ALTO M. Stiemerling
Internet-Draft NEC Europe Ltd.
Intended status: Informational S. Kiesel
Expires: July 23, 2016 University of Stuttgart
M. Scharf
H. Seidel
S. Previdi
January 20, 2016

ALTO Deployment Considerations


Many Internet applications are used to access resources such as pieces of information or server processes that are available in several equivalent replicas on different hosts. This includes, but is not limited to, peer-to-peer file sharing applications. The goal of Application-Layer Traffic Optimization (ALTO) is to provide guidance to applications that have to select one or several hosts from a set of candidates, which are able to provide a desired resource. This memo discusses deployment related issues of ALTO. It addresses different use cases of ALTO such as peer-to-peer file sharing and CDNs and presents corresponding examples. The document also includes recommendations for network administrators and application designers planning to deploy ALTO, such recommendations how to generate ALTO map information.

Status of This Memo

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This Internet-Draft will expire on July 23, 2016.

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Table of Contents

1. Introduction

Many Internet applications are used to access resources such as pieces of information or server processes that are available in several equivalent replicas on different hosts. This includes, but is not limited to, peer-to-peer (P2P) file sharing applications and Content Delivery Networks (CDNs). The goal of Application-Layer Traffic Optimization (ALTO) is to provide guidance to applications that have to select one or several hosts from a set of candidates, which are able to provide a desired resource. The basic ideas and problem space of ALTO is described in [RFC5693] and the set of requirements is discussed in [RFC6708]. The ALTO protocol is specified in [RFC7285]. An ALTO server discovery procedure is defined in [RFC7286].

This document discusses use cases and operational issues that can be expected when ALTO gets deployed. This includes, but is not limited to, location of the ALTO server, imposed load to the ALTO server, and from whom the queries are performed. This document provides guidance which ALTO services to use, and it summarizes known challenges as well as deployment experiences, including potential processes to generate ALTO network and cost maps. It thereby complements the management considerations in the protocol specification [RFC7285], which are independent of any specific use of ALTO.

2. General Considerations

2.1. ALTO Entities

2.1.1. Baseline Scenario

The ALTO protocol [RFC7285] is a client/server protocol, operating between a number of ALTO clients and an ALTO server, as sketched in Figure 1.

              |  ALTO    |
              |  Server  |
         ,-''       |       `--.
       ,'           |           `.
      (     Network |             )
       `.           |           ,'
         `--.       |       _.-'
 +----------+  +----------+   +----------+
 |  ALTO    |  |  ALTO    |...|  ALTO    |
 |  Client  |  |  Client  |   |  Client  |
 +----------+  +----------+   +----------+

Figure 1: Baseline deployment scenario of the ALTO protocol

This document uses the terminology introduced in [RFC5693]. In particular, the following terms are defined by [RFC5693]:

According to that definition, both an ALTO server and an ALTO client are logical entities. An ALTO service may be offered by more than one ALTO servers. In ALTO deployments, the functionality of an ALTO server can therefore be realized by several server instances, e.g., by using load balancing between different physical servers. The term ALTO server should not be confused with use of a single physical server.

2.1.2. Placement of ALTO Entities

The ALTO server and ALTO clients can be situated at various entities in a network deployment. The first differentiation is whether the ALTO client is located on the actual host that runs the application, as shown in Figure 2, or if the ALTO client is located on a resource directory, as shown in Figure 3.

                                           |     App      |
                                           +-----------+  |
                                       ===>|ALTO Client|  |****
                                    ===    +-----------+--+   *
                                 ===                    *     *
                              ===                       *     *
   +-------+     +-------+<===             +--------------+   *
   |       |     |       |                 |     App      |   *
   |       |.....|       |<========        +-----------+  |   *
   |       |     |       |        ========>|ALTO Client|  |   *
   +-------+     +-------+<===             +-----------+--+   *
   Source of       ALTO       ==                        *     *
   topological    Server        ==                      *     *
   information                    ==       +--------------+   *
                                    ==     |     App      |   *
                                      ==   +-----------+  |****
                                        ==>|ALTO Client|  |
   === ALTO protocol
   *** Application protocol
   ... Provisioning protocol

Figure 2: Overview of protocol interaction between ALTO elements without a resource directory

Figure 2 shows the operational model for an ALTO client running at endpoints. An example would be a peer-to-peer file sharing application that does not use a tracker, such as edonkey. In addition, ALTO clients at peers could also be used in a similar way even if there is a tracker, as further discussed in Section 4.1.2.

                                                  **| App |****
                                                **  +-----+   *
                                              **       *      *
                                            **         *      *
   +-------+     +-------+     +--------------+        *      *
   |       |     |       |     |              |     +-----+   *
   |       |.....|       |     +-----------+  |*****| App |   *
   |       |     |       |<===>|ALTO Client|  |     +-----+   *
   +-------+     +-------+     +-----------+--+        *      *
   Source of       ALTO          Resource   **         *      *
   topological    Server         directory    **       *      *
   information                                  **  +-----+   *
                                                  **| App |****
   === ALTO protocol
   *** Application protocol
   ... Provisioning protocol

Figure 3: Overview of protocol interaction between ALTO elements with a resource directory

In Figure 3, a use case with a resource directory is illustrated, e.g., a tracker in peer-to-peer file-sharing. Both deployment scenarios may differ in the number of ALTO clients that access an ALTO service: If an ALTO client is implemented in a resource directory, an ALTO server may be accessed by a limited and less dynamic set of clients, whereas in the general case any host could be an ALTO client. This use case is further detailed in Section 4.

Using ALTO in CDNs may be similar to a resource directory [I-D.jenkins-alto-cdn-use-cases]. The ALTO server can also be queried by CDN entities to get guidance about where a particular client accessing data in the CDN is exactly located in the Internet Service Provider's network, as discussed in Section 5.

2.2. Classification of Deployment Scenarios

2.2.1. Roles in ALTO Deployments

ALTO is a general-purpose protocol and it is intended to be used by a wide range of applications. This implies that there are different possibilities where the ALTO entities are actually located, i.e., if the ALTO clients and the ALTO server are in the same Internet Service Provider (ISP) domain, or if the clients and the ALTO server are managed/owned/located in different domains.

An ALTO deployment involves four kinds of entities:

  1. Source of topological information
  2. ALTO server
  3. ALTO client
  4. Resource consumer (using the ALTO guidance)

Each of these entities corresponds to a certain role, which results in requirements and constraints on the interaction between the entities.

A key design objective of the ALTO service is that each these four roles can be separated, i.e., they can be realized by different organizations or disjoint system components. ALTO is inherently designed for use in multi-domain environments. Most importantly, ALTO is designed to enable deployments in which the ALTO server and the ALTO client are not located within the same administrative domain.

As explained in [RFC5693], from this follows that at least three different kinds of entities can operate an ALTO server:

  1. Network operators. Network Service Providers (NSPs) such as Internet Service Providers (ISPs) may have detailed knowledge of their network topology and policies. In this case, the source of the topology information and the provider of the ALTO server may be part of the same organization.
  2. Third parties. Topology information could also be collected by entities separate from network operators but that may either have collected network information or have arrangements with network operators to learn the network information. Examples of such entities could be Content Delivery Network (CDN) operators or companies specialized on offering ALTO services on behalf of ISPs.
  3. User communities. User communities could run distributed measurements for estimating the topology of the Internet. In this case the topology information may not originate from ISP data.

Regarding the interaction between ALTO server and client, ALTO deployments can be differentiated according to the following aspects:

  1. Applicable trust model: The deployment of ALTO can differ depending on whether ALTO client and ALTO server are operated within the same organization and/or network, or not. This affects a lot of constraints, because the trust model is very different. For instance, as discussed later in this memo, the level-of-detail of maps can depend on who the involved parties actually are.
  2. Composition of the user group: The main use case of ALTO is to provide guidance to any Internet application. However, an operator of an ALTO server could also decide to offer guidance only to a set of well-known ALTO clients, e. g., after authentication and authorization. In the peer-to-peer application use case, this could imply that only selected trackers are allowed to access the ALTO server. The security implications of using ALTO in closed groups differ from the public Internet.
  3. Covered destinations: In general, an ALTO server has to be able to provide guidance for all potential destinations. Yet, in practice a given ALTO client may only be interested in a subset of destinations, e.g., only in the network cost between a limited set of resource providers. For instance, CDN optimization may not need the full ALTO cost maps, because traffic between individual residential users is not in scope. This may imply that an ALTO server only has to provide the costs that matter for a given user, e. g., by customized maps.

The following sections enumerate different classes of use cases for ALTO, and they discuss deployment implications of each of them. An ALTO server can in principle be operated by any organization, and there is no requirement that an ALTO server is deployed and operated by an ISP. Yet, since the ALTO solution is designed for ISPs, most examples in this document assume that the operator of an ALTO server is a network operator (e.g., an ISP or the network department in a large enterprise) that offers ALTO guidance in particular to users of this network.

It must be emphasized that any application using ALTO must also work if no ALTO servers can be found or if no responses to ALTO queries are received, e.g., due to connectivity problems or overload situations (see also [RFC6708]).

2.2.2. Information Exposure

There are basically two different approaches how an ALTO server can provide network information and guidance:

  1. The ALTO server provides maps that contain provider-defined cost values between network location groupings (e.g., sets of IP prefixes). These maps can be retrieved by clients via the ALTO protocol, and the actual processing of the map data is done inside the client. Since the maps contain (aggregated) cost information for all endpoints, the client does not have to reveal any internal operational data, such as the IP addresses of candidate resource providers. The ALTO protocol supports this mode of operation by the Network and Cost Map Service.
  2. The ALTO server provides a query interface that returns costs or rankings for explicitly specified endpoints. This means that the query of the ALTO client has to include additional information (e.g., a list of IP addresses). The server then calculates and returns costs or rankings for the endpoints specified in the request (e.g., a sorted list of the IP addresses). In ALTO, this method can be realized by the Endpoint Cost Service.

Both approaches have different privacy implications for the server and client:

For the client, approach 1 has the advantage that all operational information stays within the client and is not revealed to the provider of the server. However, this service implies that a network operator providing an ALTO server has to expose a certain amount of information about its network structure (e.g., IP prefixes or topology information in general).

For the operator of a server, approach 2 has the advantage that the query responses reveal less topology information to ALTO clients. But this method requires that client sends internal operational information to the server, such as the IP addresses of hosts also running the application. For clients, such data can be sensitive.

As a result, both approaches have their pros and cons, as further detailed in Section 3.3.

2.2.3. More Advanced Deployments

From an ALTO client's perspective, there are different ways to use ALTO:

  1. Single service instance with single metric guidance: An ALTO client only obtains guidance regarding a single metric from a single ALTO service, e.g., an ALTO server that is offered by the network service provider of the corresponding access network. Corresponding ALTO server instances can be discovered e.g. by ALTO server discovery [RFC7286] [I-D.kiesel-alto-xdom-disc]. Being a REST-ful protocol, an ALTO service can use known methods to balance the load between different server instances or between clusters of servers, i.e., an ALTO server can be realized by many instances with a load balancing scheme. The ALTO protocol also supports the use of different URIs for different ALTO features.
  2. Single service instance with multiple metric guidance: An ALTO client could also query an ALTO service for different kinds of information, e.g., cost maps with different metrics. The ALTO protocol is extensible and permits such operation. However, ALTO does not define how a client shall deal with different forms of guidance, and it is up to the client to determine what provided information may indeed be useful.
  3. Multiple service instances: An ALTO client can also decide to access multiple ALTO servers providing guidance, possibly from different operators or organizations. Each of these services may only offer partial guidance, e.g., for a certain network partition. In that case, it may be difficult for an ALTO client to compare the guidance from different services. Different organization may use different methods to determine maps, and they may also have different (possibly even contradicting or competing) guidance objectives. How to discover multiple ALTO servers and how to deal with conflicting guidance is an open issue.

There are also different options regarding the synchronization of guidance offered by an ALTO service:

  1. Authoritative servers: An ALTO server instance can provide guidance for all destinations for all kinds of ALTO clients.
  2. Cascaded servers: An ALTO server may itself include an ALTO client and query other ALTO servers, e.g., for certain destinations. This results is a cascaded deployment of ALTO servers, as further explained below.
  3. Inter-server synchronization: Different ALTO servers my communicate by other means. This approach is not further discussed in this document.

An assumption of the ALTO design is that ISP operate ALTO servers independently, irrespectively of other ISPs. This may true for most envisioned deployments of ALTO but there may be certain deployments that may have different settings. Figure 4 shows such setting with a university network that is connected to two upstream providers. NREN is a National Research and Education Network, which provides cheap high-speed connectivity to specific destinations, e.g., other universities. ISP is a commercial upstream provider from which the university buys connectivity to all destinations that cannot be reached via the NREN. The university, as well as ISP, are operating their own ALTO server. The ALTO clients, located on the peers will contact the ALTO server located at the university.

       |    ISP    |
       |   ALTO    |<==========================++
       |  Server   |                           ||
       +-----------+                           ||
         ,-------.            ,------.         ||
      ,-'         `-.      ,-'         `-.     ||
     /   Commercial  \    /               \    ||
    (    Upstream     )  (       NREN      )   ||
     \   ISP         /    \               /    ||
      `-.         ,-'      `-.         ,-'     ||
         `---+---'            `+------'        ||
             |                 |               ||
             |                 |               ||
             |,-------------.  |               \/
           ,-+               `-+          +-----------+
         ,'      University     `.        |University |
        (        Network          )       |   ALTO    |
         `.                      /        |  Server   |
           `-.               +--'         +-----------+
              `+------------'|              /\     /\
               |             |              ||     ||
      +--------+-+         +-+--------+     ||     ||
      |   Peer1  |         |   PeerN  |<====++     ||
      +----------+         +----------+            ||
           /\                                      ||
           ||                                      ||

   === ALTO protocol

Figure 4: Example of a cascaded ALTO server

In this setting all "destinations" useful for the peers within NREN are free-of-charge for the peers located in the university network (i.e., they are preferred in the rating of the ALTO server). However, all traffic that is not towards NREN will be handled by the ISP upstream provider. Therefore, the ALTO server at the university may also include the guidance given by the ISP ALTO server in its replies to the ALTO clients. This is an example for cascaded ALTO servers.

3. Deployment Considerations by ISPs

3.1. Objectives for the Guidance to Applications

3.1.1. General Objectives for Traffic Optimization

The Internet consists of many networks. The networks are operated by Network Service Providers (NSP) or Internet Service Providers (ISP), which also include e.g. universities, enterprises, or other organizations. The Internet provides network connectivity, e.g., by access networks, such as cable networks, xDSL networks, 3G/4G mobile networks, etc. Network operators need to manage, to control and to audit the traffic. Therefore, it is important to understand how to deploy an ALTO service and its expected impact.

The general objective of ALTO is to give guidance to applications on what endpoints (e.g., IP addresses or IP prefixes) are to be preferred according to the operator of the ALTO server. The ALTO protocol gives means to let the ALTO server operator express its preference, whatever this preference is.

ALTO enables ISPs to support application-level traffic engineering by influencing application resource provider selection. This traffic engineering can have different objectives:

  1. Inter-network traffic localization: ALTO can help to reduce inter-domain traffic. The networks of ISPs are interconnected through peering points. From a business view, the inter-network settlement is needed for exchanging traffic between these networks. These peering agreements can be costly. To reduce these costs, a simple objective is to decrease the traffic exchange across the peering points and thus keep the traffic in the own network or Autonomous System (AS) as far as possible.
  2. Intra-network traffic localization: In case of large ISPs, the network may be grouped into several networks, domains, or Autonomous Systems (ASs). The core network includes one or several backbone networks, which are connected to multiple aggregation, metro, and access networks. If traffic can be limited to certain areas such as access networks, this decreases the usage of backbone and thus helps to save resources and costs.
  3. Network off-loading: Compared to fixed networks, mobile networks have some special characteristics, including smaller link bandwidth, high cost, limited radio frequency resource, and limited terminal battery. In mobile networks, wireless links should be used efficiently. For example, in the case of a P2P service, it is likely that hosts should prefer retrieving data from hosts in fixed networks, and avoid retrieving data from mobile hosts.
  4. Application tuning: ALTO is also a tool to optimize the performance of applications that depend on the network and perform resource provider selection decisions among network endpoints. And example is the network-aware selection of Content Delivery Network (CDN) caches.

In the following, these objectives are explained in more detail with examples.

3.1.2. Inter-Network Traffic Localization

ALTO guidance can be used to keep traffic local in a network, for instance in order to reduce peering costs. An ALTO server can let applications prefer other hosts within the same network operator's network instead of randomly connecting to other hosts that are located in another operator's network. Here, a network operator would always express its preference for hosts in its own network, while hosts located outside its own network are to be avoided (i.e., they are undesired to be considered by the applications). Figure 5 shows such a scenario where hosts prefer hosts in the same network (e.g., Host 1 and Host 2 in ISP1 and Host 3 and Host 4 in ISP2).

                         ,-------.         +-----------+
       ,---.          ,-'         `-.      |   Host 1  |
    ,-'     `-.      /     ISP 1   ########|ALTO Client|
   /           \    /              #  \    +-----------+
  /    ISP X    \   |              #  |    +-----------+
 /               \  \              ########|   Host 2  |
;             +----------------------------|ALTO Client|
|             |   |   `-.         ,-'      +-----------+
|             |   |      `-------'                      
|     Inter-  |   |      ,-------.         +-----------+
:     network |   ;   ,-'         `########|   Host 3  |
 \    traffic |  /   /     ISP 2   # \     |ALTO Client|
  \           | /   /              #  \    +-----------+
   \          |/    |              #  |    +-----------+
    `-.     ,-|     \              ########|   Host 4  |
       `---'  +----------------------------|ALTO Client| 
                      `-.         ,-'      +-----------+ 

    ### preferred "connections"
    --- non-preferred "connections"

Figure 5: Inter-network traffic localization

Examples for corresponding ALTO maps can be found in Section 3.5. Depending on the application characteristics, it may not be possible or even not be desirable to completely localize all traffic.

3.1.3. Intra-Network Traffic Localization

The previous section describes the results of the ALTO guidance on an inter-network level. In the same way, ALTO can also be used for intra-network localization. In this case, ALTO provides guidance which internal hosts are to be preferred inside a single network (e.g., one AS). This application-level traffic engineering can reduce the capacity requirements in the core network of an ISP. Figure 6 shows such a scenario where Host 1 and Host 2 are located in an access net 1 of ISP 1 and connect via a low capacity link to the core of the same ISP 1. If Host 1 and Host 2 exchange their data with remote hosts, they would probably congest the bottleneck link.

           Bottleneck    ,-------.         +-----------+
       ,---.     |    ,-'         `-.      |   Host 1  |
    ,-'     `-.  |   /     ISP 1   ########|ALTO Client|
   /           \ |  /    (Access   #  \    +-----------+
  /    ISP 1    \|  |     net 1)   #  |    +-----------+
 /   (Core       V  \              ########|   Host 2  |
;    network) +--X~~~X---------------------|ALTO Client|
|             |   |   `-.         ,-'      +-----------+
|             |   |      `-------'                      
|             |   |      ,-------.         +-----------+
:             |   ;   ,-'         `########|   Host 3  |
 \            |  /   /     ISP 1   # \     |ALTO Client|
  \           | /   /     (Access  #  \    +-----------+
   \          |/    |      net 2)  #  |    +-----------+
    `-.     ,-X     \              ########|   Host 4  |
       `---'  ~~~~~~~X---------------------|ALTO Client| 
                ^     `-.         ,-'      +-----------+ 
                |        `-------'                       
    ### preferred "connections"
    --- non-preferred "connections"

Figure 6: Intra-network traffic localization

The operator can guide the hosts in such a situation to try first local hosts in the same network islands, avoiding or at least lowering the effect on the bottleneck link, as shown in Figure 6.

The objective is to avoid bottlenecks by optimized endpoint selection at application level. ALTO is not a method to deal with the congestion at the bottleneck.

3.1.4. Network Off-Loading

Another scenario is off-loading traffic from networks. This use of ALTO can be beneficial in particular in mobile networks. A network operator may have the desire to guide hosts in its own mobile network to use hosts outside this mobile network. One reason can be that the wireless network is not made for the load cause by, e.g., peer-to-peer applications, and it therefore makes sense when peers fetch their data from remote peers in other parts of the Internet.

                         ,-------.         +-----------+
       ,---.          ,-'         `-.      |   Host 1  |
    ,-'     `-.      /     ISP 1   +-------|ALTO Client|
   /           \    /    (Mobile   |  \    +-----------+
  /    ISP X    \   |    network)  |  |    +-----------+
 /               \  \              +-------|   Host 2  |
;             #############################|ALTO Client|
|             #   |   `-.         ,-'      +-----------+
|             #   |      `-------'                      
|             #   |      ,-------.  
:             #   ;   ,-'         `-.
 \            #  /   /     ISP 2     \ 
  \           # /   /     (Fixed      \
   \          #/    |     network)    |    +-----------+
    `-.     ,-#     \                 /    |   Host 3  |
       `---'  #############################|ALTO Client| 
                      `-.         ,-'      +-----------+ 

    ### preferred "connections"
    --- non-preferred "connections"

Figure 7: ALTO traffic network de-localization

Figure 7 shows the result of such a guidance process where Host 2 prefers a connection with Host 3 instead of Host 1, as shown in Figure 5.

A realization of this scenario may have certain limitations and may not be possible in all cases. For instance, it may require that the ALTO server can distinguish mobile and non-mobile hosts, e.g., based on their IP address. This may depend on mobility solutions and may not be possible or accurate. In general, ALTO is not intended as a fine-grained traffic engineering solution for individual hosts. Instead, it typically works on aggregates (e.g., if it is known that certain IP prefixes are often assigned to mobile users).

3.1.5. Application Tuning

ALTO can also provide guidance to optimize the application-level topology of networked applications, e.g., by exposing network performance information. Applications can often run their own measurements to determine network performance, e.g., by active delay measurements or bandwidth probing, but such measurements result in overhead and complexity. Accessing an ALTO server can be a simpler alternative. In addition, an ALTO server may also expose network information that applications cannot easily measure or reverse-engineer.

3.2. Provisioning of ALTO Topology Data

3.2.1. High-Level Process and Requirements

A process to generate ALTO topology information typically comprises several steps. The first step is to gather information, which is described in the following section. The subsequent sections then describe how the gathered data can be processed, and which methods can be applied to generate the information exposed by ALTO, such as network and cost maps.

Providing ALTO guidance can result in a win-win situation both for network providers and users of the ALTO information. Applications possibly get a better performance, while the network provider has means to optimize the traffic engineering and thus its costs. Yet, there can be security concerns with exposing topology data. Corresponding limitations are discussed in Section 7.2.

ISPs may have important privacy requirements when deploying ALTO, which have to be taken into account when processing ALTO topology data. In particular, an ISP may not be willing to expose sensitive operational details of its network. The topology abstraction of ALTO enables an ISP to expose the network topology at a desired granularity only, determined by security policies.

With the Endpoint Cost Service (ECS), the ALTO client does not have to implement any specific algorithm or mechanism in order to retrieve, maintain and process network topology information (of any kind). The complexity of the network topology (computation, maintenance and distribution) is kept in the ALTO server and ECS is delivered on demand. This allows the ALTO server to enhance and modify the way the topology information sources are used and combined. This simplifies the enforcement of privacy policies of the ISP.

The ALTO Network Map and Cost Map service expose an abstracted view on the ISP network topology. Therefore, care is needed when constructing those maps in order to take privacy policies into account, as further discussed in Section 3.2.3. The ALTO protocol also supports further features such as endpoint properties, which could also be used to expose topology guidance. The privacy considerations for ALTO maps also apply to such ALTO extensions.

3.2.2. Data Collection from Data Sources

The first step in the process of generating ALTO information is to gather the required information from the network. An ALTO server can collect topological information from a variety of sources in the network and provides a cohesive, abstracted view of the network topology to applications using an ALTO client. Topology data sources that may include routing protocols, network policies, state and performance information, geo-location, etc. An ALTO server requires at least some topology and/or routing information, i.e., information about present endpoints and their interconnection. With this information it is in principle possible to compute paths between all known endpoints. Based on such basic data, the ALTO server builds an ALTO-specific network topology that represents the network as it should be understood and utilized by applications (resource consumers) at endpoints using ALTO services (e.g., Network/Cost Map Service or ECS). A basic dataset can be extended by many other information obtainable from the network.

The ALTO protocol does not assume a specific network technology or topology. In principle, ALTO can be used with various types of addresses (Endpoint Addresses). [RFC7285] defines the use of IPv4/IPv6 addresses or prefixes in ALTO, but further address types could be added by extensions. In this document, only the use of IPv4/IPv6 addresses is considered.

The exposure of network topology information is controlled and managed by the ALTO server. ALTO abstract network topologies can be automatically generated from the physical or logical topology of the network, e.g., using "live" network data. The generation would typically be based on policies and rules set by the network operator. The maps and the guidance can significantly differ depending on the use case, the network architecture, and the trust relationship between ALTO server and ALTO client, etc. Besides the security requirements that consist of not delivering any confidential or critical information about the infrastructure, there are efficiency requirements in terms of what aspects of the network are visible and required by the given use case and/or application.

The ALTO server operator has to ensure that the ALTO topology does not reveal any details that would endanger the network integrity and security. For instance, ALTO is not intended to leak raw Interior Gateway Protocol (IGP) or Border gateway Protocol (BGP) databases to ALTO clients.

        +--------+   +--------+
        |  ALTO  |   |  ALTO  |
        | Client |   | Client |
        +--------+   +--------+
               /\     /\
               ||     || ALTO protocol
               ||     ||
               \/     \/
              |  ALTO   |
              | Server  |
               :   :   :    
               :   :   :
      +........+   :   +........+ Provisioning
      :            :            : protocol
      :            :            :
 +---------+  +---------+  +---------+
 |   BGP   |  |   I2RS  |  |   NMS   | Potential
 | Speaker |  |  Client |  |   OSS   | data sources
 +---------+  +---------+  +---------+
      ^            ^            ^
      |            |            |
 Link-State      I2RS      SNMP/NETCONF,
  NLRI for       data      traffic statistics,
  IGP/BGP                  IPFIX, etc.

Figure 8: Potential data sources for ALTO

As illustrated in Figure 8, the topology data used by an ALTO server can originate from different data sources:

  • Relevant information sources are interior gateway protocols (IGPs) or the Border Gateway Protocol (BGP). An ALTO server could get network routing information by listening to IGPs and/or peering with BGP speakers. For data collection, link-state protocols are more suitable since every router propagates its information throughout the whole network. Hence, it is possible to obtain information about all routers and their neighbors from one single router in the network. In contrast, distance-vector protocols are less suitable since routing information is only shared among neighbors. To obtain the whole topology with distance-vector routing protocols it is necessary to retrieve routing information from every router in the network.
  • The document [I-D.ietf-idr-ls-distribution] describes a mechanism by which links state and traffic engineering information can be collected from networks and shared with external components using the BGP routing protocol. This is achieved using a new BGP Network Layer Reachability Information (NLRI) encoding format. The mechanism is applicable to physical and virtual IGP links and can also include Traffic Engineering (TE) data. For instance, prefix data can be carried and originated in BGP, while TE data is originated and carried in an IGP. The mechanism described is subject to policy control.
  • The Interface to the Routing System (I2RS) is a solution for state transfer in and out of the Internet's routing system [I-D.ietf-i2rs-architecture]. An ALTO server could use an I2RS client to observe routing-related information. With the rise of Software-Defined Networking (SDN) and a decoupling of network data and control plane, topology information could also be fetched from an SDN controller. This scenario is not further discussed in the remainder of this document.
  • An ALTO server can also leverage a Network Management System (NMS) or an Operations Support System (OSS) as data sources. NMS or OSS solutions are used to control, operate, and manage a network, e.g., using the Simple Network Management Protocol (SNMP) or NETCONF. As explained for instance in [RFC7491], the NMS and OSS can be consumers of network events reported and can act on these reports as well as displaying them to users and raising alarms. The NMS and OSS can also access the Traffic Engineering Database (TED) and Label Switched Path Database (LSP-DB) to show the users the current state of the network. In addition, NMS and OSS systems may have access to IGP/BGP routing information, network inventory data (e.g., links, nodes, or link properties not visible to routing protocols, such as Shared Risk Link Groups), statistics collection system that provides traffic information, such as traffic demands or link utilization obtained from IP Flow Information Export (IPFIX), as well as other Operations, Administration, and Maintenance (OAM) information (e.g., syslog). NMS or OSS systems also may have functions to correlate and orchestrate information originating from other data sources. For instance, it could be required to correlate IP prefixes with routers (Provider, Provider Edge, Customer Edge, etc.), IGP areas, VLAN IDs, or policies.

The data sources mentioned so far are only a subset of potential topology sources and depending on the network type, (e.g. mobile, satellite network) different hardware and protocols are in operation to form and maintain the network.

In general it is challenging to gather detailed information about the whole Internet, since the network consists of multiple domains and in many cases it is not possible to collect information across network borders. Hence, potential information sources may be limited to a certain domain.

3.2.3. Partitioning and Grouping of IP Address Ranges

ALTO introduces provider-defined network location identifiers called Provider-defined Identifiers (PIDs) to aggregate network endpoints in the Map Services. Endpoints within one PID may be treated as single entity, assuming proximity based on network topology or other similarity. A key use case of PIDs is to specify network preferences (costs) between PIDs instead of individual endpoints. It is up to the operator of the ALTO server how to group endpoints and how to assign PIDs. For example, a PID may denote a subnet, a set of subnets, a metropolitan area, a POP, an autonomous system, or a set of autonomous systems.

This document only considers deployment scenarios in which PIDs expand to a set of IP address ranges (CIDR). A PID is characterized by a string identifier and its associated set of endpoint addresses [RFC7285]. If an ALTO server offers the Map Service, corresponding identifiers have to be configured.

An automated ALTO implementation may use dynamic algorithms to aggregate network topology. However, it is often desirable to have a mechanism through which the network operator can control the level and details of network aggregation based on a set of requirements and constraints. This will typically be governed by policies that enforce a certain level of abstraction and prevent leakage of sensitive operational data.

For instance, an ALTO server may leverage BGP information that is available in a networks service provider network layer and compute the group of prefix. An example are BGP communities, which are used in MPLS/IP networks as a common mechanism to aggregate and group prefixes. A BGP community is an attribute used to tag a prefix to group prefixes based on mostly any criteria (as an example, most ISP networks originate BGP prefixes with communities identifying the Point of Presence (PoP) where the prefix has been originated). These BGP communities could be used to map IP address ranges to PIDs. By an additional policy, the ALTO server operator may decide an arbitrary cost defined between groups. Alternatively, there are algorithms that allow the dynamic computation of costs between groups. The ALTO protocol itself is independent of such algorithms and policies.

3.2.4. Rating Criteria and/or Cost Calculation

An ALTO server indicates preferences amongst network locations in the form of path costs. Path costs are generic costs and can be internally computed by the operator of the ALTO server according to its own policy. For a given ALTO network map, an ALTO cost map defines directional path costs pairwise amongst the set of source and destination network locations defined by the PIDs.

The ALTO protocol permits the use of different cost types. An ALTO cost type is defined by the combination of a cost metric and a cost mode. The cost metric identifies what the costs represent. The cost mode identifies how the costs should be interpreted, e.g., whether returned costs should be interpreted as numerical values or ordinal rankings. The ALTO protocol also allows the definition of additional constraints defining which elements of a cost map shall be returned.

The ALTO protocol specification [RFC7285] defines the "routingcost" cost metric as basic set of rating criteria, which has to be supported by all implementations. This cost metric conveys a generic measure for the cost of routing traffic from a source to a destination. A lower value indicates a higher preference for traffic to be sent from a source to a destination. It is up to the ALTO server how that metric is calculated.

It is possible to calculate the "routingcost" cost metric based on actual routing protocol information. Typically, Interior Gateway Protocols (IGP) provide details about endpoints and links within the own network, while the Bordger Gateway Protocol (BGPs) is used to provide details about links to endpoints in other networks. Besides topology and routing information networks have a multitude of other attributes about its state, condition, and operation. That comprises but is not limited to attributes like link utilization, bandwidth and delay, ingress/egress points of data flows from/towards endpoints outside of the network up to the location of nodes and endpoints.

In order to enable use of extended information, there is an extension procedure for adding new ALTO cost types. The following list gives an overview on further rating criteria that have been proposed or which are in use by ALTO-related prototype implementations. This list is not intended as normative text; a definition of further metrics can be found for instance in [I-D.wu-alto-te-metrics]. Instead, the only purpose of the following list is to document and discuss rating criteria that have been proposed so far. It can also depend on the use case of ALTO whether such rating criteria are useful, and whether the corresponding information would indeed be made available by ISPs.

Distance-related rating criteria:

  • Relative topological distance: The term relative means that a larger numerical value means greater distance, but it is up to the ALTO service how to compute the values, and the ALTO client will not be informed about the nature of the information. One way of generating this kind of information may be counting AS hops, but when querying this parameter, the ALTO client must not assume that the numbers actually are AS hops. In addition to the AS path, a relative cost value could also be calculated taking into account other routing protocol parameters, such as BGP local preference or multi-exit discriminator (MED) attributes.
  • Absolute topological distance, expressed in the number of traversed autonomous systems (AS).
  • Absolute topological distance, expressed in the number of router hops (i.e., how much the TTL value of an IP packet will be decreased during transit).
  • Absolute physical distance, based on knowledge of the approximate geo-location (e.g., continent, country) of an IP address.

Performance-related rating criteria:

  • The minimum achievable throughput between the resource consumer and the candidate resource provider, which is considered useful by the application (only in ALTO queries).
  • An arbitrary upper bound for the throughput from/to the candidate resource provider (only in ALTO responses). This may be, but is not necessarily the provisioned access bandwidth of the candidate resource provider.
  • The maximum round-trip time (RTT) between resource consumer and the candidate resource provider, which is acceptable for the application for useful communication with the candidate resource provider (only in ALTO queries).
  • An arbitrary lower bound for the RTT between resource consumer and the candidate resource provider (only in ALTO responses). This may be, for example, based on measurements of the propagation delay in a completely unloaded network.

Charging-related rating criteria:

  • Traffic volume caps, in case the Internet access of the resource consumer is not charged by "flat rate". For each candidate resource provider, the ALTO service could indicate the amount of data that may be transferred from/to this resource provider until a given point in time, and how much of this amount has already been consumed. Furthermore, it would have to be indicated how excess traffic would be handled (e.g., blocked, throttled, or charged separately at an indicated price). The interaction of several applications running on a host, out of which some use this criterion while others don't, as well as the evaluation of this criterion in resource directories, which issue ALTO queries on behalf of other endpoints, are for further study.
  • Other metrics representing an abstract cost, e.g., determined by policies that distinguish "cheap" from "expensive" IP subnet ranges without detailing the cost function.

These rating criteria are subject to the remarks below:

The ALTO client must be aware that with high probability the actual performance values will differ from whatever an ALTO server exposes. In particular, an ALTO client must not consider a throughput parameter as a permission to send data at the indicated rate without using congestion control mechanisms.

The discrepancies are due to various reasons, including, but not limited to the facts that

  • the ALTO service is not an admission control system
  • the ALTO service may not know the instantaneous congestion status of the network
  • the ALTO service may not know all link bandwidths, i.e., where the bottleneck really is, and there may be shared bottlenecks
  • the ALTO service may not have all information about the actual routing
  • the ALTO service may not know whether the candidate endpoint itself is overloaded
  • the ALTO service may not know whether the candidate endpoint throttles the bandwidth it devotes for the considered application
  • the ALTO service may not know whether the candidate endpoint will throttle the data it sends to the client (e.g., because of some fairness algorithm, such as tit-for-tat).

Because of these inaccuracies and the lack of complete, instantaneous state information, which are inherent to the ALTO service, the application must use other mechanisms (such as passive measurements on actual data transmissions) to assess the currently achievable throughput, and it must use appropriate congestion control mechanisms in order to avoid a congestion collapse. Nevertheless, the rating criteria may provide a useful shortcut for quickly excluding candidate resource providers from such probing, if it is known in advance that connectivity is in any case worse than what is considered the minimum useful value by the respective application.

Rating criteria that should not be defined for and used by the ALTO service include:

  • Performance metrics that are closely related to the instantaneous congestion status. The definition of alternate approaches for congestion control is explicitly out of the scope of ALTO. Instead, other appropriate means, such as using TCP based transport, have to be used to avoid congestion.
  • Performance metrics that raise privacy concerns. For instance, it has been questioned whether an ALTO service could publicly expose the provisioned access bandwidth, e.g. of cable / DSL customers, because this could enables identification of "premium" customers.

3.3. Known Limitations of ALTO

3.3.1. General Limitations

ALTO is designed as a protocol between clients integrated in applications and servers that provide network information and guidance (e.g., basic network location structure and preferences of network paths). The objective is to modify network resource consumption patterns at application level while maintaining or improving application performance. This design focus results in a number of characteristics of ALTO:

  • Endpoint focus: In typical ALTO use cases, neither the consumer of the topology information (i.e., the ALTO client) nor the considered resources (e.g., files at endpoints) are part of the network. The ALTO server presents an abstract network topology containing only information relevant to an application overlay for better-than-random resource provider selection among its endpoints. The ALTO protocol specification [RFC7285] is not designed to expose network internals such as routing tables or configuration data that are not relevant for application-level resource provider selection decisions in network endpoints.
  • Abstraction: The ALTO services such as the Network/Cost Map Service or the ECS provide an abstract view of the network only. The operator of the ALTO server has full control over the granularity (e.g., by defining policies how to aggregate subnets into PIDs) and the level-of-detail of the abstract network representation (e.g., by deciding what cost types to support).
  • Multiple administrative domains: The ALTO protocol is designed for use cases where the ALTO server and client can be located in different organizations or trust domains. ALTO assumes a loose coupling between server and client. In addition, ALTO does not assume that an ALTO client has any a priori knowledge about the ALTO server and its supported features. An ALTO server can be discovered automatically.
  • Read-only: ALTO is a query/response protocol to retrieve guidance information. Neither network/cost map queries nor queries to the endpoint cost service are designed to affect state in the network.

If ALTO shall be deployed for use cases violating these assumptions, the protocol design may result in limitations.

For instance, in an Application-Based Network Operation (ABNO) environment the application could issue explicit service request to the network [RFC7491]. In this case, the application would require detailed knowledge about the internal network topology and the actual state. A network configuration would also require a corresponding security solution for authentication and authorization. ALTO is not designed for operations to control, operate, and manage a network.

Such deployments could be addressed by network management solutions, e.g., based on SNMP [RFC3411] or NETCONF [RFC6241] and YANG [RFC6020] that are typically designed to manipulate configuration state. Reference [RFC7491] contains a more detailed discussion of interfaces between components such as Element Management System (EMS), Network Management System (NMS), Operations Support System (OSS), Traffic Engineering Database (TED), Label Switched Path Database (LSP-DB), Path Computation Element (PCE), and other Operations, Administration, and Maintenance (OAM) components.

3.3.2. Limitations of Map-based Approaches

The specification of the Map Service in the ALTO protocol [RFC7285] is based on the concept of network maps. A network map partitions the network into Provider-defined Identifiers (PIDs) that group one or more endpoints (e.g., subnetworks) to a single aggregate. The "costs" between the various PIDs are stored in a cost map. Map-based approaches lower the signaling load on the server as maps have to be retrieved only if they change.

One main assumption for map-based approaches is that the information provided in these maps is static for a long period of time. This assumption is fine as long as the network operator does not change any parameter, e.g., routing within the network and to the upstream peers, IP address assignment stays stable (and thus the mapping to the partitions). However, there are several cases where this assumption is not valid:

  1. ISPs reallocate IP subnets from time to time;
  2. ISPs reallocate IP subnets on short notice;
  3. IP prefix blocks may be assigned to a router that serves a variety of access networks;
  4. Network costs between IP prefixes may change depending on the ISP's routing and traffic engineering.

These effects can be explained as follows:

Case 1: ISPs may reallocate IP subnets within their infrastructure from time to time, partly to ensure the efficient usage of IPv4 addresses (a scarce resource), and partly to enable efficient route tables within their network routers. The frequency of these "renumbering events" depend on the growth in number of subscribers and the availability of address space within the ISP. As a result, a subscriber's household device could retain an IP address for as short as a few minutes, or for months at a time or even longer.

It has been suggested that ISPs providing ALTO services could sub-divide their subscribers' devices into different IP subnets (or certain IP address ranges) based on the purchased service tier, as well as based on the location in the network topology. The problem is that this sub-allocation of IP subnets tends to decrease the efficiency of IP address allocation, in particular for IPv4. A growing ISP that needs to maintain high efficiency of IP address utilization may be reluctant to jeopardize their future acquisition of IP address space.

However, this is not an issue for map-based approaches if changes are applied in the order of days.

Case 2: ISPs can use techniques that allow the reallocation of IP prefixes on very short notice, i.e., within minutes. An IP prefix that has no IP address assignment to a host anymore can be reallocated to areas where there is currently a high demand for IP addresses.

Case 3: In residential access networks (e.g., DSL, cable), IP prefixes are assigned to broadband gateways, which are the first IP-hop in the access-network between the Customer Premises Equipment (CPE) and the Internet. The access-network between CPE and broadband gateway (called aggregation network) can have varying characteristics (and thus associated costs), but still using the same IP prefix. For instance one IP address IP1 out of a given CIDR prefix can be assigned to a VDSL access line (e.g., 2 MBit/s uplink) while another IP address IP2 within the same given CIDR prefix is assigned to a slow ADSL line (e.g., 128 kbit/s uplink). These IP addresses may be assigned on a first come first served basis, i.e., a single IP address out of the same CIDR prefix can change its associated costs quite fast. This may not be an issue with respect to the used upstream provider (thus the cross ISP traffic) but depending on the capacity of the aggregation-network this may raise to an issue.

Case 4: The routing and traffic engineering inside an ISP network, as well as the peering with other autonomous systems, can change dynamically and affect the information exposed by an ALTO server. As a result, cost maps and possibly also network maps can change.

3.3.3. Limitations of Non-Map-based Approaches

The specification of the ALTO protocol [RFC7285] also includes the Endpoint Cost Service (ECS) mechanism. ALTO clients can ask the ALTO server for guidance for specific IP addresses, thereby avoiding the need of processing maps. This can mitigate some of the problems mentioned in the previous section.

However frequent requests, particularly with long lists of IP addresses, may overload the ALTO server. The server has to rank each received IP address, which causes load at the server. This may be amplified when not only a single ALTO client is asking for guidance, but a larger number of them. The results of the ECS are also more difficult to cache than ALTO maps. Therefore, the ALTO client may have to await the server response before starting a communication, which results in an additional delay.

Caching of IP addresses at the ALTO client or the usage of the H12 approach [I-D.kiesel-alto-h12] in conjunction with caching may lower the query load on the ALTO server.

When ALTO server receives an ECS request, it may not have the most appropriate topology information in order to accurately determine the ranking. [RFC7285] generally assumes that a server can always offer some guidance. In such a case the ALTO server could adopt one of the following strategies:

  • Reply with available information (best effort).
  • Query another ALTO server presumed to have better topology information and return that response (cascaded servers).
  • Redirect the request to another ALTO server presumed to have better topology information (redirection).

The protocol mechanisms and decision processes that would be used to determine if redirection is necessary and which mode to use is out of the scope of this document, since protocol extensions could be required.

3.4. Monitoring ALTO

3.4.1. Impact and Observation on Network Operation

ALTO presents a new opportunity for managing network traffic by providing additional information to clients. In particular, the deployment of an ALTO server may shift network traffic patterns, and the potential impact to network operation can be large. An ISP providing ALTO may want to assess the benefits of ALTO as part of the management and operations (cf. [RFC7285]). For instance, the ISP might be interested in understanding whether the provided ALTO maps are effective, and in order to decide whether an adjustment of the ALTO configuration would be useful. Such insight can be obtained from a monitoring infrastructure. An ISP offering ALTO could consider the impact on (or integration with) traffic engineering and the deployment of a monitoring service to observe the effects of ALTO operations. The measurement of impacts can be challenging because ALTO-enabled applications may not provide related information back to the ALTO service provider.

To construct an effective monitoring infrastructure, the ALTO service provider should decide how to monitor the performance of ALTO and identify and deploy data sources to collect data to compute the performance metrics. In certain trusted deployment environments, it may be possible to collect information directly from ALTO clients. It may also be possible to vary or selectively disable ALTO guidance for a portion of ALTO clients either by time, geographical region, or some other criteria to compare the network traffic characteristics with and without ALTO. Monitoring an ALTO service could also be realized by third parties. In this case, insight into ALTO data may require a trust relationship between the monitoring system operator and the network service provider offering an ALTO service.

The required monitoring depends on the network infrastructure and the use of ALTO, and an exhaustive description is outside the scope of this document.

3.4.2. Measurement of the Impact

ALTO realizes an interface between the network and applications. This implies that an effective monitoring infrastructure may have to deal with both network and application performance metrics. This document does not comprehensively list all performance metrics that could be relevant, nor does it formally specify metrics.

The impact of ALTO can be classified regarding a number of different criteria:

  • Total amount and distribution of traffic: ALTO enables ISPs to influence and localize traffic of applications that use the ALTO service. An ISP may therefore be interested in analyzing the impact on the traffic, i.e., whether network traffic patterns are shifted. For instance, if ALTO shall be used to reduce the inter-domain P2P traffic, it makes sense to evaluate the total amount of inter-domain traffic of an ISP. Then, one possibility is to study how the introduction of ALTO reduces the total inter-domain traffic (inbound and/our outbound). If the ISPs intention is to localize the traffic inside his network, the network-internal traffic distribution will be of interest. Effectiveness of localization can be quantified in different ways, e.g., by the load on core routers and backbone links, or by considering more advanced effects, such as the average number of hops that traffic traverses inside a domain.
  • Application performance: The objective of ALTO is improve application performance. ALTO can be used by very different types applications, with different communication characteristics and requirements. For instance, if ALTO guidance achieves traffic localization, one would expect that applications achieve a higher throughput and/or smaller delays to retrieve data. If application-specific performance characteristics (e.g., video or audio quality) can be monitored, such metrics related to user experience could also help to analyze the benefit of an ALTO deployment. If available, selected statistics from the TCP/IP stack in hosts could be leveraged, too.

Of potential interest can also be the share of applications or customers that actually use an offered ALTO service, i.e., the adoption of the service.

Monitoring statistics can be aggregated, averaged, and normalized in different ways. This document does not mandate specific ways how to calculate metrics.

3.4.3. System and Service Performance

A number of interesting parameters can be measured at the ALTO server. [RFC7285] suggests certain ALTO-specific metrics to be monitored:

  • Requests and responses for each service listed in a Information Directory (total counts and size in bytes).
  • CPU and memory utilization
  • ALTO map updates
  • Number of PIDs
  • ALTO map sizes (in-memory size, encoded size, number of entries)

This data characterizes the workload, the system performance as well as the map data. Obviously, such data will depend on the implementation and the actual deployment of the ALTO service. Logging is also recommended in [RFC7285].

3.4.4. Monitoring Infrastructures

Understanding the impact of ALTO may require interaction between different systems, operating at different layers. Some information discussed in the preceding sections is only visible to an ISP, while application-level performance can hardly be measured inside the network. It is possible that not all information of potential interest can directly be measured, either because no corresponding monitoring infrastructure or measurement method exists, or because it is not easily accessible.

One way to quantify the benefit of deploying ALTO is to measure before and after enabling the ALTO service. In addition to passive monitoring, some data could also be obtained by active measurements, but due to the resulting overhead, the latter should be used with care. Yet, in all monitoring activities an ALTO service provider has to take into account that ALTO clients are not bound to ALTO server guidance as ALTO is only one source of information, and any measurement result may thus be biased.

Potential sources for monitoring the use of ALTO include:

  • Network Operations, Administration, and Maintenance (OAM) systems: Many ISPs deploy OAM systems to monitor the network traffic, which may have insight into traffic volumes, network topology, and bandwidth information inside the management area. Data can be obtained by SNMP, NETCONF, IP Flow Information Export (IPFIX), syslog, etc.
  • Applications/clients: Relevant data could be obtained by instrumentation of applications.
  • ALTO server: If available, log files or other statistics data could be analyzed.
  • Other application entities: In several use cases, there are other application entities that could provide data as well. For instance, there may be centralized log servers that collect data.

In many ALTO use cases some data sources are located within an ISP network while some other data is gathered at application level. Correlation of data could require a collaboration agreement between the ISP and an application owner, including agreements of data interchange formats, methods of delivery, etc. In practice, such a collaboration may not be possible in all use cases of ALTO, because the monitoring data can be sensitive, and because the interacting entities may have different priorities. Details of how to build an over-arching monitoring system for evaluating the benefits of ALTO are outside the scope of this memo.

3.5. Abstract Map Examples for Different Types of ISPs

3.5.1. Small ISP with Single Internet Uplink

The ALTO protocol does not mandate how to determine costs between endpoints and/or determine map data. In complex usage scenarios this can be a non-trivial problem. In order to show the basic principle, this and the following sections explain for different deployment scenarios how ALTO maps could be structured.

For a small ISP, the inter-domain traffic optimizing problem is how to decrease the traffic exchanged with other ISPs, because of high settlement costs. By using the ALTO service to optimize traffic, a small ISP can define two "optimization areas": one is its own network; the other one consists of all other network destinations. The cost map can be defined as follows: the cost of a link between clients of the inner ISP's network is lower than between clients of the outer ISP's network and clients of inner ISP's network. As a result, a host with an ALTO client inside the network of this ISP will prefer retrieving data from hosts connected to the same ISP.

An example is given in Figure 9. It is assumed that ISP A is a small ISP only having one access network. As operator of the ALTO service, ISP A can define its network to be one optimization area, named as PID1, and define other networks to be the other optimization area, named as PID2. C1 is denoted as the cost inside the network of ISP A. C2 is denoted as the cost from PID2 to PID1, and C3 from PID1 to PID2. For the sake of simplicity, in the following C2=C3 is assumed. In order to keep traffic local inside ISP A, it makes sense to define: C1<C2

       ////           \\\\
     //                   \\
   //                       \\                  /-----------\
  | +---------+               |             ////             \\\\
  | | ALTO    |  ISP A        |    C2      |    Other Networks   |
 |  | Service |  PID 1         <-----------     PID 2
  | +---------+  C1           |----------->|                     |
  |                           |  C3 (=C2)   \\\\             ////
   \\                       //                  \-----------/
     \\                   //
       \\\\           ////

Figure 9: Example ALTO deployment in small ISPs

A simplified extract of the corresponding ALTO network and cost maps is listed in Figure 10 and Figure 11, assuming that the network of ISP A has the IPv4 address ranges and In this example, the cost values C1 and C2 can be set to any number C1<C2.

   HTTP/1.1 200 OK
   Content-Type: application/alto-networkmap+json

     "network-map" : {
       "PID1" : {
         "ipv4" : [
       "PID2" : {
         "ipv4" : [
         "ipv6" : [

Figure 10: Example ALTO network map

   HTTP/1.1 200 OK
   Content-Type: application/alto-costmap+json

       "cost-type" : {"cost-mode"  : "numerical",
                      "cost-metric": "routingcost"
     "cost-map" : {
       "PID1": { "PID1": C1,  "PID2": C2 },
       "PID2": { "PID1": C2,  "PID2": 0 },

Figure 11: Example ALTO cost map

3.5.2. ISP with Several Fixed Access Networks

This example discusses a P2P application traffic optimization use case for a larger ISP with a fixed network comprising several access networks and a core network. The traffic optimizing objectives include (1) using the backbone network efficiently, (2) adjusting the traffic balance in different access networks according to traffic conditions and management policies, and (3) achieving a reduction of settlement costs with other ISPs.

Such a large ISP deploying an ALTO service may want to optimize its traffic according to the network topology of its access networks. For example, each access network could be defined to be one optimization area, i.e., traffic should be kept local withing that area if possible. This can be achieved by mapping each area to a PID. Then the costs between those access networks can be defined according to a corresponding traffic optimizing requirement by this ISP. One example setup is further described below and also shown in Figure 12.

In this example, ISP A has one backbone network and three access networks, named as AN A, AN B, and AN C. A P2P application is used in this example. For a reasonable application-level traffic optimization, the first requirement could be a decrease of the P2P traffic on the backbone network inside the Autonomous System of ISP A and the second requirement could be a decrease of the P2P traffic to other ISPs, i.e., other Autonomous Systems. The second requirement can be assumed to have priority over the first one. Also, we assume that the settlement rate with ISP B is lower than with other ISPs. ISP A can deploy an ALTO service to meet these traffic distribution requirements. In the following, we will give an example of an ALTO setting and configuration according to these requirements.

In the network of ISP A, the operator of the ALTO server can define each access network to be one optimization area, and assign one PID to each access network, such as PID 1, PID 2, and PID 3. Because of different peerings with different outer ISPs, one can define ISP B to be one additional optimization area and assign PID 4 to it. All other networks can be added to a PID to be one further optimization area (PID 5).

In the setup, costs (C1, C2, C3, C4, C5, C6, C7, C8) can be assigned as shown in Figure 12. Cost C1 is denoted as the link cost in inner AN A (PID 1), and C2 and C3 are defined accordingly. C4 is denoted as the link cost from PID 1 to PID 2, and C5 is the corresponding cost from PID 3, which is assumed to have a similar value. C6 is the cost between PID 1 and PID 3. For simplicity, this scenario assumes symmetrical costs between the AN this example. C7 is denoted as the link cost from the ISP B to ISP A. C8 is the link cost from other networks to ISP A.

According to previous discussion of the first requirement and the second requirement, the relationship of these costs will be defined as: (C1, C2, C3) < (C4, C5, C6) < (C7) < (C8)

 +------------------------------------+         +----------------+
 | ISP A   +---------------+          |         |                |
 |         |    Backbone   |          |   C7    |      ISP B     |
 |     +---+    Network    +----+     |<--------+      PID 4     |
 |     |   +-------+-------+    |     |         |                |
 |     |           |            |     |         |                |
 |     |           |            |     |         +----------------+
 | +---+--+     +--+---+     +--+---+ |
 | |AN A  |  C4 |AN B  |  C5 |AN C  | |
 | |PID 1 +<--->|PID 2 |<--->+PID 3 | |
 | |C1    |     |C2    |     |C3    | |         +----------------+
 | +---+--+     +------+     +--+---+ |         |                |
 |     ^                        ^     |   C8    | Other Networks |
 |     |                        |     |<--------+ PID 5          |
 |     +------------------------+     |         |                |
 |                  C6                |         |                |
 +------------------------------------+         +----------------+

Figure 12: ALTO deployment in large ISPs with layered fixed network structures

3.5.3. ISP with Fixed and Mobile Network

An ISP with both mobile network and fixed network may focus on optimizing the mobile traffic by keeping traffic in the fixed network as much as possible, because wireless bandwidth is a scarce resource and traffic is costly in mobile network. In such a case, the main requirement of traffic optimization could be decreasing the usage of radio resources in the mobile network. An ALTO service can be deployed to meet these needs.

Figure 13 shows an example: ISP A operates one mobile network, which is connected to a backbone network. The ISP also runs two fixed access networks AN A and AN B, which are also connected to the backbone network. In this network structure, the mobile network can be defined as one optimization area, and PID 1 can be assigned to it. Access networks AN A and B can also be defined as optimization areas, and PID 2 and PID 3 can be assigned, respectively. The cost values are then defined as shown in Figure 13.

To decrease the usage of wireless link, the relationship of these costs can be defined as follows:

From view of mobile network: C4 < C1. This means that clients in mobile network requiring data resources from other clients will prefer clients in AN A to clients in the mobile network. This policy can decrease the usage of wireless link and power consumption in terminals.

From view of AN A: C2 < C6, C5 = maximum cost. This means that clients in other optimization area will avoid retrieving data from the mobile network.

 |                                                                 |
 |  ISP A                 +-------------+                          |
 |               +--------+   ALTO      +---------+                |
 |               |        |   Service   |         |                |
 |               |        +------+------+         |                |
 |               |               |                |                |
 |               |               |                |                |
 |               |               |                |                |
 |       +-------+-------+       | C6    +--------+------+         |
 |       |     AN A      |<--------------|      AN B     |         |
 |       |     PID 2     |   C7  |       |      PID 3    |         |
 |       |     C2        |-------------->|      C3       |         |
 |       +---------------+       |       +---------------+         |
 |             ^    |            |              |     ^            |
 |             |    |            |              |     |            |
 |             |    |C4          |              |     |            |
 |          C5 |    |            |              |     |            |
 |             |    |   +--------+---------+    |     |            |
 |             |    +-->|  Mobile Network  |<---+     |            |
 |             |        |  PID 1           |          |            |
 |             +------- |  C1              |----------+            |
 |                      +------------------+                       |

Figure 13: ALTO deployment in ISPs with mobile network

These examples show that for ALTO in particular the relationships between different costs matter; the operator of the server has several degrees of freedom how to set the absolute values.

3.6. Comprehensive Example for Map Calculation

In addition to the previous, abstract examples, this section presents a more detailed scenario with a realistic IGP and BGP routing protocol configuration. This example was first described in [I-D.seidel-alto-map-calculation].

3.6.1. Example Network

Figure 14 depicts a network which is used to explain the steps carried out in the course of this example. The network consists of nine routers (R1 to R9). Two of them are border routers (R1 + R8) connected to neighbored networks (AS 2 to AS 4). Furthermore, AS 4 is not directly connected to the local network, but has AS 3 as transit network. The links between the routers are point-to-point connections, hence a /30 subnet is sufficient for each. These connections also form the core network with the subnet. This subnet is large enough to provide /30 subnets for all router interconnections. In addition to the core network, the local network also has five client networks attached to five different routers (R2, R5, R6, R7 and R9). Each client network is a /24 subnet with 100.1.10x.0 (x = [1..5]) as network address.

+--------------+      +--------+      +--------+     +--------------+
|   R6   |      |   R7   +-----+|
+--------------+      +----+---+      +----+---+     +--------------+
                           |               |
+--------------+           |               |
|     AS 2     |           |               |
| |           |               |
+-------+------+           |               |
        |                  |               |
        |                  |               |
    +---+----+        +----+---+      +----+---+     +--------------+
    |   R1   +--------+   R3   +------+   R5   |-----+|
    +---+----+        +----+---+      +----+---+     +--------------+
        |     \      /     |               |
        |      \    /      |               |
        |       \  /       |               |         +--------------+
        |        \/        |               |         |     AS 4     |
        |        /\        |               |         | |
        |       /  \       |               |         +------+-------+
        |      /    \      |               |                |
        |     /      \     |               |                |
    +---+----+        +----+---+      +----+---+     +------+-------+
    |   R2   |        |   R4   |      |   R8   +-----+     AS 3     |
    +---+----+        +----+---+      +----+---+     | |
        |                  |               |         +--------------+
        |                  |               |
        |                  |               |
+-------+------+           |          +----+---+     +--------------+
||           +----------+   R9   +-----+|
+--------------+                      +--------+     +--------------+

Figure 14: Example Network

The example network utilizes two different routing protocols, one for IGP and another for EGP routing. The used IGP is a link-state protocol such as IS-IS. The applied link weights are shown in Figure 2. To obtain the topology and routing information from the network, the topology data source must be connected directly to one of the routers (R1...R9). Furthermore, the topology data source must be enabled to communicate with the router and vice versa.

The Border Gateway Protocol (BGP) is used in this scenario to route between autonomous systems (AS). External BGP is running on the two border routers R1 and R8. Furthermore, internal BGP is used to propagate external as well as internal prefixes within the network boundaries. It is running on every router with an attached client network (R2, R5, R6, R7 and R9). Since no route reflector is present it is necessary to fetch routes from each BGP router separately.

           R1   R2   R3   R4   R5   R6   R7   R8   R9
       R1   0   15   15   20    -    -    -    -    -
       R2  15    0   20    -    -    -    -    -    -
       R3  15   20    0    5    5   10    -    -    -
       R4  20    -    5    0    5    -    -    -   20
       R5   -    -    5    5    0    -   10   10    -
       R6   -    -   10    -    -    0    -    -    -
       R7   -    -    -    -   10    -    0    -    -
       R8   -    -    -    -   10    -    -    0   10
       R9   -    -    -   20    -    -    -   10    0

Figure 15: Example Network Link Weights

For monitoring purposes it is possible to enable e.g. SNMP or NETCONF on the routers within the network. This way an ALTO server may obtain several additional information about the state of the network. As example, utilization, latency, and bandwidth information could be retrieved periodically from the network components to get and keep an up-to-date view on the network situation.

In the following, it is assumed that the listed information are collected from the network:

  • IS-IS: topology, Link weights
  • BGP: prefixes, AS numbers, AS distances, metrics
  • SNMP: latency, utilization, bandwidth

3.6.2. Potential Data Processing and Storage

Due to the variety of data source available in a network it may be necessary to aggregate the information and define a suitable data model that can hold the information efficiently and easily accessible. One potential model is an annotated directed graph that represents the topology. The attributes can be annotated at the corresponding positions in the graph. In the following it is shown how such a topology graph could describe the example topology.

In the topology graph, a node represents a router in the network, while the edges stand for the links that connect the routers. Both routers and links have a set of attributes that store information gathered from the network.

Each router could be associated with a basic set of information, such as:

  • ID: Unique ID within the network to identify the router.
  • Neighbor IDs: List of directly connected routers.
  • Endpoints: List of connected endpoints. The endpoints may also have further attributes themselves depending on the network and address type. Such potential attributes are costs for reaching the endpoint from the router, AS numbers, or AS distances. Endpoints may belong to more than one router, for example when they are assigned to link interfaces.

In addition to the basic set many more attributes may be assigned to router nodes. This mainly depends on the utilized data sources. Examples for such additional attributes are geographic location, host name and/ or interface types, just to name a few.

A suitable information set for a link is described in the following list:

  • Source ID: ID of the source router of the link.
  • Destination ID: ID of the destination router of the link.
  • Weight: The cost to cross the link defined by the used IGP.

Additional attributes that provide technical details and state information can be assigned to links as well. The availability of such additional attributes depends on the utilized data sources. Such attributes can be characteristics like maximum bandwidth, utilization, or latency on the link as well as the link type.

The example network shown in Figure 14 represents such an internal network graph where the routers R1 to R9 represent the nodes and the connections between them are the links. For instance, R2 has one directly attached IPv4 endpoint that belongs to its own AS.

   ID:  2

   Neighbor IDs:  1,3 (R1, R3)



      Metric:  10 (default client subnet metric)

      ASNumber:  1 (our own AS)

      ASDistance:  0

   Host Name:  R2

Figure 16: Example Router R2

Router R8 has two attached IPv4 endpoints. The first one belongs to a directly neighbored AS with AS number 3. The AS distance from our network to AS3 is 1. The second endpoint belongs to an AS (AS4) that is no direct neighbor but directly connected to AS3. To reach endpoints in AS4 it is necessary to cross AS3, which increases the AS distance by one.

   ID:  8

   Neighbor IDs:  5,9 (R5, R9)



      Metric:  100

      ASNumber:  3

      ASDistance:  1


      Metric:  200

      ASNumber:  4

      ASDistance:  2

   Host Name:  R8

Figure 17: Example Router R8

In the example, the link attributes are equal for all links and only their values differ. It is assumed that the attributes utilization, bandwidth, and latency are collected e.g. via SNMP or NETCONF. In the topology of Figure 14 the links between R1 and R2 would then have the following link attributes: