Multicast DNS Discovery Relay
draft-ietf-dnssd-mdns-relay-04
Network Working Group T. Lemon
Internet-Draft S. Cheshire
Intended status: Standards Track Apple Inc.
Expires: August 26, 2021 February 22, 2021
Multicast DNS Discovery Relay
draft-ietf-dnssd-mdns-relay-04
Abstract
This document complements 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 remote clients, not attached to that link, to
perform mDNS discovery operations on that link.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 26, 2021.
Copyright Notice
Copyright (c) 2021 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Connections between Clients and Relays (overview) . . . . 6
3.2. mDNS Messages On Multicast Links . . . . . . . . . . . . 7
4. Connections between Clients and Relays (details) . . . . . . 8
5. Traffic from Relays to Clients . . . . . . . . . . . . . . . 10
6. Traffic from Clients to Relays . . . . . . . . . . . . . . . 12
7. Discovery Proxy Behavior . . . . . . . . . . . . . . . . . . 13
8. DSO TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. mDNS Link Data Request . . . . . . . . . . . . . . . . . 14
8.2. mDNS Link Data Discontinue . . . . . . . . . . . . . . . 14
8.3. Link Identifier . . . . . . . . . . . . . . . . . . . . . 15
8.4. Encapsulated mDNS Message . . . . . . . . . . . . . . . . 15
8.5. IP Source . . . . . . . . . . . . . . . . . . . . . . . . 15
8.6. Link State Request . . . . . . . . . . . . . . . . . . . 16
8.7. Link State Discontinue . . . . . . . . . . . . . . . . . 16
8.8. Link Available . . . . . . . . . . . . . . . . . . . . . 16
8.9. Link Unavailable . . . . . . . . . . . . . . . . . . . . 16
8.10. Link Prefix . . . . . . . . . . . . . . . . . . . . . . . 17
9. Provisioning . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Provisioned Objects . . . . . . . . . . . . . . . . . . . 19
9.1.1. Multicast Link . . . . . . . . . . . . . . . . . . . 20
9.1.2. Discovery Proxy . . . . . . . . . . . . . . . . . . . 21
9.1.3. Discovery Relay . . . . . . . . . . . . . . . . . . . 22
9.2. Configuration Files . . . . . . . . . . . . . . . . . . . 23
9.3. Discovery Proxy Private Configuration . . . . . . . . . . 25
9.4. Discovery Relay Private Configuration . . . . . . . . . . 25
10. Security Considerations . . . . . . . . . . . . . . . . . . . 26
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
13.1. Normative References . . . . . . . . . . . . . . . . . . 28
13.2. Informative References . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Introduction
This document defines a Discovery Relay. A Discovery Relay is a
companion technology that works in conjunction with Discovery
Proxies, and other clients.
The Discovery Proxy for Multicast DNS-Based Service Discovery
[RFC8766] is a mechanism for discovering services on a subnetted
network through the use of Discovery Proxies. Discovery Proxies
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 was 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 in
conjunction with a central Discovery Proxy to provide discovery
services on a multicast link without requiring a full Discovery Proxy
on every multicast link.
The primary purpose of a Discovery Relay is providing remote virtual
interface functionality to Discovery Proxies, and this document is
written with that usage 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" somewhat interchangably; 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
a Discovery Relay Client that also happens to be a Discovery Proxy.
One example of another kind of device that can be a client of a
Discovery Relay is an Advertising Proxy [AdProx].
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) [RFC8490] is
used as a framework for conveying interface and IP header information
associated with each message. DSO formats its messages using type-
length-value (TLV) data structures. This document defines additional
DSO TLV types, used to implement the Discovery Relay functionality.
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
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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].
Use of a Discovery Relay can be considered similar to using Virtual
LAN (VLAN) trunk ports to give a Discovery Proxy device a virtual
presence on multiple links or broadcast domains. The difference is
that while a VLAN trunk port operates at the link layer and delivers
all link-layer traffic to the Discovery Proxy device, a Discovery
Relay operates further up the network stack and selectively delivers
only relevant Multicast DNS traffic. Also, VLAN trunk ports are
generally only available within a single administrative domain and
require link-layer configuration and connectivity, whereas the
Discovery Relay protocol, which runs over TCP, can be used between
any two devices with IP connectivity to each other.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.
The following definitions may be of use:
Client A network service that uses a Discovery Relay to send and
receive mDNS multicast traffic on a remote link, to enable it to
communicate with mDNS Agents on that remote link.
mDNS Agent A host which sends and/or responds to mDNS queries
directly on its local link(s). Examples include network cameras,
networked printers, networked home electronics, etc.
Discovery Proxy A network service which receives well-formed
questions using the DNS protocol, performs multicast DNS queries
to find answers to those questions, and responds with those
answers using the DNS protocol. A Discovery Proxy that can
communicate with remote mDNS Agents, using the services of a
Discovery Relay, is a Client of the Discovery Relay.
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Discovery Relay A network service which relays mDNS messages
received on a local link to a Client, and on behalf of that Client
can transmit mDNS messages on a local link.
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 sent directly between two hosts on that IPv4 subnet or
IPv6 prefix, without passing through an intervening IP-layer
router, are correctly delivered, but multicast packets are not
forwarded between the various Wi-Fi access points. Given the
slowness of Wi-Fi multicast
[I-D.ietf-mboned-ieee802-mcast-problems], having a packet that may
be of interest to only one or two end systems transmitted to
hundreds of devices, across multiple Wi-Fi access points, is
especially wasteful. Hence the common configuration decision to
not forward multicast packets between Wi-Fi access points is very
reasonable. This further motivates the need for technologies like
Discovery Proxy and Discovery Relay to facilitate discovery on
these networks.
allow-list 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.
Take care when reading this document not to confuse the terms
"Discovery Proxy" and "Discovery Relay". A Discovery Proxy [RFC8766]
provides Multicast DNS discovery service to remote clients. A
Discovery Relay is a simple software entity that provides virtual
link connectivity to one or more Discovery Proxies or other Discovery
Relay clients.
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3. Protocol Overview
This document describes a way for a Client to communicate with mDNS
agents on remote multicast links to which the client is not directly
connected, using a Discovery Relay. As such, there are two parts to
the protocol: connections between Clients and Discovery Relays, and
communications between Discovery Relays and mDNS agents.
3.1. Connections between Clients 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 [RFC8490]. 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 Encapsulated mDNS messages from Client to Relay
o Encapsulated mDNS messages from Relay to Client
Clients can send four different control messages to Relays: Link
State Request, Link State Discontinue, Link Data Request and Link
Data Discontinue. The first 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. The second 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 Client, and to subsequently stop such relaying.
Link Status messages from a Discovery Relay to the Client inform the
Client that a link has become available, or that a formerly-available
link is no longer available.
Encapsulated mDNS messages from a Discovery Relay to a Client are
sent whenever an mDNS message is received on a multicast link to
which the Discovery Relay has subscribed.
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Encapsulated mDNS messages from a Client to a Discovery Relay cause
the Discovery Relay to transmit the mDNS message on the specified
multicast link to which the Discovery Relay host is directly
attached.
During periods with no traffic flowing, Clients are responsible for
generating any necessary keepalive traffic, as stated in the DSO
specification [RFC8490].
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 Client. 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 encapsulated 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.
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4. Connections between Clients 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
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 allow-list, the next step is for
the relay to verify that that the source IP address of the connection
is on its allow-list. If the connection is not permitted either
because of the source address or the destination address, the
Discovery Relay closes the connection. If possible, before closing
the connection, the Discovery Relay first sends a TLS user_canceled
alert ([RFC8446] Section 6.1). Discovery Relays SHOULD refuse to
accept TCP connections to invalid destination addresses, rather than
accepting and then closing the connection, if this is possible.
Otherwise, the Discovery Relay will attempt to complete a TLS
handshake with the Client. Clients are required to send the
post_handshake_auth extension ([RFC8446] 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 ([RFC8446] Section 6.2).
Once the TLS handshake is complete, the Discovery Relay MUST request
post-handshake authentication ([RFC8446] 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 ([RFC8446]
Section 6.2).
Clients MUST validate server certificates. If the client is
configured with a server IP address and certificate, it can validate
the server by comparing the certificate offered by the server to the
certificate that was provided: they should be the same. If the
certificate includes a Distinguished Name that is a fully-qualified
domain name, the client SHOULD present that domain name to the server
in an SNI request.
Rather than being configured with an IP address and a certificate,
the client may be configured with the server's FQDN. In this case,
the client uses the server's FQDN as a Authentication Domain Name
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[RFC8310] Section 7.1, and uses the authentication method described
in [RFC8310] section 8.1, if the certificate is signed by a root
authority the client trusts, or the method described in section 8.2
of the same document if not. If neither method is available, then a
locally-configured copy of the server certificate can be used, as in
the previous paragraph.
Once the connection is established and authenticated, it is treated
as a DNS TCP connection [RFC7766].
Aliveness of connections between Clients and Relays is maintained as
described in Section 4 of the DSO specification [RFC8490]. Clients
must also honor the 'Retry Delay' TLV (section 5 of [RFC8490]) if
sent by the Discovery Relay.
Clients SHOULD avoid establishing more than one connection to a
specific Discovery Relay. However, there may be situations where
multiple connections to the same Discovery Relay are unavoidable, so
Discovery Relays MUST be willing to accept multiple connections from
the same Client.
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.
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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 Data Request TLV (Section 8.1)
indicating the multicast link from which traffic is requested.
When an mDNS Link Data Request message is received, the Discovery
Relay validates that it recognizes the link identifier, and that
forwarding is enabled for that link. If both checks are successful,
it MUST send a response with RCODE=0 (NOERROR). If the link
identifier is not recognized, it sends a response with RCODE=3
(NXDOMAIN/Name Error). If forwarding from that link to the Client is
not enabled, it sends a response with RCODE=5 (REFUSED). If the
relay cannot satisfy the request for some other reason, for example
resource exhaustion, it sends a response with RCODE=2 (SERVFAIL).
If the requested link is valid, the Relay begins forwarding all mDNS
messages 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 [RFC7323]. 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.
Encapsulated mDNS messages from Relays to Clients are framed within
DSO messages. Each DSO message can contain multiple TLVs, but only a
single encapsulated mDNS message is conveyed per DSO message. Each
forwarded mDNS message is sent in an Encapsulated mDNS Message TLV
(Section 8.4). The source IP address and port of the message MUST be
encoded in an IP Source TLV (Section 8.5). The multicast link on
which the message was received MUST be encoded in a Link Identifier
TLV (Section 8.3). As described in the DSO specification [RFC8490],
a Client MUST silently ignore unrecognized Additional TLVs in mDNS
messages, and MUST NOT discard mDNS messages that include
unrecognized Additional TLVs.
A Client may discontinue listening for mDNS messages on a particular
multicast link by sending a DSO message containing an mDNS Link Data
Discontinue TLV (Section 8.2). The Discovery Relay MUST discontinue
forwarding mDNS messages when the Link Data Discontinue request is
received. However, messages from that link that had previously been
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queued may arrive after the Client has discontinued its listening.
The Client should silently discard such messages. The Discovery
Relay does not respond to the Link Data Discontinue message other
than to discontinue forwarding mDNS messages from the specified
links.
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6. Traffic from Clients to Relays
Like mDNS traffic from relays, each mDNS message sent by a Client to
a Discovery Relay is communicated in an Encapsulated mDNS Message TLV
(Section 8.4) within a DSO message. Each message MUST contain
exactly one Link Identifier TLV (Section 8.3). The Discovery Relay
will transmit the mDNS message to the mDNS port and multicast address
on the 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 (or anything else
other than a DSO message) to a Relay. Therefore, if a Relay receives
any message other than a DSO message, it MUST immediately abort that
DSO session with a TCP reset (RST).
When defining this behavior, the working group considered making it
possible to specify more than one link identifier in an mDNSMessage
TLV. A superficial evaluation of this suggested that this might be a
useful optimization, since when a query is issued, it will often be
issued to all links. However, on many link types, like Wi-Fi,
multicast traffic is expensive
[I-D.ietf-mboned-ieee802-mcast-problems] and should be generated
frugally, so providing convenient ways to generate additional
multicast traffic was determined to be an unwise optimization. In
addition, because of the way mDNS handles retries, it will almost
never be the case that the exact same message will be sent on more
than one link. Therefore, the complexity that this optimization adds
is not justified by the potential benefit, and this idea has been
abandoned.
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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 remote multicast links
using multiple remote Discovery Relays behaves the same as a
Discovery Proxy serving multiple local multicast links using multiple
local 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. A relay-connected link can be
thought of as similar to a link that a Discovery Proxy connects to
using a USB Ethernet interface, just with a very long USB cable (that
runs over TCP).
When a Discovery Proxy receives a DNS query from a DNS client via
unicast, it will generate corresponding mDNS query messages on the
relevant multicast link(s) for which it is acting as a proxy. 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
Client; the Client then processes these messages using the link-
identifying information included in the encapsulation.
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 Data Request
The mDNS Link Data 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 link
identifier, a 32-bit unsigned integer in network (big endian) 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 Data Request TLV can only be used as a primary TLV, and
requires an acknowledgement.
At most one mDNS Link Data 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 Data Request
TLV.
A Client MUST NOT request a link if it already has an active
subscription to that link on the same DSO connection. If a Discovery
Relay receives a duplicate link subscription request, it MUST
immediately abort that DSO session with a TCP reset (RST).
8.2. mDNS Link Data Discontinue
The mDNS Link Data 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, a 32-bit unsigned integer in
network (big endian) byte order, as described in Section 9.
The mDNS Link Data Discontinue TLV can only be used as a DSO
unidirectional message TLV, and is not acknowledged.
At most one mDNS Link Data 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 Data
Discontinue TLV.
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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
address family followed by the link identifier, a 32-bit unsigned
integer in network (big endian) byte order, as described in
Section 9.
The Link Identifier TLV can only be used as an additional TLV. The
Link Identifier TLV can only appear at most once in a Discovery Relay
DSO message.
8.4. Encapsulated mDNS Message
The Encapsulated mDNS Message TLV is used to communicate 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 "Encapsulated 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 Encapsulated mDNS Message TLV can only be used as a DSO
unidirectional message TLV, and is not acknowledged.
8.5. 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 encapsulated
mDNS messages. DSO-TYPE is TBD-S. 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. Both port and
address are in the canonical byte order (i.e., the same
representation as used in the UDP and IP packet headers, with no byte
swapping).
The IP Source TLV can only be used as an additional TLV. The IP
Source TLV can only appear at most once in a Discovery Relay DSO
message.
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8.6. Link State Request
The Link State Request 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 Link State Request 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 Link State Request
message.
8.7. Link State Discontinue
The Link State Discontinue TLV requests that the Discovery Relay stop
reporting on the availability of links supported by the relay. This
cancels the effect of a Link State Request TLV. DSO-TYPE is TBD-Q.
DSO-LENGTH is 0.
The mDNS Link State Discontinue TLV can only be used as a DSO
unidirectional message TLV, and is not acknowledged.
8.8. 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 DSO unidirectional
message TLV, and is not acknowledged.
8.9. 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 DSO
unidirectional message TLV, and is not acknowledged.
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8.10. 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.
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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, if it is not running
on a host that is directly connected to those multicast links,
connect to Discovery Relays supporting 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 may 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
information needs to be communicated to the devices, as outlined
below.
In the example we provide here, we only refer to configuring of IP
addresses, private keys and certificates. It is also possible to use
FQDNs to identify servers; this then allows for the use of DANE
([RFC8310] Section 8.2) or PKIX authentication [RFC6125]. Which
method is used is to some extent up to the implementation, but at a
minimum, it should be possible to associate an IP address with a
self-signed certificate, and it should be possible to validate both
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self-signed and PKIX-authenticated certificates, with PKIX, DANE or a
pre-configured trust anchor.
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.
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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 [RFC8766]. 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 [RFC8766], 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
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] may be 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.
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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.
certificate a certificate that identifies the Discovery Proxy. This
certificate can be shared across services on the Discovery Proxy
Host. The public key in the certificate is used both to uniquely
identify the Discovery Proxy and to authenticate connections from
it. The certificate should be signed by its own private key.
private-key the private key corresponding to the public key in the
certificate.
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 a Discovery Proxy
listens on to receive requests from clients. This is not used for
interoperation with Discovery Relays, but is mentioned here for
completeness: the list of addresses listened on for incoming
client requests may differ from the 'source-ip-addresses' list of
addresses used for issuing outbound connection requests to
Discovery Relays. 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.
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 Discovery Proxy is configured to
communicate.
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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.
certificate a certificate that identifies the Discovery Relay. This
certificate 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 certificate can be used for
both. The public key in the certificate uniquely identifies the
Discovery Relay and is used by a Discovery Relay Client (e.g., a
Discovery Proxy) to verify that it is talking to the intended
Discovery Relay after a TLS connection has been established. The
certificate must either be signed by its own key, or have a
signature chain that can be validated using PKIX authentication
[RFC6125].
private-key the private key corresponding to the public key in the
certificate.
listen-tuple 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.
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. The details of this sort of
configuration are not specified in this document.
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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
certificate xxx
listen-tuple 192.0.2.1 1917
listen-tuple fd00::1 1917
link upstairs-wifi
link upstairs-wired
client-allow-list main
Relay downstairs
certificate yyy
listen-tuple 192.51.100.1 2088
listen-tuple fd00::2 2088
link downstairs-wifi
link downstairs-wired
client-allow-list main
Proxy main
certificate zzz
address 203.1.113.1
Link upstairs-wifi
id 1
hr-name Upstairs Wifi
Link upstairs-wired
id 2
hr-name Upstairs Wired
Link downstairs-wifi
id 3
hr-name Downstairs Wifi
Link downstairs-wired
id 4
hr-name Downstairs Wired
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9.3. Discovery Proxy Private 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 Discovery
Relays that serve the links it is configured to support.
9.4. Discovery Relay Private 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 certificate (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
Discovery 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 [RFC8446] 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 it 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 recent Client talking to an ancient
Discovery Relay. 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.
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11. IANA Considerations
The IANA is kindly requested to update the DSO Type Codes Registry
[RFC8490] 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.
+----------+----------+---------------------------+
| DSO-TYPE | Status | Name |
+----------+----------+---------------------------+
| TBD-R | Standard | Link Data Request |
| TBD-D | Standard | Link Data Discontinue |
| TBD-L | Standard | Link Identifier |
| TBD-M | Standard | Encapsulated mDNS Message |
| TBD-S | Standard | IP Source |
| TBD-P | Standard | Link State Request |
| TBD-Q | Standard | Link State Discontinue |
| TBD-V | Standard | Link Available |
| TBD-U | Standard | Link Unavailable |
| TBD-K | Standard | Link Prefix |
+----------+----------+---------------------------+
DSO Type Codes to be allocated
12. Acknowledgments
Thanks to Derek Atkins for the secdir early review.
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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>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[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>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/info/rfc7766>.
[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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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
RFC 8490, DOI 10.17487/RFC8490, March 2019,
<https://www.rfc-editor.org/info/rfc8490>.
[RFC8766] Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
2020, <https://www.rfc-editor.org/info/rfc8766>.
13.2. Informative References
[AdFam] "IANA Address Family Numbers Registry",
<https://www.iana.org/assignments/address-family-
numbers/>.
[AdProx] Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD
Service Registration Protocol", draft-sctl-advertising-
proxy-00 (work in progress), July 2020.
[I-D.ietf-mboned-ieee802-mcast-problems]
Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
Zuniga, "Multicast Considerations over IEEE 802 Wireless
Media", draft-ietf-mboned-ieee802-mcast-problems-12 (work
in progress), October 2020.
[I-D.sctl-discovery-broker]
Cheshire, S. and T. Lemon, "Service Discovery Broker",
draft-sctl-discovery-broker-00 (work in progress), July
2017.
[NOTSENT] Dumazet, E., "TCP_NOTSENT_LOWAT socket option", July 2013,
<https://lwn.net/Articles/560082/>.
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[PRIO] Chan, W., "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/>.
[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>.
Authors' Addresses
Ted Lemon
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Phone: +1 (408) 996-1010
Email: elemon@apple.com
Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Phone: +1 (408) 996-1010
Email: cheshire@apple.com
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