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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

DNSOP Working Group                                            R. Bellis
Internet-Draft                                                       ISC
Updates: 1035, 7766 (if approved)                            S. Cheshire
Intended status: Standards Track                              Apple Inc.
Expires: March 16, 2019                                     J. Dickinson
                                                            S. Dickinson
                                                                 Sinodun
                                                                T. Lemon
                                                     Nibbhaya Consulting
                                                             T. Pusateri
                                                            Unaffiliated
                                                      September 12, 2018


                        DNS Stateful Operations
                   draft-ietf-dnsop-session-signal-15

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.  This document updates RFC 1035 by adding a new
   DNS header opcode which has different message semantics, and a new
   result code.  This document updates RFC 7766 by redefining a session,
   providing new guidance on connection re-use, and providing a new
   mechanism for handling session idle timeouts.

Status of This Memo

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

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

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

   This Internet-Draft will expire on March 16, 2019.





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Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.1.1.  Session Management  . . . . . . . . . . . . . . . . .   9
       4.1.2.  Long-lived Subscriptions  . . . . . . . . . . . . . .   9
     4.2.  Applicable Transports . . . . . . . . . . . . . . . . . .  10
   5.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  DSO Session Establishment . . . . . . . . . . . . . . . .  12
       5.1.1.  Session Establishment Failure . . . . . . . . . . . .  12
       5.1.2.  Session Establishment Success . . . . . . . . . . . .  13
     5.2.  Operations After Session Establishment  . . . . . . . . .  14
     5.3.  Session Termination . . . . . . . . . . . . . . . . . . .  15
       5.3.1.  Handling Protocol Errors  . . . . . . . . . . . . . .  15
     5.4.  Message Format  . . . . . . . . . . . . . . . . . . . . .  16
       5.4.1.  DNS Header Fields in DSO Messages . . . . . . . . . .  17
       5.4.2.  DSO Data  . . . . . . . . . . . . . . . . . . . . . .  19
       5.4.3.  TLV Syntax  . . . . . . . . . . . . . . . . . . . . .  21
       5.4.4.  EDNS(0) and TSIG  . . . . . . . . . . . . . . . . . .  24
     5.5.  Message Handling  . . . . . . . . . . . . . . . . . . . .  25
       5.5.1.  Delayed Acknowledgement Management  . . . . . . . . .  26
       5.5.2.  MESSAGE ID Namespaces . . . . . . . . . . . . . . . .  27
       5.5.3.  Error Responses . . . . . . . . . . . . . . . . . . .  28
     5.6.  Responder-Initiated Operation Cancellation  . . . . . . .  29
   6.  DSO Session Lifecycle and Timers  . . . . . . . . . . . . . .  30
     6.1.  DSO Session Initiation  . . . . . . . . . . . . . . . . .  30
     6.2.  DSO Session Timeouts  . . . . . . . . . . . . . . . . . .  31
     6.3.  Inactive DSO Sessions . . . . . . . . . . . . . . . . . .  32
     6.4.  The Inactivity Timeout  . . . . . . . . . . . . . . . . .  33
       6.4.1.  Closing Inactive DSO Sessions . . . . . . . . . . . .  33



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       6.4.2.  Values for the Inactivity Timeout . . . . . . . . . .  34
     6.5.  The Keepalive Interval  . . . . . . . . . . . . . . . . .  35
       6.5.1.  Keepalive Interval Expiry . . . . . . . . . . . . . .  35
       6.5.2.  Values for the Keepalive Interval . . . . . . . . . .  35
     6.6.  Server-Initiated Session Termination  . . . . . . . . . .  37
       6.6.1.  Server-Initiated Retry Delay Message  . . . . . . . .  38
       6.6.2.  Client Reconnection . . . . . . . . . . . . . . . . .  39
   7.  Base TLVs for DNS Stateful Operations . . . . . . . . . . . .  41
     7.1.  Keepalive TLV . . . . . . . . . . . . . . . . . . . . . .  41
       7.1.1.  Client handling of received Session Timeout values  .  43
       7.1.2.  Relationship to edns-tcp-keepalive EDNS0 Option . . .  44
     7.2.  Retry Delay TLV . . . . . . . . . . . . . . . . . . . . .  45
       7.2.1.  Retry Delay TLV used as a Primary TLV . . . . . . . .  45
       7.2.2.  Retry Delay TLV used as a Response Additional TLV . .  47
     7.3.  Encryption Padding TLV  . . . . . . . . . . . . . . . . .  48
   8.  Summary Highlights  . . . . . . . . . . . . . . . . . . . . .  49
     8.1.  QR bit and MESSAGE ID . . . . . . . . . . . . . . . . . .  49
     8.2.  TLV Usage . . . . . . . . . . . . . . . . . . . . . . . .  50
   9.  Additional Considerations . . . . . . . . . . . . . . . . . .  52
     9.1.  Service Instances . . . . . . . . . . . . . . . . . . . .  52
     9.2.  Anycast Considerations  . . . . . . . . . . . . . . . . .  53
     9.3.  Connection Sharing  . . . . . . . . . . . . . . . . . . .  54
     9.4.  Zero Round-Trip Operation . . . . . . . . . . . . . . . .  55
     9.5.  Operational Considerations for Middlebox  . . . . . . . .  56
     9.6.  TCP Delayed Acknowledgement Considerations  . . . . . . .  57
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  60
     10.1.  DSO OPCODE Registration  . . . . . . . . . . . . . . . .  60
     10.2.  DSO RCODE Registration . . . . . . . . . . . . . . . . .  60
     10.3.  DSO Type Code Registry . . . . . . . . . . . . . . . . .  60
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  61
     11.1.  TCP Fast Open Considerations . . . . . . . . . . . . . .  62
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  62
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  62
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  62
     13.2.  Informative References . . . . . . . . . . . . . . . . .  63
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  65

1.  Introduction

   This document specifies a mechanism for managing stateful DNS
   connections.  DNS most commonly operates over a UDP transport, but
   can also operate over streaming transports; the original DNS RFC
   specifies DNS over TCP [RFC1035] and a profile for DNS over TLS
   [RFC7858] has been specified.  These 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




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   situations it is also advantageous to support server-initiated
   messages (such as DNS Push Notifications [I-D.ietf-dnssd-push]).

   The existing EDNS(0) Extension Mechanism for DNS [RFC6891] is
   explicitly defined to only have "per-message" semantics.  While
   EDNS(0) has been used to signal at least one session-related
   parameter (edns-tcp-keepalive EDNS0 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 ([TBA1], 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 TLVs defined here are mandatory for all implementations of
   DSO.  Further TLVs may be defined in additional specifications.

   DSO messages may or may not be acknowledged; this is signalled by
   providing a non-zero message ID for messages that must be
   acknowledged (DSO request messages) and a zero message ID for
   messages that are not to be acknowledged (DSO unidirectional
   messages), and is also specified in the definition of a particular
   DSO message type.  Messages are pipelined; answers may appear out of
   order when more than one answer is pending.

   The format for DSO messages (Section 5.4) differs somewhat from the
   traditional DNS message format used for standard queries and
   responses.  The standard twelve-byte 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.  Just
   as in traditional DNS over TCP [RFC1035] [RFC7766] the stream
   protocol carrying DSO messages (which are just another kind of DNS
   message) frames them by putting a 16-bit message length at the start,
   so the length of the DSO message is determined from that length,
   rather than from any of the DNS header counts.

   When displayed using packet analyzer tools that have not been updated
   to recognize the DSO format, this will result in the DSO data being




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   displayed as unknown additional data after the end of the DNS
   message.

   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 (see for example
   [I-D.ietf-dnsop-no-response-issue] sections 3.2 and 4).

   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 pseudo-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 DSO operation is communicated in its own
   separate DNS message, and the transport protocol can take care of
   packing several 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.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.












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3.  Terminology

   DSO:  DNS Stateful Operations.

   connection:  a bidirectional byte (or message) stream, where the
      bytes (or messages) are delivered reliably and in-order, such as
      provided by using DNS over TCP [RFC1035] [RFC7766] or DNS over TLS
      [RFC7858].

   session:  The unqualified term "session" in the context of this
      document refers to a persistent network connection between two
      endpoints which allows for the exchange of DNS messages over a
      connection where either end of the connection can send messages to
      the other end.  (The term has no relationship to the "session
      layer" of the OSI "seven-layer model".)

   DSO Session:  a session 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].

   close gracefully:  a normal session shutdown, where the client closes
      the TCP connection to the server using a graceful close, such that
      no data is lost (e.g., using TCP FIN, see Section 5.3).

   forcibly abort:  a session shutdown as a result of a fatal error,
      where the TCP connection is unilaterally aborted without regard
      for data loss (e.g., using TCP RST, see Section 5.3).

   server:  the software with a listening socket, awaiting incoming
      connection requests, in the usual DNS sense.

   client:  the software which initiates a connection to the server's
      listening socket, in the usual DNS sense.

   initiator:  the software which sends a DSO request message or a DSO
      unidirectional message during a DSO session.  Either a client or
      server can be an initiator

   responder:  the software which receives a DSO request message or a
      DSO unidirectional message during a DSO

   session.  Either a client or server can be a responder.

   sender:  the software which is sending a DNS message, a DSO message,
      a DNS response, or a DSO response.




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   receiver:  the software which is receiving a DNS message, a DSO
      message, a DNS response, or a DSO response.

   service instance:  a specific instance of server software running on
      a specific host (Section 9.1).

   long-lived operation:  a long-lived operation is an outstanding
      operation on a DSO session where either the client or server,
      acting as initiator, has requested that the responder send new
      information regarding the request, as it becomes available.

   DNS message:  any DNS message, including DNS queries, response,
      updates, DSO messages, etc.

   DNS request message:  any DNS message where the QR bit is 0.

   DNS response message:  any DNS message where the QR bit is 1.

   DSO message:  a DSO request message, DSO unidirectional message, or a
      DSO response to a DSO request message.  If the QR bit is 1 in a
      DSO message, it is a DSO response message.  If the QR bit is 0 in
      a DSO message, it is a DSO request message or DSO unidirectional
      message, as determined by the specification of its primary TLV.

   DSO response message:  a response to a DSO request message.

   DSO request message:  a DSO message that requires a response.

   DSO unidirectional message:  a DSO message that does not require and
      cannot induce a response.

   Primary TLV:  The first TLV in a DSO message or DSO response; in the
      DSO message this determines the nature of the operation being
      performed.

   Additional TLV:  Any TLVs in a DSO message response that follow the
      primary TLV.

   Response Primary TLV:  The (optional) first TLV in a DSO response.

   Response Additional TLV:  Any TLVs in a DSO response that follow the
      (optional) Response Primary TLV.

   inactivity timer:  the time since the most recent non-keepalive DNS
      message was sent or received.  (see Section 6.4)

   keepalive timer:  the time since the most recent DNS message was sent
      or received.  (see Section 6.5)



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   session timeouts:  the inactivity timer and the keepalive timer.

   inactivity timeout:  the maximum value that the inactivity timer can
      have before the connection is gracefully closed.

   keepalive interval:  the maximum value that the keepalive timer can
      have before the client is required to send a keepalive.  (see
      Section 7.1)

   resetting a timer:  setting the timer value to zero and restarting
      the timer.

   clearing a timer:  setting the timer value to zero but not restarting
      the timer.





































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4.  Applicability

   DNS Stateful Operations are applicable to several known use cases and
   are only applicable on transports that are capable of supporting a
   DSO Session.

4.1.  Use Cases

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

4.1.1.  Session Management

   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-tcp-
   keepalive EDNS0 Option [RFC7828].  The simple set of TLVs defined in
   this document is sufficient to greatly enhance connection management
   for this use case.

4.1.2.  Long-lived Subscriptions

   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.












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4.2.  Applicable Transports

   DNS Stateful Operations are applicable in cases where it is useful to
   maintain an open session between a DNS client and server, where the
   transport allows such a session to be maintained, and where the
   transport guarantees in-order delivery of messages, on which DSO
   depends.  Examples of transports that can support DNS Stateful
   Operations are DNS-over-TCP [RFC1035] [RFC7766] and DNS-over-TLS
   [RFC7858].

   Note that in the case of DNS over TLS, there is no mechanism for
   upgrading from DNS-over-TCP to DNS-over-TLS mid-connection (see
   [RFC7858] section 7).  A connection is either DNS-over-TCP from the
   start, or DNS-over-TLS from the start.

   DNS Stateful Operations are not applicable for transports that cannot
   support clean session semantics, or that do not guarantee in-order
   delivery.  While in principle such a transport could be constructed
   over UDP, the current DNS specification over UDP transport [RFC1035]
   does not provide in-order delivery or session semantics, and hence
   cannot be used.  Similarly, DNS-over-HTTP
   [I-D.ietf-doh-dns-over-https] cannot be used because HTTP has its own
   mechanism for managing sessions, and this is incompatible with the
   mechanism specified here.

   No other transports are currently defined for use with DNS Stateful
   Operations.  Such transports can be added in the future, if they meet
   the requirements set out in the first paragraph of this section.























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5.  Protocol Details

   The overall flow of DNS Stateful Operations goes through a series of
   phases:

   Connection Establishment:  A client establishes a connection to a
      server.  (Section 4.2)

   Connected but sessionless:  A connection exists, but a DSO session
      has not been established.  DNS messages can be sent from the
      client to server, and DNS responses can be sent from servers to
      clients.  In this state a client that wishes to use DSO can
      attempt to establish a DSO session (Section 5.1).  Standard DNS-
      over-TCP inactivity timeout handling is in effect [RFC7766] (see
      Section 7.1.2).

   DSO Session Establishment in Progress:  A client has sent a DSO
      request, but has not yet received a DSO response.  In this phase,
      the client may send more DSO requests and more DNS requests, but
      MUST NOT send DSO unidirectional messages (Section 5.1).

   DSO Session Establishment Failed:  The attempt to establish the DSO
      session did not succeed.  At this point, the client is permitted
      to continue operating without a DSO session (Connected but
      Sessionless) but does not send further DSO messages (Section 5.1).

   DSO Session Established:  Both client and server may send DSO
      messages and DNS messages; both may send replies in response to
      messages they receive (Section 5.2).  The inactivity timer
      (Section 6.4) is active; the keepalive timer (Section 6.5) is
      active.  Standard DNS-over-TCP inactivity timeout handling is no
      longer in effect [RFC7766] (see Section 7.1.2).

   Server Shutdown:  The server has decided to gracefully terminate the
      session, and has sent the client a Retry Delay message
      (Section 6.6.1).  There may still be unprocessed messages from the
      client; the server will ignore these.  The server will not send
      any further messages to the client (Section 6.6.1.1).

   Client Shutdown:  The client has decided to disconnect, either
      because it no longer needs service, the connection is inactive
      (Section 6.4.1), or because the server sent it a Retry Delay
      message (Section 6.6.1).  The client closes the connection
      gracefully Section 5.3.

   Reconnect:  The client disconnected as a result of a server shutdown.
      The client either waits for the server-specified Retry Delay to
      expire (Section 6.6.2), or else contacts a different server



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      instance.  If the client no longer needs service, it does not
      reconnect.

   Forcibly Abort:  The client or server detected a protocol error, and
      further communication would have undefined behavior.  The client
      or server forcibly aborts the connection (Section 5.3).

   Abort Reconnect Wait:  The client has forcibly aborted the
      connection, but still needs service.  Or, the server forcibly
      aborted the connection, but the client still needs service.  The
      client either connects to a different service instance
      (Section 9.1) or waits to reconnect (Section 6.6.2.1).

5.1.  DSO Session Establishment

   In order for a session to be established between a client and a
   server, the client must first establish a connection to the server,
   using an applicable transport (see Section 4).

   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.  In this
   case, the session is established as soon as the connection is
   established; this is referred to as implicit session establishment.

   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 or DSO unidirectional messages
   until a DSO Session has been mutually established by at least one
   successful DSO request/response exchange initiated by the client, as
   described below.  This is referred to as explicit session
   establishment.

   Until a DSO session has been implicitly or explicitly established, a
   client MUST NOT initiate DSO unidirectional messages.

   A DSO Session is established over a connection by the client sending
   a DSO request message, such as a DSO Keepalive request message
   (Section 7.1), and receiving a response, with matching MESSAGE ID,
   and RCODE set to NOERROR (0), indicating that the DSO request was
   successful.

5.1.1.  Session Establishment Failure

   If the response RCODE is set to NOTIMP (4), or in practise any value
   other than NOERROR (0) or DSOTYPENI (defined below), then the client
   MUST assume that the server does not implement DSO at all.  In this



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   case the client is permitted to continue sending DNS messages on that
   connection, but the client MUST NOT issue further DSO messages on
   that connection.

   If the RCODE in the response is set to DSOTYPENI ("DSO-TYPE Not
   Implemented", [TBA2] tentatively RCODE 11) this indicates that the
   server does support DSO, but does not implement the DSO-TYPE of the
   primary TLV in this DSO request message.  A server implementing DSO
   MUST NOT return DSOTYPENI for a DSO Keepalive request message,
   because the Keepalive TLV is mandatory to implement.  But in the
   future, if a client attempts to establish a DSO Session using a
   response-requiring DSO request message using some newly-defined DSO-
   TYPE that the server does not understand, that would result in a
   DSOTYPENI response.  If the server returns DSOTYPENI then a DSO
   Session is not considered established, but the client is permitted to
   continue sending DNS messages on the connection, including other DSO
   messages such as the DSO Keepalive, which may result in a successful
   NOERROR response, yielding the establishment of a DSO Session.

   Two other possibilities exist: the server might drop the connection,
   or the server might send no response to the DSO message.

   In the first case, the client SHOULD mark that service instance as
   not supporting DSO, and not attempt a DSO connection for some period
   of time (at least an hour) after the failed attempt.  The client MAY
   reconnect but not use DSO, if appropriate (Section 6.6.2.2).

   In the second case, the client SHOULD wait 30 seconds, after which
   time the server will be assumed not to support DSO.  If the server
   doesn't respond within 30 seconds, the client MUST forcibly abort the
   connection to the server, since the server's behavior is out of spec,
   and hence its state is undefined.  The client MAY reconnect, but not
   use DSO, if appropriate (Section 6.6.2.1).

5.1.2.  Session Establishment Success

   When the server receives a 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 appropriate DSO messages at any time, and therefore either
   client or server may be the initiator of a message.





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5.2.  Operations After Session Establishment

   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].

   Because a server that supports DNS Stateful Operations MUST return an
   RCODE of NOERROR when it receives a Keepalive TLV DSO request
   message, the Keepalive TLV is an ideal candidate for use in
   establishing a DSO session.  Any other option that can only succeed
   when sent to a server of the desired kind is also a good candidate
   for use in establishing a DSO session.  For clients that implement
   only the DSO-TYPEs defined in this base specification, sending a
   Keepalive TLV is the only DSO request message they have available to
   initiate a DSO Session.  Even for clients that do implement other
   future DSO-TYPEs, for simplicity they MAY elect to always send an
   initial DSO Keepalive request message as their way of initiating a
   DSO Session.  A future definition of a new response-requiring DSO-
   TYPE gives implementers the option of using that new DSO-TYPE if they
   wish, but does not change the fact that sending a Keepalive TLV
   remains a valid way of initiating a DSO Session.





























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5.3.  Session Termination

   A "DSO Session" is terminated when the underlying connection is
   closed.  Sessions are "closed gracefully" as a result of the server
   closing a session because it is overloaded, the client closing the
   session because it is done, or the client closing the session because
   it is inactive.  Sessions are "forcibly aborted" when either the
   client or server closes the connection because of a protocol error.

   o  Where this specification says, "close gracefully," that means
      sending a TLS close_notify (if TLS is in use) 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.

   o  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.

5.3.1.  Handling Protocol Errors

   In protocol implementation there are generally two kinds of errors
   that software writers have to deal with.  The first is situations
   that arise due to factors in the environment, such as temporary loss
   of connectivity.  While undesirable, these situations do not indicate
   a flaw in the software, and they are situations that software should
   generally be able to recover from.

   The second is situations that should never happen when communicating
   with a compliant DSO implementation.  If they do happen, they
   indicate a serious flaw in the protocol implementation, beyond what
   it is reasonable to expect software to recover from.  This document
   describes this latter form of error condition as a "fatal error" and
   specifies that an implementation encountering a fatal error condition
   "MUST forcibly abort the connection immediately".













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5.4.  Message Format

   A DSO message begins with the standard twelve-byte DNS message header
   [RFC1035] with the OPCODE field set to the DSO OPCODE.  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.

                                                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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5.4.1.  DNS Header Fields in DSO Messages

   In a DSO unidirectional message the MESSAGE ID field MUST be set to
   zero.  In a DSO 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 DSO request message for which it is still awaiting a
   response, or if the client has used it to set up 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.ietf-dnssd-mdns-relay].

   Whether a message is a DSO request message or a DSO unidirectional
   message is determined only by the specification for the Primary TLV.
   An acknowledgment cannot be requested by including a nonzero message
   ID in a message that is required according to its primary TLV to be
   unidirectional.  Nor can an acknowledgment be prevented by sending a
   message ID of zero in a message that is required to be a DSO request
   message according to its primary TLV.  A responder that receives
   either such malformed message MUST treat it as a fatal error and
   forcibly abort the connection immediately.

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

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

   The DNS Header OPCODE field holds the DSO OPCODE value.

   The Z bits are currently unused in DSO messages, and in both DSO
   request messages and DSO responses the Z bits MUST be set to zero (0)
   on transmission and MUST be ignored on reception.

   In a DSO request message (QR=0) the RCODE is set according to the
   definition of the request.  For example, in a Retry Delay message
   (Section 6.6.1) the RCODE indicates the reason for termination.
   However, in most cases, except where clearly specified otherwise, in



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   a DSO request message (QR=0) the RCODE is set to zero on
   transmission, and silently ignored on reception.

   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 DSO request message |
   |        |           | due to a problem with the server             |
   |        |           |                                              |
   |      4 | NOTIMP    | DSO not supported                            |
   |        |           |                                              |
   |      5 | REFUSED   | Operation declined for policy reasons        |
   |        |           |                                              |
   | [TBA2] | DSOTYPENI | Primary TLV's DSO-Type is not implemented    |
   |     11 |           |                                              |
   +--------+-----------+----------------------------------------------+

   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-TYPE makes use of response codes not
   defined here, then that document MUST specify the specific
   interpretation of those RCODE values in the context of that new DSO
   TLV.



















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5.4.2.  DSO Data

   The standard twelve-byte DNS message header with its zero-valued
   count fields is followed by the DSO Data, expressed using TLV syntax,
   as described below in Section 5.4.3.

   A DSO request message or DSO unidirectional message MUST contain at
   least one TLV.  The first TLV in a DSO request message or DSO
   unidirectional message is referred to as the "Primary TLV" and
   determines the nature of the operation being performed, including
   whether it is a DSO request or a DSO unidirectional operation.  In
   some cases it may be appropriate to include other TLVs in a DSO
   request message or DSO unidirectional message, such as the Encryption
   Padding TLV (Section 7.3), and these extra TLVs are referred to as
   the "Additional TLVs" and are not limited to what is defined in this
   document.  New "Additional TLVs" may be defined in the future and
   those definitions will describe when their use is appropriate.

   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.  This includes "Primary TLVs" and "Additional TLVs"
   defined in this document as well as in future TLV definitions.  It
   may be permissible for an additional TLV to appear in a response to a
   primary TLV even though the specification of that primary TLV does
   not specify it explicitly.  See Section 8.2 for more information.

   A DSO response message may contain one or more TLVs with the Primary
   TLV DSO-TYPE the same as the Primary TLV from the corresponding DSO
   request message or it may contain zero or more Additional TLVs only.
   The MESSAGE ID field in the DNS message header is sufficient to
   identify the DSO request message to which 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 most DSO operations will be specified to use
   DSO request messages, which generate corresponding DSO responses.  In
   some specialized high-traffic use cases, it may be appropriate to
   specify DSO unidirectional messages.  DSO unidirectional messages can
   be more efficient on the network, because they don't generate a
   stream of corresponding reply messages.  Using DSO unidirectional
   messages can also simplify software in some cases, by removing need



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   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 an outgoing
   DSO request message (i.e., QR=0), that message is to be
   unidirectional, the MESSAGE ID field MUST be set to zero and the
   receiver MUST NOT generate any response message corresponding to this
   DSO unidirectional message.

   The previous point, that the receiver MUST NOT generate responses to
   DSO unidirectional messages, applies even in the case of errors.

   When a DSO message is received where both the QR bit and the MESSAGE
   ID field are zero, the receiver MUST NOT generate any response.  For
   example, if the DSO-TYPE in the Primary TLV is unrecognized, then a
   DSOTYPENI error MUST NOT be returned; instead the receiver MUST
   forcibly abort the connection immediately.

   DSO unidirectional 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.  DSO unidirectional 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 DSO unidirectional 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 a Discovery Relay client has subscribed to receive
   inbound mDNS (multicast DNS, [RFC6762]) traffic from a Discovery
   Relay, the subsequent stream of received packets is then sent using
   DSO unidirectional messages, and this is appropriate because the
   client initiated the message stream by virtue of its Discovery Relay
   link subscription, thereby indicating its support of Discovery Relay,
   and its desire to receive inbound mDNS packets over that DSO session
   [I-D.ietf-dnssd-mdns-relay].












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5.4.3.  TLV Syntax

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

   Specifications that define new TLVs must specify whether the DSO-TYPE
   can be used as the Primary TLV, used as an Additional TLV, or used in
   either context, both in the case of requests and of responses.  The
   specification for a TLV must also state whether, when used as the
   Primary (i.e., first) TLV in a DSO message (i.e., QR=0), that DSO
   message is unidirectional or is a request message which requires a
   response.  If the DSO message requires a response, the specification
   must also state 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 specified 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-LENGTH                           |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                                                               |
      /                           DSO-DATA                            /
      /                                                               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

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

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

   DSO-DATA:  Type-code specific format.  The generic DSO machinery
      treats the DSO-DATA as an opaque "blob" without attempting to
      interpret it.  Interpretation of the meaning of the DSO-DATA for a
      particular DSO-TYPE is the responsibility of the software that
      implements that DSO-TYPE.











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5.4.3.1.  Request TLVs

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

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

5.4.3.2.  Response TLVs

   Depending on the operation, a DSO response message MAY contain no
   TLVs, because it is simply a response to a previous DSO request
   message, and the MESSAGE ID in the header is sufficient to identify
   the DSO 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 DSO
   request message 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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5.4.3.3.  Unrecognized TLVs

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

   If DSO unidirectional message is received containing an unrecognized
   Primary TLV, with a zero MESSAGE ID (indicating that no response is
   expected), then this is a fatal error and the recipient MUST forcibly
   abort the connection immediately.

   If a DSO request message or DSO unidirectional 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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5.4.4.  EDNS(0) and TSIG

   Since the ARCOUNT field MUST be zero, a DSO message cannot contain a
   valid 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 7.3 MUST be used.

   A DSO message can't contain a TSIG record, because a TSIG record is
   included in the additional section of the message, which would mean
   that ARCOUNT would be greater than zero.  DSO messages are required
   to have an ARCOUNT of zero.  Therefore, if use of signatures with DSO
   messages becomes necessary in the future, a new DSO TLV would have 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.

   Although messages may contain other EDNS(0) options as appropriate,
   this specification explicitly prohibits use of the edns-tcp-keepalive
   EDNS0 Option [RFC7828] in *any* messages sent on a DSO Session
   (because it is obsoleted by the functionality provided by the DSO
   Keepalive operation).  If any message sent on a DSO Session contains
   an edns-tcp-keepalive EDNS0 Option this is a fatal error and the
   recipient of the defective message MUST forcibly abort the connection
   immediately.

















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5.5.  Message Handling

   As described above in Section 5.4.1, whether an outgoing DSO message
   with the QR bit in the DNS header set to zero is a DSO request or DSO
   unidirectional message is determined by the specification for the
   Primary TLV, which in turn determines whether the MESSAGE ID field in
   that outgoing message will be zero or nonzero.

   Every DSO message with the QR bit in the DNS header set to zero and a
   nonzero MESSAGE ID field is a DSO request message, and MUST elicit a
   corresponding response, with the QR bit in the DNS header set to one
   and the MESSAGE ID field set to the value given in the corresponding
   DSO request message.

   Valid DSO request messages sent by the client with a nonzero MESSAGE
   ID field elicit a response from the server, and valid DSO request
   messages sent by the server with a nonzero MESSAGE ID field elicit a
   response from the client.

   Every DSO message with both the QR bit in the DNS header and the
   MESSAGE ID field set to zero is a DSO unidirectional message, and
   MUST NOT elicit a response.





























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5.5.1.  Delayed Acknowledgement Management

   Generally, most good TCP implementations employ a delayed
   acknowledgement timer to provide more efficient use of the network
   and better performance.

   With a DSO request message, the TCP implementation waits for the
   application-layer client software to generate the corresponding DSO
   response message, which enables the TCP implementation to send a
   single combined IP packet containing the TCP acknowledgement, the TCP
   window update, and the application-generated DSO response message.
   This is more efficient than sending three separate IP packets.

   With a DSO unidirectional message or DSO response message, there is
   no corresponding application-generated DSO response message, and
   consequently, no hint to the transport protocol about when it should
   send its acknowledgement and window update.  Some networking APIs
   provide a mechanism that allows the application-layer client software
   to signal to the transport protocol that no response will be
   forthcoming (in effect it can be thought of as a zero-length "empty"
   write).  Where available in the networking API being used, the
   recipient of a DSO unidirectional message or DSO response message,
   having parsed and interpreted the message, SHOULD then use this
   mechanism provided by the networking API to signal that no response
   for this message will be forthcoming, so that the TCP implementation
   can go ahead and send its acknowledgement and window update without
   further delay.  See Section 9.6 for further discussion of why this is
   important.























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5.5.2.  MESSAGE ID Namespaces

   The namespaces of 16-bit MESSAGE IDs are independent in each
   direction.  This means it is *not* an error for both client and
   server to send DSO request messages at the same time as each other,
   using the same MESSAGE ID, in different directions.  This
   simplification is necessary in order for the protocol to be
   implementable.  It would be infeasible to require the client and
   server to coordinate with each other regarding allocation of new
   unique MESSAGE IDs.  It is also not necessary to require the client
   and server to coordinate with each other regarding allocation of new
   unique MESSAGE IDs.  The value of the 16-bit MESSAGE ID combined with
   the identity of the initiator (client or server) is sufficient to
   unambiguously identify the operation in question.  This can be
   thought of as a 17-bit message identifier space, using message
   identifiers 0x00001-0x0FFFF for client-to-server DSO request
   messages, and message identifiers 0x10001-0x1FFFF for server-to-
   client DSO request messages.  The least-significant 16 bits are
   stored explicitly in the MESSAGE ID field of the DSO message, and the
   most-significant bit is implicit from the direction of the message.

   As described above in Section 5.4.1, an initiator MUST NOT reuse a
   MESSAGE ID that it already has in use for an outstanding DSO request
   message (unless specified otherwise by the relevant specification for
   the DSO-TYPE in question).  At the very least, this means that a
   MESSAGE ID can't be reused in a particular direction on a particular
   DSO Session while the initiator is waiting for a response to a
   previous DSO request message using that MESSAGE ID on that DSO
   Session (unless specified otherwise by the relevant specification for
   the DSO-TYPE in question), and for a long-lived operation the MESSAGE
   ID for the operation can't be reused while that operation remains
   active.

   If a client or server receives a response (QR=1) where the MESSAGE ID
   is zero, or is 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.

   If a responder receives a DSO request message (QR=0) where the
   MESSAGE ID is not zero, and the responder tracks request MESSAGE IDs,
   and the MESSAGE ID matches the MESSAGE ID of a DSO request message it
   received for which a response has not yet been sent, it MUST forcibly
   abort the connection immediately.  This behavior is required to
   prevent a hypothetical attack that takes advantage of undefined
   behavior in this case.  However, if the responder does not track
   MESSAGE IDs in this way, no such risk exists, so tracking MESSAGE IDs
   just to implement this sanity check is not required.




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5.5.3.  Error Responses

   When a DSO unidirectional message type is received (MESSAGE ID field
   is zero), the receiver should already be expecting this DSO message
   type.  Section 5.4.3.3 describes the handling of unknown DSO message
   types.  Parsing errors MUST also result in the receiver forcibly
   aborting the connection.  When a DSO unidirectional message of an
   unexpected type is received, the receiver SHOULD forcibly abort the
   connection.  Whether the connection should be forcibly aborted for
   other internal errors processing the DSO unidirectional message is
   implementation dependent, according to the severity of the error.

   When a DSO request message is unsuccessful for some reason, the
   responder returns an error code to the initiator.

   In the case of a server returning an error code to a client in
   response to an unsuccessful DSO request message, the server MAY
   choose to end the DSO Session, or MAY choose to 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 message, as described in Section 6.6.1.

   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 (Section 6.6.1)
   then the client SHOULD continue to use that DSO Session for
   subsequent operations.












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5.6.  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.ietf-dnssd-mdns-relay].

   Long-lived operations, if successful, will remain 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 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 the cancelled operation's MESSAGE ID is now free
   for reuse.















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6.  DSO Session Lifecycle and Timers

6.1.  DSO Session Initiation

   A DSO Session begins as described in Section 5.1.

   The client may perform as many DNS operations as it wishes using the
   newly created DSO Session.  When the client has multiple messages to
   send, it SHOULD NOT wait for each response before sending the next
   message.

   The server MUST act on messages in the order they are received, but
   SHOULD NOT delay sending responses to those messages as they become
   available in order to return them in the order the requests were
   received.

   Section 6.2.1.1 of the DNS-over-TCP specification [RFC7766] specifies
   this in more detail.

































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6.2.  DSO Session Timeouts

   Two timeout values are associated with a DSO Session: the inactivity
   timeout, and the keepalive interval.  Both values are communicated in
   the same TLV, the Keepalive TLV (Section 7.1).

   The first timeout value, the inactivity timeout, is the maximum time
   for which a client may speculatively keep an inactive 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 messages 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.

   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 DSO keepalive traffic to maintain that
   session.  An example of this would be a client that has subscribed to
   DNS Push notifications: in this case, the client is not sending any
   traffic to the server, but the session is not inactive, because there
   is a active request to the server to receive push notifications.

   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 DSO keepalive traffic.  This configuration
   is expected to be less common.

   In the usual case where the inactivity timeout is shorter than the
   keepalive interval, it is only when a client has a long-lived, low-
   traffic, operation that the keepalive interval comes 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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6.3.  Inactive DSO Sessions

   At both servers and clients, the generation or reception of any
   complete DNS message (including DNS requests, responses, updates, DSO
   messages, etc.) resets both timers for that DSO Session, with the one
   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.ietf-dnssd-mdns-relay]), an operation is considered
   in progress for as long as the operation is active, i.e. 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.



















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6.4.  The Inactivity Timeout

   The purpose of the inactivity timeout is for the server to balance
   the 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.

6.4.1.  Closing Inactive DSO Sessions

   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 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 a currently inactive DSO
   Session, then the client SHOULD gracefully 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 MUST consider the client delinquent, and MUST
   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 then it uses the DSO Keepalive message to
   request longer timeout values, as described in Section 7.1.






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6.4.2.  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 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.

   A server will forcibly 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.

   An inactivity timeout of 0xFFFFFFFF represents "infinity" and 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 DSO Keepalive message dictating new Session Timeout
   values to the client.

   The largest *finite* inactivity timeout supported by the current
   Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
   days).



















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6.5.  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 to establish a
   new session if appropriate.

6.5.1.  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 (Section 7.1).  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, perhaps leaving a long-lived operation
   uncancelled, the server learns of this after failing to receive the
   required DSO 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.

6.5.2.  Values for the Keepalive Interval

   For the keepalive interval value, lower values result in a higher
   volume of DSO 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 DSO 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 DSO 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 DSO
   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]
   [RFC7857].

   Note that the lower the keepalive interval value, the higher the load
   on client and server.  For example, a (hypothetical and unrealistic)
   keepalive interval value of 100 ms would result in a continuous
   stream of ten messages per second or more, 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 of over 200 ms, long enough
   to cause the server to overzealously abort the connection.

   Because of this concern, the server MUST NOT send a DSO 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 DSO Keepalive message specifying
   a keepalive interval value less than ten seconds this is a fatal
   error and the client MUST forcibly abort the connection immediately.

   A keepalive interval value of 0xFFFFFFFF represents "infinity" and
   informs the client that it should generate no DSO keepalive traffic.
   Note that after signaling that the client should generate no DSO
   keepalive traffic in this way, at any point the server may revise
   that DSO keepalive traffic requirement by sending a new DSO Keepalive
   message dictating new Session Timeout values to the client.

   The largest *finite* keepalive interval supported by the current
   Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
   days).







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6.6.  Server-Initiated Session Termination

   In addition to cancelling individual long-lived operations
   selectively (Section 5.6) there are also occasions where a server may
   need to terminate one or more entire sessions.  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.

   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.  A server only
   causes a DSO Session to be ended in the exceptional circumstances
   outlined below.  Some of the exceptional situations in which 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, causing the underlying operating
      system to send a TCP RST).

   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 the required
      DSO keepalive traffic, or to 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 6.2).

   o  The client sends a grossly invalid or malformed request that is
      indicative of a seriously defective client implementation.

   o  The server is over capacity and needs to shed some load.



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6.6.1.  Server-Initiated Retry Delay Message

   In the cases described above where a server elects to terminate a DSO
   Session, it could do so simply by forcibly aborting the connection.
   However, if it did this the likely behavior of the client might 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 message,
   with an appropriate RCODE value informing the client of the reason
   the DSO Session needs to be terminated.  The format of the Retry
   Delay TLV, and the interpretations of the various RCODE values, are
   described in Section 7.2.  After sending a Retry Delay message, the
   server MUST NOT send any further messages on that DSO Session.

   The server MAY randomize retry delays in situations where many retry
   delays are sent in quick succession, so as to avoid all the clients
   attempting to reconnect at once.  In general, implementations should
   avoid using the Retry Delay message in a way that would result in
   many clients reconnecting at the same time, if every client attempts
   to reconnect at the exact time specified.

   Upon receipt of a Retry Delay message 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 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 message MUST NOT be initiated by a client.  If a server
   receives a Retry Delay message this is a fatal error and the server
   MUST forcibly abort the connection immediately.

6.6.1.1.  Outstanding Operations

   At the instant a server chooses to initiate a Retry Delay 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 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 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




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   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 affected DSO Sessions en masse by
   sending them all a Retry Delay message, the reconnect delay MAY be
   zero, indicating that the clients SHOULD immediately attempt to re-
   establish operations.

   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).

6.6.2.  Client Reconnection

   After a DSO Session is ended by the server (either by sending the
   client a Retry Delay message, or by forcibly aborting the underlying
   transport connection) the client SHOULD try to reconnect, to that
   service instance, or to another suitable service instance, if more
   than one is available.  If reconnecting to the same service instance,
   the client MUST respect the indicated delay, if available, before
   attempting to reconnect.  Clients should not attempt to randomize the
   delay; the server will randomly jitter the retry delay values it
   sends to each client if this behavior is desired.

   If the service instance will only be out of service for a short
   maintenance period, it should use a value a little longer that the
   expected maintenance window.  It should not default to a very large
   delay value, or clients may not attempt to reconnect after it resumes
   service.

   If a particular service instance does not want a client to reconnect
   ever (perhaps the service instance is being de-commissioned), it
   SHOULD set the retry delay to the maximum value 0xFFFFFFFF (2^32-1
   milliseconds, approximately 49.7 days).  It is not possible to
   instruct a client to stay away for longer than 49.7 days.  If, after
   49.7 days, the DNS or other configuration information still indicates
   that this is the valid service instance for a particular service,



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   then clients MAY attempt to reconnect.  In reality, if a client is
   rebooted or otherwise lose state, it may well attempt to reconnect
   before 49.7 days elapses, for as long as the DNS or other
   configuration information continues to indicate that this is the
   service instance the client should use.

6.6.2.1.  Reconnecting After a Forcible Abort

   If a connection was forcibly aborted by the client, the client SHOULD
   mark that service instance as not supporting DSO.  The client MAY
   reconnect but not attempt to use DSO, or may connect to a different
   service instance, if applicable.

6.6.2.2.  Reconnecting After an Unexplained Connection Drop

   It is also possible for a server to forcibly terminate the
   connection; in this case the client doesn't know whether the
   termination was the result of a protocol error or a network outage.
   The client could determine which of the two is occurring by noticing
   if a connection is repeatedly dropped by the server; if so, the
   client can mark the server as not supporting DSO.

6.6.2.3.  Probing for Working DSO Support

   Once a server has been marked by the client as not supporting DSO,
   the client SHOULD NOT attempt DSO operations on that server until
   some time has elapsed.  A reasonable minimum would be an hour.  Since
   forcibly aborted connections are the result of a software failure,
   it's not likely that the problem will be solved in the first hour
   after it's first encountered.  However, by restricting the retry
   interval to an hour, the client will be able to notice when the
   problem has been fixed without placing an undue burden on the server.



















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7.  Base TLVs for DNS Stateful Operations

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

7.1.  Keepalive TLV

   The Keepalive TLV (DSO-TYPE=1) performs two functions.  Primarily it
   establishes the values for the Session Timeouts.  Incidentally, it
   also resets the keepalive timer for the DSO Session, meaning that it
   can be used as a kind of "no-op" message for the purpose of keeping a
   session alive.  The client will request the desired session timeout
   values and the server will acknowledge with the response values that
   it requires the client to use.

   The DSO-DATA for 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 DSO keepalive traffic to
      maintain connection state.  The keepalive interval MUST NOT be
      less than ten seconds.  If the client does not generate the
      mandated DSO 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 DSO 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



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   keepalive timer, not the inactivity timer.  The reason for this is
   that periodic DSO 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
   Session alive.  Sending DSO 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 DSO
   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 DSO Keepalive message does not reset the
   inactivity timer.

   When sent by a client, the DSO Keepalive request message MUST be sent
   as an DSO request message, with a nonzero MESSAGE ID.  If a server
   receives a DSO Keepalive message with a zero MESSAGE ID then this is
   a fatal error and the server MUST forcibly abort the connection
   immediately.  The DSO Keepalive request message resets a DSO
   Session's keepalive timer, and at the same time communicates to the
   server the client's requested Session Timeout values.  In a server
   response to a client-initiated DSO 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 DSO Keepalive
   request messages 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 (Section 5.1) a DSO Keepalive
   message MAY be initiated by a server.  When sent by a server, the DSO
   Keepalive message MUST be sent as a DSO unidirectional message, 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
   DSO Keepalive request message with a nonzero MESSAGE ID then this is
   a fatal error and the client MUST forcibly abort the connection
   immediately.  The DSO Keepalive unidirectional 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



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   use from this point forward in this DSO Session.  No client DSO
   response to this unilateral declaration is required or allowed.

   In DSO Keepalive response messages, the Keepalive TLV is REQUIRED and
   is used only as a Response Primary TLV sent as a reply to a DSO
   Keepalive request message from the client.  A Keepalive TLV MUST NOT
   be added to other responses as a Response Additional TLV.  If the
   server wishes to update a client's Session Timeout values other than
   in response to a DSO Keepalive request message from the client, then
   it does so by sending an DSO Keepalive unidirectional 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 DSO-TYPEs defined in this document, a
   DSO Keepalive request message is the only way for a client to
   initiate a DSO Session.

7.1.1.  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 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.

   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 remains as specified -- it still indicates the maximum
   permissible time allowed without useful activity on a DSO Session.
   The act of receiving the message containing the 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



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   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 less than the new
      inactivity timeout, then the DSO Session may remain open for now.
      When the inactivity timer value reaches the new inactivity
      timeout, the client MUST then begin closing the DSO Session, as
      described above.

   o  If the current inactivity timer value is equal to the new
      inactivity timeout, then this DSO Session has been inactive for
      exactly as long as the server will permit, and now the client MUST
      immediately begin closing this DSO Session.

   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.

7.1.2.  Relationship to edns-tcp-keepalive EDNS0 Option

   The inactivity timeout value in the Keepalive TLV (DSO-TYPE=1) has
   similar intent to the edns-tcp-keepalive EDNS0 Option [RFC7828].  A
   client/server pair that supports DSO MUST NOT use the edns-tcp-
   keepalive EDNS0 Option within any message after a DSO Session has
   been established.  A client that has sent a DSO message to establish
   a session MUST NOT send an edns-tcp-keepalive EDNS0 Option from this
   point on.  Once a DSO Session has been established, if either client
   or server receives a DNS message over the DSO Session that contains
   an edns-tcp-keepalive EDNS0 Option, this is a fatal error and the
   receiver of the edns-tcp-keepalive EDNS0 Option MUST forcibly abort
   the connection immediately.







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7.2.  Retry Delay TLV

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

   The DSO-DATA for 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 initiator MUST NOT retry this operation, or retry
      connecting to this server.  Recommendations for the RETRY DELAY
      value are given in Section 6.6.1.

7.2.1.  Retry Delay TLV used as a Primary TLV

   When sent from server to client, the Retry Delay TLV is used as the
   Primary TLV in a DSO unidirectional message.  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 6.6.1.
   The RCODE in the message header SHOULD indicate the principal reason
   for the termination:

   o  NOERROR indicates a routine shutdown or restart.

   o  FORMERR indicates that a client request was too badly malformed
      for the session to continue.

   o  SERVFAIL indicates that the server is overloaded due to resource
      exhaustion and needs to shed load.

   o  REFUSED indicates that the server has been reconfigured, and at
      this time it is now unable to perform one or more of the long-
      lived client operations that were previously being performed on
      this DSO Session.

   o  NOTAUTH indicates that the server has been reconfigured and at
      this time it is now unable to perform one or more of the long-
      lived client operations that were previously being performed on



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      this DSO Session because it does not have authority over the names
      in question (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 RCODE values for the Retry Delay
   message.  Servers sending Retry Delay messages SHOULD use one of
   these values.  However, future circumstances may create situations
   where other RCODE values are appropriate in Retry Delay messages, so
   clients MUST be prepared to accept Retry Delay messages with any
   RCODE value.

   In some cases, when a server sends a Retry Delay message to a client,
   there may be more than one reason for the server wanting to end the
   session.  Possibly the configuration could have been changed such
   that some long-lived client operations can no longer be continued due
   to policy (REFUSED), and other long-lived client operations can no
   longer be performed due to the server no longer being authoritative
   for those names (NOTAUTH).  In such cases the server MAY use any of
   the applicable RCODE values, or RCODE=NOERROR (routine shutdown or
   restart).

   Note that the selection of RCODE value in a Retry Delay message is
   not critical, since the RCODE value is generally used only for
   information purposes, such as writing to a log file for future human
   analysis regarding the nature of the disconnection.  Generally
   clients do not modify their behavior depending on the RCODE value.
   The RETRY DELAY in the message tells the client how long it should
   wait before attempting a new connection to this service instance.

   For clients that do in some way modify their behavior depending on
   the RCODE value, they should treat unknown RCODE values the same as
   RCODE=NOERROR (routine shutdown or restart).

   A Retry Delay message from server to client is a DSO unidirectional
   message; the MESSAGE ID MUST be set to zero in the outgoing message
   and the client MUST NOT send a response.

   A client MUST NOT send a Retry Delay DSO message to a server.  If a
   server receives a DSO message where the Primary TLV is the Retry
   Delay TLV, this is a fatal error and the server MUST forcibly abort
   the connection immediately.








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7.2.2.  Retry Delay TLV used as a Response Additional TLV

   In the case of a DSO request message that results in a nonzero RCODE
   value, the responder MAY append a Retry Delay TLV to the response,
   indicating the time interval during which the initiator SHOULD NOT
   attempt this operation again.

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









































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7.3.  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 DSO-DATA for the Padding TLV is optional and is a variable length
   field containing non-specified values.  A DSO-LENGTH of 0 essentially
   provides for 4 bytes 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
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      /                                                               /
      /              PADDING -- VARIABLE NUMBER OF BYTES              /
      /                                                               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   As specified for the EDNS(0) Padding Option [RFC7830] the PADDING
   bytes 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 bytes of any value
   MUST be accepted in the messages received.

   The Encryption Padding TLV may be included in either a DSO request
   message, response, or both.  As specified for the EDNS(0) Padding
   Option [RFC7830] if a DSO request message is received with an
   Encryption Padding TLV, then the DSO 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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8.  Summary Highlights

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

8.1.  QR bit and MESSAGE ID

   In DSO Request Messages the QR bit is 0 and the MESSAGE ID is
   nonzero.

   In DSO Response Messages the QR bit is 1 and the MESSAGE ID is
   nonzero.

   In DSO Unidirectional Messages the QR bit is 0 and the MESSAGE ID is
   zero.

   The table below illustrates which combinations are legal and how they
   are interpreted:

               +------------------------------+------------------------+
               |       MESSAGE ID zero        |   MESSAGE ID nonzero   |
      +--------+------------------------------+------------------------+
      |  QR=0  |  DSO unidirectional Message  |  DSO Request Message   |
      +--------+------------------------------+------------------------+
      |  QR=1  |    Invalid - Fatal Error     |  DSO Response Message  |
      +--------+------------------------------+------------------------+

























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8.2.  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 DSO requests or DSO unidirectional
   messages 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, sent in DSO Unidirectional 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 a DSO request message or
      DSO unidirectional message from client to server.

   o  CRP - Response Primary TLV, included in response message sent back
      to 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
      back to 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 their counterparts in the opposite
   direction: DSO requests or DSO unidirectional messages from server to
   client, and the corresponding responses from client back to server.

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

   o  S-U - Primary TLV, sent in DSO Unidirectional message, from server
      to client, with zero MESSAGE ID indicating that this request MUST
      NOT generate response message.

   o  S-A - Additional TLV, optionally added to a DSO request message or
      DSO unidirectional message from server to client.





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   o  SRP - Response Primary TLV, included in response message sent back
      to the server (in response to a server "S-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  SRA - Response Additional TLV, included in response message sent
      back to the server (in response to a server "S-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.

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

   Note that some of the columns in this table are currently empty.  The
   table provides a template for future TLV definitions to follow.  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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9.  Additional Considerations

9.1.  Service Instances

   We use the term service instance to refer to software running on a
   host which can receive connections on some set of IP address and port
   tuples.  What makes the software an instance is that regardless of
   which of these tuples the client uses to connect to it, the client is
   connected to the same software, running on the same node (but see
   Section 9.2), and will receive the same answers and the same keying
   information.

   Service instances are identified from the perspective of the client.
   If the client is configured with IP addresses and port number tuples,
   it has no way to tell if the service offered at one tuple is the same
   server that is listening on a different tuple.  So in this case, the
   client treats each such tuple as if it references a separate service
   instance.

   In some cases a client is configured with a hostname and a port
   number (either implicitly, where the port number is omitted and
   assumed, or explicitly, as in the case of DNS SRV records).  In these
   cases, the (hostname, port) tuple uniquely identifies the service
   instance (hostname comparisons are case-insensitive [RFC1034].

   It is possible that two hostnames might point to some common IP
   addresses; this is a configuration error which the client is not
   obliged to detect.  The effect of this could be that after being told
   to disconnect, the client might reconnect to the same server because
   it is represented as a different service instance.

   Implementations SHOULD NOT resolve hostnames and then perform
   matching of IP address(es) in order to evaluate whether two entities
   should be determined to be the "same service instance".

















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9.2.  Anycast Considerations

   When an anycast service is configured on a particular IP address and
   port, it must be the case that although there is more than one
   physical server responding on that IP address, each such server can
   be treated as equivalent.  What we mean by "equivalent" here is that
   both servers can provide the same service and, where appropriate, the
   same authentication information, such as PKI certificates, when
   establishing connections.

   In principle, anycast servers could maintain sufficient state that
   they can both handle packets in the same TCP connection.  In order
   for this to work with DSO, they would need to also share DSO state.
   It is unlikely that this can be done successfully, however, so we
   recommend that each anycast server instance maintain its own session
   state.

   If a change in network topology causes packets in a particular TCP
   connection to be sent to an anycast server instance that does not
   know about the connection, the new server will automatically
   terminate the connection with a TCP reset, since it will have no
   record of the connection, and then the client can reconnect or stop
   using the connection, as appropriate.

   If after the connection is re-established, the client's assumption
   that it is connected to the same service is violated in some way,
   that would be considered to be incorrect behavior in this context.
   It is however out of the possible scope for this specification to
   make specific recommendations in this regard; that would be up to
   follow-on documents that describe specific uses of DNS stateful
   operations.




















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9.3.  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
   service instance (see Section 9.1), the client SHOULD use a single
   shared DSO Session for all those operations.

   This requirement has two benefits.  First, it reduces unnecessary
   connection load on the DNS server.  Second, it avoids paying the TCP
   slow start penalty when making subsequent connections to the same
   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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9.4.  Zero Round-Trip Operation

   DSO permits zero round-trip operation using TCP Fast Open [RFC7413]
   and TLS 1.3 [RFC8446] to reduce or eliminate round trips in session
   establishment.

   A client MAY send multiple response-requiring DSO messages using TCP
   fast open or TLS 1.3 early data, without having to wait for a DSO
   response to the first DSO request message to confirm successful
   establishment of a DSO session.

   However, a client MUST NOT send DSO unidirectional 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.

   Caution must be taken to ensure that DSO messages sent before the
   first round-trip is completed are idempotent, or are otherwise immune
   to any problems that could be result from the inadvertent replay that
   can occur with zero round-trip operation.





























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9.5.  Operational Considerations for Middlebox

   Where an application-layer middlebox (e.g., a DNS proxy, forwarder,
   or session multiplexer) is in the path, care must be taken to avoid a
   configuration in which DSO traffic is mis-handled.  The simplest way
   to avoid such problems is to avoid using middleboxes.  When this is
   not possible, middleboxes should be evaluated to make sure that they
   behave correctly.

   Correct behavior for middleboxes consists of one of:

   o  The middlebox does not forward DSO messages, and responds to DSO
      messages with a response code other than NOERROR or DSOTYPENI.

   o  The middlebox acts as a DSO server and follows this specification
      in establishing connections.

   o  There is a 1:1 correspondence between incoming and outgoing
      connections, such that when a connection is established to the
      middlebox, it is guaranteed that exactly one corresponding
      connection will be established from the middlebox to some DNS
      resolver, and all incoming messages will be forwarded without
      modification or reordering.  An example of this would be a NAT
      forwarder or TCP connection optimizer (e.g. for a high-latency
      connection such as a geosynchronous satellite link).

   Middleboxes that do not meet one of the above criteria are very
   likely to fail in unexpected and difficult-to-diagnose ways.  For
   example, a DNS load balancer might unbundle DNS messages from the
   incoming TCP stream and forward each message from the stream to a
   different DNS server.  If such a load balancer is in use, and the DNS
   servers it points implement DSO and are configured to enable DSO, DSO
   session establishment will succeed, but no coherent session will
   exist between the client and the server.  If such a load balancer is
   pointed at a DNS server that does not implement DSO or is configured
   not to allow DSO, no such problem will exist, but such a
   configuration risks unexpected failure if new server software is
   installed which does implement DSO.

   It is of course possible to implement a middlebox that properly
   supports DSO.  It is even possible to implement one that implements
   DSO with long-lived operations.  This can be done either by
   maintaining a 1:1 correspondence between incoming and outgoing
   connections, as mentioned above, or by terminating incoming sessions
   at the middlebox, but maintaining state in the middlebox about any
   long-lived that are requested.  Specifying this in detail is beyond
   the scope of this document.




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9.6.  TCP Delayed Acknowledgement Considerations

   Most modern implementations of the Transmission Control Protocol
   (TCP) include a feature called "Delayed Acknowledgement" [RFC1122].

   Without this feature, TCP can be very wasteful on the network.  For
   illustration, consider a simple example like remote login, using a
   very simple TCP implementation that lacks delayed acks.  When the
   user types a keystroke, a data packet is sent.  When the data packet
   arrives at the server, the simple TCP implementation sends an
   immediate acknowledgement.  Mere milliseconds later, the server
   process reads the one byte of keystroke data, and consequently the
   simple TCP implementation sends an immediate window update.  Mere
   milliseconds later, the server process generates the character echo,
   and sends this data back in reply.  The simple TCP implementation
   then sends this data packet immediately too.  In this case, this
   simple TCP implementation sends a burst of three packets almost
   instantaneously (ack, window update, data).

   Clearly it would be more efficient if the TCP implementation were to
   combine the three separate packets into one, and this is what the
   delayed ack feature enables.

   With delayed ack, the TCP implementation waits after receiving a data
   packet, typically for 200 ms, and then send its ack if (a) more data
   packet(s) arrive (b) the receiving process generates some reply data,
   or (c) 200 ms elapses without either of the above occurring.

   With delayed ack, remote login becomes much more efficient,
   generating just one packet instead of three for each character echo.

   The logic of delayed ack is that the 200 ms delay cannot do any
   significant harm.  If something at the other end were waiting for
   something, then the receiving process should generate the reply that
   the thing at the end is waiting for, and TCP will then immediately
   send that reply (and the ack and window update).  And if the
   receiving process does not in fact generate any reply for this
   particular message, then by definition the thing at the other end
   cannot be waiting for anything, so the 200 ms delay is harmless.

   This assumption may be true, unless the sender is using Nagle's
   algorithm, a similar efficiency feature, created to protect the
   network from poorly written client software that performs many rapid
   small writes in succession.  Nagle's algorithm allows these small
   writes to be combined into larger, less wasteful packets.






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   Unfortunately, Nagle's algorithm and delayed ack, two valuable
   efficiency features, can interact badly with each other when used
   together [NagleDA].

   DSO request messages elicit responses; DSO unidirectional messages
   and DSO response messages do not.

   For DSO request messages, which do elicit responses, Nagle's
   algorithm and delayed ack work as intended.

   For DSO messages that do not elicit responses, the delayed ack
   mechanism causes the ack to be delayed by 200 ms.  The 200 ms delay
   on the ack can in turn cause Nagle's algorithm to prevent the sender
   from sending any more data for 200 ms until the awaited ack arrives.
   On an enterprise GigE backbone with sub-millisecond round-trip times,
   a 200 ms delay is enormous in comparison.

   When this issues is raised, there are two solutions that are often
   offered, neither of them ideal:

   1.  Disable delayed ack.  For DSO messages that elicit no response,
       removing delayed ack avoids the needless 200 ms delay, and sends
       back an immediate ack, which tells Nagle's algorithm that it
       should immediately grant the sender permission to send its next
       packet.  Unfortunately, for DSO messages that *do* elicit a
       response, removing delayed ack removes the efficiency gains of
       combining acks with data, and the responder will now send two or
       three packets instead of one.

   2.  Disable Nagle's algorithm.  When acks are delayed by the delayed
       ack algorithm, removing Nagle's algorithm prevents the sender
       from being blocked from sending its next small packet
       immediately.  Unfortunately, on a network with a higher round-
       trip time, removing Nagle's algorithm removes the efficiency
       gains of combining multiple small packets into fewer larger ones,
       with the goal of limiting the number of small packets in flight
       at any one time.

   For DSO messages that elicit a response, delayed ack and Nagle's
   algorithm do the right thing.

   The problem here is that with DSO messages that elicit no response,
   the TCP implementation is stuck waiting, unsure if a response is
   about to be generated, or whether the TCP implementation should go
   ahead and send an ack and window update.

   The solution is networking APIs that allow the receiver to inform the
   TCP implementation that a received message has been read, processed,



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   and no response for this message will be generated.  TCP can then
   stop waiting for a response that will never come, and immediately go
   ahead and send an ack and window update.

   For implementations of DSO, disabling delayed ack is NOT RECOMMENDED,
   because of the harm this can do to the network.

   For implementations of DSO, disabling Nagle's algorithm is NOT
   RECOMMENDED, because of the harm this can do to the network.

   At the time that this document is being prepared for publication, it
   is known that at least one TCP implementation provides the ability
   for the recipient of a TCP message to signal that it is not going to
   send a response, and hence the delayed ack mechanism can stop
   waiting.  Implementations on operating systems where this feature is
   available SHOULD make use of it.



































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10.  IANA Considerations

10.1.  DSO OPCODE Registration

   The IANA is requested to record the value [TBA1] (tentatively 6) for
   the DSO OPCODE in the DNS OPCODE Registry.  DSO stands for DNS
   Stateful Operations.

10.2.  DSO RCODE Registration

   The IANA is requested to record the value [TBA2] (tentatively 11) for
   the DSOTYPENI error code in the DNS RCODE Registry.  The DSOTYPENI
   error code ("DSO-TYPE Not Implemented") indicates that the receiver
   does implement DNS Stateful Operations, but does not implement the
   specific DSO-TYPE of the primary TLV in the DSO request message.

10.3.  DSO Type Code Registry

   The IANA is requested 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 | 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"




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   range FC00-FFFF require publication of an IETF Standards Action
   document [RFC8126].

   Requests to register additional new DSO Type Codes in the
   "Unassigned" range 0040-F7FF are to be recorded by IANA after Expert
   Review [RFC8126].  The expert review should validate that the
   requested type code is specified in a way that conforms to this
   specification, and that the intended use for the code would not be
   addressed with an experimental/local assignment.

   DSO Type Codes in the "experimental/local" range F800-FBFF may be
   used as Experimental Use or Private Use values [RFC8126] 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.

11.  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.

   When designing new DSO TLVs, the potential for data in the TLV to be
   used as a tracking identifier should be taken into consideration, and
   should be avoided when not required.

   When used without TLS or similar cryptographic protection, a
   malicious entity maybe able to inject a malicious unidirectional DSO
   Retry Delay Message into the data stream, specifying an unreasonably
   large RETRY DELAY, causing a denial-of-service attack against the
   client.

   The establishment of DSO sessions has an impact on the number of open
   TCP connections on a DNS server.  Additional resources may be used on
   the server as a result.  However, because the server can limit the
   number of DSO sessions established and can also close existing DSO
   sessions as needed, denial of service or resource exhaustion should
   not be a concern.




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11.1.  TCP Fast Open Considerations

   It would be possible to add a TLV that requires the server to do some
   significant work, and send that to the server as initial data in a
   TCP SYN packet.  A flood of such packets could be used as a DoS
   attack on the server.  None of the TLVs defined here have this
   property.  If a new TLV is specified that does have this property,
   the specification should require that some kind of exchange be done
   with the server before work is done.  That is, the TLV that requires
   work could not be processed without a round-trip from the server to
   the client to verify that the source address of the packet is
   reachable.

   One way to accomplish this would be to have the client send a TLV
   indicating that it wishes to have the server do work of this sort;
   this TLV would not actually result in work being done, but would
   request a nonce from the server.  The client could then use that
   nonce to request that work be done.

   Alternatively, the server could simply disable TCP fast open.  This
   same problem would exist for DNS-over-TLS with TLS early data; the
   same remedies would apply.

12.  Acknowledgements

   Thanks to Stephane Bortzmeyer, Tim Chown, Ralph Droms, Paul Hoffman,
   Jan Komissar, Edward Lewis, Allison Mankin, Rui Paulo, David
   Schinazi, Manju Shankar Rao, and Bernie Volz for their helpful
   contributions to this document.

13.  References

13.1.  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>.





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   [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>.

   [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>.

   [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>.

   [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>.

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

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

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

13.2.  Informative References

   [I-D.ietf-dnsop-no-response-issue]
              Andrews, M. and R. Bellis, "A Common Operational Problem
              in DNS Servers - Failure To Respond.", draft-ietf-dnsop-
              no-response-issue-11 (work in progress), July 2018.

   [I-D.ietf-dnssd-mdns-relay]
              Lemon, T. and S. Cheshire, "Multicast DNS Discovery
              Relay", draft-ietf-dnssd-mdns-relay-01 (work in progress),
              July 2018.

   [I-D.ietf-dnssd-push]
              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-14 (work in progress), March 2018.



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   [I-D.ietf-doh-dns-over-https]
              Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", draft-ietf-doh-dns-over-https-14 (work in
              progress), August 2018.

   [I-D.ietf-dprive-padding-policy]
              Mayrhofer, A., "Padding Policy for EDNS(0)", draft-ietf-
              dprive-padding-policy-06 (work in progress), July 2018.

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

   [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>.

   [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>.

   [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>.

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

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

   [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>.

   [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>.







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   [RFC7857]  Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar,
              S., and K. Naito, "Updates to Network Address Translation
              (NAT) Behavioral Requirements", BCP 127, RFC 7857,
              DOI 10.17487/RFC7857, April 2016, <https://www.rfc-
              editor.org/info/rfc7857>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

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.
   One Apple Park Way
   Cupertino  CA 95014
   USA

   Phone: +1 (408) 996-1010
   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


   Ted Lemon
   Nibbhaya Consulting
   P.O. Box 958
   Brattleboro  VT 05302-0958
   USA

   Email: mellon@fugue.com


   Tom Pusateri
   Unaffiliated
   Raleigh  NC 27608
   USA

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

























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