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Versions: (draft-sctl-dnssd-mdns-relay) 00

Network Working Group                                           T. Lemon
Internet-Draft                                       Nibbhaya Consulting
Intended status: Standards Track                             S. Cheshire
Expires: November 11, 2018                                    Apple Inc.
                                                            May 10, 2018


                     Multicast DNS Discovery Relay
                     draft-ietf-dnssd-mdns-relay-00

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, when present on a link,
   allows Discovery Proxies not attached to that link to provide mDNS
   proxy service on that link.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 11, 2018.

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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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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 Clients  . . . . . . . . . . . . . . .   8
   6.  Traffic from Clients to Relays  . . . . . . . . . . . . . . .   9
   7.  Discovery Proxy Behavior  . . . . . . . . . . . . . . . . . .   9
   8.  DSO TLVs  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  mDNS Link Request . . . . . . . . . . . . . . . . . . . .  11
     8.2.  mDNS Link Discontinue . . . . . . . . . . . . . . . . . .  11
     8.3.  Link Identifier . . . . . . . . . . . . . . . . . . . . .  11
     8.4.  mDNS Message  . . . . . . . . . . . . . . . . . . . . . .  12
     8.5.  Layer Two Source Address  . . . . . . . . . . . . . . . .  12
     8.6.  IP Source . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.7.  Report Link Changes . . . . . . . . . . . . . . . . . . .  13
     8.8.  Stop Reporting Link Changes . . . . . . . . . . . . . . .  13
     8.9.  Link Available  . . . . . . . . . . . . . . . . . . . . .  13
     8.10. Link Unavailable  . . . . . . . . . . . . . . . . . . . .  13
     8.11. Link Prefix . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Provisioning  . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Provisioned Objects . . . . . . . . . . . . . . . . . . .  15
       9.1.1.  Multicast Link  . . . . . . . . . . . . . . . . . . .  15
       9.1.2.  Discovery Proxy . . . . . . . . . . . . . . . . . . .  16
       9.1.3.  Discovery Relay . . . . . . . . . . . . . . . . . . .  17
     9.2.  Configuration Files . . . . . . . . . . . . . . . . . . .  18
     9.3.  Discovery Proxy Configuration . . . . . . . . . . . . . .  20
     9.4.  Discovery Relay Configuration . . . . . . . . . . . . . .  20
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     13.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24










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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 imagined 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 that can be used to provide
   discovery services on a multicast link without requiring a full
   Discovery Proxy on every multicast link.

   Since the primary purpose of a Discovery Relay is providing remote
   virtual interface functionality to Discovery Proxies, this document
   is written with that in mind.  However, in principle, a Discovery
   Relay could be used by any properly authorized client.  In the
   context of this specification, a Discovery Proxy is a client to the
   Discovery Relay.  This document uses the terms "Discovery Proxy" and
   "Client" to mean the same thing; the term "Client" is used when we
   are talking about the communication between the client and the Relay,
   and the term "Discovery Proxy" when we are referring specifically to
   something that is acting as a Discovery Proxy.

   The Discovery Relay operates by listening for TCP connections from
   Clients.  When a Client connects, the connection is authenticated and
   secured using TLS.  The Client can then specify one or more multicast
   links from which it wishes to receive mDNS traffic.  The Client 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 Client, 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 Client 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:

   Client  A network service that uses a Discovery Relay to receive mDNS
      multicast traffic and/or communicate with mDNS Agents.

   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.  A Discovery Proxy that can communicate with
      mDNS Agents using a Discovery Relay is also a Client.

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

   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




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      may also count such events: "silently" does not preclude such
      behavior.

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 Clients and Discovery Relays are established by
   Clients.  Connections are authenticated and encrypted using TLS, with
   both client and server certificates.  Connections are long-lived: a
   Client 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 four
   types of such messages between Discovery Relays and Clients:

   o  Control messages from Client to Relay

   o  Link status messages from Relay to Client

   o  mDNS messages from Client to Relay

   o  mDNS messages from Relay to Client

   Clients can send four different control messages to Relays: the Link
   Request, Link Discontinue, Report Link Changes and Stop Reporting
   Link Changes messages.  The first two are used by the Client to
   indicate to the Discovery Relay that mDNS messages from one or more
   specified multicast links are to be relayed to the Discovery Proxy,
   and to subsequently stop such relaying.  The second two are used by
   the Client to request that the Relay report on the set of links that
   can be requested, and to request that it discontinue such reporting.

   Link Status messages from the Relay to the Client inform the client
   that a link has become available, or that a formerly-available link
   is no longer available.




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   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.

   During periods with no traffic flowing, Clients are responsible for
   generating any necessary keepalive traffic, as stated in the DSO
   specification [I-D.ietf-dnsop-session-signal].

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 Client connection that is subscribed to mDNS
   traffic on that multicast link.  In the event of congestion, where a
   particular Client 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 practice this is not expected to be a issue.
   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.

   Clients send mDNS messages they wish to have sent on their behalf on
   remote multicast link(s) on which the Client has an active
   subscription.  A Discovery Relay will not transmit mDNS packets on
   any multicast link on which the Client does not have an active
   subscription, since it makes no sense for a Client 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 Clients.  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



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   configured on certain interfaces.  If the listener is bound to a
   specific IP address, this check is unnecessary.

   If the relay is using an IP address whitelist, the next step is for
   the relay to verify that 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
   Client 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 Client.  Clients are required to send the
   post_handshake_auth extension ([I-D.ietf-tls-tls13] Section 4.2.5).
   If a Discovery Relay 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 Client 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 Clients and Relays is maintained as
   described in Section 4 of [I-D.ietf-dnsop-session-signal].  Clients
   must also honor the 'Retry Delay' TLV (section 5 of
   [I-D.ietf-dnsop-session-signal]) if sent by the Discovery Relay.

   Clients may establish more than one connection to a specific
   Discovery Relay.  This would happen in the case that a TCP connection
   stalls, and the Client 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 Client 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 Client is that the connections
   both have source addresses that belong to the same Client, and that
   they were authenticated using the same client certificate.




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   In order to know what links to request, the Client can be configured
   with a list of links supported by the Relay.  However, in some
   networking contexts, dynamic changes in the availability of links are
   likely; therefore Clients may also use the Report Link Changes TLV to
   request that the Relay report on the availability of its links.  In
   some contexts, for example when debugging, a Client may operate with
   no information about the set of links supported by a relay, simply
   relying on the relay to provide one.

5.  Traffic from Relays to Clients

   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 Client must send one or more DSO messages, each
   containing a single mDNS Link Request TLV (Section 8.1) indicating
   the multicast link from which traffic is requested.

   When an mDNS Link Request message is received, the Discovery Relay
   validates that it is connected to the specified multicast link, and
   that forwarding is enabled for that link.  If both checks are
   successful, it MUST send a response with RCODE=0 (NOERROR).  If the
   Relay is not connected to the specified link, or forwarding from that
   link to the Client is not enabled, it sends a response with RCODE=3
   (NXDOMAIN/Name Error).  It is not an error to request the same link
   more than once; the Relay MUST NOT reject an mDNS Link Request on
   that basis.  If the relay cannot satisfy the request for some reason
   other than that the link is invalid, for example resource exhaustion,
   it sends a response with RCODE=2 (SERVFAIL).

   If the requested link is valid, the Relay begins forwarding all mDNS
   traffic from that link to the Client.  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 Clients 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 8.4).  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



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   message MUST be encoded in an IP Source TLV (Section 8.6).  The
   multicast link on which the message was received MUST be encoded in a
   Link Identifier TLV (Section 8.3).  The Client MUST silently ignore
   unrecognized TLVs in mDNS messages, and MUST NOT discard mDNS
   messages that include unrecognized TLVs.

   A Client 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 Client should silently discard 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 Clients to Relays

   Like mDNS traffic from relays, each mDNS message sent by a Client to
   a Discovery Relay is encapsulated in an mDNS Message TLV
   (Section 8.4) within a DSO message.  Each message MUST contain one or
   more Link Identifier TLVs (Section 8.3).  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.

   Although the communication between Clients and Relays uses the DNS
   stream protocol and DNS Stateless Operations, there is no case in
   which a Client would legitimately send a DNS query (something other
   than a DSO message) to a Relay.  Therefore, if a Relay receives a
   message other than a DSO message, it MUST respond with a REFUSED
   result code.  The reason not to simply drop the connection is that it
   might result in a continual reconnection loop.

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 via unicast, it will generate mDNS query messages on
   the relevant multicast link(s) for which it is acting as a proxy.



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   For locally-connected link(s), those query messages will be sent
   directly.  For relay-connected link(s), the query 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 the encapsulation.

   Discovery Proxies do not generally respond to mDNS queries on relay-
   connected links.  The one exception is responding to the Domain
   Enumeration queries used to bootstrap unicast service discovery
   ("lb._dns-sd._udp.local", etc.)  [RFC6763].  Apart from these Domain
   Enumeration queries, if any other 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 Multicast DNS, but not using
   traditional unicast DNS.  Responding to mDNS queries received from
   the Discovery Relay could address this use case.  However, continued
   reliance on multicast is counter to the goals of the current work in
   service discovery, and to benefit from wide-area service discovery
   such client devices should be updated to support service discovery
   using unicast queries.

























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

   The mDNS Link Request TLV can only be used as a primary TLV, and
   requires an acknowledgement.

   At most one mDNS Link Request TLV may appear in a DSO message.  To
   request multiple link subscriptions, multiple separate DSO messages
   are sent, each containing a single mDNS Link Request TLV.

8.2.  mDNS Link Discontinue

   The mDNS Link Discontinue TLV is used by Clients to unsubscribe to
   mDNS messages on the specified multicast link.  DSO-TYPE is TBD-D.
   DSO-LENGTH is always 5.  DSO-DATA is the 8-bit address family
   followed by the 32-bit link identifier, in network byte order, as
   described in Section 9.

   The mDNS Link Discontinue TLV can only be used as a primary TLV, and
   is not acknowledged.

   At most one mDNS Link Discontinue TLV may appear in a DSO message.
   To unsubscribe from multiple links, multiple separate DSO messages
   are sent, each containing a single mDNS Link Discontinue TLV.

8.3.  Link Identifier

   This option is used both in DSO messages from Discovery Relays to
   Clients that contain received mDNS messages, and from Clients 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.  DSO-
   TYPE is TBD-L.  DSO-LENGTH is always 5.  DSO-DATA is the 8-bit




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   address family followed by the 32-bit link identifier, in network
   byte order, as described in Section 9.

   The Link Identifier TLV can only be used as an additional TLV.

8.4.  mDNS Message

   The mDNS Message TLV is used to encapsulate an mDNS message that a
   Relay is forwarding from a multicast link to a Client, or that a
   Client is sending to a Relay 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
   DSO-TYPE for this TLV is TBD-M.  DSO-LENGTH is the length of the
   encapsulated mDNS message.  DSO-DATA is the content of the
   encapsulated mDNS message.

   The mDNS Message TLV can only be used as a primary TLV, and is not
   acknowledged.

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 Clients
   that contain mDNS messages when the source link-layer address is
   known.  The DSO-TYPE is TBD-2.  DSO-LENGTH is variable, depending on
   the length of link-layer addresses on the link from which the message
   was received.  DSO-DATA is the link-layer address as it was received
   on the link.

   The Layer Two Source Address TLV can only be used as an additional
   TLV.

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 Clients that contain mDNS messages.
   DSO-TYPE is TBD-A.  DSO-LENGTH is either 6, for an IPv4 address, or
   18, for an IPv6 address.  DSO-DATA is the two-byte source port,
   followed by the 4- or 16-byte IP Address, in network byte order.

   The IP Source TLV can only be used as an additional TLV.







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8.7.  Report Link Changes

   The Report Link Changes TLV requests that the Discovery Relay report
   link changes.  When the relay is reporting link changes and a new
   link becomes available, it sends a Link Available message to the
   Client.  When a link becomes unavailable, it sends a Link Unavailable
   message to the Client.  If there are links available when the request
   is received, then for each such link the relay immediately sends a
   Link Available Message to the Client.  DSO-TYPE is TBD-P.  DSO-LENGTH
   is 0.

   The mDNS Report Link Changes TLV can only be used as a primary TLV,
   and requires an acknowledgement.  The acknowledgment does not contain
   a Link Available TLV: it is just a response to the Report Link
   Changes message.

8.8.  Stop Reporting Link Changes

   The Stop Reporting Link Changes TLV requests that the Discovery Relay
   stop reporting on the availability of links supported by the relay.
   This cancels the effect of a Report Link Changes TLV.  DSO-TYPE is
   TBD-S.  DSO-LENGTH is 0.

   The mDNS Stop Reporting Link Changes TLV can only be used as a
   primary TLV, and is not acknowledged.

8.9.  Link Available

   The Link Available TLV is used by Discovery Relays to indicate to
   clients that a new link has become available.  The format is the same
   as the Link Identifier TLV.  DSO-TYPE is TBD-V.  The Link Available
   TLV may be accompanied by one or more Link Prefix TLVs which indicate
   IP prefixes the Relay knows to be present on the link.

   The mDNS Link Available TLV can only be used as a primary TLV, and is
   not acknowledged.

8.10.  Link Unavailable

   The Link Unavailable TLV is used by Discovery Relays to indicate to
   clients that an existing link has become unavailable.  The format is
   the same as the Link Identifier TLV.  DSO-TYPE is TBD-U.

   The mDNS Link Unavailable TLV can only be used as a primary TLV, and
   is not acknowledged.






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8.11.  Link Prefix

   The Link Prefix TLV represents an IP address or prefix configured on
   a link.  The length is 17 for an IPv6 address or prefix, and 5 for an
   IPv4 address or prefix.  The TLV consists of a prefix length, between
   0 and 32 for IPv4 or between 0 and 128 for IPv6, represented as a
   single byte.  This is followed by the IP address, either four or
   sixteen bytes.  DSO-TYPE is TBD-K.

   The Link Prefix TLV can only be used as a secondary TLV.

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 Clients 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
   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 manual configuration
   file, but regardless of provisioning mechanism, certain configuration



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   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.  No two entities can have the same human-readable
   name.  Similarly, no two entities can have the same machine-readable
   name.

9.1.1.  Multicast Link

   The description of a multicast link consists of:

   link-identifier  A 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 section 5.3 of
      the Discovery Proxy specification [I-D.ietf-dnssd-hybrid].  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 hr-name MAY be the same as the ldh-name.  (The hr-name
      is allowed to contain spaces, punctuation and rich text, but it is
      not required to do so.)

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

   Note that the ldh-name and hr-name can be used in two different ways.

   On a small home network with little or no human administrative
   configuration, link names may be directly visible to the user.  For
   example, a search in 'home.arpa' on a small home network may discover
   services on both ethernet.home.arpa and wi-fi.home.arpa.  In the case



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   of a home user who has one Ethernet-connected printer and one Wi-Fi-
   connected printer, discovering that they have one printer on
   ethernet.home.arpa and another on wi-fi.home.arpa is understandable
   and meaningful.

   On a large corporate network with hundreds of Wi-Fi Access Points,
   the individual link names of the hundreds of multicast links are less
   likely to be useful to end users.  In these cases, Discovery Broker
   functionality [I-D.sctl-discovery-broker] is used to translate the
   many link names to something more meaningful to users.  For example,
   in a building with 50 Wi-Fi Access Points, each with their own link
   names, services on all the different physical links may be presented
   to the user as appearing in 'headquarters.example.com'.  In this
   case, the individual link names can be thought of similar to MAC
   addresses or IPv6 addresses.  They are used internally by the
   software as unique identifiers, but generally are not exposed to end
   users.

9.1.2.  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.

   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




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      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.

   In some configurations it may make sense for the Discovery Relay not
   to have a list of links, but simply to support the set of all links
   available on relays to which the Proxy is configured to communicate.

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.

   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.





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   In some cases a Relay may not be configured with a static list of
   links, but may simply discover links by monitoring the set of
   available interfaces on the host on which the Relay is running.  In
   that case, the relay could be configured to identify links based on
   the names of network interfaces, or based on the set of available
   prefixes seen on those interfaces.  This sort of configuration is out
   of scope for this discussion and is left as an exercise for the
   reader.

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
   [RFC7788].

   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 proxy or relay must
   appear only in that proxy or relay's configuration file.
























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   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:

                     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 yyy
                       connect-tuple 192.51.100.1 2088
                       connect-tuple fd00::2 2088
                       link downstairs-wifi
                       link downstairs-wired
                     Proxy main
                       public-key zzz
                       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



















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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 main
                         private-key zzz
                         subscribe upstairs-wifi
                         subscribe downstairs-wifi
                         subscribe upstairs-wired
                         subscribe downstairs-wired

   When combined with the master file, this configuration is sufficient
   for the Discovery Proxy to identify and connect to the relay proxies
   that serve the links it is configured to support.

9.4.  Discovery Relay Configuration

   The discovery relay configuration just needs to tell the discovery
   relay what name to use to find its configuration in the master file,
   and what the private key is corresponding to its public key in the
   master file.  For example:

                             Relay Downstairs
                               private-key yyy



















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10.  Security Considerations

   Part of the purpose of the Multicast DNS Discovery Relay protocol is
   to place a simple relay, analogous to a BOOTP relay, into routers and
   similar devices that may not be updated frequently.  The BOOTP
   [RFC0951] protocol has been around since 1985, and continues to be
   useful today.  The BOOTP protocol uses no encryption, and in many
   enterprise networks this is considered acceptable.  In contrast, the
   relay protocol requires TLS 1.3.  A concern is that after 20 or 30
   years TLS 1.3, or some of the encryption algorithms it uses, may
   become obsolete, rendering devices that require it unusable.  Our
   assessment is that TLS 1.3 probably will be around for many years to
   come.  TLS 1.0 [RFC2246] was used for about a decade, and similarly
   TLS 1.2 [RFC5246] was also used for about a decade.  We expect TLS
   1.3 [I-D.ietf-tls-tls13] to have at least that lifespan.  In
   addition, recent IETF efforts are pushing for better software update
   practices for devices like routers, for other security reasons,
   making less likely that in ten years time it will be less common to
   be using routers that haven't had a software update for ten years.
   However, authors of encryption specifications and libraries should be
   aware of the potential backwards compatibility issues if an
   encryption algorithm becomes deprecated.  This specification
   RECOMMENDS that if an encryption algorithm becomes deprecated, then
   rather than remove that encryption algorithm entirely, encryption
   libraries should disable that encryption algorithm by default, but
   leave the code present with an option for client software to enable
   it in special cases, such as a Multicast DNS Discovery Relay client
   talking to an ancient Multicast DNS Discovery Relay server.  Using no
   encryption like BOOTP would eliminate this backwards compatibility
   concern, but we feel that in such a future hypothetical scenario,
   using even a weak encryption algorithm still makes passive
   eavesdropping and tampering harder, and is preferable to using no
   encryption at all.

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.










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            +--------+----------+-----------------------------+
            | Opcode | Status   | Name                        |
            +--------+----------+-----------------------------+
            | TBD-R  | Standard | mDNS Link Request           |
            | TBD-D  | Standard | mDNS Discontinue            |
            | TBD-L  | Standard | Link Identifier             |
            | TBD-M  | Standard | mDNS Messsage               |
            | TBD-2  | Standard | Layer Two Source Address    |
            | TBD-A  | Standard | IP Source                   |
            | TBD-P  | Standard | Report Link Changes         |
            | TBD-S  | Standard | Stop Reporting Link Changes |
            | TBD-V  | Standard | Link Available              |
            | TBD-U  | Standard | Link Unavailable            |
            | TBD-K  | Standard | Link Prefix                 |
            +--------+----------+-----------------------------+

                      DSO Type Codes to be allocated

12.  Acknowledgments
































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13.  References

13.1.  Normative References

   [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>.

   [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.sctl-discovery-broker]
              Cheshire, S. and T. Lemon, "Service Discovery Broker",
              draft-sctl-discovery-broker-00 (work in progress), July
              2017.

   [I-D.ietf-dnsop-session-signal]
              Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Lemon, T., and T. Pusateri, "DNS Stateful Operations",
              draft-ietf-dnsop-session-signal-07 (work in progress),
              March 2018.







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   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

13.2.  Informative References

   [RFC0951]  Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
              DOI 10.17487/RFC0951, September 1985, <https://www.rfc-
              editor.org/info/rfc951>.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, DOI 10.17487/RFC2246, January 1999,
              <https://www.rfc-editor.org/info/rfc2246>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008, <https://www.rfc-
              editor.org/info/rfc5246>.

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

   [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/>.

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

Authors' Addresses

   Ted Lemon
   Nibbhaya Consulting
   P.O. Box 958
   Brattleboro, Vermont  05301
   United States of America

   Email: mellon@fugue.com






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

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











































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