Multicast DNS Discovery RelayApple Inc.One Apple Park WayCupertinoCalifornia95014United States of America+1 (408) 996-1010elemon@apple.comApple Inc.One Apple Park WayCupertinoCalifornia95014United States of America+1 (408) 996-1010cheshire@apple.com
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.
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
is a mechanism for discovering
services on a subnetted network through the use of Discovery Proxies.
Discovery Proxies issue Multicast DNS (mDNS) requests
on various multicast links in the network on behalf of a remote host
performing DNS-Based Service Discovery .
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 .
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) 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 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 or configured using
some sort of orchestration software in conjunction with NETCONF
or CPE WAN Management Protocol .
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.
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
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:
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.
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.
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.
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.
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 )
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
,
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.
A list of one or more IP addresses from which a Discovery Relay may accept
connections.
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
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.
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.
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
Section 4.2.2.
However, all messages will be DSO messages .
There will be
four types of such messages between Discovery Relays and Clients:
Control messages from Client to RelayLink status messages from Relay to ClientEncapsulated mDNS messages from Client to RelayEncapsulated 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.
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
.
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.
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
( 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 ( 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 ( Section 6.2).
Once the TLS handshake is complete, the Discovery Relay MUST request post-handshake
authentication ( 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 ( 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 Section 7.1, and uses the
authentication method described in 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 .
Aliveness of connections between Clients and Relays is maintained as described
in Section 4 of the DSO specification . Clients must
also honor the 'Retry Delay' TLV (section 5 of
) 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.
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 ()
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 . The Discovery Relay should use the
TCP_NOTSENT_LOWAT mechanism 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 ().
The source IP address and port of the message MUST be encoded in an IP Source TLV
(). The multicast link on which the message was received MUST be
encoded in a Link Identifier TLV ().
As described in the DSO specification ,
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 ().
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 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.
Like mDNS traffic from relays, each mDNS message sent by a Client to a
Discovery Relay is communicated in an Encapsulated mDNS Message TLV ()
within a DSO message. Each message MUST contain exactly one Link
Identifier TLV (). 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
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.
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.
This document defines a modest number of new DSO TLVs.
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 ).
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 .
An address family value of 1 indicates IPv4 and 2 indicates IPv6,
as recorded in the IANA Registry of Address Family Numbers .
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).
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 .
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.
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 .
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.
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.
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.
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.
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.
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.
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.
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.
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 ( Section 8.2) or PKIX authentication
. 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 self-signed and
PKIX-authenticated certificates, with PKIX, DANE or a pre-configured trust anchor.
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.
The description of a multicast link consists of:
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.
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
. This name is used to identify the link
during provisioning, and must be present.
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 , 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
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.
The description of a Discovery Proxy consists of:
a machine-readable name used to reference this Discovery Proxy in provisioning.
an optional human-readable name which can appear in provisioning, monitoring and
debugging systems. Must be unique within a Discovery Domain.
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.
the private key corresponding to the public key in the certificate.
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.
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.
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.
The description of a Discovery Relay consists of:
a required machine-readable identifier used to reference the relay
an optional human-readable name which can appear in provisioning, monitoring and
debugging systems. Must be unique within a Discovery Domain.
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 .
the private key corresponding to the public key in the certificate.
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.
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.
For this discussion, we assume the simplest possible means of configuring Discovery
Proxies and Discovery Relays: the configuration file. Any environment where changes
will happen on a regular basis will either require some automatic means of generating
these configuration files as the network topology changes, or will need to use a more
automatic method for configuration, such as HNCP .
There are many different ways to organize configuration files. This discussion
assumes that multicast links, relays and proxies will be specified as objects, as described
above, perhaps in a master file, and then the specific configuration of each proxy
or relay will reference the set of objects in the master file, referencing objects
by name. This approach is not required, but is simply shown as an example.
In addition, the private keys for each proxy or relay must appear only in that
proxy or relay's configuration file.
The master file contains a list of Discovery Relays, Discovery Proxies and Multicast Links.
Each object has a name and all the other data associated with it. We do not formally
specify the format of the file, but it might look something like this:
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:
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.
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:
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 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 was used for about a decade, and similarly
TLS 1.2 was also used for about a decade.
We expect TLS 1.3 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.
The IANA is kindly requested to update the DSO Type Codes Registry
by allocating codes for each of the TBD
type codes listed in the following table, and by updating this document, here and in
. Each type code should list this document as its reference
document.
DSO-TYPEStatusNameTBD-RStandardLink Data RequestTBD-DStandardLink Data DiscontinueTBD-LStandardLink IdentifierTBD-MStandardEncapsulated mDNS MessageTBD-SStandardIP SourceTBD-PStandardLink State RequestTBD-QStandardLink State DiscontinueTBD-VStandardLink AvailableTBD-UStandardLink UnavailableTBD-KStandardLink Prefix
Thanks to Derek Atkins for the secdir early review.
Advertising Proxy for DNS-SD Service Registration ProtocolAn Advertising Proxy allows a device
that accepts service registra-tions using
Service Registration Protocol (SRP)
to make those regis-trations visible
to legacy clients that only implement Multicast DNS.CPE WAN Management ProtocolBroadband ForumTCP_NOTSENT_LOWAT socket optionPrioritization Only Works When There's Pending Data to PrioritizeIANA Address Family Numbers Registry