Network Working Group                                         C. Huitema
Internet-Draft                                      Private Octopus Inc.
Intended status: Standards Track                                M. Shore
Expires: December 31, 2018                                        Fastly
                                                               A. Mankin
                                                            S. Dickinson
                                                              Sinodun IT
                                                              J. Iyengar
                                                           June 29, 2018

          Specification of DNS over Dedicated QUIC Connections


   This document describes the use of QUIC to provide transport privacy
   for DNS.  The encryption provided by QUIC has similar properties to
   that provided by TLS, while QUIC transport eliminates the head-of-
   line blocking issues inherent with TCP and provides more efficient
   error corrections than UDP.  DNS over QUIC (DNS/QUIC) has privacy
   properties similar to DNS over TLS specified in RFC7858, and
   performance similar to classic DNS over UDP.

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

   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 December 31, 2018.

Copyright Notice

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

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.  Key Words . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Design Considerations . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Scope is Limited to the Stub to Resolver Scenario . . . .   4
     3.2.  Provide DNS Privacy . . . . . . . . . . . . . . . . . . .   5
     3.3.  Design for Minimum Latency  . . . . . . . . . . . . . . .   5
     3.4.  Development of QUIC Protocols and API . . . . . . . . . .   6
     3.5.  No Specific Middlebox Bypass Mechanism  . . . . . . . . .   6
   4.  Specifications  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Connection Establishment  . . . . . . . . . . . . . . . .   7
       4.1.1.  Draft Version Identification  . . . . . . . . . . . .   7
       4.1.2.  Port Selection  . . . . . . . . . . . . . . . . . . .   7
     4.2.  Stream Mapping and Usage  . . . . . . . . . . . . . . . .   8
       4.2.1.  Server Initiated Transactions . . . . . . . . . . . .   8
       4.2.2.  Stream Reset  . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Closing the DNS/QUIC Connection . . . . . . . . . . . . .   9
     4.4.  Connection Resume and 0-RTT . . . . . . . . . . . . . . .   9
   5.  Implementation Requirements . . . . . . . . . . . . . . . . .  10
     5.1.  Authentication  . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Fall Back to Other Protocols on Connection Failure  . . .  10
     5.3.  Response Sizes  . . . . . . . . . . . . . . . . . . . . .  10
     5.4.  DNS Message IDs . . . . . . . . . . . . . . . . . . . . .  10
     5.5.  Padding . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.6.  Connection Handling . . . . . . . . . . . . . . . . . . .  11
       5.6.1.  Connection Reuse  . . . . . . . . . . . . . . . . . .  11
       5.6.2.  Connection Close  . . . . . . . . . . . . . . . . . .  11
       5.6.3.  Idle Timeouts . . . . . . . . . . . . . . . . . . . .  12
     5.7.  Flow Control Mechanisms . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  13
     7.1.  Privacy Issues With Zero RTT data . . . . . . . . . . . .  13
     7.2.  Privacy Issues With Session Resume  . . . . . . . . . . .  13
     7.3.  Traffic Analysis  . . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Registration of DNS/QUIC Identification String  . . . . .  14
     8.2.  Reservation of Dedicated Port . . . . . . . . . . . . . .  15

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       8.2.1.  Port number 784 for experimentations  . . . . . . . .  15
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Domain Name System (DNS) concepts are specified in [RFC1034].  This
   document presents a mapping of the DNS protocol [RFC1035] over QUIC
   transport [I-D.ietf-quic-transport] [I-D.ietf-quic-tls].  The goals
   of this mapping are:

   1.  Provide the same DNS privacy protection as DNS over TLS (DNS/TLS)
       [RFC7858].  This includes an option for the client to
       authenticate the server by means of an authentication domain name

   2.  Provide an improved level of source address validation for DNS
       servers compared to DNS/UDP [RFC1035].

   3.  Provide a transport that is not constrained by path MTU
       limitations on the size of DNS responses it can send.

   4.  Explore the potential performance gains of using QUIC as a DNS
       transport, versus other solutions like DNS over UDP (DNS/UDP)
       [RFC1035] or DNS/TLS [RFC7858].

   5.  Participate in the definition of QUIC protocols and API, by
       outlining a use case for QUIC different from HTTP over QUIC

   In order to achieve these goals, the focus of this document is
   limited to the "stub to recursive resolver" scenario also addressed
   by [RFC7858].  That is, the protocol described here works for queries
   and responses between stub clients and recursive servers.  The
   specific non-goals of this document are:

   1.  No attempt is made to support zone transfers [RFC5936], as these
       are not relevant to the stub to recursive resolver scenario.

   2.  No attempt is made to evade potential blocking of DNS/QUIC
       traffic by middleboxes.

   Users interested in zone transfers should continue using TCP based
   solutions.  Users interested in evading middleboxes should consider
   using solutions like DNS/HTTPS [I-D.ietf-doh-dns-over-https].

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   Specifying the transmission of an application over QUIC requires
   specifying how the application's messages are mapped to QUIC streams,
   and generally how the application will use QUIC.  This is done for
   HTTP in [I-D.ietf-quic-http].  The purpose of this document is to
   define the way DNS messages can be transmitted over QUIC.

   In this document, Section 3 presents the reasoning that guided the
   proposed design.  Section 4 specifies the actual mapping of DNS/QUIC.
   Section 5 presents guidelines on the implementation, usage and
   deployment of DNS/QUIC.

2.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC8174].

3.  Design Considerations

   This section and its subsection present the design guidelines that
   were used for the proposed mapping of DNS/QUIC.  This section is
   informative in nature.

3.1.  Scope is Limited to the Stub to Resolver Scenario

   Usage scenarios for the DNS protocol can be broadly classified in
   three groups: stub to recursive resolver, recursive resolver to
   authoritative server, and server to server.  This design focuses only
   on the "stub to recursive resolver" scenario following the approach
   taken in [RFC7858] and [RFC8310].

   QUESTION: Should this document specify any aspects of configuration
   of discoverability differently to DNS/TLS?

   No attempt is made to address the recursive to authoritative
   scenarios.  Authoritative resolvers are discovered dynamically
   through NS records.  It is noted that at the time of writing work is
   ongoing in the DPRIVE working group to attempt to address the
   analogous problem for DNS/TLS
   [I-D.bortzmeyer-dprive-resolver-to-auth].  In the absence of an
   agreed way for authoritative to signal support for QUIC transport,
   recursive resolvers would have to resort to some trial and error
   process.  At this stage of QUIC deployment, this would be mostly
   errors, and does not seem attractive.  This could change in the

   The DNS protocol is also used for zone transfers.  In the zone
   transfer scenario ([RFC5936]), the client emits a single AXFR query,

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   and the server responds with a series of AXFR responses.  This
   creates a unique profile, in which a query results in several
   responses.  Supporting that profile would complicate the mapping of
   DNS queries over QUIC streams.  Zone transfers are not used in the
   stub to recursive scenario that is the focus here, and seem to be
   currently well served by the DNS over TCP (DNS/TCP).  There is no
   attempt to support them in this proposed mapping of DNS to QUIC.

3.2.  Provide DNS Privacy

   DNS privacy considerations are described in [RFC7626].  [RFC7858]
   defines how to mitigate some of these issues by transmitting DNS
   messages over TLS and TCP and [RFC8310] specifies Strict and
   Opportunistic Usage Profiles for DNS/TLS including how stub resolvers
   can authenticate recursive resolvers.

   QUIC connection setup includes the negotiation of security parameters
   using TLS, as specified in [I-D.ietf-quic-tls], enabling encryption
   of the QUIC transport.  Transmitting DNS messages over QUIC will
   provide essentially the same privacy protections as [RFC7858] and
   [RFC8310].  Further discussion on this is provided in Section 7.

3.3.  Design for Minimum Latency

   QUIC is specifically designed to reduce the delay between HTTP
   queries and HTTP responses.  This is achieved through three main

   1.  Support for 0-RTT data during session resumption.

   2.  Support for advanced error recovery procedures as specified in

   3.  Mitigation of head-of-line blocking by allowing parallel delivery
       of data on multiple streams.

   This mapping of DNS to QUIC will take advantage of these features in
   three ways:

   1.  Optional support for sending 0-RTT data during session resumption
       (the security and privacy implications of this are discussed in
       later sections).

   2.  Long-lived QUIC connections over which multiple DNS transactions
       are performed, generating the sustained traffic required to
       benefit from advanced recovery features.

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   3.  Mapping of each DNS Query/Response transaction to a separate
       stream, to mitigate head-of-line blocking.

   These considerations will be reflected in the mapping of DNS traffic
   to QUIC streams in Section 4.2.

3.4.  Development of QUIC Protocols and API

   QUIC is defined as a layered protocol, with application-specific
   mapping layered on top of the generic QUIC transport.  The only
   mapping defined at this stage is HTTP over QUIC [I-D.ietf-quic-http].
   Adding a different mapping for a different application contributes to
   the development of QUIC.

   HTTP/QUIC parallels the definition of HTTP/2.0, in which HTTP queries
   and responses are carried as series of frames.  The HTTP/QUIC mapping
   provide with some simplification compared to HTTP/TLS/TCP, as QUIC
   already provides concepts like stream identification or end of stream
   marks.  Dedicated control channel are used to carry connection data,
   such as settings or the relative priority of queries.  It would be
   completely possible to use the HTTP/QUIC mapping to carry DNS
   requests as HTTP queries, as specified in
   [I-D.ietf-doh-dns-over-https].  We are somewhat concerned that this
   mapping carries the overhead of HTTP into the DNS protocol, resulting
   in additional complexity and overhead.

   In this document a different design is deliberately explored, in
   which clients and servers can initiate queries as determined by the
   DNS application logic, opening new streams as necessary.  This
   provides for maximum parallelism between queries, as noted in
   Section 3.3.  It also places constraints on the API.  Client and
   servers will have to be notified of the opening of a new stream by
   their peer.  Instead of orderly closing the control stream, client
   and server will have to use orderly connection closure mechanisms and
   manage the potential loss of data if closing on one end conflicts
   with opening of a stream on the other end.

3.5.  No Specific Middlebox Bypass Mechanism

   Being different from HTTP/QUIC is a design choice.  The advantage is
   that the mapping can be defined for minimal overhead and maximum
   performance.  The downside is that the difference can be noted by
   firewalls and middleboxes.  There may be environments in which HTTP/
   QUIC will be allowed, but DNS/QUIC will be disallowed and blocked by
   these middle boxes.

   It is recognized that this might be a problem, but there is currently
   no indication on how widespread that problem might be.  The problem

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   might be acute enough that the only realistic solution would be to
   communicate with independent recursive resolvers using DNS/HTTPS, or
   maybe DNS/HTTP/QUIC.  Or the problem might be rare enough and the
   performance gains significant enough that the appropriate solution
   would be to use DNS/QUIC most of the time, and fall back to DNS/HTTPS
   some of the time.  Measurements and experimentation will inform that

   It may indeed turn out that the complexity and overhead concerns are
   negligible compared to the potential advantages of DNS/HTTPS, such as
   integration with web services or firewall traversal, and that DNS/
   QUIC does not provide sufficient performance gains to justify a new
   protocol.  We will evaluate that once implementations are available
   and can be compared.  In the meanwhile, we believe that a clean
   design is most likely to inform the QUIC development, as explained in
   Section 3.4.

4.  Specifications

4.1.  Connection Establishment

   DNS/QUIC connections are established as described in
   [I-D.ietf-quic-transport].  During connection establishment, DNS/QUIC
   support is indicated by selecting the ALPN token "dq" in the crypto

4.1.1.  Draft Version Identification

   *RFC Editor's Note:* Please remove this section prior to publication
   of a final version of this document.

   Only implementations of the final, published RFC can identify
   themselves as "dq".  Until such an RFC exists, implementations MUST
   NOT identify themselves using this string.

   Implementations of draft versions of the protocol MUST add the string
   "-" and the corresponding draft number to the identifier.  For
   example, draft-huitema-quic-dnsoquic-01 is identified using the
   string "dq-h01".

4.1.2.  Port Selection

   By default, a DNS server that supports DNS/QUIC MUST listen for and
   accept QUIC connections on the dedicated UDP port TBD (number to be
   defined in Section 8), unless it has mutual agreement with its
   clients to use a port other than TBD for DNS/QUIC.  In order to use a
   port other than TBD, both clients and servers would need a
   configuration option in their software.

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   By default, a DNS client desiring to use DNS/QUIC with a particular
   server MUST establish a QUIC connection to UDP port TBD on the
   server, unless it has mutual agreement with its server to use a port
   other than port TBD for DNS/QUIC.  Such another port MUST NOT be port
   53 or port 853.  This recommendation against use of port 53 for DNS/
   QUIC is to avoid confusion between DNS/QUIC and DNS/UDP as specified
   in [RFC1035].  Similarly, using port 853 would cause confusion
   between DNS/QUIC and DNS/DTLS as specified in [RFC8094].

4.2.  Stream Mapping and Usage

   The mapping of DNS traffic over QUIC streams takes advantage of the
   QUIC stream features detailed in Section 10 of

   The stub to resolver DNS traffic follows a simple pattern in which
   the client sends a query, and the server provides a response.  This
   design specifies that for each subsequent query on a QUIC connection
   the client MUST select the next available client-initiated
   bidirectional stream, in conformance with [I-D.ietf-quic-transport].

   The client MUST send the DNS query over the selected stream, and MUST
   indicate through the STREAM FIN mechanism that no further data will
   be sent on that stream.

   The server MUST send the response on the same stream, and MUST
   indicate through the STREAM FIN mechanism that no further data will
   be sent on that stream.

   Therefore, a single client initiated DNS transaction consumes a
   single stream.  This means that the client's first query occurs on
   QUIC stream 4, the second on 8, and so on.

4.2.1.  Server Initiated Transactions

   There are planned traffic patterns in which a server sends
   unsolicited queries to a client, such as for example PUSH messages
   defined in [I-D.ietf-dnssd-push].  These occur when a client
   subscribes to changes for a particular DNS RRset or resource record
   type.  When a PUSH server wishes to send such updates it MUST select
   the next available server initiated bidirectional stream, in
   conformance with [I-D.ietf-quic-transport].

   The server MUST send the DNS query over the selected stream, and MUST
   indicate through the STREAM FIN mechanism that no further data will
   be sent on that stream.

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   The client MUST send the response on the same stream, and MUST
   indicate through the STREAM FIN mechanism that no further data will
   be sent on that stream.

   Therefore a single server initiated DNS transaction consumes a single
   stream.  This means that the servers's first query occurs on QUIC
   stream 1, the second on 5, and so on.

4.2.2.  Stream Reset

   Stream transmission may be abandoned by either party, using the
   stream "reset" facility.  A stream reset indicates that one party is
   unwilling to continue processing the transaction associated with the
   stream.  The corresponding transaction MUST be abandoned.  A Server
   Failure (SERVFAIL, [RFC1035]) SHOULD be notified to the initiator of
   the transaction.

4.3.  Closing the DNS/QUIC Connection

   QUIC connections are closed using the CONNECTION_CLOSE mechanisms
   specified in [I-D.ietf-quic-transport].  Connections can be closed at
   the initiative of either the client or the server (also see
   Section 5.6.2).  The party initiating the connection closure SHOULD
   use the QUIC GOAWAY mechanism to initiate a graceful shutdown of a

   The transactions corresponding to stream number higher than indicated
   in the GO AWAY frames MUST be considered failed.  Similarly, if
   streams are still open when the CONNECTION_CLOSE is received, the
   corresponding transactions MUST be considered failed.  In both cases,
   a Server Failure (SERVFAIL, [RFC1035]) SHOULD be notified to the
   initiator of the transaction.

4.4.  Connection Resume and 0-RTT

   A stub resolver MAY take advantage of the connection resume
   mechanisms supported by QUIC transport [I-D.ietf-quic-transport] and
   QUIC TLS [I-D.ietf-quic-tls].  Stub resolvers SHOULD consider
   potential privacy issues associated with session resume before
   deciding to use this mechanism.  These privacy issues are detailed in
   Section 7.2.

   When resuming a session, a stub resolver MAY take advantage of the
   0-RTT mechanism supported by QUIC.  The 0-RTT mechanism MUST NOT be
   used to send data that is not "replayable" transactions.  For
   example, a stub resolver MAY transmit a Query as 0-RTT, but MUST NOT
   transmit an Update.

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5.  Implementation Requirements

5.1.  Authentication

   For the stub to recursive resolver scenario, the authentication
   requirements are the same as described in [RFC7858] and [RFC8310].
   There is no need to authenticate the client's identity in either

5.2.  Fall Back to Other Protocols on Connection Failure

   If the establishment of the DNS/QUIC connection fails, clients SHOULD
   attempt to fall back to DNS/TLS and then potentially clear text, as
   specified in [RFC7858] and [RFC8310], depending on their privacy

   DNS clients SHOULD remember server IP addresses that don't support
   DNS/QUIC, including timeouts, connection refusals, and QUIC handshake
   failures, and not request DNS/QUIC from them for a reasonable period
   (such as one hour per server).  DNS clients following an out-of-band
   key-pinned privacy profile ([RFC7858]) MAY be more aggressive about
   retrying DNS/QUIC connection failures.

5.3.  Response Sizes

   DNS/QUIC does not suffer from the limitation on the size of responses
   that can be delivered as DNS/UDP [RFC1035] does, since large
   responses will be sent in separate STREAM frames in separate packets.

   QUESTION: However, this raises a new issue because the responses sent
   over QUIC can be significantly larger than those sent over TCP
   (65,635 bytes).  According to [I-D.ietf-quic-transport] "The largest
   offset delivered on a stream - the sum of the re-constructed offset
   and data length - MUST be less than 2^62".  Should a specific limit
   be applied for DNS/QUIC responses or not?

5.4.  DNS Message IDs

   When sending multiple queries over a QUIC connection, clients MUST
   NOT reuse the DNS Message ID of an in-flight query on that connection
   in order to avoid Message ID collisions.

   Clients MUST match responses to outstanding queries using the STREAM
   ID and Message ID and if the response contains a question section,
   the client MUST match the QNAME, QCLASS, and QTYPE fields.  Failure
   to match is a DNS/QUIC protocol error.  Clients observing such errors
   SHOULD close the connection immediately, indicating the application

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   specific error code 0x00000001.  The client should also mark the
   server as inappropriate for future use of DNS/QUIC.

5.5.  Padding

   There are mechanisms specified for both padding individual DNS
   messages [RFC7830], [I-D.ietf-dprive-padding-policy] and padding
   within QUIC packets (see Section 8.6 of [I-D.ietf-quic-transport]),
   which may contain multiple frames.

   Implementations SHOULD NOT use DNS options for padding individual DNS
   messages, because QUIC transport MAY transmit multiple STREAM frames
   containing separate DNS messages in a single QUIC packet.  Instead,
   implementations SHOULD use QUIC PADDING frames to align the packet
   length to a small set of fixed sizes, aligned with the
   recommendations of [I-D.ietf-dprive-padding-policy].

5.6.  Connection Handling

   [RFC7766] provides updated guidance on DNS/TCP much of which is
   applicable to DNS/QUIC.  This section attempts to specify how those
   considerations apply to DNS/QUIC.

5.6.1.  Connection Reuse

   Historic implementations of DNS stub resolvers are known to open and
   close TCP connections for each DNS query.  To avoid excess QUIC
   connections, each with a single query, clients SHOULD reuse a single
   QUIC connection to the recursive resolver.

   In order to achieve performance on par with UDP, DNS clients SHOULD
   send their queries concurrently over the QUIC streams on a QUIC
   connection.  That is, when a DNS client sends multiple queries to a
   server over a QUIC connection, it SHOULD NOT wait for an outstanding
   reply before sending the next query.

5.6.2.  Connection Close

   In order to amortize QUIC and TLS connection setup costs, clients and
   servers SHOULD NOT immediately close a QUIC connection after each
   response.  Instead, clients and servers SHOULD reuse existing QUIC
   connections for subsequent queries as long as they have sufficient
   resources.  In some cases, this means that clients and servers may
   need to keep idle connections open for some amount of time.

   Under normal operation DNS clients typically initiate connection
   closing on idle connections; however, DNS servers can close the
   connection if the idle timeout set by local policy is exceeded.

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   Also, connections can be closed by either end under unusual
   conditions such as defending against an attack or system failure/

   Clients and servers that keep idle connections open MUST be robust to
   termination of idle connection by either party.  As with current DNS
   over TCP, DNS servers MAY close the connection at any time (perhaps
   due to resource constraints).  As with current DNS/TCP, clients MUST
   handle abrupt closes and be prepared to reestablish connections and/
   or retry queries.

5.6.3.  Idle Timeouts

   Proper management of established and idle connections is important to
   the healthy operation of a DNS server.  An implementation of DNS/QUIC
   SHOULD follow best practices for DNS/TCP, as described in [RFC7766].
   Failure to do so may lead to resource exhaustion and denial of

   This document does not make specific recommendations for timeout
   values on idle connections.  Clients and servers should reuse and/or
   close connections depending on the level of available resources.
   Timeouts may be longer during periods of low activity and shorter
   during periods of high activity.  Current work in this area may also
   assist DNS/TLS clients and servers in selecting useful timeout values
   [RFC7828] [I-D.ietf-dnsop-session-signal] [TDNS].

   TODO: Clarify what timers (idle timeouts, response timeouts) apply at
   the stream level and at the connection level.

   TODO: QUIC provides an efficient mechanism for resuming connections,
   including the possibility of sending 0-RTT data.  Does that change
   the tradeoff?  Is it plausible to use shorter timers than specified
   for TCP?

5.7.  Flow Control Mechanisms

   Servers MAY use the "maximum stream ID" option of the QUIC transport
   to limit the number of streams opened by the client.  This mechanism
   will effectively limit the number of DNS queries that a client can

6.  Security Considerations

   The security considerations of DNS/QUIC should be comparable to those
   of DNS/TLS [RFC7858].

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7.  Privacy Considerations

   DNS/QUIC is specifically designed to protect the DNS traffic between
   stub and resolver from observations by third parties, and thus
   protect the privacy of queries from the stub.  However, the recursive
   resolver has full visibility of the stub's traffic, and could be used
   as an observation point, as discussed in [RFC7626].  These
   considerations do not differ between DNS/TLS and DNS/QUIC and are not
   discussed further here.

   QUIC incorporates the mechanisms of TLS 1.3 [I-D.ietf-tls-tls13] and
   this enables QUIC transmission of "Zero-RTT" data.  This can provide
   interesting latency gains, but it raises two concerns:

   1.  Adversaries could replay the zero-RTT data and infer its content
       from the behavior of the receiving server.

   2.  The zero-RTT mechanism relies on TLS resume, which can provide
       linkability between successive client sessions.

   These issues are developed in Section 7.1 and Section 7.2.

7.1.  Privacy Issues With Zero RTT data

   The zero-RTT data can be replayed by adversaries.  That data may
   triggers a query by a recursive resolver to an authoritative
   resolvers.  Adversaries may be able to pick a time at which the
   recursive resolver outgoing traffic is observable, and thus find out
   what name was queried for in the 0-RTT data.

   This risk is in fact a subset of the general problem of observing the
   behavior of the recursive resolver discussed in [RFC7626].  The
   attack is partially mitigated by reducing the observability of this
   traffic.  However, the risk is amplified for 0-RTT data, because the
   attacker might replay it at chosen times, several times.

   The recommendation in [I-D.ietf-tls-tls13] is that the capability to
   use 0-RTT data should be turned off by default, on only enabled if
   the user clearly understands the associated risks.

   QUESTION: Should 0-RTT only be used with Opportunistic profiles (i.e.
   disabled by default for Strict only)?

7.2.  Privacy Issues With Session Resume

   The QUIC session resume mechanism reduces the cost of reestablishing
   sessions and enables zero-RTT data.  There is a linkability issue
   associated with session resume, if the same resume token is used

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   several times, but this risk is mitigated by the mechanisms
   incorporated in QUIC and in TLS 1.3.  With these mechanisms, clients
   and servers can cooperate to avoid linkability by third parties.
   However, the server will always be able to link the resumed session
   to the initial session.  This creates a virtual long duration
   session.  The series of queries in that session can be used by the
   server to identify the client.

   Enabling the server to link client sessions through session resume is
   probably not a large additional risk if the client's connectivity did
   not change between the sessions, since the two sessions can probably
   be correlated by comparing the IP addresses.  On the other hand, if
   the addresses did change, the client SHOULD consider whether the
   linkability risk exceeds the privacy benefits.  This evaluation will
   obviously depend on the level of trust between stub and recursive.

7.3.  Traffic Analysis

   Even though QUIC packets are encrypted, adversaries can gain
   information from observing packet lengths, in both queries and
   responses, as well as packet timing.  Many DNS requests are emitted
   by web browsers.  Loading a specific web page may require resolving
   dozen of DNS names.  If an application adopts a simple mapping of one
   query or response per packet, or "one QUIC STREAM frame per packet",
   then the succession of packet lengths may provide enough information
   to identify the requested site.

   Implementations SHOULD use the mechanisms defined in Section 5.5 to
   mitigate this attack.

8.  IANA Considerations

8.1.  Registration of DNS/QUIC Identification String

   This document creates a new registration for the identification of
   DNS/QUIC in the "Application Layer Protocol Negotiation (ALPN)
   Protocol IDs" registry established in [RFC7301].

   The "dq" string identifies DNS/QUIC:

   Protocol: DNS/QUIC

   Identification Sequence: 0x64 0x71 ("dq")

   Specification: This document

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8.2.  Reservation of Dedicated Port

   IANA is required to add the following value to the "Service Name and
   Transport Protocol Port Number Registry" in the System Range.  The
   registry for that range requires IETF Review or IESG Approval
   [RFC6335], and such a review was requested using the early allocation
   process [RFC7120] for the well-known UDP port in this document.
   Since port 853 is reserved for 'DNS query-response protocol run over
   TLS' consideration is requested for reserving port TBD for 'DNS
   protocol run over QUIC'.

       Service Name           domain-s
       Transport Protocol(s)  TCP/UDP
       Assignee               IESG
       Contact                IETF Chair
       Description            DNS query-response protocol run over QUIC
       Reference              This document

8.2.1.  Port number 784 for experimentations

   *RFC Editor's Note:* Please remove this section prior to publication
   of a final version of this document.

   Early experiments MAY use port 784.  This port is marked in the IANA
   registry as unassigned.

9.  Acknowledgements

   This document liberally borrows text from [I-D.ietf-quic-http] edited
   by Mike Bishop, and from [RFC7858] authored by Zi Hu, Liang Zhu, John
   Heidemann, Allison Mankin, Duane Wessels, and Paul Hoffman.

   The privacy issue with 0-RTT data and session resume were analyzed by
   Daniel Kahn Gillmor (DKG) in a message to the IETF "DPRIV" working
   group [DNS0RTT].

   Thanks to our wide cast of supporters.

10.  References

10.1.  Normative References

              Thomson, M. and S. Turner, "Using Transport Layer Security
              (TLS) to Secure QUIC", draft-ietf-quic-tls-13 (work in
              progress), June 2018.

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              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-13 (work
              in progress), June 2018.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018, <https://www.rfc-

10.2.  Informative References

   [DNS0RTT]  Kahn Gillmor, D., "DNS + 0-RTT", Message to DNS-Privacy WG
              mailing list, April 2016, <

              Bortzmeyer, S., "Encryption and authentication of the DNS
              resolver-to-authoritative communication", draft-
              bortzmeyer-dprive-resolver-to-auth-01 (work in progress),
              March 2018.

              Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Lemon, T., and T. Pusateri, "DNS Stateful Operations",
              draft-ietf-dnsop-session-signal-10 (work in progress),
              June 2018.

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

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              Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DOH)", draft-ietf-doh-dns-over-https-10 (work in
              progress), June 2018.

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

              Bishop, M., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http-13 (work in progress), June

              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", draft-ietf-quic-recovery-13 (work in
              progress), June 2018.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,

   [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,

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <>.

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015, <https://www.rfc-

   [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,

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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, <https://www.rfc-

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

   [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, <>.

   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017, <https://www.rfc-

   [TDNS]     Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
              and N. Somaiya, "Connection-Oriented DNS to Improve
              Privacy and Security", 2015 IEEE Symposium on Security and
              Privacy (SP), DOI 10.1109/SP.2015.18,

Authors' Addresses

   Christian Huitema
   Private Octopus Inc.
   Friday Harbor  WA  98250


   Melinda Shore


   Allison Mankin


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   Sara Dickinson
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA


   Jana Iyengar


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