more input parameters. Input
parameters are encoded in the HTTP request's entity body, and the
ALTO Client MUST use the HTTP POST method to send the parameters.
When requesting an ALTO Information Resource that requires input
parameters specified in a HTTP POST request, an ALTO Client MUST set
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the Content-Type HTTP header to the media type corresponding to the
format of the supplied input parameters.
8.3.3. Server Responding to IR Request
Upon receiving a request for an Information Resource that the ALTO
Server can provide, the ALTO Server normally returns the requested
Information Resource. In other cases, to be more informative
([I-D.ietf-httpbis-p2-semantics]), the ALTO Server either provides
the ALTO Client with an Information Resource Directory indicating how
to reach the desired information resource, or returns an ALTO error
object; see Section 8.5 for more details on ALTO error handling.
It is possible for an ALTO Server to leverage caching HTTP
intermediaries to respond to both GET and POST requests by including
explicit freshness information (see Section 14 of [RFC2616]).
Caching of POST requests is not widely implemented by HTTP
intermediaries, however an alternative approach is for an ALTO
Server, in response to POST requests, to return an HTTP 303 status
code ("See Other") indicating to the ALTO Client that the resulting
Information Resource is available via a GET request to an alternate
URL. HTTP intermediaries that do not support caching of POST
requests could then cache the response to the GET request from the
ALTO Client following the alternate URL in the 303 response if the
response to the subsequent GET request contains explicit freshness
information.
The ALTO Server MUST indicate the type of its response using a media
type (i.e., the Content-Type HTTP header of the response).
8.3.4. Client Handling Server Response
8.3.4.1. Using Information Resources
This specification does not indicate any required actions taken by
ALTO Clients upon successfully receiving an Information Resource from
an ALTO Server. Although ALTO Clients are suggested to interpret the
received ALTO Information and adapt application behavior, ALTO
Clients are not required to do so.
8.3.4.2. Handling Server Response and IRD
After receiving an Information Resource Directory, the Client can
consult it to determine if any of the offered URIs contain the
desired Information Resource. However, an ALTO Client MUST NOT
assume that the media type returned by the ALTO Server for a request
to a URI is the media type advertised in the IRD or specified in its
request (i.e., the client must still check the Content-Type header).
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The expectation is that the media type returned should normally be
the media type advertised and requested, but in some cases it may
legitimately not be so.
In particular, it is possible for an ALTO Client to receive an
Information Resource Directory from an ALTO Server as a response to
its request for a specific Information Resource. In this case, the
ALTO Client may ignore the response or still parse the response. To
indicate that an ALTO Client will always check if a response is an
Information Resource Directory, the ALTO Client can indicate in the
"Accept" header of a HTTP request that it can accept Information
Resource Directory; see Section 9.2 for the media type.
8.3.4.3. Handling Error Conditions
If an ALTO Client does not successfully receive a desired Information
Resource from a particular ALTO Server (i.e., server response
indicates error or there is no response), the Client can either
choose another server (if one is available) or fall back to a default
behavior (e.g., perform peer selection without the use of ALTO
information, when used in a peer-to-peer system).
8.3.5. Authentication and Encryption
ALTO server implementations as well as ALTO client implementations
MUST support the "https" URI scheme [RFC2818] and TLS [RFC5246]. See
Section 15.1.2 for security considerations and Section 16 for
manageability considerations regarding the usage of HTTPS/TLS.
For deployment scenarios where client authentication is desired, HTTP
Digest Authentication MUST be supported. TLS Client Authentication
is the preferred mechanism if it is available.
8.3.6. Information Refreshing
An ALTO Client can determine the frequency at which ALTO Information
is refreshed based on information made available via HTTP.
8.3.7. Parsing of Unknown Fields
This document only details object fields used by this specification.
Extensions may include additional fields within JSON objects defined
in this document. ALTO implementations MUST ignore unknown fields
when processing ALTO messages.
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8.4. Server Response Encoding
Though each type of ALTO Server response (i.e., an Information
Resource Directory, an individual Information Resource, or an error
message) has its distinct syntax and hence its unique Media Type,
they are designed to have a similar structure: a meta field providing
meta definitions, and another field containing the data, if needed.
Specifically, this document defines the base type of each ALTO Server
response as ResponseEntityBase:
object { ResponseMeta meta; } ResponseEntityBase;
with field:
meta meta information pertaining to the response.
8.4.1. Meta Information
Meta information is encoded as a map object for flexibility.
Specifically, ResponseMeta is defined as:
object-map { JSONString -> JSONValue } ResponseMeta;
8.4.2. Data Information
The data component of the response encodes the response-specific
data. This document derives five types from ResponseEntityBase to
add different types of data component: InfoResourceDirectory
(Section 9.2.2), InfoResourceNetworkMap (Section 11.2.1.6),
InfoResourceCostMap (Section 11.2.3.6),
InfoResourceEndpointProperties (Section 11.4.1.6), and
InfoResourceEndpointCostMap (Section 11.5.1.6).
8.5. Protocol Errors
If there is an error processing a request, an ALTO Server SHOULD
return additional ALTO-layer information, if it is available, in the
form of an ALTO Error Resource encoded in the HTTP response' entity
body. If no ALTO-layer information is available, an ALTO Server may
omit an ALTO Error resource from the response.
With or without additional ALTO-layer error information, an ALTO
Server MUST set an appropriate HTTP status code. It is important to
note that the HTTP Status Code and ALTO Error Resource have distinct
roles. An ALTO Error Resource provides detailed information about
why a particular request for an ALTO Resource was not successful.
The HTTP status code indicates to HTTP processing elements (e.g.,
intermediaries and clients) how the response should be treated.
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8.5.1. Media Type
The media type for an ALTO Error Response is "application/
alto-error+json".
8.5.2. Response Format and Error Codes
An ALTO Error Response MUST include the "code" key in the "meta"
field of the response. The value of "code" MUST be an ALTO Error
Code, encoded in string, defined in Table 1. Note that the ALTO
Error Codes defined in Table 1 are limited to support the error
conditions needed for purposes of this document. Additional status
codes may be defined in companion or extension documents.
+-----------------------+-------------------------------------------+
| ALTO Error Code | Description |
+-----------------------+-------------------------------------------+
| E_SYNTAX | Parsing error in request (including |
| | identifiers) |
| E_MISSING_FIELD | A required JSON field is missing |
| E_INVALID_FIELD_TYPE | The type of the value of a JSON field is |
| | invalid |
| E_INVALID_FIELD_VALUE | The value of a JSON field is invalid |
+-----------------------+-------------------------------------------+
Table 1: Defined ALTO Error Codes.
After an ALTO Server receives a request, it needs to verify the
syntactic and semantic validity of the request. The following
paragraphs in this section are intended to illustrate the usage of
the error codes defined above during the verification. An individual
implementation may define its message processing in a different
order.
In the first step after an ALTO Server receives a request, it checks
the syntax of the request body (i.e., whether the JSON structure can
be parsed), and indicates a syntax error using the error code
E_SYNTAX. For an E_SYNTAX error, the ALTO Server MAY provide an
optional key "syntax-error" in the "meta" field of the error
response. The objective of providing "syntax-error" is to provide
technical debugging information to developers, not end users. Hence,
it should be a human-readable, free-form text describing the syntax
error. If possible, the text should include position information
such as line number and offset within line about the syntax error.
If nothing else, "syntax-error" could have just the position. If a
syntax error occurs in a production environment, the ALTO Client
could inform the end user that there was an error communicating with
the ALTO Server, and suggest that the user submit the error
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information, which includes "syntax-error", to the developers.
A request without syntax errors may still be invalid. An error case
is that the request misses a required field. The server indicates
such an error using the error code E_MISSING_FIELD. This document
defines required fields for Network Map Filtering (Section 11.3.1.3),
Cost Map Filtering (Section 11.3.2.3), Endpoint Properties
(Section 11.4.1.3), and Endpoint Cost (Section 11.5.1.3). For an
E_MISSING_FIELD error, the server may include an optional "field" key
in the "meta" field of the error response, to indicate the missing
field. "field" should be a JSONString indicating the full path of the
missing field. For example, assume that a Filtered CostMap request
(see Section 11.3.2.3) misses the "cost-metric" field in the request.
The error response from the ALTO Server may specify the "field" key
as "cost-type/cost-mode".
A request with the correct fields might use a wrong type for the
value of a field. For example, the value of a field could be a
JSONString when a JSONNumber is expected. The server indicates such
an error using the error code E_INVALID_FIELD_TYPE. The server may
include an optional "field" key in the "meta" field of the response,
to indicate the field that contains the wrong type.
A request with the correct fields and types of values for the fields
may specify a wrong value for a field. For example, a cost map
filtering request may specify a wrong value of CostMode in the "cost-
type" field (Section 11.3.2.3). The server indicates such an error
with the error code E_INVALID_FIELD_VALUE. For an
E_INVALID_FIELD_VALUE error, the server may include an optional
"field" key in the "meta" field of the response, to indicate the
field that contains the wrong value. The server may also include an
optional "value" key in the "meta" field of the response to indicate
the wrong value that triggered the error.
A request with the correct fields and types of values for the fields
may specify a wrong value for a field. For example, a filtered cost
map request may specify a wrong value for CostMode in the "cost-type"
field (Section 11.3.2.3). The server indicates such an error with
the error code E_INVALID_FIELD_VALUE. For an E_INVALID_FIELD_VALUE
error, the server may include an optional "field" key in the "meta"
field of the response, to indicate the field that contains the wrong
value. The server may also include an optional "value" key in the
"meta" field of the response to indicate the wrong value that
triggered the error. If the "value" key is specified, the "field"
key MUST be specified. The "value" key MUST have a JSONString value.
If the invalid value is not a string, the ALTO Server MUST convert it
to a string. Below are the rules to specify the "value" key:
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o If the invalid value is a string, "value" is that string;
o If the invalid value is a number, "value" must be the invalid
number as a string;
o If the invalid value is a subfield, the server must set the
"field" key to the full path of the field name and "value" to the
invalid subfield value, converting it to a string if needed. For
example, if the "cost-mode" subfield of the "cost-type" field is
an invalid mode "foo", the server should set "value" to "foo", and
"field" to "cost-mode/cost-type";
o If an element of a JSON array has an invalid value, the server
sets "value" to the value of the invalid element, as a string, and
"field" to the name of the array. An array element of the wrong
type (e.g., a number in what is supposed to be an array of
strings) is an invalid value error, not an invalid type error.
The server sets "value" to the string version of the incorrect
element, and "field" to the name of the array.
If multiple errors are present in a single request (e.g., a request
uses a JSONString when a JSONNumber is expected and a required field
is missing), then the ALTO Server MUST return exactly one of the
detected errors. However, the reported error is implementation
defined, since specifying a particular order for message processing
encroaches needlessly on implementation techniques.
8.5.3. Overload Conditions and Server Unavailability
If an ALTO Server detects that it cannot handle a request from an
ALTO Client due to excessive load, technical problems, or system
maintenance, it SHOULD do one of the following:
o Return an HTTP 503 ("Service Unavailable") status code to the ALTO
Client. As indicated by [RFC2616], the Retry-After HTTP header
may be used to indicate when the ALTO Client should retry the
request.
o Return an HTTP 307 ("Temporary Redirect") status code indicating
an alternate ALTO Server that may be able to satisfy the request.
Using Temporary Redirect may generate infinite redirection loops.
Although [RFC2616] Section 10.3 requires that an HTTP client
SHOULD detect infinite redirection loops, it is more desirable
that multiple ALTO Servers are configured to not form redirection
loops.
The ALTO Server MAY also terminate the connection with the ALTO
Client.
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The particular policy applied by an ALTO Server to determine that it
cannot service a request is outside of the scope of this document.
9. Protocol Specification: Information Resource Directory
As already discussed, an ALTO Client starts by retrieving an
Information Resource Directory, which specifies the attributes of
individual Information Resources that an ALTO Server provides.
9.1. Information Resource Attributes
In this document, each Information Resource has five attributes
associated with it, including its assigned ID, its response format,
its capabilities, its accepted input parameters, and other resources
that it may depend on. The function of an Information Resource
Directory is to publishes these attributes.
9.1.1. Resource ID
Each Information Resource that an ALTO Client can request MUST be
assigned an ID that is unique amongst all Information Resources in
the Information Resource Closure of the client. The ID SHOULD remain
stable even when the data provided by that resource changes. For
example, even though the number of PIDs in a Network Map may be
adjusted, its Resource ID should remain the same. Similarly, if the
entries in a Cost Map are updated, its Resource ID should remain the
same. IDs SHOULD NOT be re-used for different resources over time.
9.1.2. Media Type
ALTO uses Media Type [RFC2046] to uniquely indicate the data format
used to encode the content to be transmitted between an ALTO Server
and an ALTO Client in the HTTP entity body.
9.1.3. Capabilities
The Capabilities attribute of an Information Resource indicates
specific capabilities that the server can provide. For example, if
an ALTO Server allows an ALTO Client to specify cost constraints when
the Client requests a Cost Map Information Resource, then the Server
advertises the cost-constraints capability of the Cost Map
Information Resource.
9.1.4. Accepts Input Parameters
An ALTO Server may allow an ALTO Client to supply input parameters
when requesting certain Information Resources. The associated
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accepts attribute of an Information Resource is a Media Type, which
indicates how the Client specifies the input parameters as contained
in the entity body of the HTTP POST request.
9.1.5. Dependent Resources
The information provided in an Information Resource may use
information provided in some other resources (e.g., a Cost Map uses
the PIDs defined in a Network Map). The uses attribute conveys such
information.
9.2. Information Resource Directory (IRD)
An ALTO Server uses Information Resource Directory to publish
available Information Resources and their aforementioned attributes.
Since resource selection happens after consumption of the Information
Resource Directory, the format of the Information Resource Directory
is designed to be simple with the intention of future ALTO Protocol
versions maintaining backwards compatibility. Future extensions or
versions of the ALTO Protocol SHOULD be accomplished by extending
existing media types or adding new media types, but retaining the
same format for the Information Resource Directory.
An ALTO Server MUST make an Information Resource Directory available
via the HTTP GET method to a URI discoverable by an ALTO Client.
Discovery of this URI is out of scope of this document, but could be
accomplished by manual configuration or by returning the URI of an
Information Resource Directory from the ALTO Discovery Protocol
[I-D.ietf-alto-server-discovery]. For recommendations on how the URI
may look like, see [I-D.ietf-alto-server-discovery].
9.2.1. Media Type
The media type to indicate an information directory is "application/
alto-directory+json".
9.2.2. Encoding
An Information Resource Directory response may include in "meta" the
"cost-types" key, whose value is of type IRDMetaCostTypes defined
below, where CostType is defined in Section 10.7:
object-map {
JSONString -> CostType;
} IRDMetaCostTypes;
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The function of "cost-types" is to assign names to a set of CostTypes
that can be used in one or more "resources" entries in the IRD to
simplify specification. The names defined in "cost-types" in an IRD
are local to the IRD.
For a Root IRD, "meta" MUST include the "default-alto-network-map"
key, which specifies the Resource ID of a Network Map. When there are
multiple Network Maps defined in an IRD (e.g., with different levels
of granularity), the "default-alto-network-map" key provides a
guideline to simple clients that use only one Network Map.
The data component of an Information Resource Directory response is
named "resources", which is a JSON object of type IRDResourceEntries:
object {
IRDResourceEntries resources;
} InfoResourceDirectory : ResponseEntityBase;
object-map {
ResourceID -> IRDResourceEntry;
} IRDResourceEntries;
object {
JSONString uri;
JSONString media-type;
[JSONString accepts;]
[Capabilities capabilities;]
[ResourceID uses<0..*>;]
} IRDResourceEntry;
object {
...
} Capabilities;
An IRDResourceEntries object is a dictionary map keyed by
ResourceIDs, where ResourceID is defined in Section 10.2. The value
of each entry specifies:
uri A URI at which the ALTO Server provides one or more Information
Resources, or an Information Resource Directory indicating
additional Information Resources. URIs can be relative to the URI
of the IRD and MUST be resolved according to Section 5 of
[RFC3986].
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media-type The media type of Information Resource (see
Section 9.1.2) available via GET or POST requests to the
corresponding URI or "application/alto-directory+json", which
indicates that the response for a request to the URI will be an
Information Resource Directory for URIs discoverable via the URI.
accepts The media type of input parameters (see Section 9.1.4)
accepted by POST requests to the corresponding URI. If this field
is not present, it MUST be assumed to be empty.
capabilities A JSON Object enumerating capabilities of an ALTO
Server in providing the Information Resource at the corresponding
URI and Information Resources discoverable via the URI. If this
field is not present, it MUST be assumed to be an empty object.
If a capability for one of the offered Information Resources is
not explicitly listed here, an ALTO Client may either issue an
OPTIONS HTTP request to the corresponding URI to determine if the
capability is supported, or assume its default value documented in
this specification or an extension document describing the
capability.
uses A list of Resource IDs, defined in the same IRD, that define
the resources on which this resource directly depends. An ALTO
Server SHOULD include in this list any resources that the ALTO
Client would need to retrieve in order to interpret the contents
of this resource. For example, a Cost Map resource should include
in this list the Network Map on which it depends. ALTO Clients
may wish to consult this list in order to pre-fetch necessary
resources.
If an entry has an empty list for "accepts", then the corresponding
URI MUST support GET requests. If an entry has a non-empty
"accepts", then the corresponding URI MUST support POST requests. If
an ALTO Server wishes to support both GET and POST on a single URI,
it MUST specify two entries in the Information Resource Directory.
9.2.3. Example
The following is an example Information Resource Directory returned
by an ALTO Server to an ALTO Client. Assume it is the Root IRD of
the Client.
GET /directory HTTP/1.1
Host: alto.example.com
Accept: application/alto-directory+json,application/alto-error+json
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HTTP/1.1 200 OK
Content-Length: 2333
Content-Type: application/alto-directory+json
{
"meta" : {
"cost-types": {
"num-routing": {
"cost-mode" : "numerical",
"cost-metric": "routingcost",
"description": "My default"
},
"num-hop": {
"cost-mode" : "numerical",
"cost-metric": "hopcount"
},
"ord-routing": {
"cost-mode" : "ordinal",
"cost-metric": "routingcost"
},
"ord-hop": {
"cost-mode" : "ordinal",
"cost-metric": "hopcount"
}
},
"default-alto-network-map" : "my-default-network-map"
},
"resources" : {
"my-default-network-map" : {
"uri" : "http://alto.example.com/networkmap",
"media-type" : "application/alto-networkmap+json"
},
"numerical-routing-cost-map" : {
"uri" : "http://alto.example.com/costmap/num/routingcost",
"media-type" : "application/alto-costmap+json",
"capabilities" : {
"cost-type-names" : [ "num-routing" ]
},
"uses": [ "my-default-network-map" ]
},
"numerical-hopcount-cost-map" : {
"uri" : "http://alto.example.com/costmap/num/hopcount",
"media-type" : "application/alto-costmap+json",
"capabilities" : {
"cost-type-names" : [ "num-hop" ]
},
"uses": [ "my-default-network-map" ]
},
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"custom-maps-resources" : {
"uri" : "http://custom.alto.example.com/maps",
"media-type" : "application/alto-directory+json"
},
"endpoint-property" : {
"uri" : "http://alto.example.com/endpointprop/lookup",
"media-type" : "application/alto-endpointprop+json",
"accepts" : "application/alto-endpointpropparams+json",
"capabilities" : {
"prop-types" : [ "my-default-network-map.pid",
"priv:ietf-example-prop" ]
},
},
"endpoint-cost" : {
"uri" : "http://alto.example.com/endpointcost/lookup",
"media-type" : "application/alto-endpointcost+json",
"accepts" : "application/alto-endpointcostparams+json",
"capabilities" : {
"cost-constraints" : true,
"cost-type-names" : [ "num-routing", "num-hop",
"ord-routing", "ord-hop"]
}
}
}
}
Specifically, the "cost-types" key of "meta" of the example IRD
defines names for four cost types in this IRD. For example, "num-
routing" in the example is the name that refers to a Cost Type with
Cost Mode being "numerical" and Cost Metric being "routingcost".
This name is used in the second entry of "resources", which defines a
Cost Map. In particular, the "cost-type-names" of its "capabilities"
specifies that this resource supports a Cost Type named as "num-
routing". The ALTO Client looks up the name "num-routing" in "cost-
types" of the IRD to obtain the Cost Type named as "num-routing".
The last entry of "resources" uses all four names defined in "cost-
types".
Another key defined in "meta" of the example IRD is "default-alto-
network-map", which has value "my-default-network-map", which is the
Resource ID of a Network Map that will be defined in "resources".
The "resources" field of the example IRD defines six Information
Resources. For example, the second entry, which is assigned a
Resource ID "numerical-routing-cost-map", provides a Cost Map, as
indicated by the media-type "application/alto-costmap+json". The
Cost Map is based on the Network Map defined with Resource ID "my-
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default-network-map". As another example, the last entry, which is
assigned Resource ID "endpoint-cost", provides the Endpoint Cost
Service, which is indicated by the media-type "application/
alto-endpointcost+json". An ALTO Client should use uri
"http://alto.example.com/endpointcost/lookup" to access the service.
The ALTO Client should format its request body to be the
"application/alto-endpointcostparams+json" media type, as specified
by the "accepts" attribute of the Information Resource. The "cost-
type-names" field of the "capabilities" attribute of the Information
Resource includes four defined cost types specified in the "cost-
types" key of "meta" of the IRD. Hence, one can verify that the
Endpoint Cost Information Resource supports both Cost Metrics
'routingcost' and 'hopcount', each available for both 'numerical' and
'ordinal'. When requesting the Information Resource, an ALTO Client
can specify cost constraints, as indicated by the "cost-constraints"
field of the "capabilities" attribute.
9.2.4. Delegation using IRD
ALTO Information Resource Directory provides flexibility to provide
ALTO Service (e.g., delegation to another domain). Consider the
preceding example. Assume that the ALTO Server running at
alto.example.com wants to delegate some Information Resources to a
separate subdomain: "custom.alto.example.com". In particular, assume
that the maps available via this subdomain are filtered Network Maps,
filtered Cost Maps, and some pre-generated maps for the "hopcount"
and "routingcost" Cost Metrics in the "ordinal" Cost Mode. The
fourth entry of "resources" in the preceding example IRD implements
the delegation. The entry has a media-type of "application/
alto-directory+json", and an ALTO Client can discover the Information
Resources available at "custom.alto.example.com" if its request to
"http://custom.alto.example.com/maps" is successful:
GET /maps HTTP/1.1
Host: custom.alto.example.com
Accept: application/alto-directory+json,application/alto-error+json
HTTP/1.1 200 OK
Content-Length: 1900
Content-Type: application/alto-directory+json
{
"meta" : {
"cost-types": {
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"num-routing": {
"cost-mode" : "numerical",
"cost-metric": "routingcost",
"description": "My default"
},
"num-hop": {
"cost-mode" : "numerical",
"cost-metric": "hopcount"
},
"ord-routing": {
"cost-mode" : "ordinal",
"cost-metric": "routingcost"
},
"ord-hop": {
"cost-mode" : "ordinal",
"cost-metric": "hopcount"
}
}
},
"resources" : {
"filtered-network-map" : {
"uri" : "http://custom.alto.example.com/networkmap/filtered",
"media-type" : "application/alto-networkmap+json",
"accepts" : "application/alto-networkmapfilter+json",
"uses": [ "my-default-network-map" ]
},
"filtered-cost-map" : {
"uri" : "http://custom.alto.example.com/costmap/filtered",
"media-type" : "application/alto-costmap+json",
"accepts" : "application/alto-costmapfilter+json",
"capabilities" : {
"cost-constraints" : true,
"cost-type-names" : [ "num-routing", "num-hop",
"ord-routing", "ord-hop" ]
},
"uses": [ "my-default-network-map" ]
},
"ordinal-routing-cost-map" : {
"uri" : "http://custom.alto.example.com/ord/routingcost",
"media-type" : "application/alto-costmap+json",
"capabilities" : {
"cost-type-names" : [ "ord-routing" ]
},
"uses": [ "my-default-network-map" ]
},
"ordinal-hopcount-cost-map" : {
"uri" : "http://custom.alto.example.com/ord/hopcount",
"media-type" : "application/alto-costmap+json",
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"capabilities" : {
"cost-type-names" : [ "ord-hop" ],
},
"uses": [ "my-default-network-map" ]
}
}
}
Note that the subdomain does not define Network Map, and uses the
Network Map with Resource ID "my-default-network-map" defined in the
Root IRD.
9.2.5. Considerations of Using IRD
9.2.5.1. ALTO Client
This document specifies no requirements or constraints on ALTO
Clients with regards to how they process an Information Resource
Directory to identify the URI corresponding to a desired Information
Resource. However, some advice is provided for implementors.
It is possible that multiple entries in the directory match a desired
Information Resource. For instance, in the example in Section 9.2.3,
a full Cost Map with "numerical" Cost Mode and "routingcost" Cost
Metric could be retrieved via a GET request to
"http://alto.example.com/costmap/num/routingcost", or via a POST
request to "http://custom.alto.example.com/costmap/filtered".
In general, it is preferred for ALTO Clients to use GET requests
where appropriate, since it is more likely for responses to be
cachable. However, an ALTO Client may need to use POST, for example,
to get ALTO costs or properties that are for a restricted set of PIDs
or Endpoints, or to update cached information previously acquired via
GET requests."
9.2.5.2. ALTO Server
This document indicates that an ALTO Server may or may not provide
the Information Resources specified in the Map Filtering Service. If
these resources are not provided, it is indicated to an ALTO Client
by the absence of a Network Map or Cost Map with any media types
listed under "accepts".
10. Protocol Specification: Basic Data Types
This section details the format of basic data types.
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10.1. PID Name
A PID Name is encoded as a JSON string. The string MUST be no more
than 64 characters, and MUST NOT contain characters other than US-
ASCII alphanumeric characters (U+0030-U+0039, U+0041-U+005A, and
U+0061-U+007A), the hyphen ('-', U+002D), the colon (':', U+003A),
the at ('@', code point U+0040), the low line ('_', U+005F), or the
'.' separator (U+002E). The '.' separator is reserved for future use
and MUST NOT be used unless specifically indicated in this document,
or an extension document.
The type 'PIDName' is used in this document to indicate a string of
this format.
10.2. Resource ID
A Resource ID uniquely identifies an particular resource (e.g., a
Network Map) within an ALTO Server (see Section 9.2).
A Resource ID is encoded as a JSON string with the same format as
that of PIDName.
The type 'ResourceID' is used in this document to indicate a string
of this format.
10.3. Version Tag
A Version Tag is defined as:
object {
ResourceID resource-id;
JSONString tag;
} VersionTag;
As described in Section 6.3, the 'resource-id' attribute is the
Resource ID of a resource (e.g., a Network Map) defined in the
Information Resource Directory, and 'tag' is an identifier string.
Two values of the VersionTag are equal if and only if both the
'resource-id' attributes are byte-for-byte equal and the 'tag'
attributes are byte-for-byte equal.
A 'tag' string MUST be no more than 64 characters, and MUST NOT
contain any character below U+0021 or above U+007E. It is RECOMMENDED
that the tag have a low collision probability with other tags. One
suggested mechanism is to compute it using a hash of the data
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contents of the resource.
10.4. Endpoints
This section defines formats used to encode addresses for Endpoints.
In a case that multiple textual representations encode the same
Endpoint address or prefix (within the guidelines outlined in this
document), the ALTO Protocol does not require ALTO Clients or ALTO
Servers to use a particular textual representation, nor does it
require that ALTO Servers reply to requests using the same textual
representation used by requesting ALTO Clients. ALTO Clients must be
cognizant of this.
10.4.1. Typed Endpoint Addresses
When an Endpoint Address is used, an ALTO implementation must be able
to determine its type. For this purpose, the ALTO Protocol allows
endpoint addresses to also explicitly indicate their types. This
document refers to such addresses as Typed Endpoint Addresses.
Typed Endpoint Addresses are encoded as strings of the format
'AddressType:EndpointAddr', with the ':' character as a separator.
The type 'TypedEndpointAddr' is used to indicate a string of this
format.
10.4.2. Address Type
The AddressType component of TypedEndPointAddr is defined as a string
consisting of only US-ASCII alphanumeric characters (U+0030-U+0039,
U+0041-U+005A, and U+0061-U+007A). The type 'AddressType' is used in
this document to indicate a string of this format.
This document defines two values for AddressType: 'ipv4' to refer to
IPv4 addresses, and 'ipv6' to refer to IPv6 addresses. All
AddressType identifiers appearing in an HTTP request or response with
an 'application/alto-*' media type MUST be registered in the ALTO
Address Type registry (see Section 14.4).
10.4.3. Endpoint Address
The EndpointAddr component of TypedEndPointAddr is also encoded as a
string. The exact characters and format depend on AddressType. This
document defines EndpointAddr when AddressType is 'ipv4' or 'ipv6'.
10.4.3.1. IPv4
IPv4 Endpoint Addresses are encoded as specified by the 'IPv4address'
rule in Section 3.2.2 of [RFC3986].
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10.4.3.2. IPv6
IPv6 Endpoint Addresses are encoded as specified in Section 4 of
[RFC5952].
10.4.4. Endpoint Prefixes
For efficiency, it is useful to denote a set of Endpoint Addresses
using a special notation (if one exists). This specification makes
use of the prefix notations for both IPv4 and IPv6 for this purpose.
Endpoint Prefixes are encoded as strings. The exact characters and
format depend on the type of endpoint address.
The type 'EndpointPrefix' is used in this document to indicate a
string of this format.
10.4.4.1. IPv4
IPv4 Endpoint Prefixes are encoded as specified in Section 3.1 of
[RFC4632].
10.4.4.2. IPv6
IPv6 Endpoint Prefixes are encoded as specified in Section 7 of
[RFC5952].
10.4.5. Endpoint Address Group
The ALTO Protocol includes messages that specify potentially large
sets of endpoint addresses. Endpoint Address Groups provide a more
efficient way to encode such sets, even when the set contains
endpoint addresses of different types.
An Endpoint Address Group is defined as:
object-map {
AddressType -> EndpointPrefix<0..*>;
} EndpointAddrGroup;
In particular, an Endpoint Address Group is a JSON object
representing a map, where each key is the string corresponding to an
address type, and the corresponding value is an array listing
prefixes of addresses of that type.
The following is an example with both IPv4 and IPv6 endpoint
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addresses:
{
"ipv4": [
"192.0.2.0/24",
"198.51.100.0/25"
],
"ipv6": [
"2001:db8:0:1::/64",
"2001:db8:0:2::/64"
]
}
10.5. Cost Mode
A Cost Mode is encoded as a string. The string MUST either have the
value 'numerical' or 'ordinal'.
The type 'CostMode' is used in this document to indicate a string of
this format.
10.6. Cost Metric
A Cost Metric is encoded as a string. The string MUST be no more
than 32 characters, and MUST NOT contain characters other than US-
ASCII alphanumeric characters (U+0030-U+0039, U+0041-U+005A, and
U+0061-U+007A), the hyphen ('-', U+002D), the colon (':', U+003A),
the low line ('_', U+005F), or the '.' separator (U+002E). The '.'
separator is reserved for future use and MUST NOT be used unless
specifically indicated by a companion or extension document.
Identifiers prefixed with 'priv:' are reserved for Private Use
[RFC5226] without a need to register with IANA. All other
identifiers that appear in an HTTP request or response with an
'application/alto-*' media type and indicate Cost Metrics MUST be
registered in the ALTO Cost Metrics registry Section 14.2. For an
identifier with the 'priv:' prefix, an additional string (e.g.,
company identifier or random string) MUST follow (i.e., 'priv:' only
is not a valid identifier) to reduce potential collisions.
The type 'CostMetric' is used in this document to indicate a string
of this format.
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10.7. Cost Type
The combination of a CostMetric and a CostMode defines a CostType:
object {
CostMetric cost-metric;
CostMode cost-mode;
[JSONString description;]
} CostType;
'description', if present, MUST contain a string with a human-
readable description of the cost-metric and cost-mode. An ALTO
Client MAY present this string to a developer, as part of a discovery
process. But the field is not intended to be interpreted by an ALTO
Client.
10.8. Endpoint Property
This document will distinguish two types of Endpoint Properties:
Resource Specific Endpoint Properties and Global Endpoint Properties.
The type 'EndpointPropertyType' is used in this document to indicate
a string denoting either a Resource Specific Endpoint Property or a
Global Endpoint Property.
10.8.1. Resource Specific Endpoint Properties
This document defines only one Resource Specific Endpoint Property in
this document: pid. It has the following format: a Resource ID,
followed by the '.' separator (U+002E), followed by "pid". An
example is "my-default-networkmap.pid".
10.8.2. Global Endpoint Properties
An Global Endpoint Property is encoded as a string. The string MUST
be no more than 32 characters, and MUST NOT contain characters other
than US-ASCII alphanumeric characters (U+0030-U+0039, U+0041-U+005A,
and U+0061-U+007A), the hyphen ('-', U+002D), the colon (':',
U+003A), or the low line ('_', U+005F). Note that the '.' separator
is not allowed so that there is no ambiguity on whether an endpoint
property is global or resource specific.
Identifiers prefixed with 'priv:' are reserved for Private Use
[RFC5226] without a need to register with IANA. All other
identifiers for Endpoint Properties appearing in an HTTP request or
response with an 'application/alto-*' media type MUST be registered
in the ALTO Endpoint Property registry Section 14.3. For an Endpoint
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Property identifier with the 'priv:' prefix, an additional string
(e.g., company identifier or random string) MUST follow (i.e.,
'priv:' only is not a valid Endpoint Property identifier) to reduce
potential collisions.
11. Protocol Specification: Service Information Resources
This section documents the individual Information Resources defined
to provide the services defined in this document.
11.1. Meta Information
For the "meta" field of the response to an individual Information
Resource, this document defines two generic keys: "vtag", which is
the Version Tag (see Section 10.3) of the current Information
Resource; and "dependent-vtags", which is an array of Version Tags,
to indicate the Version Tags of the resources that this resource
depends on.
11.2. Map Service
The Map Service provides batch information to ALTO Clients in the
form of two types of maps: a Network Map and Cost Map.
11.2.1. Network Map
A Network Map Information Resource defines a set of PIDs, and for
each PID, lists the network locations (endpoints) within the PID. An
ALTO Server MUST provide at least one Network Map.
11.2.1.1. Media Type
The media type of Network Map is "application/alto-networkmap+json".
11.2.1.2. HTTP Method
A Network Map resource is requested using the HTTP GET method.
11.2.1.3. Accept Input Parameters
None.
11.2.1.4. Capabilities
None.
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11.2.1.5. Uses
None.
11.2.1.6. Response
The "meta" field of a Network Map response MUST include "vtag", which
is the Version Tag of the retrieved Network Map.
The data component of a Network Map response is named "network-map",
which is a JSON object of type NetworkMapData:
object {
NetworkMapData network-map;
} InfoResourceNetworkMap : ResponseEntityBase;
object-map {
PIDName -> EndpointAddrGroup;
} NetworkMapData;
Specifically, a NetworkMapData object is a dictionary map keyed by
PIDs, and each value representing the associated set of endpoint
addresses of a PID.
The returned Network Map MUST include all PIDs known to the ALTO
Server.
11.2.1.7. Example
GET /networkmap HTTP/1.1
Host: alto.example.com
Accept: application/alto-networkmap+json,application/alto-error+json
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HTTP/1.1 200 OK
Content-Length: 449
Content-Type: application/alto-networkmap+json
{
"meta" : {
"vtag": {
"resource-id": "my-default-network-map",
"tag": "da65eca2eb7a10ce8b059740b0b2e3f8eb1d4785"
}
},
"network-map" : {
"PID1" : {
"ipv4" : [
"192.0.2.0/24",
"198.51.100.0/25"
]
},
"PID2" : {
"ipv4" : [
"198.51.100.128/25"
]
},
"PID3" : {
"ipv4" : [
"0.0.0.0/0"
],
"ipv6" : [
"::/0"
]
}
}
}
When parsing a Network Map, an ALTO Client MUST ignore any
EndpointAddressGroup whose address type it does not recognize. If as
a result a PID does not have any address types known to the client,
the client still MUST recognize that PID name as valid, even though
the PID then contains no endpoints.
Note that the encoding of a Network Map response was chosen for
readability and compactness. If lookup efficiency at runtime is
crucial, then the returned Network Map can be transformed into data
structures offering more efficient lookup. For example, one may
store the Network Map as a trie-based data structure, which may allow
efficient longest-prefix matching of IP addresses.
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11.2.2. Mapping IP Addresses to PIDs for 'ipv4'/'ipv6' Network Maps
A key usage of a Network Map is to map Endpoint Addresses to PIDs.
For Network Maps containing the 'ipv4' and 'ipv6' address types
defined in this document, when either an ALTO Client or an ALTO
Server needs to compute the mapping from IP addresses to PIDs, the
longest-prefix matching algorithm [RFC1812] MUST be used.
To ensure that the longest-prefix matching algorithm yields one and
only one PID, Network Maps containing the 'ipv4/'ipv6' address types
MUST satisfy the following two requirements.
First, such a Network Map MUST define a PID for each possible address
in the IP address space for all of the address types contained in the
map. This is defined as the completeness property of a Network Map.
A RECOMMENDED way to satisfy this property is to define a PID with
the shortest enclosing prefix of the addresses provided in the map.
For a map with full IPv4 reachability, this would mean including the
0.0.0.0/0 prefix in a PID; for full IPv6 reachability, this would be
the ::/0 prefix.
Second, such a Network Map MUST NOT define two or more PIDs that
contain an identical IP prefix, in order to ensure that the longest-
prefix matching algorithm maps each IP addresses into exactly one
PID. This is defined as the non-overlapping property of a Network
Map. Specifically, to map an IP address to its PID in a non-
overlapping Network Map, one considers the set S which consists of
all prefixes defined in the Network Map, applies the longest-prefix
mapping algorithm to S to identify the longest prefix containing the
IP address, and assigns that the IP address belongs to the PID
containing the identified longest prefix.
The following example shows a complete and non-overlapping Network
Map:
"network-map" : {
"PID0" : { "ipv6" : [ "::/0" ] },
"PID1" : { "ipv4" : [ "0.0.0.0/0" ] },
"PID2" : { "ipv4" : [ "192.0.2.0/24", "198.51.100.0/24" ] },
"PID3" : { "ipv4" : [ "192.0.2.0/25", "192.0.2.128/25" ] }
}
The IP address 192.0.2.1 should be mapped to PID3.
If, however, the two adjacent prefixes in PID3 were combined as a
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single prefix, then PID3 was changed to
"PID3" : { "ipv4" : [ "192.0.2.0/24" ] }
The new map is no longer non-overlapping, and 192.0.2.1 could no
longer be mapped unambiguously to a PID by means of longest-prefix
matching.
Extension documents may define techniques to allow a single IP
address being mapped to multiple PIDs, when a need is identified.
11.2.3. Cost Map
A Cost Map resource lists the Path Cost for each pair of source/
destination PID defined by the ALTO Server for a given Cost Metric
and Cost Mode. This resource MUST be provided for at least the
'routingcost' Cost Metric.
11.2.3.1. Media Type
The media type of Cost Map is "application/alto-costmap+json".
11.2.3.2. HTTP Method
A Cost Map resource is requested using the HTTP GET method.
11.2.3.3. Accept Input Parameters
None.
11.2.3.4. Capabilities
The capabilities of an ALTO Server URI providing an unfiltered cost
map is a JSON Object of type CostMapCapabilities:
object {
JSONString cost-type-names<1..1>;
} CostMapCapabilities;
with field:
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cost-type-names Note that the array MUST include a single CostType
name defined by key "cost-types" in "meta" of the IRD. This is
because an unfiltered Cost Map (accept == "") is requested via an
HTTP GET that accepts no input parameters. As a contrast, for
filtered cost maps (see Section 11.3.2), the array can have
multiple elements.
11.2.3.5. Uses
The Resource ID of the Network Map based on which the Cost Map will
be defined. Recall (Section 6) that the combination of a Network Map
and a CostType defines a key. In other words, an ALTO Server MUST
NOT define two Cost Maps with the same Cost Type, Network Map pair.
11.2.3.6. Response
The "meta" field of a Cost Map response MUST include the "dependent-
vtags" key, whose value is a single-element array to indicate the
Version Tag of the Network Map used, where the Network Map is
specified in "uses" of the IRD. The "meta" MUST also include "cost-
type", to indicate the Cost Type (Section 10.7) of the Cost Map.
The data component of a Cost Map response is named "cost-map", which
is a JSON object of type CostMapData:
object {
CostMapData cost-map;
} InfoResourceCostMap : ResponseEntityBase;
object-map {
PIDName -> DstCosts;
} CostMapData;
object-map {
PIDName -> JSONValue;
} DstCosts;
Specifically, a CostMapData object is a dictionary map object, with
each key being the PIDName string identifying the corresponding
Source PID, and value being a type of DstCosts, which denotes the
associated costs from the Source PID to a set of destination PIDs (
Section 6.2). An implementation of the protocol in this document
SHOULD assume that the cost is a JSONNumber and fail to parse if it
is not, unless the implementation is using an extension to this
document that indicates when and how costs of other data types are
signaled.
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The returned Cost Map MUST include the Path Cost for each (Source
PID, Destination PID) pair for which a Path Cost is defined. An ALTO
Server MAY omit entries for which a Path Cost is not defined (e.g.,
both the Source and Destination PIDs contain addresses outside of the
Network Provider's administrative domain).
Similar to Network Map, the encoding of Cost Map was chosen for
readability and compactness. If lookup efficiency at runtime is
crucial, then the returned Cost Map can be transformed into data
structures offering more efficient lookup. For example, one may
store a Cost Map as a matrix.
11.2.3.7. Example
GET /costmap/num/routingcost HTTP/1.1
Host: alto.example.com
Accept: application/alto-costmap+json,application/alto-error+json
HTTP/1.1 200 OK
Content-Length: 435
Content-Type: application/alto-costmap+json
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-default-network-map",
"tag": "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
}
],
"cost-type" : {"cost-mode" : "numerical",
"cost-metric": "routingcost"
}
},
"cost-map" : {
"PID1": { "PID1": 1, "PID2": 5, "PID3": 10 },
"PID2": { "PID1": 5, "PID2": 1, "PID3": 15 },
"PID3": { "PID1": 20, "PID2": 15 }
}
}
Similar to the Network Map case, array-based encoding for "map" was
considered, but the current encoding was chosen for clarity.
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11.3. Map Filtering Service
The Map Filtering Service allows ALTO Clients to specify filtering
criteria to return a subset of the full maps available in the Map
Service.
11.3.1. Filtered Network Map
A Filtered Network Map is a Network Map Information Resource
(Section 11.2.1) for which an ALTO Client may supply a list of PIDs
to be included. A Filtered Network Map MAY be provided by an ALTO
Server.
11.3.1.1. Media Type
Since a Filtered Network Map is still a Network Map, it uses the
media type defined for Network Map at Section 11.2.1.1.
11.3.1.2. HTTP Method
A Filtered Network Map is requested using the HTTP POST method.
11.3.1.3. Accept Input Parameters
An ALTO Client supplies filtering parameters by specifying media type
"application/alto-networkmapfilter+json" with HTTP POST body
containing a JSON Object of type ReqFilteredNetworkMap, where:
object {
PIDName pids<0..*>;
[AddressType address-types<0..*>;]
} ReqFilteredNetworkMap;
with fields:
pids Specifies list of PIDs to be included in the returned Filtered
Network Map. If the list of PIDs is empty, the ALTO Server MUST
interpret the list as if it contained a list of all currently-
defined PIDs. The ALTO Server MUST interpret entries appearing
multiple times as if they appeared only once.
address-types Specifies list of address types to be included in the
returned Filtered Network Map. If the "address-types" field is not
specified, or the list of address types is empty, the ALTO Server
MUST interpret the list as if it contained a list of all address
types known to the ALTO Server. The ALTO Server MUST interpret
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entries appearing multiple times as if they appeared only once.
11.3.1.4. Capabilities
None.
11.3.1.5. Uses
The Resource ID of the Network Map based on which the filtering is
performed.
11.3.1.6. Response
The format is the same as unfiltered Network Map. See
Section 11.2.1.6 for the format.
The ALTO Server MUST only include PIDs in the response that were
specified (implicitly or explicitly) in the request. If the input
parameters contain a PID name that is not currently defined by the
ALTO Server, the ALTO Server MUST behave as if the PID did not appear
in the input parameters. Similarly, the ALTO Server MUST only
enumerate addresses within each PID that have types which were
specified (implicitly or explicitly) in the request. If the input
parameters contain an address type that is not currently known to the
ALTO Server, the ALTO Server MUST behave as if the address type did
not appear in the input parameters.
The Version Tag included in the "vtag" of the response MUST
correspond to the full (unfiltered) Network Map Information Resource
from which the filtered information is provided. This ensures that a
single, canonical Version Tag is used independent of any filtering
that is requested by an ALTO Client.
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11.3.1.7. Example
POST /networkmap/filtered HTTP/1.1
Host: custom.alto.example.com
Content-Length: 33
Content-Type: application/alto-networkmapfilter+json
Accept: application/alto-networkmap+json,application/alto-error+json
{
"pids": [ "PID1", "PID2" ]
}
HTTP/1.1 200 OK
Content-Length: 342
Content-Type: application/alto-networkmap+json
{
"meta" : {
"vtag" : {
"resource-id": "my-default-network-map",
"tag": "c0ce023b8678a7b9ec00324673b98e54656d1f6d"
}
},
"network-map" : {
"PID1" : {
"ipv4" : [
"192.0.2.0/24",
"198.51.100.0/24"
]
},
"PID2" : {
"ipv4": [
"198.51.100.128/24"
]
}
}
}
11.3.2. Filtered Cost Map
A Filtered Cost Map is a Cost Map Information Resource
(Section 11.2.3) for which an ALTO Client may supply additional
parameters limiting the scope of the resulting Cost Map. A Filtered
Cost Map MAY be provided by an ALTO Server.
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11.3.2.1. Media Type
Since a Filtered Cost Map is still a Cost Map, it uses the media type
defined for Cost Map at Section 11.2.3.1.
11.3.2.2. HTTP Method
A Filtered Cost Map is requested using the HTTP POST method.
11.3.2.3. Accept Input Parameters
The input parameters for a Filtered Map are supplied in the entity
body of the POST request. This document specifies the input
parameters with a data format indicated by the media type
"application/alto-costmapfilter+json", which is a JSON Object of type
ReqFilteredCostMap, where:
object {
CostType cost-type;
[JSONString constraints<0..*>;]
[PIDFilter pids;]
} ReqFilteredCostMap;
object {
PIDName srcs<0..*>;
PIDName dsts<0..*>;
} PIDFilter;
with fields:
cost-type The CostType (Section 10.7) for the returned costs. The
cost-metric and cost-mode fields MUST match one of the supported
Cost Types indicated in this resource's capabilities
(Section 11.3.2.4). The ALTO Client SHOULD omit the description
field, and if present, the ALTO Server MUST ignore the description
field.
constraints Defines a list of additional constraints on which
elements of the Cost Map are returned. This parameter MUST NOT be
specified if this resource's capabilities ( Section 11.3.2.4)
indicate that constraint support is not available. A constraint
contains two entities separated by whitespace: (1) an operator,
'gt' for greater than, 'lt' for less than, 'ge' for greater than
or equal to, 'le' for less than or equal to, or 'eq' for equal to;
(2) a target cost value. The cost value is a number that MUST be
defined in the same units as the Cost Metric indicated by the
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cost-metric parameter. ALTO Servers SHOULD use at least IEEE 754
double-precision floating point [IEEE.754.2008] to store the cost
value, and SHOULD perform internal computations using double-
precision floating-point arithmetic. If multiple 'constraint'
parameters are specified, they are interpreted as being related to
each other with a logical AND.
pids A list of Source PIDs and a list of Destination PIDs for which
Path Costs are to be returned. If a list is empty, the ALTO
Server MUST interpret it as the full set of currently-defined
PIDs. The ALTO Server MUST interpret entries appearing in a list
multiple times as if they appeared only once. If the "pids" field
is not present, both lists MUST be interpreted by the ALTO Server
as containing the full set of currently-defined PIDs.
11.3.2.4. Capabilities
The URI providing this resource supports all capabilities documented
in Section 11.2.3.4 (with identical semantics), plus additional
capabilities. In particular, the capabilities are defined by a JSON
object of type FilteredCostMapCapabilities:
object {
JSONString cost-type-names<1..*>;
JSONBool cost-constraints;
} FilteredCostMapCapabilities;
with fields:
cost-type-names See Section 11.2.3.4 and note that the array can
have 1 to many cost types.
cost-constraints If true, then the ALTO Server allows cost
constraints to be included in requests to the corresponding URI.
If not present, this field MUST be interpreted as if it specified
false. ALTO Clients should be aware that constraints may not have
the intended effect for cost maps with the 'ordinal' Cost Mode
since ordinal costs are not restricted to being sequential
integers.
11.3.2.5. Uses
The Resource ID of the Network Map based on which the Cost Map will
be filtered.
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11.3.2.6. Response
The format is the same as an unfiltered Cost Map. See
Section 11.2.3.6 for the format.
The "dependent-vtags" key in the "meta" field is an array consisting
of a single element, which is the Version Tag of the Network Map used
in filtering. ALTO Clients should verify that the Version Tag
included in the response is equal to the Version Tag of the Network
Map used to generate the request (if applicable). If it is not, the
ALTO Client may wish to request an updated Network Map, identify
changes, and consider requesting a new Filtered Cost Map.
The returned Cost Map MUST contain only source/destination pairs that
have been indicated (implicitly or explicitly) in the input
parameters. If the input parameters contain a PID name that is not
currently defined by the ALTO Server, the ALTO Server MUST behave as
if the PID did not appear in the input parameters.
If any constraints are specified, Source/Destination pairs for which
the Path Costs do not meet the constraints MUST NOT be included in
the returned Cost Map. If no constraints were specified, then all
Path Costs are assumed to meet the constraints.
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11.3.2.7. Example
POST /costmap/filtered HTTP/1.1
Host: custom.alto.example.com
Content-Type: application/alto-costmapfilter+json
Content-Length: 181
Accept: application/alto-costmap+json,application/alto-error+json
{
"cost-type" : {"cost-mode": "numerical",
"cost-metric": "routingcost"
},
"pids" : {
"srcs" : [ "PID1" ],
"dsts" : [ "PID1", "PID2", "PID3" ]
}
}
HTTP/1.1 200 OK
Content-Length: 341
Content-Type: application/alto-costmap+json
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-default-network-map",
"tag": "75ed013b3cb58f896e839582504f622838ce670f"
}
],
"cost-type": {"cost-mode" : "numerical",
"cost-metric" : "routingcost"
}
},
"cost-map" : {
"PID1": { "PID1": 0, "PID2": 1, "PID3": 2 }
}
}
11.4. Endpoint Property Service
The Endpoint Property Service provides information about Endpoint
properties to ALTO Clients.
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11.4.1. Endpoint Property
An Endpoint Property resource provides information about properties
for individual endpoints. It MAY be provided by an ALTO Server.
11.4.1.1. Media Type
The media type of Endpoint Property is "application/
alto-endpointprop+json".
11.4.1.2. HTTP Method
The Endpoint Property resource is requested using the HTTP POST
method.
11.4.1.3. Accept Input Parameters
An ALTO Client supplies the endpoint properties to be queried through
a media type "application/alto-endpointpropparams+json", and
specifies in the HTTP POST entity body a JSON Object of type
ReqEndpointProp:
object {
EndpointPropertyType properties<1..*>;
TypedEndpointAddr endpoints<1..*>;
} ReqEndpointProp;
with fields:
properties List of endpoint properties to be returned for each
endpoint. Each specified property MUST be included in the list of
supported properties indicated by this resource's capabilities
(Section 11.4.1.4). The ALTO Server MUST interpret entries
appearing multiple times as if they appeared only once.
endpoints List of endpoint addresses for which the specified
properties are to be returned. The ALTO Server MUST interpret
entries appearing multiple times as if they appeared only once.
11.4.1.4. Capabilities
This resource may be defined across multiple types of endpoint
properties. The capabilities of an ALTO Server URI providing
Endpoint Properties are defined by a JSON Object of type
EndpointPropertyCapabilities:
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object {
EndpointPropertyType prop-types<1..*>;
} EndpointPropertyCapabilities;
with field:
prop-types The Endpoint Properties (see Section 10.8) supported by
the corresponding URI.
In particular, the Information Resource Closure MUST provide the look
up of pid for every Network Map defined.
11.4.1.5. Uses
None.
11.4.1.6. Response
The "dependent-vtags" key in the "meta" field of the response MUST be
an array that includes the Version Tags of all Network Maps whose
'pid' is queried.
The data component of an Endpoint Properties response is named
"endpoint-properties", which is a JSON object of type
EndpointPropertyMapData, where:
object {
EndpointPropertyMapData endpoint-properties;
} InfoResourceEndpointProperties : ResponseEntityBase;
object-map {
TypedEndpointAddr -> EndpointProps;
} EndpointPropertyMapData;
object {
EndpointPropertyType -> JSONValue;
} EndpointProps;
Specifically, an EndpointPropertyMapData object has one member for
each endpoint indicated in the input parameters (with the name being
the endpoint encoded as a TypedEndpointAddr). The requested
properties for each endpoint are encoded in a corresponding
EndpointProps object, which encodes one name/value pair for each
requested property, where the property names are encoded as strings
of type EndpointPropertyType. An implementation of the protocol in
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this document SHOULD assume that the property value is a JSONString
and fail to parse if it is not, unless the implementation is using an
extension to this document that indicates when and how property
values of other data types are signaled.
The ALTO Server returns the value for each of the requested endpoint
properties for each of the endpoints listed in the input parameters.
If the ALTO Server does not define a requested property's value for a
particular endpoint, then it MUST omit that property from the
response for only that endpoint.
11.4.1.7. Example
POST /endpointprop/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 181
Content-Type: application/alto-endpointpropparams+json
Accept: application/alto-endpointprop+json,application/alto-error+json
{
"properties" : [ "my-default-networkmap.pid",
"priv:ietf-example-prop" ],
"endpoints" : [ "ipv4:192.0.2.34",
"ipv4:203.0.113.129" ]
}
HTTP/1.1 200 OK
Content-Length: 396
Content-Type: application/alto-endpointprop+json
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-default-network-map",
"tag": "7915dc0290c2705481c491a2b4ffbec482b3cf62"
}
]
},
"endpoint-properties": {
"ipv4:192.0.2.34" : { "my-default-network-map.pid": "PID1",
"priv:ietf-example-prop": "1" },
"ipv4:203.0.113.129" : { "my-default-network-map.pid": "PID3" }
}
}
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11.5. Endpoint Cost Service
The Endpoint Cost Service provides information about costs between
individual endpoints.
In particular, this service allows lists of Endpoint prefixes (and
addresses, as a special case) to be ranked (ordered) by an ALTO
Server.
11.5.1. Endpoint Cost
An Endpoint Cost resource provides information about costs between
individual endpoints. It MAY be provided by an ALTO Server.
How an ALTO Server provides the Endpoint Cost resource is
implementation dependent. An ALTO Server may use either fine-grained
costs among individual endpoints or coarse-grained costs based on the
costs between the PIDs corresponding to the endpoints. See
Section 15.3 for additional details.
11.5.1.1. Media Type
The media type of Endpoint Cost is "application/
alto-endpointcost+json".
11.5.1.2. HTTP Method
The Endpoint Cost resource is requested using the HTTP POST method.
11.5.1.3. Accept Input Parameters
An ALTO Client supplies the endpoint cost parameters through a media
type "application/alto-endpointcostparams+json", with an HTTP POST
entity body of a JSON Object of type ReqEndpointCostMap:
object {
CostType cost-type;
[JSONString constraints<0..*>;]
EndpointFilter endpoints;
} ReqEndpointCostMap;
object {
[TypedEndpointAddr srcs<0..*>;]
[TypedEndpointAddr dsts<0..*>;]
} EndpointFilter;
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with fields:
cost-type The Cost Type (Section 10.7) to use for returned costs.
The cost-metric and cost-mode fields MUST match one of the
supported Cost Types indicated in this resource's capabilities (
Section 11.5.1.4). The ALTO Client SHOULD omit the description
field, and if present, the ALTO Server MUST ignore the description
field.
constraints Defined equivalently to the "constraints" input
parameter of a Filtered Cost Map (see Section 11.3.2).
endpoints A list of Source Endpoints and Destination Endpoints for
which Path Costs are to be returned. If the list of Source or
Destination Endpoints is empty (or not included), the ALTO Server
MUST interpret it as if it contained the Endpoint Address
corresponding to the client IP address from the incoming
connection (see Section 13.3 for discussion and considerations
regarding this mode). The Source and Destination Endpoint lists
MUST NOT be both empty. The ALTO Server MUST interpret entries
appearing multiple times in a list as if they appeared only once.
11.5.1.4. Capabilities
This document defines EndpointCostCapabilities as the same as
FilteredCostMapCapabilities. See Section 11.3.2.4.
11.5.1.5. Uses
It is important to note that although this resource allows an ALTO
Server to reveal costs between individual endpoints, an ALTO Server
is not required to do so. A simple implementation of an ECS resource
may compute the cost between two endpoints as the cost between the
PIDs corresponding to the endpoints, using one of the exposed network
and cost maps defined by the server. ECS MUST NOT use a Network or
Cost Map. Hence, the ECS cost is the cost from the source to
destination endpoint. A future extension may allow ECS to state that
it "uses" a Network Map. The extension then will need to define the
semantics.
11.5.1.6. Response
The "meta" field of an Endpoint Cost response MUST include the "cost-
type" key, to indicate the Cost Type used.
The data component of an Endpoint Cost response is named "endpoint-
cost-map", which is a JSON object of type EndpointCostMapData:
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object {
EndpointCostMapData endpoint-cost-map;
} InfoResourceEndpointCostMap : ResponseEntityBase;
object-map {
TypedEndpointAddr -> EndpointDstCosts;
} EndpointCostMapData;
object-map {
TypedEndpointAddr -> JSONValue;
} EndpointDstCosts;
Specifically, an EndpointCostMapData object is a dictionary map with
each key representing a TypedEndpointAddr string identifying the
Source Endpoint specified in the input parameters. For each Source
Endpoint, a EndpointDstCosts dictionary map object denotes the
associated cost to each Destination Endpoint specified in input
parameters. An implementation of the protocol in this document
SHOULD assume that the cost value is a JSONNumber and fail to parse
if it is not, unless the implementation is using an extension to this
document that indicates when and how costs of other data types are
signaled. If the ALTO Server does not define a cost value from a
Source Endpoint to a particular Destination Endpoint, it MAY be
omitted from the response.
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11.5.1.7. Example
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 248
Content-Type: application/alto-endpointcostparams+json
Accept: application/alto-endpointcost+json,application/alto-error+json
{
"cost-type": {"cost-mode" : "ordinal",
"cost-metric" : "routingcost"},
"endpoints" : {
"srcs": [ "ipv4:192.0.2.2" ],
"dsts": [
"ipv4:192.0.2.89",
"ipv4:198.51.100.34",
"ipv4:203.0.113.45"
]
}
}
HTTP/1.1 200 OK
Content-Length: 274
Content-Type: application/alto-endpointcost+json
{
"meta" : {
"cost-type": {"cost-mode" : "ordinal",
"cost-metric" : "routingcost"
}
},
"endpoint-cost-map" : {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89" : 1,
"ipv4:198.51.100.34" : 2,
"ipv4:203.0.113.45" : 3
}
}
}
12. Use Cases
The sections below depict typical use cases. While these use cases
focus on peer-to-peer applications, ALTO can be applied to other
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environments such as CDNs [I-D.jenkins-alto-cdn-use-cases].
12.1. ALTO Client Embedded in P2P Tracker
Many currently-deployed P2P systems use a Tracker to manage swarms
and perform peer selection. Such a P2P Tracker can already use a
variety of information to perform peer selection to meet application-
specific goals. By acting as an ALTO Client, the P2P Tracker can use
ALTO information as an additional information source to enable more
network-efficient traffic patterns and improve application
performance.
A particular requirement of many P2P trackers is that they must
handle a large number of P2P clients. A P2P tracker can obtain and
locally store ALTO information (the Network Map and Cost Map) from
the ISPs containing the P2P clients, and benefit from the same
aggregation of network locations done by ALTO Servers.
.---------. (1) Get Network Map .---------------.
| | <----------------------> | |
| ALTO | | P2P Tracker |
| Server | (2) Get Cost Map | (ALTO Client) |
| | <----------------------> | |
`---------' `---------------'
^ |
(3) Get Peers | | (4) Selected Peer
| v List
.---------. .-----------.
| Peer 1 | <-------------- | P2P |
`---------' | Client |
. (5) Connect to `-----------'
. Selected Peers /
.---------. /
| Peer 50 | <------------------
`---------'
Figure 4: ALTO Client Embedded in P2P Tracker
Figure 4 shows an example use case where a P2P tracker is an ALTO
Client and applies ALTO information when selecting peers for its P2P
clients. The example proceeds as follows:
1. The P2P Tracker requests from the ALTO Server using the Network
Map query the Network Map covering all PIDs. The Network Map
includes the IP prefixes contained in each PID, allowing the P2P
tracker to locally map P2P clients into PIDs.
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2. The P2P Tracker requests from the ALTO Server the Cost Map
amongst all PIDs identified in the preceding step.
3. A P2P Client joins the swarm, and requests a peer list from the
P2P Tracker.
4. The P2P Tracker returns a peer list to the P2P client. The
returned peer list is computed based on the Network Map and Cost
Map returned by the ALTO Server, and possibly other information
sources. Note that it is possible that a tracker may use only
the Network Map to implement hierarchical peer selection by
preferring peers within the same PID and ISP.
5. The P2P Client connects to the selected peers.
Note that the P2P tracker may provide peer lists to P2P clients
distributed across multiple ISPs. In such a case, the P2P tracker
may communicate with multiple ALTO Servers.
12.2. ALTO Client Embedded in P2P Client: Numerical Costs
P2P clients may also utilize ALTO information themselves when
selecting from available peers. It is important to note that not all
P2P systems use a P2P tracker for peer discovery and selection.
Furthermore, even when a P2P tracker is used, the P2P clients may
rely on other sources, such as peer exchange and DHTs, to discover
peers.
When an P2P Client uses ALTO information, it typically queries only
the ALTO Server servicing its own ISP. The my-Internet view provided
by its ISP's ALTO Server can include preferences to all potential
peers.
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.---------. (1) Get Network Map .---------------.
| | <----------------------> | |
| ALTO | | P2P Client |
| Server | (2) Get Cost Map | (ALTO Client) |
| | <----------------------> | | .---------.
`---------' `---------------' <- | P2P |
.---------. / | ^ ^ | Tracker |
| Peer 1 | <-------------- | | \ `---------'
`---------' | (3) Gather Peers
. (4) Select Peers | | \
. and Connect / .--------. .--------.
.---------. / | P2P | | DHT |
| Peer 50 | <---------------- | Client | `--------'
`---------' | (PEX) |
`--------'
Figure 5: ALTO Client Embedded in P2P Client
Figure 5 shows an example use case where a P2P Client locally applies
ALTO information to select peers. The use case proceeds as follows:
1. The P2P Client requests the Network Map covering all PIDs from
the ALTO Server servicing its own ISP.
2. The P2P Client requests the Cost Map amongst all PIDs from the
ALTO Server. The Cost Map by default specifies numerical costs.
3. The P2P Client discovers peers from sources such as Peer Exchange
(PEX) from other P2P Clients, Distributed Hash Tables (DHT), and
P2P Trackers.
4. The P2P Client uses ALTO information as part of the algorithm for
selecting new peers, and connects to the selected peers.
12.3. ALTO Client Embedded in P2P Client: Ranking
It is also possible for a P2P Client to offload the selection and
ranking process to an ALTO Server. In this use case, the ALTO Client
embedded in the P2P Client gathers a list of known peers in the
swarm, and asks the ALTO Server to rank them. This document limits
the use case to when the P2P Client and the ALTO Server are deployed
by the same entity, and hence the P2P Client uses the ranking
provided by the ALTO Server directly.
As in the use case using numerical costs, the P2P Client typically
only queries the ALTO Server servicing its own ISP.
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.---------. .---------------.
| | | |
| ALTO | (2) Get Endpoint Ranking | P2P Client |
| Server | <----------------------> | (ALTO Client) |
| | | | .---------.
`---------' `---------------' <- | P2P |
.---------. / | ^ ^ | Tracker |
| Peer 1 | <-------------- | | \ `---------'
`---------' | (1) Gather Peers
. (3) Connect to | | \
. Selected Peers / .--------. .--------.
.---------. / | P2P | | DHT |
| Peer 50 | <---------------- | Client | `--------'
`---------' | (PEX) |
`--------'
Figure 6: ALTO Client Embedded in P2P Client: Ranking
Figure 6 shows an example of this scenario. The use case proceeds as
follows:
1. The P2P Client discovers peers from sources such as Peer Exchange
(PEX) from other P2P Clients, Distributed Hash Tables (DHT), and
P2P Trackers.
2. The P2P Client queries the ALTO Server's Ranking Service,
including discovered peers as the set of Destination Endpoints,
and indicates the 'ordinal' Cost Mode. The response indicates
the ranking of the candidate peers.
3. The P2P Client connects to the peers in the order specified in
the ranking.
13. Discussions
13.1. Discovery
The discovery mechanism by which an ALTO Client locates an
appropriate ALTO Server is out of scope for this document. This
document assumes that an ALTO Client can discover an appropriate ALTO
Server. Once it has done so, the ALTO Client may use the Information
Resource Directory (see Section 9.2) to locate an Information
Resource with the desired ALTO Information.
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13.2. Hosts with Multiple Endpoint Addresses
In practical deployments, a particular host can be reachable using
multiple addresses (e.g., a wireless IPv4 connection, a wireline IPv4
connection, and a wireline IPv6 connection). In general, the
particular network path followed when sending packets to the host
will depend on the address that is used. Network providers may
prefer one path over another. An additional consideration may be how
to handle private address spaces (e.g., behind carrier-grade NATs).
To support such behavior, this document allows multiple endpoint
addresses and address types. With this support, the ALTO Protocol
allows an ALTO Service Provider the flexibility to indicate
preferences for paths from an endpoint address of one type to an
endpoint address of a different type.
13.3. Network Address Translation Considerations
At this day and age of NAT v4<->v4, v4<->v6 [RFC6144], and possibly
v6<->v6 [RFC6296], a protocol should strive to be NAT friendly and
minimize carrying IP addresses in the payload, or provide a mode of
operation where the source IP address provide the information
necessary to the server.
The protocol specified in this document provides a mode of operation
where the source network location is computed by the ALTO Server
(i.e., the the Endpoint Cost Service) from the source IP address
found in the ALTO Client query packets. This is similar to how some
P2P Trackers (e.g., BitTorrent Trackers - see "Tracker HTTP/HTTPS
Protocol" in [BitTorrent]) operate.
There may be cases where an ALTO Client needs to determine its own IP
address, such as when specifying a source Endpoint Address in the
Endpoint Cost Service. It is possible that an ALTO Client has
multiple network interface addresses, and that some or all of them
may require NAT for connectivity to the public Internet.
If a public IP address is required for a network interface, the ALTO
Client SHOULD use the Session Traversal Utilities for NAT (STUN)
[RFC5389]. If using this method, the host MUST use the "Binding
Request" message and the resulting "XOR-MAPPED-ADDRESS" parameter
that is returned in the response. Using STUN requires cooperation
from a publicly accessible STUN server. Thus, the ALTO Client also
requires configuration information that identifies the STUN server,
or a domain name that can be used for STUN server discovery. To be
selected for this purpose, the STUN server needs to provide the
public reflexive transport address of the host.
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ALTO Clients should be cognizant that the network path between
Endpoints can depend on multiple factors, e.g., source address, and
destination address used for communication. An ALTO Server provides
information based on Endpoint Addresses (more generally, Network
Locations), but the mechanisms used for determining existence of
connectivity or usage of NAT between Endpoints are out of scope of
this document.
13.4. Endpoint and Path Properties
An ALTO Server could make available many properties about Endpoints
beyond their network location or grouping. For example, connection
type, geographical location, and others may be useful to
applications. This specification focuses on network location and
grouping, but the protocol may be extended to handle other Endpoint
properties.
14. IANA Considerations
This document defines registries for application/alto-* Media Types,
ALTO Cost Metric, ALTO Endpoint Property Types, ALTO Address Types,
and ALTO Error Codes. Initial values for the registries and the
process of future assignments are given below.
14.1. application/alto-* Media Types
This document requests the registration of multiple media types,
listed in Table 2.
+-------------+------------------------------+----------------+
| Type | Subtype | Specification |
+-------------+------------------------------+----------------+
| application | alto-directory+json | Section 9.2 |
| application | alto-networkmap+json | Section 11.2.1 |
| application | alto-networkmapfilter+json | Section 11.3.1 |
| application | alto-costmap+json | Section 11.2.3 |
| application | alto-costmapfilter+json | Section 11.3.2 |
| application | alto-endpointprop+json | Section 11.4.1 |
| application | alto-endpointpropparams+json | Section 11.4.1 |
| application | alto-endpointcost+json | Section 11.5.1 |
| application | alto-endpointcostparams+json | Section 11.5.1 |
| application | alto-error+json | Section 8.5 |
+-------------+------------------------------+----------------+
Table 2: ALTO Protocol Media Types.
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Type name: application
Subtype name: This documents requests the registration of multiple
subtypes, as listed in Table 2.
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: Encoding considerations are identical to
those specified for the 'application/json' media type. See
[RFC4627].
Security considerations: Security considerations relating to the
generation and consumption of ALTO Protocol messages are discussed
in Section 15.
Interoperability considerations: This document specifies format of
conforming messages and the interpretation thereof.
Published specification: This document is the specification for
these media types; see Table 2 for the section documenting each
media type.
Applications that use this media type: ALTO Servers and ALTO Clients
either standalone or embedded within other applications.
Additional information:
Magic number(s): n/a
File extension(s): This document uses the mime type to refer to
protocol messages and thus does not require a file extension.
Macintosh file type code(s): n/a
Person & email address to contact for further information: See
"Authors' Addresses" section.
Intended usage: COMMON
Restrictions on usage: n/a
Author: See "Authors' Addresses" section.
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Change controller: Internet Engineering Task Force
(mailto:iesg@ietf.org).
14.2. ALTO Cost Metric Registry
This document requests the creation of an ALTO Cost Metric registry,
listed in Table 3, to be maintained by IANA.
+-------------+---------------------+
| Identifier | Intended Semantics |
+-------------+---------------------+
| routingcost | See Section 6.1.1.1 |
| priv: | Private use |
+-------------+---------------------+
Table 3: ALTO Cost Metrics.
This registry serves two purposes. First, it ensures uniqueness of
identifiers referring to ALTO Cost Metrics. Second, it provides
references to particular semantics of allocated Cost Metrics to be
applied by both ALTO Servers and applications utilizing ALTO Clients.
New ALTO Cost Metrics are assigned after IETF Review [RFC5226] to
ensure that proper documentation regarding ALTO Cost Metric semantics
and security considerations has been provided. The RFCs documenting
the new metrics should be detailed enough to provide guidance to both
ALTO Service Providers and applications utilizing ALTO Clients as to
how values of the registered ALTO Cost Metric should be interpreted.
Updates and deletions of ALTO Cost Metrics follow the same procedure.
Registered ALTO Cost Metric identifiers MUST conform to the
syntactical requirements specified in Section 10.6. Identifiers are
to be recorded and displayed as strings.
As specified in Section 10.6, identifiers prefixed with 'priv:' are
reserved for Private Use.
Requests to add a new value to the registry MUST include the
following information:
o Identifier: The name of the desired ALTO Cost Metric.
o Intended Semantics: ALTO Costs carry with them semantics to guide
their usage by ALTO Clients. For example, if a value refers to a
measurement, the measurement units must be documented. For proper
implementation of the ordinal Cost Mode (e.g., by a third-party
service), it should be documented whether higher or lower values
of the cost are more preferred.
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o Security Considerations: ALTO Costs expose information to ALTO
Clients. As such, proper usage of a particular Cost Metric may
require certain information to be exposed by an ALTO Service
Provider. Since network information is frequently regarded as
proprietary or confidential, ALTO Service Providers should be made
aware of the security ramifications related to usage of a Cost
Metric.
This specification requests registration of the identifier
'routingcost'. Semantics for the this Cost Metric are documented in
Section 6.1.1.1, and security considerations are documented in
Section 15.3.
14.3. ALTO Endpoint Property Type Registry
This document requests the creation of an ALTO Endpoint Property
Types registry, listed in Table 4, to be maintained by IANA.
+------------+--------------------+
| Identifier | Intended Semantics |
+------------+--------------------+
| pid | See Section 7.1.1 |
| priv: | Private use |
+------------+--------------------+
Table 4: ALTO Endpoint Property Types.
The maintenance of this registry is similar to that of the preceding
ALTO Cost Metrics. That is, the registry will be maintained by IANA,
subject to the description in Section 10.8.2.
New Endpoint Property Types are assigned after IETF Review [RFC5226]
to ensure that proper documentation regarding ALTO Endpoint Property
Type semantics and security considerations has been provided.
Updates and deletions of ALTO Endpoint Property Type follow the same
procedure.
Registered ALTO Endpoint Property Type identifiers MUST conform to
the syntactical requirements specified in Section 10.8.1.
Identifiers are to be recorded and displayed as strings.
As specified in Section 10.8.1, identifiers prefixed with 'priv:' are
reserved for Private Use.
Requests to add a new value to the registry MUST include the
following information:
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o Identifier: The name of the desired ALTO Endpoint Property Type.
o Intended Semantics: ALTO Endpoint Properties carry with them
semantics to guide their usage by ALTO Clients. Hence, a document
defining a new type should provide guidance to both ALTO Service
Providers and applications utilizing ALTO Clients as to how values
of the registered ALTO Endpoint Property should be interpreted.
For example, if a value refers to a measurement, the measurement
units must be documented.
o Security Considerations: ALTO Endpoint Properties expose
information to ALTO Clients. ALTO Service Providers should be
made aware of the security ramifications related to the exposure
of an Endpoint Property.
In particular, the request should discuss the sensitivity of the
information, and why such sensitive information is required for ALTO-
based operations. It may recommend that ISP provide mechanisms for
users to grant or deny consent to such information sharing.
Limitation to a trust domain being a type of consent bounding.
A request defining new Endpoint Properties should focus on exposing
attributes of endpoints that are related to the goals of ALTO --
optimization of application-layer traffic -- as opposed to more
general properties of endpoints. Maintaining this focus on
technical, network-layer data will also help extension developers
avoid the privacy concerns associated with publishing information
about endpoints. For example:
o An extension to indicate the capacity of a server would likely be
appropriate, since server capacities can be used by a client to
choose between multiple equivalent servers. In addition, these
properties are unlikely to be viewed as private information.
o An extension to indicate the geolocation of endpoints might be
appropriate. In some cases, a certain level of geolocation (e.g.,
to the country level) can be useful for selecting content sources.
More precise geolocation, however, is not relevant to content
delivery, and is typically considered private.
o An extension indicating demographic attributes of the owner of an
endpoint (e.g., age, sex, income) would not be appropriate,
because these attributes are not related to delivery optimization,
and because they are clearly private data.
This specification requests registration of the identifier 'pid'.
Semantics for this property is documented in Section 7.1.1, and
security considerations are documented in Section 15.4.
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14.4. ALTO Address Type Registry
This document requests the creation of an ALTO Address Type registry,
listed in Table 5, to be maintained by IANA.
+------------+-----------------+-----------------+------------------+
| Identifier | Address | Prefix Encoding | Mapping to/from |
| | Encoding | | IPv4/v6 |
+------------+-----------------+-----------------+------------------+
| ipv4 | See | See | Direct mapping |
| | Section 10.4.3 | Section 10.4.4 | to IPv4 |
| ipv6 | See | See | Direct mapping |
| | Section 10.4.3 | Section 10.4.4 | to IPv6 |
+------------+-----------------+-----------------+------------------+
Table 5: ALTO Address Types.
This registry serves two purposes. First, it ensures uniqueness of
identifiers referring to ALTO Address Types. Second, it states the
requirements for allocated Address Type identifiers.
New ALTO Address Types are assigned after IETF Review [RFC5226] to
ensure that proper documentation regarding the new ALTO Address Types
and their security considerations has been provided. RFCs defining
new Address Types should indicate how an address of a registered type
is encoded as an EndpointAddr and, if possible, a compact method
(e.g., IPv4 and IPv6 prefixes) for encoding a set of addresses as an
EndpointPrefix. Updates and deletions of ALTO Address Types follow
the same procedure.
Registered ALTO Address Type identifiers MUST conform to the
syntactical requirements specified in Section 10.4.2. Identifiers
are to be recorded and displayed as strings.
Requests to add a new value to the registry MUST include the
following information:
o Identifier: The name of the desired ALTO Address Type.
o Endpoint Address Encoding: The procedure for encoding an address
of the registered type as an EndpointAddr (see Section 10.4.3).
o Endpoint Prefix Encoding: The procedure for encoding a set of
addresses of the registered type as an EndpointPrefix (see
Section 10.4.4). If no such compact encoding is available, the
same encoding used for a singular address may be used. In such a
case, it must be documented that sets of addresses of this type
always have exactly one element.
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o Mapping to/from IPv4/IPv6 Addresses: If possible, a mechanism to
map addresses of the registered type to and from IPv4 or IPv6
addresses should be specified.
o Security Considerations: In some usage scenarios, Endpoint
Addresses carried in ALTO Protocol messages may reveal information
about an ALTO Client or an ALTO Service Provider. Applications
and ALTO Service Providers using addresses of the registered type
should be made aware of how (or if) the addressing scheme relates
to private information and network proximity.
This specification requests registration of the identifiers 'ipv4'
and 'ipv6', as shown in Table 5.
14.5. ALTO Error Code Registry
This document requests the creation of an ALTO Error Code registry,
to be maintained by IANA. Initial values are listed in Table 1, and
recommended usage of the Error Codes is specified in Section 8.5.2.
Although the Error Codes defined in Table 1 are already quite
complete, future extensions may define new Error Codes. The ALTO
Error Code registry ensures the uniqueness of Error Codes when new
Error Codes are added.
New ALTO Error Codes are assigned after IETF Review [RFC5226] to
ensure that proper documentation regarding the new ALTO Error Codes
and their usage has been provided.
A request to add a new ALTO Error Code to the registry MUST include
the following information:
o Error Code: A string starting with E_ to indicate the error.
o Intended Usage: ALTO Error Codes carry with them semantics to
guide their usage by ALTO Servers and Clients. In particular, if
a new Error Code indicates conditions that overlap with those of
an existing ALTO Error Code, recommended usage of the new Error
Code should be specified.
15. Security Considerations
Some environments and use cases of ALTO require consideration of
security attacks on ALTO Servers and Clients. In order to support
those environments interoperably, the ALTO requirements document
[RFC6708] outlines minimum-to-implement authentication and other
security requirements. This document considers the following threats
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and protection strategies.
15.1. Authenticity and Integrity of ALTO Information
15.1.1. Risk Scenarios
An attacker may want to provide false or modified ALTO Information
Resources or Information Resource Directory to ALTO Clients to
achieve certain malicious goals. As an example, an attacker may
provide false endpoint properties. For example, suppose that a
network supports an endpoint property named "hasQuota" which reports
whether an endpoint has usage quota. An attacker may want to
generate a false reply to lead to unexpected charges to the endpoint.
An attack may also want to provide false Cost Map. For example, by
faking a Cost Map that highly prefers a small address range or a
single address, the attacker may be able to turn a distributed
application into a Distributed Denial of Service (DDoS) tool.
Depending on the network scenario, an attacker can attack
authenticity and integrity of ALTO Information Resources using
various techniques, including, but not limited to, sending forged
DHCP replies in an Ethernet, DNS poisoning, and installing a
transparent HTTP proxy that does some modifications.
15.1.2. Protection Strategies
ALTO protects the authenticity and integrity of ALTO Information
(both Information Directory and individual Information Resources) by
leveraging the authenticity and integrity mechanisms in TLS (see
Section 8.3.5).
ALTO Providers who request server certificates and certification
authorities who issue ALTO-specific certificates SHOULD consider the
recommendations and guidelines defined in [RFC6125].
Software engineers developing and service providers deploying ALTO
should make themselves familiar with possibly updated standards
documents as well as up-to-date Best Current Practices on configuring
HTTP over TLS.
15.1.3. Limitations
The protection of HTTP over TLS for ALTO depends on that the domain
name in the URI for the Information Resources is not comprised. This
will depend on the protection implemented by service discovery.
A deployment scenario may require redistribution of ALTO information
to improve scalability. When authenticity and integrity of ALTO
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information are still required, then ALTO Clients obtaining ALTO
information through redistribution must be able to validate the
received ALTO information. Support for this validation is not
provided in this document, but may be provided by extension
documents.
15.2. Potential Undesirable Guidance from Authenticated ALTO
Information
15.2.1. Risk Scenarios
The ALTO Service makes it possible for an ALTO Provider to influence
the behavior of network applications. An ALTO Provider may be
hostile to some applications and hence try to use ALTO Information
Resources to achieve certain goals [RFC5693]: "redirecting
applications to corrupted mediators providing malicious content, or
applying policies in computing Cost Map based on criteria other than
network efficiency." See [I-D.ietf-alto-deployments] for additional
discussions on faked ALTO Guidance.
A related scenario is that an ALTO Server could unintentionally give
"bad" guidance. For example, if many ALTO Clients follow the Cost
Map or Endpoint Cost guidance without doing additional sanity checks
or adaptation, more preferable hosts and/or links could get
overloaded while less preferable ones remain idle; see AR-14 of
[RFC6708] for related application considerations.
15.2.2. Protection Strategies
To protect applications from undesirable ALTO Information Resources,
it is important to note that there is no protocol mechanism to
require conforming behaviors on how applications use ALTO Information
Resources. An application using ALTO may consider including a
mechanism to detect misleading or undesirable results from using ALTO
Information Resources. For example, if throughput measurements do
not show "better-than-random" results when using the Cost Map to
select resource providers, the application may want to disable ALTO
usage or switch to an external ALTO Server provided by an
"independent organization" (see AR-20 and AR-21 in [RFC6708]). If
the first ALTO Server is provided by the access network service
provider and the access network service provider tries to redirect
access to the external ALTO Server back to the provider's ALTO Server
or try to tamper with the responses, the preceding authentication and
integrity protection can detect such a behavior.
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15.3. Confidentiality of ALTO Information
15.3.1. Risk Scenarios
Although in many cases ALTO Information Resources may be regarded as
non-confidential information, there are deployment cases where ALTO
Information Resources can be sensitive information that can pose
risks if exposed to unauthorized parties. This document discusses
the risks and protection strategies for such deployment scenarios.
For example, an attacker may infer details regarding the topology,
status, and operational policies of a network through the Network and
Cost Maps. As a result, a sophisticated attacker may be able to
infer more fine-grained topology information than an ISP hosting an
ALTO Server intends to disclose. The attacker can leverage the
information to mount effective attacks such as focusing on high-cost
links.
Revealing some endpoint properties may also reveal additional
information than the Provider intended. For example, when adding the
line bitrate as one endpoint property, such information may be
potentially linked to the income of the habitants at the network
location of an endpoint.
In [RFC6708] Section 5.2.1, three types of risks associated with the
confidentiality of ALTO Information Resources are identified: risk
type (1) Excess disclosure of the ALTO service provider's data to an
authorized ALTO Client; risk type (2) Disclosure of the ALTO service
provider's data (e.g., network topology information or endpoint
addresses) to an unauthorized third party; and risk type (3) Excess
retrieval of the ALTO service provider's data by collaborating ALTO
Clients. [I-D.ietf-alto-deployments] also discusses information
leakage from ALTO.
15.3.2. Protection Strategies
To address risk types (1) and (3), the Provider of an ALTO Server
must be cognizant that the network topology and provisioning
information provided through ALTO may lead to attacks. ALTO does not
require any particular level of details of information disclosure,
and hence the Provider should evaluate how much information is
revealed and the associated risks.
To address risk type (2), the ALTO Protocol needs confidentiality.
Since ALTO requires that HTTP over TLS must be supported, the
confidentiality mechanism is provided by HTTP over TLS.
For deployment scenarios where client authentication is desired to
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address risk type (2), ALTO requires that HTTP Digestion
Authentication is supported to achieve ALTO Client Authentication to
limit the number of parties with whom ALTO information is directly
shared. TLS Client Authentication may also be supported. Depending
on the use-case and scenario, an ALTO Server may apply other access
control techniques to restrict access to its services. Access
control can also help to prevent Denial-of-Service attacks by
arbitrary hosts from the Internet. See [I-D.ietf-alto-deployments]
for a more detailed discussion on this issue.
See Section 14.3 on guidelines when registering Endpoint Properties
to protect endpoint privacy.
15.3.3. Limitations
ALTO Information Providers should be cognizant that encryption only
protects ALTO information until it is decrypted by the intended ALTO
Client. Digital Rights Management (DRM) techniques and legal
agreements protecting ALTO information are outside of the scope of
this document.
15.4. Privacy for ALTO Users
15.4.1. Risk Scenarios
The ALTO Protocol provides mechanisms in which the ALTO Client
serving a user can send messages containing Network Location
Identifiers (IP addresses or fine-grained PIDs) to the ALTO Server.
This is particularly true for the Endpoint Property, Endpoint Cost,
and fine-grained Filtered Map services. The ALTO Server or a third-
party who is able to intercept such messages can store and process
obtained information in order to analyze user behaviors and
communication patterns. The analysis may correlate information
collected from multiple clients to deduce additional application/
content information. Such analysis can lead to privacy risks. For a
more comprehensive classification of related risk scenarios, see
cases 4, 5, and 6 in [RFC6708], Section 5.2.
15.4.2. Protection Strategies
To protect user privacy, an ALTO Client should be cognizant about
potential ALTO Server tracking through client queries, e.g., by using
HTTP cookies. The ALTO Protocol as defined by this document does not
rely on HTTP cookies. ALTO Clients MAY decide to not return cookies
received from the server, in order to make tracking more difficult.
However, this might break protocol extensions that are beyond the
scope of this document.
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An ALTO Client may consider the possibility of relying only on
Network Map for PIDs and Cost Map amongst PIDs to avoid passing IP
addresses of other endpoints (e.g., peers) to the ALTO Server. When
specific IP addresses are needed (e.g., when using the Endpoint Cost
Service), an ALTO Client SHOULD minimize the amount of information
sent in IP addresses. For example, the ALTO Client may consider
obfuscation techniques such as specifying a broader address range
(i.e., a shorter prefix length) or by zeroing out or randomizing the
last few bits of IP addresses. Note that obfuscation may yield less
accurate results.
15.5. Availability of ALTO Service
15.5.1. Risk Scenarios
An attacker may want to disable ALTO Service as a way to disable
network guidance to large scale applications.In particular, queries
which can be generated with low effort but result in expensive
workloads at the ALTO Server could be exploited for Denial-of-Service
attacks. For instance, a simple ALTO query with n Source Network
Locations and m Destination Network Locations can be generated fairly
easily but results in the computation of n*m Path Costs between pairs
by the ALTO Server (see Section 5.2).
15.5.2. Protection Strategies
ALTO Provider should be cognizant of the workload at the ALTO Server
generated by certain ALTO Queries, such as certain queries to the Map
Service, the Map Filtering Service and the Endpoint Cost (Ranking)
Service. One way to limit Denial-of-Service attacks is to employ
access control to the ALTO Server. The ALTO Server can also indicate
overload and reject repeated requests that can cause availability
problems. More advanced protection schemes such as computational
puzzles [I-D.jennings-sip-hashcash] may be considered in an extension
document.
An ALTO Provider should also leverage the fact that the Map Service
allows ALTO Servers to pre-generate maps that can be distributed to
many ALTO Clients.
16. Manageability Considerations
This section details operations and management considerations based
on existing deployments and discussions during protocol development.
It also indicates where extension documents are expected to provide
appropriate functionality discussed in [RFC5706] as additional
deployment experience becomes available.
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16.1. Operations
16.1.1. Installation and Initial Setup
The ALTO Protocol is based on HTTP. Thus, configuring an ALTO Server
may require configuring the underlying HTTP server implementation to
define appropriate security policies, caching policies, performance
settings, etc.
Additionally, an ALTO Service Provider will need to configure the
ALTO information to be provided by the ALTO Server. The granularity
of the topological map and the cost map is left to the specific
policies of the ALTO Service Provider. However, a reasonable default
may include two PIDs, one to hold the endpoints in the provider's
network and the second PID to represent full IPv4 and IPv6
reachability (see Section 11.2.2), with the cost between each source/
destination PID set to 1. Another operational issue that the ALTO
Service Provider needs to consider is that the filtering service can
degenerate into a full map service when the filtering input is empty.
Although this choice as the degeneration behavior provides
continuity, the computational and network load of serving full maps
to a large number of ALTO Clients should be considered.
Implementers employing an ALTO Client should attempt to automatically
discover an appropriate ALTO Server. Manual configuration of the
ALTO Server location may be used where automatic discovery is not
appropriate. Methods for automatic discovery and manual
configuration are discussed in [I-D.ietf-alto-server-discovery].
Specifications for underlying protocols (e.g., TCP, HTTP, TLS) should
be consulted for their available settings and proposed default
configurations.
16.1.2. Migration Path
This document does not detail a migration path for ALTO Servers since
there is no previous standard protocol providing the similar
functionality.
There are existing applications making use of network information
discovered from other entities such as whois, geo-location databases,
or round-trip time measurements, etc. Such applications should
consider using ALTO as an additional source of information; ALTO need
not be the sole source of network information.
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16.1.3. Dependencies on Other Protocols and Functional Components
The ALTO Protocol assumes that HTTP client and server implementations
exist. It also assumes that JSON encoder and decoder implementations
exist.
An ALTO Server assumes that it can gather sufficient information to
populate Network and Cost maps. "Sufficient information" is
dependent on the information being exposed, but likely includes
information gathered from protocols such as IGP and EGP Routing
Information Bases (see Figure 1). Specific mechanisms have been
proposed (e.g., [I-D.medved-alto-svr-apis]) and are expected to be
provided in extension documents.
16.1.4. 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 ALTO
Service Provider should ensure that appropriate information is being
exposed. Privacy implications for ISPs are discussed in
Section 15.3.
An ALTO Service Provider should consider how to measure impacts on
(or integration with) traffic engineering, in addition to monitoring
correctness and responsiveness of ALTO Servers. The measurement of
impacts can be challenging because ALTO-enabled applications may not
provide related information back to the ALTO Service Provider.
Furthermore, the measurement of an ALTO Service Provider may show
that ALTO Clients are not bound to ALTO Server guidance as ALTO is
only one source of information.
While it can be challenging to measure the impact of ALTO guidance,
there exist some possible techniques. 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.
Both ALTO Service Providers and those using ALTO Clients should be
aware of the impact of incorrect or faked guidance (see
[I-D.ietf-alto-deployments]).
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16.2. Management
16.2.1. Management Interoperability
A common management API would be desirable given that ALTO Servers
may typically be configured with dynamic data from various sources,
and ALTO Servers are intended to scale horizontally for fault-
tolerance and reliability. A specific API or protocol is outside the
scope of this document, but may be provided by an extension document.
Logging is an important functionality for ALTO Servers and, depending
on the deployment, ALTO Clients. Logging should be done via syslog
[RFC5424].
16.2.2. Management Information
A Management Information Model (see Section 3.2 of [RFC5706]) is not
provided by this document, but should be included or referenced by
any extension documenting an ALTO-related management API or protocol.
16.2.3. Fault Management
An ALTO Service Provider should monitor whether any ALTO Servers have
failed. See Section 16.2.5 for related metrics which may indicate
server failures.
16.2.4. Configuration Management
Standardized approaches and protocols to configuration management for
ALTO are outside the scope of this document, but this document does
outline high-level principles suggested for future standardization
efforts.
An ALTO Server requires at least the following logical inputs:
o Data sources from which ALTO Information is derived. This can
either be raw network information (e.g., from routing elements) or
pre-processed ALTO-level information in the form of a Network Map,
Cost Map, etc.
o Algorithms for computing the ALTO information returned to clients.
These could either return information from a database, or
information customized for each client.
o Security policies mapping potential clients to the information
that they have privilege to access.
Multiple ALTO Servers can be deployed for scalability. A centralized
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configuration database may be used to ensure they are providing the
desired ALTO information with appropriate security controls. The
ALTO information (e.g., Network Maps and Cost Maps) being served by
each ALTO Server, as well as security policies (HTTP authentication,
TLS client and server authentication, TLS encryption parameters)
intended to serve the same information should be monitored for
consistency.
16.2.5. Performance Management
An exhaustive list of desirable performance information from ALTO
Servers and ALTO Clients are outside of the scope of this document.
The following is a list of suggested ALTO-specific metrics to be
monitored based on the existing deployment and protocol development
experience:
o Requests and responses for each service listed in a 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)
16.2.6. Security Management
Section 15 documents ALTO-specific security considerations.
Operators should configure security policies with those in mind.
Readers should refer to HTTP [RFC2616] and TLS [RFC5246] and related
documents for mechanisms available for configuring security policies.
Other appropriate security mechanisms (e.g., physical security,
firewalls, etc) should also be considered.
17. References
17.1. Normative References
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
17.2. Informative References
[BitTorrent]
"Bittorrent Protocol Specification v1.0",
<http://wiki.theory.org/BitTorrentSpecification>.
[Fielding-Thesis]
Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", University of
California, Irvine, Dissertation 2000, 2000.
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[I-D.akonjang-alto-proxidor]
Akonjang, O., Feldmann, A., Previdi, S., Davie, B., and D.
Saucez, "The PROXIDOR Service",
draft-akonjang-alto-proxidor-00 (work in progress),
March 2009.
[I-D.ietf-alto-deployments]
Stiemerling, M., Kiesel, S., Previdi, S., and M. Scharf,
"ALTO Deployment Considerations",
draft-ietf-alto-deployments-09 (work in progress),
February 2014.
[I-D.ietf-alto-server-discovery]
Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
S. Yongchao, "ALTO Server Discovery",
draft-ietf-alto-server-discovery-10 (work in progress),
September 2013.
[I-D.ietf-httpbis-p2-semantics]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-26 (work in progress),
February 2014.
[I-D.jenkins-alto-cdn-use-cases]
Niven-Jenkins, B., Watson, G., Bitar, N., Medved, J., and
S. Previdi, "Use Cases for ALTO within CDNs",
draft-jenkins-alto-cdn-use-cases-03 (work in progress),
June 2012.
[I-D.jennings-sip-hashcash]
Jennings, C., "Computational Puzzles for SPAM Reduction in
SIP", draft-jennings-sip-hashcash-06 (work in progress),
July 2007.
[I-D.medved-alto-svr-apis]
Medved, J., Ward, D., Peterson, J., Woundy, R., and D.
McDysan, "ALTO Network-Server and Server-Server APIs",
draft-medved-alto-svr-apis-00 (work in progress),
March 2011.
[I-D.p4p-framework]
Alimi, R., Pasko, D., Popkin, L., Wang, Y., and Y. Yang,
"P4P: Provider Portal for P2P Applications",
draft-p4p-framework-00 (work in progress), November 2008.
[I-D.saumitra-alto-multi-ps]
Das, S., Narayanan, V., and L. Dondeti, "ALTO: A Multi
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Dimensional Peer Selection Problem",
draft-saumitra-alto-multi-ps-00 (work in progress),
October 2008.
[I-D.saumitra-alto-queryresponse]
Das, S. and V. Narayanan, "A Client to Service Query
Response Protocol for ALTO",
draft-saumitra-alto-queryresponse-00 (work in progress),
March 2009.
[I-D.shalunov-alto-infoexport]
Shalunov, S., Penno, R., and R. Woundy, "ALTO Information
Export Service", draft-shalunov-alto-infoexport-00 (work
in progress), October 2008.
[I-D.wang-alto-p4p-specification]
Wang, Y., Alimi, R., Pasko, D., Popkin, L., and Y. Yang,
"P4P Protocol Specification",
draft-wang-alto-p4p-specification-00 (work in progress),
March 2009.
[IEEE.754.2008]
Institute of Electrical and Electronics Engineers,
"Standard for Binary Floating-Point Arithmetic", IEEE
Standard 754, August 2008.
[P4P-SIGCOMM08]
Xie, H., Yang, Y., Krishnamurthy, A., Liu, Y., and A.
Silberschatz, "P4P: Provider Portal for (P2P)
Applications", SIGCOMM 2008, August 2008.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
October 2009.
[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
RFC 5706, November 2009.
[RFC6144] Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation", RFC 6144, April 2011.
[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
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Translation", RFC 6296, June 2011.
[RFC6708] Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
Y. Yang, "Application-Layer Traffic Optimization (ALTO)
Requirements", RFC 6708, September 2012.
Appendix A. Acknowledgments
Thank you to Sebastian Kiesel (University of Stuttgart) and Jan
Seedorf (NEC) for substantial contributions to the Security
Considerations section. Ben Niven-Jenkins (Velocix), Michael Scharf
and Sabine Randriamasy (Alcatel-Lucent) gave substantial feedback and
suggestions on the protocol design. We are particularly grateful to
the substantial contributions of Wendy Roome (Alcatel-Lucent).
We would like to thank the following people whose input and
involvement was indispensable in achieving this merged proposal:
Obi Akonjang (DT Labs/TU Berlin),
Saumitra M. Das (Qualcomm Inc.),
Syon Ding (China Telecom),
Doug Pasko (Verizon),
Laird Popkin (Pando Networks),
Satish Raghunath (Juniper Networks),
Albert Tian (Ericsson/Redback),
Yu-Shun Wang (Microsoft),
David Zhang (PPLive),
Yunfei Zhang (China Mobile).
We would also like to thank the following additional people who were
involved in the projects that contributed to this merged document:
Alex Gerber (ATT), Chris Griffiths (Comcast), Ramit Hora (Pando
Networks), Arvind Krishnamurthy (University of Washington), Marty
Lafferty (DCIA), Erran Li (Bell Labs), Jin Li (Microsoft), Y. Grace
Liu (IBM Watson), Jason Livingood (Comcast), Michael Merritt (ATT),
Ingmar Poese (DT Labs/TU Berlin), James Royalty (Pando Networks),
Damien Saucez (UCL) Thomas Scholl (ATT), Emilio Sepulveda
(Telefonica), Avi Silberschatz (Yale University), Hassan Sipra (Bell
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Canada), Georgios Smaragdakis (DT Labs/TU Berlin), Haibin Song
(Huawei), Oliver Spatscheck (ATT), See-Mong Tang (Microsoft), Jia
Wang (ATT), Hao Wang (Yale University), Ye Wang (Yale University),
Haiyong Xie (Yale University).
Appendix B. Design History and Merged Proposals
The ALTO Protocol specified in this document consists of
contributions from
o P4P [I-D.p4p-framework], [P4P-SIGCOMM08],
[I-D.wang-alto-p4p-specification];
o ALTO Info-Export [I-D.shalunov-alto-infoexport];
o Query/Response [I-D.saumitra-alto-queryresponse],
[I-D.saumitra-alto-multi-ps]; and
o Proxidor [I-D.akonjang-alto-proxidor].
Appendix C. Authors
[[CmtAuthors: RFC Editor: Please move information in this section to
the Authors' Addresses section at publication time.]]
Sebastian Kiesel
University of Stuttgart Computing Center
Networks and Communication Systems Department
Allmandring 30
70550 Stuttgart
Germany
Email: ietf-alto@skiesel.de
Stefano Previdi
Cisco
Email: sprevidi@cisco.com
Wendy Roome
Alcatel Lucent
Email: w.roome@alcatel-lucent.com
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Stanislav Shalunov
BitTorrent
Email: shalunov@bittorrent.com
Richard Woundy
Comcast
Richard_Woundy@cable.comcast.com
Authors' Addresses
Richard Alimi (editor)
Google
1600 Amphitheatre Parkway
Mountain View CA
USA
Email: ralimi@google.com
Reinaldo Penno (editor)
Cisco Systems
170 West Tasman Dr
San Jose CA
USA
Email: repenno@cisco.com
Y. Richard Yang (editor)
Yale University
51 Prospect St
New Haven CT
USA
Email: yry@cs.yale.edu
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