wdiff draft-ietf-dnssd-push-13.txt draft-ietf-dnssd-push-14.txt

Internet Engineering Task Force T. Pusateri
Internet-Draft Unaffiliated
Intended status: Standards Track S. Cheshire
Expires: May 2, September 19, 2018 Apple Inc.
October 29, 2017
March 18, 2018

DNS Push Notifications
draft-ietf-dnssd-push-13
draft-ietf-dnssd-push-14

Abstract

The Domain Name System (DNS) was designed to return matching records
efficiently for queries for data that is are relatively static. When
those records change frequently, DNS is still efficient at returning
the updated results when polled, as long as the polling rate is not
too high. 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/. https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on May 2, September 19, 2018.

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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5
4. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7
5. State Considerations . . . . . . . . . . . . . . . . . . . . 7 8
6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 8 9
6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 9 10
6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 12 13
6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 13
6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 16
6.3. DNS Push Notification Updates . . . . . . . . . . . . . . 19
6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 20
6.3.2. PUSH Response . . . . . . . . . . . . . . . . . . . . 23 19
6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 24 22
6.4.1. UNSUBSCRIBE Request . . . . . . . . . . . . . . . . . 25
6.4.2. UNSUBSCRIBE Response . . . . . . . . . . . . . . . . 27 22
6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 29 24
6.5.1. RECONFIRM Request . . . . . . . . . . . . . . . . . . 30 24
6.5.2. RECONFIRM Response . . . . . . . . . . . . . . . . . 32 26
6.6. Client-Initiated Termination . . . . . . . . . . . . . . 34 28
7. Security Considerations . . . . . . . . . . . . . . . . . . . 35 29
7.1. Security Services . . . . . . . . . . . . . . . . . . . . 35 29
7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 35 29
7.3. TLS Compression . . . . . . . . . . . . . . . . . . . . . 36 30
7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 36 30
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 30
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37 31
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 37 31
10.1. Normative References . . . . . . . . . . . . . . . . . . 37 31
10.2. Informative References . . . . . . . . . . . . . . . . . 39 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 35

1. Introduction

DNS

Domain Name System (DNS) records may be updated using DNS Update
[RFC2136]. Other mechanisms such as a Discovery Proxy [DisProx] can
also generate changes to a DNS zone. This document specifies a
protocol for 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, and provides a general DNS mechanism for
DNS record change notifications. Familiarity with the DNS protocol
and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6895].

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]. Levels",
when, and only when, they appear in all capitals, as shown here
[RFC2119] [RFC8174].

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 [obs]. 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
deliver 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 record changes are sent to that
multicast group address 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 Discovery Proxy [DisProx]) 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.

The DNS Long-Lived Queries (LLQ) [I-D.sekar-dns-llq] mechanism [LLQ] is an existing
deployed solution to provide asynchronous change notifications, used
by Apple's Back to My Mac Service [RFC6281] introduced in Mac OS X
10.5 Leopard in 2007. Back to My Mac was designed in an era when the
data centre center operations staff asserted that it was impossible for a
server to handle large numbers of mostly-idle TCP connections, so LLQ had to
was defined as a UDP-based protocol, effectively replicating much of
TCP's connection state management logic in user space, and creating
its own poor imitations of existing TCP features like the three-way
handshake, flow control, and reliability.

This document builds on experience gained with the LLQ protocol, with
an improved design. Instead of using UDP, this specification uses
DNS Stateful Operations (DSO) [DSO] running over TLS over TCP, and
therefore doesn't need to reinvent existing TCP functionality. Using
TCP also gives long-lived low-traffic connections better longevity
through NAT gateways without resorting to excessive keepalive
traffic. Instead of inventing a new vocabulary of messages to
communicate DNS zone changes as LLQ did, this specification adopts borrows
the established syntax and semantics of DNS Update messages
[RFC2136].

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.

This specification adopts a simplified subset of these existing
syntax and semantics, and uses them for DNS Push Notification
messages going in the opposite direction, from server to client, to
communicate changes to a zone. The client 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 receiving further 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 [RFC7719].
Consequently, the "_dns-push-tls._tcp.<zone>" SRV record for a
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 and DNS Push Notification Queries.

Supporting DNS Updates and DNS Push Notifications on the same server
is OPTIONAL. A DNS Push Notification server does NOT also have 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. If a query is
received with the RD bit set, matching records for names falling
within the server's zones should be returned with the RA (Recursion
Available) bit clear. If the query is for a name not in the server's
zone, an error with RCODE NOTAUTH (Not Authoritative) should be
returned.

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 a server, and that concept has no applicability when it
comes to an authoritative server unilaterally 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 is the reality today with existing DNS servers and
clients. A DNS Push Notification server MAY choose to implement only
DNS class "IN". If messages are received for a class other than
"IN", and that class is not supported, an error with RCODE NOTIMPL
(Not Implemented) should be returned.

DNS Push Notifications impose less load on the responding server than
rapid polling would, but Push Notifications do still have a cost, so
DNS Push Notification clients must not recklessly create an excessive
number of Push Notification subscriptions. Specifically:

(a) 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 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 smartphone, 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.

(b) A DNS Push Notification client must not SHOULD 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 if the user does
choose to bring up an on-screen display of that data, it can be
displayed really fast. 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.

Generally, as described in the DNS Stateful Operations specification
[StatefulOp],
[DSO], a client must not keep a connection session to a server open indefinitely
if it has no subscriptions (or other operations) active on that connection.
session. A client MAY close a connection session as soon as it becomes idle,
and then if needed in the future, open a new connection session when required.
Alternatively, a client MAY speculatively keep an idle connection session open
for some time, subject to the constraint that it MUST NOT keep a connection
session open that has been idle for more than the session's idle
timeout (15 seconds by default).

4. Transport

Implementations of

Other DNS operations like 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
[RFC7766].

In keeping with the more recent precedent, DNS Push Notification is
defined only for TCP. DNS Push Notification clients MUST use DNS
Stateful Operations (DSO) [DSO] running over TLS over TCP [RFC7858].

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. Flooding attacks
are possible with any protocol, and a benefit of TCP is that there
are already established industry best practices to guard against SYN
flooding and similar attacks [IPJ.9-4-TCPSYN] [SYN] [RFC4953].

Use of TCP also allows DNS Push Notifications to take advantage of
current and future developments in TCP, such as Multipath TCP (MPTCP)
[RFC6824], TCP Fast Open (TFO) [RFC7413], Tail Loss Probe (TLP)
[I-D.dukkipati-tcpm-tcp-loss-probe], and so on.

Transport Layer Security (TLS) [RFC5246] is well understood and
deployed across many protocols running over TCP. It is designed to
prevent eavesdropping, tampering, and 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.

5. State Considerations

Each 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 a client establishes a connection session to a DNS server,
each subscription is individually accepted or rejected. Servers may
employ various techniques to limit subscriptions to a manageable
level. Correspondingly, the client is free to establish simultaneous
connections
sessions to alternate DNS servers that support DNS Push Notifications
for the zone and distribute subscriptions at its the client's discretion.
In this way, both clients and servers can react to resource
constraints. Token bucket rate limiting schemes are also effective
in providing fairness by a server across numerous client requests.

6. Protocol Operation

The DNS Push Notification protocol is a session-oriented protocol,
and makes use of DNS Stateful Operations [StatefulOp]. (DSO) [DSO].

For details of the DNS Stateful Operations DSO message format refer to the DNS Stateful
Operations specification [StatefulOp]. [DSO]. Those details are not repeated here.

DNS Push Notification clients and servers MUST support DNS Stateful
Operations, DSO, but (as
stated in the DSO specification [DSO]) the server SHOULD NOT issue
any DNS Stateful
Operations DSO messages until after the client has first initiated a DNS
Stateful Operation an
acknowledged DSO message of its own. A single server can support DNS
Queries, DNS Updates, and DNS Push Notifications (using DNS Stateful
Operations) DSO) on the
same TCP port, and until the client has sent at least one DNS Stateful Operations DSO
message, the server does not know what kind of client has connected
to it. Once the client has indicated willingness to use DNS Stateful Operations DSO by
sending one of its own, either side of the connection session may then initiate
further
Stateful Operations DSO messages at any time.

A DNS Push Notification exchange begins with the client discovering
the appropriate server, using the procedure described in Section 6.1,
and then making a TLS/TCP connection to it.

A typical DNS Push Notification client will immediately issue a DNS
Stateful Operations DSO
Keepalive operation to request a session timeout or keepalive
interval longer than the the 15-second defaults, but this is not
required. A DNS Push Notification client MAY issue other requests on
the connection session first, and only issue a DNS Stateful
Operations DSO Keepalive operation later if
it determines that to be necessary.

Once the connection session is made, the client may then add and remove Push
Notification subscriptions. 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.

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 begins by opening a DNS DSO Session to its normal configured
DNS recursive resolver and requesting a Push Notification
subscription. If this is successful, then the recursive resolver
will make appropriate Push Notification subscriptions on the client's
behalf, and the client will recieve receive appropriate results. If the
recursive resolver does not support Push Notification subscriptions,
then it will return an error code, and the client should proceed to
discover the appropriate server to talk to directly. for direct communication. 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, or TCP port 853 used for DNS over
TLS [RFC7858].

The client begins the discovery by sending a DNS query to its
local resolver, algorithm described here is an iterative algorithm, which starts
with record type SOA [RFC1035] for the full name of the record to which it the client wishes to
subscribe. As an example, if it wishes to
subscribe to PTR records Successive SOA queries are then issued, trimming one
label each time, until the closest enclosing authoritative server is
discovered. There is also an optimization to enable the client to
take a "short cut" directly to the SOA record of the closest
enclosing authoritative server in many cases.

1. The client begins the discovery by sending a DNS query to its
local resolver, with record type SOA [RFC1035] for the record
name
_printers._tcp.foo.example.com, to which it sends wishes to subscribe. As an example, suppose the
client wishes to subscribe to PTR records with the name
_ipp._tcp.foo.example.com (to discover Internet Printing Protocol
(IPP) printers [RFC8010] [RFC8011] being advertised at
"foo.example.com"). The client begins by sending an SOA query
for
_printers._tcp.foo.example.com. _ipp._tcp.foo.example.com to the local recursive resolver.
The goal is to determine the
authoritative server authoritative for _printers._tcp.foo.example.com. the name
_ipp._tcp.foo.example.com. The DNS zone containing the name
_ipp._tcp.foo.example.com could be example.com, or
foo.example.com, or _tcp.foo.example.com, or even
_ipp._tcp.foo.example.com. The client does not know in advance
where the closest enclosing zone cut occurs, which is why it uses
the procedure described here to discover this information.

2. If the requested SOA record exists as exactly specified in the query, exists, it is
expected to will be returned in the
Answer section with a NOERROR response code. code, and the client has
succeeded in discovering the information it needs. (This text is
not placing any new requirements on DNS recursive resolvers. It
is merely describing the existing operation of the DNS protocol
[RFC1034] [RFC1035].)

3. If the exact requested SOA record does not exist, the client may will get
back a NOERROR/NODATA response or it may get back
a an NXDOMAIN/Name Error
response. This depends on In either case, the local resolver
implementation and whether SHOULD include the domain exists. The client is
looking for an
SOA record to be returned for the zone of the requested name in either the Answer
section or Authority
Section. If the SOA record is received in the Authority section with a NOERROR response code. Section,
then the client has succeeded in discovering the information it
needs. (This text is not placing any new requirements on DNS
recursive resolvers. It is merely describing the existing
operation of the DNS protocol regarding negative responses
[RFC2308].)

4. If the client receives an NXDOMAIN/Name Error response code or a response containing no SOA record, then
it should strip proceeds with the iterative approach. The client strips the
leading label from the current query name and if the resulting
name has at least one label in it, the client should send sends a new SOA
query, and processing continues at step 2 above, repeating this the
iterative search until a NOERROR response code either an SOA is received received, or the query
name is empty. In the case of an empty name, this is a network
configuration error which should not happen and the client gives
up. The client may retry the operation at a later time, of the
client's choosing, such after a change in network attachment.

3. In the example above, if an SOA record query is sent for
_printers._tcp.foo.example.com and an NXDOMAIN/Name Error is
returned with an SOA record in the Authority section for
foo.example.com, the client should strip the leading label and
query an SOA record for _tcp.foo.example.com. If a NOERROR/
NODATA response is received with an SOA record in the Authority
section for foo.example.com, this is sufficent. If an NXDOMAIN/
Name Error response is received, the client should again strip
the leading label and query an SOA record for foo.example.com.
If the foo.example.com domain exists, this should result in a
NOERROR response with the SOA record in the Answer section. If
the domain foo.example.com does not exist, the response will
likely be an NXDOMAIN/Name Error with an SOA record for
example.com in the Authority section. This means the subdomain
foo.example.com has not been properly delegated by example.com.

4. If a NOERROR/NODATA response is received but contains no SOA in
the Authority section, the client could try stripping the leading
label and issuing another SOA query. Additional information
about negative responses can be found in Section 2 of [RFC2308].

5. Once the SOA is known (either by virtue of being seen in the
Answer Section, or in the Authority Section), 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.

6. For implementors of this specification, an authoritative answer
for that SRV record, and only such an answer, will determine
whether If the zone supports in question does not offer DNS Push Notifications.

7. If the Notifications
then SRV record does MUST NOT exist, and the SRV "target" contains the name
of the server providing DNS Push Notifications query will return a
negative answer. (The "_dns-push-tls._tcp" service type is
allocated by IANA for the this purpose, and, like any allocated IANA
service type, MUST NOT be used for other services. Other
services that require an IANA service type should use a unique
service type allocated by IANA for that service [RFC6335][ST].)

7. If the zone in question is set up to offer DNS Push Notifications
then this SRV record MUST exist. (If this SRV record does not
exist then the zone is not correctly configured for DNS Push
Notifications as specified in this document.) 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 and in the specification for using DANE TLSA
Records with SRV Records [RFC7673].

8. 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, or is not reachable within a
reasonable time, as determined by the client, then a subsequent
server is to be contacted.

Each time a client makes a new DNS Push Notification subscription
connection,
session, it SHOULD repeat the discovery process in order to determine
the preferred DNS server for subscriptions at that time. However,
the client device MUST respect the DNS TTL values on records it
receives, and store them in its local cache with this lifetime. This
means that, as long as the DNS TTL values on the authoritative
records were set to reasonable values, repeated application of this
discovery process can be completed nearly instantaneously by the
client, using only locally-stored cached data.

6.2. DNS Push Notification SUBSCRIBE

After connecting, and requesting a longer idle timeout and/or
keepalive interval if necessary, a DNS Push Notification client then
indicates its desire to receive DNS Push Notifications for a given
domain name by sending a SUBSCRIBE request over the established TLS
connection DSO
session to the server. A SUBSCRIBE request is encoded in a DNS
Stateful Operations [StatefulOp] DSO [DSO]
message. This specification defines a DNS Stateful Operations DSO TLV for DNS Push
Notification SUBSCRIBE Requests/Responses (tentatively Stateful Operations DSO Type Code
0x40).

The entity that initiates a SUBSCRIBE request is by definition the
client. A server should not MUST NOT send a SUBSCRIBE request over an existing connection
session from a client. If a server does send a SUBSCRIBE request
over the connection a DSO session initiated by a client, it this is an a fatal error and
the client should acknowledge immediately abort the request connection with the error response
RCODE NOTAUTH (Not Authoritative). a TCP RST (or
equivalent for other protocols).

6.2.1. SUBSCRIBE Request

A SUBSCRIBE request message begins with the standard DNS Stateful
Operations DSO 12-byte header [StatefulOp],
[DSO], followed by the SUBSCRIBE TLV. A SUBSCRIBE request message is
illustrated below:

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| MESSAGE ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode | Z | RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| SSOP-TYPE = SUBSCRIBE (tentatively 0x40) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| SSOP-LENGTH (number of octets in SSOP-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| | \
\ NAME \ |
\ \ |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > SSOP-DATA
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /

Figure 1

The in Figure 1.

The MESSAGE ID field MUST be set to a unique value, that the client
is not using for any other active operation on this connection. session. For the
purposes here, a MESSAGE ID is in use on this connection session if the client
has used it in a request for which it has not yet received a
response, or if the client has used it for a subscription which it
has not yet cancelled using UNSUBSCRIBE. In the SUBSCRIBE response
the server MUST echo back the MESSAGE ID value unchanged.

The other header fields MUST be set as described in the DNS Stateful
Operations DSO
specification [StatefulOp]. [DSO]. The DNS Opcode is the
Stateful Operations DSO Opcode (tentatively
6). The four count fields MUST be zero, and the corresponding four
sections MUST be empty (i.e., absent).

The SSOP-TYPE DSO-TYPE is SUBSCRIBE (tentatively 0x40). The SSOP-LENGTH DSO-LENGTH is the
length of the SSOP-DATA DSO-DATA that follows, which specifies the name, type,
and class of the record(s) being sought.

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| MESSAGE ID | \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
|QR| Opcode | Z | RCODE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| QDCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = SUBSCRIBE (tentatively 0x40) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| | \
\ NAME \ |
\ \ |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /

Figure 1: SUBSCRIBE Request

The SSOP-DATA DSO-DATA for a SUBSCRIBE request MUST contain exactly one
question. The SSOP-DATA DSO-DATA for a SUBSCRIBE request has no QDCOUNT field
to specify more than one question. Since SUBSCRIBE requests are sent
over TCP, multiple SUBSCRIBE request messages can be concatenated in
a single TCP stream and packed efficiently into TCP segments.

If accepted, the subscription will stay in effect until the client
cancels the subscription using UNSUBSCRIBE or until the connection DSO session
between the client and the server is closed.

SUBSCRIBE requests on a given connection session 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. DSO session. For the
purpose of this matching, the established DNS case-
insensitivity case-insensitivity for
US-ASCII letters applies (e.g., "example.com" and "Example.com" are
the same). If a server receives such a duplicate SUBSCRIBE message
this is an error and the server MUST immediately terminate the
connection with a TCP RST (or equivalent for other protocols).

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 server is responsible for) and this is not an error.
The server MUST accept these requests and send Push Notifications if
and when matching records are found in the future.

If neither TYPE nor CLASS are ANY (255) then this is a specific
subscription to changes for the given NAME, TYPE and CLASS. If one
or both of TYPE or CLASS are ANY (255) then this subscription matches
any type and/or any class, as appropriate.

NOTE: A little-known quirk of DNS is that in DNS QUERY requests,
QTYPE and QCLASS 255 mean "ANY" not "ALL". They indicate that the
server should respond with ANY matching records of its choosing, not
necessarily ALL matching records. This can lead to some surprising
and unexpected results, where a query returns some valid answers but
not all of them, and makes QTYPE=ANY queries less useful than people
sometimes imagine.

When used in conjunction with SUBSCRIBE, TYPE and CLASS 255 should be
interpreted to mean "ALL", not "ANY". After accepting a subscription
where one or both of TYPE or CLASS are 255, the server MUST send Push
Notification Updates for ALL record changes that match the
subscription, not just some of them.

6.2.2. SUBSCRIBE Response

Each SUBSCRIBE request generates exactly one SUBSCRIBE response from
the server.

A SUBSCRIBE response message begins with the standard DNS Stateful
Operations DSO 12-byte
header [StatefulOp], [DSO], possibly followed by one or more optional Modifier TLVs, such as
a Retry Delay Modifier TLV.

The MESSAGE ID field MUST echo the value given in the ID field of the
SUBSCRIBE request. This is how the client knows which request is
being responded to.

A SUBSCRIBE response message MUST NOT contain include a Stateful Operations
Operation SUBSCRIBE TLV. The Stateful Operations Operation If a
client receives a SUBSCRIBE response message containing a SUBSCRIBE
TLV then the response message is NOT copied
from processed but the SUBSCRIBE request. TLV MUST
be silently ignored.

In the SUBSCRIBE response the RCODE indicates whether or not the
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.

If the server sends a nonzero RCODE in the SUBSCRIBE response, either that
means (a) the client is (at least partially) misconfigured, (b) the
server resources are exhausted, or (c) there is some other unknown
failure on the server. In any case, the client shouldn't retry the
subscription right away. Either end can terminate the connection, session, but
the client may want to try this subscription again, or it may have
other successful subscriptions that it doesn't want to abandon. If
the server sends a nonzero RCODE then it SHOULD append a Retry Delay
Modifier
TLV [StatefulOp] [DSO] to the response specifying a delay before the client
attempts this operation again. Recommended values for the delay for
different RCODE values are given below: below. These recommended values
apply both to the default values a server should place in the Retry
Delay TLV, and the default values a client should assume if the
server provides no Retry Delay TLV.

For RCODE = 1 (FORMERR) the delay may be any value selected by the
implementer. A value of five minutes is RECOMMENDED, to reduce
the risk of high load from defective clients.

For RCODE = 2 (SERVFAIL) the delay should be chosen according to
the level of server overload and the anticipated duration of that
overload. By default, a value of one minute is RECOMMENDED. If a
more serious server failure occurs, the delay may be longer in
accordance with the specific problem encountered.

For RCODE = 4 (NOTIMP), which occurs on a server that doesn't
implement DNS Stateful Operations [StatefulOp], DSO [DSO], it is unlikely that the server will begin
supporting DNS Stateful Operations DSO in the next few minutes, so the retry delay SHOULD
be one hour.

For RCODE = 5 (REFUSED), which occurs on Note that in such a case, a server that implements
DNS doesn't
implement DSO is unlikely to place a Retry Delay TLV in its
response, so this recommended value in particular applies to what
a client should assume by default.

For RCODE = 5 (REFUSED), which occurs on a server that implements
DNS Push Notifications, but is currently configured to disallow
DNS Push Notifications, the 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 possible that the misconfiguration may be repaired at any
time, the retry delay should not be set too high. By default, a
value of 5 minutes is RECOMMENDED.

For RCODE = 9 (NOTAUTH), which occurs on a server that implements
DNS Push Notifications, but is not configured to be authoritative
for the requested name, the 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 for
that zone. Since it is possible that the misconfiguration may be
repaired at any time, the retry delay should not be set too high.
By default, a value of 5 minutes is RECOMMENDED.

For RCODE = 11 (DNS Push SUBSCRIBE operation not supported), which
occurs on a server that doesn't implement DNS Push Notifications,
it is unlikely that the server will begin supporting DNS Push
Notifications in the next few minutes, so the retry delay SHOULD
be one hour.

For other RCODE values, the retry delay should be set by the
server as appropriate for that error condition. By default, a
value of 5 minutes is RECOMMENDED.

For RCODE = 9 (NOTAUTH), the time delay applies to requests for other
names falling within the same zone. Requests for names falling
within other zones are not subject to the delay. For all other
RCODEs the time delay applies to all subsequent requests to this
server.

After sending an error response the server MAY allow the connection session to
remain open, or MAY send a DNS Push Notification Retry Delay
Operation TLV instructing the client to close the TCP connection, session, as
described in the DNS Stateful Operations DSO specification [StatefulOp]. [DSO]. Clients MUST correctly
handle both cases.

6.3. DNS Push Notification Updates

Once a subscription has been successfully established, the server
generates PUSH messages to send to the client as appropriate. In the
case that the answer set was non-empty at the moment the subscription
was established, an initial PUSH message will be sent immediately
following the SUBSCRIBE Response. Subsequent changes to the answer
set are then communicated to the client in subsequent PUSH messages.

6.3.1. PUSH Message

A PUSH message begins with the standard DNS Stateful Operations DSO 12-byte header [StatefulOp], [DSO],
followed by the PUSH TLV. A PUSH message is illustrated below:

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| MESSAGE ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode | Z | RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| SSOP-TYPE = PUSH (tentatively 0x42) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| SSOP-LENGTH (number of octets in SSOP-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| RDLEN | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
\ RDATA \ |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > SSOP-DATA
\ NAME \ |
+--+--+--+--+--+--+- | |
| TYPE Repeated | |
+--+--+--+--+--+--+- | |
| CLASS As | |
+--+--+--+--+--+--+- | |
| RDLEN Necessary | |
+--+--+--+--+--+--+- | |
\ RDATA \ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / Figure 2 2.

The MESSAGE ID field MUST be set zero. There is no client response to a unique value, that the server
is not currently using for any other active outgoing request that it
has sent on this connection.
PUSH message.

The MESSAGE ID other header fields MUST be set as described in the outgoing PUSH
message DSO
specification [DSO]. The DNS Opcode is selected by the server and has no relationship to the
MESSAGE ID in any of the client subscriptions it may relate to. In
the PUSH response the client MUST echo back the MESSAGE ID value
unchanged.

The other header fields MUST be set as described in the DNS Stateful
Operations specification [StatefulOp]. The DNS Opcode is the
Stateful Operations Opcode (tentatively 6). The four count fields
MUST be zero, DSO Opcode (tentatively
6). The four count fields MUST be zero, and the corresponding four
sections MUST be empty (i.e., absent).

The SSOP-TYPE DSO-TYPE is PUSH (tentatively 0x41). The SSOP-LENGTH DSO-LENGTH is the
length of the SSOP-DATA DSO-DATA that follows, which specifies the changes
being communicated.

The SSOP-DATA DSO-DATA contains one or more Update records. A PUSH Message
MUST contain at least one Update record. If a PUSH Message is
received that contains no Update records, this is a fatal error, and
the receiver MUST immediately terminate the connection with a TCP RST
(or equivalent for other protocols). The Update records are
formatted in the customary way for Resource Records in DNS messages
with the stipulation that DNS name compression is not permitted in
DNS Stateful Operations TLVs. messages.
Update records in a PUSH Message are interpreted according to the
same rules as for DNS Update [RFC2136] messages, namely:

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

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| MESSAGE ID | \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
|QR| Opcode | Z | RCODE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| QDCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = PUSH (tentatively 0x41) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TTL | |
| (32 bits) | > DSO-DATA
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| RDLEN | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
\ RDATA \ |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
: NAME, TYPE, CLASS, TTL, RDLEN, RDATA : |
: Repeated As Necessary : /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /

Figure 2: PUSH Message

When processing the records received in a PUSH Message, the receiving
client MUST validate that the records being added or deleted
correspond with at least one currently active subscription on that
connection.
session. 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 TYPE in the SUBSCRIBE
request was not ANY (255) then the TYPE of the record must match the
TYPE given in the SUBSCRIBE request. If the CLASS in the SUBSCRIBE
request was not ANY (255) then the CLASS of the record must match the
CLASS given in the SUBSCRIBE request. If a matching active
subscription on that connection session 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 DSO
session is not closed. This is to allow for the unavoidable race
condition where a client sends an outbound UNSUBSCRIBE while inbound
PUSH messages 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 PUSH message is sent. For example, a PUSH
message adding a given record may match both a SUBSCRIBE request with
the same TYPE and a different SUBSCRIBE request with TYPE=ANY. It is
not the case that two PUSH messages 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" PUSH message, not
three separate "delete an individual RR from a name" PUSH messages.
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" PUSH message,
not two separate "delete an RRset from a name" PUSH messages.

A server SHOULD combine multiple change notifications in a single
PUSH message when possible, even if those change notifications apply
to different subscriptions. Conceptually, a PUSH message is a
connection-level
session-level mechanism, not a subscription-level mechanism.

Reception of a PUSH message by a client generates a PUSH response
back to the server.

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 DSO session being closed) record
aging resumes and records are removed from the local cache when their
TTL reaches zero.

6.3.2. PUSH Response

Each PUSH message generates exactly one PUSH response from the
receiver.

A PUSH response message begins with the standard DNS Stateful
Operations 12-byte header [StatefulOp], possibly followed by one or
more optional Modifier TLVs.

The MESSAGE ID field MUST echo the value given in the ID field of the
PUSH message.

A PUSH response message MUST NOT contain a Stateful Operations
Operation TLV. The Stateful Operations Operation TLV is NOT copied
from the PUSH message.

In a PUSH response the RCODE MUST be zero. Receiving a PUSH response
with a nonzero RCODE is a fatal error, and the receiver MUST
immediately terminate the connection with a TCP RST (or equivalent
for other protocols).

6.4. DNS Push Notification UNSUBSCRIBE

To cancel an individual subscription without closing the entire
connection, DSO
session, the client sends an UNSUBSCRIBE message over the established TCP connection
DSO session to the server. The UNSUBSCRIBE message is encoded in a DNS Stateful Operations [StatefulOp]
DSO [DSO] message. This specification defines a DNS Stateful Operations DSO TLV for DNS Push
Notification UNSUBSCRIBE Requests/Responses (tentatively Stateful
Operations DSO Type
Code 0x42).

A server MUST NOT initiate an UNSUBSCRIBE request. If a server does
send a an UNSUBSCRIBE request over the connection a DSO session initiated by a client,
it
this is an a fatal error and the client should acknowledge immediately abort the request
connection with the
error response RCODE NOTAUTH (Not Authoritative). a TCP RST (or equivalent for other protocols).

6.4.1. UNSUBSCRIBE Request

An UNSUBSCRIBE request message begins with the standard DNS Stateful
Operations DSO 12-byte header [StatefulOp],
[DSO], followed by the UNSUBSCRIBE TLV.

Figure 3

In the UNSUBSCRIBE TLV the SSOP-TYPE is An UNSUBSCRIBE (tentatively
0x42). The SSOP-LENGTH request
message is 2 octets. illustrated in Figure 3.

The SSOP-DATA contains the MESSAGE ID field of the value given in the
ID field of an active SUBSCRIBE request. This is how the server
knows which SUBSCRIBE request is being cancelled. After receipt of
the UNSUBSCRIBE request, the SUBSCRIBE request MUST be zero. There is no longer active.
If a server receives an UNSUBSCRIBE message where the MESSAGE ID does
not match the ID of an active SUBSCRIBE request the server MUST
return a response containing RCODE = 3 (NXDOMAIN).

It is allowable for the client to issue an UNSUBSCRIBE request for a
previous SUBSCRIBE request for which the client has not yet received
a SUBSCRIBE response. This is to allow for the case where a client
starts and stops a subscription
UNSUBSCRIBE message.

The other header fields MUST be set as described in less than the round-trip time to the server. DSO
specification [DSO]. The client DNS Opcode is NOT required to wait for the SUBSCRIBE
response before issuing DSO Opcode (tentatively
6). The four count fields MUST be zero, and the UNSUBSCRIBE request. A consequence of
this is that if corresponding four
sections MUST be empty (i.e., absent).

In the client issues an UNSUBSCRIBE request for an as-
yet unacknowledged SUBSCRIBE request, and TLV the SUBSCRIBE request DSO-TYPE is
subsequently unsuccessful for some reason, then when the UNSUBSCRIBE
request (tentatively
0x42). The DSO-LENGTH is eventually processed it will be an UNSUBSCRIBE request for
a nonexistent subscription, which will result NXDOMAIN response.

6.4.2. UNSUBSCRIBE Response

Each UNSUBSCRIBE request generates exactly one UNSUBSCRIBE response
from the server.

An UNSUBSCRIBE response message begins with the standard DNS Stateful
Operations 12-byte header [StatefulOp], possibly followed by one or
more optional Modifier TLVs, such as a Retry Delay Modifier TLV. 2 octets.

The DSO-DATA contains the MESSAGE ID field MUST echo of the value given in the
ID field of the
UNSUBSCRIBE an active SUBSCRIBE request. This is how the client server
knows which request is
being responded to.

An UNSUBSCRIBE response message MUST NOT contain a Stateful
Operations Operation TLV. The Stateful Operations Operation TLV is
NOT copied from the UNSUBSCRIBE request.

In the UNSUBSCRIBE response the RCODE indicates whether or not the
unsubscribe request was successful. Supported RCODEs are as follows:

UNSUBSCRIBE Response codes

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

Having being successfully revoked with a correctly-formatted
UNSUBSCRIBE message (resulting in a response with RCODE NOERROR) 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.

Nonzero RCODE values signal some kind of error.

RCODE value FORMERR indicates a message format error.

RCODE value NXDOMAIN indicates a MESSAGE ID that does not correspond
to any active subscription.

RCODE values NOTIMP and SSOPNOTIMP should not occur in practice.

A server would only generate NOTIMP if it did not support Stateful
Operations, and if SUBSCRIBE request is being cancelled. After receipt of
the server does not support Stateful Operations
then it should not be possible UNSUBSCRIBE request, the SUBSCRIBE request is no longer active.

It is allowable for a the client to have issue an active
subscription to cancel.

Similarly, UNSUBSCRIBE request for a server would only generate SSOPNOTIMP if it did not
support Push Notifications, and if
previous SUBSCRIBE request for which the server does not support Push
Notifications then it should client has not be possible yet received
a SUBSCRIBE response. This is to allow for the case where a client to have an
active
starts and stops a subscription to cancel.

Nonzero RCODE values other in less than NXDOMAIN indicate a serious problem
with the client. After sending an error response other than
NXDOMAIN, round-trip time to
the server SHOULD send a DNS Stateful Operations Retry
Delay Operation TLV and then close server. The client is NOT required to wait for the TCP connection, as described
in SUBSCRIBE
response before issuing the DNS Stateful Operations specification [StatefulOp]. UNSUBSCRIBE request.

Figure 3: UNSUBSCRIBE Request

6.5. DNS Push Notification RECONFIRM

Sometimes, particularly when used with a Discovery Proxy [DisProx], 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
can send a RECONFIRM request to ask the server to re-verify that the
data is still valid. For a Discovery Proxy, this causes it to issue
new Multicast DNS requests to ascertain whether the target device is
still present. For other types of DNS server, the RECONFIRM
operation is currently undefined, and SHOULD result in a NOERROR
response, but otherwise need not cause any action to occur. Frequent
RECONFIRM operations may be a sign of network unreliability, or some
kind of misconfiguration, so RECONFIRM operations MAY be logged or
otherwise communicated to a human administrator to assist in
detecting, and remedying, such network problems.

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

6.5.1. RECONFIRM Request

A RECONFIRM request message begins with the standard DNS Stateful
Operations DSO 12-byte header [StatefulOp],
[DSO], followed by the RECONFIRM TLV. A RECONFIRM request message is
illustrated below:

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| MESSAGE ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode | Z | RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT (MUST BE ZERO) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| SSOP-TYPE = RECONFIRM (tentatively 0x43) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| SSOP-LENGTH (number of octets in SSOP-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | > SSOP-DATA
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| RDLEN | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
\ RDATA \ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / Figure 4 4.

The MESSAGE ID field MUST be set to a unique value, that the client
is not using for any other active operation on this connection. DSO session. For
the purposes here, a MESSAGE ID is in use on this connection session if the
client has used it in a request for which it has not yet received a
response, or if the client has used it for a subscription which it
has not yet cancelled using UNSUBSCRIBE. In the RECONFIRM response
the server MUST echo back the MESSAGE ID value unchanged.

The SSOP-TYPE DSO-TYPE is RECONFIRM (tentatively 0x43). The SSOP-LENGTH is
the length of the data that follows, which specifies the name, type,
class, and content of the record being disputed. (tentatively 0x43). The DSO-LENGTH is the
length of the data that follows, which specifies the name, type,
class, and content of the record being disputed.

Figure 4: RECONFIRM Request

The SSOP-DATA DSO-DATA for a RECONFIRM request MUST contain exactly one record.
The SSOP-DATA DSO-DATA for a RECONFIRM request has no count field to specify
more than one record. Since RECONFIRM requests are sent over TCP,
multiple RECONFIRM request messages can be concatenated in a single
TCP stream and packed efficiently into TCP segments.

TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value
ANY (255).

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

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

6.5.2. RECONFIRM Response

Each RECONFIRM request generates exactly one RECONFIRM response from
the server.

A RECONFIRM response message begins with the standard DNS Stateful
Operations DSO 12-byte
header [StatefulOp], [DSO], possibly followed by one or more optional Modifier TLVs, such as
a Retry Delay Modifier TLV. For suggested values for the Retry Delay TLV, see
Section 6.2.2.

The MESSAGE ID field MUST echo the value given in the ID field of the
RECONFIRM request. This is how the client knows which request is
being responded to.

A RECONFIRM response message MUST NOT contain include a Stateful Operations
Operation DSO RECONFIRM TLV. The Stateful Operations Operation
If a client receives a RECONFIRM response message containing a
RECONFIRM TLV then the response message is NOT copied
from processed but the
RECONFIRM request. TLV MUST be silently ignored.

In the RECONFIRM response the RCODE confirms receipt of the
reconfirmation request. Supported RCODEs are as follows:

+------------+-------+----------------------------------------------+

RECONFIRM Response codes

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

Nonzero RCODE values signal some kind of error.

RCODE value FORMERR indicates a message format error, for example
TYPE or CLASS being ANY (255).

RCODE value SERVFAIL indicates that the server has exhausted its
resources or other serious problem occurred.

RCODE values NOTIMP indicates that the server does not support
Stateful Operations, DSO,
and Stateful Operations DSO is required for RECONFIRM requests.

RCODE value REFUSED indicates that the server supports RECONFIRM
requests but is currently not configured to accept them from this
client.

RCODE value NOTAUTH indicates that the server is not authoritative
for the requested name, and can do nothing to remedy the apparent
error. Note that there may be future cases in which a server is able
to pass on the RECONFIRM request to the ultimate source of the
information, and in these cases the server should return NOERROR.

RCODE value SSOPNOTIMP DSOTYPENI indicates that the server does not support
RECONFIRM requests.

Nonzero RCODE values SERVFAIL, REFUSED and SSOPNOTIMP DSOTYPENI are benign from
the client's point of view. The client may log them to aid in
debugging, but otherwise they require no special action.

Nonzero RCODE values other than these three indicate a serious
problem with the client. After sending an error response other than
one of these three, the server SHOULD send a DNS Stateful Operations DSO Retry Delay Operation TLV and then close to
end the TCP connection, DSO session, as described in the DNS Stateful Operations DSO specification [StatefulOp]. [DSO].

6.6. Client-Initiated Termination

An individual subscription is terminated by sending an UNSUBSCRIBE
TLV for that specific subscription, or all subscriptions can be
cancelled at once by the client closing the connection. DSO session. When a
client terminates an individual subscription (via UNSUBSCRIBE) or all
subscriptions on that connection DSO session (by closing ending the connection) session) it is
signaling to the server that it is longer interested in 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 session via UNSUBSCRIBE,
a client MAY close the connection session immediately, or it may keep it open if
it anticipates performing further operations on that
connection session in the
future. If a client wishes to keep an idle
connection session open, it MUST
respect the maximum idle time required by the server [StatefulOp]. [DSO].

If a client plans to terminate one or more subscriptions on a
connection session
and doesn't intend to keep that connection session open, then as an efficiency
optimization it MAY instead choose to simply close the
connection, session, which
implicitly terminates all subscriptions on that
connection. session. 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 connection session has been
cancelled.

When closing a connection, session, a client will generally do an abortive
disconnect, sending a TCP RST. This immediately discards all
remaining inbound and outbound data, which is appropriate if the
client no longer has any interest in this data. In the BSD Sockets
API, sending a TCP RST is achieved by setting the SO_LINGER option
with a time of 0 seconds and then closing the socket.

If a client has performed operations on this connection session that it would
not want lost (like DNS updates) then the client SHOULD do an orderly
disconnect, sending a TLS close_notify followed by a TCP FIN. In (In
the BSD Sockets API, sending a TCP FIN is achieved by calling
"shutdown(s,SHUT_WR)" and keeping the socket open until all remaining
data has been read from it. it.)

7. Security Considerations

The Strict Privacy Usage Profile for DNS over TLS is strongly
recommended for DNS Push Notifications as defined in Authentication "Authentication
and (D)TLS Profile for DNS-over-(D)TLS DNS-over-(D)TLS"
[I-D.ietf-dprive-dtls-and-tls-profiles]. The Opportunistic Privacy
Usage Profile is permissible as a way to support incremental
deployment of security capabilities. Cleartext connections for DNS
Push Notifications are not permissible.

DNSSEC is RECOMMENDED for the authentication of DNS Push Notification
servers. 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: 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: Any changes made to the communication in
transit are detectable by the receiver.

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.

See Authentication and (D)TLS Profile for DNS-over-(D)TLS
[I-D.ietf-dprive-dtls-and-tls-profiles] for more information on
authenticating domain names. Also note that a DNS Push server is an
authoritative server and a DNS Push client is a standard DNS client.
While the terminology in Authentication and (D)TLS Profile for DNS-
over-(D)TLS [I-D.ietf-dprive-dtls-and-tls-profiles] explicitly states
it does not apply to authoritative servers, it does in this case
apply to DNS Push Notification clients and servers.

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 DSO session 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. DSO
session. 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
[RFC6335][SN]. Service Types
[RFC6335][ST].

This document defines four DNS Stateful Operations TLV types:
SUBSCRIBE with (tentative) value 0x40 (64), PUSH with (tentative)
value 0x41 (65), UNSUBSCRIBE with (tentative) value 0x42 (66), and
RECONFIRM with (tentative) value 0x43 (67).

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 Volz, Jan Komissar, Manju
Shankar Rao, Markus Stenberg, Dave Thaler, Soraia Zlatkovic, Sara
Dickinson, and Andrew Sullivan.

10. References

10.1. Normative References

[DSO] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Mankin, A., and T. Pusateri, "DNS Stateful Operations",
draft-ietf-dnsop-session-signal-05 (work in progress),
January 2018.

[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-21 draft-ietf-tls-tls13-26 (work in progress),
July 2017.
March 2018.

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

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

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

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

[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989, <https://www.rfc-
editor.org/info/rfc1123>.
<https://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, <https://www.rfc-
editor.org/info/rfc2119>.
<https://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,
<https://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, <https://www.rfc-
editor.org/info/rfc2782>.
<https://www.rfc-editor.org/info/rfc2782>.

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

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

[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.

[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.

[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, <https://www.rfc-editor.org/info/rfc7673>.

[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/info/rfc7766>.

[SN]

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[ST] "Service Name and Transport Protocol Port Number
Registry", <http://www.iana.org/assignments/
service-names-port-numbers/>.

[StatefulOp]
Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Mankin, A., and T. Pusateri, "DNS Stateful Operations",
draft-ietf-dnsop-session-signal-04 (work in progress),
September 2017.

10.2. Informative References

[DisProx] Cheshire, S., "Hybrid Unicast/Multicast "Discovery Proxy for Multicast DNS-Based
Service Discovery", draft-ietf-dnssd-hybrid-07 draft-ietf-dnssd-hybrid-08 (work in
progress),
September 2017. March 2018.

[I-D.dukkipati-tcpm-tcp-loss-probe]
Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
"Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
in progress), February 2013.

[I-D.ietf-dprive-dtls-and-tls-profiles]
Dickinson, S., Gillmor, D., and T. Reddy, "Usage and
(D)TLS Profiles for DNS-over-(D)TLS", draft-ietf-dprive-
dtls-and-tls-profiles-11 (work in progress), September
2017.

[I-D.sekar-dns-llq]

[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.

[obs] "Observer Pattern", <https://en.wikipedia.org/wiki/
Observer_pattern>.
<https://en.wikipedia.org/wiki/Observer_pattern>.

[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/info/rfc2308>.

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

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

[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, <https://www.rfc-editor.org/info/rfc5077>.

[RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
"Understanding Apple's Back to My Mac (BTMM) Service",
RFC 6281, DOI 10.17487/RFC6281, June 2011,
<https://www.rfc-editor.org/info/rfc6281>.

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

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

[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/info/rfc6824>.

[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.

[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, <https://www.rfc-editor.org/info/rfc7525>.

[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>.

[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.

[RFC8010] Sweet, M. and I. McDonald, "Internet Printing
Protocol/1.1: Encoding and Transport", RFC 8010,
DOI 10.17487/RFC8010, January 2017,
<https://www.rfc-editor.org/info/rfc8010>.

[RFC8011] Sweet, M. and I. McDonald, "Internet Printing
Protocol/1.1: Model and Semantics", RFC 8011,
DOI 10.17487/RFC8011, January 2017,
<https://www.rfc-editor.org/info/rfc8011>.

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

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

Authors' Addresses

Tom Pusateri
Unaffiliated
Raleigh, NC 27608
USA

Phone: +1 919 867 1330
Email: pusateri@bangj.com

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

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