Network Working Group
Request for Comments: 5679
Category: Standards Track
G. Bajko
December 2009

Locating IEEE 802.21 Mobility Services Using DNS


This document defines application service tags that allow service location without relying on rigid domain naming conventions, and DNS procedures for discovering servers that provide IEEE 802.21-defined Mobility Services. Such Mobility Services are used to assist a Mobile Node (MN) supporting IEEE 802.21, in handover preparation (network discovery) and handover decision (network selection). The services addressed by this document are the Media Independent Handover Services defined in IEEE 802.21.

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © 2009 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 ( 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 BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................3
      1.2. Terminology ................................................3
   2. Discovering a Mobility Server ...................................3
      2.1. Selecting a Mobility Service ...............................5
      2.2. Selecting the Transport Protocol ...........................5
      2.3. Determining the IP Address and Port ........................6
   3. IANA Considerations .............................................7
   4. Security Considerations .........................................8
   5. Normative References ............................................8
   6. Informative References ..........................................9

1. Introduction

IEEE 802.21 [IEEE802.21] defines three distinct service types to facilitate link-layer handovers across heterogeneous technologies:

   a) MIH Information Service (MIHIS)
      IS provide a unified framework to the higher-layer entities across
      the heterogeneous network environment to facilitate discovery and
      selection of multiple types of networks existing within a
      geographical area, with the objective to help the higher-layer
      mobility protocols to acquire a global view of the heterogeneous
      networks and perform seamless handover across these networks.
   b) MIH Event Service (MIHES)
      Events may indicate changes in state and transmission behavior of
      the physical, data link and logical link layers, or predict state
      changes of these layers.  The Event Services may also be used to
      indicate management actions or command status on the part of the
      network or some management entity.
   c) MIH Command Service (MIHCS)
      The command service enables higher layers to control the physical,
      data link, and logical link layers.  The higher layers may control
      the reconfiguration or selection of an appropriate link through a
      set of handover commands.

In IEEE terminology, these services are called Media Independent Handover (MIH) services. While these services may be co-located, the different pattern and type of information they provide do not necessitate the co-location.

"Service Management" service messages, i.e., MIH registration, MIH capability discovery and MIH event subscription messages, are considered as MIHES and MIHCS when transporting MIH messages over L3 transport.

A Mobile Node (MN) may make use of any of these MIH service types separately or any combination of them.

It is anticipated that a Mobility Server will not necessarily host all three of these MIH services together, thus there is a need to discover the MIH service types separately.

This document defines a number of application service tags that allow service location without relying on rigid domain naming conventions.

1.1. Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

1.2. Terminology

Mobility Services: composed of a set of different services provided by the network to mobile nodes to facilitate handover preparation and handover decision, as described in [IEEE802.21] and [RFC5164].

Mobility Server: a network node providing IEEE 802.21 Mobility Services.


Media Independent Handover, as defined in [IEEE802.21].

   Application service:  is a generic term for some type of application,
   independent of the protocol that may be used to offer it.  Each
   application service will be associated with an IANA-registered tag.

Application protocol: is used to implement the application service. These are also associated with IANA-registered tags.

Home domain: the DNS suffix of the operator with which the Mobile Node has a subscription service. The suffix is usually stored in the Mobile Node as part of the subscription.

2. Discovering a Mobility Server

The Dynamic Delegation Discovery System (DDDS) [RFC3401] is used to implement lazy binding of strings to data, in order to support dynamically configured delegation systems. The DDDS functions by mapping some unique string to data stored within a DDDS database by iteratively applying string transformation rules until a terminal condition is reached. When DDDS uses DNS as a distributed database of rules, these rules are encoded using the Naming Authority Pointer (NAPTR) Resource Record (RR). One of these rules is the First Well Known Rule, which says where the process starts.

In current specifications, the First Well Known Rule in a DDDS application [RFC3403] is assumed to be fixed, i.e., the domain in the tree where the lookups are to be routed to, is known. This document proposes the input to the First Well Known Rule to be dynamic, based on the search path the resolver discovers or is configured with.

The search path of the resolver can either be pre-configured, discovered using DHCP, or learned from a previous MIH Information Services (IS) query [IEEE802.21] as described in [RFC5677].

When the MN needs to discover Mobility Services in its home domain, the input to the First Well Known Rule MUST be the MN's home domain, which is assumed to be pre-configured in the MN.

When the MN needs to discover Mobility Services in a local (visited) domain, it SHOULD use DHCP as described in [RFC5678] to discover the IP address of the server hosting the desired service, and contact it directly. In some instances, the discovery may result in a per protocol/application list of domain names that are then used as starting points for the subsequent NAPTR lookups. If neither the IP address or domain name can be discovered with the above procedure, the MN MAY request a domain search list, as described in [RFC3397] and [RFC3646], and use it as input to the DDDS application.

The MN may also have a list of cached domain names of Service Providers, learned from a previous MIH Information Services (IS) query [IEEE802.21]. If the cache entries have not expired, they can be used as input to the DDDS application.

When the MN does not find valid domain names using the procedures above, it MUST stop any attempt to discover MIH services.

The dynamic rule described above SHOULD NOT be used for discovering services other than MIH services described in this document, unless stated otherwise by a future specification.

The procedures defined here result in an IP address, port, and transport protocol where the MN can contact the Mobility Server that hosts the service the MN is looking for.

2.1. Selecting a Mobility Service

The MN should know the characteristics of the Mobility Services defined in [IEEE802.21], and based on that, it should be able to select the service it wants to use to facilitate its handover. The services it can choose from are:

      - Information Services (MIHIS)
      - Event Services (MIHES)
      - Command Services (MIHCS)

The service identifiers for the services are "MIHIS", "MIHES", and "MIHCS", respectively. The server supporting any of the above services MUST support at least UDP and TCP as transport, as described in [RFC5677]. SCTP and other transport protocols MAY also be supported.

2.2. Selecting the Transport Protocol

After the desired service has been chosen, the client selects the transport protocol it prefers to use. Note that transport selection may impact the handover performance.

The services relevant for the task of transport protocol selection are those with NAPTR service fields with values "ID+M2X", where ID is the service identifier defined in the previous section, and X is a letter that corresponds to a transport protocol supported by the domain. This specification defines M2U for UDP, M2T for TCP and M2S for SCTP. This document also establishes an IANA registry for mappings of NAPTR service name to transport protocol.

These NAPTR [RFC3403] records provide a mapping from a domain to the SRV [RFC2782] record for contacting a server with the specific transport protocol in the NAPTR services field. The resource record MUST contain an empty regular expression and a replacement value, which indicates the domain name where the SRV record for that particular transport protocol can be found. If the server supports multiple transport protocols, there will be multiple NAPTR records, each with a different service value. As per [RFC3403], the client discards any records whose services fields are not applicable.

The MN MUST discard any service fields that identify a resolution service whose value is not "M2X", for values of X that indicate transport protocols supported by the client. The NAPTR processing as described in RFC 3403 will result in the discovery of the most preferred transport protocol of the server that is supported by the client, as well as an SRV record for the server.

As an example, consider a client that wishes to find MIHIS service in the domain. The client performs a NAPTR query for that domain, and the following NAPTR records are returned:

           Order Pref Flags  Service     Regexp       Replacement
   IN NAPTR  50   50   "s"  "MIHIS+M2T"    ""
   IN NAPTR  90   50   "s"  "MIHIS+M2U"    ""

This indicates that the domain does have a server providing MIHIS services over TCP and UDP, in that order of preference. Since the client supports TCP and UDP, TCP will be used, targeted to a host determined by an SRV lookup of That lookup would return:

   ;;          Priority  Weight    Port        Target
        IN  SRV    0        1      XXXX
        IN  SRV    0        2      XXXX

where XXXX represents the port number at which the service is reachable.

If no NAPTR records are found, the client constructs SRV queries for those transport protocols it supports, and does a query for each. Queries are done using the service identifier "_MIHIS" for the MIH Information Service, "_MIHES" for the MIH Event Service and "_MIHCS" for the MIH Command Service. A particular transport is supported if the query is successful. The client MAY use any transport protocol it desires that is supported by the server.

Note that the regexp field in the NAPTR example above is empty. The regexp field MUST NOT be used when discovering MIH services, as its usage can be complex and error prone. Also, the discovery of the MIH services does not require the flexibility provided by this field over a static target present in the TARGET field.

If the client is already configured with the information about which transport protocol is used for a mobility service in a particular domain, it can directly perform an SRV query for that specific transport using the service identifier of the Mobility Service. For example, if the client knows that it should be using TCP for MIHIS service, it can perform a SRV query directly for

2.3. Determining the IP Address and Port

Once the server providing the desired service and the transport protocol has been determined, the next step is to determine the IP address and port.

The response to the SRV DNS query contains the port number in the Port field of the SRV RDATA.

According to the specification of SRV RRs in [RFC2782], the TARGET field is a fully qualified domain name (FQDN) that MUST have one or more address records; the FQDN must not be an alias, i.e., there MUST NOT be a CNAME or DNAME RR at this name. Unless the SRV DNS query already has reported a sufficient number of these address records in the Additional Data section of the DNS response (as recommended by [RFC2782]), the MN needs to perform A and/or AAAA record lookup(s) of the domain name, as appropriate. The result will be a list of IP addresses, each of which can be contacted using the transport protocol determined previously.

3. IANA Considerations

The usage of NAPTR records described here requires well-known values for the service fields for each transport supported by Mobility Services. The table of mappings from service field values to transport protocols is to be maintained by IANA.

The registration in the RFC MUST include the following information:

Service Field: The service field being registered.

Protocol: The specific transport protocol associated with that service field. This MUST include the name and acronym for the protocol, along with reference to a document that describes the transport protocol.

Name and Contact Information: The name, address, email address, and telephone number for the person performing the registration.

The following values have been placed into the registry:

      Service Fields                    Protocol
         MIHIS+M2T                        TCP
         MIHIS+M2U                        UDP
         MIHIS+M2S                       SCTP
         MIHES+M2T                        TCP
         MIHES+M2U                        UDP
         MIHES+M2S                       SCTP
         MIHCS+M2T                        TCP
         MIHCS+M2U                        UDP
         MIHCS+M2S                       SCTP

New Service Fields are to be added via Standards Action as defined in [RFC5226].

New entries to the table that specify additional transport protocols for the existing Service Fields may similarly be registered by IANA through Standards Action [RFC5226].

IANA is also requested to register MIHIS, MIHES, MIHCS as service names in the Protocol and Service Names registry.

4. Security Considerations

A list of known threats to services using DNS is documented in [RFC3833]. For most of those identified threats, the DNS Security Extensions [RFC4033] does provide protection. It is therefore recommended to consider the usage of DNSSEC [RFC4033] and the aspects of DNSSEC Operational Practices [RFC4641] when deploying IEEE 802.21 Mobility Services.

In deployments where DNSSEC usage is not feasible, measures should be taken to protect against forged DNS responses and cache poisoning as much as possible. Efforts in this direction are documented in [RFC5452].

Where inputs to the procedure described in this document are fed via DHCP, DHCP vulnerabilities can also cause issues. For instance, the inability to authenticate DHCP discovery results may lead to the mobility service results also being incorrect, even if the DNS process was secured.

5. Normative References

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
   [RFC2782]    Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
                specifying the location of services (DNS SRV)", RFC
                2782, February 2000.
   [RFC3397]    Aboba, B. and S. Cheshire, "Dynamic Host Configuration
                Protocol (DHCP) Domain Search Option", RFC 3397,
                November 2002.
   [RFC3403]    Mealling, M., "Dynamic Delegation Discovery System
                (DDDS) Part Three: The Domain Name System (DNS)
                Database", RFC 3403, October 2002.
   [RFC3646]    Droms, R., Ed., "DNS Configuration options for Dynamic
                Host Configuration Protocol for IPv6 (DHCPv6)", RFC
                3646, December 2003.
   [RFC4033]    Arends, R., Austein, R., Larson, M., Massey, D., and S.
                Rose, "DNS Security Introduction and Requirements", RFC
                4033, March 2005.
   [RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
                IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                May 2008.
   [RFC5677]    Melia, T., Ed., Bajko, G., Das, S., Golmie, N., and JC.
                Zuniga, "IEEE 802.21 Mobility Services Framework Design
                (MSFD)", RFC 5677, December 2009.
   [RFC5678]    Bajko, G. and S. Das, "Dynamic Host Configuration
                Protocol (DHCPv4 and DHCPv6) Options for IEEE 802.21
                Mobility Services (MoS) Discovery", RFC 5678, December

6. Informative References

[IEEE802.21] "IEEE Standard for Local and Metropolitan Area Networks

- Part 21: Media Independent Handover Services", IEEE LAN/MAN Std 802.21-2008, January 2009, 802.21-2008.pdf (access to the document requires membership).

   [RFC3401]    Mealling, M., "Dynamic Delegation Discovery System
                (DDDS) Part One: The Comprehensive DDDS", RFC 3401,
                October 2002.
   [RFC3833]    Atkins, D. and R. Austein, "Threat Analysis of the
                Domain Name System (DNS)", RFC 3833, August 2004.
   [RFC4641]    Kolkman, O. and R. Gieben, "DNSSEC Operational
                Practices", RFC 4641, September 2006.
   [RFC5164]    Melia, T., Ed., "Mobility Services Transport: Problem
                Statement", RFC 5164, March 2008.
   [RFC5452]    Hubert, A. and R. van Mook, "Measures for Making DNS
                More Resilient against Forged Answers", RFC 5452,
                January 2009.

Author's Address

   Gabor Bajko