Network Working Group T. Lemon
Internet-Draft Nibbhaya Consulting
Intended status: Standards Track S. Cheshire
Expires: September 12, 2019 Apple Inc.
March 11, 2019
Multicast DNS Discovery Relay
draft-ietf-dnssd-mdns-relay-02
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 12, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Lemon & Cheshire Expires September 12, 2019 [Page 1]
Internet-Draft mDNS Discovery Relay March 2019
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . 15
8.7. Link State Discontinue . . . . . . . . . . . . . . . . . 16
8.8. Link Available . . . . . . . . . . . . . . . . . . . . . 16
8.9. Link Unavailable . . . . . . . . . . . . . . . . . . . . 16
8.10. Link Prefix . . . . . . . . . . . . . . . . . . . . . . . 16
9. Provisioning . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Provisioned Objects . . . . . . . . . . . . . . . . . . . 19
9.1.1. Multicast Link . . . . . . . . . . . . . . . . . . . 19
9.1.2. Discovery Proxy . . . . . . . . . . . . . . . . . . . 20
9.1.3. Discovery Relay . . . . . . . . . . . . . . . . . . . 21
9.2. Configuration Files . . . . . . . . . . . . . . . . . . . 22
9.3. Discovery Proxy Private Configuration . . . . . . . . . . 24
9.4. Discovery Relay Private Configuration . . . . . . . . . . 24
10. Security Considerations . . . . . . . . . . . . . . . . . . . 25
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
13.1. Normative References . . . . . . . . . . . . . . . . . . 27
13.2. Informative References . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
Lemon & Cheshire Expires September 12, 2019 [Page 2]
Internet-Draft mDNS Discovery Relay March 2019
1. Introduction
The Discovery Proxy for Multicast DNS-Based Service Discovery
[I-D.ietf-dnssd-hybrid] is a mechanism for discovering services on a
subnetted network through the use of Discovery Proxies, which issue
Multicast DNS (mDNS) requests [RFC6762] on various multicast links in
the network on behalf of a remote host performing DNS-Based Service
Discovery [RFC6763].
In the original Discovery Proxy specification, it is imagined that
for every multicast link on which services will be discovered, a host
will be present running a full Discovery Proxy. This document
introduces a lightweight Discovery Relay that can be used to provide
discovery services on a multicast link without requiring a full
Discovery Proxy on every multicast link.
Since the primary purpose of a Discovery Relay is providing remote
virtual interface functionality to Discovery Proxies, this document
is written with that 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
acting as a Discovery Proxy.
The Discovery Relay operates by listening for TCP connections from
Clients. When a Client connects, the connection is authenticated and
secured using TLS. The Client can then specify one or more multicast
links from which it wishes to receive mDNS traffic. The Client can
also send messages to be transmitted on its behalf on one or more of
those multicast links. DNS Stateful Operations (DSO)
[I-D.ietf-dnsop-session-signal] is used as a framework for conveying
interface and IP header information associated with each message.
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.
Lemon & Cheshire Expires September 12, 2019 [Page 3]
Internet-Draft mDNS Discovery Relay March 2019
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].
2. Terminology
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 answer 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.
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
Lemon & Cheshire Expires September 12, 2019 [Page 4]
Internet-Draft mDNS Discovery Relay March 2019
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.
whitelist A list of one or more IP addresses from which a Discovery
Relay may accept connections.
silently discard When a message that is not supported or not
permitted is received, and the required response to that message
is to "silently discard" it, that means that no response is sent
by the service that is discarding the message to the service that
sent it. The service receiving the message may log the event, and
may also count such events: "silently" does not preclude such
behavior.
Lemon & Cheshire Expires September 12, 2019 [Page 5]
Internet-Draft mDNS Discovery Relay March 2019
3. Protocol Overview
This document describes a way for Clients to communicate with mDNS
agents on remote multicast links to which they are not directly
connected, using a Discovery Relay. As such, there are two parts to
the protocol: connections between 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 [I-D.ietf-dnsop-session-signal]. There will be four
types of such messages between Discovery Relays and Clients:
o Control messages from Client to Relay
o Link status messages from Relay to Client
o Encapsulated mDNS query messages from Client to Relay
o Encapsulated mDNS response 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 response messages from a Discovery Relay to a
Client are sent whenever an mDNS response message is received on a
multicast link to which the Discovery Relay has subscribed.
Lemon & Cheshire Expires September 12, 2019 [Page 6]
Internet-Draft mDNS Discovery Relay March 2019
Encapsulated query mDNS messages from a Client to a Discovery Relay
cause the Discovery Relay to transmit the mDNS query 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 [I-D.ietf-dnsop-session-signal].
3.2. mDNS Messages On Multicast Links
Discovery Relays listen for mDNS traffic on all configured multicast
links that have at least one active subscription from a Client. When
an mDNS response 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 query 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
query 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.
Lemon & Cheshire Expires September 12, 2019 [Page 7]
Internet-Draft mDNS Discovery Relay March 2019
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 whitelist, the next step is for
the relay to verify that that the source IP address of the connection
is on its whitelist. If the connection is not permitted either
because of the source address or the destination address, the
Discovery Relay closes the connection. If possible, before closing
the connection, the Discovery Relay first sends a TLS user_canceled
alert ([I-D.ietf-tls-tls13] 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 ([I-D.ietf-tls-tls13] Section 4.2.5).
If a Discovery Relay receives a ClientHello message with no
post_handshake_auth extension, the Discovery Relay rejects the
connection with a certificate_required alert ([I-D.ietf-tls-tls13]
Section 6.2).
Once the TLS handshake is complete, the Discovery Relay MUST request
post-handshake authentication as described in ([I-D.ietf-tls-tls13]
Section 4.6.2). If the Client refuses to send a certificate, or the
key presented does not match the key associated with the IP address
from which the connection originated, or the CertificateVerify does
not validate, the connection is dropped with the TLS access_denied
alert ([I-D.ietf-tls-tls13] Section 6.2).
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.
Lemon & Cheshire Expires September 12, 2019 [Page 8]
Internet-Draft mDNS Discovery Relay March 2019
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
[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 [I-D.ietf-dnsop-session-signal]. Clients
must also honor the 'Retry Delay' TLV (section 5 of
[I-D.ietf-dnsop-session-signal]) if sent by the Discovery Relay.
Clients 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.
Lemon & Cheshire Expires September 12, 2019 [Page 9]
Internet-Draft mDNS Discovery Relay March 2019
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). It
is not an error to request a recognized link identifier which is not
yet available; the Discovery Relay accepts the request, and begins
forwarding packets when the link becomes available.
If the requested link is valid, the Relay begins forwarding all mDNS
response 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 [RFC1323]. The
Discovery Relay should use the TCP_NOTSENT_LOWAT mechanism
[NOTSENT][PRIO] or equivalent, to avoid building up a backlog of data
in excess of the amount necessary to have in flight to fill the
bandwidth delay product of the TCP connection.
Encapsulated mDNS response 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 response 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 [I-D.ietf-dnsop-session-signal], 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
Lemon & Cheshire Expires September 12, 2019 [Page 10]
Internet-Draft mDNS Discovery Relay March 2019
Discontinue TLV (Section 8.2). Subsequent messages from that link
that had previously been queued may arrive after listening has been
discontinued. The Client should silently discard such messages. The
Discovery Relay MUST discontinue generating such messages as soon as
the request is received. The Discovery Relay does not respond to
this message other than to discontinue forwarding mDNS messages from
the specified links.
Lemon & Cheshire Expires September 12, 2019 [Page 11]
Internet-Draft mDNS Discovery Relay March 2019
6. Traffic from Clients to Relays
Like mDNS traffic from relays, each mDNS query 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 query 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 (something other
than a DSO message) to a Relay. Therefore, if a Relay receives a
message other than a DSO message, it MUST respond with a REFUSED
result code. The reason not to simply drop the connection is that it
might result in a continual reconnection loop.
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 suggests that this would be a
useful optimization, since when a query is issued, it will often be
issued to all links. However, 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 in no way justified by the potential
benefit, and this idea has been abandoned.
Lemon & Cheshire Expires September 12, 2019 [Page 12]
Internet-Draft mDNS Discovery Relay March 2019
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 Discovery Relays behaves the same as a Discovery Proxy
serving multiple local multicast links using multiple physical
network interfaces. In this section we refer to multicast links
served directly by the Discovery Proxy as locally-connected links,
and multicast links served through the Discovery Relay as relay-
connected links.
When a Discovery Proxy receives a DNSSD query from a Client via
unicast, it will generate mDNS query messages on the relevant
multicast link(s) for which it is acting as a proxy. 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.
Discovery Proxies do not generally respond to mDNS queries on relay-
connected links. The one exception is responding to the Domain
Enumeration queries used to bootstrap unicast service discovery
("lb._dns-sd._udp.local", etc.) [RFC6763]. Apart from these Domain
Enumeration queries, if any other mDNS query is received from a
Discovery Relay, the Discovery Proxy silently discards it.
In principle it could be the case that some device is capable of
performing service discovery using Multicast DNS, but not using
traditional unicast DNS. Responding to mDNS queries received from
the Discovery Relay could address this use case. However, continued
reliance on multicast is counter to the goals of the current work in
service discovery, and to benefit from wide-area service discovery
such client devices should be updated to support service discovery
using unicast queries.
Lemon & Cheshire Expires September 12, 2019 [Page 13]
Internet-Draft mDNS Discovery Relay March 2019
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 SHOULD
immediately abort that DSO session.
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 primary 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.
Lemon & Cheshire Expires September 12, 2019 [Page 14]
Internet-Draft mDNS Discovery Relay March 2019
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.
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 primary 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 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.
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
Lemon & Cheshire Expires September 12, 2019 [Page 15]
Internet-Draft mDNS Discovery Relay March 2019
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 primary
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 primary 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 primary TLV, and
is not acknowledged.
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
Lemon & Cheshire Expires September 12, 2019 [Page 16]
Internet-Draft mDNS Discovery Relay March 2019
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.
Lemon & Cheshire Expires September 12, 2019 [Page 17]
Internet-Draft mDNS Discovery Relay March 2019
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
Lemon & Cheshire Expires September 12, 2019 [Page 18]
Internet-Draft mDNS Discovery Relay March 2019
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.
9.1.1. Multicast Link
The description of a multicast link consists of:
link-identifier A 32-bit identifier that uniquely identifies that
link within the Discovery Domain. Each link MUST have exactly one
such identifier. Link Identifiers do not have any special
semantics, and are not intended to be human-readable.
ldh-name A fully-qualified domain name for the multicast link that
is used to form an LDH domain name as described in section 5.3 of
the Discovery Proxy specification [I-D.ietf-dnssd-hybrid]. This
name is used to identify the link during provisioning, and must be
present.
hr-name A human-readable user-friendly fully-qualified domain name
for the multicast link. This name MUST be unique within the
Discovery Domain. Each multicast link MUST have exactly one such
name. The hr-name MAY be the same as the ldh-name. (The hr-name
is allowed to contain spaces, punctuation and rich text, but it is
not required to do so.)
The ldh-name and hr-name can be used to form the LDH and human-
readable domain names as described in [I-D.ietf-dnssd-hybrid],
section 5.3.
Note that the ldh-name and hr-name can be used in two different ways.
On a small home network with little or no human administrative
configuration, link names may be directly visible to the user. For
example, a search in 'home.arpa' on a small home network may discover
services on both ethernet.home.arpa and wi-fi.home.arpa. In the case
Lemon & Cheshire Expires September 12, 2019 [Page 19]
Internet-Draft mDNS Discovery Relay March 2019
of a home user who has one Ethernet-connected printer and one Wi-Fi-
connected printer, discovering that they have one printer on
ethernet.home.arpa and another on wi-fi.home.arpa is understandable
and meaningful.
On a large corporate network with hundreds of Wi-Fi access points,
the individual link names of the hundreds of multicast links are less
likely to be useful to end users. In these cases, Discovery Broker
functionality [I-D.sctl-discovery-broker] is used to translate the
many link names to something more meaningful to users. For example,
in a building with 50 Wi-Fi access points, each with their own link
names, services on all the different physical links may be presented
to the user as appearing in 'headquarters.example.com'. In this
case, the individual link names can be thought of similar to MAC
addresses or IPv6 addresses. They are used internally by the
software as unique identifiers, but generally are not exposed to end
users.
9.1.2. Discovery Proxy
The description of a Discovery Proxy consists of:
name a machine-readable name used to reference this Discovery Proxy
in provisioning.
hr-name an optional human-readable name which can appear in
provisioning, monitoring and debugging systems. Must be unique
within a Discovery Domain.
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
Lemon & Cheshire Expires September 12, 2019 [Page 20]
Internet-Draft mDNS Discovery Relay March 2019
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.
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 the 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.
Lemon & Cheshire Expires September 12, 2019 [Page 21]
Internet-Draft mDNS Discovery Relay March 2019
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.
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.
Lemon & Cheshire Expires September 12, 2019 [Page 22]
Internet-Draft mDNS Discovery Relay March 2019
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-whitelist main
Relay downstairs
certificate yyy
listen-tuple 192.51.100.1 2088
listen-tuple fd00::2 2088
link downstairs-wifi
link downstairs-wired
client-whitelist main
Proxy main
certificate zzz
address 203.1.113.1
Link upstairs-wifi
id 1
name Upstairs Wifi
Link upstairs-wired
id 2
hr-name Upstairs Wired
Link downstairs-wifi
id 3
name Downstairs Wifi
Link downstairs-wired
id 4
hr-name Downstairs Wired
Lemon & Cheshire Expires September 12, 2019 [Page 23]
Internet-Draft mDNS Discovery Relay March 2019
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
Lemon & Cheshire Expires September 12, 2019 [Page 24]
Internet-Draft mDNS Discovery Relay March 2019
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 [I-D.ietf-tls-tls13] to have at least that lifespan.
In addition, recent IETF efforts are pushing for better software
update practices for devices like routers, for other security
reasons, making 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.
Lemon & Cheshire Expires September 12, 2019 [Page 25]
Internet-Draft mDNS Discovery Relay March 2019
11. IANA Considerations
The IANA is kindly requested to update the DSO Type Codes Registry
[I-D.ietf-dnsop-session-signal] by allocating codes for each of the
TBD type codes listed in the following table, and by updating this
document, here and in Section 8. Each type code should list this
document as its reference document.
+--------+----------+---------------------------+
| Opcode | Status | Name |
+--------+----------+---------------------------+
| TBD-R | Standard | 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
To be completed...
Lemon & Cheshire Expires September 12, 2019 [Page 26]
Internet-Draft mDNS Discovery Relay March 2019
13. References
13.1. Normative References
[I-D.ietf-dnsop-session-signal]
Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
draft-ietf-dnsop-session-signal-20 (work in progress),
December 2018.
[I-D.ietf-dnssd-hybrid]
Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", draft-ietf-dnssd-hybrid-08 (work in
progress), March 2018.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
[I-D.sctl-discovery-broker]
Cheshire, S. and T. Lemon, "Service Discovery Broker",
draft-sctl-discovery-broker-00 (work in progress), July
2017.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, DOI 10.17487/RFC1323, May
1992, <https://www.rfc-editor.org/info/rfc1323>.
[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>.
Lemon & Cheshire Expires September 12, 2019 [Page 27]
Internet-Draft mDNS Discovery Relay March 2019
[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>.
[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>.
[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>.
13.2. Informative References
[AdFam] "IANA Address Family Numbers Registry",
<https://www.iana.org/assignments/
address-family-numbers/>.
[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-04 (work
in progress), November 2018.
[NOTSENT] Dumazet, E., "TCP_NOTSENT_LOWAT socket option", July 2013,
<https://lwn.net/Articles/560082/>.
[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>.
Lemon & Cheshire Expires September 12, 2019 [Page 28]
Internet-Draft mDNS Discovery Relay March 2019
[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
Nibbhaya Consulting
P.O. Box 958
Brattleboro, Vermont 05301
United States of America
Email: mellon@fugue.com
Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, California 95014
USA
Phone: +1 (408) 996-1010
Email: cheshire@apple.com
Lemon & Cheshire Expires September 12, 2019 [Page 29]
