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Versions: 00 01 02

Network Working Group                                        S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                                T. Lemon
Expires: May 16, 2018                                Barefoot Consulting
                                                       November 12, 2017


                     Multicast DNS Discovery Relay
                     draft-sctl-dnssd-mdns-relay-02

Abstract

   This document extends the specification of the Discovery Proxy for
   Multicast DNS-Based Service Discovery.  It describes a lightweight
   relay mechanism, a Discovery Relay, which allows Discovery Proxies to
   provide service on multicast links to which they are not directly
   attached.

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 May 16, 2018.

Copyright Notice

   Copyright (c) 2017 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
   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




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Connections between Proxies and Relays (overview) . . . .   5
     3.2.  mDNS Messages On Multicast Links  . . . . . . . . . . . .   6
   4.  Connections between Proxies and Relays (details)  . . . . . .   6
   5.  Traffic from Relays to Proxies  . . . . . . . . . . . . . . .   7
   6.  Traffic from Proxies to Relays  . . . . . . . . . . . . . . .   8
   7.  Discovery Proxy Behavior  . . . . . . . . . . . . . . . . . .   9
   8.  DSO TLVs  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  mDNS Link Request . . . . . . . . . . . . . . . . . . . .  10
     8.2.  mDNS Discontinue  . . . . . . . . . . . . . . . . . . . .  10
     8.3.  mDNS Message  . . . . . . . . . . . . . . . . . . . . . .  10
     8.4.  Link Identifier . . . . . . . . . . . . . . . . . . . . .  10
     8.5.  Layer Two Source Address  . . . . . . . . . . . . . . . .  11
     8.6.  IP Source . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Provisioning  . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Provisioned Objects . . . . . . . . . . . . . . . . . . .  12
       9.1.1.  Discovery Proxy . . . . . . . . . . . . . . . . . . .  12
       9.1.2.  Multicast Link  . . . . . . . . . . . . . . . . . . .  13
       9.1.3.  Discovery Relay . . . . . . . . . . . . . . . . . . .  13
     9.2.  Configuration Files . . . . . . . . . . . . . . . . . . .  14
     9.3.  Discovery Proxy Configuration . . . . . . . . . . . . . .  15
     9.4.  Discovery Relay Configuration . . . . . . . . . . . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     13.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18















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1.  Introduction

   The Discovery Proxy for Multicast DNS-Based Service Discovery
   [I-D.ietf-dnssd-hybrid] is a mechanism for discovering services on a
   subnetted network through the use of Discovery Proxies, which issue
   Multicast DNS (mDNS) requests [RFC6762] on various multicast links in
   the network on behalf of a remote host performing DNS-Based Service
   Discovery [RFC6763].

   In the original Discovery Proxy specification, it is assumed that for
   every multicast link on which services will be discovered, a host
   will be present running a full Discovery Proxy.  This document
   introduces a lightweight Discovery Relay which can be used to provide
   discovery services on a multicast link without requiring a full
   Discovery Proxy on every multicast link.

   The Discovery Relay operates by listening for TCP connections from
   Discovery Proxies.  When a Discovery Proxy connects, the connection
   is authenticated and secured using TLS.  The Discovery Proxy can then
   specify one or more multicast links from which it wishes to receive
   mDNS traffic.  The Discovery Proxy can also send messages to be
   transmitted on its behalf on one or more of those multicast links.
   DNS Stateful Operations (DSO) [I-D.ietf-dnsop-session-signal] is used
   as a framework for conveying interface and IP header information
   associated with each message.

   The Discovery Relay functions essentially as a set of one or more
   remote virtual interfaces for the Discovery proxy, one on each
   multicast link to which the Discovery Relay is connected.  In a
   complex network, it is possible that more than one Discovery Relay
   will be connected to the same multicast link; in this case, the
   Discovery Proxy ideally should only be using one such Relay Proxy per
   multicast link, since using more than one will generate duplicate
   traffic.

   How such duplication is detected and avoided is out of scope for this
   document: in principle it could be detected using HNCP [RFC7788] or
   configured using some sort of orchestration software in conjunction
   with NETCONF [RFC6241] or CPE WAN Management Protocol [TR-069].












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2.  Terminology

   The following definitions may be of use:

   mDNS Agent  A host which sends and/or responds to mDNS queries.

   Discovery Proxy  A network service which receives well-formed
      questions using the DNS protocol, performs multicast DNS queries
      to answer those questions, and responds with those answers using
      the DNS protocol.

   Discovery Relay  A network service which relays received mDNS
      messages to a Discovery Proxy, and can transmit mDNS messages on
      behalf of that Discovery Proxy.

   multicast link  A maximal set of network connection points, such that
      any host connected to any connection point in the set may send a
      packet with a link-local multicast destination address
      (specifically the mDNS link-local multicast destination address
      [RFC6762]) that will be received by all hosts connected to all
      other connection points in the set.  Note that it is becoming
      increasingly common for a multicast link to be smaller than its
      corresponding unicast link.  For example it is becoming common to
      have multiple Wi-Fi Access Points on a shared Ethernet backbone,
      where the multiple Wi-Fi Access Points and their shared Ethernet
      backbone form a single unicast link (a single IPv4 subnet, or
      single IPv6 prefix) but not a single multicast link.  Unicast
      packets between two hosts on that IPv4 subnet or IPv6 prefix are
      correctly delivered, but multicast packets are not forwarded
      between the various Wi-Fi Access Points.  Given the slowness of
      Wi-Fi multicast, the decision to not forward multicast packets
      between Wi-Fi Access Points is reasonable, and that further
      supports the need for technologies like Discovery Proxy and
      Discovery Relay to facilitate discovery on these networks.

   whitelist  A list of one or more IP addresses from which a Discovery
      Relay may accept connections.

   silently discard  When a message that is not supported or not
      permitted is received, and the required response to that message
      is to "silently discard" it, that means that no response is sent
      by the service that is discarding the message to the service that
      sent it.  The service receiving the message may log the event, and
      may also count such events: "silently" does not preclude such
      behavior.






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3.  Protocol Overview

   This document describes a way for Discovery Proxies to communicate
   with mDNS agents on remote multicast links to which they are not
   directly connected, using a Discovery Relay.  As such, there are two
   parts to the protocol: connections between Discovery Proxies and
   Discovery Relays, and communications between Discovery Relays and
   mDNS agents.

3.1.  Connections between Proxies and Relays (overview)

   Discovery Relays listen for incoming connection requests.
   Connections between Discovery Proxies and Discovery Relays are
   established by Discovery Proxies.  Connections are authenticated and
   encrypted using TLS, with both client and server certificates.
   Connections are long-lived: a Discovery Proxy is expected to send
   many queries over a single connection, and Discovery Relays will
   forward all mDNS traffic from subscribed interfaces over the
   connection.

   The stream encapsulated in TLS will carry DNS frames as in the DNS
   TCP protocol [RFC1035] Section 4.2.2.  However, all messages will be
   DSO messages [I-D.ietf-dnsop-session-signal].  There will be three
   types of such messages:

   o  Control messages from Discovery Proxy to Discovery Relay

   o  mDNS messages from Discovery Relay to Discovery Proxy

   o  mDNS messages from Discovery Proxy to Discovery Relay

   Subscribe messages from the Discovery Proxy to the Discovery Relay
   indicate to the Discovery Relay that mDNS messages from one or more
   specified multicast links are to be relayed to the Discovery Proxy.

   mDNS messages from a Discovery Proxy to a Discovery Relay cause the
   Discovery Relay to transmit the mDNS message on one or more multicast
   links to which the Discovery Relay host is directly attached.

   mDNS messages from a Discovery Relay to a Discovery Proxy are sent
   whenever an mDNS message is received on a multicast link to which the
   Discovery Relay has subscribed.

   Discovery Relays are responsible for keeping connections alive when
   no traffic has been sent during a keepalive period (See
   [I-D.ietf-dnsop-session-signal] Section 4).





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3.2.  mDNS Messages On Multicast Links

   Discovery Relays listen for mDNS traffic on all configured multicast
   links that have at least one active subscription from a Discovery
   Proxy.  When an mDNS message is received on a multicast link, it is
   forwarded on every open Discovery Proxy connection that is subscribed
   to mDNS traffic on that multicast link.  In the event of congestion,
   where a particular Discovery Proxy connection has no buffer space for
   an mDNS message that would otherwise be forwarded to it, the mDNS
   message is not forwarded to it.  Normal mDNS retry behavior is used
   to recover from this sort of packet loss.  Discovery Relays are not
   expected to buffer more than a few mDNS packets.  Excess mDNS packets
   are silently discarded.  In reality this is expected to be a
   nonissue.  Particularly on networks like Wi-Fi, multicast packets are
   transmitted at rates ten or even a hundred times slower than unicast
   packets.  This means that even at peak multicast packets rates, it is
   likely that a unicast TCP connection will able to carry those packets
   with ease.

   Discovery Proxies send mDNS messages they wish to have sent on their
   behalf on remote multicast link(s) on which the Discovery Proxy has
   an active subscription.  A Discovery Relay will not transmit mDNS
   packets on any multicast link on which the remote Discovery Proxy
   does not have an active subscription, since it makes no sense for a
   Discovery Proxy to ask to have a query sent on its behalf if it's not
   able to receive the responses to that query.

4.  Connections between Proxies and Relays (details)

   When a Discovery Relay starts, it opens a passive TCP listener to
   receive incoming connection requests from Discovery Proxies.  This
   listener may be bound to one or more source IP addresses, or to the
   wildcard address, depending on the implementation.  When a connection
   is received, the relay must first validate that it is a connection to
   an IP address to which connections are allowed.  For example, it may
   be that only connections to ULAs are allowed, or to the IP addresses
   configured on certain interfaces.  If the listener is bound to a
   specific IP address, this check is unnecessary.

   The relay must then validate that the source IP address of the
   connection is on its whitelist.  If the connection is not permitted
   either because of the source address or the destination address, the
   Discovery Relay responds to the TLS Client Hello message from the
   Discovery Proxy with a TLS user_canceled alert ([I-D.ietf-tls-tls13]
   Section 6.1).

   Otherwise, the Discovery Relay will attempt to complete a TLS
   handshake with the Discovery Proxy.  Discovery Proxies are required



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   to send the post_handshake_auth extension ([I-D.ietf-tls-tls13]
   Section 4.2.5).  If a relay proxy receives a ClientHello message with
   no post_handshake_auth extension, the Discovery Relay rejects the
   connection with a certificate_required alert ([I-D.ietf-tls-tls13]
   Section 6.2).

   Once the TLS handshake is complete, the Discovery Relay MUST request
   post-handshake authentication as described in ([I-D.ietf-tls-tls13]
   Section 4.6.2).  If the Discovery Proxy refuses to send a
   certificate, or the key presented does not match the key associated
   with the IP address from which the connection originated, or the
   CertificateVerify does not validate, the connection is dropped with
   the TLS access_denied alert ([I-D.ietf-tls-tls13] Section 6.2).

   Once the connection is established and authenticated, it is treated
   as a DNS TCP connection [RFC1035].

   Aliveness of connections between Discovery Proxies and Relays is
   maintained as described in Section 4 of
   [I-D.ietf-dnsop-session-signal].  Discovery Proxies must also honor
   the 'Retry Delay' TLV (section 5 of [I-D.ietf-dnsop-session-signal])
   if sent by the Discovery Relay.

   Discovery Proxies may establish more than one connection to a
   specific Discovery Relay.  This would happen in the case that a TCP
   connection stalls, and the Discovery Proxy is able to reconnect
   before the previous connection has timed out.  It could also happen
   as a result of a server restart.  It is not likely that two active
   connections from the same Discovery Proxy would be present at the
   same time, but it must be possible for additional connections to be
   established.  The Discovery Relay may drop the old connection when
   the new one has been fully established, including a successful TLS
   handshake.  What it means for two connections to be from the same
   Discovery Proxy is that the connections both have source addresses
   that belong to the same proxy, and that they were authenticated using
   the same client certificate.

5.  Traffic from Relays to Proxies

   The mere act of connecting to a Discovery Relay does not result in
   any mDNS traffic being forwarded.  In order to request that mDNS
   traffic from a particular multicast link be forwarded on a particular
   connection, the Discovery Proxy must send one or more DSO messages
   containing mDNS a Link Request TLV (Section 8.1) indicating the
   multicast link from which traffic is requested.

   When such a message is received, the Discovery Relay validates that
   the specified multicast link is available for forwarding, and that



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   forwarding is enabled for that multicast link.  For each such message
   the Discovery Relay validates the multicast link specified and
   includes in a single response RCODE 0 if the multicast link specified
   is valid, or RCODE 11 (DSOFAIL) otherwise.  For each valid multicast
   link, it begins forwarding all mDNS traffic from that link to the
   Discovery Proxy.  Delivery is not guaranteed: if there is no buffer
   space, packets will be dropped.  It is expected that regular mDNS
   retry processing will take care of retransmission of lost packets.
   The amount of buffer space is implementation dependent, but generally
   should not be more than the bandwidth delay product of the TCP
   connection [RFC1323].  The Discovery Relay should use the
   TCP_NOTSENT_LOWAT mechanism [NOTSENT][PRIO] or equivalent, to avoid
   building up a backlog of data in excess of the amount necessary to
   have in flight to fill the bandwidth delay product of the TCP
   connection.

   mDNS messages from Relays to Proxies are framed within DSO messages.
   Each DSO message can contain multiple TLVs, but only a single mDNS
   message is conveyed per DSO message.  Each forwarded mDNS message is
   contained in an mDNS Message TLV (Section 8.3).  The layer two source
   address of the message, if known, MAY be encoded in a Layer Two
   Source TLV (Section 8.5).  The source IP address and port of the
   message MUST be encoded in a IP Source (Section 8.6).  The multicast
   link on which the message was received MUST be encoded in a Link
   Identifier TLV (Section 8.4).  The Discovery Proxy MUST silently
   ignore unrecognized TLVs in mDNS messages, and MUST NOT discard mDNS
   messages that include unrecognized TLVs.

   A Discovery Proxy may discontinue listening for mDNS messages on a
   particular multicast link by sending a DSO message containing an mDNS
   Link Discontinue TLV (Section 8.2).  Subsequent messages from that
   link that had previously been queued may arrive after listening has
   been discontinued.  The Discovery Proxy should silently ignore such
   messages.  The Discovery Relay MUST discontinue generating such
   messages as soon as the request is received.  The Discovery Relay
   does not respond to this message other than to discontinue forwarding
   mDNS messages from the specified links.

6.  Traffic from Proxies to Relays

   Like mDNS traffic from relays, each mDNS message sent by a Discovery
   Proxy to a Discovery Relay is encapsulated in an mDNS Message TLV
   (Section 8.3) within a DSO message.  Each message MUST contain one or
   more Link Identifier TLVs (Section 8.4).  The Discovery Relay will
   transmit the message to the mDNS port and multicast address on each
   link specified in the message using the specified IP address family.





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7.  Discovery Proxy Behavior

   Discovery Proxies treat multicast links for which Discovery Relay
   service is being used as if they were virtual interfaces; in other
   words, a Discovery Proxy serving multiple multicast links using
   multiple Discovery Relays behaves the same as a Discovery Proxy
   serving multiple multicast links using multiple physical network
   interfaces.  In this section we refer to multicast links served
   directly by the Discovery Proxy as locally-connected links, and
   multicast links served through the Discovery Relay as relay-connected
   links.

   What this means is that when a Discovery Proxy receives a DNSSD query
   from a client, it will generate mDNS messages on the relevant
   multicast link for which it is acting as a proxy.  For locally-
   connected links, those messages will be sent directly.  For relay-
   connected links, the messages will be sent through the Discovery
   Relay that is being used to serve that multicast link.

   Responses from devices on locally-connected links are processed
   normally.  Responses from devices on relay-connected links are
   received by the Discovery Relay, encapsulated, and forwarded to the
   Discovery Proxy; the discovery proxy then processes these messages
   using the link-identifying information included in encapsulation.

   Discovery Proxies do not respond to mDNS queries on relay-connected
   links.  If an mDNS query is received from a Discovery Relay, the
   Discovery Proxy silently discards it.

   In principle it could be the case that some device is capable of
   performing service discovery using mDNS, but not using the DNS
   protocol.  Responding to mDNS queries received from the Discovery
   Relay could address this use case.  However, it is believed that no
   such devices exist, and therefore the preferred behavior is that all
   queries be resolved with unicast rather than multicast.
















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8.  DSO TLVs

   This document defines a modest number of new DSO TLVs.

8.1.  mDNS Link Request

   The mDNS Link Request TLV conveys a link identifier from which a
   Discovery Proxy is requesting that a Discovery Relay forward mDNS
   traffic.  The link identifier comes from the provisioning
   configuration (see Section 9).  The SSOP-TYPE for this TLV is TBD-R.
   The SSOP-LENGTH is always 5.  The SSOP-DATA is the 8-bit address
   family followed by the 32-bit link identifier, in network byte order.
   An address family value of 1 indicates IPv4 and 2 indicates IPv6, as
   recorded in the IANA Registry of Address Family Numbers [AdFam].

8.2.  mDNS Discontinue

   This option is used by Discovery Proxies to unsubscribe to mDNS
   messages on the specified multicast link.  More than one may be
   present in a single DSO message.  SSOP-TYPE is TBD-D.  SSOP-LENGTH is
   5.  SSOP-DATA is the 8-bit address family followed by the 32-bit link
   identifier, in network byte order, as described in Section 9.

8.3.  mDNS Message

   The mDNS Message TLV is used to encapsulate an mDNS message that is
   being forwarded from a multicast link to a Discovery Proxy, or is
   being sent from a Discovery Proxy for transmission on a multicast
   link.  Only the application layer payload of the mDNS message is
   carried in the DSO mDNS Message TLV, i.e., just the DNS message
   itself, beginning with the DNS Message ID, not the IP or UDP headers.
   The SSOP-TYPE for this TLV is TBD-M.  SSOP-LENGTH is the length of
   the encapsulated mDNS message.  SSOP-DATA is the content of the
   encapsulated mDNS message.

8.4.  Link Identifier

   This option is used both in DSO messages from Discovery Relays to
   Discovery Proxies that contain received mDNS messages, and from
   Discovery Proxies to Discovery Relays that contain mDNS messages to
   be transmitted on the multicast link.  In the former case, it
   indicates the multicast link on which the message was received; in
   the latter case, it indicates the multicast link on which the message
   should be transmitted.  SSOP-TYPE is TBD-L.  SSOP-LENGTH is 5.  SSOP-
   DATA is the 8-bit address family followed by the 32-bit link
   identifier, in network byte order, as described in Section 9.





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8.5.  Layer Two Source Address

   The Layer Two Source Address TLV is used to report the link-layer
   address from which an mDNS message was received.  This TLV is
   optionally present in DSO messages from Discovery Relays to Discovery
   Proxies that contain mDNS messages when the source link-layer address
   is known.  The SSOP-TYPE is TBD-2.  SSOP-LENGTH is variable,
   depending on the length of link-layer addresses on the link from
   which the message was received.  SSOP-data is the link-layer address
   as it was received on the link.

8.6.  IP Source

   The IP Source TLV is used to report the IP source address and port
   from which an mDNS message was received.  This TLV is present in DSO
   messages from Discovery Relays to Discovery Proxies that contain mDNS
   messages.  SSOP-TYPE is TBD-A.  SSOP-LENGTH is either 6, for an IPv4
   address, or 18, for an IPv6 address.  SSOP-DATA is the source port,
   followed by the IP Address, in network byte order.

9.  Provisioning

   In order for a Discovery Proxy to use Discovery Relays, it must be
   configured with sufficient information to identify multicast links on
   which service discovery is to be supported and connect to discovery
   relays supporting those multicast links, if it is not running on a
   host that is directly connected to those multicast links.

   A Discovery Relay must be configured both with a set of multicast
   links to which the host on which it is running is connected, on which
   mDNS relay service is to be provided, and also with a list of one or
   more Discovery Proxies authorized to use it.

   On a network supporting DNS Service Discovery using Discovery Relays,
   more than one different Discovery Relay implementation is likely be
   present.  While it may be that only a single Discovery Proxy is
   present, that implementation will need to be able to be configured to
   interoperate with all of the Discovery Relays that are present.
   Consequently, it is necessary that a standard set of configuration
   parameters be defined for both Discovery Proxies and Discovery
   Relays.

   DNS Service Discovery generally operates within a constrained set of
   links, not across the entire internet.  This section assumes that
   what will be configured will be a limited set of links operated by a
   single entity or small set of cooperating entities, among which
   services present on each link should be available to users on that
   link and every other link.  This could be, for example, a home



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   network, a small office network, or even a network covering an entire
   building or small set of buildings.  The set of Discovery Proxies and
   Discovery Relays within such a network will be referred to in this
   section as a 'Discovery Domain'.

   Depending on the context, several different candidates for
   configuration of Discovery Proxies and Discovery relays may be
   applicable.  The simplest such mechanism is a configuration file, but
   regardless of provisioning mechanism, certain configuration
   information needs to be communicated to the devices, as outlined
   below.

9.1.  Provisioned Objects

   Three types of objects must be described in order for Discovery
   Proxies and Discovery Relays to be provisioned: Discovery Proxies,
   Multicast Links, and Discovery Relays.  "Human-readable" below means
   actual words or proper names that will make sense to an untrained
   human being.  "Machine-readable" means a name that will be used by
   machines to identify the entity to which the name refers.  Each
   entity must have a machine-readable name and may have a human-
   readable name.  Every name must be unique: no two entities can have
   the same human-readable name nor machine-readable name.

9.1.1.  Discovery Proxy

   The description of a Discovery Proxy consists of:

   name  a machine-readable name used to reference this discovery proxy
      in provisioning.

   hr-name  an optional human-readable name which can appear in
      provisioning, monitoring and debugging systems.  Must be unique
      within a Discovery Domain.

   public-key  a public key that identifies the Discovery Proxy.  This
      key can be shared across services on the Discovery Proxy Host.
      The public key is used both to uniquely identify the Discovery
      Proxy and to authenticate connections from it.

   private-key  the private key corresponding to the public key.

   source-ip-addresses  a list of IP addresses that may be used by the
      Discovery Proxy when connecting to Discovery Relays.  These
      addresses should be addresses that are configured on the Discovery
      Proxy Host.  They should not be temporary addresses.  All such
      addresses must be reachable within the Discovery Domain.




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   public-ip-addresses  a list of IP addresses that may be used to
      submit DNS queries to the Discovery Proxy.  This is not used for
      interoperation with Discovery Relays, but is mentioned here for
      completeness: this list of addresses may differ from the 'source-
      ip-addresses' list.  If any of these addresses are reachable from
      outside of the Discovery Domain, services in that domain will be
      discoverable outside of the domain.

   multicast links  a list of multicast links on which this Discovery
      Proxy is expected to provide service

   The private key should never be distributed to other hosts; all of
   the other information describing a Discovery Proxy can be safely
   shared with Discovery Relays.

9.1.2.  Multicast Link

   The description of a multicast link consists of:

   link-identifier  An 32-bit identifier that uniquely identifies that
      link within the Discovery Domain.  Each link MUST have exactly one
      such identifier.  Link Identifiers do not have any special
      semantics, and are not intended to be human-readable.

   ldh-name  A fully-qualified domain name for the multicast link that
      is used to form an LDH domain name as described in
      [I-D.ietf-dnssd-hybrid], section 5.3.  This name is used to
      identify the link during provisioning, and must be present.

   hr-name  A human-readable user-friendly fully-qualified domain name
      for the multicast link.  This name MUST be unique within the
      Discovery Domain.  Each multicast link MUST have exactly one such
      name.

   The 'ldh-name' and 'hr-name' names can be used to form the LDH and
   human-readable domain names as described in [I-D.ietf-dnssd-hybrid],
   section 5.3.

9.1.3.  Discovery Relay

   The description of a Discovery Relay consists of:

   name  a required machine-readable identifier used to reference the
      relay

   hr-name  an optional human-readable name which can appear in
      provisioning, monitoring and debugging systems.  Must be unique
      within a Discovery Domain.



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   public-key  a public key that identifies the Discovery Relay.  This
      key can be shared across services on the Discovery Relay Host.
      Indeed, if a Discovery Proxy and Discovery Relay are running on
      the same host, the same key may be used for both.  The public key
      uniquely identifies the Discovery Relay and is used by the
      Discovery Proxy to verify that it is talking to the intended
      Discovery Relay after a TLS connection has been established.

   private-key  the private key corresponding to the public key.

   connect-tuples  a list of IP address/port tuples that may be used to
      connect to the Discovery Relay.  The relay may be configured to
      listen on all addresses on a single port, but this is not
      required, so the port as well as the address must be specified.

   multicast links  a list of multicast links to which this relay is
      physically connected.

   The private key should never be distributed to other hosts; all of
   the other information describing a Discovery Relay can be safely
   shared with Discovery Proxies.

9.2.  Configuration Files

   For this discussion, we assume the simplest possible means of
   configuring Discovery Proxies and Discovery Relays: the configuration
   file.  Any environment where changes will happen on a regular basis
   will either require some automatic means of generating these
   configuration files as the network topology changes, or will need to
   use a more automatic method for configuration, such as HNCP.

   There are many different ways to organize configuration files.  This
   discussion assumes that multicast links, relays and proxies will be
   specified as objects, as described above, perhaps in a master file,
   and then the specific configuration of each proxy or relay will
   reference the set of objects in the master file, referencing objects
   by name.  This approach is not required, but is simply shown as an
   example.  In addition, the private keys for each relay and proxy must
   appear only in that relay or proxy's configuration file.

   The master file contains a list of Discovery Relays, Discovery
   Proxies and Multicast Links.  Each object has a name and all the
   other data associated with it.  We do not formally specify the format
   of the file, but it might look something like this:







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                     Relay upstairs
                       public-key xxx
                       connect-tuple 192.0.2.1 1917
                       connect-tuple fd00::1 1917
                       link upstairs-wifi
                       link upstairs-wired
                     Relay downstairs
                       public-key xxx
                       connect-tuple 192.51.100.1 2088
                       connect-tuple fd00::2 2088
                       link downstairs-wifi
                       link downstairs-wired
                     Proxy main
                       public-key xxx
                       address 203.1.113.1
                     Link upstairs-wifi
                       id 1
                       name "Upstairs Wifi"
                     Link upstairs-wired
                       id 2
                       hr-name "Upstairs Wired"
                     Link downstairs-wifi
                       id 3
                       name "Downstairs Wifi"
                     Link downstairs-wired
                       id 4
                       hr-name "Downstairs Wired"

9.3.  Discovery Proxy Configuration

   The Discovery Proxy configuration contains enough information to
   identify which Discovery Proxy is being configured, enumerate the
   list of multicast links it is intended to serve, and provide keying
   information it can use to authenticate to Discovery Relays.  It may
   also contain custom information about the port and/or IP address(es)
   on which it will respond to DNS queries.

   An example configuration, following the convention used in this
   section, might look something like this:

                   Proxy
                   [placeholder, to be completed later]

9.4.  Discovery Relay Configuration







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10.  Security Considerations

11.  IANA Considerations

   The IANA is kindly requested to update the DSO Type Codes Registry
   [I-D.ietf-dnsop-session-signal] by allocating codes for each of the
   TBD type codes listed in the following table, and by updating this
   document, here and in Section 8.  Each type code should list this
   document as its reference document.

             +--------+----------+--------------------------+
             | Opcode | Status   | Name                     |
             +--------+----------+--------------------------+
             | TBD-R  | Standard | mDNS Link Request        |
             | TBD-D  | Standard | mDNS Discontinue         |
             | TBD-M  | Standard | mDNS Messsage            |
             | TBD-2  | Standard | Layer Two Source Address |
             | TBD-A  | Standard | IP Source                |
             | TBD-L  | Standard | Link Identifier          |
             +--------+----------+--------------------------+

                      DSO Type Codes to be allocated

12.  Acknowledgments



























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13.  References

13.1.  Normative References

   [I-D.ietf-dnsop-session-signal]
              Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Mankin, A., and T. Pusateri, "DNS Stateful Operations",
              draft-ietf-dnsop-session-signal-04 (work in progress),
              September 2017.

   [I-D.ietf-dnssd-hybrid]
              Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", draft-ietf-dnssd-hybrid-07 (work in
              progress), September 2017.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
              July 2017.

   [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>.

   [RFC1323]  Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, DOI 10.17487/RFC1323, May
              1992, <https://www.rfc-editor.org/info/rfc1323>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

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

   [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>.

   [RFC7788]  Stenberg, M., Barth, S., and P. Pfister, "Home Networking
              Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
              2016, <https://www.rfc-editor.org/info/rfc7788>.







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13.2.  Informative References

   [AdFam]    "IANA Address Family Numbers Registry",
              <https://www.iana.org/assignments/
              address-family-numbers/>.

   [NOTSENT]  "TCP_NOTSENT_LOWAT socket option", July 2013,
              <https://lwn.net/Articles/560082/>.

   [PRIO]     "Prioritization Only Works When There's Pending Data to
              Prioritize", January 2014, <https://insouciant.org/tech/
              prioritization-only-works-when-theres-pending-data-to-
              prioritize/>.

   [TR-069]   Broadband Forum, "CPE WAN Management Protocol", November
              2013, <https://www.broadband-forum.org/technical/download/
              TR-069_Amendment-5.pdf>.

Authors' Addresses

   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014
   USA

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


   Ted Lemon
   Barefoot Consulting
   Brattleboro, Vermont  05301
   United States of America

   Email: mellon@fugue.com















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