wdiff draft-ietf-dnssd-push-05.txt draft-ietf-dnssd-push-06.txt

Internet Engineering Task Force T. Pusateri
Internet-Draft Seeking affiliation
Intended status: Standards Track S. Cheshire
Expires: August 1, September 22, 2016 Apple Inc.
January 29,
March 21, 2016

DNS Push Notifications
draft-ietf-dnssd-push-05
draft-ietf-dnssd-push-06

Abstract

The Domain Name System (DNS) was designed to return matching records
efficiently for queries for data that is relatively static. When
those records change frequently, DNS is still efficient at returning
the updated results when polled. But there exists no mechanism for a
client to be asynchronously notified when these changes occur. This
document defines a mechanism for a client to be notified of such
changes to DNS records, called DNS Push Notifications.

Status of This Memo

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

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

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

This Internet-Draft will expire on August 1, September 22, 2016.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5
4. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6
4.1. Client-Initiated Termination . . . . . . . . . . . . . . 7
4.2. Server-Initiated Termination . . . . . . . . . . . . . . 7
5. State Considerations . . . . . . . . . . . . . . . . . . . . 6 9
6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 7 10
6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 8 11
6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 10 13
6.3. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 13 18
6.4. DNS Push Notification Update Messages . . . . . . . . . . 14 19
6.5. DNS RECONFIRM . . . . . . . . . . . . . . . . . . . . . . 17 22
6.6. DNS Push Notification Termination Message . . . . . . . . 18 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19 26
7.1. Security Services . . . . . . . . . . . . . . . . . . . . 19 26
7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 20 27
7.3. TLS Compression . . . . . . . . . . . . . . . . . . . . . 20 27
7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 20 27
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 27
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21 28
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 28
10.1. Normative References . . . . . . . . . . . . . . . . . . 21 28
10.2. Informative References . . . . . . . . . . . . . . . . . 23 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 31

1. Introduction

DNS records may be updated using DNS Update [RFC2136]. Other
mechanisms such as a Hybrid Proxy [I-D.ietf-dnssd-hybrid] can also
generate changes to a DNS zone. This document specifies a protocol
for Unicast DNS clients to subscribe to receive asynchronous
notifications of changes to RRSets of interest. It is immediately
relevant in the case of DNS Service Discovery [RFC6763] but is not
limited to that use case case, and provides a general DNS mechanism for
DNS record change notifications. Familiarity with the DNS protocol
and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6195].

1.1. Requirements Language

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
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

2. Motivation

As the domain name system continues to adapt to new uses and changes
in deployment, polling has the potential to burden DNS servers at
many levels throughout the network. Other network protocols have
successfully deployed a publish/subscribe model to state changes
following the Observer design pattern. XMPP Publish-Subscribe
[XEP0060] and Atom [RFC4287] are examples. While DNS servers are
generally highly tuned and capable of a high rate of query/response
traffic, adding a publish/subscribe model for tracking changes to DNS
records can result in more timely notification of changes with
reduced CPU usage and lower network traffic.

Multicast DNS [RFC6762] implementations always listen on a well known
link-local IP multicast group, and new services and updates are sent
for all group members to receive. Therefore, Multicast DNS already
has asynchronous change notification capability. However, when DNS
Service Discovery [RFC6763] is used across a wide area network using
Unicast DNS (possibly facilitated via a Hybrid Proxy
[I-D.ietf-dnssd-hybrid]) it would be beneficial to have an equivalent
capability for Unicast DNS, to allow clients to learn about DNS
record changes in a timely manner without polling.

DNS Long-Lived Queries (LLQ) [I-D.sekar-dns-llq] is an existing
deployed solution to provide asynchronous change notifications. Even
though it can be used over TCP, LLQ is defined primarily as a UDP-
based protocol, and as such it defines its own equivalents of
existing TCP features like the three-way handshake. This document
builds on experience gained with the LLQ protocol, with an improved
design that uses long-lived TCP connections instead of UDP (and
therefore doesn't need to duplicate existing TCP functionality), and
adopts the syntax and semantics of DNS Update messages [RFC2136]
instead of inventing a new vocabulary of messages to communicate DNS
zone changes.

Because DNS Push Notifications impose a certain load on the
responding server (though less load than rapid polling of that
server) DNS Push Notification clients SHOULD exercise restraint in
issuing DNS Push Notification subscriptions. A subscription SHOULD
only be active when there is a valid reason to need live data (for
example, an on-screen display is currently showing the results of
that subscription to the user) and the subscription SHOULD be
cancelled as soon as the need for that data ends (for example, when
the user dismisses that display). Implementations MAY want to
implement idle timeouts, so that if the user ceases interacting with
the device, the display showing the result of the DNS Push
Notification subscription is automatically dismissed after a certain
period of inactivity. For example, if a user presses the "Print"
button on their phone, and then leaves the phone showing the printer
discovery screen until the phone goes to sleep, then the printer
discovery screen should be automatically dismissed as the device goes
to sleep. If the user does still intend to print, this will require
them to press the "Print" button again when they wake their phone up.

A DNS Push Notification client MUST NOT routinely keep a DNS Push
Notification subscription active 24 hours a day 7 days a week just to
keep a list in memory up to date so that it will be really fast if
the user does choose to bring up an on-screen display of that data.
DNS Push Notifications are designed to be fast enough that there is
no need to pre-load a "warm" list in memory just in case it might be
needed later.

3. Overview

The existing DNS Update protocol [RFC2136] provides a mechanism for
clients to add or delete individual resource records (RRs) or entire
resource record sets (RRSets) on the zone's server. Adopting this

This specification adopts a simplified subset of these existing
syntax and semantics semantics, and uses them for DNS Push Notifications allows for Notification
messages going in the other opposite direction, from server to client, to
communicate changes to a zone. The client first must subscribe subscribes for Push
Notifications by connecting to the server and sending DNS message(s)
indicating the RRSet(s) of interest. When the client loses interest
in updates to these records, it unsubscribes.

The DNS Push Notification server for a zone is any server capable
of generating the correct change notifications for a name.
It may be a master, slave, or stealth name server [RFC1996].
Consequently, the "_dns-push-tls._tcp.<zone>" SRV record for a <zone>
zone MAY reference the same target host and port as that zone's
"_dns-update-tls._tcp.<zone>" SRV record. When the same target host
and port is offered for both DNS Updates and DNS Push Notifications,
a client MAY use a single TCP connection to that server for both DNS
Updates, DNS Queries,
Updates and DNS Push Notification Queries.

Supporting DNS Updates and DNS Push Notifications on the same server
is OPTIONAL. A DNS Push Notification server is not REQUIRED to
support DNS Update.

DNS Updates and DNS Push Notifications may be handled on different
ports on the same target host, in which case they are not considered
to be the "same server" for the purposes of this specification, and
communications with these two ports are handled independently.

Standard DNS Queries MAY be sent over a DNS Push Notification
connection, provided that these are queries for names falling within
the server's zone (the <zone> in the "_dns-push-tls._tcp.<zone>" SRV
record). The RD (Recursion Desired) bit MUST be zero.

DNS Push Notification clients are NOT required to implement DNS
Update Prerequisite processing. Prerequisites are used to perform
tentative atomic test-and-set type operations when a client updates
records on the a server, and that concept has no application applicability when it
comes to an authoritative server informing a client of changes to DNS
records.

This DNS Push Notification specification includes support for DNS
classes, for completeness. However, in practice, it is anticipated
that for the foreseeable future the only DNS class in use will be DNS
class "IN", as it is today with existing DNS servers and clients. A
DNS Push Notification server MAY choose to implement only DNS class
"IN".

4. Transport

Implementations of DNS Update [RFC2136] MAY use either User Datagram
Protocol (UDP) [RFC0768] or Transmission Control Protocol (TCP)
[RFC0793] as the transport protocol, in keeping with the historical
precedent that DNS queries must first be sent over UDP [RFC1123].
This requirement to use UDP has subsequently been relaxed
[RFC5966][I-D.ietf-dnsop-5966bis]. Following that precendent,

In keeping with the more recent precedent, DNS Push Notification is
defined only for TCP. DNS Push Notification clients MUST use TLS
over TCP.

Either end of the TCP connection can terminate all of the
subscriptions on that connection by simply closing the connection
abruptly with a TCP FIN or RST. (An individual subscription is
terminated by sending an UNSUBSCRIBE message for that specific
subscription.)

If a client closes the connection, it is signaling that it is no
longer interested in receiving updates to any of the records it has
subscribed. It is informing the server that the server may release
all state information it has been keeping with regards to this
client. This may occur because the client computer has been
disconnected from the network, has gone to sleep, or the application
requiring the records has terminated.

If a server closes the connection, it is informing the client that it
can no longer provide updates for the subscribed records. This may
occur because the server application software or operating system is
restarting, the application terminated unexpectedly, the server is
undergoing maintenance procedures, or the server is overloaded and
can no longer provide the information to all the clients that wish to
receive it. The client can try to re-subscribe at a later time or
connect to another server supporting DNS Push Notifications for the
zone.

Connection setup over TCP ensures return reachability and alleviates
concerns of state overload at the server through anonymous
subscriptions. All subscribers are guaranteed to be reachable by the
server by virtue of the TCP three-way handshake. Because TCP SYN
flooding attacks are possible with any protocol over TCP,
implementers are encouraged to use industry best practices to guard
against such attacks [IPJ.9-4-TCPSYN] [RFC4953].

Transport Layer Security (TLS) [RFC5246] is well understood and
deployed across many protocols running over TCP. It is designed to
prevent eavesdropping, tampering, or message forgery. TLS is
REQUIRED for every connection between a client subscriber and server
in this protocol specification. Additional security measures such as
client authentication during TLS negotiation MAY also be employed to
increase the trust relationship between client and server.
Additional authentication of the SRV target using DNSSEC verification
and DANE TLSA records [RFC7673] is strongly encouraged. See below in
Section 7.2 for details.

5. State Considerations

Each

A DNS Push Notification server is capable of handling some finite
number of Push Notification subscriptions. This number will vary
from server to server and is based on physical machine
characteristics, network bandwidth, and operating system resource
allocation. After session begins with a client establishes a connection connecting to a
DNS server,
each record subscription is individually accepted or rejected.
Servers may employ various techniques to limit subscriptions to a
manageable level. Correspondingly, Push Notification server. Over that connection the client is free to establish
simultaneous connections to alternate DNS servers that support then
issues DNS
Push Notifications operation requests, such as SUBSCRIBE.

4.1. Client-Initiated Termination

An individual subscription is terminated by sending an UNSUBSCRIBE
message for the zone and distribute record that specific subscription, or all subscriptions
at its discretion. In this way, both clients and servers can react
to resource constraints. Token bucket rate limiting schemes are also
effective in providing fairness be
cancelled at once by a server across numerous client
requests.

6. Protocol Operation

A DNS Push Notification exchange begins with the client discovering closing the appropriate server, and then making a TLS/TCP connection to it.
The client may then add and remove Push Notification subscriptions
over this connection. In accordance with the current set of active
subscriptions the server sends relevant asynchronous Push
Notifications to the client. Note that a TCP
RST. When a client MUST be prepared to
receive (and silently discard) Push Notifications for terminates an individual subscription (via
UNSUBSCRIBE) or all subscriptions on that connection (by closing the
connection) it has previously removed, since there is no way signalling to prevent the
situation where a Push Notification is in flight from server to
client while the client's UNSUBSCRIBE message cancelling that
subscription it is simultaneously longer
interested in flight from client to server.

The exchange between client and server terminates when either end
closes receiving those particular updates. It is informing
the server that the server may release any state information it has
been keeping with regards to these particular subscriptions.

After terminating its last subscription on a connection via
UNSUBSCRIBE, a client MAY close the TCP connection immediately with a TCP FIN
FIN, or RST.

A it may keep it open if it anticipates performing further
operations on that connection in the future. If a client SHOULD NOT make multiple TLS/TCP connections wishes to
keep an idle connection open, it MUST meet its keepalive obligations
[I-D.ietf-dnsop-edns-tcp-keepalive] or the same DNS
Push Notification server. A server is entitled to
close the connection (see below).

If a client SHOULD share plans to terminate one or more subscriptions on a single TLS/TCP
connection for all requests and doesn't intend to keep that connection open, then as
an efficiency optimization it MAY instead choose to simply close the same DNS Push Notification server.
This shared
connection should be used for with a TCP RST, which implicitly terminates all DNS Queries and DNS
Push Notification Queries queries
subscriptions on that connection. This may occur because the client
computer is being shut down, is going to sleep, the application
requiring the subscriptions has terminated, or simply because the
last active subscription on that server, connection has been cancelled.

4.2. Server-Initiated Termination

If a client makes a connection and for then fails to send any DNS Update
requests too when the "_dns-update-tls._tcp.<zone>" SRV record
indicates message
that uses EDNS(0) TCP Keepalive [I-D.ietf-dnsop-edns-tcp-keepalive]
(either SUBSCRIBE, where Keepalive is implicit, or some other DNS
message, with an explicit an EDNS(0) TCP Keepalive option) then after
30 seconds of inactivity the same server also handles DNS Update requests.
This is to reduce unnecessary load SHOULD close the connection. If
no data has been sent on the DNS Push Notification
server.

For connection the purposes here, server MAY abort the determination of "same server" is made by
inspecting
connection with a TCP RST. If data has been sent on the target hostname and port, regardless of connection
then the name being
queried, or what zone if falls within. A given server may support
Push Notifications (and possibly DNS Updates too) for multiple DNS
zones. When SHOULD close the connection gracefully with a client discovers TCP FIN
so that the DNS Push Notification server
(and/or DNS Update server) for several different names (including
names that fall within different zones) data is reliably delivered.

In the same target hostname
and port, the client SHOULD use a single shared TCP connection for
all relevant operations on those names. A client SHOULD NOT open
multiple TCP connections response to the same target host and port just
because first successful SUBSCRIBE, the names being queried (or updated) happen to fall within
different zones.

Note included
EDNS(0) TCP Keepalive option specifies the idle timeout so that the "same server" determination described here is made
using
client knows the target hostname given in frequency of traffic it must generate to keep the SRV record, not
connection alive. If the IP
address(es) idle timeout for that connection changes,
then the hostname resolves to. If two different target
hostnames happen to resolve server communicates this by placing an updated EDNS(0) TCP
Keepalive option in a subsequent message to the same IP address(es), then client.

At both servers and clients, the
client SHOULD NOT recognize these as generation or reception of any
request, response, update, or keepalive message resets the "same server" keepalive
timer for that connection.

In the
purposes absence of using any requests, responses, or update messages on a single shared connection
connection, a client MUST generate keepalive traffic before the idle
timeout expires, or the server is entitled to that server. close the connection.

If an
administrator wishes to use a single client disconnects from the network abruptly, without closing
its connection, the server for multiple zones and/or
multiple roles (e.g., both DNS Push Notifications and DNS Updates),
and wishes learns of this after failing to have clients use receive
further traffic from that client. If no requests, responses, update
messages or keepalive traffic occurs on a single shared connection for
operations on that server, 1.5 times
the idle timeout, then this indicates that the administrator MUST use client is probably no
longer on the same
target hostname in network, and the appropriate SRV records.

However, a single client device may be home to multiple independent
client software instances that don't know about each other, so a DNS
Push Notification server MUST be prepared to accept multiple
connections from SHOULD abort the same client IP address. This is undesirable
from an efficiency stanpoint, but may be unavoidable in some
situations, so connection
with a DNS Push Notification server MUST be prepared TCP RST.

[We need to
accept multiple connections from discuss the same client IP address.

6.1. Discovery

The first step in DNS Push Notification subscription nature of "the required keepalives". Are
they TCP-layer keepalives? DNS-layer keepalives? There is to discover
an appropriate DNS server currently
no DNS-layer keepalive or 'no-op' operation defined. What would that supports
operation be? A DNS Push Notifications for QUERY containing zero questions? A DNS
SUBSCRIBE containing zero questions? An "empty" DNS message over the desired zone. The client MUST also determine which
TCP port on
the server connection (just a pair of zero bytes, signifying a zero-length
message)? One benefit of TCP-layer keepalives is listening that they transmit
fewer bytes, and involve less software overhead for connections, processing those
bytes. Another benefit is that it is more feasible to implement
these in networking offload hardware, which need not be (and often can allow devices to meet
their TCP keepalive obligations while sleeping. This is not) particularly
important for battery-powered devices like mobile phones and tablets.
On the typical TCP port 53 used other hand, using TCP-layer keepalives requires an API for conventional DNS.

1. The a
client begins to tell the discovery by sending a DNS query networking stack at what frequency to the
local resolver with record type SOA [RFC1035] perform TCP-
layer keepalives, and an API for the name of the
record it wishes a server to subscribe.

2. If request the SOA record exists, networking
stack to inform it MUST be returned in when TCP-layer keepalives are not received by the Answer
Section
required deadline. TCP-layer keepalives also only verify liveness of
the reply. If not, the server SHOULD include the SOA
record for the zone remote networking stack, whereas DNS-layer keepalives provide
higher assurance of liveness of the requested name in the Authority
Section.

3. If no SOA record remote server application
software -- though this a limited benefit, since there is returned, the client then strips off the
leading label from the requested name. If no reason
to expect that DNS Push Notification server software will routinely
become wedged and unresponsive.]

After sending an error response to a client, the resulting name has
at least one label in it, server MAY close the client sends
connection with a new SOA query and
processing continues at step 2 above. TCP FIN.

If the resulting name is
empty (the root label) then this server is a network configuration error overloaded and needs to shed load, it MAY send a
Termination Message to the client gives up. The client MAY retry and close the operation at connection with a
later time.

4. Once the SOA is known, TCP
FIN.

Apart from the client sends cases described above, a DNS query server MUST NOT close a
connection with type SRV
[RFC2782] for a DNS Push Notification client, except in
extraordinary error conditions. Closing the record name "_dns-push-tls._tcp.<zone>", where
<zone> connection is the owner name of the discovered SOA record.

5. If
client's responsibility, to be done at the zone in question does not offer client's discretion, when
it so chooses. A DNS Push Notifications
then SRV record MUST NOT exist and the SRV query will return Notification server only closes a
negative answer.

6. If the zone in question is set up to offer DNS Push Notifications
then this SRV record MUST exist. The SRV "target" contains the
name of
Notification connection under exceptional circumstances, such as when
the server providing DNS Push Notifications for application software or underlying operating system is
restarting, the zone.

The port number on which server application terminated unexpectedly (perhaps
due to contact a bug that makes it crash), or the server is in the SRV
record "port" field. The address(es) of the target host MAY be
included in the Additional Section, however, the address records undergoing
maintenance procedures. When possible, a DNS Push Notification
server SHOULD be authenticated before use as described below in
Section 7.2 [RFC7673].

7. More than one SRV record may be returned. In this case, the
"priority" and "weight" values in the returned SRV records are
used to determine send a Termination Message (Section 6.6 ) informing the order in which to contact
client of the servers reason for
subscription requests. As described in the SRV specification
[RFC2782], the server with the lowest "priority" is first
contacted. If more than one server has the same "priority", the
"weight" connection being closed.

After a connection is indicates closed by the weighted probability that server, the client
should contact SHOULD try to
reconnect, to that server. Higher weights have higher
probabilities of being selected. If a server is not reachable server, or
is not willing to accept a subscription request, then a
subsequent server is to be contacted.

If a another server closes a supporting DNS Push Notification subscription connection,
the client SHOULD repeat
Notifications for the discovery process in order zone. If reconnecting to determine the preferred DNS server for subscriptions at that time.

6.2. same server, and
there was a Termination Message or error response containing a
EDNS(0) TCP Keepalive option, the client MUST respect the indicated
delay before attempting to reconnect.

5. State Considerations

Each DNS Push Notification SUBSCRIBE

A DNS server is capable of handling some finite
number of Push Notification client indicates its desire subscriptions. This number will vary
from server to receive DNS
Push Notifications for server and is based on physical machine
characteristics, network bandwidth, and operating system resource
allocation. After a given domain name by sending client establishes a SUBSCRIBE
request over the established TCP connection to the server. A
SUBSCRIBE request a DNS server,
each record subscription is formatted identically individually accepted or rejected.
Servers may employ various techniques to limit subscriptions to a conventional DNS
QUERY request [RFC1035], except that
manageable level. Correspondingly, the opcode is SUBSCRIBE (6)
instead of QUERY (0). If neither QTYPE nor QCLASS are ANY (255) then
this client is a specific subscription free to changes establish
simultaneous connections to alternate DNS servers that support DNS
Push Notifications for the given name, type zone and class. If one or distribute record subscriptions
at its discretion. In this way, both of QTYPE or QCLASS clients and servers can react
to resource constraints. Token bucket rate limiting schemes are ANY (255) then this
subscription matches any type and/or any class, as appropriate.

In also
effective in providing fairness by a SUBSCRIBE request the server across numerous client
requests.

6. Protocol Operation

A DNS Header QR bit MUST be zero.
If Push Notification exchange begins with the QR bit is not zero client discovering
the message is not appropriate server, and then making a SUBSCRIBE request.

The AA, TC, TLS/TCP connection to it.
The client may then add and remove Push Notification subscriptions
over this connection. In accordance with the current set of active
subscriptions the server sends relevant asynchronous Push
Notifications to the client. Note that a client MUST be prepared to
receive (and silently ignore) Push Notifications for subscriptions it
has previously removed, since there is no way to prevent the
situation where a Push Notification is in flight from server to
client while the client's UNSUBSCRIBE message cancelling that
subscription is simultaneously in flight from client to server.

The exchange between client and server terminates when either end
closes the TCP connection with a TCP FIN or RST.

A client SHOULD NOT make multiple TLS/TCP connections to the same DNS
Push Notification server. A client SHOULD share a single TLS/TCP
connection for all requests to the same DNS Push Notification server.
This shared connection should be used for all DNS Queries and DNS
Push Notification Queries queries to that server, and for DNS Update
requests too when the "_dns-update-tls._tcp.<zone>" SRV record
indicates that the same server also handles DNS Update requests.
This is to reduce unnecessary load on the DNS Push Notification
server.

For the purposes here, the determination of "same server" is made by
inspecting the target hostname and port, regardless of the name being
queried, or what zone if falls within. A given server may support
Push Notifications (and possibly DNS Updates too) for multiple DNS
zones. When a client discovers that the DNS Push Notification server
(and/or DNS Update server) for several different names (including
names that fall within different zones) is the same target hostname
and port, the client SHOULD use a single shared TCP connection for
all relevant operations on those names. A client SHOULD NOT open
multiple TCP connections to the same target host and port just
because the names being queried (or updated) happen to fall within
different zones.

Note that the "same server" determination described here is made
using the target hostname given in the SRV record, not the IP
address(es) that the hostname resolves to. If two different target
hostnames happen to resolve to the same IP address(es), then the
client SHOULD NOT recognize these as the "same server" for the
purposes of using a single shared connection to that server. If an
administrator wishes to use a single server for multiple zones and/or
multiple roles (e.g., both DNS Push Notifications and DNS Updates),
and wishes to have clients use a single shared connection for
operations on that server, then the administrator MUST use the same
target hostname in the appropriate SRV records.

However, server implementers and operators should be aware that this
connection sharing may not be possible in all cases. A single client
device may be home to multiple independent client software instances
that don't know about each other, so a DNS Push Notification server
MUST be prepared to accept multiple connections from the same client
IP address. This is undesirable from an efficiency standpoint, but
may be unavoidable in some situations, so a DNS Push Notification
server MUST be prepared to accept multiple connections from the same
client IP address.

6.1. Discovery

The first step in DNS Push Notification subscription is to discover
an appropriate DNS server that supports DNS Push Notifications for
the desired zone. The client MUST also determine which TCP port on
the server is listening for connections, which need not be (and often
is not) the typical TCP port 53 used for conventional DNS.

1. The client begins the discovery by sending a DNS query to the
local resolver with record type SOA [RFC1035] for the name of the
record it wishes to subscribe.

2. If the SOA record exists, it MUST be returned in the Answer
Section of the reply. If not, the local resolver SHOULD include
the SOA record for the zone of the requested name in the
Authority Section.

3. If no SOA record is returned, the client then strips off the
leading label from the requested name. If the resulting name has
at least one label in it, the client sends a new SOA query and
processing continues at step 2 above. If the resulting name is
empty (the root label) then this is a network configuration error
and the client gives up. The client MAY retry the operation at a
later time.

4. Once the SOA is known, the client sends a DNS query with type SRV
[RFC2782] for the record name "_dns-push-tls._tcp.<zone>", where
<zone> is the owner name of the discovered SOA record.

5. If the zone in question does not offer DNS Push Notifications
then SRV record MUST NOT exist and the SRV query will return a
negative answer.

6. If the zone in question is set up to offer DNS Push Notifications
then this SRV record MUST exist. The SRV "target" contains the
name of the server providing DNS Push Notifications for the zone.
The port number on which to contact the server is in the SRV
record "port" field. The address(es) of the target host MAY be
included in the Additional Section, however, the address records
SHOULD be authenticated before use as described below in
Section 7.2 [RFC7673].

7. More than one SRV record may be returned. In this case, the
"priority" and "weight" values in the returned SRV records are
used to determine the order in which to contact the servers for
subscription requests. As described in the SRV specification
[RFC2782], the server with the lowest "priority" is first
contacted. If more than one server has the same "priority", the
"weight" indicates the weighted probability that the client
should contact that server. Higher weights have higher
probabilities of being selected. If a server is not reachable or
is not willing to accept a subscription request, then a
subsequent server is to be contacted.

Each time a client makes a new DNS Push Notification subscription
connection, it SHOULD repeat the discovery process in order to
determine the preferred DNS server for subscriptions at that time.

6.2. DNS Push Notification SUBSCRIBE

A DNS Push Notification client indicates its desire to receive DNS
Push Notifications for a given domain name by sending a SUBSCRIBE
request over the established TCP connection to the server. A
SUBSCRIBE request is formatted identically to a conventional DNS
QUERY request [RFC1035], except that the opcode is SUBSCRIBE (6)
instead of QUERY (0). If neither QTYPE nor QCLASS are ANY (255) then
this is a specific subscription to changes for the given name, type
and class. If one or both of QTYPE or QCLASS are ANY (255) then this
subscription matches any type and/or any class, as appropriate.

In a SUBSCRIBE request the DNS Header QR bit MUST be zero.
If the QR bit is not zero the message is not a SUBSCRIBE request.

The AA, TC, RD, RA, Z, AD, and CD bits, the ID field, and the RCODE
field, MUST be zero on transmission, and MUST be silently ignored on
reception.

Like a DNS QUERY request, a SUBSCRIBE request MUST contain exactly
one question. Since SUBSCRIBE requests are sent over TCP, multiple
SUBSCRIBE requests can be concatenated in a single TCP stream and
packed efficiently into TCP segments, so the ability to pack multiple
SUBSCRIBE operations into a single DNS message within that TCP stream
would add extra complexity for little benefit.

ANCOUNT MUST be zero, and the Answer Section MUST be empty.
Any records in the Answer Section MUST be silently ignored.

NSCOUNT MUST be zero, and the Authority Section MUST be empty.
Any records in the Authority Section MUST be silently ignored.

ARCOUNT MUST be zero, and Authority Section MUST be silently ignored.

ARCOUNT specifies the number of records in the Additional Data
Section. Typically this is zero, but it may be nonzero in some
cases, such as when the request includes an EDNS(0) OPT record.

If accepted, the subscription will stay in effect until the client
revokes the subscription or until the connection between the client
and the server is closed.

SUBSCRIBE requests on a given connection MUST be unique. A client
MUST NOT send a SUBSCRIBE message that duplicates the name, type and
class of an existing active subscription on that TLS/TCP connection.
For the purpose of this matching, the established DNS case-
insensitivity for US-ASCII letters applies (e.g., "foo.com" and
"Foo.com" are the same). If a server receives such a duplicate
SUBSCRIBE message this is an error and the server MUST immediately
close the TCP connection.

DNS wildcarding is not supported. That is, a wildcard ("*") in a
SUBSCRIBE message matches only a literal wildcard character ("*") in
the zone, and nothing else.

Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message
matches only a literal CNAME record in the zone, and nothing else.

A client may SUBSCRIBE to records that are unknown to the server at
the time of the request (providing that the name falls within one of
the zone(s) the Additional Section server is responsible for) and this is not an error.
The server MUST be empty.
Any records accept these requests and send Push Notifications if
and when matches are found in the Additional Section future.

Since all SUBSCRIBE operations are implicitly long-lived operations,
the server MUST be silently ignored.

Each interpret a SUBSCRIBE request generates exactly one SUBSCRIBE response from as if it contained an
EDNS(0) TCP Keepalive option [I-D.ietf-dnsop-edns-tcp-keepalive]. A
client MUST NOT include an actual EDNS(0) TCP Keepalive option in the server.

In
request, since it is automatic, and implied by the semantics of
SUBSCRIBE. If a server receives a SUBSCRIBE response request that does
contain an actual EDNS(0) TCP Keepalive option this is an error and
the RCODE indicates whether or not server MUST immediately close the
subscription was accepted. Supported RCODEs are as follows:

A SUBSCRIBE successful |
| FORMERR | 1 | Server failed to process request due operation MAY include an explicit EDNS(0) [RFC6891] OPT
record where necessary to carry additional information.

Table 1: Response codes generates exactly one SUBSCRIBE response from
the server.

In a SUBSCRIBE response the DNS Header QR bit MUST be one.
If the QR bit is not one the message is not a SUBSCRIBE response.

The AA, TC, RD, RA, Z, AD, and CD bits, and the ID field, MUST be
zero on transmission, and MUST be silently ignored on reception.

The Question Section MUST echo back the values provided by the client
in the SUBSCRIBE request that generated this SUBSCRIBE response.

ANCOUNT MUST be zero, and the Answer Section MUST be empty.
Any records in the Answer Section MUST be silently ignored.
If the subscription was accepted and there are positive answers for
the requested name, type and class, then these positive answers MUST
be communicated to the client in an immediately following Push
Notification Update, not in the Answer Section of the SUBSCRIBE
response. This simplifying requirement is made so that there is only
a single way that information is communicated to a DNS Push
Notification client. Since a DNS Push Notification client has to
parse information received via Push Notification Updates anyway, it
is simpler if it does not also have to parse information received via
the Answer Section of a SUBSCRIBE response.

NSCOUNT MUST be zero, and the Authority Section MUST be empty.
Any records in the Authority Section MUST be silently ignored.

ARCOUNT MUST be zero, and specifies the Additional Section MUST be empty.

Any number of records in the Additional Section MUST be silently ignored.

If accepted, Data
Section, e.g., the subscription will stay in effect until EDNS(0) OPT record.

In the client
revokes SUBSCRIBE response the subscription RCODE indicates whether or until not the connection between
subscription was accepted. Supported RCODEs are as follows:

SUBSCRIBE Response codes

This document specifies only these RCODE values for SUBSCRIBE
Responses. Servers sending SUBSCRIBE Responses SHOULD use one of
these values. However, future circumstances may create situations
where other RCODE values are appropriate in SUBSCRIBE Responses, so
clients MUST be prepared to accept SUBSCRIBE Responses with any RCODE
value.

In the server is closed. first SUBSCRIBE requests response on a given connection MUST be unique. A client
MUST NOT send a SUBSCRIBE message that duplicates connection, the name, type and
class of server MUST
include an existing active subscription on that TLS/TCP connection.
For the purpose of this matching, the established DNS case-
insensitivity for US-ASCII letters applies (e.g., "foo.com" and
"Foo.com" are the same). explicit EDNS(0) TCP Keepalive option. If a server receives such a duplicate the first
SUBSCRIBE message response does not include an explicit EDNS(0) TCP Keepalive
option this is an error and the server client MUST immediately close the TCP
connection.

DNS wildcarding is not supported. That is, In this case the client should act as if the response
contained an EDNS(0) TCP Keepalive option with a wildcard ("*") in value of one hour,
and not attempt any further DNS Push Notification requests to that
server until one hour has passed. This situation may occur if a
SUBSCRIBE message matches only
client connects to a wildcard ("*") in the zone, server that doesn't implement DNS Push
Notifications at all, and
nothing else.

Aliasing it is important not supported. That is, a CNAME in a SUBSCRIBE message
matches only a CNAME to burden such servers
with continuous retries.

The server MAY include EDNS(0) TCP Keepalive options in subsequent
messages, if the idle timeout changes. If the client receives
subsequent messages that do not contain an explicit EDNS(0) TCP
Keepalive option then the idle timeout for that connection remains
unchanged at that time.

In an error response, with nonzero RCODE, the server MUST contain an
EDNS(0) TCP Keepalive option specifying the delay before the zone, and nothing else.

A client
tries again:

For RCODE = 1 (FORMERR) the delay may SUBSCRIBE be any value selected by the
implementer. A value of one minute is RECOMMENDED, to records that are unknown avoid high
load from defective clients.

For RCODE = 2 (SERVFAIL), which occurs due to resource exhaustion,
the server at delay should be chosen according to the time level of server
overload and the request (providing anticipated duration of that the name falls within one overload. By
default, a value of
the zone(s) the server one minute is responsible for) and this RECOMMENDED.

For RCODE = 4 (NOTIMP), which occurs on a server that doesn't
implement DNS Push Notifications, it is not an error.
The unlikely that the server MUST accept these requests and send
will begin supporting DNS Push Notifications if
and when matches are found in the future.

Since all SUBSCRIBE operations are implicitly long-lived operations, next few
minutes, so the server MUST interpret retry delay SHOULD be one hour. Note that a SUBSCRIBE request as if it contained an
EDNS0
server that doesn't implement DNS Push Notifications will most
likely not implement this retry delay mechanism using the EDNS(0)
TCP Keepalive option [I-D.ietf-dnsop-edns-tcp-keepalive]. A either, and in this case the client MUST NOT include will fall
back to the case described above specifying how to handle
SUBSCRIBE responses that do not contain an actual EDNS0 EDNS(0) TCP Keepalive option in
option.

For RCODE = 5 (REFUSED), which occurs on a server that implements
DNS Push Notifications, but is currently configured to disallow
DNS Push Notifications, the
request, retry delay may be any value selected
by the implementer and/or configured by the operator.
This is a misconfiguration, since this server is listed in a
"_dns-push-tls._tcp.<zone>" SRV record, but the server itself is
not currently configured to support DNS Push Notifications. Since
it is automatic, and implied by possible that the semantics misconfiguration may be repaired at any
time, the retry delay should not be set too high. By default, a
value of
SUBSCRIBE. If 5 minutes is RECOMMENDED.

For RCODE = 9 (NOTAUTH), which occurs on a server receives a SUBSCRIBE request that does
contain an actual EDNS0 TCP Keepalive option this implements
DNS Push Notifications, but is an error and not configured to be authoritative
for the
server MUST immediately close requested name, the TCP connection. In retry delay may be any value selected
by the implementer and/or configured by the operator.
This is a misconfiguration, since this server is listed in a SUBSCRIBE
response
"_dns-push-tls._tcp.<zone>" SRV record, but the server MUST include an EDNS0 TCP Keepalive option
specifying the idle timeout so itself is
not currently configured to support DNS Push Notifications for
that zone. Since it is possible that the client knows misconfiguration may be
repaired at any time, the frequency retry delay should not be set too high.
By default, a value of
keepalives it must generate to keep 5 minutes is RECOMMENDED.

For other RCODE values, the connection alive. If retry delay should be set by the
client receives a SUBSCRIBE response
server as appropriate for that does not contain an EDNS0
TCP Keepalive option this error condition. By default, a
value of 5 minutes is RECOMMENDED.

After sending an error and response the client MUST immediately server MAY close the TCP connection.
connection with a FIN, or MAY allow it to remain open. Clients MUST
correctly handle both cases.

6.3. DNS Push Notification UNSUBSCRIBE

To cancel an individual subscription without closing the entire
connection, the client sends an UNSUBSCRIBE message over the
established TCP connection to the server. The UNSUBSCRIBE message is
formatted identically to the SUBSCRIBE message which created the
subscription, with the exact same name, type and class, except that
the opcode is UNSUBSCRIBE (7) instead of SUBSCRIBE (6).

A client MUST NOT send an UNSUBSCRIBE message that does not exactly
match the name, type and class of an existing active subscription on
that TLS/TCP connection. If a server receives such an UNSUBSCRIBE
message this is an error and the server MUST immediately close the
connection.

No response message is generated as a result of processing an
UNSUBSCRIBE message.

Having being successfully revoked with a correctly-formatted
UNSUBSCRIBE message, the previously referenced subscription is no
longer active and the server MAY discard the state associated with it
immediately, or later, at the server's discretion.

6.4. DNS Push Notification Update Messages

Once a subscription has been successfully established, the server
generates Push Notification Updates to send to the client as
appropriate. An initial Push Notification Update will be sent
immediately in the case that the answer set was non-empty at the
moment the subscription was established. Subsequent changes to the
answer set are then communicated to the client in subsequent Push
Notification Updates.

The format of Push Notification Updates borrows from the existing DNS
Update [RFC2136] protocol, with some simplifications.

The following figure shows the existing DNS Update header format:

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode | Z | RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ZOCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PRCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| UPCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ADCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Figure 1

For DNS Push Notifications the following rules apply:

The QR bit MUST be zero, and the Opcode MUST be UPDATE (5).
Messages received where this is not true are not Push Notification
Update Messages and should be silently ignored for the purposes of
Push Notification Update Message handling.

ID, the Z bits, and RCODE MUST be zero on transmission,
and MUST be silently ignored on reception.

ZOCOUNT MUST be zero, and the Zone Section MUST be empty.
Any records in the Zone Section MUST be silently ignored.

PRCOUNT MUST be zero, and the Prerequisite Section MUST be empty.
Any records in the Prerequisite Section MUST be silently ignored.

UPCOUNT specifies the number of records in the Update Section.

ADCOUNT MUST be zero, and specifies the Additional Data Section MUST be empty.
Any number of records in the Additional Data Section MUST
Section. Typically this is zero, but it may be silently ignored. nonzero in some
cases, such as when the message includes an EDNS(0) OPT record.

The Update Section contains the relevant change information for the
client, formatted identically to a DNS Update [RFC2136]. To recap:

Delete all RRsets from a name:
TTL=0, CLASS=ANY, RDLENGTH=0, TYPE=ANY.

Delete an RRset from a name:
TTL=0, CLASS=ANY, RDLENGTH=0;
TYPE specifies the RRset being deleted.

Delete an individual RR from a name:
TTL=0, CLASS=NONE;
TYPE, RDLENGTH and RDATA specifies the RR being deleted.

Add an individual RR to a name: an RRset:
TTL, CLASS, TYPE, RDLENGTH and RDATA specifies the RR being added.

Upon reception of

When processing the records received in a Push Notification Update
Message, the client receiving the message client MUST validate that the records being
added or deleted correspond with at least one currently active
subscription on that connection. Specifically, the record name MUST
match the name given in the SUBSCRIBE request, subject to the usual
established DNS case-insensitivity for US-ASCII letters. If the
QTYPE in the SUBSCRIBE request was not ANY (255) then the TYPE of the
record must match the QTYPE given in the SUBSCRIBE request. If the
QCLASS in the SUBSCRIBE request was not ANY (255) then the CLASS of
the record must match the QCLASS given in the SUBSCRIBE request. If
a matching active subscription on that connection is not found, then
that individual record addition/deletion is silently ignored.
Processing of other additions and deletions in this message is not
affected. The TCP connection is not closed. This is to allow for
the race condition where a client sends an outbound UNSUBSCRIBE while
inbound Push Notification Updates for that subscription from the
server are still in flight.

In the case where a single change affects more than one active
subscription, only one update is sent. For example, an update adding
a given record may match both a SUBSCRIBE request with the same QTYPE
and a different SUBSCRIBE request with QTYPE=ANY. It is not the case
that two updates are sent because the new record matches two active
subscriptions.

The server SHOULD encode change notifications in the most efficient
manner possible. For example, when three AAAA records are deleted
from a given name, and no other AAAA records exist for that name, the
server SHOULD send a "delete an RRset from a name" update, not three
separate "delete an individual RR from a name" updates. Similarly,
when both an SRV and a TXT record are deleted from a given name, and
no other records of any kind exist for that name, the server SHOULD
send a "delete all RRsets from a name" update, not two separate
"delete an RRset from a name" updates.

All

A server SHOULD combine multiple change notifications in a single
Update Message when possible, even if those change notifications
apply to different subscriptions. Conceptually, a Push Notification
Update Message is a connection-level concept, not a subscription-
level concept.

Push Notification Update Messages MUST MAY contain an EDNS0 EDNS(0) TCP
Keepalive option [I-D.ietf-dnsop-edns-tcp-keepalive] specifying if the idle
timeout so has changed since the last time the server sent an EDNS(0)
TCP Keepalive option on this connection.

In the event that the server wishes to inform a client knows of a new idle
timeout for the connection, the server MAY combine that with the frequency of keepalives next
message it
must generate sends to keep the connection alive. If client, or the client receives a server MAY send an empty Push
Notification Update Message that does not contain an EDNS0 (zero records in the Update Section) to
carry the EDNS(0) TCP Keepalive option this is an error and the client option. Clients MUST immediately
close correctly
receive and process the EDNS(0) TCP connection. Keepalive option in both cases.

Reception of a Push Notification Update Message results in no does not directly
generate a response back to the server. (Updates may indirectly
generate other operations; e.g., a Push Notification Update Message
declaring the appearance of a PTR record could lead to a query for
the SRV record named in the rdata of that PTR record[RFC6763].

The TTL of an added record is stored by the client and decremented as
time passes, with the caveat that for as long as a relevant
subscription is active, the TTL does not decrement below 1 second.
For as long as a relevant subscription remains active, the client
SHOULD assume that when a record goes away the server will notify it
of that fact. Consequently, a client does not have to poll to verify
that the record is still there. Once a subscription is cancelled
(individually, or as a result of the TCP connection being closed)
record aging resumes and records are removed from the local cache
when their TTL reaches zero.

6.5. DNS RECONFIRM

Sometimes, particularly when used with a Hybrid Proxy
[I-D.ietf-dnssd-hybrid], a DNS Zone may contain stale data. When a
client encounters data that it believe may be stale (e.g., an SRV
record referencing a target host+port that is not responding to
connection requests) the client sends a DNS RECONFIRM message to
request that the server re-verify that the data is still valid. For
a Hybrid Proxy, this causes it to issue new Multicast DNS requests to
ascertain whether the target device is still present. For other
kinds of DNS server the RECONFIRM operation is currently undefined
and should SHOULD be sliently silently ignored.

A RECONFIRM request is formatted similarly to a conventional DNS
QUERY request [RFC1035], except that the opcode is RECONFIRM (8)
instead of QUERY (0). QTYPE MUST NOT be the value ANY (255). QCLASS
MUST NOT be the value ANY (255).

In a RECONFIRM request the DNS Header QR bit MUST be zero.
If the QR bit is not zero the message is not a RECONFIRM request.

The AA, TC, RD, RA, Z, AD, and CD bits, the ID field, and the RCODE
field, MUST be zero on transmission, and MUST be silently ignored on
reception.

Like a DNS QUERY request, a RECONFIRM request MUST contain exactly
one question. Since RECONFIRM requests are sent over TCP, multiple
RECONFIRM requests can be concatenated in a single TCP stream and
packed efficiently into TCP segments, so the ability to pack multiple
RECONFIRM operations into a single DNS message within that TCP stream
would add extra complexity for little benefit.

ANCOUNT MUST be nonzero, and the Answer Section MUST contain the
rdata for the record(s) that the client believes to be in doubt.

NSCOUNT MUST be zero, and the Authority Section MUST be empty.
Any records in the Authority Section MUST be silently ignored.

ARCOUNT MUST be zero, and specifies the Additional Section MUST be empty.
Any number of records in the Additional Section MUST Data
Section. Typically this is zero, but it may be silently ignored. nonzero in some
cases, such as when the request includes an EDNS(0) OPT record.

DNS wildcarding is not supported. That is, a wildcard ("*") in a
SUBSCRIBE message matches only a wildcard ("*") in the zone, and
nothing else.

Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message
matches only a CNAME in the zone, and nothing else.

No response message is generated as a result of processing a
RECONFIRM message.

If the server receiving the RECONFIRM request determines that the
records are in fact no longer valid, then subsequent DNS Push
Notification Update Messages will be generated to inform interested
clients. Thus, one client discovering that a previously-advertised
printer is no longer present has the side effect of informing all
other interested clients that the printer in question is now gone.

6.6. DNS Push Notification Termination Message

If a server is low on resources it MAY simply terminate a client
connection with a TCP RST. However, the likely behavour behaviour of the
client may be simply to reconnect immediately, putting more burden on
the server. Therefore, a server MAY SHOULD instead choose to shed client
load by (a) sending a DNS Push Notification Termination Message and
then (b) immediately closing the client connection with a TCP FIN
instead of RST, thereby facilitating reliable delivery of the
Termination Message.

The format of a Termination Message is similar to a Push Notification
Update.

The following figure shows the existing DNS Update header format:

Figure 2

For Termination Messages the following rules apply:

The QR bit MUST be zero, and the Opcode MUST be UPDATE (5).
Messages received where this is not true are not Termination Messages
and should be silently ignored.

ID and the Z bits MUST be zero on transmission,
and MUST be silently ignored on reception.

ZOCOUNT MUST be zero, and the Zone Section MUST be empty.
Any records in the Zone Section MUST be silently ignored.

PRCOUNT MUST be zero, and the Prerequisite Section MUST be empty.
Any records in the Prerequisite Section MUST be silently ignored.

UPCOUNT the Prerequisite Section MUST be silently ignored.

UPCOUNT MUST be zero, and the Update Section MUST be empty.
Any records in the Update Section MUST be silently ignored.

ADCOUNT specifies the number of records in the Additional Data
Section, e.g., the EDNS(0) OPT record..

The RCODE MUST contain a nonzero code giving the reason for
termination, as indicated below:

Termination Response codes

This document specifies only these two RCODE values for Termination
Messages. Servers sending Termination Messages SHOULD use one of
these two values. However, future circumstances may create
situations where other RCODE values are appropriate in Termination
Messages, so clients MUST be zero, prepared to accept Termination Messages
with any RCODE value. In particular, a Termination Message with
RCODE value zero (NOERROR) is still a Termination Message and the Update Section MUST should
be empty.
Any records in the Update Section treated as such.

The Termination Message MUST be silently ignored.

ADCOUNT contain an EDNS(0) TCP Keepalive option
[I-D.ietf-dnsop-edns-tcp-keepalive]. The client MUST be zero, and wait for the Additional Data Section MUST be empty.
Any records
time indicated in the Additional Data Section MUST be silently ignored.

The RCODE MUST contain a code giving the reason for termination.
[Codes EDNS(0) TCP Keepalive option's idle timeout
before attempting any new connections to be determined.] The this server. A client that
receives a Termination Message MUST contain without an
EDNS0 EDNS(0) TCP Keepalive
option [I-D.ietf-dnsop-edns-tcp-keepalive] SHOULD treat it as equivalent to a TCP Keepalive option with a
zero timeout value.

In the case where the server is rejecting some, but not all, of the
existing subscriptions (perhaps because it has been reconfigured and
is no longer authoritative for those names) with a REFUSED (5) RCODE,
the EDNS(0) TCP Keepalive option's idle timeout indicates the time MAY be zero,
indicating that the client SHOULD wait before
attempting attempt to reconnect. re-establish its
subscriptions immediately.

In the case where a server is terminating a large number of
connections at once (e.g., if the system is restarting) and the
server doesn't want to be inundated with a flood of simultaneous
retries, it SHOULD send different EDNS(0) TCP Keepalive values to
each client. These adjustments MAY be selected randomly,
pseudorandomly, or deterministically (e.g., incrementing the time
value by one for each successive client, yielding a post-restart
reconnection rate of ten clients per second).

7. Security Considerations

TLS support is REQUIRED in DNS Push Notifications. There is no
provision for opportunistic encryption using a mechanism like
"STARTTLS".

DNSSEC is RECOMMENDED for DNS Push Notifications. TLS alone does not
provide complete security. TLS certificate verification can provide
reasonable assurance that the client is really talking to the server
associated with the desired host name, but since the desired host
name is learned via a DNS SRV query, if the SRV query is subverted
then the client may have a secure connection to a rogue server.
DNSSEC can provided added confidence that the SRV query has not been
subverted.

7.1. Security Services

It is the goal of using TLS to provide the following security
services:

Confidentiality

Confidentiality: All application-layer communication is encrypted
with the goal that no party should be able to decrypt it except
the intended receiver.

Data integrity protection protection: Any changes made to the communication in
transit are detectable by the receiver.

Authentication

Authentication: An end-point of the TLS communication is
authenticated as the intended entity to communicate with.

Deployment recommendations on the appropriate key lengths and cypher
suites are beyond the scope of this document. Please refer to TLS
Recommendations [RFC7525] for the best current practices. Keep in
mind that best practices only exist for a snapshot in time and
recommendations will continue to change. Updated versions or errata
may exist for these recommendations.

7.2. TLS Name Authentication

As described in Section 6.1, the client discovers the DNS Push
Notification server using an SRV lookup for the record name
"_dns-push-tls._tcp.<zone>". The server connection endpoint SHOULD
then be authenticated using DANE TLSA records for the associated SRV
record. This associates the target's name and port number with a
trusted TLS certificate [RFC7673]. This procedure uses the TLS Sever
Name Indication (SNI) extension [RFC6066] to inform the server of the
name the client has authenticated through the use of TLSA records.
Therefore, if the SRV record passes DNSSEC validation and a TLSA
record matching the target name is useable, an SNI extension MUST be
used for the target name to ensure the client is connecting to the
server it has authenticated. If the target name does not have a
usable TLSA record, then the use of the SNI extension is optional.

7.3. TLS Compression

In order to reduce the chances of compression related attacks, TLS-
level compression SHOULD be disabled when using TLS versions 1.2 and
earlier. In the draft version of TLS 1.3 [I-D.ietf-tls-tls13], TLS-
level compression has been removed completely.

7.4. TLS Session Resumption

TLS Session Resumption is permissible on DNS Push Notification
servers. The server may keep TLS state with Session IDs [RFC5246] or
operate in stateless mode by sending a Session Ticket [RFC5077] to
the client for it to store. However, once the connection is closed,
any existing subscriptions will be dropped. When the TLS session is
resumed, the DNS Push Notification server will not have any
subscription state and will proceed as with any other new connection.
Use of TLS Session Resumption allows a new TLS connection to be set
up more quickly, but the client will still have to recreate any
desired subscriptions.

8. IANA Considerations

This document defines the service name: "_dns-push-tls._tcp".
It is only applicable for the TCP protocol.
This name is to be published in the IANA Service Name Registry.

This document defines three DNS OpCodes: SUBSCRIBE with (tentative)
value 6, UNSUBSCRIBE with (tentative) value 7, and RECONFIRM with
(tentative) value 8.

9. Acknowledgements

The authors would like to thank Kiren Sekar and Marc Krochmal for
previous work completed in this field.

This draft has been improved due to comments from Ran Atkinson, Tim
Chown, Mark Delany, Ralph Droms, Bernie Holz, Jan Komissar, Manju
Shankar Rao, and Markus Stenberg. Stenberg, and Dave Thaler.

10. References

10.1. Normative References

[I-D.ietf-dnsop-5966bis]
Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", draft-ietf-dnsop-5966bis-06 (work in
progress), January 2016.

[I-D.ietf-dnsop-edns-tcp-keepalive]
Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", draft-ietf-dnsop-edns-
tcp-keepalive-05
tcp-keepalive-06 (work in progress), January February 2016.

[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-11 (work in progress),
December 2015.

[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI
10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.

[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, DOI 10.17487/
RFC1123, October 1989,
<http://www.rfc-editor.org/info/rfc1123>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<http://www.rfc-editor.org/info/rfc2136>.

[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<http://www.rfc-editor.org/info/rfc2782>.

[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", RFC
4953, DOI 10.17487/RFC4953, July 2007,
<http://www.rfc-editor.org/info/rfc4953>.

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

[RFC5966] Bellis, R., "DNS Transport over TCP - Implementation
Requirements", RFC 5966, DOI 10.17487/RFC5966, August
2010, <http://www.rfc-editor.org/info/rfc5966>.

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, DOI
10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.

[RFC6195] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", RFC 6195, DOI 10.17487/RFC6195, March
2011, <http://www.rfc-editor.org/info/rfc6195>.

[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/
RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.

[RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October
2015, <http://www.rfc-editor.org/info/rfc7673>.

10.2. Informative References

[I-D.ietf-dnssd-hybrid]
Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service
Discovery", draft-ietf-dnssd-hybrid-02 draft-ietf-dnssd-hybrid-03 (work in progress),
November 2015.

[I-D.sekar-dns-llq]
Sekar, K., "DNS Long-Lived Queries", draft-sekar-dns-
llq-01 (work in progress), August 2006.

[IPJ.9-4-TCPSYN]
Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The
Internet Protocol Journal, Cisco Systems, Volume 9, Number
4, December 2006.

[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
August 1996, <http://www.rfc-editor.org/info/rfc1996>.

[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <http://www.rfc-editor.org/info/rfc4287>.

[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <http://www.rfc-editor.org/info/rfc5077>.

[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.

[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.

[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.

[XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
Subscribe", XSF XEP 0060, July 2010.

Authors' Addresses

Tom Pusateri
Seeking affiliation
Hilton Head Island, SC
USA

Phone: +1 843 473 7394
Email: pusateri@bangj.com

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

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