[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-bellis-dnsop-session-signal) 00 01 02 03 04 05 06 07 08 09

DNSOP Working Group                                            R. Bellis
Internet-Draft                                                       ISC
Updates: 1035, 7766 (if approved)                            S. Cheshire
Intended status: Standards Track                              Apple Inc.
Expires: November 17, 2018                                  J. Dickinson
                                                            S. Dickinson
                                                                 Sinodun
                                                                T. Lemon
                                                     Barefoot Consulting
                                                             T. Pusateri
                                                            Unaffiliated
                                                            May 16, 2018


                        DNS Stateful Operations
                   draft-ietf-dnsop-session-signal-09

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 and result code which has different message
   semantics.  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 https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on November 17, 2018.





Bellis, et al.          Expires November 17, 2018               [Page 1]


Internet-Draft           DNS Stateful Operations                May 2018


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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  DSO Session Establishment . . . . . . . . . . . . . . . .  10
       5.1.1.  Connection Sharing  . . . . . . . . . . . . . . . . .  12
       5.1.2.  Zero Round-Trip Operation . . . . . . . . . . . . . .  12
       5.1.3.  Middlebox Considerations  . . . . . . . . . . . . . .  13
     5.2.  Message Format  . . . . . . . . . . . . . . . . . . . . .  14
       5.2.1.  DNS Header Fields in DSO Messages . . . . . . . . . .  15
       5.2.2.  DSO Data  . . . . . . . . . . . . . . . . . . . . . .  17
       5.2.3.  EDNS(0) and TSIG  . . . . . . . . . . . . . . . . . .  22
     5.3.  Message Handling  . . . . . . . . . . . . . . . . . . . .  23
       5.3.1.  Error Responses . . . . . . . . . . . . . . . . . . .  24
     5.4.  DSO Response Generation . . . . . . . . . . . . . . . . .  25
     5.5.  Responder-Initiated Operation Cancellation  . . . . . . .  26
   6.  DSO Session Lifecycle and Timers  . . . . . . . . . . . . . .  27
     6.1.  DSO Session Initiation  . . . . . . . . . . . . . . . . .  27
     6.2.  DSO Session Timeouts  . . . . . . . . . . . . . . . . . .  27
     6.3.  Inactive DSO Sessions . . . . . . . . . . . . . . . . . .  28
     6.4.  The Inactivity Timeout  . . . . . . . . . . . . . . . . .  29
       6.4.1.  Closing Inactive DSO Sessions . . . . . . . . . . . .  29
       6.4.2.  Values for the Inactivity Timeout . . . . . . . . . .  30
     6.5.  The Keepalive Interval  . . . . . . . . . . . . . . . . .  31
       6.5.1.  Keepalive Interval Expiry . . . . . . . . . . . . . .  31
       6.5.2.  Values for the Keepalive Interval . . . . . . . . . .  31
     6.6.  Server-Initiated Session Termination  . . . . . . . . . .  33
       6.6.1.  Server-Initiated Retry Delay Message  . . . . . . . .  34
   7.  Base TLVs for DNS Stateful Operations . . . . . . . . . . . .  37
     7.1.  Keepalive TLV . . . . . . . . . . . . . . . . . . . . . .  37



Bellis, et al.          Expires November 17, 2018               [Page 2]


Internet-Draft           DNS Stateful Operations                May 2018


       7.1.1.  Client handling of received Session Timeout values  .  39
       7.1.2.  Relation to EDNS(0) TCP Keepalive Option  . . . . . .  41
     7.2.  Retry Delay TLV . . . . . . . . . . . . . . . . . . . . .  42
       7.2.1.  Retry Delay TLV used as a Primary TLV . . . . . . . .  42
       7.2.2.  Retry Delay TLV used as a Response Additional TLV . .  44
     7.3.  Encryption Padding TLV  . . . . . . . . . . . . . . . . .  44
   8.  Summary Highlights  . . . . . . . . . . . . . . . . . . . . .  45
     8.1.  QR bit and MESSAGE ID . . . . . . . . . . . . . . . . . .  45
     8.2.  TLV Usage . . . . . . . . . . . . . . . . . . . . . . . .  46
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  48
     9.1.  DSO OPCODE Registration . . . . . . . . . . . . . . . . .  48
     9.2.  DSO RCODE Registration  . . . . . . . . . . . . . . . . .  48
     9.3.  DSO Type Code Registry  . . . . . . . . . . . . . . . . .  48
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  49
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  49
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     12.2.  Informative References . . . . . . . . . . . . . . . . .  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  52

1.  Introduction

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

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

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





Bellis, et al.          Expires November 17, 2018               [Page 3]


Internet-Draft           DNS Stateful Operations                May 2018


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

   The format for DSO messages (Section 5.2) 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.  When displayed
   using packet analyzer tools that have not been updated to recognize
   the DSO format, this will result in the DSO data being displayed as
   unknown additional data after the end of the DNS message.  It is
   likely that future updates to these tools will add the ability to
   recognize, decode, and display the DSO data.

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

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








Bellis, et al.          Expires November 17, 2018               [Page 4]


Internet-Draft           DNS Stateful Operations                May 2018


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.

3.  Terminology

   "DSO" is used to mean DNS Stateful Operation.

   The term "connection" means 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].

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

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

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

   In this document the term "session" is used exclusively as described
   above.  The term has no relationship to the "session layer" of the
   OSI "seven-layer model" popularized in the 1980s.

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

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

   Where this specification says, "close gracefully," that means sending
   a TLS close_notify (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.

   Where this specification says, "forcibly abort," that means sending a
   TCP RST, or the equivalent for other protocols.  In the BSD Sockets




Bellis, et al.          Expires November 17, 2018               [Page 5]


Internet-Draft           DNS Stateful Operations                May 2018


   API this is achieved by setting the SO_LINGER option to zero before
   closing the socket.

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

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

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

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

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

   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 correctly-implemented
   peer.  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".  Given that these fatal error
   conditions signify defective software, and given that defective
   software is likely to remain defective for some time until it is
   fixed, after forcibly aborting a connection, a client SHOULD refrain
   from automatically reconnecting to that same server instance for at
   least one hour.

   This document uses the term "same server instance" as follows:

   o  In cases where a server is specified or configured using an IP
      address and TCP port number, two different configurations are
      referring to the same server instance if they contain the same IP
      address and TCP port number.




Bellis, et al.          Expires November 17, 2018               [Page 6]


Internet-Draft           DNS Stateful Operations                May 2018


   o  In cases where a server is specified or configured using a
      hostname and TCP port number, such as in the content of a DNS SRV
      record [RFC2782], two different configurations (or DNS SRV
      records) are considered to be referring to the same server
      instance if they contain the same hostname (subject to the usual
      case insensitive DNS name matching rules [RFC1034] [RFC1035]) and
      TCP port number.  In these cases, configurations with different
      hostnames are considered to be referring to different server
      instances, even if those different hostnames happen to be aliases,
      or happen to resolve to the same IP address(es).  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 server instance".

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

   DNS Stateful Operations uses three kinds of message: "DSO request
   messages", "DSO response messages", and "DSO unacknowledged
   messages".  A DSO request message elicits a DSO response message.
   DSO unacknowledged messages are unidirectional messages and do not
   generate any response.

   Both DSO request messages and DSO unacknowledged messages are
   formatted as DNS request messages (the header QR bit is set to zero,
   as described in Section 5.2).  One difference is that in DSO request
   messages the MESSAGE ID field is nonzero; in DSO unacknowledged
   messages it is zero.

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

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



Bellis, et al.          Expires November 17, 2018               [Page 7]


Internet-Draft           DNS Stateful Operations                May 2018


   A DSO response message may contain no TLVs, or it may contain one or
   more TLVs as appropriate to the information being communicated.  In
   the context of DSO response messages, one or more TLVs with the same
   DSO-TYPE as the Primary TLV in the corresponding DSO request message
   are referred to as "Response Primary TLVs".  Any other TLVs with
   different DSO-TYPEs are referred to as "Response Additional TLVs".
   The Response Primary TLV(s), if present, MUST occur first in the
   response message, before any Response Additional TLVs.

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

   o  an inactivity timer (see Section 6.4 and Section 7.1)

   o  a keepalive timer (see Section 6.5 and Section 7.1)

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

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




























Bellis, et al.          Expires November 17, 2018               [Page 8]


Internet-Draft           DNS Stateful Operations                May 2018


4.  Discussion

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

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

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

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






















Bellis, et al.          Expires November 17, 2018               [Page 9]


Internet-Draft           DNS Stateful Operations                May 2018


5.  Protocol Details

5.1.  DSO Session Establishment

   DSO messages MUST be carried in only protocols and in environments
   where a session may be established according to the definition given
   above in the Terminology section (Section 3).

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

   At the time of publication, DSO is specified only for DNS over TCP
   [RFC1035] [RFC7766], and for DNS over TLS over TCP [RFC7858].  Any
   use of DSO over some other connection technology needs to be
   specified in an appropriate future document.

   Determining whether a given connection is using DNS over TCP, or DNS
   over TLS over TCP, is outside the scope of this specification, and
   must be determined using some out-of-band configuration information.
   There is no provision within the DSO specification to turn TLS on or
   off during the lifetime of a connection.  For service types where the
   service instance is discovered using a DNS SRV record [RFC2782], the
   specification for that service type SRV name [RFC6335] will state
   whether the connection uses plain TCP, or TLS over TCP.  For example,
   the specification for the "_dns-push-tls._tcp" service
   [I-D.ietf-dnssd-push], states that it uses TLS.  It is a common
   convention that protocols specified to run over TLS are given IANA
   service type names ending in "-tls".

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

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





Bellis, et al.          Expires November 17, 2018              [Page 10]


Internet-Draft           DNS Stateful Operations                May 2018


   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.

   If the RCODE in the response is set to DSOTYPENI ("DSO-TYPE Not
   Implemented", 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.

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

   When the server receives a 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.

   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].

   Note that for clients that implement only the DSO-TYPEs defined in
   this base specification, sending a DSO 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



Bellis, et al.          Expires November 17, 2018              [Page 11]


Internet-Draft           DNS Stateful Operations                May 2018


   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 DSO Keepalive TLV remains a valid way
   of initiating a DSO Session.

5.1.1.  Connection Sharing

   As previously specified for DNS over TCP [RFC7766]:

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

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

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

5.1.2.  Zero Round-Trip Operation

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



Bellis, et al.          Expires November 17, 2018              [Page 12]


Internet-Draft           DNS Stateful Operations                May 2018


   mechanisms to reduce or eliminate round trips in session
   establishment.

   Similarly, DSO supports zero round-trip operation.

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

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

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

   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.

5.1.3.  Middlebox Considerations

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

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





Bellis, et al.          Expires November 17, 2018              [Page 13]


Internet-Draft           DNS Stateful Operations                May 2018


5.2.  Message Format

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

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

                                                1   1   1   1   1   1
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                          MESSAGE ID                           |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |QR |    OPCODE     |            Z              |     RCODE     |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                     QDCOUNT (MUST be zero)                    |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                     ANCOUNT (MUST be zero)                    |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                     NSCOUNT (MUST be zero)                    |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                     ARCOUNT (MUST be zero)                    |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                                                               |
      /                           DSO Data                            /
      /                                                               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+



















Bellis, et al.          Expires November 17, 2018              [Page 14]


Internet-Draft           DNS Stateful Operations                May 2018


5.2.1.  DNS Header Fields in DSO Messages

   In an unacknowledged message the MESSAGE ID field MUST be set to
   zero.  In an acknowledged request message the MESSAGE ID field MUST
   be set to a unique nonzero value, that the initiator is not currently
   using for any other active operation on this connection.  For the
   purposes here, a MESSAGE ID is in use in this DSO Session if the
   initiator has used it in a request for which it is still awaiting a
   response, or if the client has used it to 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 acknowledged or unacknowledged is determined
   only by the specification for the Primary TLV.  An acknowledgment
   cannot be requested by including a nonzero message ID in a message
   the primary TLV of which is specified to be unacknowledged, nor can
   an acknowledgment be prevented by sending a message ID of zero in a
   message with a primary TLV that is specified to be acknowledged.  A
   responder that receives either such malformed message MUST treat it
   as a fatal error and forcibly abort the connection immediately.

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

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

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

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

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







Bellis, et al.          Expires November 17, 2018              [Page 15]


Internet-Draft           DNS Stateful Operations                May 2018


   In a DNS 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
   a DNS 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 request due to a      |
   |      |           | problem with the server                        |
   |      |           |                                                |
   |    3 | NXDOMAIN  | Name Error -- Named entity does not exist      |
   |      |           | (TLV-dependent)                                |
   |      |           |                                                |
   |    4 | NOTIMP    | DSO not supported                              |
   |      |           |                                                |
   |    5 | REFUSED   | Operation declined for policy reasons          |
   |      |           |                                                |
   |    9 | NOTAUTH   | Not Authoritative (TLV-dependent)              |
   |      |           |                                                |
   |   11 | DSOTYPENI | Primary TLV's DSO-Type is not implemented      |
   +------+-----------+------------------------------------------------+

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











Bellis, et al.          Expires November 17, 2018              [Page 16]


Internet-Draft           DNS Stateful Operations                May 2018


5.2.2.  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 Section 5.2.2.1.

   A DSO message may be a request message, a response message, or an
   unacknowledged message.

   A DSO request message or DSO unacknowledged message MUST contain at
   least one TLV.  The first TLV in a DSO request message or DSO
   unacknowledged message is referred to as the "Primary TLV" and
   determines the nature of the operation being performed, including
   whether it is an acknowledged or unacknowledged operation.  In some
   cases it may be appropriate to include other TLVs in a request
   message or unacknowledged message, such as the Encryption Padding TLV
   (Section 7.3), and these extra TLVs are referred to as the
   "Additional TLVs".

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

   A DSO response message may contain one or more TLVs with DSO-TYPE the
   same as the Primary TLV from the corresponding DSO request message,
   in which case those TLV(s) are referred to as "Response Primary
   TLVs".  A DSO response message is not required to carry Response
   Primary TLVs.  The MESSAGE ID field in the DNS message header is
   sufficient to identify 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
   request messages, which generate corresponding responses.  In some
   specialized high-traffic use cases, it may be appropriate to specify
   unacknowledged messages.  Unacknowledged messages can be more
   efficient on the network, because they don't generate a stream of
   corresponding reply messages.  Using unacknowledged messages can also
   simplify software in some cases, by removing need for an initiator to
   maintain state while it waits to receive replies it doesn't care
   about.  When the specification for a particular TLV states that, when



Bellis, et al.          Expires November 17, 2018              [Page 17]


Internet-Draft           DNS Stateful Operations                May 2018


   used as a Primary TLV (i.e., first) in an outgoing DNS request
   message (i.e., QR=0), that message is to be unacknowledged, the
   MESSAGE ID field MUST be set to zero and the receiver MUST NOT
   generate any response message corresponding to this unacknowledged
   message.

   The previous point, that the receiver MUST NOT generate responses to
   unacknowledged 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.

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

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

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














Bellis, et al.          Expires November 17, 2018              [Page 18]


Internet-Draft           DNS Stateful Operations                May 2018


5.2.2.1.  TLV Syntax

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

   The specification for a TLV states whether that DSO-TYPE may be used
   in "Primary", "Additional", "Response Primary", or "Response
   Additional" TLVs.  The specification for a TLV also states whether,
   when used as the Primary (i.e., first) TLV in a DNS request message
   (i.e., QR=0), that DSO message is to be acknowledged.  If the DSO
   message is to be acknowledged, the specification also states which
   TLVs, if any, are to be included in the response.  The Primary TLV
   may or may not be contained in the response, depending on what is
   stated in the specification for that TLV.

                                                1   1   1   1   1   1
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                           DSO-TYPE                            |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                          DSO-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.












Bellis, et al.          Expires November 17, 2018              [Page 19]


Internet-Draft           DNS Stateful Operations                May 2018


5.2.2.2.  Request TLVs

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

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

5.2.2.3.  Response TLVs

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

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






















Bellis, et al.          Expires November 17, 2018              [Page 20]


Internet-Draft           DNS Stateful Operations                May 2018


5.2.2.4.  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 (tentatively 11).  The error
   response MUST NOT contain a copy of the unrecognized Primary TLV.

   If DSO unacknowledged 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 unacknowledged 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.




























Bellis, et al.          Expires November 17, 2018              [Page 21]


Internet-Draft           DNS Stateful Operations                May 2018


5.2.3.  EDNS(0) and TSIG

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

   For example, the EDNS(0) Padding Option [RFC7830] used for security
   purposes is not permitted in a DSO message, so if message padding is
   desired for DSO messages then the Encryption Padding TLV described in
   Section 7.3 MUST be used.

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

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

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















Bellis, et al.          Expires November 17, 2018              [Page 22]


Internet-Draft           DNS Stateful Operations                May 2018


5.3.  Message Handling

   The initiator MUST set the value of the QR bit in the DNS header to
   zero (0), and the responder MUST set it to one (1).

   As described above in Section 5.2.1 whether an outgoing message with
   QR=0 is unacknowledged or acknowledged 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.

   A DSO unacknowledged message has both the QR bit and the MESSAGE ID
   field set to zero, and MUST NOT elicit a response.

   Every DSO request message (QR=0) with a nonzero MESSAGE ID field is
   an acknowledged DSO request, and MUST elicit a corresponding response
   (QR=1), which MUST have the same MESSAGE ID in the DNS message header
   as in the corresponding request.

   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.

   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 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 5.2.1, an initiator MUST NOT reuse a
   MESSAGE ID that it already has in use for an outstanding request
   (unless specified otherwise by the relevant specification for the
   DSO-TYPE in question).  At the very least, this means that a MESSAGE
   ID MUST NOT be reused in a particular direction on a particular DSO



Bellis, et al.          Expires November 17, 2018              [Page 23]


Internet-Draft           DNS Stateful Operations                May 2018


   Session while the initiator is waiting for a response to a previous
   request using that MESSAGE ID on that DSO Session (unless specified
   otherwise by the relevant specification for the DSO-TYPE in
   question), and for a long-lived operation the MESSAGE ID for the
   operation MUST NOT 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.

5.3.1.  Error Responses

   When a DSO unacknowledged message is unsuccessful for some reason,
   the responder immediately aborts the connection.

   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.








Bellis, et al.          Expires November 17, 2018              [Page 24]


Internet-Draft           DNS Stateful Operations                May 2018


5.4.  DSO Response Generation

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

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

   Possible mitigations for this problem include:

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

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

   o  Use a networking API that lets the receiver signal to the TCP
      implementation that the receiver has received and processed a
      client request for which it will not be generating any immediate
      response.  This allows the TCP implementation to operate
      efficiently in both cases; for requests that generate a response,
      the TCP ACK, window update, and DSO response are transmitted
      together in a single TCP segment, and for requests that do not
      generate a response, the application-layer software informs the
      TCP implementation that it should go ahead and send the TCP ACK
      and window update immediately, without waiting for the Delayed ACK
      timer.  Unfortunately it is not known at this time which (if any)



Bellis, et al.          Expires November 17, 2018              [Page 25]


Internet-Draft           DNS Stateful Operations                May 2018


      of the widely-available networking APIs currently include this
      capability.

5.5.  Responder-Initiated Operation Cancellation

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

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

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

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

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

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

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

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










Bellis, et al.          Expires November 17, 2018              [Page 26]


Internet-Draft           DNS Stateful Operations                May 2018


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.  Operations SHOULD be pipelined (i.e., the
   client doesn't need wait for a response before sending the next
   message).  The server MUST act on messages in the order they are
   transmitted, but responses to those messages SHOULD be sent out of
   order when appropriate.

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 DSO Keepalive TLV (Section 7.1).

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

   The second timeout value, the keepalive interval, is the maximum
   permitted interval between 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 keepalive traffic to maintain that
   session.

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

   In the usual case where the inactivity timeout is shorter than the
   keepalive interval, it is only when a client has a very long-lived,
   low-traffic, operation that the keepalive interval comes into play,



Bellis, et al.          Expires November 17, 2018              [Page 27]


Internet-Draft           DNS Stateful Operations                May 2018


   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.

6.3.  Inactive DSO Sessions

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

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

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

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









Bellis, et al.          Expires November 17, 2018              [Page 28]


Internet-Draft           DNS Stateful Operations                May 2018


6.4.  The Inactivity Timeout

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

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 SHOULD consider the client delinquent, and SHOULD
   forcibly abort the DSO Session.

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

   If the client wishes to keep an inactive DSO Session open for longer
   than the default duration then it uses the DSO Keepalive message to
   request longer timeout values, as described in Section 7.1.






Bellis, et al.          Expires November 17, 2018              [Page 29]


Internet-Draft           DNS Stateful Operations                May 2018


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 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 Keepalive message dictating new Session Timeout
   values to the client.

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



















Bellis, et al.          Expires November 17, 2018              [Page 30]


Internet-Draft           DNS Stateful Operations                May 2018


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

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

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

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




Bellis, et al.          Expires November 17, 2018              [Page 31]


Internet-Draft           DNS Stateful Operations                May 2018


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

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

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

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

   Because of this concern, the server MUST NOT send a Keepalive message
   (either a response to a client-initiated request, or a server-
   initiated message) with a keepalive interval value less than ten
   seconds.  If a client receives a Keepalive message specifying a
   keepalive interval value less than ten seconds this is 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 keepalive traffic.
   Note that after signaling that the client should generate no
   keepalive traffic in this way, at any point the server may revise
   that keepalive traffic requirement by sending a new Keepalive message
   dictating new Session Timeout values to the client.

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








Bellis, et al.          Expires November 17, 2018              [Page 32]


Internet-Draft           DNS Stateful Operations                May 2018


6.6.  Server-Initiated Session Termination

   In addition to cancelling individual long-lived operations
   selectively (Section 5.5) 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.

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

   Some 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).

   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
      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.



Bellis, et al.          Expires November 17, 2018              [Page 33]


Internet-Draft           DNS Stateful Operations                May 2018


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

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.

   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.





















Bellis, et al.          Expires November 17, 2018              [Page 34]


Internet-Draft           DNS Stateful Operations                May 2018


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
   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).


















Bellis, et al.          Expires November 17, 2018              [Page 35]


Internet-Draft           DNS Stateful Operations                May 2018


6.6.1.2.  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
   server instance, or to another suitable server instance, if more than
   one is available.  If reconnecting to the same server instance, the
   client MUST respect the indicated delay, if available, before
   attempting to reconnect.

   If the server 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 server instance does not want a client to reconnect
   ever (perhaps the server 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 server instance for a particular service, 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
   server instance the client should use.























Bellis, et al.          Expires November 17, 2018              [Page 36]


Internet-Draft           DNS Stateful Operations                May 2018


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: to reset the
   keepalive timer for the DSO Session, and to establish the values for
   the Session Timeouts.

   The DSO-DATA for the the Keepalive TLV is as follows:

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

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

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

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



Bellis, et al.          Expires November 17, 2018              [Page 37]


Internet-Draft           DNS Stateful Operations                May 2018


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

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

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

   Once a DSO Session is in progress (Section 5.1) a Keepalive message
   MAY be initiated by a server.  When sent by a server, the Keepalive
   message MUST be sent as an unacknowledged 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 Keepalive
   request message with a nonzero MESSAGE ID then this is a fatal error
   and the client MUST forcibly abort the connection immediately.  The
   Keepalive unacknowledged message from the server resets a DSO
   Session's keepalive timer, and at the same time unilaterally informs
   the client of the new Session Timeout values to use from this point
   forward in this DSO Session.  No client DSO response message to this
   unilateral declaration is required or allowed.

   The Keepalive TLV is not used as an Additional TLV.





Bellis, et al.          Expires November 17, 2018              [Page 38]


Internet-Draft           DNS Stateful Operations                May 2018


   In response messages the Keepalive TLV is used only as a Response
   Primary TLV, replying to a Keepalive request message from the client.
   A Keepalive TLV MUST NOT be added as to other responses a Response
   Additional TLV.  If the server wishes to update a client's Session
   Timeout values other than in response to a Keepalive request message
   from the client, then it does so by sending an unacknowledged
   Keepalive 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
   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 DSO Keepalive
   TLV from the server may each be higher, lower, or the same as the
   respective Session Timeout values the client previously had for this
   DSO Session.

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
















Bellis, et al.          Expires November 17, 2018              [Page 39]


Internet-Draft           DNS Stateful Operations                May 2018


   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 DSO Keepalive TLV
   does not itself reset the inactivity timer.  The time elapsed since
   the last useful activity on this DSO Session is unaffected by
   exchange of DSO Keepalive messages.  The new inactivity timeout value
   in the DSO Keepalive TLV in the received message does update the
   timeout associated with the running inactivity timer; that becomes
   the new maximum permissible time without activity on a DSO Session.

   o  If the current inactivity timer value is 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.













Bellis, et al.          Expires November 17, 2018              [Page 40]


Internet-Draft           DNS Stateful Operations                May 2018


7.1.2.  Relation to EDNS(0) TCP Keepalive Option

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








































Bellis, et al.          Expires November 17, 2018              [Page 41]


Internet-Draft           DNS Stateful Operations                May 2018


7.2.  Retry Delay TLV

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

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

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

   RETRY DELAY:  A time value, specified as a 32-bit unsigned integer,
      in network (big endian) byte order, in units of milliseconds,
      within which the 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 an unacknowledged 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 the client requests are 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



Bellis, et al.          Expires November 17, 2018              [Page 42]


Internet-Draft           DNS Stateful Operations                May 2018


      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 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 server 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 an unacknowledged
   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 request message or DSO
   unacknowledged message to a server.  If a server receives a DNS
   request message (i.e., QR=0) where the Primary TLV is the Retry Delay
   TLV, this is a fatal error and the server MUST forcibly abort the
   connection immediately.







Bellis, et al.          Expires November 17, 2018              [Page 43]


Internet-Draft           DNS Stateful Operations                May 2018


7.2.2.  Retry Delay TLV used as a Response Additional TLV

   In the case of a request that returns 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.

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 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
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      /                                                               /
      /                   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,
   response, or both.  As specified for the EDNS(0) Padding Option
   [RFC7830] if a request is received with an Encryption Padding TLV,
   then the response MUST also include an Encryption Padding TLV.

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







Bellis, et al.          Expires November 17, 2018              [Page 44]


Internet-Draft           DNS Stateful Operations                May 2018


8.  Summary Highlights

   This section summarizes some noteworthy highlights about various
   components 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 Unacknowledged 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  |  Unacknowledged Message  |    Request Message     |
      +--------+--------------------------+------------------------+
      |  QR=1  |  Invalid - Fatal Error   |    Response Message    |
      +--------+--------------------------+------------------------+

























Bellis, et al.          Expires November 17, 2018              [Page 45]


Internet-Draft           DNS Stateful Operations                May 2018


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 requests or unacknowledged 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 Unacknowledged message, from client
      to server, with zero MESSAGE ID indicating that this request MUST
      NOT generate response message.

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

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

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

   The second five contexts are their counterparts in the opposite
   direction: requests or unacknowledged messages from server to client,
   and the corresponding responses from client back to server.

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

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



Bellis, et al.          Expires November 17, 2018              [Page 46]


Internet-Draft           DNS Stateful Operations                May 2018


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

















































Bellis, et al.          Expires November 17, 2018              [Page 47]


Internet-Draft           DNS Stateful Operations                May 2018


9.  IANA Considerations

9.1.  DSO OPCODE Registration

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

9.2.  DSO RCODE Registration

   The IANA is requested to record the value (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.

9.3.  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 | Reserved for                   |          |           |
   |           | experimental/local use         |          |           |
   |           |                                |          |           |
   | FC00-FFFF | Reserved for future expansion  |          |           |
   +-----------+--------------------------------+----------+-----------+

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

   Registrations of new DSO Type Codes in the "Reserved for DSO session-
   management" range 0004-003F and the "Reserved for future expansion"



Bellis, et al.          Expires November 17, 2018              [Page 48]


Internet-Draft           DNS Stateful Operations                May 2018


   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].  At the time of publication of this document, the
   Designated Expert for the newly created DSO Type Code registry is
   [*TBD*].

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

10.  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 Retry Delay
   Unacknowledged Message into the data stream, specifying an
   unreasonably large RETRY DELAY, causing a denial-of-service attack
   against the client.

11.  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.





Bellis, et al.          Expires November 17, 2018              [Page 49]


Internet-Draft           DNS Stateful Operations                May 2018


12.  References

12.1.  Normative References

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

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

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

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

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

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

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

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

   [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>.






Bellis, et al.          Expires November 17, 2018              [Page 50]


Internet-Draft           DNS Stateful Operations                May 2018


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

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

   [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>.

12.2.  Informative References

   [I-D.ietf-dnssd-mdns-relay]
              Lemon, T. and S. Cheshire, "Multicast DNS Discovery
              Relay", draft-ietf-dnssd-mdns-relay-00 (work in progress),
              May 2018.

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

   [I-D.ietf-dprive-padding-policy]
              Mayrhofer, A., "Padding Policy for EDNS(0)", draft-ietf-
              dprive-padding-policy-05 (work in progress), April 2018.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 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/>.

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





Bellis, et al.          Expires November 17, 2018              [Page 51]


Internet-Draft           DNS Stateful Operations                May 2018


   [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>.

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

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

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

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

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

Authors' Addresses

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

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











Bellis, et al.          Expires November 17, 2018              [Page 52]


Internet-Draft           DNS Stateful Operations                May 2018


   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


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

   Email: sara@sinodun.com


   Ted Lemon
   Barefoot Consulting
   Brattleboro
   VT 05301
   USA

   Email: mellon@fugue.com


   Tom Pusateri
   Unaffiliated
   Raleigh  NC 27608
   USA

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





Bellis, et al.          Expires November 17, 2018              [Page 53]


Html markup produced by rfcmarkup 1.126, available from https://tools.ietf.org/tools/rfcmarkup/