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Versions: (draft-bellis-dnsop-session-signal) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20

DNSOP Working Group                                            R. Bellis
Internet-Draft                                                       ISC
Updates: RFC 7766, RFC 1035 (if                              S. Cheshire
         approved)                                            Apple Inc.
Intended status: Standards Track                            J. Dickinson
Expires: July 30, 2018                                      S. Dickinson
                                                                 Sinodun
                                                               A. Mankin
                                                              Salesforce
                                                             T. Pusateri
                                                            Unaffiliated
                                                        January 26, 2018


                        DNS Stateful Operations
                   draft-ietf-dnsop-session-signal-05

Abstract

   This document defines a new DNS OPCODE for DNS Stateful Operations
   (DSO).  DSO messages communicate operations within persistent
   stateful sessions, using type-length-value (TLV) syntax.  Three TLVs
   are defined that manage session timeouts, termination, and encryption
   padding, and a framework is defined for extensions to enable new
   stateful operations.

Status of This Memo

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

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

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

   This Internet-Draft will expire on July 30, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  DSO Session Establishment . . . . . . . . . . . . . . . .   9
       4.1.1.  Connection Sharing  . . . . . . . . . . . . . . . . .  11
       4.1.2.  Zero Round-Trip Operation . . . . . . . . . . . . . .  12
       4.1.3.  Middlebox Considerations  . . . . . . . . . . . . . .  12
     4.2.  Message Format  . . . . . . . . . . . . . . . . . . . . .  13
       4.2.1.  DNS Header Fields in DSO Messages . . . . . . . . . .  14
       4.2.2.  DSO Data  . . . . . . . . . . . . . . . . . . . . . .  16
       4.2.3.  EDNS(0) and TSIG  . . . . . . . . . . . . . . . . . .  21
     4.3.  Message Handling  . . . . . . . . . . . . . . . . . . . .  22
     4.4.  DSO Response Generation . . . . . . . . . . . . . . . . .  23
     4.5.  Responder-Initiated Operation Cancellation  . . . . . . .  24
   5.  DSO Session Lifecycle and Timers  . . . . . . . . . . . . . .  25
     5.1.  DSO Session Initiation  . . . . . . . . . . . . . . . . .  25
     5.2.  DSO Session Timeouts  . . . . . . . . . . . . . . . . . .  25
     5.3.  Inactive DSO Sessions . . . . . . . . . . . . . . . . . .  26
     5.4.  The Inactivity Timeout  . . . . . . . . . . . . . . . . .  26
       5.4.1.  Closing Inactive DSO Sessions . . . . . . . . . . . .  27
       5.4.2.  Values for the Inactivity Timeout . . . . . . . . . .  27
     5.5.  The Keepalive Interval  . . . . . . . . . . . . . . . . .  29
       5.5.1.  Keepalive Interval Expiry . . . . . . . . . . . . . .  29
       5.5.2.  Values for the Keepalive Interval . . . . . . . . . .  29
     5.6.  Server-Initiated Session Termination  . . . . . . . . . .  31
       5.6.1.  Server-Initiated Session Termination on Error . . . .  31
       5.6.2.  Server-Initiated Session Termination on Overload  . .  32
       5.6.3.  Server-Initiated Retry Delay Request Message  . . . .  33
   6.  Base TLVs for DNS Stateful Operations . . . . . . . . . . . .  35
     6.1.  Keepalive TLV . . . . . . . . . . . . . . . . . . . . . .  35
       6.1.1.  Client handling of received Session Timeout values  .  37
       6.1.2.  Relation to EDNS(0) TCP Keepalive Option  . . . . . .  38
     6.2.  Retry Delay TLV . . . . . . . . . . . . . . . . . . . . .  39
       6.2.1.  Retry Delay TLV used as a Primary TLV . . . . . . . .  39
       6.2.2.  Retry Delay TLV used as a Response Additional TLV . .  40



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       6.2.3.  Retry Delay TLV is used by server only  . . . . . . .  40
     6.3.  Encryption Padding TLV  . . . . . . . . . . . . . . . . .  41
   7.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  42
     7.1.  MESSAGE ID  . . . . . . . . . . . . . . . . . . . . . . .  42
     7.2.  TLV Usage . . . . . . . . . . . . . . . . . . . . . . . .  43
     7.3.  Inactivity Timeout  . . . . . . . . . . . . . . . . . . .  44
     7.4.  Keepalive Interval  . . . . . . . . . . . . . . . . . . .  44
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  45
     8.1.  DSO OPCODE Registration . . . . . . . . . . . . . . . . .  45
     8.2.  DSO RCODE Registration  . . . . . . . . . . . . . . . . .  45
     8.3.  DSO Type Code Registry  . . . . . . . . . . . . . . . . .  45
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  46
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  46
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  47
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  47
     11.2.  Informative References . . . . . . . . . . . . . . . . .  48
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  49

1.  Introduction

   The use of transports for DNS other than UDP is being increasingly
   specified, for example, DNS over TCP [RFC1035][RFC7766] and DNS over
   TLS [RFC7858].  Such transports can offer persistent, long-lived
   sessions and therefore when using them for transporting DNS messages
   it is of benefit to have a mechanism that can establish parameters
   associated with those sessions, such as timeouts.  In such situations
   it is also advantageous to support server initiated messages.

   The existing EDNS(0) Extension Mechanism for DNS [RFC6891] is
   explicitly defined to only have "per-message" semantics.  Whilst
   EDNS(0) has been used to signal at least one session-related
   parameter (the EDNS(0) TCP Keepalive option [RFC7828]) the result is
   less than optimal due to the restrictions imposed by the EDNS(0)
   semantics and the lack of server-initiated signalling.  For example,
   a server cannot arbitrarily instruct a client to close a connection
   because the server can only send EDNS(0) options in responses to
   queries that contained EDNS(0) options.

   This document defines a new DNS OPCODE, DSO (tentatively 6), for DNS
   Stateful Operations.  DSO messages are used to communicate operations
   within persistent stateful sessions, expressed using type-length-
   value (TLV) syntax.  This document defines an initial set of three
   TLVs, used to manage session timeouts, termination, and encryption
   padding.

   All three of the TLVs defined here are mandatory for all
   implementations of DSO.  Further TLVs may be defined in additional
   specifications.



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   The format for DSO messages (see Section 4.2) differs somewhat from
   the traditional DNS message format used for standard queries and
   responses.  The standard twelve-octet header is used, but the four
   count fields (QDCOUNT, ANCOUNT, NSCOUNT, ARCOUNT) are set to zero and
   accordingly their corresponding sections are not present.  The actual
   data pertaining to DNS Stateful Operations (expressed in TLV syntax)
   is appended to the end of the DNS message header.  When displayed
   using packet analyzer tools that have not been updated to recognize
   the DSO format, this will result in the DSO data being displayed as
   unknown additional data after the end of the DNS message.  It is
   likely that future updates to these tools will add the ability to
   recognize, decode, and display the DSO data.

   This new format has distinct advantages over an RR-based format
   because it is more explicit and more compact.  Each TLV definition is
   specific to its use case, and as a result contains no redundant or
   overloaded fields.  Importantly, it completely avoids conflating DNS
   Stateful Operations in any way with normal DNS operations or with
   existing EDNS(0)-based functionality.  A goal of this approach is to
   avoid the operational issues that have befallen EDNS(0), particularly
   relating to middlebox behaviour.

   With EDNS(0), multiple options may be packed into a single OPT
   pseudo-RR, and there is no generalized mechanism for a client to be
   able to tell whether a server has processed or otherwise acted upon
   each individual option within the combined OPT RR.  The
   specifications for each individual option need to define how each
   different option is to be acknowledged, if necessary.

   In contrast to EDNS(0), with DSO there is no compelling motivation to
   pack multiple operations into a single message for efficiency
   reasons, because DSO always operates using a connection-oriented
   transport protocol.  Each Stateful operation is communicated in its
   own separate DNS message, and the transport protocol can take care of
   packing separate DNS messages into a single IP packet if appropriate.
   For example, TCP can pack multiple small DNS messages into a single
   TCP segment.  This simplification allows for clearer semantics.  Each
   DSO request message communicates just one primary operation, and the
   RCODE in the corresponding response message indicates the success or
   failure of that operation.











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2.  Terminology

   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].

   "DSO" is used to mean DNS Stateful Operation.

   The term "connection" means a bidirectional byte stream of reliable,
   in-order messages, such as provided by using DNS over TCP
   [RFC1035][RFC7766] or DNS over TLS [RFC7858].

   The unqualified term "session" in the context of this document means
   the exchange of DNS messages over a connection where:

   o  The connection between client and server is persistent and
      relatively long-lived (i.e., minutes or hours, rather than
      seconds).

   o  Either end of the connection may initiate messages to the other.

   A "DSO Session" is established between two endpoints that acknowledge
   persistent DNS state via the exchange of DSO messages over the
   connection.  This is distinct from a DNS-over-TCP session as
   described in the previous specification for DNS over TCP [RFC7766].

   A "DSO Session" is terminated when the underlying connection is
   closed.  The underlying connection can be closed in two ways.

   Where this specification says, "close gracefully," that means sending
   a TLS close_notify followed by a TCP FIN, or the equivalents for
   other protocols.  Where this specification requires a connection to
   be closed gracefully, the requirement to initiate that graceful close
   is placed on the client, to place the burden of TCP's TIME-WAIT state
   on the client rather than the server.

   Where this specification says, "forcibly abort," that means sending a
   TCP RST, or the equivalent for other protocols.  In the BSD Sockets
   API this is achieved by setting the SO_LINGER option to zero before
   closing the socket.

   The term "server" means the software with a listening socket,
   awaiting incoming connection requests.

   The term "client" means the software which initiates a connection to
   the server's listening socket.




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   The terms "initiator" and "responder" correspond respectively to the
   initial sender and subsequent receiver of a DSO request message,
   regardless of which was the "client" and "server" in the usual DNS
   sense.

   The term "sender" may apply to either an initiator (when sending a
   DSO request message) or a responder (when sending a DSO response
   message).

   Likewise, the term "receiver" may apply to either a responder (when
   receiving a DSO request message) or an initiator (when receiving a
   DSO response message).

   The term "long-lived operations" refers to operations such as Push
   Notification subscriptions [I-D.ietf-dnssd-push], Discovery Relay
   interface subscriptions [I-D.sctl-dnssd-mdns-relay], and other future
   long-lived DNS operations that choose to use DSO as their basis, that
   establish state that persists beyond the lifetime of a traditional
   brief request/response transaction.  This document, the base
   specification for DNS Stateful Operations, defines a framework for
   supporting long-lived operations, but does not itself define any
   long-lived operations.  Nonetheless, to appreciate the design
   rationale behind DNS Stateful Operations, it is helpful to understand
   the long-lived operations that it is intended to support.

   DNS Stateful Operations uses "DSO request messages" and "DSO response
   messages".  DSO request messages are further subdivided into two
   variants, "acknowledged request messages" (which generate a
   corresponding response message) and "unacknowledged request messages"
   (which do not generate any corresponding response message).

   The content of DSO messages is expressed using type-length-value
   (TLV) syntax.

   In a DSO request message the first TLV is referred to as the "Primary
   TLV" and determines the nature of the operation being performed,
   including whether it is an acknowledged or unacknowledged operation;
   any other TLVs in a DSO request message are referred to as
   "Additional TLVs" and serve additional non-primary purposes, which
   may be related to the primary purpose, or not, as in the case of the
   encryption padding TLV.

   A DSO response message may contain no TLVs, or it may contain one or
   more TLVs as appropriate to the information being communicated.  In
   the context of DSO response messages, one or more TLVs with the same
   DSO-TYPE as the Primary TLV in the corresponding DSO request message
   are referred to as "Response Primary TLVs".  Any other TLVs with
   different DSO-TYPEs are referred to as "Response Additional TLVs".



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   The Response Primary TLV(s), if present, MUST occur first in the
   response message, before any Response Additional TLVs.

   Two timers (elapsed time since an event) are defined in this
   document:

   o  an inactivity timer (see Section 6.1 and Section 5.3)

   o  a keepalive timer (see Section 6.1 and Section 5.5)

   The timeouts associated with these timers are called the inactivity
   timeout and the keepalive interval, respectively.  The term "Session
   Timeouts" is used to refer to this pair of timeout values.

   Resetting a timer means resetting the timer value to zero and
   starting the timer again.  Clearing a timer means resetting the timer
   value to zero but NOT starting the timer again.


































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3.  Discussion

   There are several use cases for DNS Stateful operations that can be
   described here.

   Firstly, establishing session parameters such as server-defined
   timeouts is of great use in the general management of persistent
   connections.  For example, using DSO sessions for stub to recursive
   DNS-over-TLS [RFC7858] is more flexible for both the client and the
   server than attempting to manage sessions using just the EDNS(0) TCP
   Keepalive option [RFC7828].  The simple set of TLVs defined in this
   document is sufficient to greatly enhance connection management for
   this use case.

   Secondly, DNS-SD [RFC6763] has evolved into a naturally session based
   mechanism where, for example, long-lived subscriptions lend
   themselves to 'push' mechanisms as opposed to polling.  Long-lived
   stateful connections and server initiated messages align with this
   use case [I-D.ietf-dnssd-push].

   A general use case is that DNS traffic is often bursty but session
   establishment can be expensive.  One challenge with long-lived
   connections is to maintain sufficient traffic to maintain NAT and
   firewall state.  To mitigate this issue this document introduces a
   new concept for the DNS, that is DSO "Keepalive traffic".  This
   traffic carries no DNS data and is not considered 'activity' in the
   classic DNS sense, but serves to maintain state in middleboxes, and
   to assure client and server that they still have connectivity to each
   other.

   There are a myriad of other potential use cases for DSO given the
   versatility and extensibility of this specification.

   Section 4 of this document describes the protocol details of DNS
   Stateful Operations including definitions of three TLVs for session
   management and encryption padding.  Section 5 presents a detailed
   discussion of the DSO Session lifecycle including an in-depth
   discussion of keepalive traffic and session termination.













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4.  Protocol Details

4.1.  DSO Session Establishment

   DSO messages MUST only be carried in protocols and in environments
   where a session may be established according to the definition above.
   Standard DNS over TCP [RFC1035][RFC7766], and DNS over TLS [RFC7858]
   are suitable protocols.

   DNS over plain UDP [RFC0768] is not appropriate since it fails on the
   requirement for in-order message delivery, and, in the presence of
   NAT gateways and firewalls with short UDP timeouts, it fails to
   provide a persistent bi-directional communication channel unless an
   excessive amount of keepalive traffic is used.

   In some environments it may be known in advance by external means
   that both client and server support DSO, and in these cases either
   client or server may initiate DSO messages at any time.

   However, in the typical case a server will not know in advance
   whether a client supports DSO, so in general, unless it is known in
   advance by other means that a client does support DSO, a server MUST
   NOT initiate DSO request messages until a DSO Session has been
   mutually established, as described below.  Similarly, unless it is
   known in advance by other means that a server does support DSO, a
   client MUST NOT initiate non-response-requiring DSO request messages
   until after a DSO Session has been mutually established.

   Whether or not a given DSO request message elicits a response is
   determined by whether or not the first DSO TLV (see Section 4.2.2.1)
   in the message (the Primary TLV) is one that is specified to generate
   a response.  Whether a Primary TLV will be specified to elicit a
   response will depend on the intended use pattern for that particular
   TLV.

   A DSO Session is established over a connection by the client sending
   a DSO request message of a kind that requires a response, such as the
   DSO Keepalive TLV (see Section 6.1), and receiving a response, with
   matching MESSAGE ID, and RCODE set to NOERROR (0), indicating that
   the DSO request was successful.

   If the RCODE is set to DSONOTIMP (tentatively 11) this indicates that
   the server does support DSO, but does not support the particular
   operation the client requested.  A server MUST NOT return DSONOTIMP
   for the DSO Keepalive TLV, but a DSONOTIMP response could happen in
   the future, if a client attempts to establish a DSO Session using a
   future response-requiring DSO TLV that the server does not
   understand.  If the server returns DSONOTIMP then a DSO Session is



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   not considered established, but the client is permitted to continue
   sending DNS messages on the connection, including other response-
   requiring DSO messages such as the DSO Keepalive, which may result in
   a successful NOERROR response, yielding the establishment of a DSO
   Session.

   If the RCODE is set to any value other than NOERROR (0) or DSONOTIMP
   (tentatively 11), then the client should assume that the server does
   not support DSO.  In this case the client is permitted to continue
   sending DNS messages on that connection, but the client SHOULD NOT
   issue further DSO messages on that connection.

   When the server receives a response-requiring DSO request message
   from a client, and transmits a successful NOERROR response to that
   request, the server considers the DSO Session established.

   When the client receives the server's NOERROR response to its DSO
   request message, the client considers the DSO Session established.

   Once a DSO Session has been established, either end may unilaterally
   send DSO messages at any time, and therefore either client or server
   may be the initiator of a message.

   Once a DSO Session has been established, clients and servers should
   behave as described in this specification with regard to inactivity
   timeouts and session termination, not as previously prescribed in the
   earlier specification for DNS over TCP [RFC7766].
























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4.1.1.  Connection Sharing

   As previously specified for DNS over TCP [RFC7766], to mitigate the
   risk of unintentional server overload, DNS clients MUST take care to
   minimize the number of concurrent TCP connections made to any
   individual server.  It is RECOMMENDED that for any given client/
   server interaction there SHOULD be no more than one connection for
   regular queries, one for zone transfers, and one for each protocol
   that is being used on top of TCP (for example, if the resolver was
   using TLS).  However, it is noted that certain primary/secondary
   configurations with many busy zones might need to use more than one
   TCP connection for zone transfers for operational reasons (for
   example, to support concurrent transfers of multiple zones).

   A single server may support multiple services, including DNS Updates
   [RFC2136], DNS Push Notifications [I-D.ietf-dnssd-push], and other
   services, for one or more DNS zones.  When a client discovers that
   the target server for several different operations is the same target
   hostname and port, the client SHOULD use a single shared DSO Session
   for all those operations.  A client SHOULD NOT open multiple
   connections to the same target host and port just because the names
   being operated on are different or happen to fall within different
   zones.  This is to reduce unnecessary connection load on the DNS
   server.

   However, server implementers and operators should be aware that
   connection sharing may not be possible in all cases.  A single host
   device may be home to multiple independent client software instances
   that don't coordinate with each other.  Similarly, multiple
   independent client devices behind the same NAT gateway will also
   typically appear to the DNS server as different source ports on the
   same client IP address.  Because of these constraints, a DNS server
   MUST be prepared to accept multiple connections from different source
   ports on the same client IP address.

















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4.1.2.  Zero Round-Trip Operation

   There is increased awareness today of the performance benefits of
   eliminating round trips in session establishment.  Technologies like
   TCP Fast Open [RFC7413] and TLS 1.3 [I-D.ietf-tls-tls13] provide
   mechanisms to reduce or eliminate round trips in session
   establishment.

   Similarly, DSO supports zero round-trip operation.

   Having initiated a connection to a server, possibly using zero round-
   trip TCP Fast Open and/or zero round-trip TLS 1.3, a client MAY send
   multiple response-requiring DSO request messages to the server in
   succession without having to wait for a response to the first request
   message to confirm successful establishment of a DSO session.

   However, a client MUST NOT send non-response-requiring DSO request
   messages until after a DSO Session has been mutually established.

   Similarly, a server MUST NOT send DSO request messages until it has
   received a response-requiring DSO request message from a client and
   transmitted a successful NOERROR response for that request.

4.1.3.  Middlebox Considerations

   Where an application-layer middlebox (e.g., a DNS proxy, forwarder,
   or session multiplexer) is in the path the middlebox MUST NOT blindly
   forward DSO messages in either direction, and MUST treat the inbound
   and outbound connections as separate sessions.  This does not
   preclude the use of DSO messages in the presence of an IP-layer
   middlebox, such as a NAT that rewrites IP-layer and/or transport-
   layer headers but otherwise preserves the effect of a single session
   between the client and the server.

   To illustrate the above, consider a network where a middlebox
   terminates one or more TCP connections from clients and multiplexes
   the queries therein over a single TCP connection to an upstream
   server.  The DSO messages and any associated state are specific to
   the individual TCP connections.  A DSO-aware middlebox MAY in some
   circumstances be able to retain associated state and pass it between
   the client and server (or vice versa) but this would be highly TLV-
   specific.  For example, the middlebox may be able to maintain a list
   of which clients have made Push Notification subscriptions
   [I-D.ietf-dnssd-push] and make its own subscription(s) on their
   behalf, relaying any subsequent notifications to the client (or
   clients) that have subscribed to that particular notification.





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4.2.  Message Format

   A DSO message begins with the standard twelve-octet DNS message
   header [RFC1035] with the OPCODE field set to the DSO OPCODE
   (tentatively 6).  However, unlike standard DNS messages, the question
   section, answer section, authority records section and additional
   records sections are not present.  The corresponding count fields
   (QDCOUNT, ANCOUNT, NSCOUNT, ARCOUNT) MUST be set to zero on
   transmission.

   If a DSO message is received where any of the count fields are not
   zero, then a FORMERR MUST be returned, unless a future IETF Standard
   specifies otherwise.

                                                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)                    |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                                                               |
      /                           DSO Data                            /
      /                                                               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


















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4.2.1.  DNS Header Fields in DSO Messages

   In an unacknowledged request message the MESSAGE ID field MUST be set
   to zero.  In an acknowledged request message the MESSAGE ID field
   MUST be set to a unique nonzero value, that the initiator is not
   currently using for any other active operation on this connection.
   For the purposes here, a MESSAGE ID is in use in this DSO Session if
   the initiator has used it in a request for which it is still awaiting
   a response, or if the client has used it to setup a long-lived
   operation that has not yet been cancelled.  For example, a long-lived
   operation could be a Push Notification subscription
   [I-D.ietf-dnssd-push] or a Discovery Relay interface subscription
   [I-D.sctl-dnssd-mdns-relay].

   Whether a message is acknowledged or unacknowledged is determined
   only by the specification for the Primary TLV.  An acknowledgment
   cannot be requested by including a nonzero message ID in a message
   the primary TLV of which is specified to be unacknowledged, nor can
   an acknowledgment be prevented by sending a message ID of zero in a
   message with a primary TLV that is specified to be acknowledged.  A
   responder that receives either such malformed message MUST treat it
   as a programming error and terminate the connection.

   In a request message the DNS Header QR bit MUST be zero (QR=0).
   If the QR bit is not zero the message is not a request message.

   In a response message the DNS Header QR bit MUST be one (QR=1).
   If the QR bit is not one the message is not a response message.

   In a response message (QR=1) the MESSAGE ID field MUST contain a copy
   of the value of the MESSAGE ID field in the acknowledged request
   message being responded to.  In a response message (QR=1) the MESSAGE
   ID field MUST NOT be zero.  If a response message (QR=1) is received
   where the MESSAGE ID is zero this is a fatal error and the receiver
   MUST forcibly abort the connection immediately.

   The DNS Header OPCODE field holds the DSO OPCODE value (tentatively
   6).

   The Z bits are currently unused in DSO messages, and in both DSO
   requests and DSO responses the Z bits MUST be set to zero (0) on
   transmission and MUST be silently ignored on reception, unless a
   future IETF Standard specifies otherwise.








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   In a request message (QR=0) the RCODE is generally set to zero on
   transmission, and silently ignored on reception, except where
   specified otherwise (for example, the Retry Delay request message
   (see Section 5.6.3), where the RCODE indicates the reason for
   termination).

   The RCODE value in a response message (QR=1) may be one of the
   following values:

   +------+-----------+------------------------------------------------+
   | Code | Mnemonic  | Description                                    |
   +------+-----------+------------------------------------------------+
   |    0 | NOERROR   | Operation processed successfully               |
   |      |           |                                                |
   |    1 | FORMERR   | Format error                                   |
   |      |           |                                                |
   |    2 | SERVFAIL  | Server failed to process request due to a      |
   |      |           | problem with the server                        |
   |      |           |                                                |
   |    3 | NXDOMAIN  | Name Error -- Named entity does not exist      |
   |      |           | (TLV-dependent)                                |
   |      |           |                                                |
   |    4 | NOTIMP    | DSO not supported                              |
   |      |           |                                                |
   |    5 | REFUSED   | Operation declined for policy reasons          |
   |      |           |                                                |
   |    9 | NOTAUTH   | Not Authoritative (TLV-dependent)              |
   |      |           |                                                |
   |   11 | DSONOTIMP | DSO type code not supported                    |
   +------+-----------+------------------------------------------------+

   Use of the above RCODEs is likely to be common in DSO but does not
   preclude the definition and use of other codes in future documents
   that make use of DSO.

   If a document defining a new DSO TLV makes use of NXDOMAIN (Name
   Error) or NOTAUTH (Not Authoritative) then that document MUST specify
   the specific interpretation of these RCODE values in the context of
   that new DSO TLV.












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4.2.2.  DSO Data

   The standard twelve-octet DNS message header with its zero-valued
   count fields is followed by the DSO Data, expressed using TLV syntax,
   as described below Section 4.2.2.1.

   A DSO message may be either a request message or a response message,
   as indicated by the QR bit in the DNS message header.  DSO request
   messages are further subdivided into two variants, acknowledged
   request messages (which generate a corresponding response message)
   and unacknowledged request messages (which do not generate any
   corresponding response message).

   A DSO request message MUST contain at least one TLV.  The first TLV
   in a DSO request message is referred to as the "Primary TLV" and
   determines the nature of the operation being performed, including
   whether it is an acknowledged or unacknowledged operation.  In some
   cases it may be appropriate to include other TLVs in a request
   message, such as the Encryption Padding TLV (Section 6.3), and these
   extra TLVs are referred to as the "Additional TLVs".

   A DSO response message may contain no TLVs, or it may be specified to
   contain one or more TLVs appropriate to the information being
   communicated.

   A DSO response message may contain one or more TLVs with DSO-TYPE the
   same as the Primary TLV from the corresponding DSO request message,
   in which case those TLV(s) are referred to as "Response Primary
   TLVs".  A DSO response message is not required to carry Response
   Primary TLVs.  The MESSAGE ID field in the DNS message header is
   sufficient to identify to which DSO request message this response
   message relates.

   A DSO response message may contain one or more TLVs with DSO-TYPEs
   different from the Primary TLV from the corresponding DSO request
   message, in which case those TLV(s) are referred to as "Response
   Additional TLVs".

   Response Primary TLV(s), if present, MUST occur first in the response
   message, before any Response Additional TLVs.

   It is anticipated that by default most DSO request messages will be
   specified to be acknowledged request messages, which generate
   corresponding responses.  In some specialized high-traffic use cases,
   it may be appropriate to specify unacknowledged request messages.
   Unacknowledged request messages can be more efficient on the network,
   because they don't generate a stream of corresponding reply messages.
   Using unacknowledged request messages can also simplify software in



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   some cases, by removing need for an initiator to maintain state while
   it waits to receive replies it doesn't care about.  When the
   specification for a particular TLV states that, when used as a
   Primary TLV (i.e., first) in a request message, that request message
   is to be unacknowledged, the MESSAGE ID field MUST be set to zero and
   the receiver MUST NOT generate any response message corresponding to
   this unacknowledged request message.

   The previous point, that the receiver MUST NOT generate responses to
   unacknowledged request messages, applies even in the case of errors.
   When a DSO request message is received with the MESSAGE ID field set
   to zero, the receiver MUST NOT generate any response.  For example,
   if the DSO-TYPE in the Primary TLV is unrecognized, then a DSONOTIMP
   error MUST NOT be returned; instead the receiver MUST forcibly abort
   the connection immediately.

   Unacknowledged request messages MUST NOT be used "speculatively" in
   cases where the sender doesn't know if the receiver supports the
   Primary TLV in the message, because there is no way to receive any
   response to indicate success or failure of the request message (the
   request message does not contain a unique MESSAGE ID with which to
   associate a response with its corresponding request).  Unacknowledged
   request messages are only appropriate in cases where the sender
   already knows that the receiver supports and wishes to receive these
   messages.

   For example, after a client has subscribed for Push Notifications
   [I-D.ietf-dnssd-push], the subsequent event notifications are then
   sent as unacknowledged messages, and this is appropriate because the
   client initiated the message stream by virtue of its Push
   Notification subscription, thereby indicating its support of Push
   Notifications, and its desire to receive those notifications.

   Similarly, after an mDNS Relay client has subscribed to receive
   inbound mDNS traffic from an mDNS Relay, the subsequent stream of
   received packets is then sent using unacknowledged messages, and this
   is appropriate because the client initiated the message stream by
   virtue of its mDNS Relay link subscription, thereby indicating its
   support of mDNS Relay, and its desire to receive inbound mDNS packets
   over that DSO session [I-D.sctl-dnssd-mdns-relay].











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4.2.2.1.  TLV Syntax

   All TLVs, whether used as "Primary", "Additional", "Response
   Primary", or "Response Additional", use the same encoding syntax.

   The specification for a TLV determines whether, when used as the
   Primary (i.e., first) TLV in a request message, that request message
   is to be acknowledged.  If the request message is to be acknowledged,
   the specification also states which TLVs, if any, are to be included
   in the response.  The Primary TLV may or may not be contained in the
   response, depending on what is stated in the specification for that
   TLV.

                                                1   1   1   1   1   1
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                            DSO-TYPE                           |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                        DSO DATA LENGTH                        |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                                                               |
      /                      TYPE-DEPENDENT DATA                      /
      /                                                               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   DSO-TYPE:  A 16-bit unsigned integer in network (big endian) byte
      order giving the type of the current DSO TLV per the IANA DSO Type
      Code Registry.

   DSO DATA LENGTH:  A 16-bit unsigned integer in network (big endian)
      byte order giving the size in octets of the TYPE-DEPENDENT DATA.

   TYPE-DEPENDENT DATA:  Type-code specific format.

   Where domain names appear within TYPE-DEPENDENT DATA, they MAY
   be compressed using standard DNS name compression [RFC1035].
   However, the compression offsets MUST be relative to the start of the
   TYPE-DEPENDENT DATA and MUST NOT extend beyond the end of the TYPE-
   DEPENDENT DATA.












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4.2.2.2.  Request TLVs

   The first TLV in a DSO request message is the "Primary TLV" and
   indicates the operation to be performed.  A DSO request message MUST
   contain at at least one TLV, the Primary TLV.

   Immediately following the Primary TLV, a DSO request message MAY
   contain one or more "Additional TLVs", which specify additional
   parameters relating to the operation.

4.2.2.3.  Response TLVs

   Depending on the operation, a DSO response message MAY contain no
   TLVs, because it is simply a response to a previous request message,
   and the MESSAGE ID in the header is sufficient to identify the
   request in question.  Or it may contain a single response TLV, with
   the same DSO-TYPE as the Primary TLV in the request message.
   Alternatively it may contain one or more TLVs of other types, or a
   combination of the above, as appropriate for the information that
   needs to be communicated.  The specification for each DSO TLV
   determines what TLVs are required in a response to a request using
   that TLV.

   If a DSO response is received for an operation where the
   specification requires that the response carry a particular TLV or
   TLVs, and the required TLV(s) are not present, then this is a fatal
   error and the recipient of the defective response message MUST
   forcibly abort the connection immediately.























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4.2.2.4.  Unrecognized TLVs

   If DSO request is received containing an unrecognized Primary TLV,
   with a nonzero MESSAGE ID (indicating that a response is expected),
   then the receiver MUST send a response with matching MESSAGE ID, and
   RCODE DSONOTIMP (tentatively 11).  The response MUST NOT contain a
   copy of the unrecognized Primary TLV.

   If DSO request is received containing an unrecognized Primary TLV,
   with a zero MESSAGE ID (indicating that no response is expected), the
   receiver MUST silently ignore the message.  A response MUST NOT be
   sent.

   If a DSO request message is received where the Primary TLV is
   recognized, containing one or more unrecognized Additional TLVs, the
   unrecognized Additional TLVs MUST be silently ignored, and the
   remainder of the message is interpreted and handled as if the
   unrecognized parts were not present.

   Similarly, if a DSO response message is received containing one or
   more unrecognized TLVs, the unrecognized TLVs MUST be silently
   ignored, and the remainder of the message is interpreted and handled
   as if the unrecognized parts were not present.




























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4.2.3.  EDNS(0) and TSIG

   Since the ARCOUNT field MUST be zero, a DSO message MUST NOT contain
   an EDNS(0) option in the additional records section.  If
   functionality provided by current or future EDNS(0) options is
   desired for DSO messages, one or more new DSO TLVs need to be defined
   to carry the necessary information.

   For example, the EDNS(0) Padding Option [RFC7830] used for security
   purposes is not permitted in a DSO message, so if message padding is
   desired for DSO messages then the Encryption Padding TLV described in
   Section 6.3 MUST be used.

   Similarly, a DSO message MUST NOT contain a TSIG record.  A TSIG
   record in a conventional DNS message is added as the last record in
   the additional records section, and carries a signature computed over
   the preceding message content.  Since DSO data appears after the
   additional records section, it would not be included in the signature
   calculation.  If use of signatures with DSO messages becomes
   necessary in the future, a new DSO TLV needs to be defined to perform
   this function.

   Note however that, while DSO *messages* cannot include EDNS(0) or
   TSIG records, a DSO *session* is typically used to carry a whole
   series of DNS messages of different kinds, including DSO messages,
   and other DNS message types like Query [RFC1034] [RFC1035] and Update
   [RFC2136], and those messages can carry EDNS(0) and TSIG records.

   This specification explicitly prohibits use of the EDNS(0) TCP
   Keepalive Option [RFC7828] in *any* messages sent on a DSO Session
   (because it is obsoleted by the functionality provided by the DSO
   Keepalive operation), but messages may contain other EDNS(0) options
   as appropriate.


















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4.3.  Message Handling

   The initiator MUST set the value of the QR bit in the DNS header to
   zero (0), and the responder MUST set it to one (1).  Every DSO
   request message (QR=0) with a nonzero MESSAGE ID field MUST elicit a
   corresponding response (QR=1), which MUST have the same MESSAGE ID in
   the DNS message header as in the corresponding request.  DSO request
   messages sent by the client with a nonzero MESSAGE ID field elicit a
   response from the server, and DSO request messages sent by the server
   with a nonzero MESSAGE ID field elicit a response from the client.

   A DSO request message (QR=0) with a zero MESSAGE ID field MUST NOT
   elicit a response.

   The namespaces of 16-bit MESSAGE IDs are disjoint in each direction.
   For example, it is *not* an error for both client and server to send
   a request message with the same ID.  In effect, the 16-bit MESSAGE ID
   combined with the identity of the initiator (client or server) serves
   as a 17-bit unique identifier for a particular operation on a DSO
   Session.

   As described in Section 4.2.1 An initiator MUST NOT reuse a MESSAGE
   ID that is already in use for an outstanding request, unless
   specified otherwise by the relevant specification for the DSO in
   question.  At the very least, this means that a MESSAGE ID MUST NOT
   be reused in a particular direction on a particular DSO Session while
   the initiator is waiting for a response to a previous request using
   that MESSAGE ID on that DSO Session, unless specified otherwise by
   the relevant specification for the DSO in question.  (For a long-
   lived operation the MESSAGE ID for the operation MUST NOT be reused
   whilst that operation remains active.)

   If a client or server receives a response (QR=1) where the MESSAGE ID
   is zero, or any other value that does not match the MESSAGE ID of any
   of its outstanding operations, this is a fatal error and the
   recipient MUST forcibly abort the connection immediately.















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4.4.  DSO Response Generation

   With most TCP implementations, for DSO requests that generate a
   response, the TCP data acknowledgement (generated because data has
   been received by TCP), the TCP window update (generated because TCP
   has delivered that data to the receiving software), and the DSO
   response (generated by the receiving application-layer software
   itself) are all combined into a single IP packet.  Combining these
   three elements into a single IP packet gives a potentially
   significant improvement in network efficiency.

   For DSO requests that do not generate a response, the TCP
   implementation generally doesn't have any way to know that no
   response will be forthcoming, so it waits fruitlessly for the
   application-layer software to generate a response, until the Delayed
   ACK timer fires [RFC1122] (typically 200 milliseconds) and only then
   does it send the TCP ack and window update.  In conjunction with
   Nagle's Algorithm at the sender, this can delay the sender's
   transmission of its next (non-full-sized) TCP segment, while the
   sender is waiting for its previous (non-full-sized) TCP segment to be
   acknowledged, which won't happen until the Delayed ACK timer fires.
   Nagle's Algorithm exists to combine multiple small application writes
   into more efficient large TCP segments, to guard against wasteful use
   of the network by applications that would otherwise transmit a stream
   of small TCP segments, but in this case Nagle's Algorithm (created to
   improve network efficiency) can interact badly with TCP's Delayed ACK
   feature (also created to improve network efficiency) [NagleDA] with
   the result of delaying some messages by up to 200 milliseconds.

   Possible mitigations for this problem include:

   o  Disabling Nagle's Algorithm at the sender.
      This is not great, because it results
      in less efficient use of the network.

   o  Disabling Delayed ACK at the receiver.
      This is not great, because it results
      in less efficient use of the network.

   o  Using a networking API that lets the receiver signal to the TCP
      implementation that the receiver has received and processed a
      client request for which it will not be generating any immediate
      response.  This allows the TCP implementation to operate
      efficiently in both cases; for requests that generate a response,
      the TCP ack, window update, and DSO response are transmitted
      together in a single TCP segment, and for requests that do not
      generate a response, the application-layer software informs the
      TCP implementation that it should go ahead and send the TCP ack



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      and window update immediately, without waiting for the Delayed ACK
      timer.  Unfortunately it is not known at this time which (if any)
      of the widely-available networking APIs currently include this
      capability.

4.5.  Responder-Initiated Operation Cancellation

   This document, the base specification for DNS Stateful Operations,
   does not itself define any long-lived operations, but it defines a
   framework for supporting long-lived operations such as Push
   Notification subscriptions [I-D.ietf-dnssd-push] and Discovery Relay
   interface subscriptions [I-D.sctl-dnssd-mdns-relay].

   Generally speaking, a long-lived operation is initiated by the
   initiator, and, if successful, remains active until the initiator
   terminates the operation.

   However, it is possible that a long-lived operation may be valid at
   the time it was initiated, but then a later change of circumstances
   may render that previously valid operation invalid.

   For example, a long-lived client operation may pertain to a name that
   the server is authoritative for, but then the server configuration is
   changed such that it is no longer authoritative for that name.

   In such cases, instead of terminating the entire session it may be
   desirable for the responder to be able to cancel selectively only
   those operations that have become invalid.

   The responder performs this selective cancellation by sending a new
   response message, with the MESSAGE ID field containing the MESSAGE ID
   of the long-lived operation that is to be terminated (that it had
   previously acknowledged with a NOERROR RCODE), and the RCODE field of
   the new response message giving the reason for cancellation.

   After a response message with nonzero RCODE has been sent, that
   operation has been terminated from the responder's point of view, and
   the responder sends no more messages relating to that operation.

   After a response message with nonzero RCODE has been received by the
   initiator, that operation has been terminated from the initiator's
   point of view, and its MESSAGE ID is now free for reuse.









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5.  DSO Session Lifecycle and Timers

5.1.  DSO Session Initiation

   A DSO Session begins as described in Section 4.1.

   The client may perform as many DNS operations as it wishes using the
   newly created DSO Session.  Operations SHOULD be pipelined (i.e., the
   client doesn't need wait for a response before sending the next
   message).  The server MUST act on messages in the order they are
   transmitted, but responses to those messages SHOULD be sent out of
   order when appropriate.

5.2.  DSO Session Timeouts

   Two timeout values are associated with a DSO Session: the inactivity
   timeout, and the keepalive interval.

   The first timeout value, the inactivity timeout, is the maximum time
   for which a client may speculatively keep a DSO Session open in the
   expectation that it may have future requests to send to that server.

   The second timeout value, the keepalive interval, is the maximum
   permitted interval between client messages to the server if the
   client wishes to keep the DSO Session alive.

   The two timeout values are independent.  The inactivity timeout may
   be lower, the same, or higher than the keepalive interval, though in
   most cases the inactivity timeout is expected to be shorter than the
   keepalive interval.

   Only when the client has a very long-lived low-traffic operation does
   the keepalive interval come into play, to ensure that a sufficient
   residual amount of traffic is generated to maintain NAT and firewall
   state and to assure client and server that they still have
   connectivity to each other.

   On a new DSO Session, if no explicit DSO Keepalive message exchange
   has taken place, the default value for both timeouts is 15 seconds.
   For both timeouts, lower values of the timeout result in higher
   network traffic and higher CPU load on the server.










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5.3.  Inactive DSO Sessions

   At both servers and clients, the generation or reception of any
   complete DNS message, including DNS requests, responses, updates, or
   DSO messages, resets both timers for that DSO Session, with the
   exception that a DSO Keepalive message resets only the keepalive
   timer, not the inactivity timeout timer.

   In addition, for as long as the client has an outstanding operation
   in progress, the inactivity timer remains cleared, and an inactivity
   timeout cannot occur.

   For short-lived DNS operations like traditional queries and updates,
   an operation is considered in progress for the time between request
   and response, typically a period of a few hundred milliseconds at
   most.  At the client, the inactivity timer is cleared upon
   transmission of a request and remains cleared until reception of the
   corresponding response.  At the server, the inactivity timer is
   cleared upon reception of a request and remains cleared until
   transmission of the corresponding response.

   For long-lived DNS Stateful operations (such as a Push Notification
   subscription [I-D.ietf-dnssd-push] or a Discovery Relay interface
   subscription [I-D.sctl-dnssd-mdns-relay]), an operation is considered
   in progress for as long as the operation is active, until it is
   cancelled.  This means that a DSO Session can exist, with active
   operations, with no messages flowing in either direction, for far
   longer than the inactivity timeout, and this is not an error.  This
   is why there are two separate timers: the inactivity timeout, and the
   keepalive interval.  Just because a DSO Session has no traffic for an
   extended period of time does not automatically make that DSO Session
   "inactive", if it has an active operation that is awaiting events.

5.4.  The Inactivity Timeout

   The purpose of the inactivity timeout is for the server to balance
   its trade off between the costs of setting up new DSO Sessions and
   the costs of maintaining inactive DSO Sessions.  A server with
   abundant DSO Session capacity can offer a high inactivity timeout, to
   permit clients to keep a speculative DSO Session open for a long
   time, to save the cost of establishing a new DSO Session for future
   communications with that server.  A server with scarce memory
   resources can offer a low inactivity timeout, to cause clients to
   promptly close DSO Sessions whenever they have no outstanding
   operations with that server, and then create a new DSO Session later
   when needed.





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5.4.1.  Closing Inactive DSO Sessions

   A client is NOT required to wait until the inactivity timeout expires
   before closing a DSO Session.  A client MAY close a DSO Session at
   any time, at the client's discretion.  If a client determines that it
   has no current or reasonably anticipated future need for an inactive
   DSO Session, then the client SHOULD close that connection.

   If, at any time during the life of the DSO Session, the inactivity
   timeout value (i.e., 15 seconds by default) elapses without there
   being any operation active on the DSO Session, the client MUST close
   the connection gracefully.

   If, at any time during the life of the DSO Session, twice the
   inactivity timeout value (i.e., 30 seconds by default), or five
   seconds, if twice the inactivity timeout value is less than five
   seconds, elapses without there being any operation active on the DSO
   Session, the server SHOULD consider the client delinquent, and SHOULD
   forcibly abort the DSO Session.

   In this context, an operation being active on a DSO Session includes
   a query waiting for a response, an update waiting for a response, or
   an active long-lived operation, but not a DSO Keepalive message
   exchange itself.  A DSO Keepalive message exchange resets only the
   keepalive interval timer, not the inactivity timeout timer.

   If the client wishes to keep an inactive DSO Session open for longer
   than the default duration without having to send traffic every 15
   seconds, then it uses the DSO Keepalive message to request longer
   timeout values, as described in Section 6.1.

5.4.2.  Values for the Inactivity Timeout

   For the inactivity timeout value, lower values result in more
   frequent DSO Session teardown and re-establishment.  Higher values
   result in lower traffic and lower CPU load on the server, but higher
   memory burden to maintain state for inactive DSO Sessions.

   A server may dictate (in a server-initiated Keepalive message, or in
   a response to a client-initiated Keepalive request message) any value
   it chooses for the inactivity timeout.  When a connection's
   inactivity timeout is reached the client MUST begin closing the idle
   connection, but a client is NOT REQUIRED to keep an idle connection
   open until the inactivity timeout is reached -- a client SHOULD begin
   closing the connection sooner if it has no reason to expect future
   operations with that server before the inactivity timeout is reached.





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   A shorter inactivity timeout with a longer keepalive interval signals
   to the client that it should not speculatively keep an inactive DSO
   Session open for very long without reason, but when it does have an
   active reason to keep a DSO Session open, it doesn't need to be
   sending an aggressive level of keepalive traffic to maintain that
   session.

   A longer inactivity timeout with a shorter keepalive interval signals
   to the client that it may speculatively keep an inactive DSO Session
   open for a long time, but to maintain that inactive DSO Session it
   should be sending a lot of keepalive traffic.  This configuration is
   expected to be less common.

   A server may dictate any value it chooses for the inactivity timeout
   (either in a response to a client-initiated request, or in a server-
   initiated message) including values under one second, or even zero.

   An inactivity timeout of zero informs the client that it should not
   speculatively maintain idle connections at all, and as soon as the
   client has completed the operation or operations relating to this
   server, the client should immediately begin closing this session.

   An inactivity timeout of 0xFFFFFFFF (2^32-1 milliseconds,
   approximately 49.7 days) informs the client that it may keep an idle
   connection open as long as it wishes.  Note that after granting an
   unlimited inactivity timeout in this way, at any point the server may
   revise that inactivity timeout by sending a new Keepalive TLV
   dictating new Session Timeout values to the client.

   A server will abort an idle client session after twice the inactivity
   timeout value, or five seconds, whichever is greater.  In the case of
   a zero inactivity timeout value, this means that if a client fails to
   close an idle client session then the server will forcibly abort the
   idle session after five seconds.

















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5.5.  The Keepalive Interval

   The purpose of the keepalive interval is to manage the generation of
   sufficient messages to maintain state in middleboxes (such at NAT
   gateways or firewalls) and for the client and server to periodically
   verify that they still have connectivity to each other.  This allows
   them to clean up state when connectivity is lost, and attempt re-
   connection if appropriate.

5.5.1.  Keepalive Interval Expiry

   If, at any time during the life of the DSO Session, the keepalive
   interval value (i.e., 15 seconds by default) elapses without any DNS
   messages being sent or received on a DSO Session, the client MUST
   take action to keep the DSO Session alive, by sending a DSO Keepalive
   message (see Section 6.1).  A DSO Keepalive message exchange resets
   only the keepalive timer, not the inactivity timer.

   If a client disconnects from the network abruptly, without cleanly
   closing its DSO Session, leaving a long-lived operation uncanceled,
   the server learns of this after failing to receive the required
   keepalive traffic from that client.  If, at any time during the life
   of the DSO Session, twice the keepalive interval value (i.e., 30
   seconds by default) elapses without any DNS messages being sent or
   received on a DSO Session, the server SHOULD consider the client
   delinquent, and SHOULD forcibly abort the DSO Session.

5.5.2.  Values for the Keepalive Interval

   For the keepalive interval value, lower values result in a higher
   volume of keepalive traffic.  Higher values of the keepalive interval
   reduce traffic and CPU load, but have minimal effect on the memory
   burden at the server, because clients keep a DSO Session open for the
   same length of time (determined by the inactivity timeout) regardless
   of the level of keepalive traffic required.

   It may be appropriate for clients and servers to select different
   keepalive interval values depending on the nature of the network they
   are on.

   A corporate DNS server that knows it is serving only clients on the
   internal network, with no intervening NAT gateways or firewalls, can
   impose a higher keepalive interval, because frequent keepalive
   traffic is not required.

   A public DNS server that is serving primarily residential consumer
   clients, where it is likely there will be a NAT gateway on the path,




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   may impose a lower keepalive interval, to generate more frequent
   keepalive traffic.

   A smart client may be adaptive to its environment.  A client using a
   private IPv4 address [RFC1918] to communicate with a DNS server at an
   address outside that IPv4 private address block, may conclude that
   there is likely to be a NAT gateway on the path, and accordingly
   request a lower keepalive interval.

   By default it is RECOMMENDED that clients request, and servers grant,
   a keepalive interval of 60 minutes.  This keepalive interval provides
   for reasonably timely detection if a client abruptly disconnects
   without cleanly closing the session, and is sufficient to maintain
   state in firewalls and NAT gateways that follow the IETF recommended
   Best Current Practice that the "established connection idle-timeout"
   used by middleboxes be at least 2 hours 4 minutes [RFC5382].

   Note that the lower the keepalive interval value, the higher the load
   on client and server.  For example, a hypothetical keepalive interval
   value of 100ms would result in a continuous stream of at least ten
   messages per second, in both directions, to keep the DSO Session
   alive.  And, in this extreme example, a single packet loss and
   retransmission over a long path could introduce a momentary pause in
   the stream of messages, long enough to cause the server to
   overzealously abort the connection.

   Because of this concern, the server MUST NOT send a Keepalive message
   (either a response to a client-initiated request, or a server-
   initiated message) with a keepalive interval value less than ten
   seconds.  If a client receives a Keepalive message specifying a
   keepalive interval value less than ten seconds this is an error and
   the client MUST forcibly abort the connection immediately.

   A keepalive interval value of 0xFFFFFFFF (2^32-1 milliseconds,
   approximately 49.7 days) informs the client that it should generate
   no keepalive traffic.  Note that after signaling that the client
   should generate no keepalive traffic in this way, at any point the
   server may revise that keepalive traffic requirement by sending a new
   Keepalive TLV dictating new Session Timeout values to the client.












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5.6.  Server-Initiated Session Termination

   In addition to cancelling individual operations selectively (see
   Section 4.5) there are also occasions where a server may need to
   terminate one or more entire sessions wholesale.  An entire session
   may need to be terminated if the client is defective in some way, or
   departs from the network without closing its session.  Sessions may
   also need to be terminated if the server becomes overloaded, or if
   the server is reconfigured and lacks the ability to be selective
   about which operations need to be cancelled.

   This section discusses various reasons a session may be terminated,
   and the mechanisms for doing so.

5.6.1.  Server-Initiated Session Termination on Error

   After sending an error response to a client, the server MAY end the
   DSO Session, or may allow the DSO Session to remain open.  For error
   conditions that only affect the single operation in question, the
   server SHOULD return an error response to the client and leave the
   DSO Session open for further operations.  For error conditions that
   are likely to make all operations unsuccessful in the immediate
   future, the server SHOULD return an error response to the client and
   then end the DSO Session by sending a Retry Delay request message, as
   described in Section 5.6.3.

   Upon receiving an error response from the server, a client SHOULD NOT
   automatically close the DSO Session.  An error relating to one
   particular operation on a DSO Session does not necessarily imply that
   all other operations on that DSO Session have also failed, or that
   future operations will fail.  The client should assume that the
   server will make its own decision about whether or not to end the DSO
   Session, based on the server's determination of whether the error
   condition pertains to this particular operation, or would also apply
   to any subsequent operations.  If the server does not end the DSO
   Session by sending the client a Retry Delay message (see
   Section 5.6.3) then the client SHOULD continue to use that DSO
   Session for subsequent operations.













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5.6.2.  Server-Initiated Session Termination on Overload

   A server MUST NOT close a DSO Session with a client, except in
   certain exceptional circumstances, as outlined below.  In normal
   operation, closing a DSO Session is the client's responsibility.  The
   client makes the determination of when to close a DSO Session based
   on an evaluation of both its own needs, and the inactivity timeout
   value dictated by the server.

   Some exceptional situations where a server may terminate a DSO
   Session include:

   o  The server application software or underlying operating system is
      shutting down or restarting.

   o  The server application software terminates unexpectedly (perhaps
      due to a bug that makes it crash).

   o  The server is undergoing a reconfiguration or maintenance
      procedure, that, due to the way the server software is
      implemented, requires clients to be disconnected.  For example,
      some software is implemented such that it reads a configuration
      file at startup, and changing the server's configuration entails
      modifying the configuration file and then killing and restarting
      the server software, which generally entails a loss of network
      connections.

   o  The client fails to meets its obligation to generate keepalive
      traffic or close an inactive session by the prescribed time (twice
      the time interval dictated by the server, or five seconds,
      whichever is greater, as described in Section 5.2).

   o  The client sends a grossly invalid or malformed request that is
      indicative of a seriously defective client implementation (see
      Section 5.6.1).

   o  The server is over capacity and needs to shed some load (see
      Section 5.6.3).

   When a server has to close a DSO Session with a client (because of
   exceptional circumstances such as those outlined above) the server
   SHOULD, whenever possible, send a Retry Delay request message (see
   below) informing the client of the reason for the DSO Session being
   closed, and allow the client five seconds to receive it before the
   server resorts to forcibly aborting the connection.






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5.6.3.  Server-Initiated Retry Delay Request Message

   There may be rare cases where a server is overloaded and wishes to
   shed load.  If a server is low on resources it MAY simply terminate a
   client connection by forcibly aborting it.  However, the likely
   behavior of the client may be simply to to treat this as a network
   failure and reconnect immediately, putting more burden on the server.

   Therefore to avoid this reconnection implosion, a server SHOULD
   instead choose to shed client load by sending a Retry Delay request
   message, with an RCODE of SERVFAIL, to inform the client of the
   overload situation.  The format of the Retry Delay TLV is described
   in Section 6.2.  After sending a Retry Delay request message, the
   server MUST NOT send any further messages on that DSO Session.

   Upon receipt of a Retry Delay request from the server, the client
   MUST make note of the reconnect delay for this server, and then
   immediately close the connection gracefully.

   After sending a Retry Delay request message the server SHOULD allow
   the client five seconds to close the connection, and if the client
   has not closed the connection after five seconds then the server
   SHOULD forcibly abort the connection.

   A Retry Delay request message MUST NOT be initiated by a client.  If
   a server receives a Retry Delay request message this is an error and
   the server MUST forcibly abort the connection immediately.

5.6.3.1.  Outstanding Operations

   At the moment a server chooses to initiate a Retry Delay request
   message there may be DNS requests already in flight from client to
   server on this DSO Session, which will arrive at the server after its
   Retry Delay request message has been sent.  The server MUST silently
   ignore such incoming requests, and MUST NOT generate any response
   messages for them.  When the Retry Delay request message from the
   server arrives at the client, the client will determine that any DNS
   requests it previously sent on this DSO Session, that have not yet
   received a response, now will certainly not be receiving any
   response.  Such requests should be considered failed, and should be
   retried at a later time, as appropriate.

   In the case where some, but not all, of the existing operations on a
   DSO Session have become invalid (perhaps because the server has been
   reconfigured and is no longer authoritative for some of the names),
   but the server is terminating all DSO Sessions en masse with a
   REFUSED (5) RCODE, the RECONNECT DELAY MAY be zero, indicating that
   the clients SHOULD immediately attempt to re-establish operations.



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   It is likely that some of the attempts will be successful and some
   will not, depending on the nature of the reconfiguration.

   In the case where a server is terminating a large number of DSO
   Sessions 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 RECONNECT delay values to each client.  These
   adjustments MAY be selected randomly, pseudorandomly, or
   deterministically (e.g., incrementing the time value by one tenth of
   a second for each successive client, yielding a post-restart
   reconnection rate of ten clients per second).

5.6.3.2.  Client Reconnection

   After a DSO Session is closed by the server, the client SHOULD try to
   reconnect, to that server, or to another suitable server, if more
   than one is available.  If reconnecting to the same server, the
   client MUST respect the indicated delay before attempting to
   reconnect.

   If a particular server does not want a client to reconnect (the
   server is being de-commissioned), it SHOULD set the retry delay to
   the maximum value (which is approximately 49.7 days).  If the server
   will only be out of service for a maintenance period, it should use a
   value closer to the expected maintenance window and not default to a
   very large delay value or clients may not attempt to reconnect after
   it resumes service.
























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6.  Base TLVs for DNS Stateful Operations

   This section describes the three base TLVs for DNS Stateful
   Operations: Keepalive, Retry Delay, and Encryption Padding.

6.1.  Keepalive TLV

   The Keepalive TLV (DSO-TYPE=1) performs two functions: to reset the
   keepalive timer for the DSO Session, and to establish the values for
   the Session Timeouts.

   The TYPE-DEPENDENT DATA for the the Keepalive TLV is as follows:

                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 INACTIVITY TIMEOUT (32 bits)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 KEEPALIVE INTERVAL (32 bits)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   INACTIVITY TIMEOUT:  The inactivity timeout for the current DSO
      Session, specified as a 32-bit unsigned integer in network (big
      endian) byte order in units of milliseconds.  This is the timeout
      at which the client MUST begin closing an inactive DSO Session.
      The inactivity timeout can be any value of the server's choosing.
      If the client does not gracefully close an inactive DSO Session,
      then after twice this interval, or five seconds, whichever is
      greater, the server will forcibly abort the connection.

   KEEPALIVE INTERVAL:  The keepalive interval for the current DSO
      Session, specified as a 32-bit unsigned integer in network (big
      endian) byte order in units of milliseconds.  This is the interval
      at which a client MUST generate keepalive traffic to maintain
      connection state.  The keepalive interval MUST NOT be less than
      ten seconds.  If the client does not generate the mandated
      keepalive traffic, then after twice this interval the server will
      forcibly abort the connection.  Since the minimum allowed
      keepalive interval is ten seconds, the minimum time at which a
      server will forcibly disconnect a client for failing to generate
      the mandated keepalive traffic is twenty seconds.

   The transmission or reception of DSO Keepalive messages (i.e.,
   messages where the Keepalive TLV is the first TLV) reset only the
   keepalive timer, not the inactivity timer.  The reason for this is
   that periodic Keepalive messages are sent for the sole purpose of
   keeping a DSO Session alive, when that DSO Session has current or
   recent non-maintenance activity that warrants keeping that DSO



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   Session alive.  Sending keepalive traffic itself is not considered a
   client activity; it is considered a maintenance activity that is
   performed in service of other client activities.  If keepalive
   traffic itself were to reset the inactivity timer, then that would
   create a circular livelock where keepalive traffic would be sent
   indefinitely to keep a DSO Session alive, where the only activity on
   that DSO Session would be the keepalive traffic keeping the DSO
   Session alive so that further keepalive traffic can be sent.  For a
   DSO Session to be considered active, it must be carrying something
   more than just keepalive traffic.  This is why merely sending or
   receiving a Keepalive message does not reset the inactivity timer.

   When sent by a client, the Keepalive request message MUST be sent as
   an acknowledged request, with a nonzero MESSAGE ID.  If a server
   receives a Keepalive request message with a zero MESSAGE ID then this
   is a fatal error and the server MUST forcibly abort the connection
   immediately.  The Keepalive request message resets a DSO Session's
   keepalive timer, and at the same time communicates to the server the
   the client's requested Session Timeout values.  In a server response
   to a client-initiated Keepalive request message, the Session Timeouts
   contain the server's chosen values from this point forward in the DSO
   Session, which the client MUST respect.  This is modeled after the
   DHCP protocol, where the client requests a certain lease lifetime
   using DHCP option 51 [RFC2132], but the server is the ultimate
   authority for deciding what lease lifetime is actually granted.

   When a client is sending its second and subsequent Keepalive DSO
   requests to the server, the client SHOULD continue to request its
   preferred values each time.  This allows flexibility, so that if
   conditions change during the lifetime of a DSO Session, the server
   can adapt its responses to better fit the client's needs.

   Once a DSO Session is in progress (see Section 4) a Keepalive request
   message MAY be initiated by a server.  When sent by a server, the
   Keepalive request message MUST be sent as an unacknowledged request,
   with the MESSAGE ID set to zero.  The client MUST NOT generate a
   response to a server-initiated DSO Keepalive message.  If a client
   receives a Keepalive request message with a nonzero MESSAGE ID then
   this is a fatal error and the client MUST forcibly abort the
   connection immediately.  The Keepalive request message from the
   server resets a DSO Session's keepalive timer, and at the same time
   unilaterally informs the client of the new Session Timeout values to
   use from this point forward in this DSO Session.  No client DSO
   response message to this unilateral declaration is required or
   allowed.

   The Keepalive TLV is not used as a request message Additional TLV.




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   In response messages the Keepalive TLV is used only as a Response
   Primary TLV, replying to a Keepalive request message from the client.
   A Keepalive TLV MUST NOT be added as to other responses a Response
   Additional TLV.  If the server wishes to update a client's Session
   Timeout values other than in response to a Keepalive request message
   from the client, then it does so by sending an unacknowledged
   Keepalive request message of its own, as described above.

   It is not required that the Keepalive TLV be used in every DSO
   Session.  While many DNS Stateful operations will be used in
   conjunction with a long-lived session state, not all DNS Stateful
   operations require long-lived session state, and in some cases the
   default 15-second value for both the inactivity timeout and keepalive
   interval may be perfectly appropriate.  However, note that for
   clients that implement only the TLVs defined in this document it is
   the only way for a client to initiate a DSO Session.

6.1.1.  Client handling of received Session Timeout values

   When a client receives a response to its client-initiated DSO
   Keepalive message, or receives a server-initiated DSO Keepalive
   message, the client has then received Session Timeout values dictated
   by the server.  The two timeout values contained in the DSO Keepalive
   TLV from the server may each be higher, lower, or the same as the
   respective Session Timeout values the client previously had for this
   DSO Session.

   In the case of the keepalive timer, the handling of the received
   value is straightforward.  The act of receiving the message
   containing the DSO Keepalive TLV itself resets the keepalive timer
   and updates the keepalive interval for the DSO Session.  The new
   keepalive interval indicates the maximum time that may elapse before
   another message must be sent or received on this DSO Session, if the
   DSO Session is to remain alive.

















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   In the case of the inactivity timeout, the handling of the received
   value is a little more subtle, though the meaning of the inactivity
   timeout is unchanged -- it still indicates the maximum permissible
   time allowed without useful activity on a DSO Session.  The act of
   receiving the message containing the DSO Keepalive TLV does not
   itself reset the inactivity timer.  The time elapsed since the last
   useful activity on this DSO Session is unaffected by exchange of DSO
   Keepalive messages.  The new inactivity timeout value in the DSO
   Keepalive TLV in the received message does update the timeout
   associated with the running inactivity timer; that becomes the new
   maximum permissible time without activity on a DSO Session.

   o  If the current inactivity timer value is not greater than the new
      inactivity timeout, then the DSO Session may remain open for now.
      When the inactivity timer value exceeds the new inactivity
      timeout, the client MUST then begin closing the DSO Session, as
      described above.

   o  If the current inactivity timer value is already greater than the
      new inactivity timeout, then this DSO Session has already been
      inactive for longer than the server permits, and the client MUST
      immediately begin closing this DSO Session.

   o  If the current inactivity timer value is already more than twice
      the new inactivity timeout, then the client is immediately
      considered delinquent (this DSO Session is immediately eligible to
      be forcibly terminated by the server) and the client MUST
      immediately begin closing this DSO Session.  However if a server
      abruptly reduces the inactivity timeout in this way, then, to give
      the client time to close the connection gracefully before the
      server resorts to forcibly aborting it, the server SHOULD give the
      client an additional grace period of one quarter of the new
      inactivity timeout, or five seconds, whichever is greater.

6.1.2.  Relation to EDNS(0) TCP Keepalive Option

   The inactivity timeout value in the Keepalive TLV (DSO-TYPE=1) has
   similar intent to the EDNS(0) TCP Keepalive Option [RFC7828].  A
   client/server pair that supports DSO MUST NOT use the EDNS(0) TCP
   KeepAlive option within any message after a DSO Session has been
   established.  Once a DSO Session has been established, if either
   client or server receives a DNS message over the DSO Session that
   contains an EDNS(0) TCP Keepalive option, this is an error and the
   receiver of the EDNS(0) TCP Keepalive option MUST forcibly abort the
   connection immediately.






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6.2.  Retry Delay TLV

   The Retry Delay TLV (DSO-TYPE=2) can be used as a Primary TLV
   (unacknowledged) in a server-to-client message, or as a Response
   Additional TLV in a server-to-client response to a client-to-server
   request message.

   The TYPE-DEPENDENT DATA for the the Retry Delay TLV is as follows:

                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     RETRY DELAY (32 bits)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   RETRY DELAY:  A time value, specified as a 32-bit unsigned integer in
      network (big endian) byte order in units of milliseconds, within
      which the client MUST NOT retry this operation, or retry
      connecting to this server.

   The RECOMMENDED value is 10 seconds.

6.2.1.  Retry Delay TLV used as a Primary TLV

   When sent in a DSO request message, from server to client, the Retry
   Delay TLV (0) is used as a Primary TLV.  It is used by a server to
   instruct a client to close the DSO Session and underlying connection,
   and not to reconnect for the indicated time interval.

   In this case it applies to the DSO Session as a whole, and the client
   MUST begin closing the DSO Session, as described in Section 5.6.3.
   The RCODE in the message header MUST indicate the reason for the
   termination:

   o  NOERROR indicates a routine shutdown.

   o  SERVFAIL indicates that the server is overloaded due to resource
      exhaustion.

   o  REFUSED indicates that the server has been reconfigured and is no
      longer able to perform one or more of the functions currently
      being performed on this DSO Session (for example, a DNS Push
      Notification server could be reconfigured such that is is no
      longer accepting DNS Push Notification requests for one or more of
      the currently subscribed names).

   This document specifies only these three RCODE values for Retry Delay
   request.  Servers sending Retry Delay requests SHOULD use one of



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   these three values.  However, future circumstances may create
   situations where other RCODE values are appropriate in Retry Delay
   requests, so clients MUST be prepared to accept Retry Delay requests
   with any RCODE value.

   A Retry Delay request is an unacknowledged request message; the
   MESSAGE ID MUST be set to zero in the request and the client MUST NOT
   send a response.

6.2.2.  Retry Delay TLV used as a Response Additional TLV

   In the case of a client request that returns a nonzero RCODE value,
   the server MAY append a Retry Delay TLV (0) to the response,
   indicating the time interval during which the client SHOULD NOT
   attempt this operation again.

   The indicated time interval during which the client SHOULD NOT retry
   applies only to the failed operation, not to the DSO Session as a
   whole.

6.2.3.  Retry Delay TLV is used by server only

   A client MUST NOT send a Retry Delay TLV to a server, either in a DSO
   request message, or in a DSO response message.  If a server receives
   a DSO message containing a Retry Delay TLV, this is a fatal error and
   the server MUST forcibly abort the connection immediately.

























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6.3.  Encryption Padding TLV

   The Encryption Padding TLV (DSO-TYPE=3) can only be used as an
   Additional or Response Additional TLV.  It is only applicable when
   the DSO Transport layer uses encryption such as TLS.

   The TYPE-DEPENDENT DATA for the the Padding TLV is optional and is a
   variable length field containing non-specified values.  A DATA LENGTH
   of 0 essentially provides for 4 octets of padding (the minimum
   amount).

                                                1   1   1   1   1   1
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      /                                                               /
      /                   VARIABLE NUMBER OF OCTETS                   /
      /                                                               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   As specified for the EDNS(0) Padding Option [RFC7830] the PADDING
   octets SHOULD be set to 0x00.  Other values MAY be used, for example,
   in cases where there is a concern that the padded message could be
   subject to compression before encryption.  PADDING octets of any
   value MUST be accepted in the messages received.

   The Encryption Padding TLV may be included in either a DSO request,
   response, or both.  As specified for the EDNS(0) Padding Option
   [RFC7830] if a request is received with an Encryption Padding TLV,
   then the response MUST also include an Encryption Padding TLV.

   The length of padding is intentionally not specified in this document
   and is a function of current best practices with respect to the type
   and length of data in the preceding TLVs
   [I-D.ietf-dprive-padding-policy].

















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7.  Summary

   This section summarizes some noteworthy highlights about various
   components of the DSO protocol.

7.1.  MESSAGE ID

   In DSO Request Messages the MESSAGE ID may be either nonzero
   (signaling that the responder MUST generate a response) or zero
   (signaling that the responder MUST NOT generate a response).

   In DSO Response Messages the MESSAGE ID MUST NOT be zero (since this
   would be a response to a request that had indicated that a response
   is not allowed).

   The table below illustrates the legal combinations:

                             +--------------------+-------------------+
                             | Nonzero MESSAGE ID |  Zero MESSAGE ID  |
      +----------------------+--------------------+-------------------+
      | DSO Request Message  |         X          |         X         |
      +----------------------+--------------------+-------------------+
      | DSO Response Message |         X          |                   |
      +----------------------+--------------------+-------------------+



























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7.2.  TLV Usage

   The table below indicates, for each of the three TLVs defined in this
   document, whether they are valid in each of ten different contexts.

   The first five contexts are requests from client to server, and the
   corresponding responses from server back to client:

   o  C-P - Primary TLV, sent in DSO Request message, from client to
      server, with nonzero MESSAGE ID indicating that this request MUST
      generate response message.

   o  C-U - Primary TLV (unacknowledged), sent in DSO Request message,
      from client to server, with zero MESSAGE ID indicating that this
      request MUST NOT generate response message.

   o  C-A - Additional TLV, optionally added to request message from
      client to server.

   o  CRP - Response Primary TLV, included in response message sent to
      back the client (in response to a client "C-P" request with
      nonzero MESSAGE ID indicating that a response is required) where
      the DSO-TYPE of the Response TLV matches the DSO-TYPE of the
      Primary TLV in the request.

   o  CRA - Response Additional TLV, included in response message sent
      to back the client (in response to a client "C-P" request with
      nonzero MESSAGE ID indicating that a response is required) where
      the DSO-TYPE of the Response TLV does not match the DSO-TYPE of
      the Primary TLV in the request.

   The second five contexts are the reverse: requests from server to
   client, and the corresponding responses from client back to server.

                 +-------------------------+-------------------------+
                 | C-P  C-U  C-A  CRP  CRA | S-P  S-U  S-A  SRP  SRA |
    +------------+-------------------------+-------------------------+
    | KeepAlive  |  X              X       |       X                 |
    +------------+-------------------------+-------------------------+
    | RetryDelay |                      X  |       X                 |
    +------------+-------------------------+-------------------------+
    | Padding    |            X         X  |            X         X  |
    +------------+-------------------------+-------------------------+

   It is recommended that definitions of future TLVs include a similar
   table summarizing the contexts where the new TLV is valid.





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7.3.  Inactivity Timeout

   The Inactivity Timeout may have any 32-bit unsigned integer value.

   The value zero informs the client that it should not speculatively
   maintain idle connections at all, and as soon as the client has
   completed the operation or operations relating to this server, the
   client should immediately begin closing this session.

   The maximum possible value, 0xFFFFFFFF (2^32-1 milliseconds,
   approximately 49.7 days), informs the client that it may keep an idle
   connection open as long as it wishes.

   The Inactivity timer is reset by any message *except* the Keepalive
   TLV, and remains cleared any time that an operation is outstanding.

7.4.  Keepalive Interval

   The Keepalive Interval is a 32-bit unsigned integer value, with a
   minimum value of 10,000 milliseconds (10 seconds).

   The maximum possible value, 0xFFFFFFFF (2^32-1 milliseconds,
   approximately 49.7 days), informs the client that it should generate
   no keepalive traffic.

   Any message exchange (including the Keepalive TLV) resets the
   Keepalive timer.
























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8.  IANA Considerations

8.1.  DSO OPCODE Registration

   The IANA is directed to record the value (tentatively) 6 for the
   DSO OPCODE in the DNS OPCODE Registry.

8.2.  DSO RCODE Registration

   The IANA is directed to record the value (tentatively) 11 for the
   DSONOTIMP error code in the DNS RCODE Registry.

8.3.  DSO Type Code Registry

   The IANA is directed to create the 16-bit DSO Type Code Registry,
   with initial (hexadecimal) values as shown below:

   +-----------+--------------------------------+----------+-----------+
   | Type      | Name                           | Status   | Reference |
   +-----------+--------------------------------+----------+-----------+
   | 0000      | Reserved                       | Standard | RFC-TBD   |
   |           |                                |          |           |
   | 0001      | KeepAlive                      | Standard | RFC-TBD   |
   |           |                                |          |           |
   | 0002      | RetryDelay                     | Standard | RFC-TBD   |
   |           |                                |          |           |
   | 0003      | EncryptionPadding              | Standard | RFC-TBD   |
   |           |                                |          |           |
   | 0004-003F | Unassigned, reserved for       |          |           |
   |           | DSO session-management TLVs    |          |           |
   |           |                                |          |           |
   | 0040-F7FF | Unassigned                     |          |           |
   |           |                                |          |           |
   | F800-FBFF | Reserved for                   |          |           |
   |           | experimental/local use         |          |           |
   |           |                                |          |           |
   | FC00-FFFF | Reserved for future expansion  |          |           |
   +-----------+--------------------------------+----------+-----------+

   DSO Type Code zero is reserved and is not currently intended for
   allocation.

   Registrations of new DSO Type Codes in the "Reserved for DSO session-
   management" range 0004-003F and the "Reserved for future expansion"
   range FC00-FFFF require publication of an IETF Standards Action
   document [RFC5226].





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   Requests to register additional new DSO Type Codes in the
   "Unassigned" range 0040-F7FF are to be recorded by IANA after
   consultation with the registry's Designated Expert [RFC5226] at that
   time.  At the time of publication of this document, the Designated
   Expert for the newly created DSO Type Code registry is [*TBD*].

   DSO Type Codes in the "experimental/local" range F800-FBFF may be
   used as Experimental Use or Private Use values [RFC5226] and may be
   used freely for development purposes, or for other purposes within a
   single site.  No attempt is made to prevent multiple sites from using
   the same value in different (and incompatible) ways.  There is no
   need for IANA to review such assignments (since IANA does not record
   them) and assignments are not generally useful for broad
   interoperability.  It is the responsibility of the sites making use
   of "experimental/local" values to ensure that no conflicts occur
   within the intended scope of use.

9.  Security Considerations

   If this mechanism is to be used with DNS over TLS, then these
   messages are subject to the same constraints as any other DNS-over-
   TLS messages and MUST NOT be sent in the clear before the TLS session
   is established.

   The data field of the "Encryption Padding" TLV could be used as a
   covert channel.

10.  Acknowledgements

   Thanks to Tim Chown, Ralph Droms, Jan Komissar, Manju Shankar Rao,
   and Ted Lemon for their helpful contributions to this document.




















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11.  References

11.1.  Normative References

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

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
              <https://www.rfc-editor.org/info/rfc1918>.

   [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>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <https://www.rfc-editor.org/info/rfc2132>.

   [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>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, DOI 10.17487/RFC5382, October 2008,
              <https://www.rfc-editor.org/info/rfc5382>.

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






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   [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>.

   [RFC7828]  Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", RFC 7828,
              DOI 10.17487/RFC7828, April 2016,
              <https://www.rfc-editor.org/info/rfc7828>.

   [RFC7830]  Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
              DOI 10.17487/RFC7830, May 2016,
              <https://www.rfc-editor.org/info/rfc7830>.

11.2.  Informative References

   [I-D.ietf-dnssd-push]
              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-13 (work in progress), October 2017.

   [I-D.ietf-dprive-padding-policy]
              Mayrhofer, A., "Padding Policy for EDNS(0)", draft-ietf-
              dprive-padding-policy-03 (work in progress), January 2018.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-23 (work in progress),
              January 2018.

   [I-D.sctl-dnssd-mdns-relay]
              Cheshire, S. and T. Lemon, "Multicast DNS Discovery
              Relay", draft-sctl-dnssd-mdns-relay-02 (work in progress),
              November 2017.

   [NagleDA]  Cheshire, S., "TCP Performance problems caused by
              interaction between Nagle's Algorithm and Delayed ACK",
              May 2005,
              <http://www.stuartcheshire.org/papers/nagledelayedack/>.

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

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.




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   [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>.

   [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>.

   [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>.

Authors' Addresses

   Ray Bellis
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City  CA 94063
   USA

   Phone: +1 650 423 1200
   Email: ray@isc.org


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

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


   John Dickinson
   Sinodun Internet Technologies
   Magadalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   United Kingdom

   Email: jad@sinodun.com








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   Sara Dickinson
   Sinodun Internet Technologies
   Magadalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   United Kingdom

   Email: sara@sinodun.com


   Allison Mankin
   Salesforce

   Email: allison.mankin@gmail.com


   Tom Pusateri
   Unaffiliated
   Raleigh  NC 27608
   USA

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




























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