





Internet Engineering Task Force (IETF)                         S. Kiesel
Request for Comments: 7286                       University of Stuttgart
Category: Standards Track                                 M. Stiemerling
ISSN: 2070-1721                                          NEC Europe Ltd.
                                                               N. Schwan
                                                      Thales Deutschland
                                                               M. Scharf
                                                Alcatel-Lucent Bell Labs
                                                                 H. Song
                                                                  Huawei
                                                           November 2014


     Application-Layer Traffic Optimization (ALTO) Server Discovery

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.

   This document specifies a procedure for resource-consumer-initiated
   ALTO server discovery, which can be used if the ALTO client is
   embedded in the resource consumer.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7286.










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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology and Requirements Language . . . . . . . . . .   3
   2.  ALTO Server Discovery Procedure Overview  . . . . . . . . . .   3
   3.  ALTO Server Discovery Procedure Specification . . . . . . . .   4
     3.1.  Step 1: Retrieving the Domain Name  . . . . . . . . . . .   5
       3.1.1.  Step 1, Option 1: Local Configuration . . . . . . . .   5
       3.1.2.  Step 1, Option 2: DHCP  . . . . . . . . . . . . . . .   5
     3.2.  Step 2: U-NAPTR Resolution  . . . . . . . . . . . . . . .   6
   4.  Deployment Considerations . . . . . . . . . . . . . . . . . .   7
     4.1.  Issues with Home Gateways . . . . . . . . . . . . . . . .   7
     4.2.  Issues with Multihoming, Mobility, and Changing IP
           Addresses . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     6.1.  Integrity of the ALTO Server's URI  . . . . . . . . . . .   9
     6.2.  Availability of the ALTO Server Discovery Procedure . . .  11
     6.3.  Confidentiality of the ALTO Server's URI  . . . . . . . .  11
     6.4.  Privacy for ALTO Clients  . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Acknowledgments   . . . . . . . . . . . . . . . . . . . . . . . .  14
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15










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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 a client-server protocol;
   see requirement AR-1 in [RFC6708].  Before an ALTO client can ask for
   guidance it needs to discover one or more ALTO servers that can
   provide guidance to this specific client.

   This document specifies a procedure for resource-consumer-initiated
   ALTO server discovery, which can be used if the ALTO client is
   embedded in the resource consumer.  In other words, this document
   meets requirement AR-32 in [RFC6708] while AR-33 is out of scope.  A
   different approach, which tries to meet requirement AR-33, i.e.,
   third-party ALTO server discovery, is addressed in [3PDISC].

   A more detailed discussion of various options on where to place the
   functional entities comprising the overall ALTO architecture can be
   found in [ALTO-DEPLOY].

   The ALTO protocol specification [RFC7285] is based on HTTP and
   expects the discovery procedure to yield the HTTP(S) URI of an ALTO
   server's Information Resource Directory (IRD).  Therefore, this
   procedure is based on a combination of the Dynamic Host Configuration
   Protocol (DHCP) or local configuration and URI-enabled Naming
   Authority Pointer (U-NAPTR) resource records in the Domain Name
   System (DNS), in order to deliver such URIs.

1.1.  Terminology and Requirements Language

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

2.  ALTO Server Discovery Procedure Overview

   The ALTO protocol specification [RFC7285] expects that the ALTO
   discovery procedure yields the HTTP(S) URI [RFC7230] of the ALTO
   server's Information Resource Directory (IRD), which gives further
   information about the capabilities and services provided by that ALTO
   server.

   On hosts with more than one interface or address family (IPv4/v6),
   the ALTO server discovery procedure has to be run for every interface
   and address family.  For more details see Section 4.2.



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   The ALTO server discovery procedure is performed in two steps:

   1.  One DNS domain name is retrieved for each combination of
       interface and address family, either by local configuration
       (e.g., manual input into a menu or configuration file) or by
       means of DHCP.

   2.  These DNS domain names are used for U-NAPTR lookups yielding one
       or more URIs.  Further DNS lookups may be necessary to determine
       the ALTO server's IP address(es).

   The primary means for retrieving the DNS domain name is DHCP.
   However, there may be situations where DHCP is not available or does
   not return a suitable value.  Furthermore, there might be situations
   in which the user wishes to override the value that could be
   retrieved from DHCP.  In these situations, local configuration may be
   used.  Consequently, the algorithm first checks for a locally
   configured override, before it tries to retrieve a value from DHCP.

   Typically, but not necessarily, the DNS domain name is the domain
   name in which the client is located, i.e., a PTR lookup on the
   client's IP address (according to [RFC1035], Section 3.5 for IPv4 or
   [RFC3596], Section 2.5 for IPv6) would yield a similar name.
   However, due to the widespread use of Network Address Translation
   (NAT), trying to determine the DNS domain name through a PTR lookup
   on an interface's IP address is not recommended for resource consumer
   initiated ALTO server discovery (see also [RFC3424]).

3.  ALTO Server Discovery Procedure Specification

   As already outlined in Section 2, the ALTO server discovery procedure
   is performed for every address family on every interface the
   application considers for communicating with resource providers.

   First, the algorithm checks for a locally configured domain name, as
   specified in Section 3.1.1.  If no such name was configured, it tries
   to retrieve one from DHCP, as specified in Section 3.1.2.  If still
   no domain name could be found, the procedure has failed and
   terminates with an appropriate error code.

   If one or more domain names were found, they will be used as U-NAPTR/
   DDDS (URI-Enabled NAPTR/Dynamic Delegation Discovery Service)
   [RFC4848] application-unique strings for a DNS lookup, as specified
   in Section 3.2.







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3.1.  Step 1: Retrieving the Domain Name

3.1.1.  Step 1, Option 1: Local Configuration

   The preferred way to acquire a domain name related to an interface's
   point of network attachment is the use of DHCP (see Section 3.1.2).
   However, in some network deployment scenarios, there is no DHCP
   server available.  Furthermore, a user may want to use an ALTO
   service instance provided by an entity that is not the operator of
   the underlying IP network.  Therefore, we allow the user to specify a
   DNS domain name, for example, in a configuration file option.  An
   example domain name is:

      my-alternative-alto-provider.example.org

   Implementations MAY give the user the opportunity (e.g., by means of
   configuration file options or menu items) to specify an individual
   domain name for every address family on every interface.
   Implementations SHOULD allow the user to specify a default name that
   is used if no more specific name has been configured.

3.1.2.  Step 1, Option 2: DHCP

   Network operators may provide the domain name to be used for service
   discovery within an access network using DHCP.

   RFC 5986 [RFC5986] defines DHCP IPv4 and IPv6 access network domain
   name options to identify a domain name that is suitable for service
   discovery within the access network.  RFC 2132 [RFC2132] defines the
   DHCP IPv4 domain name option.  While this option is less suitable, it
   still may be useful if the RFC 5986 option is not available.

   For IPv6, the ALTO server discovery procedure MUST try to retrieve
   DHCP option 57 (OPTION_V6_ACCESS_DOMAIN).  If no such option can be
   retrieved the procedure fails for this interface.  For IPv4, the ALTO
   server discovery procedure MUST try to retrieve DHCP option 213
   (OPTION_V4_ACCESS_DOMAIN).  If no such option can be retrieved, the
   procedure SHOULD try to retrieve option 15 (Domain Name).  If neither
   option can be retrieved, the procedure fails for this interface.  If
   a result can be retrieved, it will be used as an input for the next
   step (U-NAPTR resolution).  One example result could be:

      example.net








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3.2.  Step 2: U-NAPTR Resolution

   The first step of the ALTO server discovery procedure (see
   Section 3.1) retrieved one or -- in case of multiple interfaces and/
   or IPv4/v6 dual-stack operation -- several domain names, which will
   be used as U-NAPTR/DDDS (URI-Enabled NAPTR/Dynamic Delegation
   Discovery Service) [RFC4848] application unique strings.  An example
   is:

      example.net

   In the second step, the ALTO server discovery procedure uses a
   U-NAPTR [RFC4848] lookup with the "ALTO" Application Service Tag and
   either the "http" or the "https" Application Protocol Tag to obtain
   one or more URIs (indicating protocol, host, and possibly path
   elements) for the ALTO server's Information Resource Directory.  In
   this document, only the HTTP and HTTPS URI schemes are defined, as
   the ALTO protocol specification defines the access over both
   protocols, but no other [RFC7285].  Note that the result can be any
   valid HTTP(S) URI.

   The following two U-NAPTR resource records can be used for mapping
   "example.net" to the HTTPS URIs "https://alto1.example.net/ird" and
   "https://alto2.example.net/ird", with the former being preferred.

       example.net.

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

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


   If no ALTO-specific U-NAPTR records can be retrieved, the discovery
   procedure fails for this domain name (and the corresponding interface
   and IP protocol version).  If further domain names retrieved by Step
   1 are known, the discovery procedure may perform the corresponding
   U-NAPTR lookups immediately.  However, before retrying a lookup that
   has failed, a client MUST wait a time period that is appropriate for
   the encountered error (NXDOMAIN, timeout, etc.).










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4.  Deployment Considerations

4.1.  Issues with Home Gateways

   Section 3.1.2 describes the usage of a DHCP option that provides a
   means for the network operator of the network in which the ALTO
   client is located to provide a DNS domain name.  However, this
   assumes that this particular DHCP option is correctly passed from the
   DHCP server to the actual host with the ALTO client, and that the
   particular host understands this DHCP option.  This memo assumes the
   client to be able to understand the proposed DHCP option; otherwise,
   there is no further use of the DHCP option, but the client has to use
   the other proposed mechanisms.

   There are well-known issues with the handling of DHCP options in home
   gateways.  One issue is that unknown DHCP options are not passed
   through some home gateways, effectively eliminating the DHCP option.

   Another well-known issue is the use of home-gateway-specific DNS
   domain names that "override" the DNS domain name provided by the
   network operator.  For instance, a host behind a home gateway may
   receive a DNS domain name ".local" instead of "example.net".  In
   general, this domain name is not usable for the server discovery
   procedure, unless a DNS server in the home gateway resolves the
   corresponding NAPTR lookup correctly, e.g., by means of a DNS split
   horizon approach.

4.2.  Issues with Multihoming, Mobility, and Changing IP Addresses

   If the user decides to enter only one (default) DNS domain name in
   the local configuration facility (see Section 3.1.1), only one set of
   ALTO servers will be discovered, irrespective of multihoming and
   mobility.  Particularly in mobile scenarios, this can lead to
   undesirable results.

   The DHCP-based discovery method can discover different sets of ALTO
   servers for each interface and address family (i.e., IPv4/v6).  In
   general, if a client wishes to communicate using one of its
   interfaces and using a specific IP address family, it SHOULD query
   the ALTO server or servers that have been discovered for this
   specific interface and address family.  How to select an interface
   and IP address family as well as how to compare results returned from
   different ALTO servers are out of the scope of this document.








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   A change of the IP address at an interface invalidates the result of
   the ALTO server discovery procedure.  For instance, if the IP address
   assigned to a mobile host changes due to host mobility, it is
   required to re-run the ALTO server discovery procedure without
   relying on earlier gained information.

   There are several challenges with DNS on hosts with multiple
   interfaces [RFC6418], which can affect the ALTO server discovery.  If
   the DNS resolution is performed on the wrong interface, it can return
   an ALTO server that could provide suboptimal or wrong guidance.
   Finding the best ALTO server for multi-interfaced hosts is outside
   the scope of this document.

   When using Virtual Private Network (VPN) connections, there is
   usually no DHCP.  The user has to enter the DNS domain name in the
   local configuration facility.  For good optimization results, a DNS
   domain name corresponding to the VPN concentrator, not corresponding
   to the user's current location, has to be entered.  Similar
   considerations apply for Mobile IP.

5.  IANA Considerations

   IANA has registered the following U-NAPTR [RFC4848] application
   service tag for ALTO:

   Application Service Tag:  ALTO

   Intended usage:  see [RFC5693]  or: "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."

   Defining Publication:  The specification contained within this
      document

   Contact information:  The authors of this document

   Author/Change controller:  The IESG

   Interoperability considerations:  No interoperability issues are
      known or expected.  This tag is to be registered specifically for
      ALTO, which is a new application without any legacy deployments.

   Security considerations:  see Section 6 of this document.







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   Related publications:  This document specifies a procedure for
      discovering an HTTP or HTTPS URI of an ALTO server.  HTTP and
      HTTPS are specified in [RFC7230].  The HTTP(S)-based ALTO protocol
      is specified in [RFC7285].

   Application Protocol Tag:  This document specifies how to use the
      application service tag "ALTO" with the application protocol tags
      "http" and "https", which have already been registered in the
      relevant IANA registry.  Therefore, IANA is not requested by this
      document to register any new application protocol tag.

6.  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 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.

6.1.  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 DHCP and/or DNS servers used for ALTO server discovery
      (see below), (s)he could also launch significantly more serious
      other attacks (e.g., redirecting various application protocols).



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   Protection Strategies and Mechanisms
      The ALTO server discovery procedure consists of three building
      blocks (local configuration, DHCP, and DNS) and each of them is a
      possible attack vector.

      The problem of users possibly following "bad advice" that tricks
      them into manually configuring unsuitable ALTO servers cannot be
      solved by technical means and is out of the scope of this
      document.

      Due to the nature of the protocol, DHCP is rather prone to
      attacks.  As already mentioned, an attacker that is able to inject
      forged DHCP replies into the network may do significantly more
      harm than only configuring a wrong ALTO server.  Best current
      practices for safely operating DHCP should be followed.

      A further threat is the possible alteration of the DNS records
      used in U-NAPTR resolution.  If an attacker was able to modify or
      spoof any of the DNS records used in the DDDS resolution, this 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 used in U-NAPTR
      resolution.  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 Transport Layer Security (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 U-NAPTR based mapping within the 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 outcomes, the use of the ALTO server
      may be discontinued (see also Section 15.2 of [RFC7285]).









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6.2.  Availability of the ALTO Server Discovery Procedure

   Scenario Description
      An attacker could compromise the 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
      DHCP, DNS, and ALTO (see Section 15.5 of [RFC7285]) servers
      against Denial-of-Service (DoS) attacks.

6.3.  Confidentiality of the ALTO Server's URI

   Scenario Description
      An unauthorized party could invoke the ALTO server discovery
      procedure, or intercept discovery messages between an authorized
      ALTO client and the DHCP and DNS servers, in order to acquire
      knowledge of the ALTO server's URI.

   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.










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6.4.  Privacy for ALTO Clients

   Scenario Description
      An unauthorized party could intercept discovery messages between
      an ALTO client and the DHCP and DNS servers, and thereby find out
      the fact that said ALTO client uses (or at least tries to use) the
      ALTO service.

   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.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997,
              <http://www.rfc-editor.org/info/rfc2132>.

   [RFC4848]  Daigle, L., "Domain-Based Application Service Location
              Using URIs and the Dynamic Delegation Discovery Service
              (DDDS)", RFC 4848, April 2007,
              <http://www.rfc-editor.org/info/rfc4848>.

   [RFC5986]  Thomson, M. and J. Winterbottom, "Discovering the Local
              Location Information Server (LIS)", RFC 5986, September
              2010, <http://www.rfc-editor.org/info/rfc5986>.

   [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, <http://www.rfc-editor.org/info/rfc7285>.






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7.2.  Informative References

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

   [ALTO-DEPLOY]
              Stiemerling, M., Kiesel, S., Previdi, S., and M. Scharf,
              "ALTO Deployment Considerations", Work in Progress,
              draft-ietf-alto-deployments-10, July 2014.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987,
              <http://www.rfc-editor.org/info/rfc1035>.

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002,
              <http://www.rfc-editor.org/info/rfc3424>.

   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              October 2003, <http://www.rfc-editor.org/info/rfc3596>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005,
              <http://www.rfc-editor.org/info/rfc4033>.

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693, October
              2009, <http://www.rfc-editor.org/info/rfc5693>.

   [RFC6418]  Blanchet, M. and P. Seite, "Multiple Interfaces and
              Provisioning Domains Problem Statement", RFC 6418,
              November 2011, <http://www.rfc-editor.org/info/rfc6418>.

   [RFC6708]  Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
              Y. Yang, "Application-Layer Traffic Optimization (ALTO)
              Requirements", RFC 6708, September 2012,
              <http://www.rfc-editor.org/info/rfc6708>.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014, <http://www.rfc-editor.org/info/rfc7230>.






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Acknowledgments

   Olafur Gudmundsson provided an excellent DNS expert review on an
   earlier version of this document.  Thanks to Tina Tsou for an
   accurate security review.

   Michael Scharf is supported by the German-Lab project
   <http://www.german-lab.de> funded by the German Federal Ministry of
   Education and Research (BMBF).

   Martin Stiemerling is partially supported by the CHANGE project
   <http://www.change-project.eu>, a research project supported by the
   European Commission under its 7th Framework Program (contract no.
   257422).  The views and conclusions contained herein are those of the
   authors and should not be interpreted as necessarily representing the
   official policies or endorsements, either expressed or implied, of
   the CHANGE project or the European Commission.

Contributors

   The initial version of this document was coauthored by Marco Tomsu.

   Hannes Tschofenig provided the initial input to the U-NAPTR solution
   part.  Hannes and Martin Thomson provided excellent feedback and
   input to the server discovery.

   The authors would also like to thank the following persons for their
   contribution to this document or its predecessors: Richard Alimi,
   David Bryan, Roni Even, Gustavo Garcia, Jay Gu, Xingfeng Jiang,
   Enrico Marocco, Victor Pascual, Y. Richard Yang, Yu-Shun Wang, Yunfei
   Zhang, Ning Zong.




















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Authors' Addresses

   Sebastian Kiesel
   University of Stuttgart Information Center
   Networks and Communication Systems Department
   Allmandring 30
   Stuttgart  70550
   Germany

   EMail: ietf-alto@skiesel.de
   URI:   http://www.rus.uni-stuttgart.de/nks/


   Martin Stiemerling
   NEC Laboratories Europe
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   Phone: +49 6221 4342 113
   EMail: mls.ietf@gmail.com
   URI:   http://ietf.stiemerling.org


   Nico Schwan
   Thales Deutschland
   Thalesplatz 1
   Ditzingen  71254
   Germany

   EMail: ietf@nico-schwan.de


   Michael Scharf
   Alcatel-Lucent Bell Labs
   Lorenzstrasse 10
   Stuttgart  70435
   Germany

   EMail: michael.scharf@alcatel-lucent.com


   Haibin Song
   Huawei

   EMail: haibin.song@huawei.com





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