Internet DRAFT - draft-kiesel-alto-xdom-disc-alg

draft-kiesel-alto-xdom-disc-alg






ALTO                                                           S. Kiesel
Internet-Draft                                                 K. Krause
Intended status: Experimental                    University of Stuttgart
Expires: January 5, 2015                                  M. Stiemerling
                                                                    H-DA
                                                            July 4, 2014


   Application Layer Traffic Optimization (ALTO) Cross-Domain Server
            Discovery - Experimental Algorithm Specification
                   draft-kiesel-alto-xdom-disc-alg-00

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.

   This document contains a strawman proposal for an ALTO Cross-Domain
   Server Discovery procedure (also known as Third-Party Discovery).
   Technically, the algorithm specified in this document takes one
   IP address and a U-NAPTR Service Parameter (i.e., "ALTO:http" or
   "ALTO:https") as parameters.  It performs several DNS lookups (for
   U-NAPTR and SOA resource records) and returns one or more URI(s) of
   information resources related to that IP address.

   The functionality has been validated in a lab environment.  However,
   the feasibility and possible side-effects of Internet-wide
   "production use" are not yet understood.  The purpose of this
   document is to foster further discussion within the ALTO working
   group.  Readers of this document should exercise caution in
   evaluating its value for implementation and deployment.
















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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 January 5, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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
     1.1.  Document History . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Feedback . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  ALTO Cross-Domain Server Discovery Procedure Specification . .  5
     2.1.  Interface  . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Basic Principle  . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Overall Procedure  . . . . . . . . . . . . . . . . . . . .  6
     2.4.  Specification of Tasks and Conditional Branches  . . . . .  7
       2.4.1.  T1: Prepare Domain Name for Reverse DNS Lookup . . . .  7
       2.4.2.  T2/B1: U-NAPTR Lookup in Reverse Zone  . . . . . . . .  7
       2.4.3.  B2/T3/B3: Acquire SOA Record for Reverse Zone  . . . .  8
       2.4.4.  T4/B4: U-NAPTR Lookup on SOA-MNAME . . . . . . . . . .  9
   3.  Implementation, Deployment, and Operational Considerations . . 10
     3.1.  Considerations for ALTO Clients  . . . . . . . . . . . . . 10
       3.1.1.  Resource Consumer Initiated Discovery  . . . . . . . . 10
       3.1.2.  IPv4/v6 Dual Stack, Multihoming, NAT, and Host
               Mobility . . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.  Deployment Considerations for Network Operators  . . . . . 11
       3.2.1.  NAPTR in Reverse Tree vs. SOA-based discovery  . . . . 11
       3.2.2.  Separation of Interests  . . . . . . . . . . . . . . . 11
     3.3.  Impact on DNS  . . . . . . . . . . . . . . . . . . . . . . 12
       3.3.1.  Non-PTR Resource Records in Reverse Tree . . . . . . . 12
       3.3.2.  Usage with DNS Hidden Master Servers . . . . . . . . . 12
       3.3.3.  Load on the DNS  . . . . . . . . . . . . . . . . . . . 12
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
     4.1.  Integrity of the ALTO Server's URI . . . . . . . . . . . . 13
     4.2.  Availability of the ALTO Server Discovery Procedure  . . . 14
     4.3.  Confidentiality of the ALTO Server's URI . . . . . . . . . 14
     4.4.  Privacy for ALTO Clients . . . . . . . . . . . . . . . . . 15
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19















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1.  Introduction

   The motivation for Application-Layer Traffic Optimization (ALTO)
   Cross-Domain Server Discovery (also known as Third-Party Server
   Discovery), as well as applicable scenarios and requirements for a
   solution are documented in draft-kiesel-alto-xdom-disc-00.  In the
   following, we assume that the reader is familiar with said document.

   This document presents the specification of an DNS-based procedure
   that satisfies these requirements.  The functionality has been
   validated in a lab environment.  However, the feasibility and
   possible side-effects of Internet-wide "production use" are not yet
   understood.

   The purpose of this document is to foster further discussion within
   the ALTO working group and the IETF community in general.  Readers of
   this document should exercise caution in evaluating its value for
   implementation and deployment.

1.1.  Document History

   This document is a direct successor of [I-D.kiesel-alto-3pdisc] and
   [I-D.kist-alto-3pdisc].  The scenario and mechanisms described here
   and in these documents have been referred to as "third-party server
   discovery" in the past.  However, to avoid ambiguities with a
   completely different scenario, it has been renamed to "ALTO Cross-
   Domain Server Discovery".

1.2.  Feedback

   Comments and discussions about this document should be directed to
   the ALTO working group: alto@ietf.org.



















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2.  ALTO Cross-Domain Server Discovery Procedure Specification

2.1.  Interface

   The algorithm specified in this document takes one IP address and a
   U-NAPTR Service Parameter (i.e., "ALTO:http" or "ALTO:https") as
   parameters.  It performs several DNS lookups (for U-NAPTR and SOA
   resource records) and returns one or more URI(s) of information
   resources related to that IP address.

2.2.  Basic Principle

   The algorithm sequentially tries two different lookup strategies.
   First, an ALTO-specific U-NAPTR lookup is performed in the "reverse
   tree", i.e., in subdomains of in-addr.arpa. or ip6.arpa.,
   respectively.  If this lookup does not yield a usable result, the SOA
   record for the reverse zone is acquired, its master name server
   (MNAME) value is extracted and used for a further ALTO-specific
   U-NAPTR lookup.

   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.



























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2.3.  Overall Procedure

   This figure gives an overview on the discovery procedure.  All tasks
   (T) and conditional branches (B) are specified below.

                (---------------------------------------)
                ( START ALTO XDOM disc with parameters  )
                ( IP_address IP, Service_Parameter SP   )
                (-------------------+-------------------)
                                    V
                +- T1 --------------+-------------------+
                | R:=<IP>.in-addr.arpa. / <IP>.ip6.arpa.|
                +-------------------+-------------------+
                                    V
                +- T2 --------------+-------------------+
                | X:=DNSlookup(R,U-NAPTR,SP)            |
                +-------------------+-------------------+
                                    V
                 / B1 --------------+------------------\
      /---------< One or more U-NAPTR results in X      >
      |      yes \------------------+------------------/
      |                             V no
      |          /- B2 -------------+------------------\
      |    /----< Authority sect. with SOA record in X  >
      |    | yes \------------------+------------------/
      |    |                        V no
      |    |    +- T3 --------------+-------------------+
      |    |    | X:=DNSlookup(R,SOA)                   |
      |    |    +-------------------+-------------------+
      |    |                        V
      |    |     /- B3 -------------+------------------\
      |    |    < Lookup OK, SOA record present in X    >----\
      |    |     \------------------+------------------/ no  |
      |    |                        V yes                    |
      |    \----------------------->+                        |
      |                             V                        |
      |         +- T4 --------------+-------------------+    |
      |         | M:=extract MNAME from SOA record in X |    |
      |         | X:=DNSlookup(M,U-NAPTR,SP)            |    |
      |         +-------------------+-------------------+    |
      |                             V                        |
      |          /- B4 -------------+------------------\     V
      \--->+<---< One or more U-NAPTR results in X      >--->+
           | yes \-------------------------------------/ no  |
           V                                                 V
   (-------+-------)                                 (-------+-------)
   ( END, result X )                                 ( END, failure  )
   (---------------)                                 (---------------)



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2.4.  Specification of Tasks and Conditional Branches

2.4.1.  T1: Prepare Domain Name for Reverse DNS Lookup

   Task T1 takes the IP address parameter the procedure was called with
   and constructs a domain name, which is stored in variable "R" for use
   in subsequent tasks.

   If the IP address given as a parameter to the procedure is an IPv4
   address, the domain name is constructed according to the rules
   specified in Section 3.5 of [RFC1035] and it is rooted in the in the
   special domain "IN-ADDR.ARPA.".  For IPv6 addresses, the construction
   rules in Section 2.5 of [RFC3596] apply and the special domain
   "IP6.ARPA." is used.

   Example values for "R" for IPv4 and IPv6 addresses could be (Note: a
   line break was added in the IPv6 example):

       R:="3.100.51.198.in-addr.arpa."

       R:="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."


2.4.2.  T2/B1: U-NAPTR Lookup in Reverse Zone

   Task T1 performs a U-NAPTR lookup as specified in [RFC4848] on "R",
   in order to get service-specific U-NAPTR resource records that are
   directly associated with the IP address in question.

   The ALTO protocol specification defines HTTP and HTTPS as transport
   mechanisms and URI schemes for ALTO.  Consequently, the U-NAPTR
   lookup is performed with the "ALTO" Application Service Tag and
   either the "http" or the "https" Application Protocol Tag.
   Application Service Tag and Application Protocol Tag are concatenated
   to form the Service Parameter SP, i.e., either "ALTO:http" or "ALTO:
   https".

   The goal of said U-NAPTR lookup is to obtain one or more URIs for the
   ALTO server's Information Resource Directory.  If two or more URIs
   are found they are sorted according to their order and preference
   fields as specified in [RFC4848] and [RFC3403].

   The lookup result, including a SOA record that may or may not be
   present in the authority section, is stored in variable "X".

   As an example, the following two U-NAPTR resource records can be used
   for mapping "3.100.51.198.in-addr.arpa." to the HTTPS URI



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   https://altoserver.isp.example.net/secure/directory or the HTTP URI
   http://altoserver.isp.example.net/directory, with the former being
   preferred.

   3.100.51.198.in-addr.arpa.

   IN NAPTR 100  10   "u"    "ALTO:https"
        "!.*!https://altoserver.isp.example.net/secure/directory!"  ""

   IN NAPTR 200  10   "u"    "ALTO:http"
        "!.*!http://altoserver.isp.example.net/directory!"  ""

   Conditional Branch B1 checks whether at least one U-NAPTR record
   matching the service parameter SP could be retrieved.  If so, the
   procedure ends successfully and the sorted list of U-NAPTR records is
   the result.  Otherwise, if no U-NAPTR records could be retrieved, we
   continue with B2.

   Note: The U-NAPTR lookup in Task T2 is identical to Step 2 specified
   in [RFC7286], which specifies with "manual input" and "DHCP" two
   alternatives for acquiring the name to be looked up.  Therefore, it
   is possible to merge both documents into a common ALTO server
   discovery framework.

2.4.3.  B2/T3/B3: Acquire SOA Record for Reverse Zone

   The task of B2/T3/B3 is to acquire the SOA record for the "reverse
   zone", i.e., the zone in the in-addr.arpa. or ip6.arpa. domain that
   contains the IP address in question.

   A sample SOA record could be:

   100.51.198.in-addr.arpa
   IN  SOA dns1.isp.example.net.   hostmaster.isp.example.net. (
                                 1         ; Serial
                            604800         ; Refresh
                             86400         ; Retry
                           2419200         ; Expire
                            604800 )       ; Negative Cache TTL

   Conditional Branch B2 checks whether the SOA record was present in
   the authority section of X, i.e., the result of Task T2.  If not, an
   explicit lookup is done in Task T3.  If Conditional Branch B3
   determines that this explicit lookup failed, the discovery procedure
   is aborted without a result; otherwise we continue with T4.






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2.4.4.  T4/B4: U-NAPTR Lookup on SOA-MNAME

   Now that the SOA record is available, Task T4 first extracts the
   MNAME field, i.e., the responsible master name server from the SOA
   record.  An example MNAME could be:

       dns1.isp.example.net.

   Then, a U-NAPTR lookup as specified in Task T2 is performed on this
   MNAME and the result is stored in variable "X".

   Conditional Branch B4 checks whether at least one U-NAPTR record
   matching the service parameter SP could be retrieved.  If so, the
   procedure ends successfully and the sorted list of U-NAPTR records is
   the result.  Otherwise, if no U-NAPTR records could be retrieved, the
   discovery procedure is aborted without a result.



































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3.  Implementation, Deployment, and Operational Considerations

3.1.  Considerations for ALTO Clients

3.1.1.  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 resource 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].

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

   The algorithm specified in this document can discover ALTO server
   URIs for a given IP address.  The intention is, that an entity taking
   part in the application signaling (e.g., a resource directory such as
   a P2P tracker) that receives signaling messages from a resource
   consumer can use the source address contained 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.

3.2.  Deployment Considerations for Network Operators

3.2.1.  NAPTR in Reverse Tree vs. SOA-based discovery

   As already outlined in Section 2.2, the ALTO cross-domain server
   discovery procedure sequentially tries two different lookup
   strategies, thus giving network operators the choice of two different
   deployment options:

   o  Individual NAPTR records in the in-addr.arpa or ip6.arpa domains
      allow very fine-grained discovery of ALTO "entry point" URIs on a
      per-IP-address basis.  This method also gives the fastest response
      times and causes a comparatively low load on the DNS, as the
      algorithm terminates successfully after the first DNS query.  DNS
      operators that already maintain reverse zones (e.g., for PTR
      records) should prefer this option, possibly using DNS server
      implementation-specific methods for mass deployment (e.g., BIND9's
      $GENERATE statement).

   o  If a DNS operator considers the first option too cumbersome, or if
      IPv6 privacy extensions is to be used without dynamic PTR updates,
      setting up SOA records in the in-addr.arpa. or ip6.arpa.
      subdomains plus setting up corresponding ALTO-specific U-NAPTR
      records will also give reasonable, yet less fine-grained results
      at the cost of slightly higher delay and load on the DNS.

3.2.2.  Separation of Interests

   We assume that if two organizations share parts of their DNS
   infrastructure, i.e., have a common SOA record in their in-addr.arpa.
   or ip6.arpa. subdomain(s), 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



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   redirect the client to another ALTO server using mechanisms of the
   ALTO protocol (see Sect. 9 of [RFC7285]).

3.3.  Impact on DNS

3.3.1.  Non-PTR Resource Records in Reverse Tree

   Installing NAPTR records, i.e., a record type other than PTR records,
   in the in-addr.arpa or ip6.arpa domain may seem uncommon, but it is
   not a new concept.  Earlier documents that specify the usage of Non-
   PTR resource records in the reverse tree include RFC 4025 [RFC4025],
   RFC 4255 [RFC4255], and RFC 4322 [RFC4322].

3.3.2.  Usage with DNS Hidden Master Servers

   In some deployment scenarios, the Master DNS server for a in-
   addr.arpa. or ip6.arpa. subdomain, as indicated in the respective SOA
   record, may not be reachable due to traffic restrictions ("hidden
   master").  This does not cause any problems with the algorithm
   described here, as the MNAME is only used for further DNS lookups;
   but it is never attempted to contact this server directly.

3.3.3.  Load on the DNS

   The procedure described in this document features several nested
   conditional branches, but no loops.  Each time being called it
   attempts one to three DNS lookups.
























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4.  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 ALTO cross-domain 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.

4.1.  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 ALTO cross-domain
      server discovery (see below), (s)he could also launch
      significantly more serious other attacks (e.g., redirecting
      various application protocols).

   Protection Strategies and Mechanisms
      The ALTO cross-domain 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 ALTO cross-domain 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]).

4.2.  Availability of the ALTO Server Discovery Procedure

   Scenario Description
      An attacker could compromise the ALTO cross-domain 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.



4.3.  Confidentiality of the ALTO Server's URI







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   Scenario Description
      An unauthorized party could invoke the ALTO cross-domain 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 resource consumer.

   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.

4.4.  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
      on behalf of a specific resource consumer.

   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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5.  IANA Considerations

   This document does not require any IANA action.

   This document specifies an algorithm that uses U-NAPTR lookups
   [RFC4848] with the Application Service Tag "ALTO" and the Application
   Protocol Tags "http" and "https".  These tags have already been
   registered with IANA.  In particular, for the registration of the
   Application Service Tag "ALTO", see [RFC7286].










































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6.  References

6.1.  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.

   [RFC3403]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, October 2002.

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

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

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

6.2.  Informative References

   [I-D.kiesel-alto-3pdisc]
              Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M.,
              Tomsu, M., and H. Song, "ALTO Server Discovery Protocol",
              draft-kiesel-alto-3pdisc-05 (work in progress),
              March 2011.

   [I-D.kist-alto-3pdisc]
              Kiesel, S., Krause, K., and M. Stiemerling, "Third-Party
              ALTO Server Discovery (3pdisc)", draft-kist-alto-3pdisc-05
              (work in progress), January 2014.

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

   [RFC4025]  Richardson, M., "A Method for Storing IPsec Keying
              Material in DNS", RFC 4025, March 2005.

   [RFC4255]  Schlyter, J. and W. Griffin, "Using DNS to Securely
              Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,



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              January 2006.

   [RFC4322]  Richardson, M. and D. Redelmeier, "Opportunistic
              Encryption using the Internet Key Exchange (IKE)",
              RFC 4322, December 2005.

   [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.

   [RFC7285]  Alimi, R., Penno, R., and Y. Yang, "Application-Layer
              Traffic Optimization (ALTO) Protocol", RFC 7285,
              June 2014.

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


























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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.rus.uni-stuttgart.de/nks/


   Kilian Krause
   University of Stuttgart Information Center
   Allmandring 30
   Stuttgart  70550
   Germany

   Email: schreibt@normalerweise.net
   URI:   http://www.rus.uni-stuttgart.de/nks/


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

   Phone: +49 6151 16 7938
   Email: mls.ietf@gmail.com
   URI:   http://ietf.stiemerling.org




















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