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

DNSOP Working Group                                            R. Bellis
Internet-Draft                                                       ISC
Updates: RFC 7766 (if approved)                              S. Cheshire
Intended status: Standards Track                              Apple Inc.
Expires: January 4, 2018                                    J. Dickinson
                                                            S. Dickinson
                                                                 Sinodun
                                                               A. Mankin
                                                              Salesforce
                                                             T. Pusateri
                                                            Unaffiliated
                                                           July 03, 2017


                         DNS Session Signaling
                   draft-ietf-dnsop-session-signal-03

Abstract

   This document defines a new Session Signaling OPCODE used to
   communicate persistent "per-session" operations, expressed using
   type-length-value (TLV) syntax, and defines an initial set of TLVs
   used to manage session timeouts and termination.  This mechanism is
   intended to reduce the overhead of existing "per-packet" signaling
   mechanisms with "per-message" semantics as well as defining new
   signaling operations not defined in EDNS(0).

Status of This Memo

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

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

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

   This Internet-Draft will expire on January 4, 2018.








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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Message Format  . . . . . . . . . . . . . . . . . . . . .   6
       4.1.1.  Header  . . . . . . . . . . . . . . . . . . . . . . .   7
       4.1.2.  Session Signaling Data  . . . . . . . . . . . . . . .   8
       4.1.3.  EDNS(0) and TSIG  . . . . . . . . . . . . . . . . . .  10
     4.2.  TLV Format  . . . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Message Handling  . . . . . . . . . . . . . . . . . . . .  11
   5.  Keepalive Operation TLV . . . . . . . . . . . . . . . . . . .  12
     5.1.  Relation to EDNS(0) TCP Keepalive Option  . . . . . . . .  14
   6.  Retry Delay TLV . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Use as an Operational TLV . . . . . . . . . . . . . . . .  14
     6.2.  Use as a Modifier TLV . . . . . . . . . . . . . . . . . .  15
   7.  Session Lifecycle and Timers  . . . . . . . . . . . . . . . .  15
     7.1.  Session Initiation  . . . . . . . . . . . . . . . . . . .  15
     7.2.  Timers  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.3.  Inactive Sessions . . . . . . . . . . . . . . . . . . . .  16
     7.4.  The Inactive Timeout  . . . . . . . . . . . . . . . . . .  17
       7.4.1.  Closing Inactive Sessions . . . . . . . . . . . . . .  17
       7.4.2.  Values for the Inactive Timeout . . . . . . . . . . .  18
     7.5.  The Keepalive Interval  . . . . . . . . . . . . . . . . .  18
       7.5.1.  Keepalive Interval Expiry . . . . . . . . . . . . . .  19
       7.5.2.  Values for the Keepalive Interval . . . . . . . . . .  19
     7.6.  Server-Initiated Termination on Error . . . . . . . . . .  20
     7.7.  Client Behaviour in Receiving an Error  . . . . . . . . .  20
     7.8.  Server-Initiated Termination on Overload  . . . . . . . .  21
     7.9.  Retry Delay Operation TLV . . . . . . . . . . . . . . . .  21
       7.9.1.  Outstanding Operations  . . . . . . . . . . . . . . .  22
       7.9.2.  Client Reconnection . . . . . . . . . . . . . . . . .  22



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   8.  Connection Sharing  . . . . . . . . . . . . . . . . . . . . .  23
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  DNS Session Signaling OPCODE Registration . . . . . . . .  23
     9.2.  DNS Session Signaling RCODE Registration  . . . . . . . .  24
     9.3.  DNS Session Signaling Type Codes Registry . . . . . . . .  24
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     12.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

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

   The existing EDNS(0) Extension Mechanism for DNS [RFC6891] is
   explicitly defined to only have "per-message" semantics.  Whilst
   EDNS(0) has been used to signal at least one session related
   parameter (the EDNS(0) TCP Keepalive option [RFC7828]) the result is
   less than optimal due to the restrictions imposed by the EDNS(0)
   semantics and the lack of server-initiated signalling.

   This document defines a new Session Signaling OPCODE used to carry
   persistent "per-session" operations, expressed using type-length-
   value (TLV) syntax, and defines an initial set of TLVs used to manage
   session timeouts and termination.

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

   With Session Signaling, in contrast, there is no compelling
   motivation to pack multiple operations into a single message for
   efficiency reasons.  Each Session Signaling operation is communicated
   in its own separate DNS message, and the transport protocol can take
   care of packing separate DNS messages into a single IP packet if
   appropriate.  For example, TCP can pack multiple small DNS messages




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   into a single TCP segment.  The RCODE in each response message
   indicates the success or failure of the operation in question.

   It should be noted that the message format for Session Signaling
   operations (see Section 4.1) differs from the traditional DNS packet
   format used for standard queries and responses.  The standard twelve-
   octet header is used, but the four count fields (QDCOUNT, ANCOUNT,
   NSCOUNT, ARCOUNT) are set to zero and the corresponding sections are
   not present.  The actual data pertaining to Session Signaling
   operations is appended to the end of the DNS message header.  When
   displayed using today's packet analyser tools that have not been
   updated to recognize the DNS Session Signaling format, this will
   result in the Session Signaling 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 Session Signaling data.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

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

   The 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.

   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 Session Signaling request
   message, regardless of which was the "client" and "server" in the
   usual DNS sense.




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

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

   Session Signaling operations are expressed using type-length-value
   (TLV) syntax.

   "SSOP" is used to mean Session Signalling Operation.

   A Session Signaling "Session" is established between two endpoints
   that acknowledge persistent DNS state via the exchange of Session
   Signalling messages over the connection.  This is distinct from, for
   example a DNS-over-TCP session as described in RC7766.

   Two timers are defined in this document: an inactive timeout and a
   keepalive interval.  The term "Session Timers" is used to refer to
   this pair of values.

3.  Discussion

   TODO: Discuss that this draft introduces 2 session timers and their
   functions.  Discuss that this draft introduces "Keepalive traffic"
   this is special because it does not reset the inactive timeout.
   Possibly move some of the text from "Session Lifestyle and Timers"
   here.

4.  Protocol Details

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

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

   There are discussions about using DNS over the QUIC transport
   protocol [I-D.ietf-quic-transport].  Specifications for DNS over QUIC
   are still preliminary and it is not yet known whether QUIC will
   provide a suitable transport for Session Signaling.




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   Session Signaling messages relate only to the specific "session" in
   which they are being carried.  A "session" is established over a
   connection when either side of the connection sends the first session
   signaling operation TLV and it is acknowledged by the other side.
   While this specification defines and initial set of two operations,
   additional operations may be defined in additional specifications.

   Where an application-layer middle box (e.g., a DNS proxy, forwarder,
   or session multiplexer) is in the path the middle box MUST NOT
   blindly forward the message in either direction.  This does not
   preclude the use of these messages in the presence of an IP-layer
   middle box such as a NAT that rewrites IP-layer and/or transport-
   layer headers, but otherwise preserves the effect of a single
   session.

   TODO: State clearly what a proxy should do when in the path.

   A client MAY attempt to initiate Session Signaling messages at any
   time on a connection; receiving a NOTIMP response in reply indicates
   that the server does not implement Session Signaling, and the client
   SHOULD NOT issue further Session Signaling messages on that
   connection.

   A server SHOULD NOT initiate Session Signaling messages until a
   client-initiated Session Signaling message is received first, unless
   in an environment where it is known in advance by other means that
   the client supports Session Signaling.  This requirement is to ensure
   that the clients that do not support Session Signaling do not receive
   unsolicited inbound Session Signaling messages that they would not
   know how to handle.

   On a session between a client and server that support Session
   Signaling, once the client has sent at least one Session Signaling
   message (or it is known in advance by other means that the client
   supports Session Signaling) either end may unilaterally send Session
   Signaling messages at any time, and therefore either client or server
   may be the initiator of a message.

   From this point on it is considered that a "Session Signalling
   session"" is in progress.  Clients and servers should behave as
   described in this specification with regard to inactive timeouts and
   connection close, not as prescribed in [RFC7766].

4.1.  Message Format

   A Session Signaling message begins with the standard twelve-octet DNS
   message header [RFC1035] with the OPCODE field set to the Session
   Signaling OPCODE (tentatively 6).  However, unlike standard DNS



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   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 Session Signaling message is received where any of the count
   fields are not zero, then a FORMERR MUST be returned.

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

4.1.1.  Header

   In a request the MESSAGE ID field MUST be set to a unique 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 session if the initiator has used it in a request for which it
   has not yet received a response, or if the client has used it for a
   subscription which it has not yet cancelled [I-D.ietf-dnssd-push].

   In a response the MESSAGE ID field MUST contain a copy of the value
   of the MESSAGE ID field in the request being responded to.

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

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

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



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

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

   The RCODE value in a response may be one of the following values:

   +------+------------+-----------------------------------------------+
   | Code | Mnemonic   | Description                                   |
   +------+------------+-----------------------------------------------+
   |    0 | NOERROR    | Operation processed successfully              |
   |      |            |                                               |
   |    1 | FORMERR    | Format error                                  |
   |      |            |                                               |
   |    4 | NOTIMP     | Session Signaling not supported               |
   |      |            |                                               |
   |    5 | REFUSED    | Operation declined for policy reasons         |
   |      |            |                                               |
   |   11 | SSOPNOTIMP | Session Signaling operation type code not     |
   |      |            | supported                                     |
   +------+------------+-----------------------------------------------+

4.1.2.  Session Signaling Data

   The standard twelve-octet DNS message header is followed by the
   Session Signaling Data.

   The first TLV in a Session Signaling request message is the Operation
   TLV.  Any subsequent TLVs after this initial Operation TLV are
   Modifier TLVs.

   Depending on the operation a Session Signaling response can contain:

   o  No TLVs

   o  Only an Operation TLV

   o  An Operation TLV followed by one or more Modifier TLVs

   o  Only Modifier TLVs






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4.1.2.1.  Operation TLVs

   A "Session Signaling Operation TLV" specifies the operation to be
   performed.

   A Session Signaling message MUST contain at most one Operation TLV.

   In all cases a Session Signaling request message MUST contain exactly
   one Operation TLV, indicating the operation to be performed.

   Depending on the operation, a Session Signaling response message MAY
   contain no Operation TLV, 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 corresponding response Operation TLV, with the same SIGNALING-
   TYPE as in the request message.  The specification for each Session
   Signaling operation type determines whether a response for that
   operation type is required to carry the Operation TLV.

   If a Session Signaling response is received for an operation which
   requires that the response carry an Operation TLV, and the required
   Operation TLV is not the first Session Signaling TLV in the response
   message, then this is a fatal error and the recipient of the
   defective response message MUST immediately terminate the connection
   with a TCP RST (or equivalent for other protocols).

4.1.2.2.  Modifier TLVs

   A "Session Signaling Modifier TLV" specifies additional parameters
   relating to the operation.  Immediately following the Operation TLV,
   if present, a Session Signaling message MAY contain one or more
   Modifier TLVs.

4.1.2.3.  Unrecognised TLVs

   If a Session Signaling request is received containing an unrecognized
   Operation TLV, the receiver MUST send a response with matching
   MESSAGE ID, and RCODE SSOPNOTIMP (tentatively 11).  The response MUST
   NOT contain an Operation TLV.

   If a Session Signaling message (request or response) is received
   containing one or more unrecognized Modifier TLVs, the unrecognized
   Modifier TLVs MUST be silently ignored, and the remainder of the
   message is interpreted and handled as if the unrecognized parts were
   not present.






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4.1.3.  EDNS(0) and TSIG

   Since the ARCOUNT field MUST be zero, a Session Signaling 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 Session Signaling messages, a Session Signaling Operation
   TLV or Modifier TLV needs 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 Session Signaling message, so if
   message padding is desired for Session Signaling messages, a Session
   Signaling Modifier TLV needs to be defined to perform this function.

   Similarly, a Session Signaling 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 Session
   Signaling data appears after the additional records section, it would
   not be included in the signature calculation.  If use of signatures
   with Session Signaling messages becomes necessary in the future, an
   explicit Session Signaling Modifier TLV needs to be defined to
   perform this function.

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

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

4.2.  TLV Format

   Operation and modifier TLVs both use the same encoding format.











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

   SIGNALING-TYPE:  A 16 bit field in network order giving the type of
      the current Session Signaling TLV per the IANA DNS Session
      Signaling Type Codes Registry.

   SIGNALING DATA LENGTH:  A 16 bit field in network order giving the
      size in octets of the TYPE-DEPENDENT DATA.

   TYPE-DEPENDENT DATA:  Type-code specific format.

   Where domain names appear within TYPE-DEPENDENT DATA, they MUST NOT
   be compressed using standard DNS name compression.

4.3.  Message Handling

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

   With most TCP implementations, 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 DNS Session Signaling response (generated
   by the receiving software itself) are all combined into a single
   packet, so in practice the requirement that every Session Signaling
   request message MUST elicit a Session Signaling response incurs
   minimal extra cost on the network.  Requiring that every request
   elicit a corresponding response also avoids performance problems
   caused by interaction between Nagle's Algorithm and Delayed Ack
   [NagleDA].

   The namespaces of 16-bit MESSAGE IDs are disjoint in each direction.
   For example, it is _not_ an error for both client and server to send



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   a request message with the same ID.  In effect, the 16-bit MESSAGE ID
   combined with the identity of the initiator (client or server) serves
   as a 17-bit unique identifier for a particular operation on a
   session.

   As described in Section 4.1.1 An initiator MUST NOT reuse a MESSAGE
   ID that is already in use for an outstanding request, unless
   specified otherwise by the relevant specification for the Session
   Signaling operation in question.  At the very least, this means that
   a MESSAGE ID MUST NOT be reused in a particular direction on a
   particular session while the initiator is waiting for a response to a
   previous request on that session, unless specified otherwise by the
   relevant specification for the Session Signaling operation in
   question.  (For a long-lived operation, such as a DNS Push
   Notification subscription [I-D.ietf-dnssd-push] the MESSAGE ID for
   the operation MUST NOT be reused for a new subscription as long as
   the existing subscription using that MESSAGE ID remains active.)

   If a client or server receives a response (QR=1) where the MESSAGE ID
   does not match any of its outstanding operations, this is a fatal
   error and it MUST immediately terminate the connection with a TCP RST
   (or equivalent for other protocols).

5.  Keepalive Operation TLV

   The Keepalive Operation TLV (SIGNALING-TYPE=1) performs two
   functions: to reset the keepalive timer for the session and to
   establish the values for the Session Timers.

   When sent by a client, it resets a session's keepalive timer, and at
   the same time requests what the Session Timer values should be from
   this point forward in the session.

   Once a Session Signalling session is in progress (see Section 4) the
   Keepalive TLV also MAY be initiated by a server.  When sent by a
   server, it resets a session's keepalive timer, and unilaterally
   informs the client of the new Session Timer values to use from this
   point forward in this session.

   It is not required that the Keepalive TLV be used in every session.
   While many Session Signaling operations (such as DNS Push
   Notifications [I-D.ietf-dnssd-push]) will be used in conjunction with
   a long-lived session, not all Session Signaling operations require a
   long-lived session, and in some cases the default 15-second value for
   both inactive timeout and keepalive interval may be perfectly
   appropriate.

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



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                           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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  INACTIVE TIMEOUT (32 bits)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 KEEPALIVE INTERVAL (32 bits)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   INACTIVE TIMEOUT:  the inactive timeout for the current session,
      specified as a 32 bit word in network (big endian) order in units
      of milliseconds.  This is the timeout at which the client MUST
      close an inactive session.  If the client does not gracefully
      close an inactive session then after twice this interval the
      server will forcibly terminate the connection with a TCP RST (or
      equivalent for other protocols).

   KEEPALIVE INTERVAL:  the keepalive interval for the current session,
      specified as a 32-bit word, in network (big endian) order, in
      units of milliseconds.  This is the interval at which a client
      MUST generate keepalive traffic to maintain connection state.  If
      the client does not generate the necessary keepalive traffic then
      after twice this interval the server will forcibly terminate the
      connection with a TCP RST (or equivalent for other protocols).

   In a client-initiated Session Signaling Keepalive message, the
   inactive timeout and keepalive interval contain the client's
   requested values.  In a server response to a client-initiated
   message, the inactive timeout and keepalive interval contain the
   server's chosen values, 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.

   In a server-initiated Session Signaling Keepalive message, the
   inactive timeout and keepalive interval unilaterally inform the
   client of the new values from this point forward in this session.
   The client MUST generate a response to the server-initiated Session
   Signaling Keepalive message.  The Message ID in the response message
   MUST match the ID from the server-initiated Session Signaling
   Keepalive message, and the response message MUST NOT contain any
   Operation TLV.

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



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5.1.  Relation to EDNS(0) TCP Keepalive Option

   The inactive timeout value in the Keepalive TLV (SIGNALING-TYPE=1)
   has similar intent to the EDNS(0) TCP Keepalive Option [RFC7828].  A
   client/server pair that supports Session Signaling MUST NOT use the
   EDNS(0) TCP KeepAlive option within any message after a Session
   Signalling session has been established.  Once a Session Signalling
   session has been established, if either client or server receives a
   DNS message over the session that contains an EDNS(0) TCP Keepalive
   option, this is an error and the receiver of the EDNS(0) TCP
   Keepalive option MUST immediately terminate the connection with a TCP
   RST (or equivalent for other protocols).

6.  Retry Delay TLV

   The Retry Delay TLV (SIGNALING-TYPE=0) can be used as an Operation
   TLV or as a Modifier TLV.

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

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

   RETRY DELAY:  a time value, specified as a 32 bit word in network
      order in units of milliseconds, within which the client MUST NOT
      retry this operation, or retry connecting to this server.

   The RECOMMENDED value is 10 seconds.

6.1.  Use as an Operational TLV

   When sent in a Session Signaling request message, from server to
   client, the Retry Delay TLV (0) is considered an Operation TLV.  It
   is used by a server to request that a client close the session, and
   not to reconnect for the indicated time interval.

   In this case it applies to the session as a whole, and the client
   MUST close the session, as described in section Section 7.9.  The
   RCODE in the message header MUST indicate the reason for the
   termination:

   o  NOERROR indicates a routine shutdown.

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



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

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

6.2.  Use as a Modifier TLV

   When appended to a Session Signaling response message for some client
   request, the Retry Delay TLV (0) is considered a Modifier TLV.  The
   indicated time interval during which the client SHOULD NOT retry
   applies only to the failed operation, not to the session as a whole.

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

7.  Session Lifecycle and Timers

7.1.  Session Initiation

   A session begins when a client makes a new connection to a server.

   A Session Signalling session MAY begin as described in Section 4.....

   The client may perform as many DNS operations as it wishes using the
   newly created 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
   received, but responses to those messages MAY be sent out of order,
   if appropriate.

7.2.  Timers

   Two timer values are associated with a session: the inactive timeout,
   and the keepalive interval.






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

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

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

   Only when the client has a very long-lived low-traffic operation
   outstanding like a Push Notification subscription, does the keepalive
   interval timer come into play, to ensure that a sufficient residual
   amount of traffic is generated to maintain NAT and firewall state.

   On a new session, before any explicit Session Signaling Keepalive
   message exchange, the default value for both timers is 15 seconds.
   For both timers, lower values of the timer result in higher network
   traffic and higher CPU load on the server.

7.3.  Inactive Sessions

   At both servers and clients, the generation or reception of any
   complete DNS message, including DNS requests, responses, updates, or
   Session Signaling messages, resets both timers for that session
   [RFC7766], with the exception that a Session Signaling Keepalive
   message resets only the keepalive interval timer, not the inactive
   timeout timer.

   In addition, for as long as the client has an outstanding operation
   in progress, the inactive timeout timer remains fixed at zero, and an
   inactive 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 inactive timeout timer is cleared upon
   transmission of a request and remains cleared until reception of the
   corresponding response.  At the server, the inactive timeout timer is
   cleared upon reception of a request and remains cleared until
   transmission of the corresponding response.

   For long-lived DNS operations like Push Notification subscriptions
   [I-D.ietf-dnssd-push], an operation is considered in progress for as
   long as the subscription is active, until it is cancelled.  This



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   means that a session can exist, with a Push Notification subscription
   active, with no messages flowing in either direction, for far longer
   than the inactive timeout, and this is not an error.  This is why
   there are two separate timers: the inactive timeout, and the
   keepalive interval.  Just because a session has no traffic for an
   extended period of time does not automatically make that session
   "inactive", if it has an active Push Notification subscription that
   is awaiting notification events.

7.4.  The Inactive Timeout

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

7.4.1.  Closing Inactive Sessions

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

   If, at any time during the life of the session, the inactive timeout
   value (i.e., 15 seconds by default) elapses without there being any
   operation active on the session, the client MUST gracefully close the
   connection with a TCP FIN (or equivalent for other protocols).

   If, at any time during the life of the session, twice the inactive
   timeout value (i.e., 30 seconds by default) elapses without there
   being any operation active on the session, the server SHOULD consider
   the client delinquent, and forcibly abort the session.  For sessions
   over TCP (or over TLS over TCP), to avoid the burden of having a
   connection in TIME-WAIT state, instead of closing the connection
   gracefully with a TCP FIN the server SHOULD abort the connection with
   a TCP RST (or equivalent for other protocols).  (In the BSD Sockets
   API this is achieved by setting the SO_LINGER option to zero before
   closing the socket.)

   In this context, an operation being active on a session includes a
   query waiting for a response, an update waiting for a response, or an



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   outstanding Push Notification subscription [I-D.ietf-dnssd-push], but
   not a Session Signaling Keepalive message exchange itself.  A Session
   Signaling Keepalive message exchange resets only the keepalive
   interval timer, not the inactive timeout timer.

   If the client wishes to keep an inactive session open for longer than
   the default duration without having to send traffic every 15 seconds,
   then it uses the Session Signaling Keepalive message to request
   longer timeout values, as described in Section 5.

7.4.2.  Values for the Inactive Timeout

   For the inactive timeout value, lower values result in more frequent
   session teardown and re-establishment.  Higher values result in lower
   traffic and CPU load on the server, but a larger memory burden to
   maintain state for inactive sessions.

   A shorter inactive timeout with a longer keepalive interval signals
   to the client that it should not speculatively keep inactive sessions
   open for very long for no reason, but when it does have an active
   reason to keep a session open, it doesn't need to be sending an
   agressive level of keepalive traffic.  Only when the client has a
   very long-lived low-traffic operation outstanding like a Push
   Notification subscription, does the keepalive interval timer come
   into play, to ensure that a sufficient residual amount of traffic is
   generated to maintain NAT and firewall state.

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

   To avoid excessive traffic the server MUST NOT send a Keepalive
   message (either a response to a client-initiated request, or a
   server-initiated message) with an inactive timeout value less than
   ten seconds.  If a client receives an Keepalive message specifying an
   inactive timeout value less than ten seconds this is an error and the
   client MUST immediately terminate the connection with a TCP RST (or
   equivalent for other protocols).

7.5.  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




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   them to clean up state when connectivity is lost, and attempt re-
   connection if appropriate.

7.5.1.  Keepalive Interval Expiry

   If, at any time during the life of the session, the keepalive
   interval value (i.e., 15 seconds by default) elapses without any DNS
   messages being sent or received on a session, the client MUST take
   action to keep the session alive.  To keep the session alive the
   client MUST send a Session Signaling Keepalive message (see
   Section 5).  A Session Signaling Keepalive message exchange resets
   only the keepalive interval timer, not the inactive timeout timer.

   If a client disconnects from the network abruptly, without cleanly
   closing its session, leaving long-lived outstanding operations like
   Push Notification subscriptions uncanceled, the server learns of this
   after failing to receive the required keepalive traffic from that
   client.  If, at any time during the life of the session, twice the
   keepalive interval value (i.e., 30 seconds by default) elapses
   without any DNS messages being sent or received on a session, the
   server SHOULD consider the client delinquent, and forcibly abort the
   connection with a TCP RST (or equivalent for other protocols).

7.5.2.  Values for the Keepalive Interval

   For the keepalive interval value, lower values result in higher
   volume 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 session open for the
   same length of time (determined by the inactive 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,
   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



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   address that is not in the same 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.

   For environments where there is a NAT gateway or firewalls on the
   path, it is RECOMMENDED that clients request, and servers grant, a
   keepalive interval of 15 minutes.  In other environments it is
   RECOMMENDED that clients request, and servers grant, a keepalive
   interval of 60 minutes.

   Note that the lower the keepalive interval value, the higher the load
   on client and server.  For example, an keepalive interval value of
   100ms would result in a continuous stream of at least ten messages
   per second, in both directions, to keep the 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 an keepalive interval value less than ten
   seconds.  If a client receives an Keepalive message specifying an
   keepalive interval value less than ten seconds this is an error and
   the client MUST immediately terminate the connection with a TCP RST
   (or equivalent for other protocols).

7.6.  Server-Initiated Termination on Error

   After sending an error response to a client, the server MAY close the
   session, or may allow the 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
   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
   close the session by sending a Retry Delay request message, as
   described in Section 6.

7.7.  Client Behaviour in Receiving an Error

   Upon receiving an error response from the server, a client SHOULD NOT
   automatically close the session.  An error relating to one particular
   operation on a session does not necessarily imply that all other
   operations on that 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 close the session,
   based on the server's determination of whether the error condition



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   pertains to this particular operation, or would also apply to any
   subsequent operations.  If the server does not close the session then
   the client SHOULD continue to use that session for subsequent
   operations.

7.8.  Server-Initiated Termination on Overload

   Apart from the cases where:

   o  Session Timer expire (see Section xx)

   o  On error (see Section xx)

   o  When under load (see below)

   a server MUST NOT close a session with a client, except in
   extraordinary error conditions.  Closing the session is the client's
   responsibility, to be done at the client's discretion, when it so
   chooses.  A server only closes a session under exceptional
   circumstances, such as when the server application software or
   underlying operating system is restarting, the server application
   terminated unexpectedly (perhaps due to a bug that makes it crash),
   or the server is undergoing maintenance procedures.  When possible, a
   server SHOULD send a Retry Delay message informing the client of the
   reason for the session being closed, and allow the client five
   seconds to receive it before the server resorts to forcibly aborting
   the connection.

7.9.  Retry Delay Operation TLV

   There may be rare cases where a server is overloaded and wishes to
   shed load.  If a server is low on resources it MAY simply terminate a
   client connection with a TCP RST (or equivalent for other protocols).
   However, the likely behaviour of the client may be simply to to treat
   this as a network failure and connect immediately, putting more
   burden on the server.

   Therefore to avoid this reconnection implosion, a server SHOULD
   instead choose to shed client load by sending a Retry Delay request
   message, with an RCODE of SERVFAIL, to inform the client of the
   overload situation.  After sending a Retry Delay request message, the
   server MUST NOT send any further messages on that session.

   After sending the Retry Delay request 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
   abort the connection with a TCP RST (or equivalent for other
   protocols).



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   Upon receipt of a Retry Delay request from the server, the client
   MUST make note of the reconnect delay for this server, and then
   immediately close the connection.  This is to place the burden of
   TCP's TIME-WAIT state on the client.

   A Retry Delay request message MUST NOT be initiated by a client.  If
   a server receives a Retry Delay request message this is an error and
   the server MUST immediately terminate the connection with a TCP RST
   (or equivalent for other protocols).

7.9.1.  Outstanding Operations

   At the moment a server chooses to initiate a Retry Delay request
   message there may be DNS requests already in flight from client to
   server on this session, which will arrive at the server after its
   Retry Delay request message has been sent.  The server MUST silently
   ignore such incoming requests, and MUST NOT generate any response
   messages for them.  When the Retry Delay request message from the
   server arrives at the client, the client will determine that any DNS
   requests it previously sent on this 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
   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 sessions en masse with a REFUSED
   (5) RCODE, the RECONNECT DELAY MAY be zero, indicating that the
   clients SHOULD immediately attempt to re-establish operations.  It is
   likely that some of the attempts will be successful and some will
   not.

   In the case where a server is terminating a large number of 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).

7.9.2.  Client Reconnection

   After a session is closed by the server, the client SHOULD try to
   reconnect, to that server, or to another suitable server, if more
   than one is available.  If reconnecting to the same server, the




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   client MUST respect the indicated delay before attempting to
   reconnect.

   If a particular server does not want a client to reconnect (it is
   being de-commissioned), it SHOULD set the retry delay to the maximum
   value (which is approximately 497 days).  If the server will only be
   out of service for a maintenance period, it should use a value closer
   to the expected maintenance window and not default to a very large
   delay value or clients may not attempt to reconnect after it resumes
   service.

8.  Connection Sharing

   (QUESTION: RFC7766 already has Section 6.2.2 that specifies
   "Concurrent Connections".  I think we should align this section with
   that so any updates are explicit.)

   A client that supports Session Signaling SHOULD NOT make multiple
   connections to the same DNS server.

   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 session for
   all those operations.  A client SHOULD NOT open multiple connections
   to the same target host and port just because the names being
   operated on are different or happen to fall within different zones.
   This is to reduce unnecessary connection load on the DNS server.

   However, server implementers and operators should be aware that
   connection sharing may not be possible in all cases.  A single client
   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.

9.  IANA Considerations

9.1.  DNS Session Signaling OPCODE Registration

   IANA are directed to assign a value (tentatively 6) in the DNS
   OPCODEs Registry for the Session Signaling OPCODE.





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9.2.  DNS Session Signaling RCODE Registration

   IANA are directed to assign a value (tentatively 11) in the DNS RCODE
   Registry for the SSOPNOTIMP error code.

9.3.  DNS Session Signaling Type Codes Registry

   IANA are directed to create the DNS Session Signaling Type Codes
   Registry, with initial values as follows:

   +------------+-------------------------------+----------+-----------+
   |       Type | Name                          | Status   | Reference |
   +------------+-------------------------------+----------+-----------+
   |     0x0000 | RetryDelay                    | Standard | RFC-TBD   |
   |            |                               |          |           |
   |     0x0001 | KeepAlive                     | Standard | RFC-TBD   |
   |            |                               |          |           |
   |   0x0002 - | Unassigned, reserved for      |          |           |
   |     0x003F | session management TLVs       |          |           |
   |            |                               |          |           |
   |   0x0040 - | Unassigned                    |          |           |
   |     0xF7FF |                               |          |           |
   |            |                               |          |           |
   |   0xF800 - | Reserved for local /          |          |           |
   |     0xFBFF | experimental use              |          |           |
   |            |                               |          |           |
   |   0xFC00 - | Reserved for future expansion |          |           |
   |     0xFFFF |                               |          |           |
   +------------+-------------------------------+----------+-----------+

   Registration of additional Session Signaling Type Codes requires
   publication of an appropriate IETF "Standards Action" or "IESG
   Approval" document [RFC5226].

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.

11.  Acknowledgements

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






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12.  References

12.1.  Normative References

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

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

   [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,
              <http://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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc2136>.

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

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

   [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,
              <http://www.rfc-editor.org/info/rfc7766>.






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

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

12.2.  Informative References

   [I-D.ietf-dnssd-push]
              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-11 (work in progress), June 2017.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-04 (work
              in progress), June 2017.

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

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

   [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, <http://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







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

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


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

   Email: jad@sinodun.com


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

   Email: sara@sinodun.com


   Allison Mankin
   Salesforce

   Email: allison.mankin@gmail.com


   Tom Pusateri
   Unaffiliated

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










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