ALTO Protocol
draft-ietf-alto-protocol-15
The information below is for an old version of the document.
| Document | Type | This is an older version of an Internet-Draft that was ultimately published as an RFC. | |
|---|---|---|---|
| Authors | Richard Alimi , Reinaldo Penno , Y. Richard Yang | ||
| Last updated | 2013-05-08 | ||
| Replaces | draft-penno-alto-protocol | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
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| Stream | WG state | In WG Last Call | |
| Document shepherd | (None) | ||
| IESG | IESG state | AD is watching | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Spencer Dawkins | ||
| IESG note | Enrico Marocco (enrico.marocco@telecomitalia.it) is the document shepherd. | ||
| Send notices to | alto-chairs@tools.ietf.org, draft-ietf-alto-protocol@tools.ietf.org |
draft-ietf-alto-protocol-15
ALTO WG R. Alimi, Ed.
Internet-Draft Google
Intended status: Standards Track R. Penno, Ed.
Expires: November 9, 2013 Cisco Systems
Y. Yang, Ed.
Yale University
May 8, 2013
ALTO Protocol
draft-ietf-alto-protocol-15.txt
Abstract
Applications using the Internet already have access to 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 application
clients. What is missing is knowledge of the underlying network
topologies from the point of view of ISPs (henceforth referred as
Providers). In other words, what a Provider 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 the network
service providers (e.g., Internet service providers), content service
providers and third parties 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 is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 9, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1. Background and Problem Statement . . . . . . . . . . . . . 6
1.2. Solution Benefits . . . . . . . . . . . . . . . . . . . . 6
1.2.1. Service Providers . . . . . . . . . . . . . . . . . . 6
1.2.2. Applications . . . . . . . . . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Endpoint . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Endpoint Address . . . . . . . . . . . . . . . . . . . . . 7
2.3. ASN . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4. Network Location . . . . . . . . . . . . . . . . . . . . . 8
2.5. ALTO Information . . . . . . . . . . . . . . . . . . . . . 8
2.6. ALTO Information Base . . . . . . . . . . . . . . . . . . 8
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. ALTO Service and Protocol Scope . . . . . . . . . . . . . 8
3.2. ALTO Information Reuse and Redistribution . . . . . . . . 10
4. ALTO Information Service Framework . . . . . . . . . . . . . . 10
4.1. ALTO Information Services . . . . . . . . . . . . . . . . 11
4.1.1. Map Service . . . . . . . . . . . . . . . . . . . . . 11
4.1.2. Map Filtering Service . . . . . . . . . . . . . . . . 11
4.1.3. Endpoint Property Service . . . . . . . . . . . . . . 12
4.1.4. Endpoint Cost Service . . . . . . . . . . . . . . . . 12
5. Network Map . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Provider-defined Identifier (PID) . . . . . . . . . . . . 12
5.2. Endpoint Addresses . . . . . . . . . . . . . . . . . . . . 13
5.2.1. IP Addresses . . . . . . . . . . . . . . . . . . . . . 13
5.3. Example Network Map . . . . . . . . . . . . . . . . . . . 13
6. Cost Map . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Cost Types . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1.1. Cost Metric . . . . . . . . . . . . . . . . . . . . . 15
6.1.2. Cost Mode . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Cost Map Structure . . . . . . . . . . . . . . . . . . . . 16
6.3. Network Map and Cost Map Dependency . . . . . . . . . . . 17
7. Endpoint Properties . . . . . . . . . . . . . . . . . . . . . 17
7.1. Endpoint Property Type . . . . . . . . . . . . . . . . . . 17
7.1.1. Endpoint Property Type: pid . . . . . . . . . . . . . 18
8. Protocol Specification: General Processing . . . . . . . . . . 18
8.1. Overall Design . . . . . . . . . . . . . . . . . . . . . . 18
8.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.3. Basic Operation . . . . . . . . . . . . . . . . . . . . . 19
8.3.1. Client Discovering Information Resources . . . . . . . 19
8.3.2. Client Requesting Information Resources . . . . . . . 20
8.3.3. Server Responding to IR Request . . . . . . . . . . . 20
8.3.4. Client Handling Server Response . . . . . . . . . . . 21
8.3.5. Authentication and Encryption . . . . . . . . . . . . 21
8.3.6. HTTP Cookies . . . . . . . . . . . . . . . . . . . . . 21
8.3.7. Parsing . . . . . . . . . . . . . . . . . . . . . . . 22
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8.4. Information Resource: Attributes . . . . . . . . . . . . . 22
8.4.1. Media Type . . . . . . . . . . . . . . . . . . . . . . 22
8.4.2. Capabilities . . . . . . . . . . . . . . . . . . . . . 22
8.4.3. Accepts Input Parameters . . . . . . . . . . . . . . . 22
8.5. Information Resource Directory . . . . . . . . . . . . . . 22
8.5.1. Media Type . . . . . . . . . . . . . . . . . . . . . . 23
8.5.2. Encoding . . . . . . . . . . . . . . . . . . . . . . . 23
8.5.3. Example . . . . . . . . . . . . . . . . . . . . . . . 24
8.5.4. Multiple Choices and OPTIONS . . . . . . . . . . . . . 26
8.5.5. Usage Considerations . . . . . . . . . . . . . . . . . 29
8.6. Information Resource: Content Encoding . . . . . . . . . . 29
8.6.1. Meta Information . . . . . . . . . . . . . . . . . . . 30
8.6.2. Data Information . . . . . . . . . . . . . . . . . . . 30
8.6.3. Example . . . . . . . . . . . . . . . . . . . . . . . 30
8.7. Protocol Errors . . . . . . . . . . . . . . . . . . . . . 30
8.7.1. Media Type . . . . . . . . . . . . . . . . . . . . . . 31
8.7.2. Resource Format and Error Codes . . . . . . . . . . . 31
8.7.3. Overload Conditions and Server Unavailability . . . . 32
9. Protocol Specification: Basic ALTO Data Types . . . . . . . . 32
9.1. PID Name . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.2. Version Tag . . . . . . . . . . . . . . . . . . . . . . . 32
9.3. Endpoints . . . . . . . . . . . . . . . . . . . . . . . . 33
9.3.1. Address Type . . . . . . . . . . . . . . . . . . . . . 33
9.3.2. Endpoint Address . . . . . . . . . . . . . . . . . . . 33
9.3.3. Endpoint Prefixes . . . . . . . . . . . . . . . . . . 34
9.3.4. Endpoint Address Group . . . . . . . . . . . . . . . . 34
9.4. Cost Mode . . . . . . . . . . . . . . . . . . . . . . . . 35
9.5. Cost Metric . . . . . . . . . . . . . . . . . . . . . . . 35
9.6. Cost Type . . . . . . . . . . . . . . . . . . . . . . . . 36
9.7. Endpoint Property . . . . . . . . . . . . . . . . . . . . 36
10. Protocol Specification: Service Information Resources . . . . 36
10.1. Map Service . . . . . . . . . . . . . . . . . . . . . . . 37
10.1.1. Network Map . . . . . . . . . . . . . . . . . . . . . 37
10.1.2. Cost Map . . . . . . . . . . . . . . . . . . . . . . . 39
10.2. Map Filtering Service . . . . . . . . . . . . . . . . . . 42
10.2.1. Filtered Network Map . . . . . . . . . . . . . . . . . 42
10.2.2. Filtered Cost Map . . . . . . . . . . . . . . . . . . 44
10.3. Endpoint Property Service . . . . . . . . . . . . . . . . 48
10.3.1. Endpoint Property . . . . . . . . . . . . . . . . . . 48
10.4. Endpoint Cost Service . . . . . . . . . . . . . . . . . . 51
10.4.1. Endpoint Cost . . . . . . . . . . . . . . . . . . . . 51
11. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 54
11.1. ALTO Client Embedded in P2P Tracker . . . . . . . . . . . 55
11.2. ALTO Client Embedded in P2P Client: Numerical Costs . . . 56
11.3. ALTO Client Embedded in P2P Client: Ranking . . . . . . . 57
12. Discussions . . . . . . . . . . . . . . . . . . . . . . . . . 58
12.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 58
12.2. Hosts with Multiple Endpoint Addresses . . . . . . . . . . 59
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12.3. Network Address Translation Considerations . . . . . . . . 59
12.4. Endpoint and Path Properties . . . . . . . . . . . . . . . 60
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 60
13.1. application/alto-* Media Types . . . . . . . . . . . . . . 60
13.2. ALTO Cost Metric Registry . . . . . . . . . . . . . . . . 61
13.3. ALTO Endpoint Property Type Registry . . . . . . . . . . . 63
13.4. ALTO Address Type Registry . . . . . . . . . . . . . . . . 63
13.5. ALTO Error Code Registry . . . . . . . . . . . . . . . . . 64
14. Security Considerations . . . . . . . . . . . . . . . . . . . 65
14.1. Authenticity and Integrity of ALTO Information . . . . . . 65
14.1.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 65
14.1.2. Protection Strategies . . . . . . . . . . . . . . . . 65
14.1.3. Limitations . . . . . . . . . . . . . . . . . . . . . 66
14.2. Potential Undesirable Guidance from Authenticated ALTO
Information . . . . . . . . . . . . . . . . . . . . . . . 66
14.2.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 66
14.2.2. Protection Strategies . . . . . . . . . . . . . . . . 66
14.3. Confidentiality of ALTO Information . . . . . . . . . . . 67
14.3.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 67
14.3.2. Protection Strategies . . . . . . . . . . . . . . . . 67
14.3.3. Limitations . . . . . . . . . . . . . . . . . . . . . 68
14.4. Privacy for ALTO Users . . . . . . . . . . . . . . . . . . 68
14.4.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 68
14.4.2. Protection Strategies . . . . . . . . . . . . . . . . 68
14.5. Availability of ALTO Service . . . . . . . . . . . . . . . 69
14.5.1. Risk Scenarios . . . . . . . . . . . . . . . . . . . . 69
14.5.2. Protection Strategies . . . . . . . . . . . . . . . . 69
15. Manageability Considerations . . . . . . . . . . . . . . . . . 69
15.1. Operations . . . . . . . . . . . . . . . . . . . . . . . . 69
15.1.1. Installation and Initial Setup . . . . . . . . . . . . 70
15.1.2. Migration Path . . . . . . . . . . . . . . . . . . . . 70
15.1.3. Requirements on Other Protocols and Functional
Components . . . . . . . . . . . . . . . . . . . . . . 70
15.1.4. Impact and Observation on Network Operation . . . . . 71
15.2. Management . . . . . . . . . . . . . . . . . . . . . . . . 71
15.2.1. Management Interoperability . . . . . . . . . . . . . 71
15.2.2. Management Information . . . . . . . . . . . . . . . . 72
15.2.3. Fault Management . . . . . . . . . . . . . . . . . . . 72
15.2.4. Configuration Management . . . . . . . . . . . . . . . 72
15.2.5. Performance Management . . . . . . . . . . . . . . . . 72
15.2.6. Security Management . . . . . . . . . . . . . . . . . 73
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 73
16.1. Normative References . . . . . . . . . . . . . . . . . . . 73
16.2. Informative References . . . . . . . . . . . . . . . . . . 74
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 76
Appendix B. Design History and Merged Proposals . . . . . . . . . 77
Appendix C. Authors . . . . . . . . . . . . . . . . . . . . . . . 78
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 78
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1. Introduction
1.1. Background and Problem Statement
Today, network information available to applications is mostly from
the view of endhosts. There is no clear mechanism for a network to
convey to network applications its point of view on its network
topological structures and path preferences, forcing applications to
make approximations using data sources such as BGP Looking Glass
and/or applications' own measurements, which can be misleading or
inaccurate. On the other hand, modern 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 better
network-provided information.
This document defines the ALTO protocol to implement the ALTO
Service, which provides a simple mechanism to convey useful network
information such as topological and path preference information to
applications from the underlying network Providers' points of view.
The ALTO protocol meets the ALTO requirements [I-D.ietf-alto-reqs],
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.
1.2. Solution Benefits
At a high level, the ALTO Service allows a Service Provider (e.g., an
ISP) to publish network information such as network locations, costs
between them at configurable granularities, and endhost properties.
A mechanism to publish such information can benefit both Service
Providers (providers of the information) and Applications (consumers
of the information). We enumerate some benefits below.
1.2.1. Service Providers
A Service Provider 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 Service Provider is
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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.
1.2.2. Applications
Applications that use an ALTO Service can benefit from better
knowledge of the network to avoid network bottlenecks. For example,
a peer-to-peer overlay application can use information provided by an
ALTO Service to avoid selecting peers connected with low bandwidth
links. By using ALTO information, applications can reduce the
reliance on obtaining network information through third-party
databases; applications relying on measuring path performance metrics
themselves can reduce the measurement overhead by conducting only
fine-tuning or fault-tolerant measurements on top of ALTO
information.
2. Terminology
We use 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.
We also use the following additional terms: Endpoint Address,
Autonomous System Number (ASN), Network Location, ALTO Information,
and ALTO Information Base.
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 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, overlay ID, and phone number. An Endpoint Address can be
network-attachment based (e.g., IP address) or network-attachment
agnostic (e.g., MAC address).
Each Endpoint Address has an associated Address Type, which indicates
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both its syntax and semantics.
2.3. ASN
An Autonomous System Number.
2.4. 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.
2.5. ALTO Information
ALTO Information is a generic term referring to the network
information sent by an ALTO Server.
2.6. ALTO Information Base
Internal representation of the ALTO Information maintained by the
ALTO Server. Note that the structure of this internal representation
is not defined by this document.
3. Architecture
We now define 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.
Specifically, the ALTO Service of a network region 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. Hence, we say that the network
information provided by the ALTO Service of a network region
represents the "my-Internet View" of the network region.
To better understand the ALTO Service and the role of the ALTO
Protocol, we show in Figure 1 the overall ALTO system architecture.
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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.
+-------------------------------------------------------------------+
| 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
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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
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 ALTO 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 query 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.
In this document, we focus on achieving the goal of conveying (1)
Network Locations, which denote the locations of Endpoints at a
network, (2) provider-defined costs for paths between pairs of
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Network Locations, and (3) network related properties of endhosts.
We achieve the goal 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.
Below we give an overview of the services. Details of the services
will be presented in the following sections.
.-----------------------------------------.
| 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. The Network Map (See Section 5)
provides the full set of Network Location groupings defined by the
ALTO Server and the Endpoints contained within each grouping. The
Cost Map (see Section 6) provides costs between the defined
groupings.
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 spends
network bandwidth and CPU cycles to collect results and then perform
client-side filtering. The Map Filtering Service allows ALTO Clients
to query for the ALTO Server Network Map and Cost Map based on
additional parameters.
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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).
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. In this
document, we use 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 US-ASCII string of type
PIDName (see Section 9.1) and its associated set of Endpoint
Addresses. As we discussed above, there can be many different ways
of grouping the endpoints and assigning PIDs. For example, a PID may
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denote a subnet, a set of subnets, a metropolitan area, a PoP, an
autonomous system, or a set of autonomous systems.
A key use case of PIDs is to specify network preferences (costs)
between PIDs instead of individual endpoints. This allows cost
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 specification only considers IP
addresses.
5.2.1. IP Addresses
When either an ALTO Client or ALTO Server needs to determine which
PID in a Network Map contains a particular IP address, longest-prefix
matching MUST be used.
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. 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.
Each endpoint MUST map into exactly one PID. Since longest-prefix
matching is used to map an endpoint to a PID, this can be
accomplished by ensuring that no two PIDs contain an identical IP
prefix.
5.3. Example Network Map
Figure 3 illustrates an example Network Map. PIDs are used to
identify network-agnostic aggregations.
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.-----------------------------------------------------------.
| ALTO Network Map |
| |
| .-----------------------------------. .---------------. |
| | NetLoc: PID-1 | | NetLoc: PID-2 | |
| | .------------------------------. | | ... | |
| | | 192.0.2.0/24 | | `---------------` |
| | | .--------------------------. | | |
| | | | Endpoint: 192.0.2.34 | | | .---------------. |
| | | `--------------------------` | | | NetLoc: PID-3 | |
| | `------------------------------` | | ... | |
| | .------------------------------. | `---------------` |
| | | 198.51.100.0/25 | | |
| | | .--------------------------. | | .---------------. |
| | | | Endpoint: 198.51.100.100 | | | | NetLoc: PID-4 | |
| | | `--------------------------` | | | ... | |
| | `------------------------------` | `---------------` |
| `-----------------------------------` |
| |
`-----------------------------------------------------------`
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 needs.
An ALTO Cost Map defines Path Costs pairwise amongst sets of source
and destination Network Locations defined by PIDs. Each Path Cost is
the end-to-end cost when a unit of traffic goes from the source to
the destination.
As 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
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.
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As used in this document, the 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.
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. Lower values indicate 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
it chooses (e.g., air-miles or hop-count) as long as it conforms to
these semantics.
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
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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 SHOULD be cognizant of operations when a
desired Cost Mode is not supported. For example, 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 given 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 are a ranking (relative
to all other values in a Cost Map), with lower values indicating a
higher preference. The values are non-negative integers. 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.
It is important to note that the values in the Cost Map provided with
the ordinal Cost Mode are not necessarily the actual cost known to
the ALTO Server.
6.2. Cost Map Structure
A query for a Cost Map either explicitly or implicitly includes a
list of Source Network Locations and a list of Destination Network
Locations. (Recall that a Network Location can be an endpoint
address or a PID.)
Specifically, assume that a query has 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, ...,
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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. We
refer 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
If a Cost Map contains PIDs in the list of Source Network Locations
or the list of Destination Network Locations, the Path Costs are
generated based on a particular Network Map (which defines the PIDs).
Version Tags are introduced to ensure that ALTO Clients are able to
use consistent information even though the information is provided in
two maps.
A Version Tag is an opaque string associated with a Network Map
maintained by the ALTO Server. Two Version Tags match only if their
strings are the same. Whenever the content of the Network Map
maintained by the ALTO Server changes, the Version Tag MUST also be
changed. Possibilities for generating a Version Tag 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.
A Network Map distributed by the ALTO Server includes its Version
Tag. A Cost Map referring to PIDs also includes the Version Tag of
the Network Map on which it is based.
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.
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7.1.1. Endpoint Property Type: pid
An ALTO Server MUST define the 'pid' Endpoint Property Type, which
provides the PID of an endpoint. Since the PID of an endpoint
depends on the Network Map, Version Tag of the Network Map used to
return the pid property MUST be included.
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: Each service provides network information
as a set of information resources, which are distinguished by
their media types [RFC2046]. An ALTO Client may construct an HTTP
request for a particular information resource (including any
parameters, if necessary), and an 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.
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].
We use an adaptation of the C-style struct notation to define the
members (names/values) of JSON objects. An optional member is
enclosed by [ ], and an array is indicated by two numbers, m and n,
where m indicates the minimal number of values, and n is the maximum.
When we write .. for n, it means no upper bound. In the definitions,
the JSON names of the members are case sensitive.
For example, the definition below defines a new type Type4, with
three members named "name1", "name2", and "name3" respectively. The
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member named "name3" is optional, and the member named "name2" is an
array of at least one value.
object {
Type1 name1;
Type2 name2<1..*>;
[Type3 name3;]
} Type4;
We also define 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
any type.
object-map {
Type1 -> Type2;
} Type3;
Note that despite the notation, no standard, machine-readable
interface definition or schema is provided. Extension documents may
document these as necessary.
8.3. Basic Operation
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
encoding as specified in this document, and MUST be compliant with
[RFC2616].
8.3.1. Client Discovering Information Resources
To discover available Information Resources, an ALTO Client requests
the Information Resource Directory, which an ALTO Server provides at
the URI found by the ALTO Discovery protocol.
Informally, an Information Resource Directory enumerates URIs at
which an ALTO Server offers Information Resources. Each entry in the
directory indicates a URI at which an ALTO Server accepts requests,
and returns either the requested Information Resource or an
Information Resource Directory that references additional Information
Resources. See Section 8.5 for a detailed specification.
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8.3.2. Client Requesting Information Resources
Through the retrieved Information Resource Directories, 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.
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
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 MUST return the requested
Information Resource. In other cases, to be more informative
([I-D.ietf-httpbis-p2-semantics]), the ALTO server MAY provide the
ALTO Client with an Information Resource Directory indicating how to
reach the desired information resource, or return an ALTO error
object; see Section 8.7 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).
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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 IRD as a Response
After receiving an Information Resource Directory, the Client can
consult it to determine if any of the offered URIs contain the
desired Information Resource. Note that 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 8.5
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
When server and/or client authentication, encryption, and/or
integrity protection are required, an ALTO Server MUST support SSL/
TLS [RFC5246] as a mechanism. For cases such as a public ALTO
service or deployment scenarios where there is an implicit trust
relationship between the client and the server and the network
infrastructure connecting them is secure, SSL/TLS may not be
necessary. See [RFC6125] for considerations regarding verification
of server identity.
8.3.6. HTTP Cookies
If cookies are included in an HTTP request received by an ALTO
Server, they MUST be ignored.
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8.3.7. Parsing
This document only details object members used by this specification.
Extensions may include additional members within JSON objects defined
in this document. ALTO implementations MUST ignore such unknown
fields when processing ALTO messages.
8.4. Information Resource: Attributes
An Information Resource encodes the ALTO Information desired by an
ALTO Client. This document specifies multiple Information Resources
that can be provided by an ALTO Server.
Each Information Resource has certain attributes associated with it,
including its data format, its capabilities, and its accepted input
parameters. These attributes are published by an ALTO Server in its
Information Resource Directory.
8.4.1. Media Type
A Media Type [RFC2046] uniquely indicates a data format used to
encode the content of an Information Resource that an ALTO Server
returns to an ALTO Client in the HTTP entity body.
8.4.2. Capabilities
The Capabilities associated with an Information Resource announced by
an ALTO Server 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, the Server advertises the cost-constraints
capability for its Cost Map Information Resource.
8.4.3. Accepts Input Parameters
An ALTO Server may allow an ALTO Client to supply input parameters
when requesting certain Information Resources. The associated
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.
8.5. Information Resource Directory
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
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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].
8.5.1. Media Type
The media type to indicate an information directory is "application/
alto-directory+json".
8.5.2. Encoding
An Information Resource Directory is a JSON object of type
InfoResourceDirectory:
object {
IRDMeta meta;
IRDResourceEntry resources<0..*>;
} InfoResourceDirectory;
object-map {
JSONString -> JSONValue;
} IRDMeta;
object {
JSONString uri;
JSONString media-types<1..*>;
[JSONString accepts<0..*>;]
[Capabilities capabilities;]
} IRDResourceEntry;
object {
...
} Capabilities;
where the "meta" member provides definitions related with the IRD
itself, or can be used when defining multiple individual Information
resources;
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the "resources" array indicates a list of Information Resources
provided by an ALTO Server. Note that the list of available
resources is enclosed in a JSON object for extensibility; future
protocol versions may specify additional members in the
InfoResourceDirectory object.
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 and MUST
be resolved according to Section 5 of [RFC3986].
media-types The list of all media types of Information Resources
(see Section 8.4.1) available via GET or POST requests to the
corresponding URI or URIs discoverable via the URI.
accepts The list of all media types of input parameters (see
Section 8.4.3) accepted by POST requests to the corresponding URI
or URIs discoverable via the URI. If this member is not present,
it MUST be assumed to be an empty array.
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
member 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.
If an entry has an empty list for "accepts", then the corresponding
URI MUST support GET requests. If an entry has a non-empty list for
"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.
8.5.3. Example
The following is an example Information Resource Directory returned
by an ALTO Server.
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: TBA
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"}
}
},
"resources" : [
{
"uri" : "http://alto.example.com/networkmap",
"media-types" : [ "application/alto-networkmap+json" ]
}, {
"uri" : "http://alto.example.com/costmap/num/routingcost",
"media-types" : [ "application/alto-costmap+json" ],
"capabilities" : {
"cost-type-names" : [ "num-routing" ]
}
}, {
"uri" : "http://alto.example.com/costmap/num/hopcount",
"media-types" : [ "application/alto-costmap+json" ],
"capabilities" : {
"cost-type-names" : [ "num-hop" ]
}
}, {
"uri" : "http://custom.alto.example.com/maps",
"media-types" : [
"application/alto-networkmap+json",
"application/alto-costmap+json"
],
"accepts" : [
"application/alto-networkmapfilter+json",
"application/alto-costmapfilter+json"
]
}, {
"uri" : "http://alto.example.com/endpointprop/lookup",
"media-types" : [ "application/alto-endpointprop+json" ],
"accepts" : [ "application/alto-endpointpropparams+json" ],
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"capabilities" : {
"prop-types" : [ "pid" ]
}
}, {
"uri" : "http://alto.example.com/endpointcost/lookup",
"media-types" : [ "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 "meta" member of the example IRD defines four cost
types so that each Information Resource can use them.
The "resources" array of the example IRD defines six Information
Resources. For example, the last entry is to provide 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" member of the "capabilities" attribute of the Information
Resource includes 4 defined cost types from the "meta" member 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" member of the
"capabilities" attribute.
8.5.4. Multiple Choices and OPTIONS
ALTO Information Resource Directory provides flexibility to an ALTO
Server (e.g., delegation) so that it MAY indicate multiple
Information Resources using one URI endpoint. In the example above,
the ALTO Server provides additional Network and Cost Maps via a
separate subdomain, "custom.alto.example.com". In particular, the
maps available via this subdomain are Filtered Network and Cost Maps
as well as pre-generated maps for the "hopcount" and "routingcost"
Cost Metrics in the "ordinal" Cost Mode.
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If an ALTO Client requests one URI that provides multiple Information
Resources, the ALTO Server replies with an HTTP 300 status code
("Multiple Choices"). The ALTO Client can discover the specific
Information Resources indicated by the URI using an OPTIONS request.
The ALTO Server SHOULD use the Information Resource Directory format
in its reply to an OPTIONS request.
Consider the preceding example. The ALTO Client can discover the
maps available at "custom.alto.example.com" by successfully
performing an OPTIONS request to
"http://custom.alto.example.com/maps":
OPTIONS /maps HTTP/1.1
Host: custom.alto.example.com
Accept: application/alto-directory+json,application/alto-error+json
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HTTP/1.1 200 OK
Content-Length: TBA
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"}
}
},
"resources" : [
{
"uri" : "http://custom.alto.example.com/networkmap/filtered",
"media-types" : [ "application/alto-networkmap+json" ],
"accepts" : [ "application/alto-networkmapfilter+json" ]
}, {
"uri" : "http://custom.alto.example.com/costmap/filtered",
"media-types" : [ "application/alto-costmap+json" ],
"accepts" : [ "application/alto-costmapfilter+json" ],
"capabilities" : {
"cost-constraints" : true,
"cost-type-names" : [ "num-routing", "num-hop",
"ord-routing", "ord-hop" ]
}
}, {
"uri" : "http://custom.alto.example.com/ord/routingcost",
"media-types" : [ "application/alto-costmap+json" ],
"capabilities" : {
"cost-type-names" : [ "ord-routing" ]
}
}, {
"uri" : "http://custom.alto.example.com/ord/hopcount",
"media-types" : [ "application/alto-costmap+json" ],
"capabilities" : {
"cost-type-names" : [ "ord-hop" ],
}
}
]
}
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8.5.5. Usage Considerations
8.5.5.1. 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 8.5.3,
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.
8.5.5.2. 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".
8.6. Information Resource: Content Encoding
Though each Information Resource may have a distinct syntax and hence
its unique Media Type, they are designed to have a common structure
containing generic ALTO-layer metadata about the resource, as well as
data itself.
Specifically, each Information Resource has a single top-level JSON
object of type InfoResourceEntity:
object {
InfoResourceMeta meta;
InfoResourceDataType data;
} InfoResourceEntity;
with members:
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meta meta-information pertaining to the Information Resource;
data the data contained in the Information Resource.
8.6.1. Meta Information
Meta information is encoded as a JSON object. This document does not
specify any members, but it is defined here as a standard container
for extensibility. Specifically, InfoResourceMetaData is defined as:
object-map {
JSONString -> JSONValue
} InfoResourceMetaData;
8.6.2. Data Information
The "data" member of the InfoResourceEntity encodes the resource-
specific data. In this document, we define four specific
InfoResourceDataType: InfoResourceNetworkMap, InfoResourceCostMap,
InfoResourceEndpointProperty, and InfoResourceEndpointCostMap, whose
structures will be detailed below.
8.6.3. Example
The following is an example of the encoding for an Information
Resource:
HTTP/1.1 200 OK
Content-Length: 40
Content-Type: application/alto-costmap+json
{
"meta" : {},
"data" : {
...
}
}
8.7. 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
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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.
8.7.1. Media Type
The media type for an ALTO Error Resource is "application/
alto-error+json".
8.7.2. Resource Format and Error Codes
An ALTO Error Resource has the format:
object {
JSONString code;
} ErrorResourceEntity;
where:
code An ALTO Error Code 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_JSON_FIELD_MISSING | Required field missing |
| E_JSON_VALUE_TYPE | JSON Value of unexpected type |
| E_INVALID_COST_MODE | Invalid cost mode |
| E_INVALID_COST_METRIC | Invalid cost metric |
| E_INVALID_PROPERTY_TYPE | Invalid property type |
+-------------------------+-----------------------------------------+
Table 1: Defined ALTO Error Codes.
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
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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 technique.
8.7.3. 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], a 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.
The ALTO Server MAY also terminate the connection with the ALTO
Client.
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: Basic ALTO Data Types
This section details the format for particular data values used in
the ALTO Protocol.
9.1. PID Name
A PID Name is encoded as a US-ASCII string. The string MUST be no
more than 64 characters, and MUST NOT contain any ASCII character
below 0x21 or above 0x7E or the '.' separator (0x2E). The '.'
separator is reserved for future use and MUST NOT be used unless
specifically indicated by a companion or extension document.
The type 'PIDName' is used in this document to indicate a string of
this format.
9.2. Version Tag
A Version Tag is encoded as a case-sensitive US-ASCII string. The
string MUST be no more than 64 characters, and MUST NOT contain any
ASCII character below 0x21 or above 0x7E.
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The type 'VersionTag' is used in this document to indicate a string
of this type. Two tags are the same if and only if they are byte for
byte equal.
9.3. 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.
9.3.1. Address Type
Address Types are encoded as US-ASCII strings consisting of only
alphanumeric characters (code points 0x30-0x39, 0x41-0x5A, and 0x61-
0x7A). This document defines the address type 'ipv4' to refer to
IPv4 addresses, and 'ipv6' to refer to IPv6 addresses. All Address
Type 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 13.4).
The type 'AddressType' is used in this document to indicate a string
of this format.
9.3.2. Endpoint Address
Endpoint Addresses are encoded as US-ASCII strings. The exact
characters and format depend on the type of endpoint address.
The type 'EndpointAddr' is used in this document to indicate a string
of this format.
9.3.2.1. IPv4
IPv4 Endpoint Addresses are encoded as specified by the 'IPv4address'
rule in Section 3.2.2 of [RFC3986].
9.3.2.2. IPv6
IPv6 Endpoint Addresses are encoded as specified in Section 4 of
[RFC5952].
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9.3.2.3. 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 type.
Typed Endpoint Addresses are encoded as US-ASCII strings of the
format 'AddressType:EndpointAddr' (with the ':' character as a
separator). The type 'TypedEndpointAddr' is used to indicate a
string of this format.
9.3.3. 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 US-ASCII 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.
9.3.3.1. IPv4
IPv4 Endpoint Prefixes are encoded as specified in Section 3.1 of
[RFC4632].
9.3.3.2. IPv6
IPv6 Endpoint Prefixes are encoded as specified in Section 7 of
[RFC5952].
9.3.4. 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;
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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
addresses:
{
"ipv4": [
"192.0.2.0/24",
"198.51.100.0/25"
],
"ipv6": [
"2001:db8:0:1::/64",
"2001:db8:0:2::/64"
]
}
9.4. Cost Mode
A Cost Mode is encoded as a US-ASCII 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.
9.5. Cost Metric
A Cost Metric is encoded as a US-ASCII string. The string MUST be no
more than 32 characters, and MUST NOT contain characters other than
alphanumeric characters (code points 0x30-0x39, 0x41-0x5A, and 0x61-
0x7A), the hyphen ('-', code point 0x2D), or the colon (':', code
point 0x3A).
Identifiers prefixed with 'priv:' are reserved for Private Use
[RFC5226]. Identifiers prefixed with 'exp:' are reserved for
Experimental use. For an identifier with the 'priv:' or 'exp:'
prefix, an additional string (e.g., company identifier or random
string) MUST follow to reduce potential collisions. For example, a
short string after 'exp:' to indicate the starting time of a specific
experiment is recommended. All other identifiers appearing in an
HTTP request or response with an 'application/alto-*' media type MUST
be registered in the ALTO Cost Metrics registry Section 13.2.
The type 'CostMetric' is used in this document to indicate a string
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of this format.
9.6. Cost Type
The combinaton of CostType and a CostMode defines a CostType:
object {
CostMetric cost-metric;
CostModel cost-mode;
[JSONString description;]
} CostType;
'description', if present, MUST contain a US-ASCII string with a
human-readable description of the cost-metric and cost-mode. The
field SHOULD NOT be interpreted by an ALTO Client.
9.7. Endpoint Property
An Endpoint Property is encoded as a US-ASCII string. The string
MUST be no more than 32 characters, and MUST NOT contain characters
other than alphanumeric characters (code points 0x30-0x39, 0x41-0x5A,
and 0x61-0x7A), the hyphen ('-', code point 0x2D), or the colon (':',
code point 0x3A).
Identifiers prefixed with 'priv:' are reserved for Private Use
[RFC5226]. Identifiers prefixed with 'exp:' are reserved for
Experimental use. For an identifier with the 'priv:' or 'exp:'
prefix, an additional string (e.g., company identifier or random
string) MUST follow to reduce potential collisions. For example, a
short string after 'exp:' to indicate the starting time of a specific
experiment is recommended. All other identifiers appearing in an
HTTP request or response with an 'application/alto-*' media type MUST
be registered in the ALTO Endpoint Property registry Section 13.3.
The type 'EndpointPropertyType' is used in this document to indicate
a string of this format.
10. Protocol Specification: Service Information Resources
This section documents the individual Information Resources defined
to provide the services define in this document.
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10.1. 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.
10.1.1. Network Map
The Network Map Information Resource lists for each PID, the network
locations (endpoints) within the PID. It MUST be provided by an ALTO
Server.
10.1.1.1. Media Type
The media type of Network Map is "application/alto-networkmap+json".
10.1.1.2. HTTP Method
The Network Map resource is requested using the HTTP GET method.
10.1.1.3. Accept Input Parameters
None.
10.1.1.4. Capabilities
None.
10.1.1.5. Response
The "data" member of the returned InfoResourceEntity for a Network
Map is an object of type InfoResourceNetworkMap:
object {
VersionTag map-vtag;
NetworkMapData map;
} InfoResourceNetworkMap;
object-map {
PIDName -> EndpointAddrGroup;
} NetworkMapData;
with members:
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map-vtag The Version Tag (Section 6.3) of the Network Map.
map The Network Map data itself.
NetworkMapData is a JSON object representing a dictionary map with
each key representing a single PID, and the value the associated set
of endpoint addresses.
The returned Network Map MUST include all PIDs known to the ALTO
Server.
10.1.1.6. 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: 370
Content-Type: application/alto-networkmap+json
{
"meta" : {},
"data" : {
"map-vtag" : "1266506139",
"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"
]
}
}
}
}
The encodings were chosen for readability and compactness. If lookup
efficiency at runtime is crucial, then the returned Network Map and
Cost Map can be transformed into data structures offering more
efficient lookup, such as transforming the Network Map into a IP trie
for longest-prefix matching and the Cost Map into a matrix.
10.1.2. Cost Map
The 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.
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10.1.2.1. Media Type
The media type of Cost Map is "application/alto-costmap+json".
10.1.2.2. HTTP Method
The Cost Map resource is requested using the HTTP GET method.
10.1.2.3. Accept Input Parameters
None.
10.1.2.4. 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..*>;
} CostMapCapabilities;
with member:
cost-type-names A sequence of CostType names defined in "cost-types"
of the "meta" member of an IRD. These represent the Cost Types
that are supported via the corresponding URI in the IRD. If there
is more than one Cost Type in this list, then the ALTO Server
SHOULD return an IRD to the client to lead it towards the URIs for
the corresponding Cost Maps. Since an unfiltered Cost Map is
requested via an HTTP GET that accepts no input parameters, an
ALTO Client MUST be led towards a resource that has a single
element in the 'cost-type-names' list.
10.1.2.5. Response
The "data" member of the returned InfoResourceEntity for a Cost Map
is an object of type InfoResourceCostMap:
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object {
CostType cost-type;
VersionTag map-vtag;
CostMapData map;
} InfoResourceCostMap;
object-map {
PIDName -> DstCosts;
} CostMapData;
object-map {
PIDName -> JSONValue;
} DstCosts;
with members:
cost-type Cost Type (Section 9.6) used in the Cost Map.
map-vtag The Version Tag (Section 6.3) of the Network Map used to
generate the Cost Map.
map The Cost Map data itself.
CostMapData 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.
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).
10.1.2.6. Example
GET /costmap/num/routingcost HTTP/1.1
Host: alto.example.com
Accept: application/alto-costmap+json,application/alto-error+json
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HTTP/1.1 200 OK
Content-Length: TBA
Content-Type: application/alto-costmap+json
{
"meta" : {},
"data" : {
"cost-type" : {"cost-mode" : "numerical",
"cost-metric": "routingcost"},
"map-vtag" : "1266506139",
"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, we considered array-based encoding
for "map", but chose the current encoding for clarity.
10.2. 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.
10.2.1. Filtered Network Map
A Filtered Network Map is a Network Map Information Resource
(Section 10.1.1) 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.
10.2.1.1. Media Type
As a Filtered Network Map is a Network Map, it uses the media type
defined for Network Map at Section 10.1.1.1.
10.2.1.2. HTTP Method
A Filtered Network Map is requested using the HTTP POST method.
10.2.1.3. Accept Input Parameters
An ALTO Client supplies filtering parameters by specifying media type
"application/alto-networkmapfilter+json" with HTTP POST body
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containing a JSON Object of type ReqFilteredNetworkMap, where:
object {
PIDName pids<0..*>;
AddressType address-types<0..*>;
} ReqFilteredNetworkMap;
with members:
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 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 entries appearing multiple times as if they
appeared only once.
10.2.1.4. Capabilities
None.
10.2.1.5. Response
See Section 10.1.1.5 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.
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10.2.1.6. Example
POST /networkmap/filtered HTTP/1.1
Host: custom.alto.example.com
Content-Length: 27
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: 255
Content-Type: application/alto-networkmap+json
{
"meta" : {},
"data" : {
"map-vtag" : "1266506139",
"map" : {
"PID1" : {
"ipv4" : [
"192.0.2.0/24",
"198.51.100.0/24"
]
},
"PID2" : {
"ipv4": [
"198.51.100.128/24"
]
}
}
}
}
10.2.2. Filtered Cost Map
A Filtered Cost Map is a Cost Map Information Resource
(Section 10.1.2) 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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10.2.2.1. Media Type
As a Filtered Cost Map is a Cost Map, it uses the media type defined
for Cost Map at Section 10.1.2.1.
10.2.2.2. HTTP Method
A Filtered Cost Map is requested using the HTTP POST method.
10.2.2.3. 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 members:
cost-type The CostType (Section 9.6) for the returned costs. This
MUST be one of the supported Cost Types indicated in this
resource's capabilities ( Section 10.2.2.4).
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 10.2.2.4)
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
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-
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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"
member is not present, both lists MUST be interpreted by the ALTO
Server as containing the full set of currently-defined PIDs.
10.2.2.4. Capabilities
The URI providing this resource supports all capabilities documented
in Section 10.1.2.4 (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 members:
cost-type-names See Section 10.1.2.4 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 member 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.
10.2.2.5. Response
See Section 10.1.2.5 for the format.
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.
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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.
Note that ALTO Clients should verify that the Version Tag included in
the response is consistent with 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.
10.2.2.6. Example
POST /costmap/filtered HTTP/1.1
Host: custom.alto.example.com
Content-Type: application/alto-costmapfilter+json
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: 177
Content-Type: application/alto-costmap+json
{
"meta" : {},
"data" : {
"cost-type": {"cost-mode" : "numerical",
"cost-metric" : "routingcost"},
"map-vtag" : "1266506139",
"map" : {
"PID1": { "PID1": 0, "PID2": 1, "PID3": 2 }
}
}
}
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10.3. Endpoint Property Service
The Endpoint Property Service provides information about Endpoint
properties to ALTO Clients.
10.3.1. Endpoint Property
The Endpoint Property resource provides information about properties
for individual endpoints. It MAY be provided by an ALTO Server. If
an ALTO Server provides one or more Endpoint Property resources, then
at least one MUST provide the 'pid' property.
10.3.1.1. Media Type
The media type of Endpoint Property is "application/
alto-endpointprop+json".
10.3.1.2. HTTP Method
The Endpoint Property resource is requested using the HTTP POST
method.
10.3.1.3. 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 members:
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 10.3.1.4). 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.
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10.3.1.4. 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:
object {
EndpointPropertyType prop-types<0..*>;
} EndpointPropertyCapabilities;
with members:
prop-types The Endpoint Properties (see Section 9.7) supported by
the corresponding URI. If not present, this member MUST be
interpreted as an empty array.
10.3.1.5. Response
The returned InfoResourceEntity object has "data" member of type
InfoResourceEndpointProperty, where:
object {
VersionTag map-vtag; [DEPEND ON PROPERTIES]
EndpointPropertyMapData map;
} InfoResourceEndpointProperty;
object-map {
TypedEndpointAddr -> EndpointProps;
} EndpointPropertyMapData;
object {
EndpointPropertyType -> JSONValue;
} EndpointProps;
EndpointPropertyMapData 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 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
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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.
The ALTO Server MAY include the Version Tag (Section 6.3) of the
Network Map used to generate the response (if desired and applicable)
as the 'map-vtag' member in the response. If the 'pid' property is
returned for any endpoints in the response, the 'map-vtag' member is
REQUIRED. Otherwise, it is OPTIONAL.
10.3.1.6. Example
POST /endpointprop/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 96
Content-Type: application/alto-endpointpropparams+json
Accept: application/alto-endpointprop+json,application/alto-error+json
{
"properties" : [ "pid", "example-prop" ],
"endpoints" : [ "ipv4:192.0.2.34", "ipv4:203.0.113.129" ]
}
HTTP/1.1 200 OK
Content-Length: 149
Content-Type: application/alto-endpointprop+json
{
"meta" : {},
"data": {
"map-vtag" : "1266506139",
"map" : {
"ipv4:192.0.2.34" : { "pid": "PID1", "example-prop": "1" },
"ipv4:203.0.113.129" : { "pid": "PID3" }
}
}
}
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10.4. 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.
10.4.1. Endpoint Cost
The Endpoint Cost resource provides information about costs between
individual endpoints. It MAY be provided by an ALTO Server.
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 alternative would be to compute
the cost between two endpoints as the cost between the PIDs
corresponding to the endpoints. See Section 14.3 for additional
details.
10.4.1.1. Media Type
The media type of Endpoint Cost is "application/
alto-endpointcost+json".
10.4.1.2. HTTP Method
The Endpoint Cost resource is requested using the HTTP POST method.
10.4.1.3. 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<1..*>;
} EndpointFilter;
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with members:
cost-type The Cost Type ( Section 9.6) to use for returned costs.
This MUST be one of the CostType indicated in this resource's
capabilities ( Section 10.4.1.4).
constraints Defined equivalently to the "constraints" input
parameter of a Filtered Cost Map (see Section 10.2.2).
endpoints A list of Source Endpoints and Destination Endpoints for
which Path Costs are to be returned. If the list of Source
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 12.3 for discussion and considerations regarding this
mode). The list of destination Endpoints MUST NOT be empty. The
ALTO Server MUST interpret entries appearing multiple times in a
list as if they appeared only once.
10.4.1.4. Capabilities
In this document, we define EndpointCostCapabilities the same as
FilteredCostMapCapabilities. See Section 10.2.2.4.
10.4.1.5. Response
The returned InfoResourceEntity object has "data" member equal to
InfoResourceEndpointCostMap, where:
object {
CostType cost-type;
EndpointCostMapData map;
} InfoResourceEndpointCostMap;
object-map {
TypedEndpointAddr -> EndpointDstCosts;
} EndpointCostMapData;
object-map {
TypedEndpointAddr -> JSONValue;
} EndpointDstCosts;
InfoResourceEndpointCostMap has members:
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cost-type The Cost Type used in the returned Cost Map.
map The Endpoint Cost Map data itself.
EndpointCostMapData is a dictionary map object with each key
representing a TypedEndpointAddr string identifying the Source
Endpoint specified in the input parameters; the name for a member is.
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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10.4.1.6. Example
POST /endpointcost/lookup HTTP/1.1
Host: alto.example.com
Content-Length: 195
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: 231
Content-Type: application/alto-endpointcost+json
{
"meta" : {},
"data" : {
"cost-type": {"cost-mode" : "ordinal",
"cost-metric" : "routingcost"},
"map" : {
"ipv4:192.0.2.2": {
"ipv4:192.0.2.89" : 1,
"ipv4:198.51.100.34" : 2,
"ipv4:203.0.113.45" : 3
}
}
}
}
11. 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].
11.1. 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.
11.2. 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.
11.3. 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
gathers a list of known peers in the swarm, and asks the ALTO Server
to rank them.
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.
12. Discussions
12.1. 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 8.5) to locate an Information
Resource with the desired ALTO Information.
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12.2. 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.
12.3. Network Address Translation Considerations
At this day and age of NAT v4<->v4, v4<->v6 [RFC6144], and possibly
v6<->v6[I-D.mrw-nat66], 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.
12.4. 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.
13. IANA Considerations
13.1. 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 8.5 |
| application | alto-networkmap+json | Section 10.1.1 |
| application | alto-networkmapfilter+json | Section 10.2.1 |
| application | alto-costmap+json | Section 10.1.2 |
| application | alto-costmapfilter+json | Section 10.2.2 |
| application | alto-endpointprop+json | Section 10.3.1 |
| application | alto-endpointpropparams+json | Section 10.3.1 |
| application | alto-endpointcost+json | Section 10.4.1 |
| application | alto-endpointcostparams+json | Section 10.4.1 |
| application | alto-error+json | Section 8.7 |
+-------------+------------------------------+----------------+
Table 2: ALTO Protocol Media Types.
Type name: application
Subtype name: This documents requests the registration of multiple
subtypes, as listed in Table 2.
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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 14.
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.
Change controller: Internet Engineering Task Force
(mailto:iesg@ietf.org).
13.2. ALTO Cost Metric Registry
This document requests the creation of an ALTO Cost Metric registry,
listed in Table 3, to be maintained by IANA.
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+-------------+---------------------+
| Identifier | Intended Semantics |
+-------------+---------------------+
| routingcost | See Section 6.1.1.1 |
| priv: | Private use |
| exp: | Experimental 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 Expert Review [RFC5226].
The Expert Reviewer will generally consult the ALTO Working Group or
its successor. Expert Review is used to ensure that proper
documentation regarding ALTO Cost Metric semantics and security
considerations has been provided. The provided documentation 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 9.5. Identifiers are
to be recorded and displayed as ASCII strings.
Identifiers prefixed with 'priv:' are reserved for Private Use.
Identifiers prefixed with 'exp:' are reserved for Experimental 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.
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
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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 14.3.
13.3. 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 |
| exp: | Experimental use |
+------------+--------------------+
Table 4: ALTO Endpoint Property Types.
The maintenance of this registry is similar to that of the preceding
ALTO Cost Metrics.
13.4. 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 | Mapping to/from |
| | Encoding | Encoding | IPv4/v6 |
+------------+----------------+----------------+--------------------+
| ipv4 | See | See | Direct mapping to |
| | Section 9.3.2 | Section 9.3.3 | IPv4 |
| ipv6 | See | See | Direct mapping to |
| | Section 9.3.2 | Section 9.3.3 | 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.
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New ALTO Address Types are assigned after Expert Review [RFC5226].
The Expert Reviewer will generally consult the ALTO Working Group or
its successor. Expert Review is used to ensure that proper
documentation regarding the new ALTO Address Types and their security
considerations has been provided. The provided documentation 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 9.3.1. Identifiers are
to be recorded and displayed as ASCII 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 9.3.2).
o Endpoint Prefix Encoding: The procedure for encoding a set of
addresses of the registered type as an EndpointPrefix (see
Section 9.3.3). 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.
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.
13.5. ALTO Error Code Registry
This document requests the creation of an ALTO Error Code registry,
listed in Table 1, to be maintained by IANA.
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14. 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
<xref target="RFC6708"/> outlines minimum-to-implement authentication
and other security requirements. Below we consider the threats and
protection strategies.
14.1. Authenticity and Integrity of ALTO Information
14.1.1. 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 if
the 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.
14.1.2. 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. In
particular, the ALTO Protocol requires that HTTP over TLS <xref
target="RFC2818"/> MUST be supported, when protecting the
authenticity and integrity of ALTO Information is required. The
rules in <xref target="RFC2818"/> for a client to verify server
identity using server certificates MUST be supported. ALTO Providers
who request server certificates and certification authorities who
issue ALTO-specific certificates SHOULD consider the recommendations
and guidelines defined in <xref target="RFC6125"/>.
Software engineers developing and service providers deploying ALTO
with should make themselves familar with up-to-date Best Current
Practices on configuring HTTP over TLS.
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14.1.3. 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
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.
14.2. Potential Undesirable Guidance from Authenticated ALTO
Information
14.2.1. 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 <xref target="RFC5693"/>:
"redirecting applications to corrupted mediators providing malicious
content, or applying policies in computing Cost Map based on criteria
other than network efficiency." See <xref target="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.
14.2.2. 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 [RFC 6708]). If
the first ALTO server is provided by the access network service
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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.
14.3. Confidentiality of ALTO Information
14.3.1. 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. We discuss 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-graind 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 <xref target="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) to an unauthorized third party; and risk type
(3) Excess retrieval of the ALTO service provider's data by
collaborating ALTO clients. Section 10 of <xref target="I-D.ietf-
alto-deployments"/>also discusses information leakage from ALTO.
14.3.2. Protection Strategies
To address risk type (1), 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.
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To address risk type (2), the ALTO Protocol need 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
address risk type (2), ALTO requires that HTTP Digestion
Authentication MUST be supported to achieve ALTO Client
Authentication to limit the parties with whom ALTO information is
directly shared. 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 <xref
target="I-D.ietf-alto-deployments"/> for a more detailed discussion
on this issue.
14.3.3. 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.
14.4. Privacy for ALTO Users
14.4.1. 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 [RFC 6708], Section 5.2.
14.4.2. Protection Strategies
To protect user privacy, an ALTO Client should be cognizant about
potential ALTO Server tracking through client queries. 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
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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.
14.5. Availability of ALTO Service
14.5.1. 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).
14.5.2. 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.
15. 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.
15.1. Operations
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15.1.1. 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 5.2.1), 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 operational impact 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, SSL/TLS)
should be consulted for their available settings and proposed default
configurations.
15.1.2. 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.
15.1.3. Requirements 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.
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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.
15.1.4. Impact and Observation on Network Operation
ALTO presents a new opportunity for managing network traffic by
providing additional information to clients. The potential impact to
network operation is large.
Deployment of an ALTO Server may shift network traffic patterns.
Thus, an ALTO Service Provider should consider impacts on (or
integration with) traffic engineering and the deployment of a
monitoring service to observe the effects of ALTO operations. Note
that ALTO-specific monitoring and metrics are discussed in 6.3 of
[I-D.ietf-alto-deployments] and future versions of that document. In
particular, an ALTO Service Provider may observe that ALTO Clients
are not bound to ALTO Server guidance as ALTO is only one source of
information.
An ALTO Service Provider should ensure that appropriate information
is being exposed. Privacy implications for ISPs are discussed in
Section 14.3. Both ALTO Service Providers and those using ALTO
Clients should be aware of the impact of incorrect or faked guidance
(see Section 10.3 of [I-D.ietf-alto-deployments] and future versions
of that document).
15.2. Management
15.2.1. 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].
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15.2.2. 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.
15.2.3. Fault Management
Monitoring ALTO Servers and Clients is described in Section 6.3 of
[I-D.ietf-alto-deployments] and future versions of that document.
15.2.4. 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
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,
SSL/TLS client and server authentication, SSL/TLS encryption
parameters) intended to serve the same information should be
monitored for consistency.
15.2.5. Performance Management
An exhaustive list of desirable performance information from a ALTO
Servers and ALTO Clients are outside of the scope of this document.
The following is a list of suggested ALTO-specific to be monitored
based on the existing deployment and protocol development experience:
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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)
15.2.6. Security Management
Section 14 documents ALTO-specific security considerations.
Operators should configure security policies with those in mind.
Readers should refer to HTTP [RFC2616] and SSL/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.
16. References
16.1. Normative References
[IEEE.754.2008]
Institute of Electrical and Electronics Engineers,
"Standard for Binary Floating-Point Arithmetic", IEEE
Standard 754, August 2008.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[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.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
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[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.
[RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic
Optimization (ALTO) Problem Statement", RFC 5693,
October 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.
[RFC6708] Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
Y. Yang, "Application-Layer Traffic Optimization (ALTO)
Requirements", RFC 6708, September 2012.
16.2. 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.
[I-D.akonjang-alto-proxidor]
Akonjang, O., Feldmann, A., Previdi, S., Davie, B., and D.
Saucez, "The PROXIDOR Service",
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draft-akonjang-alto-proxidor-00 (work in progress),
March 2009.
[I-D.ietf-alto-deployments]
Stiemerling, M., Kiesel, S., and S. Previdi, "ALTO
Deployment Considerations", draft-ietf-alto-deployments-06
(work in progress), February 2013.
[I-D.ietf-alto-reqs]
Previdi, S., Stiemerling, M., Woundy, R., and Y. Yang,
"Application-Layer Traffic Optimization (ALTO)
Requirements", draft-ietf-alto-reqs-08 (work in progress),
March 2011.
[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-08 (work in progress),
March 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-22 (work in progress),
February 2013.
[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.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.mrw-nat66]
Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", draft-mrw-nat66-16 (work in progress),
April 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.
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[I-D.saumitra-alto-multi-ps]
Das, S., Narayanan, V., and L. Dondeti, "ALTO: A Multi
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.
[P4P-SIGCOMM08]
Xie, H., Yang, Y., Krishnamurthy, A., Liu, Y., and A.
Silberschatz, "P4P: Provider Portal for (P2P)
Applications", SIGCOMM 2008, August 2008.
[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.
Appendix A. Acknowledgments
Thank you to Jan Seedorf and Sebastian Kiesel for contributions to
the Security Considerations section. Ben Niven-Jenkins, Wendy Roome
and Michael Scharf gave substantial feedback and suggestions on the
protocol design.
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),
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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
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];
o ATTP [ATTP]; and
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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.]]
Stefano Previdi
Cisco
Email: sprevidi@cisco.com
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
Alimi, et al. Expires November 9, 2013 [Page 78]
Internet-Draft ALTO Protocol May 2013
Y. Richard Yang (editor)
Yale University
51 Prospect St
New Haven CT
USA
Email: yry@cs.yale.edu
Alimi, et al. Expires November 9, 2013 [Page 79]