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Versions: (draft-kiesel-alto-xdom-disc) 00 01 02

ALTO                                                           S. Kiesel
Internet-Draft                                   University of Stuttgart
Intended status: Standards Track                          M. Stiemerling
Expires: September 6, 2018                                          H-DA
                                                           March 5, 2018


   Application Layer Traffic Optimization (ALTO) Cross-Domain Server
                               Discovery
                      draft-ietf-alto-xdom-disc-02

Abstract

   The goal of Application-Layer Traffic Optimization (ALTO) is to
   provide guidance to applications that have to select one or several
   hosts from a set of candidates capable of providing a desired
   resource.  ALTO is realized by a client-server protocol.  Before an
   ALTO client can ask for guidance it needs to discover one or more
   ALTO servers that can provide suitable guidance.

   In some deployment scenarios, in particular if the information about
   the network topology is partitioned and distributed over several ALTO
   servers, it may be needed to discover an ALTO server outside of the
   own network domain, in order to get appropriate guidance.  This
   document details applicable scenarios, itemizes requirements, and
   specifies a procedure for ALTO cross-domain server discovery.

   Technically, the procedure specified in this document takes one
   IP address or prefix and a U-NAPTR Service Parameter (i.e., "ALTO:
   http" or "ALTO:https") as parameters.  It performs DNS lookups (for
   NAPTR resource records in the in-addr.arpa. or ip6.arpa. tree) and
   returns one or more URI(s) of information resources related to that
   IP address or prefix.


















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Terminology and Requirements Language

   This document makes use of the ALTO terminology defined in RFC 5693
   [RFC5693].

   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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 6, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.











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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  ALTO Cross-Domain Server Discovery Procedure Specification . .  5
     2.1.  Interface  . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Step 1: Prepare Domain Name for Reverse DNS Lookup . . . .  6
     2.3.  Step 2: Prepare Shortened Domain Names . . . . . . . . . .  6
     2.4.  Step 3: Perform DNS U-NAPTR lookups  . . . . . . . . . . .  7
   3.  Using the ALTO Protocol with ALTO Cross-Domain Server
       Discovery  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Network and Cost Map Service . . . . . . . . . . . . . . .  8
     3.2.  Map-Filtering Service  . . . . . . . . . . . . . . . . . .  9
     3.3.  Endpoint Property Service  . . . . . . . . . . . . . . . .  9
     3.4.  Endpoint Cost Service  . . . . . . . . . . . . . . . . . . 10
   4.  Implementation, Deployment, and Operational Considerations . . 12
     4.1.  Considerations for ALTO Clients  . . . . . . . . . . . . . 12
     4.2.  Deployment Considerations for Network Operators  . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
     5.1.  Integrity of the ALTO Server's URI . . . . . . . . . . . . 14
     5.2.  Availability of the ALTO Server Discovery Procedure  . . . 15
     5.3.  Confidentiality of the ALTO Server's URI . . . . . . . . . 16
     5.4.  Privacy for ALTO Clients . . . . . . . . . . . . . . . . . 16
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Multiple Information Sources and Partitioned
                Knowledge . . . . . . . . . . . . . . . . . . . . . . 20
     A.1.  Classification of Solution Approaches  . . . . . . . . . . 20
     A.2.  Discussion of Solution Approaches  . . . . . . . . . . . . 21
     A.3.  The Need for Cross-Domain ALTO Server Discovery  . . . . . 21
     A.4.  Our Solution Approach  . . . . . . . . . . . . . . . . . . 22
     A.5.  Relation to the ALTO Requirements  . . . . . . . . . . . . 22
   Appendix B.  Requirements for ALTO Cross-Domain Server
                Discovery . . . . . . . . . . . . . . . . . . . . . . 23
     B.1.  Discovery Client Application Programming Interface . . . . 23
     B.2.  Data Storage and Authority Requirements  . . . . . . . . . 23
     B.3.  Cross-Domain Operations Requirements . . . . . . . . . . . 23
     B.4.  Protocol Requirements  . . . . . . . . . . . . . . . . . . 24
     B.5.  Further Requirements . . . . . . . . . . . . . . . . . . . 24
   Appendix C.  ALTO and Tracker-based Peer-to-Peer Applications  . . 25
   Appendix D.  Contributors List and Acknowledgments . . . . . . . . 30
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31








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1.  Introduction

   The goal of Application-Layer Traffic Optimization (ALTO) is to
   provide guidance to applications that have to select one or several
   hosts from a set of candidates capable of providing a desired
   resource [RFC5693].  ALTO is realized by an HTTP-based client-server
   protocol [RFC7285], which can be used in various scenarios [RFC7971].

   The ALTO base protocol document [RFC7285] specifies the communication
   between an ALTO client and one ALTO server.  In principle, the client
   may send any ALTO query.  For example, it might ask for the routing
   cost between any two IP addresses, or it might request network and
   cost maps for the whole network, which might be the worldwide
   Internet.  It is assumed that the server can answer any query,
   possibly with some kind of default value if no exact data is known.

   No special provisions were made for deployment scenarios with
   multiple ALTO servers, with some servers having more accurate
   information about some parts of the network topology while others
   having better information about other parts of the network
   ("partitioned knowledge").  Various ALTO use cases have been studied
   in the context of such scenarios.  In some cases, one cannot assume
   that a topologically nearby ALTO server (e.g., a server discovered
   with the procedure specified in [RFC7286]) will always provide useful
   information to the client.  One such scenario is detailed in
   Appendix C.  Several solution approaches, such as redirecting a
   client to a server that has more accurate information or forwarding
   the request to it on behalf of the client, have been proposed and
   analyzed (see Appendix A), but none has been specified so far.

   This document specifies an ALTO server discovery procedure that runs
   on the client side.  An ALTO client, which wants to send a query
   related to a specific IP address or prefix X, may use the procedure
   specified in Section 2 with X as a parameter, in order to perform DNS
   lookups and find an ALTO server that can provide a competent answer.
   The wording "related to" in the previous sentence is intentionally
   kept somewhat vague, as the exact semantics depends on the ALTO
   service to be used; see Section 3 for details.

   Those who are in control of the "reverse DNS" (i.e., the
   corresponding subdomain of in-addr.arpa. or ip6.arpa.) for a given IP
   address or prefix - typically an Internet Service Provider (ISP), a
   corporate IT department, or a university's computing center - may add
   resource records to the DNS that point to a suitable ALTO server.  In
   many cases, it may be an ALTO server run by that ISP or IT
   department, as they naturally have good insight into routing costs
   from and to their networks.  However, they may also refer to an ALTO
   server run by a different organization, e.g., their upstream ISP.



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2.  ALTO Cross-Domain Server Discovery Procedure Specification

   This procedure was inspired by [RFC7216] and re-uses parts of
   [RFC7286].

   The procedure sequentially tries two different lookup strategies.
   First, an ALTO-specific U-NAPTR record is searched in the "reverse
   tree", i.e., in subdomains of in-addr.arpa. or ip6.arpa.
   corresponding to the given IP address or prefix.  If this lookup does
   not yield a usable result, further lookups with truncated domain
   names may be tried.  The goal is to allow deployment scenarios that
   require fine-grained discovery on a per-IP basis, as well as large-
   scale scenarios where discovery is to be enabled for a large number
   of IP addresses with a small number of additional DNS resource
   records.

2.1.  Interface

   The procedure specified in this document takes one IP address or
   prefix X and a U-NAPTR Service Parameter as parameters.

   The parameter X may be an IPv4 or an IPv6 address or prefix in CIDR
   notation (see [RFC4632] for the IPv4 CIDR notation and [RFC4291] for
   IPv6).  In both cases, it consists of an IP address A and a prefix
   length L. For IPv4, it holds: 0 <= L <= 32 and for IPv6, it holds: 0
   <= L <= 128.

   For example, for X=198.51.100.0/24, we get A=198.51.100.0 and L=24.
   Similarly, for X=2001:0DB8::20/128, we get A=2001:0DB8::20 and L=128.

   The procedure SHOULD always be called with the U-NAPTR Service
   Parameter [RFC4848] set to "ALTO:https".  However, other parameter
   values MAY be used in some scenarios, e.g., "ALTO:http" to request
   unencrypted transmission for debugging purposes, or other application
   protocol or service tags if applicable.

   The procedure performs DNS lookups and returns one or more URI(s) of
   information resources related to that IP address or prefix, usually
   the URI(s) of one or more ALTO Information Resource Directory (IRD,
   see Section 9 of [RFC7285]).

   For the remainder of the document, we use the notation:
   IRD_URIS_X := XDOMDISC(X,"ALTO:https")








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2.2.  Step 1: Prepare Domain Name for Reverse DNS Lookup

   If A is an IPv4 address, a domain name R32 is constructed according
   to the rules specified in Section 3.5 of [RFC1035] and it is rooted
   in the special domain "IN-ADDR.ARPA.".

   For example, A=198.51.100.3 yields R32="3.100.51.198.IN-ADDR.ARPA.".

   If A is an IPv6 address, the domain name R128 is constructed
   according to the rules specified in Section 2.5 of [RFC3596] and the
   special domain "IP6.ARPA." is used.

   For example (note: a line break was added after the second line),
   A = 2001:0DB8::20    yields
   R128 = "0.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.B.D.0.
           1.0.0.2.IP6.ARPA."

2.3.  Step 2: Prepare Shortened Domain Names

   For this step, an auxiliary function "skip" is defined as follows:
   skip(str,n) will skip all characters in the string str, up to and
   including the n-th dot, and return the remaining part of str.  For
   example, skip("foo.bar.baz.qux.quux.",2) will return "baz.qux.quux.".

   If A is an IPv4 address, the following additional domain names are
   generated from the result of the previous step: R24=skip(R32,1),
   R16=skip(R32,2), and R8=skip(R32,3).  Removing one label from a
   domain name (i.e., one number of the "dotted quad notation")
   corresponds to shortening the prefix length by 8 bits.

   For example, R32="3.100.51.198.IN-ADDR.ARPA." yields
   R24="100.51.198.IN-ADDR.ARPA.", R16="51.198.IN-ADDR.ARPA.", and
   R8="198.IN-ADDR.ARPA.".

   If A is an IPv6 address, the following additional domain names are
   generated from the result of the previous step: R64=skip(R128,16),
   R56=skip(R128,18), R48=skip(R128,20), and R32=skip(R128,24).
   Removing one label from a domain name (i.e., one hex digit)
   corresponds to shortening the prefix length by 4 bits.

   For example (note: a line break was added after the first line),
   R128 = "0.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.B.D.0.
           1.0.0.2.IP6.ARPA."    yields
   R64  = "0.0.0.0.0.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.",
   R56  = "0.0.0.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.",
   R48  = "0.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA.", and
   R32  = "8.B.D.0.1.0.0.2.IP6.ARPA."




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2.4.  Step 3: Perform DNS U-NAPTR lookups

   The address type of A (i.e., IPv4 or IPv6) and the value of L are
   matched against the first and the second column of the following
   table, respectively:

   +-------------+-------------+-------------+-----------------------+
   | 1: Addresss | 2: Prefix   | 3: MUST     | 4: SHOULD do further  |
   | Type of A   | Length L    | lookup first| lookups in that order |
   +-------------+-------------+-------------+-----------------------+
   | IPv4        |        32   |  R32        | R24, R16, R8          |
   | IPv4        |  24 .. 31   |  R24        | R16, R8               |
   | IPv4        |  16 .. 23   |  R16        | R8                    |
   | IPv4        |   8 .. 15   |   R8        | (none)                |
   | IPv4        |   0 ..  7   | (none, procedure fails w/o result)  |
   +-------------+-------------+-------------+-----------------------+
   | IPv6        |       128   | R128        | R64, R56, R48, R32    |
   | IPv6        | 64 (..127)  |  R64        | R56, R48, R32         |
   | IPv6        |  56 .. 63   |  R56        | R48, R32              |
   | IPv6        |  48 .. 55   |  R48        | R32                   |
   | IPv6        |  32 .. 47   |  R32        | (none)                |
   | IPv6        |   0 .. 31   | (none, procedure fails w/o result)  |
   +-------------+-------------+-------------+-----------------------+

   Then, the domain name given in column 3 and the U-NAPTR Service
   Parameter the procedure was called with (usually "ALTO:https") MUST
   be used for an U-NAPTR [RFC4848] lookup, in order to obtain one or
   more URIs (indicating protocol, host, and possibly path elements) for
   the ALTO server's Information Resource Directory (IRD).  If such
   URI(s) can be found, the ALTO Cross-Domain Server Discovery Procedure
   returns that information to the caller and terminates successfully.

   For example, the following two U-NAPTR resource records can be used
   for mapping "100.51.198.IN-ADDR.ARPA." (i.e., R24 from the example in
   the previous step) to the HTTPS URIs "https://alto1.example.net/ird"
   and "https://alto2.example.net/ird", with the former being preferred.

       100.51.198.IN-ADDR.ARPA.  IN NAPTR 100  10  "u"  "ALTO:https"
            "!.*!https://alto1.example.net/ird!"  ""

       100.51.198.IN-ADDR.ARPA.  IN NAPTR 100  20  "u"  "ALTO:https"
            "!.*!https://alto2.example.net/ird!"  ""

   If no matching U-NAPTR records can be found, the procedure SHOULD try
   further lookups, using the domain names from column 4 in the
   indicated order, until one lookup succeeds.  If no IRD URI could be
   found after looking up all domain names from column 3 and column 4,
   the procedure terminates unsuccessfully, without producing a result.



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3.  Using the ALTO Protocol with ALTO Cross-Domain Server Discovery

   Based on a modular design principle, ALTO provides several ALTO
   services, each consisting of a set of information resouces that can
   be accessed using the ALTO protocol.  The ALTO protocol specification
   defines the following ALTO services and their corresponding
   information resouces:

   o  Network and Cost Map Service, see Section 11.2 of [RFC7285]

   o  Map-Filtering Service, see Section 11.3 of [RFC7285]

   o  Endpoint Property Service, see Section 11.4 of [RFC7285]

   o  Endpoint Cost Service, see Section 11.5 of [RFC7285]

   Extension documents may specify further information resources;
   however, these are out of scope of this document.  The information
   resources that are available at a specific ALTO server are listed in
   its Information Resource Directory (IRD, see Section 9 of [RFC7285]).

   The ALTO Cross-Domain Server Discovery Procedure is most useful in
   conjunction with the Endpoint Property Service and the Endpoint Cost
   Service.  However, for the sake of completeness, possible interaction
   with all four services is discussed below.

3.1.  Network and Cost Map Service

   An ALTO client may invoke the ALTO Cross-Domain Server Discovery
   Procedure (as specified in Section 2) for an IP address or prefix "X"
   and get a list of one or more IRD URI(s):
   IRD_URIS_X := XDOMDISC(X,"ALTO:https").  These IRD(s) will always
   contain a network and a cost map, as these are mandatory information
   ressources (see Section 11.2 of [RFC7285]).  However, the cost matrix
   may be very sparse.  If, according to the network map, PID_X is the
   PID that contains the IP address or prefix X, and PID_1, PID_2,
   PID_3, ... are other PIDS, the cost map may look like this:

   From \ To PID_1    PID_2    PID_X    PID_3
   ------+-----------------------------------
   PID_1 |                        92
   PID_2 |                         6
   PID_X |      46        3        1       19
   PID_3 |                        38

   In this example, all cells outside column "X" and row "X" are
   unspecified.  A cost map with this structure contains the same
   information as what could be retrieved using the ECS, cases 1 and 2



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   in Section 3.4.  Accessing cells outside column "X" and row "X" may
   not yield useful results.

   Trying to assemble a more densely populated cost map from several
   cost maps with this very sparse structure may be a non-trivial task,
   as different ALTO servers may use different PID definitions (i.e.,
   network maps) and incompatible scales for the costs, in particular
   for the "routingcost" metric.

3.2.  Map-Filtering Service

   An ALTO client may invoke the ALTO Cross-Domain Server Discovery
   Procedure (as specified in Section 2) for an IP address or prefix "X"
   and get a list of one or more IRD URI(s): IRD_URIS_X :=
   XDOMDISC(X,"ALTO:https").  These IRD(s) may provide the optional Map-
   Filtering Service (see Section 11.3 of [RFC7285]).  This service
   returns a subset of the full map, as specified by the client.  As
   discussed in Section 3.1, a cost map may be very sparse in the
   envisioned deployment scenario.  Therefore, depending on the
   filtering criteria provided by the client, this service may return
   results similar to the Endpoint Cost Service, or it may not return
   any useful result.

3.3.  Endpoint Property Service

   If an ALTO client wants to query an Endpoint Property Service (see
   Section 11.4 of RFC 7285 [RFC7285]) about an endpoint with IP address
   "X" or a group of endpoints within IP prefix "X", respectively, it
   has to perform the following steps:

   1.  Invoke the ALTO Cross-Domain Server Discovery Procedure (as
       specified in Section 2): IRD_URIS_X := XDOMDISC(X,"ALTO:https")

   2.  The result IRD_URIS_X is a list of one or more Information
       Resource Directories (IRD, see Section 9 of [RFC7285]).  Check
       each of these IRDs for a suitable Endpoint Property Service and
       query it.

   If the ALTO client wants to do a similar Endpoint Property query for
   a different IP address or prefix "Y", the whole procedure has to be
   repeated, as IRD_URIS_Y := XDOMDISC(Y,"ALTO:https") may yield a
   different list of IRDs.  Of course, the results of individual DNS
   queries may be cached as indicated by their respective time-to-live
   (TTL) values.







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3.4.  Endpoint Cost Service

   The ALTO Endpoint Cost Service (ECS, see Section 11.5 of RFC 7285
   [RFC7285]) provides information about costs between individual
   endpoints and it also supports ranking.  The ECS allows that
   endpoints may be denoted by IP addresses or prefixes.  The ECS is
   called with a list of one or more source IP addresses or prefixes,
   which we will call (S1, S2, S3, ...), and a list of one or more
   destination IP addresses or prefixes, which we will call (D1, D2, D3,
   ...).

   This specification distinguishes several cases, regarding the number
   of elements in the list of source and destination addresses,
   respectively:

   1.  Exactly one source address S1 and more than one destination
       addresses D1, D2, D3, ...  In this case, the ALTO client has to
       perform the following steps:

       1.  Invoke the ALTO Cross-Domain Server Discovery Procedure (as
           specified in Section 2):
           IRD_URIS_S1 := XDOMDISC(S1,"ALTO:https")

       2.  The result IRD_URIS_S1 is a list of one or more Information
           Resource Directories (IRD, see Section 9 of [RFC7285]).
           Check each of these IRDs for a suitable ECS and query it.

   2.  More than one source addresses S1, S2, S3, ... and exactly one
       destination address D1.  In this case, the ALTO client has to
       perform the following steps:

       1.  Invoke the ALTO Cross-Domain Server Discovery Procedure (as
           specified in Section 2):
           IRD_URIS_D1 := XDOMDISC(D1,"ALTO:https")

       2.  The result IRD_URIS_D1 is a list of one or more Information
           Resource Directories (IRD, see Section 9 of [RFC7285]).
           Check each of these IRDs for a suitable ECS and query it.

   3.  Exactly one source address S1 and exactly one destination address
       D1.  The ALTO client may perform the same steps as in case 1, as
       specified above.  As an alternative, it may also perform the same
       steps as in case 2, as specified above.

   4.  More than one source addresses S1, S2, S3, ... and more than one
       destination addresses D1, D2, D3, ...  In this case, the ALTO
       client should split the list of source addresses, and perform
       separately for each source address the same steps as in case 1,



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       as specified above.  As an alternative, the ALTO client may also
       split the list of destination addresses, and perform separately
       for each destination address the same steps as in case 2, as
       specified above.  However, comparing results between these sub-
       queries may be difficult, in particular if the cost metric is a
       relative preference without a well-defined base unit (e.g., the
       "routingcost" metric).












































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4.  Implementation, Deployment, and Operational Considerations

4.1.  Considerations for ALTO Clients

4.1.1.  Resource Consumer Initiated Discovery

   To some extent, ALTO requirement AR-32 [RFC6708], i.e., resource
   consumer initiated ALTO server discovery, can be seen as a special
   case of cross-domain ALTO server discovery.  To that end, an ALTO
   client embedded in a resouce consumer would have to figure out its
   own "public" IP address and perform the procedures described in this
   document on that address.  However, due to the widespread deployment
   of Network Address Translators (NAT), additional protocols and
   mechanisms such as STUN [RFC5389] would be needed and considerations
   for UNSAF [RFC3424] apply.  Therefore, using the procedures specified
   in this document for resource consumer based ALTO server discovery is
   generally NOT RECOMMENDED.  Note that a less versatile yet simpler
   approach for resource consumer initiated ALTO server discovery is
   specified in [RFC7286].

4.1.2.  IPv4/v6 Dual Stack, Multihoming, NAT, and Host Mobility

   The procedure specified in this document can discover ALTO server
   URIs for a given IP address or prefix.  The intention is, that a
   third party (e.g., a resource directory) that receives query messages
   from a resource consumer can use the source address in these messages
   to discover suitable ALTO servers for this specific resource
   consumer.

   However, resource consumers (as defined in Section 2 of [RFC5693])
   may reside on hosts with more than one IP address, e.g., due to
   IPv4/v6 dual stack operation and/or multihoming.  IP packets sent
   with different source addresses may be subject to different routing
   policies and path costs.  In some deployment scenarios, it may even
   be required to ask different sets of ALTO servers for guidance.
   Furthermore, source addresses in IP packets may be modified en-route
   by Network Address Translators (NAT).

   If a resource consumer queries a resource directory for candidate
   resource providers, the locally selected (and possibly en-route
   translated) source address of the query message - as observed by the
   resource directory - will become the basis for the ALTO server
   discovery and the subsequent optimization of the resource directory's
   reply.  If, however, the resource consumer then selects different
   source addresses to contact returned resource providers, the desired
   better-than-random "ALTO effect" may not occur.

   Therefore, a dual stack or multihomed resource consumer SHOULD either



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   always use the same address for contacting the resource directory and
   the resource providers, i.e., overriding the operating system's
   automatic source IP address selection, or use resource consumer based
   ALTO server discovery [RFC7286] to discover suitable ALTO servers for
   every local address and then locally perform ALTO-influenced resource
   consumer selection and source address selection.  Similarly, resource
   consumers on mobile hosts SHOULD query the resource directory again
   after a change of IP address, in order to get a list of candidate
   resource providers that is optimized for the new IP address.

4.2.  Deployment Considerations for Network Operators

4.2.1.  Separation of Interests

   We assume that if two organizations share parts of their DNS
   infrastructure, i.e., have common in-addr.arpa. and/or ip6.arpa.
   subdomains, they will also be able to operate a common ALTO server,
   which still may do redirections if desired or required by policies.

   Note that the ALTO server discovery procedure is supposed to produce
   only a first URI of an ALTO server that can give reasonable guidance
   to the client.  An ALTO server can still return different results
   based on the client's address (or other identifying properties) or
   redirect the client to another ALTO server using mechanisms of the
   ALTO protocol (see Sect. 9 of [RFC7285]).


























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5.  Security Considerations

   A high-level discussion of security issues related to ALTO is part of
   the ALTO problem statement [RFC5693].  A classification of unwanted
   information disclosure risks, as well as specific security-related
   requirements can be found in the ALTO requirements document
   [RFC6708].

   The remainder of this section focuses on security threats and
   protection mechanisms for the cross-domain ALTO server discovery
   procedure as such.  Once the ALTO server's URI has been discovered
   and the communication between the ALTO client and the ALTO server
   starts, the security threats and protection mechanisms discussed in
   the ALTO protocol specification [RFC7285] apply.

5.1.  Integrity of the ALTO Server's URI

   Scenario Description
      An attacker could compromise the ALTO server discovery procedure
      or infrastructure in a way that ALTO clients would discover a
      "wrong" ALTO server URI.

   Threat Discussion
      This is probably the most serious security concern related to ALTO
      server discovery.  The discovered "wrong" ALTO server might not be
      able to give guidance to a given ALTO client at all, or it might
      give suboptimal or forged information.  In the latter case, an
      attacker could try to use ALTO to affect the traffic distribution
      in the network or the performance of applications (see also
      Section 15.1. of [RFC7285]).  Furthermore, a hostile ALTO server
      could threaten user privacy (see also Section 5.2.1, case (5a) in
      [RFC6708]).

      However, it should also be noted that, if an attacker was able to
      compromise the DNS infrastructure used for cross-domain ALTO
      server discovery, (s)he could also launch significantly more
      serious other attacks (e.g., redirecting various application
      protocols).

   Protection Strategies and Mechanisms
      The cross-domain ALTO server discovery procedure relies on a
      series of DNS lookups.  If an attacker was able to modify or spoof
      any of the DNS records, the resulting URI could be replaced by a
      forged URI.  The application of DNS security (DNSSEC) [RFC4033]
      provides a means to limit attacks that rely on modification of the
      DNS records while in transit.  Additional operational precautions
      for safely operating the DNS infrastructure are required in order
      to ensure that name servers do not sign forged (or otherwise



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      "wrong") resource records.  Security considerations specific to
      U-NAPTR are described in more detail in [RFC4848].

      A related risk is the impersonation of the ALTO server (i.e.,
      attacks after the correct URI has been discovered).  This threat
      and protection strategies are discussed in Section 15.1 of
      [RFC7285].  Note that if TLS is used to protect ALTO, the server
      certificate will contain the host name (CN).  Consequently, only
      the host part of the HTTPS URI will be authenticated, i.e., the
      result of the ALTO server discovery procedure.  The DNS/U-NAPTR
      based mapping within the cross-domain ALTO server discovery
      procedure needs to be secured as described above, e.g., by using
      DNSSEC.

      In addition to active protection mechanisms, users and network
      operators can monitor application performance and network traffic
      patterns for poor performance or abnormalities.  If it turns out
      that relying on the guidance of a specific ALTO server does not
      result in better-than-random results, the usage of the ALTO server
      may be discontinued (see also Section 15.2 of [RFC7285]).

5.2.  Availability of the ALTO Server Discovery Procedure

   Scenario Description
      An attacker could compromise the cross-domain ALTO server
      discovery procedure or infrastructure in a way that ALTO clients
      would not be able to discover any ALTO server.

   Threat Discussion
      If no ALTO server can be discovered (although a suitable one
      exists) applications have to make their decisions without ALTO
      guidance.  As ALTO could be temporarily unavailable for many
      reasons, applications must be prepared to do so.  However, The
      resulting application performance and traffic distribution will
      correspond to a deployment scenario without ALTO.

   Protection Strategies and Mechanisms
      Operators should follow best current practices to secure their DNS
      and ALTO (see Section 15.5 of [RFC7285]) servers against Denial-
      of-Service (DoS) attacks.











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5.3.  Confidentiality of the ALTO Server's URI

   Scenario Description
      An unauthorized party could invoke the cross-domain ALTO server
      discovery procedure, or intercept discovery messages between an
      authorized ALTO client and the DNS servers, in order to acquire
      knowledge of the ALTO server URI for a specific IP address.

   Threat Discussion
      In the ALTO use cases that have been described in the ALTO problem
      statement [RFC5693] and/or discussed in the ALTO working group,
      the ALTO server's URI as such has always been considered as public
      information that does not need protection of confidentiality.

   Protection Strategies and Mechanisms
      No protection mechanisms for this scenario have been provided, as
      it has not been identified as a relevant threat.  However, if a
      new use case is identified that requires this kind of protection,
      the suitability of this ALTO server discovery procedure as well as
      possible security extensions have to be re-evaluated thoroughly.

5.4.  Privacy for ALTO Clients

   Scenario Description
      An unauthorized party could intercept messages between an ALTO
      client and the DNS servers, and thereby find out the fact that
      said ALTO client uses (or at least tries to use) the ALTO service
      in order to optimize traffic from/to a specific IP address.

   Threat Discussion
      In the ALTO use cases that have been described in the ALTO problem
      statement [RFC5693] and/or discussed in the ALTO working group,
      this scenario has not been identified as a relevant threat.

   Protection Strategies and Mechanisms
      No protection mechanisms for this scenario have been provided, as
      it has not been identified as a relevant threat.  However, if a
      new use case is identified that requires this kind of protection,
      the suitability of this ALTO server discovery procedure as well as
      possible security extensions have to be re-evaluated thoroughly.











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6.  IANA Considerations

   This document does not require any IANA action.
















































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7.  References

7.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              October 2003.

   [RFC4848]  Daigle, L., "Domain-Based Application Service Location
              Using URIs and the Dynamic Delegation Discovery Service
              (DDDS)", RFC 4848, April 2007.

7.2.  Informative References

   [I-D.kiesel-alto-alto4alto]
              Kiesel, S., "Using ALTO for ALTO server selection",
              draft-kiesel-alto-alto4alto-00 (work in progress),
              July 2010.

   [I-D.kiesel-alto-ip-based-srv-disc]
              Kiesel, S. and R. Penno, "Application-Layer Traffic
              Optimization (ALTO) Anycast Address",
              draft-kiesel-alto-ip-based-srv-disc-03 (work in progress),
              July 2014.

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291,
              February 2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632,
              August 2006, <http://www.rfc-editor.org/info/rfc4632>.




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   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693,
              October 2009.

   [RFC6708]  Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
              Y. Yang, "Application-Layer Traffic Optimization (ALTO)
              Requirements", RFC 6708, September 2012.

   [RFC7216]  Thomson, M. and R. Bellis, "Location Information Server
              (LIS) Discovery Using IP Addresses and Reverse DNS",
              RFC 7216, April 2014.

   [RFC7285]  Alimi, R., Penno, R., Yang, Y., Kiesel, S., Previdi, S.,
              Roome, W., Shalunov, S., and R. Woundy, "Application-Layer
              Traffic Optimization (ALTO) Protocol", RFC 7285,
              September 2014.

   [RFC7286]  Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
              H. Song, "Application-Layer Traffic Optimization (ALTO)
              Server Discovery", RFC 7286, June 2014.

   [RFC7971]  Stiemerling, M., Kiesel, S., Scharf, M., Seidel, H., and
              S. Previdi, "Application-Layer Traffic Optimization (ALTO)
              Deployment Considerations", RFC 7971, DOI 10.17487/
              RFC7971, October 2016,
              <http://www.rfc-editor.org/info/rfc7971>.





















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Appendix A.  Multiple Information Sources and Partitioned Knowledge

   The ALTO base protocol document [RFC7285] specifies the communication
   between an ALTO client and a single ALTO server.  It is implicitly
   assumed that this server can answer any query, possibly with some
   kind of default value if no exact data is known.  No special
   provisions were made for the case that the ALTO information
   originates from multiple sources, which are possibly under the
   control of different administrative entities (e.g., different ISPs)
   or that the overall ALTO information is partitioned and stored on
   several ALTO servers.

A.1.  Classification of Solution Approaches

   Various protocol extensions and other solutions have been proposed to
   deal with multiple information sources and partitioned knowledge.
   They can be classified as follows:

   1    Ensure that all ALTO servers have the same knowlegde

   1.1  Ensure data replication and synchronization within the
        provisioning protocol (cf. RFC 5693, Fig 1 [RFC5693]).

   1.2  Use an Inter-ALTO-server data replication protocol.  Possibly,
        the ALTO protocol itself - maybe with some extensions - could be
        used for that purpose; however, this has not been studied in
        detail so far.

   2    Accept that different ALTO servers (possibly operated by
        different organizations, e.g., ISPs) do not have the same
        knowledge

   2.1  Allow ALTO clients to send arbitrary queries to any ALTO server
        (e.g. the one discovered using [RFC7286]).  If this server
        cannot answer the query itself, it will fetch the data on behalf
        of the client, using the ALTO protocol or a to-be-defined inter-
        ALTO-server request forwarding protocol.

   2.2  Allow ALTO clients to send arbitrary queries to any ALTO server
        (e.g. the one discovered using [RFC7286]).  If this server
        cannot answer the query itself, it will redirect the client to
        the "right" ALTO server that has the desired information, using
        a small to-be-defined extension of the ALTO protocol.

   2.3  ALTO clients need to use some kind of "search engine" that
        indexes ALTO servers and redirects and/or gives cached results.





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   2.4  ALTO clients need to use a new discovery mechanism to discover
        the ALTO server that has the desired information and contact it
        directly.

A.2.  Discussion of Solution Approaches

   The provisioning or initialization protocol for ALTO servers (cf. RFC
   5693, Fig 1 [RFC5693]) is currently not standardized.  It was a
   conscious decision not to include this in the scope of the IETF ALTO
   working group.  The reason is that there are many different kinds of
   information sources.  This implementation specific protocol will
   adapt them to the ALTO server, which offers a standardized protocol
   to the ALTO clients.  However, adding the task of synchronization
   between ALTO servers to this protocol (i.e., approach 1.1) would
   overload this protocol with a second functionality that requires
   standardization for seamless multi-domain operation.

   For the 1.? solution approaches, in addition to general technical
   feasibility and issues like overhead and caching efficiency, another
   aspect to consider is legal liability.  Operator "A" might prefer not
   to publish information about nodes in or paths between the networks
   of operators "B" and "C" through A's ALTO server, even if A knew that
   information.  This is not only a question of map size and processing
   load on A's ALTO server.  Operator A could also face legal liability
   issues if that information had a bad impact on the traffic
   engineering between B's and C's networks, or on their business
   models.

   No specific actions to build a "search engine" based solution
   (approach 2.3) are currently known and it is unclear what could be
   the incentives to operate such an engine.  Therefore, this approach
   is not considered in the remainder of this document.

A.3.  The Need for Cross-Domain ALTO Server Discovery

   Approaches 1.1, 1.2, 2.1, and 2.2 do not only require the
   specification of an ALTO protocol extension or a new protocol that
   runs between ALTO servers.  A large-scale, maybe Internet-wide,
   multi-domain deployment would also need mechanisms by which an ALTO
   server could discover other ALTO servers, learn which information is
   available where, and ideally also who is authorized to publish
   information related to a given part of the network.  Approach 2.4
   needs the same mechanisms, except that they are used on the client-
   side instead of the server-side.

   It is sometimes questioned whether there is a need for a solution
   that allows clients to ask arbitrary queries, even if the ALTO
   information is partitioned and stored on many ALTO servers.  The main



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   argument is, that clients are supposed to optimize the traffic from
   and to themselves, and that the information needed for that is most
   likely stored on a "nearby" ALTO server, i.e., the one that can be
   discovered using [RFC7286].  However, there are scenarios where the
   ALTO client is not co-located with an endpoint of the to-be-optimized
   data transmission.  Instead, the ALTO client is located at a third
   party, which takes part in the application signaling, e.g., a so-
   called "tracker" in a peer-to-peer application.  One such scenario,
   where it is advantageous to place the ALTO client not at an endpoint
   of the user data transmission, is analyzed in Appendix C.

A.4.  Our Solution Approach

   Several solution approaches for cross-domain ALTO server discovery
   have been evaluated, using the criteria documented in Appendix B.
   One of them was to use the ALTO protocol itself for the exchange of
   information availability [I-D.kiesel-alto-alto4alto].  However, the
   drawback of that approach is that a new registration administration
   authority would have to be established.

   This document specifies a DNS-based procedure for cross-domain ALTO
   server discovery, which was inspired by "Location Information Server
   (LIS) Discovery Using IP Addresses and Reverse DNS" [RFC7216].  The
   primary goal is that this procedure can be used on the client-side
   (i.e., approach 2.4), but together with new protocols or protocol
   extensions it could also be used to implement the other solution
   approaches itemized above.

A.5.  Relation to the ALTO Requirements

   During the design phase of the overall ALTO solution, two different
   server discovery scenarios have been identified and documented in the
   ALTO requirements document [RFC6708].  The first scenario, documented
   in Req. AR-32, can be supported using the discovery mechanisms
   specified in [RFC7286].  An alternative approach, based on IP anycast
   [I-D.kiesel-alto-ip-based-srv-disc], has also been studied.  This
   document, in contrast, tries to address Req. AR-33.














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Appendix B.  Requirements for ALTO Cross-Domain Server Discovery

   This appendix itemizes requirements that have been collected before
   the design phase and that are reflected by the design of the ALTO
   Cross-Domain Server Discovery Procedure.

B.1.  Discovery Client Application Programming Interface

   The discovery client will be called through some kind of application
   programming interface (API) and the parameters will be an IP address
   and, for purposes of extensibility, a service identifier such as
   "ALTO".  It will return one or more URI(s) that offers the requested
   service ("ALTO") for the given IP address.

   In other words, the client would be used to retrieve a mapping:

   (IP address, "ALTO") -> IRD-URI(s)

   where IRD-URI(s) is one or more URI(s) of Information Resource
   Directories (IRD, see Section 9 of [RFC7285]) of ALTO server(s) that
   can give reasonable guidance to a resource consumer with the
   indicated IP address.

B.2.  Data Storage and Authority Requirements

   The information for mapping IP addresses and service parameters to
   URIs should be stored in a - preferably distributed - database.  It
   must be possible to delegate administration of parts of this
   database.  Usually, the mapping from a specific IP address to an URI
   is defined by the authority that has administrative control over this
   IP address, e.g., the ISP in residential access networks or the IT
   department in enterprise, university, or similar networks.

B.3.  Cross-Domain Operations Requirements

   The cross-domain server discovery mechanism should be designed in
   such a way that it works across the public Internet and also in other
   IP-based networks.  This in turn means that such mechanisms cannot
   rely on protocols that are not widely deployed across the Internet or
   protocols that require special handling within participating
   networks.  An example is multicast, which is not generally available
   across the Internet.

   The ALTO Cross-Domain Server Discovery protocol must support gradual
   deployment without a network-wide flag day.  If the mechanism needs
   some kind of well-known "rendezvous point", re-using an existing
   infrastructure (such as the DNS root servers or the WHOIS database)
   should be preferred over establishing a new one.



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B.4.  Protocol Requirements

   The protocol must be able to operate across middleboxes, especially
   across NATs and firewalls.

   The protocol shall not require any pre-knowledge from the client
   other than any information that is known to a regular IP host on the
   Internet.

B.5.  Further Requirements

   The ALTO cross domain server discovery cannot assume that the server
   discovery client and the server discovery responding entity are under
   the same administrative control.





































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Appendix C.  ALTO and Tracker-based Peer-to-Peer Applications

   This appendix illustrates one ALTO use case and shows that ALTO
   Cross-Domain Server Discovery is beneficial in that scenario.

   The ALTO protocol specification [RFC7285] details how an ALTO client
   can query an ALTO server for guiding information and receive the
   corresponding replies.  However, in the considered scenario of a
   tracker-based P2P application, there are two fundamentally different
   possibilities where to place the ALTO client:

   1.  ALTO client in the resource consumer ("peer")

   2.  ALTO client in the resource directory ("tracker")

   In the following, both scenarios are compared in order to explain the
   need for ALTO queries on behalf of remote resource consumers.

   In the first scenario (see Figure 2), the resource consumer queries
   the resource directory for the desired resource (F1).  The resource
   directory returns a list of potential resource providers without
   considering ALTO (F2).  It is then the duty of the resource consumer
   to invoke ALTO (F3/F4), in order to solicit guidance regarding this
   list.

   In the second scenario (see Figure 4), the resource directory has an
   embedded ALTO client.  After receiving a query for a given resource
   (F1) the resource directory invokes this ALTO client to evaluate all
   resource providers it knows (F2/F3).  Then it returns a, possibly
   shortened, list containing the "best" resource providers to the
   resource consumer (F4).




















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    .............................          .............................
    : Tracker                   :          : Peer                      :
    :   ______                  :          :                           :
    : +-______-+                :          :            k good         :
    : |        |     +--------+ : P2P App. : +--------+ peers +------+ :
    : |   N    |     | random | : Protocol : | ALTO-  |------>| data | :
    : | known  |====>| pre-   |*************>| biased |       | ex-  | :
    : | peers, |     | selec- | : transmit : | peer   |------>| cha- | :
    : | M good |     | tion   | : n peer   : | select | n-k   | nge  | :
    : +-______-+     +--------+ : IDs      : +--------+ bad p.+------+ :
    :...........................:          :.....^.....................:
                                                 |
                                                 | ALTO
                                                 | client protocol
                                               __|___
                                             +-______-+
                                             |        |
                                             | ALTO   |
                                             | server |
                                             +-______-+

   Figure 1: Tracker-based P2P Application with random peer preselection


   Peer w. ALTO cli.            Tracker               ALTO Server
   --------+--------       --------+--------       --------+--------
           | F1 Tracker query      |                       |
           |======================>|                       |
           | F2 Tracker reply      |                       |
           |<======================|                       |
           | F3 ALTO client protocol query                 |
           |---------------------------------------------->|
           | F4 ALTO client protocol reply                 |
           |<----------------------------------------------|
           |                       |                       |

   ====  Application protocol (i.e., tracker-based P2P app protocol)
   ----  ALTO client protocol

       Figure 2: Basic message sequence chart for resource consumer-
                           initiated ALTO query










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    .............................          .............................
    : Tracker                   :          : Peer                      :
    :   ______                  :          :                           :
    : +-______-+                :          :                           :
    : |        |     +--------+ : P2P App. :  k good peers &  +------+ :
    : |   N    |     | ALTO-  | : Protocol :  n-k bad peers   | data | :
    : | known  |====>| biased |******************************>| ex-  | :
    : | peers, |     | peer   | : transmit :                  | cha- | :
    : | M good |     | select | : n peer   :                  | nge  | :
    : +-______-+     +--------+ : IDs      :                  +------+ :
    :.....................^.....:          :...........................:
                          |
                          | ALTO
                          | client protocol
                        __|___
                      +-______-+
                      |        |
                      | ALTO   |
                      | server |
                      +-______-+

    Figure 3: Tracker-based P2P Application with ALTO client in tracker


         Peer             Tracker w. ALTO cli.       ALTO Server
   --------+--------       --------+--------       --------+--------
           | F1 Tracker query      |                       |
           |======================>|                       |
           |                       | F2 ALTO cli. p. query |
           |                       |---------------------->|
           |                       | F3 ALTO cli. p. reply |
           |                       |<----------------------|
           | F4 Tracker reply      |                       |
           |<======================|                       |
           |                       |                       |

   ====  Application protocol (i.e., tracker-based P2P app protocol)
   ----  ALTO client protocol

    Figure 4: Basic message sequence chart for ALTO query on behalf of
                         remote resource consumer

   Note: the message sequences depicted in Figure 2 and Figure 4 may
   occur both in the target-aware and the target-independent query mode
   (c.f.  [RFC6708]).  In the target-independent query mode no message
   exchange with the ALTO server might be needed after the tracker
   query, because the candidate resource providers could be evaluated
   using a locally cached "map", which has been retrieved from the ALTO



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   server some time ago.

   The problem with the first approach is, that while the resource
   directory might know thousands of peers taking part in a swarm, the
   list returned to the resource consumer is usually shortened for
   efficiency reasons.  Therefore, the "best" (in the sense of ALTO)
   potential resource providers might not be contained in that list
   anymore, even before ALTO can consider them.

   For illustration, consider a simple model of a swarm, in which all
   peers fall into one of only two categories: assume that there are
   "good" ("good" in the sense of ALTO's better-than-random peer
   selection, based on an arbitrary desired rating criterion) and "bad'
   peers only.  Having more different categories makes the maths more
   complex but does not change anything to the basic outcome of this
   analysis.  Assume that the swarm has a total number of N peers, out
   of which are M "good" and N-M "bad" peers, which are all known to the
   tracker.  A new peer wants to join the swarm and therefore asks the
   tracker for a list of peers.

   If, according to the first approach, the tracker randomly picks n
   peers from the N known peers, the result can be described with the
   hypergeometric distribution.  The probability that the tracker reply
   contains exactly k "good" peers (and n-k "bad" peers) is:



               / M \   / N - M \
               \ k /   \ n - k /
   P(X=k) =  ---------------------
                     / N \
                     \ n /


           / n \        n!
   with    \ k /  = -----------    and   n! = n * (n-1) * (n-2) * .. * 1
                     k! (n-k)!


   The probability that the reply contains at most k "good" peers is:
   P(X<=k)=P(X=0)+P(X=1)+..+P(X=k).

   For example, consider a swarm with N=10,000 peers known to the
   tracker, out of which M=100 are "good" peers.  If the tracker
   randomly selects n=100 peers, the formula yields for the reply:
   P(X=0)=36%, P(X<=4)=99%.  That is, with a probability of approx. 36%
   this list does not contain a single "good" peer, and with 99%
   probability there are only four or less of the "good" peers on the



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   list.  Processing this list with the guiding ALTO information will
   ensure that the few favorable peers are ranked to the top of the
   list; however, the benefit is rather limited as the number of
   favorable peers in the list is just too small.

   Much better traffic optimization could be achieved if the tracker
   would evaluate all known peers using ALTO, and return a list of 100
   peers afterwards.  This list would then include a significantly
   higher fraction of "good" peers.  (Note, that if the tracker returned
   "good" peers only, there might be a risk that the swarm might
   disconnect and split into several disjunct partitions.  However,
   finding the right mix of ALTO-biased and random peer selection is out
   of the scope of this document.)

   Therefore, from an overall optimization perspective, the second
   scenario with the ALTO client embedded in the resource directory is
   advantageous, because it is ensured that the addresses of the "best"
   resource providers are actually delivered to the resource consumer.
   An architectural implication of this insight is that the ALTO server
   discovery procedures must support ALTO queries on behalf of remote
   resource consumers.  That is, as the tracker issues ALTO queries on
   behalf of the peer which contacted the tracker, the tracker must be
   able to discover an ALTO server that can give guidance suitable for
   that respective peer.



























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Appendix D.  Contributors List and Acknowledgments

   The initial version of this document was co-authored by Marco Tomsu
   (Alcatel-Lucent).

   This document borrows some text from [RFC7286], as historically, it
   has been part of the draft that eventually became said RFC.  Special
   thanks to Michael Scharf and Nico Schwan.











































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Authors' Addresses

   Sebastian Kiesel
   University of Stuttgart Information Center
   Allmandring 30
   Stuttgart  70550
   Germany

   Email: ietf-alto@skiesel.de
   URI:   http://www.izus.uni-stuttgart.de


   Martin Stiemerling
   University of Applied Sciences Darmstadt,  Computer Science Dept.
   Haardtring 100
   Darmstadt  64295
   Germany

   Phone: +49 6151 16 37938
   Email: mls.ietf@gmail.com
   URI:   http://ietf.stiemerling.org






























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