Multicast DNS conflict resolution using the Time Since Received (TSR) EDNS option
draft-ietf-dnssd-tsr-02
| Document | Type | Active Internet-Draft (dnssd WG) | |
|---|---|---|---|
| Authors | Ted Lemon , Esko Dijk | ||
| Last updated | 2026-04-22 | ||
| Replaces | draft-tllq-tsr | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-dnssd-tsr-02
dnssd T. Lemon
Internet-Draft Apple Inc.
Intended status: Standards Track E. Dijk
Expires: 24 October 2026 IoTconsultancy.nl
22 April 2026
Multicast DNS conflict resolution using the Time Since Received (TSR)
EDNS option
draft-ietf-dnssd-tsr-02
Abstract
This document specifies a new conflict resolution mechanism for DNS,
for use in cases where the advertisement is being proxied, rather
than advertised directly, e.g. when using a combined DNS-SD
advertising proxy and SRP registrar. A new EDNS option is defined
that communicates the time at which the set of resource records on a
particular DNS owner name was most recently updated.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://datatracker.ietf.org/doc/html/draft-ietf-dnssd-tsr. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-dnssd-tsr/.
Discussion of this document takes place on the WG Working Group
mailing list (mailto:dnssd@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/dnssd/. Subscribe at
https://www.ietf.org/mailman/listinfo/dnssd/.
Source for this draft and an issue tracker can be found at
https://github.com/dnssd-wg/draft-ietf-dnssd-tsr.
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/.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on 24 October 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Current Behavior . . . . . . . . . . . . . . . . . . . . 4
1.2. Problem Statement . . . . . . . . . . . . . . . . . . . . 5
1.3. Conventions, Terms and Definitions . . . . . . . . . . . 6
2. Time Since Received EDNS Option . . . . . . . . . . . . . . . 7
3. mDNS Registrar Behavior . . . . . . . . . . . . . . . . . . . 9
3.1. Validating requested local RR registrations that include a
TSR option . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Probing resource records on names for which TSR data has
been proposed . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Processing mDNS questions for which TSR data exists . . . 11
3.4. Receiving mDNS messages that may contain TSR options . . 11
3.5. Processing mDNS messages thay may contain TSR options . . 11
3.6. Duplicate answer suppression behavior . . . . . . . . . . 13
3.7. Suppression of Goodbye announcements . . . . . . . . . . 14
3.8. Suppression of redundant probing . . . . . . . . . . . . 14
3.9. Constructing a mDNS message with TSR options . . . . . . 16
4. The effect of network latency on time computations . . . . . 17
5. Internal Handling of TSR data . . . . . . . . . . . . . . . . 17
6. Timeliness of Conflict Resolution . . . . . . . . . . . . . . 18
7. Legacy Behavior . . . . . . . . . . . . . . . . . . . . . . . 18
8. When to Use TSR . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Use of TSR with redundant proxies . . . . . . . . . . . . 18
8.2. Use of TSR with multihomed devices . . . . . . . . . . . 19
8.3. TSR in networks with non-compliant mDNS caches . . . . . 19
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9. Registrant API considerations . . . . . . . . . . . . . . . . 20
9.1. Removing data that is still valid . . . . . . . . . . . . 20
9.2. Primary/Secondary indication . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Duplicate Answer Suppression Example . . . . . . . . 23
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
Unlike the Domain Name System [RFC1034], with its authority servers
and delegation of authority, Multicast DNS has no single source of
authority. Because of this, mDNS has a mechanism, conflict
resolution (Section 9 of [RFC6762]) for detecting and fixing
conflicts in mDNS advertisements.
The current goal of mDNS conflict resolution is to prevent a newly
advertised service from taking the place of an existing service with
the same name that is already being advertised. This goal, however,
assumes that the entity advertising an mDNS service is in fact
authoritative for that service. For mDNS proxies, such as an
advertising proxy [I-D.ietf-dnssd-advertising-proxy], this is not the
case: the source of truth for the service being advertised is an
DNSSD Service Registration Protocol (SRP) [RFC9665] requester.
On a link with more than one SRP registrar, an SRP requester may
register with one SRP registrar, and then subsequently update its
registration on a different SRP registrar. Both SRP registrars may
be acting as advertising proxies. If so, the original SRP registrar
may still be advertising the old SRP registration using mDNS. If the
information in the new SRP registration is identical to that in the
old registration, this is often not a problem. However if some
information has changed (e.g., a new IP address has been added, or a
TXT record updated), then the new registration will be seen to be in
conflict with the old registration. In addition, the method used in
mDNS to detect conflicts can sometimes produce apparent conflicts
where no actual conflict exists because of the way records in mDNS
packets are marshalled.
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In the case of such an apparent conflict, the current behavior of
mDNS is for the older (stale) registration to win, and the newer
(current) information to be discarded. This behavior, which is
entirely correct for services that are advertising on their own
behalf, is exactly wrong when a service advertisement is being
proxied.
1.1. Current Behavior
When a new service is to be advertised, the registrant (the entity
requesting the registration) typically registers the service with a
central mDNS registrar on the host on which it is running. This mDNS
registrar locally stores services that have already been registered,
and may detect a conflict with one of those registered services.
This can be true whether the conflicting service entry is data for
which the mDNS registrar is authoritative (stored in its local
registration database), or an entry it has received via mDNS (stored
in its cache).
In the case of such a conflict, no network transaction is required:
the mDNS registrar detects it locally. It addresses the conflict in
one of two ways. The first alternative is that the mDNS registrar
will report the conflict to the registrant as an error, which it must
fix. Alternatively, if the registrant has indicated that the mDNS
registrar should automatically choose a new name for the service in
case of conflict, the mDNS registrar does so automatically, without
necessarily notifying the registrant.
Once any locally-detectable conflicts have been resolved, the mDNS
registrar probes (see Section 8.1 of [RFC6762]) the local network to
see if any other host has already registered a service name that
conflicts with the proposed new service name. If such a service name
is present on the network, the mDNS registrar follows the same
process previously described: either report the error to the mDNS
registrant or automatically choose a new name.
The effect of this approach is that generally whichever registrant
first registers a service under a particular name wins. If another
registrant comes along later and registers the same service name with
conflicting service information, the newcomer’s information is
rejected.
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1.2. Problem Statement
The current behavior works well for registrants registering services
on their own behalf. However, for example in the case of an SRP
registrar, it works poorly: an SRP registrar acting as an advertising
proxy publishes the contents of its DNS zone using mDNS. The sources
of truth for this information are the SRP requesters, not the SRP
registrar itself. The SRP registrar in this case acts as a proxy for
the SRP requesters.
In the case of an advertising proxy publishing records originating
from an SRP Update, the most recent information is correct; the older
information is simply stale, and not competing. When the SRP
requester is able to continue registering with the same SRP
registrar, this works well: stale data is automatically removed and
replaced with current data. However, if more than one SRP registrar
is available, the SRP requester may wind up sending its SRP Updates
to a different SRP registrar. This can happen as a result of a
network partition event, or in cases where the SRP registrar is
contacted using an anycast address.
When the SRP requester sends its SRP Update to a different SRP
registrar, the behavior of the mDNS conflict resolution approach
without TSR is that the SRP requester's service will be given a new
name, and both the old (stale) service advertisement (A) and the new
(more recent) service advertisement (A’) will be discoverable as
separate services.
This creates a new burden on consumers of such services: they need to
parse through the whole list of services of their type, using
metadata from the TXT record in the service instance data, if
possible, to determine that service A and service A’ are the same
service. If no such information is present in the TXT record, the
only way to determine that one of these two advertised services is
stale is to attempt to use the advertised service, which may no
longer be reachable if, for example, the change that produced the
conflict was an IP address change. When the SRP lease for the stale
service expires, that service's mDNS advertisement will be removed,
and the service will no longer be discoverable under the original
name, even if its IP address hasn't changed.
This document proposes an enhancement to the current conflict
resolution algorithm for mDNS, which allows an mDNS proxy to report
the time at which it received the registration for DNS records it is
advertising, and the source from which these records were received.
This is done using a new Time Since Received (TSR) EDNS(0)
([RFC6891]) option, of which there must be exactly one per name being
advertised by the mDNS proxy.
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1.3. Conventions, Terms and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
*absolute time* a node-local timestamp that is derived from a local
monotonic clock. This clock is not required to be synchronized
with any other clock, nor is it an absolute time in the sense of
UTC. The clock may restart upon boot of the node but not during
regular operation.
*authoritative data* DNS records stored in the local registration
database, for which an mDNS registrar is authoritative (as defined
by [RFC6762]).
*cache* each mDNS registrar maintains a cache of DNS records
received from hosts on the local network, as described in
[RFC6762]. Information in the cache represents the mDNS
registrar's current understanding of what records are advertised
on the network. Authoritative data may be present in the cache,
but the presence of data in the cache does not in itself indicate
that the mDNS registrar is authoritative for that data.
*local registration database* a database of DNS records maintained
by the mDNS registrar. Records in this database are registered
locally with the mDNS registrar. The mDNS registrar is
authoritative for all DNS records in this database. When the mDNS
registrar receives queries that match records in this database,
the mDNS registrar can respond to these queries with the matching
records.
*mDNS registrar* an mDNS [RFC6762] implementation on a host that
accepts local requests for querying, advertising and registering
DNS records from one or more requesters and/or registrants. This
could for example be an mDNS daemon process running in an
operating system, accepting API calls from local processes to
register or update DNS records for that process. It stores the
locally-registered records in its local registration database.
*mDNS registrant* an entity or software process that is attempting
to register records for advertisement to a (local) mDNS registrar.
*mDNS requester* an entity or software process that issues mDNS
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queries to a local mDNS registrar, via an API call. The mDNS
registrar is responsible for executing these queries and notifying
the mDNS requester about the answer(s).
*mDNS proxy* a host that runs an mDNS registrar and at least one
mDNS registrant acting as a proxy. That is, it needs to advertise
mDNS records on behalf of one or more entities not located on the
host itself. The advertising proxy
[I-D.ietf-dnssd-advertising-proxy] is an example of an mDNS proxy.
*SRP requester* a client that uses the Service Registration Protocol
(SRP) [RFC9665] to send an SRP Update to an SRP registrar.
*SRP registrar* a server that accepts SRP Updates sent by SRP
requesters using the SRP [RFC9665]. DNS records registered via
SRP to an SRP registrar may then be advertised by mDNS using an
advertising proxy [I-D.ietf-dnssd-advertising-proxy] located on
the same host. In that case, the SRP registrar process acts as an
mDNS registrant towards its local mDNS registrar process.
*TSR data* locally stored data, associated with a single DNS owner
name, that keeps track of the absolute time when a set of resource
records were last updated and includes a key checksum to identify
the owner of these records.
*TSR option* an EDNS0 Time Since Received (TSR) option as defined by
this specification.
*TSR time* the (node-local) absolute time indicated by a particular
TSR data.
2. Time Since Received EDNS Option
Each Time Since Received (TSR) EDNS option is applicable to exactly
one DNS owner name. So all the records for that owner name that
appear in the answer, authority and/or additional sections of an mDNS
message would be covered by a single TSR option.
The TSR EDNS option has the following format:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-------------------------------+-------------------------------+
| OPTION-CODE = TBD1 | OPTION-LENGTH = 10 |
+-------------------------------+-------------------------------+
| RR Index | Key Checksum ... |
+-------------------------------+-------------------------------+
| ... Key Checksum | Time Offset ... |
+-------------------------------+-------------------------------+
| ... Time Offset |
+-------------------------------+
The TSR EDNS option includes three fields in the OPTION-DATA
([RFC6891]): 'RR Index' (two byte integer in network byte order),
'Key Checksum' (four bytes), and 'Time Offset' (four bytes).
The 'RR Index' is the number of the RR in the mDNS packet. Question
RRs are not counted. So if the message includes two answer RRs, one
authority RR and two additional RRs, an index of 0 would refer to the
first answer, an index of 1 to the second answer, and index of 2 to
the single authority record, and so on. Questions are excluded
because they have no data associated with them, and so it makes no
sense for them to have TSR options associated with them.
If there is more than one record in the mDNS Message with the same
owner name, only one TSR option is emitted for that name, and it
applies to every RR in the mDNS Message with that owner name. It is
not possible in the SRP protocol for two updates at two different
times to contain records that apply to the same name: in such a
situation, the second update completely replaces the first, so all
data in the first update is then rendered stale.
The second field, 'Key Checksum', is a simple 32-bit checksum of the
public key that the owner of the data (for example the SRP requester)
used to authenticate itself. The key checksum is computed by
treating the key as a series of 32-bit unsigned integers in network
byte order, and adding these integers together to produce a 32-bit
unsigned checksum. Overflow is not considered. This checksum need
not be cryptographically secure: mDNS messages are not authenticated,
so an attacker on the local link can always cause problems with mDNS
by providing spurious responses. The purpose of the checksum is
simply to notice whether, for a specific owner name, two different
authoritative sources have provided information.
The 'Time Offset' field contains the difference, in seconds, between
the the time at which the TSR option is being generated and the time
of receipt of resource records for that owner name.
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The time offset is represented in the mDNS message as a time in
seconds relative to the time when the mDNS message is sent. If this
difference is greater than seven days (7 * 24 * 60 * 60), the mDNS
registrar MUST use a value of seven days rather than the larger
value. The relative time value in the TSR option is converted to an
absolute (local) time when stored in a cache or a local registration
database on an mDNS registrar as TSR data, and is converted back to a
relative time whenever an mDNS message with a TSR option is generated
from local data.
3. mDNS Registrar Behavior
3.1. Validating requested local RR registrations that include a TSR
option
When a local mDNS registrant asks an mDNS registrar to register one
or more records on an owner name, and provides TSR data for that
name, the mDNS registrar first checks that none of the records are
marked shared. If any record is marked shared, the mDNS registrar
MUST respond with an error indicating that the registration is
invalid.
It then checks to see if there are any records either in cache or in
its local registration database on that owner name. If no such data
exists, the mDNS registrar stores the record(s) in this registration
in its local registration database and puts them in the probing
state.
When such data exists, the registrar MUST check to see if it has TSR
data for that owner name. If it does not, or if there is TSR data on
that name but the key checksum does not match, the registrar MUST
treat this registration as a conflict and return an appropriate error
to the registrant.
If such data exists and the key checksums match, there are three
possibilities based on the known TSR time (from the existing data)
and the proposed TSR time (from the TSR data in the registration
request):
*Known TSR time is more recent* In this case, the registrar MUST
treat the requested data as stale, and return an indication to the
registrant that its registration is stale. This indication must
be distinct from the "appropriate error" indication of conflict
that was defined above.
*Both TSR times are the same* In this case, cached data (if any) on
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the owner name is discarded first. Then, the requested records
are added to the local registration database, updating existing
records (if any). Since the requested records are considered
identical to the known records, based on TSR time, no probing or
announcing is performed for these records.
*Proposed TSR time is more recent* In this case, cached data (if
any) on the owner name is discarded first. The registrant for any
existing locally-registered data is notified that the data they
had previously registered is stale, and the stale data is removed
from the local registration database. The requested records are
added to the local registration database and put in the probing
state, and the TSR data is replaced by the proposed TSR data.
It is in principle possible for two different mDNS registrants to ask
the same mDNS registrar to publish different RRs on the same name,
some of which are shared and some of which are unique (see Section 2
of [RFC6762]). If an mDNS registrant tries to register an RR on a
name for which the registrar already has data, cached or
authoritative, on the same name, whether of the same RR type or a
different RR type, for which there is no TSR data, or for which the
key checksum in the TSR data being registered does not match what is
already known, the registrar MUST treat this as an immediate
conflict, and MUST NOT add the data to its local registration
database and MUST NOT probe.
For the third case, as with any local mDNS registration, the mDNS
registrar treats all of the records in the registration request as
tentative (that is, they are put in the probing state) until they
have been probed and no conflicting answers from other mDNS hosts
have been received.
3.2. Probing resource records on names for which TSR data has been
proposed
Section 8.1 of [RFC6762] describes how an mDNS registrar probes to
ensure that there is no conflicting data for records in the probing
state. The behavior for records that are in the probing state with
owner names to which no TSR data applies remains unchanged. When
there is TSR data on a name for which records are being probed, the
mDNS registar MUST include TSR options for each such name as
described in Section 2. Handling of responses is described in
Section 3.5.
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3.3. Processing mDNS questions for which TSR data exists
When processing a question for which local TSR data is present, the
mDNS registrar MUST first check to see if there is corresponding data
in the mDNS message being processed. In this case, before
constructing a response, the mDNS registrar MUST process the non-
question records in the packet, since this could result in stale data
being flushed. Processing is performed as described in Section 3.5.
Once all non-question records have been processed, the registrar MUST
respond to any questions that match locally-registered resource
records for which a known answer is not present in the query.
Responses are constructed as described in Section 2.
3.4. Receiving mDNS messages that may contain TSR options
mDNS registrars that support TSR need to compute an absolute time
based on a time offset. This means that registrars need to know when
the packet was received. A naive implementation might assume that
the time that the packet is read off the input queue by the registrar
is close enough. However, in practice it can be the case on a
heavily loaded system that the time of receipt and the time of
processing are far enough apart to create the appearance of
staleness.
To avoid this, mDNS registrars that have an API available to get the
actual time of receipt of a packet should make use of that API. For
example, the SO_TIMESTAMP_CONTINUOUS socket option is available on
Linux and BSD Unix platforms, including MacOS. When such APIs are
not available, another option is to receive such packets on a high
priority thread and queue them for later processing.
3.5. Processing mDNS messages thay may contain TSR options
mDNS registrars that support the TSR option MUST check incoming
messages for the presence of an EDNS(0) option containing TSR
options. mDNS registrars that do not support TSR will not do this
check, and will behave as if no TSR options are present. For non-
proxy use cases, this should make no difference, since in such cases
if multiple devices advertise records on the same owner name, these
are actually in conflict. However, for the proxy use case, what this
means is that two proxies that are proxying the same data cannot
interoperate if one supports TSR and the other doesn't.
It is important to note that mDNS messages, particularly in the case
of proxies, can contain combined information answering multiple
queries that may be outstanding, and as such, it's entirely possible
for a mDNS message sent by an mDNS registrar that supports TSR to
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contain some answers for which there is TSR data, and some answers
for which there is not. It's equally possible that such a registrar
will send mDNS packets containing no TSR options at all.
When an mDNS message contains TSR options, for each TSR option in an
mDNS message, the mDNS registrar first determines the owner name of
the TSR option by assigning an index to each non-question resource
record in the mDNS message. The 'RR index' of each TSR option is
then matched to the index of a resource record, and the owner name
for that resource record is applied to the TSR option. The time on
the TSR option is then computed by taking the current local clock
time and subtracting from it the 'Time Offset' value in the TSR
option.
If there is a TSR option in an mDNS message for which there is no
matching resource record in the mDNS message, the mDNS registrar MUST
ignore that TSR option. The mDNS registrar MUST NOT use the 'RR
Index' value in the TSR option to search across the mDNS packet since
such an index can easily be out of bounds.
Now, for each record in the mDNS message, the mDNS registrar first
determines whether the record is an OPT record, is in the question
section, or is a known answer (QD bit = 0 and it's a record in the
answer section). For all such records, no special processing is done
for TSRs, since no TSR should exist in the mDNS message.
For each remaining resource record in the mDNS message, the mDNS
registrar MUST check to see if there is a TSR option in the mDNS
message for that owner name. If there is not, the mDNS registrar
MUST check to see if there is TSR data with that owner name locally.
If there is not, the record is processed normally.
If there is local TSR data for the record's owner name, but no TSR
data for that owner name in the mDNS message, the mDNS registrar
checks to see if there are any resource records in the local
registration database on that name. If there are, all such records
are treated as in conflict. This conflict exists even if the locally
registered records are all shared records. In cases where there are
records on the name in the cache, those records are all discarded,
because they are in conflict with the new data.
In the case that there is TSR data for the record in the mDNS
message, and there are local records on the same owner name for which
there is no local TSR data, this always means that any data is in
conflict. How that conflict is addressed depends on the data.
First, note that resource records in the answer section of an mDNS
Query (QR bit in the header is 0) are "known answers" and therefore
are not relevant when adding data to the cache. Such records can
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never have TSR options associated with them. However, resource
records in the authority and additional sections of a query do need
to be processed (but in the case of authority records, are not added
to the cache).
In cases where the TSR data for a particular name is present both
locally and in the mDNS message, the mDNS registrar MUST compare the
key checksums. If these are different, then the records are always
in conflict, and are handled according to the context of the
conflict, as described in Section 9 of [RFC6762].
In cases where the key checksums match, the mDNS registrar MUST
compare the times. When the TSR time from the mDNS message is more
recent than the local TSR time, the local data in the cache is
flushed. Stale data in the local registration database is removed,
and the mDNS registrant is informed that this data is stale.
When the TSR times are the same, any resource records on that name in
the answer section and additional section are added to the cache.
When the local TSR time is more recent, the data in the message is
not added to the cache, and no action is taken with respect to any
locally-registered data.
3.6. Duplicate answer suppression behavior
Section 7.4 of [RFC6762], Duplicate Answer Suppression, describes
behavior intended to prevent the redundant transmission of duplicate
answers. When an mDNS query is received for which the mDNS registrar
has authoritative data, the mDNS registrar will wait for a random
amount of time before sending a response. If, during that time, a
response is received that contains all the answers it would have
sent, it suppresses sending these answers, since they are redundant.
Such a response is referred to as a "pre-empting response".
In the case where TSR data is present locally or in a pre-empting
response, it can be the case that the data in a pre-empting response
is stale or conflicting. For this reason, an mDNS registrar MUST NOT
suppress duplicate answers when:
* the TSR key checksum for the owner name in a pre-empting response
does not match the local TSR key checksum for that owner name
* TSR data is present locally for that owner name but is not present
in the pre-empting response
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* A TSR option is present for that owner name in the pre-empting
response, but local authoritative data for that owner name is
present but has no associated TSR data
* local authoritative data for that owner name is present and
includes TSR data, a TSR option for that owner name is present in
the pre-empting response, but the local TSR time for that owner
name is more recent than the TSR time for that owner name in the
pre-empting response
Appendix A shows what this might look like from the perspective of an
mDNS requester.
3.7. Suppression of Goodbye announcements
When authoritative data is removed on an mDNS registrar because an
mDNS message has been received with more recent data, the mDNS
registrar MUST NOT send a "goodbye" announcement for any RR on that
owner name as a result of flushing this stale data.
The reason for this is that in the case where an mDNS registrant
updates one or more RRsets on an owner name covered by TSR data, and
as a result of this some records are removed, but some remain, the
mDNS registrar that was directly updated will either send "goodbye"
announcement or an announcement with the cache flush bit set as
specified in Section 8.4 of [RFC6762]. Since the mDNS registrar with
the most current information has already done what is needed, if an
mDNS registrar that is flushing locally-registered data were to send
a "goodbye" announcement this would at best be redundant and hence
wasteful use of multicast, and at worse might cause valid data to be
flushed from the cache on some mDNS registrar.
3.8. Suppression of redundant probing
When mDNS proxies are doing any form of replication of the records
they are publishing, it can be the case that one proxy sends its
probes for a new set of records first. If this is the case, some or
all proxies on the same multicast link that receive the replicated
data may already have (at the time of probing) the correct data in
their local registration database with matching TSR times, waiting to
be probed.
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In such case, these proxies will determine that the records in the
received probe message are identical to the locally-registered set of
records, and therefore per Section 8.2.1 of [RFC6762] no conflict is
detected and no response to the probe will be sent. An mDNS
registrar that receives a probe, for a new set of records that it has
just placed in the probing state, MUST suppress sending its own probe
messages as this would be redundant: the probing is already being
performed on the link.
[TBD for WG: for the above, the response to a probe in case of a true
conflict (by another mDNS host) will typically be 'fast' with unicast
as per RFC 6762. For this reason, the suppression of probing would
be dangerous in the above case, as most of the proxies that suppress
their probing would not see the actual conflict (if any). Unless the
replication algorithm takes care of withdrawing the records on all
proxies.]
Conversely, it can also be the case that one proxy already sent its
mDNS probes and announced the new records before some other proxy on
the same multicast link fully completed the replication process for
these records. In that case, the announced records will be stored
already in the cache of this other proxy at the time that its local
mDNS registrant (i.e. the replicating proxy process) performs the
registration of these records and the associated probing.
To avoid redundant probing for such cases, when an mDNS registrant
registers data with an mDNS registrar for which the same data is
already cached with the same TSR key checksum and an equal TSR time,
the mDNS registrar MUST skip probing. This requirement follows the
second case ("Both TSR times are the same") in Section 3.1.
[TBD for WG: the below 'recent' rule text instead of 'equal' as used
in above sentence seems to conflict the section 'locval' Section 3.1
3 cases. Only when TSR time is equal, will the probing be
suppressed.
Recent here should take into account network delays: a difference of
less than ten seconds between the cached TSR time and the
registrant's TSR time should be considered "recent." ]
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In addition, when the TSR time for a set of RRs is updated by an mDNS
registrant, but nothing else changes, the mDNS registrar MUST NOT re-
probe those RRs. In this situation, if some RRs are removed, then a
goodbye announcement should be sent for such RRs, but no probe is
sent for RRs that are not removed. If some RRs are added, the
probing stage can be skipped because the registrar already knows it
is up to date. So the new RRs can simply be announced immediately.
One can assume that updates do not happen frequently enough for there
to be competing mDNS updates being probed or announced at the same
time.
3.9. Constructing a mDNS message with TSR options
For each non-question record that is added to the mDNS message, one
of three things must be true:
* The mDNS registrar has locally-registered resource record(s) on
that owner name, which may or may not be in the probing state.
* It is sending an answer which is either an announcement or a
response containing data it has already validated and for which it
is authoritative.
* The message is a query (QD=0) and the record is in the answer
section, and is therefore a "known answer."
As described in Section 7.1 of [RFC6762], an mDNS registrar asking a
question about one or more RRs on a particular name populates the
answer section of its mDNS message with the answers it already knows,
to avoid unnecessary responses. However, in this case it can't also
be probing for records on the same name, because probes are only done
for unique (non-shared) records.
The requirements in Section 3.1 mean that there can never be an mDNS
probe that contains known answers on an owner name for which any RR
is being probed to which a TSR option applies.
This means that for any particular owner name that might be
represented in an mDNS packet, it must be the case either that it is
not a known answer, or that it is a known answer and no other records
exist in the mDNS packet with the same owner name to which a TSR
option would apply. That is, one of two things must be true about
the set of all records with a particular owner name being added to
the mDNS packet: either a TSR option applies to all of the records,
or it applies to none of the records. Furthermore, either a record
is a known answer from cache, or it is a locally-registered record.
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When constructing an mDNS message, the mDNS registrar maintains a set
of names and associated TSR data. Initially this set is empty. When
the registrar adds a record to the mDNS message, if that record is
locally registered, and if the registrar has TSR data for that name,
it first checks to see if it has already added TSR data for that name
to the set. If not, then it adds a new entry to the set containing
the TSR data for the owner name of the RR. The data added consists
of the owner name, the index of the record being added (since it is
the first), the key checksum, and the time offset.
Once the mDNS registrar has finished adding resource records to the
mDNS message, it adds an OPT record in the additional section. In
this OPT record it adds a TSR option for every name in the set that
was generated when adding resource records to the message. The time
of receipt is subtracted from the current time to produce the value
for the 'Time Offset' field, and this value is clamped to a maximum
of seven days (604,800 seconds) Section 2.
4. The effect of network latency on time computations
Because TSR computations are affected by network latency, comparisons
can’t be considered accurate. It is therefore necessary to tolerate
some amount of error. In practice, however, it should generally not
be the case that two advertising proxies receive SRP updates from the
same SRP requester at nearly the same time. So it should always be
the case either that there is a clear ordering to the TSR 'Time
Offset' values, or that there is no conflict in the data. For
example with anycast, a retransmission could go to a different SRP
registrar, but in this case both SRP registrars would simultaneously
receive identical data, so the close ordering or even equality of the
TSR time offsets should not affect the outcome.
5. Internal Handling of TSR data
The TSR 'Time Offset' value that is sent on the wire is expressed in
seconds relative to the time of receipt of the registration. In
order to derive this value, the mDNS registrar must remember the
(local) time at which the registration occurred. This time is
recorded as an absolute time, not a relative time. We refer to this
as the time of receipt. When constructing a TSR option, the
registrar computes the difference between the current time and the
time of receipt, which must always be in the past. This difference,
which should be a positive integer, is converted to seconds, and that
unsigned value is then used to synthesize the TSR option.
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6. Timeliness of Conflict Resolution
If a conflict exists in which some mDNS registrant(s) store stale
data, it is expected that the conflict is recent, and that it will be
resolved quickly since mDNS message exchanges with TSR options cause
removal of the stale data. Different hosts may be able to record
shorter or longer time offsets, depending on their implementation.
However, because of this expectation of recentness, mDNS registrars
should never need to report a TSR 'Time Offset' value of longer than
seven days. It’s reasonable to expect that every mDNS implementation
should be able to remember time intervals of at least seven days.
This maximum time interval is normatively defined in Section 2.
7. Legacy Behavior
mDNS registrars and queriers that do not support the TSR option are
expected to ignore the option, so they will behave as if no TSR
option was sent. This may result in such registrars temporarily
caching stale data. However, in the normal course of processing,
more recent data will win. In cases where it does not, the Reconfirm
process which is part of [RFC6762] already works to clear stale data:
since SRP registrars are expected to implement TSR, by the time a
Reconfirm is attempted, all authoritative stale data should have been
cleared.
8. When to Use TSR
There are no cases where using TSR is harmful, but in the case of
individual devices advertising individual services with mDNS, it may
be of little benefit. The reason for this is that when two devices
both claim the same name to use for advertising services, their
advertisements will be seen as in conflict whether or not TSR is
present. In this case, TSR does no harm, but most likely does not
help.
8.1. Use of TSR with redundant proxies
Use of TSR is most beneficial in the case of redundant mDNS proxies.
The reason for this is that such proxies tend to publish data that
could potentially produce name conflicts as a result of updates: when
one proxy is publishing an old version of the data and another proxy
is publishing a new version of the data, and these data differ, this
can appear as a name conflict and result in renaming. So for this
use case, the use of TSR is very beneficial.
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8.2. Use of TSR with multihomed devices
It can also happen that a multihomed device uses SRP to register when
connected to one sort of network, and uses mDNS to advertise the same
service when connected to another sort of network. For example, the
device may have capabilities to connect to a constrained network to
reduce power use, but also to occasionally connect to a WiFi network
either for backup or for bulk data transfers. In this situation we
have two potentially competing mDNS advertisements. The first is
that of the mDNS registrar on the multihomed device, which is
directly advertising its own service using mDNS. The second is the
one or more advertising proxies that are advertising services
registered via SRP using mDNS. Both datasets are coming from the
same source, and the advertising proxy will use TSR to identify the
source. To avoid spurious conflicts, a device that sometimes
registers with SRP and sometimes advertises with mDNS SHOULD use TSR
when advertising with mDNS.
8.3. TSR in networks with non-compliant mDNS caches
Some network infrastructure devices available commercially provide
"mDNS cache" services or "mDNS proxy" services that purport to allow
mDNS discovery across links that are separate at the IP layer and do
not share a multicast domain. These services generally cause mDNS to
become unreliable in various ways, and it would be helpful to be able
to use TSR to distinguish between stale data advertised by such
devices, and fresh data advertised directly from the device that is
authoritative for that data.
Unfortunately, for the most part these devices do not cache mDNS
packets, but rather cache data advertised using mDNS. As a result,
if such devices do not support TSR, we get no benefit from TSR.
That said, such devices SHOULD support TSR, and if they do this might
prevent some of their failure modes. What this would mean would be
that any DNS RRs cached or forwarded by such devices would be
accompanied by any TSR data applicable to them. When forwarded with
no time delay, the TSR data could be copied verbatim (following the
rules for generating packets with TSR given in Section 3.9).
When such RRs are cached and later resent, they would also be
accompanied by their TSR data. The mDNS cache service would need to
record the time at which they were received. When retransmitting
such cached data, the cache service would need to adjust the time
offset in the TSR option, increasing it by the subtracting the time
at which the cached RRs were received from the current time, and then
increasing the offset in the applicable TSR option by that amount, up
to the limit of 7 days.
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The use of such caches and forwarders is NOT RECOMMENDED. We mention
the use of TSR for such use cases because it can mitigate some of the
failure modes of such caches and forwarders. However, the use of
Discovery Proxy [RFC8766] in such instances is preferred: as the size
of the network covered by such caching and forwarding services
increases, the amount of mDNS traffic they create increases, and this
is often addressed by limiting which services are available for
discovery. Discovery Proxy uses unicast DNS, and therefore does not
suffer from these limitations.
9. Registrant API considerations
When a registrant registers a service at its mDNS registrar and
requests the use of a time of receipt, the registrant MUST also
specify when it received the original registration. In order to
support this, the API is required not only to allow the registrant to
specify that TSR conflict resolution is wanted, but also to provide a
way for the registrant to specify an absolute time at which the
original registration was received, and the key checksum used to
identify the entity that's actually authoritative for the data.
This is important, for example, in the case of SRP Replication
[I-D.ietf-dnssd-srp-replication], where an SRP registrar may receive
a registration from a peer during startup synchronization. This
registration will have occurred at some significant amount of time in
the past, and so it would be incorrect for the mDNS registrar
receiving the registration to use the time that the registrant
registers the service as the time of receipt.
9.1. Removing data that is still valid
In some cases, a proxy may need to stop being a proxy, but may be
proxying RRs that are also being proxied by one or more other
proxies. In this case, if the proxy sends a "goodbye" announcement
for such RRs, they will be removed from the caches of mDNS registrars
that receive such announcements.
To prevent this, an mDNS registrar implementation that implements TSR
MUST provide a way for an mDNS registrant to indicate that such data
is being withdrawn from publication by that registrant, but is still
valid. When the registrant indicates that this is the case, the mDNS
registrar MUST NOT send goodbye announcements for such data.
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9.2. Primary/Secondary indication
When more than one proxy is authoritative for a particular RR, this
can generate excessive answer traffic, and also redundant goodbye
announcements. mDNS registrar implementations that support TSR MUST
provide a way for a new proxy mDNS registrant to indicate that it is
primary or secondary. When an RR registered by a secondary proxy is
later removed, the mDNS registrant MUST NOT send a goodbye packet for
that RR.
Similarly, when an RR is registered by a registrant that indicates
that it is secondary, the mDNS registrar MUST NOT respond to the
initial mDNS query for that RR. Only if a second mDNS query
retransmission is received within 5 seconds of the initial query
reception, should it respond.
Note that any change of indication from primary to secondary, or vice
versa, while an mDNS registrant already has records registered on the
mDNS registrar is out of scope of this specification and an API to
indicate such changes is not required on the mDNS registrar.
10. Security Considerations
The TSR option is an optimization: it ameliorates an edge case for
mDNS proxies. A malicious host on the same link could use the TSR
option to win conflict resolution processes. However, because TSR is
only used by proxies, this technique will not work for normal mDNS
service registrations: in that case, normal mDNS conflict resolution
is done, and the attacker gains no benefit from using TSR.
Whether or not an mDNS registration has a recorded time of receipt,
an attacker can deny service by announcing its own conflicting data
and then answering the subsequent probe as described in Section 9 of
[RFC6762]. Because it does not include a TSR option in its authority
section, it can win the simultaneous conflict resolution process that
follows its bogus announcement.
So the TSR-based conflict resolution process creates no new
vulnerability. Addressing the existing vulnerability is out of scope
for this document. Protocols that rely on mDNS MUST NOT assume that
mDNS service is secure or private. If security (authentication,
authorization and/or secrecy) are needed, these must be provided at
the application layer, or by using DNSSEC rather than mDNS for
service discovery.
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11. IANA Considerations
IANA is requested to allocate a new OPT RR option code from the DNS
EDNS0 Option Codes (OPT) registry for the 'Time Since Received'
Option. The Name shall be 'mDNS-TSR'. The value shall be allocated
by IANA. The meaning shall be 'Multicast DNS Time Since Received".
Reference shall refer to this document, once published. IANA shall
determine the registration date.
12. References
12.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/rfc/rfc1034>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/rfc/rfc6762>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/rfc/rfc6891>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8766] Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
2020, <https://www.rfc-editor.org/rfc/rfc8766>.
[RFC9665] Lemon, T. and S. Cheshire, "Service Registration Protocol
for DNS-Based Service Discovery", RFC 9665,
DOI 10.17487/RFC9665, June 2025,
<https://www.rfc-editor.org/rfc/rfc9665>.
12.2. Informative References
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[I-D.ietf-dnssd-advertising-proxy]
Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD
Service Registration Protocol", Work in Progress,
Internet-Draft, draft-ietf-dnssd-advertising-proxy-04, 4
March 2024, <https://datatracker.ietf.org/doc/html/draft-
ietf-dnssd-advertising-proxy-04>.
[I-D.ietf-dnssd-srp-replication]
Lemon, T., Keshavarzian, A., and J. Hui, "Automatic
Replication of DNS-SD Service Registration Protocol
Zones", Work in Progress, Internet-Draft, draft-ietf-
dnssd-srp-replication-02, 4 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnssd-
srp-replication-02>.
Appendix A. Duplicate Answer Suppression Example
As described in Section 3.6, it is possible that two proxies may
respond to an mDNS query with answers to the same question, where the
data for a particular owner name in both answers is authoritative and
unique, but where the TSR time in the earlier message is earlier than
the TSR time in the later message. In this case, we will see the
data from the earlier message added and then removed, followed by the
data in the later message being added. Consider the following
example using Apple's mDNSResponder implementation.
1. mDNS requester A sends a multicast query for AAAA records on the
name "example.local"
2. mDNS registrar for proxy B sends a multicast response of
2001:db8:0:42::1 with TSR time T
3. mDNS registrar for proxy C sends a multicast response of
2001:db8:0:17::1 with TSR time T+300
Note that TSR times are absolute times, but these are represented in
the mDNS message as relative times, so for example "TSR time T+300"
when sent at time T+400 would be represented in the mDNS message as a
time offset of 100 seconds, and if the message in (2) were sent at
nearly the same time, it would have a time offset of about 400
seconds.
In between (2) and (3) we would expect mDNS Requester A to see a
callback to its DNSServiceQueryRecord call, providing a AAAA record
of 2001:db8:0:42::1 with the kDNSServiceFlagsAdd bit set and the
kDNSServiceFlagsMoreComing bit clear.
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After (3) we would expect mDNS Requester A to see two callbacks to
its DNSServiceQueryRecord call. The first would provide a AAAA
record of 2001:db8:0:42::1 with the kDNSServiceFlagsAdd clear and the
kDNSServiceFlagsMoreComing bit set. The second would provide a AAAA
record of 2001:db8:0:17::1 with the kDNSServiceFlagsAdd bit set and
the kDNSServiceFlagsMoreComing bit clear.
Note that we would not normally see the reverse sequence:
1. mDNS requester A sends a multicast query for AAAA records on the
name "example.local"
2. mDNS registrar for proxy C sends a multicast response of
2001:db8:0:17::1 with TSR time T+300
3. mDNS registrar for proxy B sends a multicast response of
2001:db8:0:42::1 with TSR time T
This is because in this case we would expect known answer suppression
to result in the mDNS registrar for proxy B suppressing its response.
However, it is possible that B might not see the response from C. In
that situation, A would receive both responses, but since the TSR
time on the second response is earlier than the TSR time on the first
response, we would see only one callback, providing an AAAA record of
2001:db8:0:17::1 with the kDNSServiceFlagsAdd bit set and the
kDNSServiceFlagsMoreComing bit clear.
Acknowledgments
TODO acknowledge reviewers and contributors.
Authors' Addresses
Ted Lemon
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: mellon@fugue.com
Esko Dijk
IoTconsultancy.nl
Utrecht
Netherlands
Email: esko.dijk@iotconsultancy.nl
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