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Versions: (draft-sctl-service-registration) 00

Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Informational                                  T. Lemon
Expires: April 26, 2019                              Nibbhaya Consulting
                                                        October 23, 2018


     Service Registration Protocol for DNS-Based Service Discovery
                        draft-ietf-dnssd-srp-00

Abstract

   The Service Registration Protocol for DNS-Based Service Discovery
   uses the standard DNS Update mechanism to enable DNS-Based Service
   Discovery using only unicast packets.  This eliminates the dependency
   on Multicast DNS as the foundation layer, which greatly improves
   scalability and improves performance on networks where multicast
   service is not an optimal choice, particularly 802.11 (Wi-Fi) and
   802.15.4 (IoT) networks.  DNS-SD Service registration uses public
   keys and SIG(0) to allow services to defend their registrations
   against attack.

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 https://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 April 26, 2019.

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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Service Registration Protocol . . . . . . . . . . . . . . . .   4
     2.1.  What to publish . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Where to publish it . . . . . . . . . . . . . . . . . . .   6
     2.3.  How to publish it . . . . . . . . . . . . . . . . . . . .   6
       2.3.1.  How DNS-SD Service Registration differs from standard
               RFC2136 DNS Update  . . . . . . . . . . . . . . . . .   7
       2.3.2.  Testing using standard RFC2136-compliant servers  . .   7
       2.3.3.  How to allow services to update standard
               RFC2136-compliant servers . . . . . . . . . . . . . .   7
     2.4.  How to secure it  . . . . . . . . . . . . . . . . . . . .   8
       2.4.1.  First-Come First-Served Naming  . . . . . . . . . . .   8
       2.4.2.  SRP Server Behavior . . . . . . . . . . . . . . . . .   9
     2.5.  TTL Consistency . . . . . . . . . . . . . . . . . . . . .  12
     2.6.  Maintenance . . . . . . . . . . . . . . . . . . . . . . .  12
       2.6.1.  Cleaning up stale data  . . . . . . . . . . . . . . .  12
       2.6.2.  Sleep Proxy . . . . . . . . . . . . . . . . . . . . .  13
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  15
   5.  Delegation of 'services.arpa.'  . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   DNS-Based Service Discovery [RFC6763] is a component of Zero
   Configuration Networking [RFC6760] [ZC] [I-D.cheshire-dnssd-roadmap].

   This document describes an enhancement to DNS-Based Service Discovery
   [RFC6763] that allows services to automatically register their
   services using the DNS protocol rather than using Multicast DNS
   [RFC6762] (mDNS).  There is already a large installed base of DNS-SD
   clients that can do service discovery using the DNS protocol.  This
   extension makes it much easier to take advantage of this existing
   functionality.




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   This document is intended for three audiences: implementors of
   software that provides services that should be advertised using DNS-
   SD, implementors of DNS servers that will be used in contexts where
   DNS-SD registration is needed, and administrators of networks where
   DNS-SD service is required.  The document is intended to provide
   sufficient information to allow interoperable implementation of the
   registration protocol.

   DNS-Based Service Discovery (DNS-SD) allows services to advertise the
   fact that they provide service, and to provide the information
   required to access that service.  Clients can then discover the set
   of services of a particular type that are available.  They can then
   select a service from among those that are available and obtain the
   information required to use it.

   The Service Registration Protocol for DNS-SD (SRP), described in this
   document, provides a reasonably secure mechanism for publishing this
   information.  Once published, these services can be readily
   discovered by clients using standard DNS lookups.

   The DNS-SD specification [RFC6763], Section 10 ("Populating the DNS
   with Information"), briefly discusses ways that services can publish
   their information in the DNS namespace.  In the case of mDNS, it
   allows services to publish their information on the local link, using
   names in the ".local" namespace, which makes their services directly
   discoverable by peers attached to that same local link.

   RFC6763 also allows clients to discover services using the DNS
   protocol [RFC1035].  This can be done by having a system
   administrator manually configure service information in the DNS, but
   manually populating DNS authoritative server databases is costly and
   potentially error-prone, and requires a knowledgable network
   administrator.  Consequently, although all DNS-SD client
   implementations of which we are aware support DNS-SD using DNS
   queries, in practice it is used much less frequently than mDNS.

   The Discovery Proxy [I-D.ietf-dnssd-hybrid] provides one way to
   automatically populate the DNS namespace, but is only appropriate on
   networks where services are easily advertised using mDNS.  This
   document describes a solution more suitable for networks where
   multicast is inefficient, or where sleepy devices are common, by
   supporting both offering of services, and discovery of services,
   using unicast.








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2.  Service Registration Protocol

   Services that implement SRP use DNS Update [RFC2136] [RFC3007] to
   publish service information in the DNS.  Two variants exist, one for
   full-featured hosts, and one for devices designed for "Constrained-
   Node Networks" [RFC7228].

   Full-featured hosts are either configured manually with a
   registration domain, or use the "dr._dns-sd._udp.<domain>" query
   ([RFC6763] Section 11) to learn the default registration domain from
   the network.  RFC6763 says to discover the registration domain using
   either ".local" or a network-supplied domain name for <domain>.
   Services using SRP MUST use the domain name received through the
   DHCPv4 Domain Name option ([RFC2132] section 3.17), if available, or
   the Neighbor Discovery DNS Search List option [RFC8106].  If the DNS
   Search List option contains more than one domain name, it MUST NOT be
   used.  If neither option is available, the Service Registration
   protocol is not available on the local network.

   Manual configuration of the registraton domain can be done either by
   querying the list of available registration zones ("r._dns-sd._udp")
   and allowing the user to select one from the UI, or by any other
   means appropriate to the particular use case being addressed.  Full-
   featured devices construct the names of the SRV, TXT, and PTR records
   describing their service(s) as subdomains of the chosen service
   registration domain.  For these names they then discover the zone
   apex of the closest enclosing DNS zone using SOA queries
   [I-D.ietf-dnssd-push].  Having discovered the enclosing DNS zone,
   they query for the "_dnssd-srp._tcp<zone>" SRV record to discover the
   server to which they should send DNS updates.

   For devices designed for Constrained-Node Networks [RFC7228] some
   simplifications are used.  Instead of being configured with (or
   discovering) the service registration domain, the (proposed) special-
   use domain name [RFC6761] "services.arpa" is used.  Instead of
   learning the server to which they should send DNS updates, a fixed
   IPv6 anycast address is used (value TBD).  Anycasts are sent using
   UDP unless TCP is required due to the size of the update.  It is the
   responsibility of a Constrained-Node Network supporting SRP to
   provide appropriate anycast routing to deliver the DNS updates to the
   appropriate server.  It is the responsibility of the SRP server
   supporting a Constrained-Node Network to handle the updates
   appropriately.  In some network environments, updates may be accepted
   directly into a local "services.arpa" zone, which has only local
   visibility.  In other network environments, updates for names ending
   in "services.arpa" may be rewritten internally to names with broader
   visibility.




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   The reason for these different assumptions is that Constrained-Node
   Networks generally require special egress support, and Anycast
   packets captured at the Constrained-Node Network egress can be
   assumed to have originated locally.  Low-power devices that typically
   use Constrained-Node Networks may have very limited battery power.
   The additional DNS lookups required to discover an SRP server and
   then communicate with it will increase the power required to
   advertise a service; for low-power devices, the additional
   flexibility this provides does not justify the additional use of
   power.

   General networks have the potential to have more complicated
   topologies at the Internet layer, which makes anycast routing more
   difficult.  Such networks may or may not have the infrastructure
   required to route anycast to a server that can process it.  However,
   they can be assumed to be able to provide registration domain
   discovery and routing.  By requiring the use of TCP, the possibility
   of off-network spoofing is eliminated.

   We will discuss several parts to this process: how to know what to
   publish, how to know where to publish it (under what name), how to
   publish it, how to secure its publication, and how to maintain the
   information once published.

2.1.  What to publish

   We refer to the DNS Update message sent by services using SRP as an
   SRP update.  Three types of updates appear in an SRP update: Service
   Discovery records, Service Description records, and Host Description
   records.

   o  Service Discovery records are one or more PTR RRs, mapping from
      the generic service type (or subtype) to the specific Service
      Instance Name.

   o  Service Description records are exactly one SRV RR, exactly one
      KEY RR, and one or more TXT RRs, both with the same name, the
      Service Instance Name ([RFC6763] section 4.1).  In principle
      Service Description records can include other record types, with
      the same Service Instance Name, though in practice they rarely do.
      The Service Instance Name MUST be referenced by one or more
      Service Discovery PTR records, unless it is a placeholder service
      registration for an intentionally non-discoverable service name.

   o  The Host Description records for a service are a KEY RR, used to
      claim exclusive ownership of the service registration, and one or
      more RRs of type A or AAAA, giving the IPv4 or IPv6 address(es) of
      the host where the service resides.



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   RFC 6763 describes the details of what each of these types of updates
   contains and is the definitive source for information about what to
   publish; the reason for summarizing this here is to provide the
   reader with enough information about what will be published that the
   service registration process can be understood at a high level
   without first learning the full details of DNS-SD.  Also, the
   "Service Instance Name" is an important aspect of first-come, first-
   serve naming, which we describe later on in this document.

2.2.  Where to publish it

   Multicast DNS uses a single namespace, ".local", which is valid on
   the local link.  This convenience is not available for DNS-SD using
   the DNS protocol: services must exist in some specific unicast
   namespace.

   As described above, full-featured devices are responsible for knowing
   in what domain they should register their services.  Devices made for
   Constrained-Node Networks register in the (proposed) special use
   domain name [RFC6761] "services.arpa", and let the SRP server handle
   rewriting that to a different domain if necessary.

2.3.  How to publish it

   It is possible to issue a DNS Update that does several things at
   once; this means that it's possible to do all the work of adding a
   PTR resource record to the PTR RRset on the Service Name if it
   already exists, or creating one if it doesn't, and creating or
   updating the Service Instance Name and Host Description in a single
   transaction.

   An SRP update is therefore implemented as a single DNS Update message
   that contains a service's Service Discovery records, Service
   Description records, and Host Description records.

   Updates done according to this specification are somewhat different
   than regular DNS Updates as defined in RFC2136.  RFC2136 uses a
   fairly heavyweight process for updating: you might first attempt to
   add a name if it doesn't exist; if that fails, then in a second
   message you might update the name if it does exist but matches
   certain preconditions.  Because the registration protocol uses a
   single transaction, some of this adaptability is lost.

   In order to allow updates to happen in a single transaction, SRP
   updates do not include update constraints.  The constraints specified
   in Section 2.4.2 are implicit in the processing of SRP updates, and
   so there is no need for the service sending the SRP update to put in
   any explicit constraints.



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2.3.1.  How DNS-SD Service Registration differs from standard RFC2136
        DNS Update

   DNS-SD Service Registration is based on standard RFC2136 DNS Update,
   with some differences:

   o  It implements first-come first-served name allocation, protected
      using SIG(0) [RFC2931].

   o  It enforces policy about what updates are allowed.

   o  It optionally performs rewriting of "services.arpa" to some other
      domain.

   o  It optionally performs automatic population of the address-to-name
      reverse mapping domains.

   o  An SRP server is not required to implement general DNS Update
      prerequsite processing.

   o  Simplified clients are allowed to send updates to an anycast
      address, for names ending in "services.arpa"

2.3.2.  Testing using standard RFC2136-compliant servers

   It may be useful to set up a DNS server for testing that does not
   implement SRP.  This can be done by configuring the server to listen
   on the anycast address, or advertising it in the _dnssd-srp._tcp SRV
   record.  It must be configured to be authoritative for
   "services.arpa", and to accept updates from hosts on local networks
   for names under "services.arpa" without authentication, since such
   servers will not have support for FCFS authentication Section 2.4.1.

   A server configured in this way will be able to successfully accept
   and process SRP updates from services that send SRP updates.
   However, no constraints will be applied, and this means that the test
   server will accept internally inconsistent SRP updates, and will not
   stop two SRP updates, sent by different services, that claim the same
   name(s), from overwriting each other.

2.3.3.  How to allow services to update standard RFC2136-compliant
        servers

   Ordinarily SRP updates will fail when sent to an RFC 2136-compliant
   server that does not implement SRP because the zone being updated is
   "services.arpa", and no DNS server that is not an SRP server should
   normally be configured to be authoritative for "services.arpa".
   Therefore, a service that sends an SRP update can tell that the



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   receiving server does not support SRP, but does support RFC2136,
   because the RCODE will either be NOTZONE, NOTAUTH or REFUSED, or
   because there is no response to the update request (when using the
   anycast address)

   In this case a service MAY attempt to register itself using regular
   RFC2136 DNS updates.  To do so, it must discover the default
   registration zone and the DNS server designated to receive updates
   for that zone, as described earlier using the _dns-update._udp SRV
   record.  It can then make the update using the port and host pointed
   to by the SRV record, and should use appropriate constraints to avoid
   overwriting competing records.  Such updates are out of scope for
   SRP, and a service that implements SRP MUST first attempt to use SRP
   to register itself, and should only attempt to use RFC2136 backwards
   compatibility if that fails.  Although the owner name for the SRV
   record specifies the UDP protocol for updates, it is also possible to
   use TCP, when the update is too large.

2.4.  How to secure it

   Traditional DNS update is secured using the TSIG protocol, which uses
   a secret key shared between the client (which issues the update) and
   the server (which authenticates it).  This model does not work for
   automatic service registration.

   The goal of securing the DNS-SD Registration Protocol is to provide
   the best possible security given the constraint that service
   registration has to be automatic.  It is possible to layer more
   operational security on top of what we describe here, but what we
   describe here improves upon the security of mDNS.  The goal is not to
   provide the level of security of a network managed by a skilled
   operator.

2.4.1.  First-Come First-Served Naming

   First-Come First-Serve naming provides a limited degree of security:
   a service that registers its service using DNS-SD Registration
   protocol is given ownership of a name for an extended period of time
   based on the key used to authenticate the DNS Update.  As long as the
   registration service remembers thename and the key used to register
   that name, no other service can add or update the information
   associated with that.  FCFS naming is used to protect both the
   Service Description and the Host Description.








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2.4.1.1.  Service Behavior

   The service generates a public/private key pair.  This key pair MUST
   be stored in stable storage; if there is no writable stable storage
   on the client, the client MUST be pre-configured with a public/
   private key pair in read-only storage that can be used.  This key
   pair MUST be unique to the device.

   When sending DNS updates, the service includes a KEY record
   containing the public portion of the key in each Host Description
   update and each Service Description update.  Each KEY record MUST
   contain the same public key.  The update is signed using SIG(0),
   using the private key that corresponds to the public key in the KEY
   record.  The lifetimes of the records in the update is set using the
   EDNS(0) Update Lease option [I-D.sekar-dns-ul].

   The lifetime of the DNS-SD PTR, SRV, A, AAAA and TXT records
   [RFC6763] uses the LEASE field of the Update Lease option, and is
   typically set to two hours.  This means that if a device is
   disconnected from the network, it does not appear in the user
   interfaces of devices looking for services of that type for too long.

   The lifetime of the KEY records is set using the KEY-LEASE field of
   the Update Lease Option, and should be set to a much longer time,
   typically 14 days.  The result of this is that even though a device
   may be temporarily unplugged, disappearing from the network for a few
   days, it makes a claim on its name that lasts much longer.

   This means that even if a device is unplugged from the network for a
   few days, and its services are not available for that time, no other
   rogue device can come along and immediately claim its name the moment
   it disappears from the network.  In the event that a device is
   unplugged from the network and permanently discarded, then its name
   is eventually cleaned up and made available for re-use.

2.4.2.  SRP Server Behavior

   The SRP server first validates that the SRP update is a syntactically
   and semantically valid DNS Update according to the rules specified in
   RFC2136.

   The SRP server checks each update in the SRP update to see that it
   contains a Service Discovery update, a Service Description update,
   and a Host Description update.  Order matters in DNS updates.
   Specifically, deletes must precede adds for records that the deletes
   would affect; otherwise the add will have no effect.  This is the
   only ordering constraint; aside from this constraint, updates may
   appear in whatever order is convenient when constructing the update.



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   An update is a Service Discovery update if it contains

   o  exactly one RRset update,
   o  which is for a PTR RR,
   o  which points to a Service Instance Name
   o  for which an update is present in the SRP update.
   o  Service Discovery updates do not contain any deletes, and do not
      contain any other updates.

   An update is a Service Description update if, for the appropriate
   Service Instance Name, it contains

   o  exactly one "Delete all RRsets from a name" update,
   o  exactly one SRV RRset update,
   o  exactly one KEY RR update that adds a KEY RR that contains the
      public key corresponding to the private key that was used to sign
      the message,
   o  one or more TXT RRset updates,
   o  and the target of the SRV record update references a hostname for
      which there is a Host Description update in the SRP update.
   o  Service Descriptions do not update any other records.

   An update is a Host Description update if, for the appropriate
   hostname, it contains

   o  exactly one "Delete all RRsets from a name" update,
   o  one or more A or AAAA RR update(s)
   o  exactly one KEY RR update that adds a KEY RR that contains the
      public key corresponding to the private key that was used to sign
      the message,
   o  there is a Service Instance Name update in the SRP update that
      updates an SRV RR so that it points to the hostname being updated
      by this update.
   o  Host Description updates do not update any other records.

   An SRP update MUST include at least one Service Discovery update, at
   least one Service Description update, and exactly one Host
   Description update.  An update message that does not is not an SRP
   update.  An update message that contains any other updates, or any
   update constraints, is not an SRP update.  Such messages should
   either be processed as regular RFC2136 updates, including access
   control checks and constraint checks, if supported, or else rejected
   with RCODE=REFUSED.

   Note that if the definitions of each of these update types are
   followed carefully, this means that many things that look very much
   like SRP updates nevertheless are not.  For example, a DNS update
   that contains an update to a Service Name and an update to a Service



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   Instance Name, where the Service Name does not reference the Service
   Instance Name, is not a valid SRP update message, but may be a valid
   RFC2136 update.

   Assuming that an update message has been validated with these
   conditions and is a valid SRP update, the server checks that the name
   in the Host Description update exists.  If so, then the server checks
   to see if the KEY record on the name is the same as the KEY record in
   the update.  The server performs the same check for the KEY records
   in any Service Description update.  If any existing KEY record
   corresponding to a KEY record in the SRP update does not match the
   KEY record in the SRP update, then the server MUST reject the SRP
   update with the YXDOMAIN RCODE.

   Otherwise, the server validates the SRP update using SIG(0) on the
   public key in the KEY record of the Host Description update.  If the
   validation fails, the server MUST reject the SRP Update with the
   REFUSED RCODE.  Otherwise, the SRP update is considered valid and
   authentic, and is processed according to the method described in
   RFC2136.  The status that is returned depends on the result of
   processing the update.

   The server MAY add a Reverse Mapping that corresponds to the Host
   Description.  This is not required because the Reverse Mapping serves
   no protocol function, but it may be useful for debugging, e.g. in
   annotating network packet traces or logs.

   The server MAY apply additional criteria when accepting updates.  In
   some networks, it may be possible to do out-of-band registration of
   keys, and only accept updates from pre-registered keys.  In this
   case, an update for a key that has not been registered should be
   rejected with the REFUSED RCODE.

   There are at least two benefits to doing this rather than simply
   using normal SIG(0) DNS updates.  First, the same registration
   protocol can be used in both cases, so both use cases can be
   addressed by the same service implementation.  Second, the
   registration protocol includes maintenance functionality not present
   with normal DNS updates.

   Note that the semantics of using SRP in this way are different than
   for typical RFC2136 implementations: the KEY used to sign the SRP
   update only allows the client to update records that refer to its
   Host Description.  RFC2136 implementations do not normally provide a
   way to enforce a constraint of this type.

   The server may also have a dictionary of names or name patterns that
   are not permitted.  If such a list is used, updates for Service



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   Instance Names that match entries in the dictionary are rejected with
   YXDOMAIN.

2.5.  TTL Consistency

   All RRs within an RRset are required to have the same TTL
   (Clarifications to the DNS Specification [RFC2181], Section 5.2).  In
   order to avoid inconsistencies, SRP places restrictions on TTLs sent
   by services and requires that SRP Servers enforce consistency.

   Services sending SRP updates MUST use consistent TTLs in all RRs
   within the SRP update.

   SRP update servers MUST check that the TTLs for all RRs within the
   SRP update are the same.  If they are not, the SRP update MUST be
   rejected with a REFUSED RCODE.

   Additionally, when adding RRs to an RRset, for example when
   processing Service Discovery records, the server MUST use the same
   TTL on all RRs in the RRset.  How this consistency is enforced is up
   to the implementation.

   TTLs sent in SRP updates are advisory: they indicate the client's
   guess as to what a good TTL would be.  SRP servers may override these
   TTLs.  SRP servers SHOULD ensure that TTLs are reasonable: neither
   too long nor too short.  The TTL should never be longer than the
   lease time Section 2.6.1.  Shorter TTLs will result in more frequent
   data refreshes; this increases latency on the client side, and
   increases load on any caching resolvers and on the authoritative
   server.  Longer TTLs will increase the likelihood that data in caches
   will be stale.  TTL minimums and maximums SHOULD be configurable by
   the operator of the SRP server.

2.6.  Maintenance

2.6.1.  Cleaning up stale data

   Because the DNS-SD registration protocol is automatic, and not
   managed by humans, some additional bookkeeping is required.  When an
   update is constructed by the client, it MUST include include an
   EDNS(0) Update Lease Option [I-D.sekar-dns-ul].  The Update Lease
   Option contains two lease times: the Lease Time and the Key Lease
   Time.

   These leases are promises, similar to DHCP leases [RFC2131], from the
   client that it will send a new update for the service registration
   before the lease time expires.  The Lease time is chosen to represent
   the time after the update during which the registered records other



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   than the KEY record should be assumed to be valid.  The Key Lease
   time represents the time after the update during which the KEY record
   should be assumed to be valid.

   The reasoning behind the different lease times is discussed in the
   section on first-come, first-served naming Section 2.4.1.  SRP
   servers may be configured with limits for these values.  A default
   limit of two hours for the Lease and 14 days for the SIG(0) KEY are
   currently thought to be good choices.  Clients that are going to
   continue to use names on which they hold leases should update well
   before the lease ends, in case the registration service is
   unavailable or under heavy load.

   The SRP server MUST include an EDNS(0) Update Lease option in the
   response if the lease time proposed by the service has been shortened
   or lengthened.  The service MUST check for the EDNS(0) Update Lease
   option in the response and MUST use the lease times from that option
   in place of the options that it sent to the server when deciding when
   to update its registration.  The times may be shorter or longer than
   those specified in the SRP update; the client must honor them in
   either case.

   Clients should assume that each lease ends N seconds after the update
   was first transmitted, where N is the lease duration.  Servers should
   assume that each lease ends N seconds after the update that was
   successfully processed was received.  Because the server will always
   receive the update after the client sent it, this avoids the
   possibility of misunderstandings.

   SRP servers MUST reject updates that do not include an EDNS(0) Update
   Lease option.  Dual-use servers MAY accept updates that don't include
   leases, but SHOULD differentiate between SRP updates and other
   updates, and MUST reject updates that would otherwise be SRP updates
   updates if they do not include leases.

   Lease times have a completely different function than TTLs.  On an
   authoritative DNS server, the TTL on a resource record is a constant:
   whenever that RR is served in a DNS response, the TTL value sent in
   the answer is the same.  The lease time is never sent as a TTL; its
   sole purpose is to determine when the authoritative DNS server will
   delete stale records.  It is not an error to send a DNS response with
   a TTL of 'n' when the remaining time on the lease is less than 'n'.

2.6.2.  Sleep Proxy

   Another use of SRP is for devices that sleep to reduce power
   consumption.




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   In this case, in addition to the DNS Update Lease option
   [I-D.sekar-dns-ul] described above, the device includes an EDNS(0)
   OWNER Option [I-D.cheshire-edns0-owner-option].

   The EDNS(0) Update Lease option constitutes a promise by the device
   that it will wake up before this time elapses, to renew its
   registration and thereby demonstrate that it is still attached to the
   network.  If it fails to renew the registration by this time, that
   indicates that it is no longer attached to the network, and its
   registration (except for the KEY in the Host Description) should be
   deleted.

   The EDNS(0) OWNER Option indicates that the device will be asleep,
   and will not be receptive to normal network traffic.  When a DNS
   server receives a DNS Update with an EDNS(0) OWNER Option, that
   signifies that the SRP server should set up a proxy for any IPv4 or
   IPv6 address records in the DNS Update message.  This proxy should
   send ARP or ND messages claiming ownership of the IPv4 and/or IPv6
   addresses in the records in question.  In addition, proxy should
   answer future ARP or ND requests for those IPv4 and/or IPv6
   addresses, claiming ownership of them.  When the DNS server receives
   a TCP SYN or UDP packet addressed to one of the IPv4 or IPv6
   addresses for which it proxying, it should then wake up the sleeping
   device using the information in the EDNS(0) OWNER Option.  At present
   version 0 of the OWNER Option specifies the "Wake-on-LAN Magic
   Packet" that needs to be sent; future versions could be extended to
   specify other wakeup mechanisms.

   Note that although the authoritative DNS server that implements the
   SRP function need not be on the same link as the sleeping host, the
   Sleep Proxy must be on the same link.

   It is not required that sleepy nodes on a Constrained-Node Network
   support sleep proxy.  Such devices may have different mechanisms for
   dealing with sleep and wakeup.  An SRP registration for such a device
   will be useful regardless of the mechanism whereby messages are
   delivered to the sleepy end device.  For example, the message might
   be held in a buffer for an extended period of time by an intermediate
   device on a mesh network, and then delivered to the device when it
   wakes up.  The exact details of such behaviors are out of scope for
   this document.

3.  Security Considerations

   SRP updates have no authorization semantics other than first-come,
   first-served.  This means that if an attacker from outside of the
   administrative domain of the server knows the server's IP address, it
   can in principle send updates to the server that will be processed



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   successfully.  Servers should therefore be configured to reject
   updates from source addresses outside of the administrative domain of
   the server.

   For Anycast updates, this validation must be enforced by every router
   that connects the Constrained-Device Network to the unconstrained
   portion of the network.  For TCP updates, the initial SYN-SYN+ACK
   handshake prevents updates being forged by an off-network attacker.
   In order to ensure that this handshake happens, Service Discovery
   Protocol servers MUST NOT accept TCP Fast Open payloads.

   Note that these rules only apply to the validation of SRP updates.  A
   server that accepts updates from DNS-SD registration protocol clients
   may also accept other DNS updates, and those DNS updates may be
   validated using different rules.  However, in the case of a DNS
   service that accepts SRP updates, the intersection of the SRP update
   rules and whatever other update rules are present must be considered
   very carefully.

   For example, a normal, authenticated RFC2136 update to any RR that
   was added using SRP, but that is authenticated using a different key,
   could be used to override a promise made by the registration
   protocol, by replacing all or part of the service registration
   information with information provided by a different client.  An
   implementation that allows both kinds of updates should not allow
   updates to records added by SRP updates using different
   authentication and authorization credentials.

4.  Privacy Considerations

5.  Delegation of 'services.arpa.'

   In order to be fully functional, there must be a delegation of
   'services.arpa.' in the '.arpa.' zone [RFC3172].  This delegation
   should be set up as was done for 'home.arpa', as a result of the
   specification in [RFC8375]Section 7.

6.  IANA Considerations

   IANA is requested to record the domain name 'services.arpa.' in the
   Special-Use Domain Names registry [SUDN].  IANA is requested, with
   the approval of IAB, to implement the delegation requested in
   Section 5.

   IANA is further requested to add a new entry to the "Transport-
   Independent Locally-Served Zones" subregistry of the the "Locally-
   Served DNS Zones" registry[LSDZ].  The entry will of for




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   'services.arpa.' with the description "DNS-SD Registration Protocol
   Special-Use Domain", listing this document as the reference.

   IANA is also requested to add a new entry to the Service Names and
   Port Numbers registry for dnssd-srp with a transport type of tcp.  No
   port number is to be assigned.  The reference should be to this
   document, and the Assignee and Contact information should reference
   the authors of this document.  The Description should be as follows:

   Availability of DNS Service Discovery Service Registration Protocol
   Service for a given domain is advertised using the "_dnssd-
   srp._tcp.<domain>."  SRV record gives the target host and port where
   DNSSD Service Registration Service is provided for the named domain.

7.  Acknowledgments

   Thanks to Toke Hoeiland-Joergensen for a thorough technical review,
   to Tamara Kemper for doing a nice developmental edit, Tim Wattenberg
   for doing a service implementation at the Montreal Hackathon at IETF
   102, Tom Pusateri for reviewing during the hackathon and afterwards,
   and [...] more reviewers to come, hopefully.

8.  References

8.1.  Normative References

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [I-D.sekar-dns-ul]
              Cheshire, S. and T. Lemon, "Dynamic DNS Update Leases",
              draft-sekar-dns-ul-02 (work in progress), August 2018.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <https://www.rfc-editor.org/info/rfc2132>.

   [RFC3172]  Huston, G., Ed., "Management Guidelines & Operational
              Requirements for the Address and Routing Parameter Area
              Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
              September 2001, <https://www.rfc-editor.org/info/rfc3172>.

   [RFC8106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 8106, DOI 10.17487/RFC8106, March 2017,
              <https://www.rfc-editor.org/info/rfc8106>.




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   [RFC8375]  Pfister, P. and T. Lemon, "Special-Use Domain
              'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018,
              <https://www.rfc-editor.org/info/rfc8375>.

   [SUDN]     "Special-Use Domain Names Registry", July 2012,
              <https://www.iana.org/assignments/special-use-domain-
              names/special-use-domain-names.xhtml>.

   [LSDZ]     "Locally-Served DNS Zones Registry", July 2011,
              <https://www.iana.org/assignments/locally-served-dns-
              zones/locally-served-dns-zones.xhtml>.

8.2.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/info/rfc2181>.

   [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
              ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September
              2000, <https://www.rfc-editor.org/info/rfc2931>.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
              <https://www.rfc-editor.org/info/rfc3007>.

   [RFC3152]  Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152,
              DOI 10.17487/RFC3152, August 2001,
              <https://www.rfc-editor.org/info/rfc3152>.





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   [RFC6760]  Cheshire, S. and M. Krochmal, "Requirements for a Protocol
              to Replace the AppleTalk Name Binding Protocol (NBP)",
              RFC 6760, DOI 10.17487/RFC6760, February 2013,
              <https://www.rfc-editor.org/info/rfc6760>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <https://www.rfc-editor.org/info/rfc6761>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [I-D.ietf-dnssd-hybrid]
              Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", draft-ietf-dnssd-hybrid-08 (work in
              progress), March 2018.

   [I-D.ietf-dnssd-push]
              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-15 (work in progress), September
              2018.

   [I-D.cheshire-dnssd-roadmap]
              Cheshire, S., "Service Discovery Road Map", draft-
              cheshire-dnssd-roadmap-02 (work in progress), October
              2018.

   [I-D.cheshire-edns0-owner-option]
              Cheshire, S. and M. Krochmal, "EDNS0 OWNER Option", draft-
              cheshire-edns0-owner-option-01 (work in progress), July
              2017.

   [ZC]       Cheshire, S. and D. Steinberg, "Zero Configuration
              Networking: The Definitive Guide", O'Reilly Media, Inc. ,
              ISBN 0-596-10100-7, December 2005.

Authors' Addresses








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   Stuart Cheshire
   Apple Inc.
   One Apple Park Way
   Cupertino, California  95014
   USA

   Phone: +1 408 974 3207
   Email: cheshire@apple.com


   Ted Lemon
   Nibbhaya Consulting
   P.O. Box 958
   Brattleboro, Vermont  05302
   United States of America

   Email: mellon@fugue.com


































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