dnsop                                                       J. Dickinson
Internet-Draft                                              S. Dickinson
Obsoletes: 5966 (if approved)                                    Sinodun
Intended status: Standards Track                               R. Bellis
Expires: January 7, March 23, 2016                                              ISC
                                                               A. Mankin
                                                              D. Wessels
                                                           Verisign Labs
                                                            July 6,
                                                      September 20, 2015

          DNS Transport over TCP - Implementation Requirements
                      draft-ietf-dnsop-5966bis-02
                      draft-ietf-dnsop-5966bis-03

Abstract

   This document specifies the requirement for support of TCP as a
   transport protocol for DNS implementations and provides guidelines
   towards DNS-over-TCP performance on par with that of 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 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 7, March 23, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Terminology  . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3   4
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Transport Protocol Selection  . . . . . . . . . . . . . . . .   5
   6.  Connection Handling . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Current practices . . . . . . . . . . . . . . . . . . . .   6
       6.1.1.  Clients . . . . . . . . . . . . . . . . . . . . . . .   6   7
       6.1.2.  Servers . . . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  Recommendations . . . . . . . . . . . . . . . . . . . . .   7
       6.2.1.  Connection Re-use . . . . . . . . . . . . . . . . . .   7
         6.2.1.1.  Query Pipelining  . . . . . . . . . . . . . . . .   8
       6.2.2.  Concurrent connections  . . . . . . . . . . . . . . .   8
       6.2.3.  Idle Timeouts . . . . . . . . . . . . . . . . . . . .   8   9
       6.2.4.  Tear Down . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Response Reordering . . . . . . . . . . . . . . . . . . . . .   9
   8.  TCP Message Length Field  . . . . . . . . . . . . . . . . . .  10
   9.  TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . .  10
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     13.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Summary of Advantages and Disadvantages to using TCP
                for DNS  . . . . . . . . . . . . . . . . . . . . . .  13  14
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  15
     B.1.  Changes -02 to -03  . . . . . . . . . . . . . . . . . . .  15
     B.2.  Changes -01 to -02  . . . . . . . . . . . . . . . . .  14
   Appendix C. . .  15
     B.3.  Changes -00 to -01  . . . . . . . . . . . . . . . . .  15
   Appendix D. . .  16
     B.4.  Changes to RFC 5966 . . . . . . . . . . . . . . . .  15 . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16  17

1.  Introduction

   Most DNS [RFC1034] transactions take place over UDP [RFC0768].  TCP
   [RFC0793] is always used for full zone transfers (AXFR) and is often
   used for messages whose sizes exceed the DNS protocol's original
   512-byte limit.  The growing deployment of DNSSEC and IPv6 has
   increased response sizes and therefore the use of TCP.  The need for
   increased TCP use has also been driven by the protection it provides
   against address spoofing and therefore exploitation of DNS in
   reflection/amplification attacks.  It is now widely used in Response
   Rate Limiting [RRL] Response Rate Limiting [RRL].

   Section 6.1.3.2 of [RFC1123] states:

      DNS resolvers and recursive servers MUST support UDP, and SHOULD
      support TCP, for sending (non-zone-transfer) queries.

   However, some implementors have taken the text quoted above to mean
   that TCP support is an optional feature of the DNS protocol.

   The majority of DNS server operators already support TCP and the
   default configuration for most software implementations is to support
   TCP.  The primary audience for this document is those implementors
   whose limited support for TCP restricts interoperability and hinders
   deployment of new DNS features.

   This document therefore updates the core DNS protocol specifications
   such that support for TCP is henceforth a REQUIRED part of a full DNS
   protocol implementation.

   There are several advantages and disadvantages to the increased use
   of TCP (see Appendix A) as well as implementation details that need
   to be considered.  This document addresses these issues and therefore presents
   TCP as a valid transport alternative for DNS.  It extends the content
   of [RFC5966], with additional considerations and lessons learned from
   research, developments and implementation of TCP in DNS and in other
   internet protocols.

   Whilst this document makes no specific requirements for operators of
   DNS servers to meet, it does offer some suggestions to operators to
   help ensure that support for TCP on their servers and network is
   optimal.  It should be noted that failure to support TCP (or the
   blocking of DNS over TCP at the network layer) may result in
   resolution failure and/or application-level timeouts.

2.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Terminology

   o  Persistent connection: a TCP connection that is not closed either
      by the server after sending the first response nor by the client
      after receiving the first response.

   o  Connection Reuse: the sending of multiple queries and responses
      over a single TCP connection.

   o  Idle DNS-over-TCP session: Clients and servers view application
      level idleness differently.  A DNS client considers a an established
      DNS-over-TCP session to be idle when it has no pending queries to
      send and there are no outstanding responses.  A DNS server
      considers a DNS-
      over-TCP an established DNS-over-TCP session to be idle when it
      has sent responses to all the queries it has received on that
      connection.

   o  Pipelining: the sending of multiple queries and responses over a
      single TCP connection but not waiting for any outstanding replies
      before sending another query.

   o  Out-Of-Order Processing: The processing of queries concurrently
      and the returning of individual responses as soon as they are
      available, possibly out-of-order.  This will most likely occur in
      recursive servers, however it is possible in authoritative servers
      that, for example, have different backend data stores.

4.  Discussion

   In the absence of EDNS0 (Extension Mechanisms for DNS 0) (see below),
   the normal behaviour of any DNS server needing to send a UDP response
   that would exceed the 512-byte limit is for the server to truncate
   the response so that it fits within that limit and then set the TC
   flag in the response header.  When the client receives such a
   response, it takes the TC flag as an indication that it should retry
   over TCP instead.

   RFC 1123 also says:

      ... it is also clear that some new DNS record types defined in the
      future will contain information exceeding the 512 byte limit that
      applies to UDP, and hence will require TCP.  Thus, resolvers and
      name servers should implement TCP services as a backup to UDP
      today, with the knowledge that they will require the TCP service
      in the future.

   Existing deployments of DNS Security (DNSSEC) [RFC4033] have shown
   that truncation at the 512-byte boundary is now commonplace.  For
   example, a Non-Existent Domain (NXDOMAIN) (RCODE == 3) response from
   a DNSSEC-signed zone using NextSECure 3 (NSEC3) [RFC5155] is almost
   invariably larger than 512 bytes.

   Since the original core specifications for DNS were written, the
   Extension Mechanisms for DNS (EDNS0 [RFC6891]) have been introduced.
   These extensions can be used to indicate that the client is prepared
   to receive UDP responses larger than 512 bytes.  An EDNS0-compatible
   server receiving a request from an EDNS0-compatible client may send
   UDP packets up to that client's announced buffer size without
   truncation.

   However, transport of UDP packets that exceed the size of the path
   MTU causes IP packet fragmentation, which has been found to be
   unreliable in many circumstances.  Many firewalls routinely block
   fragmented IP packets, and some do not implement the algorithms
   necessary to reassemble fragmented packets.  Worse still, some
   network devices deliberately refuse to handle DNS packets containing
   EDNS0 options.  Other issues relating to UDP transport and packet
   size are discussed in [RFC5625].

   The MTU most commonly found in the core of the Internet is around
   1500 bytes, and even that limit is routinely exceeded by DNSSEC-
   signed responses.

   The future that was anticipated in RFC 1123 has arrived, and the only
   standardised UDP-based mechanism that may have resolved the packet
   size issue has been found inadequate.

5.  Transport Protocol Selection

   All general-purpose DNS implementations MUST support both UDP and TCP
   transport.

   o  Authoritative server implementations MUST support TCP so that they
      do not limit the size of responses to what fits in a single UDP
      packet.

   o  Recursive server (or forwarder) implementations MUST support TCP
      so that they do not prevent large responses from a TCP-capable
      server from reaching its TCP-capable clients.

   o  Stub resolver implementations (e.g., an operating system's DNS
      resolution library) MUST support TCP since to do otherwise would
      limit their interoperability with their own clients and with
      upstream servers.

   Regarding the choice of when to use UDP or TCP, Section 6.1.3.2 of
   RFC 1123 also says:

      ... a DNS resolver or server that is sending a non-zone-transfer
      query MUST send a UDP query first.

   This requirement is hereby relaxed.  A resolver MAY elect to send
   either TCP or UDP queries depending on local operational reasons.
   TCP MAY be used before sending any UDP queries.  If it already has an
   open TCP connection to the server it SHOULD reuse this connection.
   In essence, TCP should ought to be considered a valid alternative transport
   to UDP, not purely a fallback option.

   In addition it is noted that all Recursive and Authoritative servers
   MUST send responses using the same transport as the query arrived on.
   In the case of TCP this MUST also be the same connection.

6.  Connection Handling

6.1.  Current practices

   Section 4.2.2 of [RFC1035] says:

   o  The server should assume that the client will initiate connection
      closing, and should delay closing its end of the connection until
      all outstanding client requests have been satisfied.

   o  If the server needs to close a dormant connection to reclaim
      resources, it should wait until the connection has been idle for a
      period on the order of two minutes.  In particular, the server
      should allow the SOA and AXFR request sequence (which begins a
      refresh operation) to be made on a single connection.  Since the
      server would be unable to answer queries anyway, a unilateral
      close or reset may be used instead of graceful close.

   Other more modern protocols (e.g., HTTP/1.1 [RFC7230]) have support
   by default for persistent TCP connections for all requests.
   Connections are then normally closed via a 'connection close' signal
   from one party.

   The description in [RFC1035] is clear that servers should view
   connections as persistent (particularly after receiving an SOA), but
   unfortunately does not provide enough detail for an unambiguous
   interpretation of client behaviour for queries other than a SOA.
   Additionally, DNS does not yet have a signalling mechanism for
   connection timeout or close, although some have been proposed.

6.1.1.  Clients

   There is no clear guidance today in any RFC as to when a DNS client
   should close a TCP connection, and there are no specific
   recommendations with regard to DNS client idle timeouts.  However it
   is common practice for clients to close the TCP connection after
   sending a single request (apart from the SOA/AXFR case).

6.1.2.  Servers

   Many DNS server implementations use a long fixed idle timeout and
   default to a small number of TCP connections.  They also offer little
   by the way of TCP connection management options.  The disadvantages
   of this include:

   o  Operational experience has shown that long server timeouts can
      easily cause resource exhaustion and poor response under heavy
      load.

   o  Intentionally opening many connections and leaving them idle can
      trivially create a TCP "denial-of-service" attack as many DNS
      servers are poorly equipped to defend against this by modifying
      their idle timeouts or other connection management policies.

   o  A modest number of clients that all concurrently attempt to use
      persistent connections with non-zero idle timeouts to such a
      server could unintentionally cause the same "denial-of-service"
      problem.

   Note that this denial-of-service is only on the TCP service.
   However, in these cases it affects not only clients wishing to use
   TCP for their queries for operational reasons, but all clients who
   must
   choose to fall back to TCP from UDP after receiving a TC=1 flag.

6.2.  Recommendations

   The following sections include recommendations that are intended to
   result in more consistent and scalable implementations of DNS-over-
   TCP.

6.2.1.  Connection Re-use

   One perceived disadvantage to DNS over TCP is the added connection
   setup latency, generally equal to one RTT.  To amortize connection
   setup costs, both clients and servers SHOULD support connection reuse
   by sending multiple queries and responses over a single persistent
   TCP connection.

   When sending multiple queries over a TCP connection clients MUST take
   care to avoid Message ID collisions.  In other words, they MUST not
   re-use the DNS Message ID of an in-flight query.  This is especially
   important if the server could be performing out-of-order processing
   (see Section 7).

6.2.1.1.  Query Pipelining

   Due to the historical use of TCP primarily for zone transfer and
   truncated responses, no existing RFC discusses the idea of pipelining
   DNS queries over a TCP connection.

   In order to achieve performance on par with UDP DNS clients SHOULD
   pipeline their queries.  When a DNS client sends multiple queries to
   a server, it should SHOULD not wait for an outstanding reply before sending
   the next query.  Clients should SHOULD treat TCP and UDP equivalently when
   considering the time at which to send a particular query.

   DNS clients should note will benefit from noting that DNS servers that do not
   both process pipelined queries concurrently and send out-of-order
   responses will likely not provide performance on a par with UDP.  IF  If
   TCP performance is of importance, clients may might find it useful to use
   server processing times as input to server and transport selection
   algorithms.

   DNS servers (especially recursive) SHOULD expect to receive pipelined
   queries.  The server should SHOULD process TCP queries concurrently, just as
   it would for UDP.  The server SHOULD answer all pipelined queries,
   even if they are sent in quick succession.  The handling of responses
   to pipelined queries is covered in Section 7.

6.2.2.  Concurrent connections

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

   Similarly, servers MAY impose limits on the number of concurrent TCP
   connections being handled for any particular client. client IP address or
   subnet.  These limits SHOULD be much looser than the client
   guidelines above, because the server does not know know, for example, if the a
   client IP address belongs to a single client or is, for example, is multiple resolvers
   on a single machine, or multiple clients behind NAT.

6.2.3.  Idle Timeouts

   To mitigate the risk of unintentional server overload, DNS clients
   MUST take care to minimize the idle time of established DNS-over-TCP
   sessions made to any individual server.  DNS clients SHOULD close the
   TCP connection of an idle session, unless an idle timeout has been
   established using some other signalling mechanism. mechanism, for example,
   [edns-tcp-keepalive].

   To mitigate the risk of unintentional server overload it is
   RECOMMENDED that the default server application-level idle period be
   of the order of seconds, but no particular value is specified.  In
   practice, the idle period may can vary dynamically, and servers MAY allow
   idle connections to remain open for longer periods as resources
   permit.  A timeout of at least a few seconds is advisable for normal
   operations to support those clients that expect the SOA and AXFR
   request sequence to be made on a single connection as originally
   specified in [RFC1035].  Servers MAY use zero timeouts when
   experiencing heavy load or are under attack.

6.2.4.  Tear Down

   Under normal operation clients should typically initiate connection closing
   on idle connections however servers may can close the connection if their
   local idle timeout policy is exceeded.  Connections may can be also
   closed by either end under unusual conditions such as defending
   against an attack or system failure/reboot.

   Clients SHOULD retry unanswered queries if the connection closes
   before receiving all outstanding responses.  No specific retry
   algorithm is specified in this document.

   If a server finds that a client has closed a TCP session, or if the
   session has been otherwise interrupted, before all pending responses
   have been sent then the server MUST NOT attempt to send those
   responses.  Of course the server MAY cache those responses.

7.  Response Reordering
   RFC 1035 is ambiguous on the question of whether TCP responses may be
   reordered -- the only relevant text is in Section 4.2.1, which
   relates to UDP:

      Queries or their responses may be reordered by the network, or by
      processing in name servers, so resolvers should not depend on them
      being returned in order.

   For the avoidance of future doubt, this requirement is clarified.
   Authoritative servers and recursive resolvers are RECOMMENDED to
   support the sending of responses in parallel and/or out-of-order,
   regardless of the transport protocol in use.  Stub and recursive
   resolvers MUST be able to process responses that arrive in a
   different order to that in which the requests were sent, regardless
   of the transport protocol in use.

   In order to achieve performance on par with UDP, recursive resolvers
   SHOULD process TCP queries in parallel and return individual
   responses as soon as they are available, possibly out-of-order.

   Since pipelined responses may can arrive out-of-order, clients must take
   care to MUST match
   responses to outstanding queries, queries using the ID field,
   port number, query name/type/class, field and any other relevant protocol
   features. port number.
   Failure by clients to properly match responses to outstanding queries
   can have serious consequences for inter-
   operability. interoperability.

8.  TCP Message Length Field

   For reasons of efficiency, DNS clients and servers SHOULD transmit pass the
   two-octet length field, and the message described by that length
   field, to the TCP layer at the same time (e.g., in a single "write"
   system call) to make it more likely that all the data will be
   transmitted in a single TCP segment.  This additionally avoids
   problems due to some DNS servers being very sensitive to timeout
   conditions on receiving messages (they may might abort a TCP session if
   the first TCP segment does not contain both the length field and the
   entire message).

9.  TCP Fast Open

   This section is non-normative.

   TCP fastopen Fast Open [RFC7413] (TFO) allows data to be carried in the SYN
   packet.
   packet, reducing the cost of re-opening TCP connections.  It also
   saves up to one RTT compared to standard TCP.

   TFO mitigates the security vulnerabilities inherent in sending data
   in the SYN, especially on a system like DNS where amplification
   attacks are possible, by use of a server-supplied cookie.  TFO
   clients request a server cookie in the initial SYN packet at the
   start of a new connection.  The server returns a cookie in its SYN-
   ACK.  The client caches the cookie and reuses it when opening
   subsequent connections to the same server.

   The cookie is stored by the client's TCP stack (kernel) and persists
   if either the client or server processes are restarted.  TFO also
   falls back to a regular TCP handshake gracefully.

   DNS services taking advantage of IP anycast [RFC4786] may might need to
   take additional steps when enabling TFO.From TFO.  From [RFC7413]:

      Servers that accept connection requests to the same server IP
      address should use the same key such that they generate identical
      Fast Open Cookies for a particular client IP address.  Otherwise a
      client may get different cookies across connections; its Fast Open
      attempts would fall back to regular 3WHS.

10.  IANA Considerations

   This memo includes no request to IANA.

11.  Security Considerations

   Some DNS server operators have expressed concern that wider promotion
   and use of DNS over TCP will expose them to a higher risk of denial-of-service denial-
   of-service (DoS) attacks. attacks on TCP (both accidental and deliberate).

   Although there is a higher risk of such some specific attacks against TCP-enabled TCP-
   enabled servers, techniques for the mitigation of DoS attacks at the
   network level have improved substantially since DNS was first
   designed.

   Readers are advised to familiarise themselves with [CPNI-TCP]. [CPNI-TCP], a
   security assessment of TCP detailing known TCP attacks and
   countermeasures which references most of the relevant RFCs on this
   topic.

   To mitigate the risk of DoS attacks, DNS servers should are advised to
   engage in TCP connection management.  This may could include maintaining
   state on existing connections, re-using existing connections and
   controlling request queues to enable fair use.  It is likely to be
   advantageous to provide configurable connection management options,
   for example:

   o  total number of TCP connections
   o  maximum TCP connections per source IP address or subnet

   o  TCP connection idle timeout

   o  maximum DNS transactions per TCP connection

   o  maximum TCP connection duration

   No specific values are recommended for these parameters.

   Operators are advised to familiarise themselves with the
   configuration and tuning parameters available in the operating system
   TCP stack.  However detailed advice on this is outside the scope of
   this document.

   Operators of recursive servers should are advised to ensure that they only
   accept connections from expected clients, and do not accept them from
   unknown sources.  In the case of UDP traffic, this will help protect
   against reflector attacks [RFC5358] and in the case of TCP traffic it
   will prevent an unknown client from exhausting the server's limits on
   the number of concurrent connections.

12.  Acknowledgements

   The authors would like to thank Francis Dupont and Paul Vixie for
   detailed review, Andrew Sullivan, Tony Finch, Stephane Bortzmeyer and
   the many others who contributed to the mailing list discussion.  Also
   Liang Zhu, Zi Hu, and John Heidemann for extensive DNS-over-TCP
   discussions and code.  Lucie Guiraud and Danny McPherson for
   reviewing early versions of this document.  We would also like to
   thank all those who contributed to RFC 5966.

13.  References

13.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

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

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

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, December 2006.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, March 2008.

   [RFC5358]  Damas, J. and F. Neves, "Preventing Use of Recursive
              Nameservers in Reflector Attacks", BCP 140, RFC 5358,
              October 2008.

   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines", BCP
              152, RFC 5625, August 2009.

   [RFC5966]  Bellis, R., "DNS Transport over TCP - Implementation
              Requirements", RFC 5966, August 2010.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014.

13.2.  Informative References

   [CPNI-TCP]
              CPNI, "Security Assessment of the Transmission Control
              Protocol (TCP)", 2009, <http://www.gont.com.ar/papers/
              tn-03-09-security-assessment-TCP.pdf>.

   [Connection-Oriented-DNS]
              Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
              and N. Somaiya, "Connection-Oriented DNS to Improve
              Privacy and Security",
              <http://www.isi.edu/~johnh/PAPERS/Zhu15b.pdf>.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, DOI
              10.17487/RFC5405, November 2008,
              <http://www.rfc-editor.org/info/rfc5405>.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, December 2014.

   [RRL]      Vixie, P. and V. Schryver, "DNS Response Rate Limiting
              (DNS RRL)", ISC-TN 2012-1-Draft1, April 2012.

   [edns-tcp-keepalive]
              Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", draft-ietf-dnsop-edns-
              tcp-keepalive-02 (work in progress), May 2015.

   [fragmentation-considered-poisonous]
              Herzberg, A. and H. Shulman, "Fragmentation Considered
              Poisonous", May 2012, <http://arxiv.org/abs/1205.4011>.

Appendix A.  Summary of Advantages and Disadvantages to using TCP for
             DNS

   The TCP handshake generally prevents address spoofing and, therefore,
   the reflection/amplification attacks which plague UDP.

   IP fragmentation is less of a problem for TCP than it is for UDP.
   TCP stacks generally implement Path MTU Discovery so they can avoid
   IP fragmentation of TCP segments.  UDP, on the other hand, does not suffer from UDP's issues with fragmentation.
   provide reassembly, which means datagrams that exceed the path MTU
   size must experience fragmentation [RFC5405].  Middleboxes are known
   to block IP fragments, leading to timeouts and forcing client
   implementations to "hunt" for EDNS0 reply size values supported by
   the network path.  Additionally, fragmentation may lead to cache
   poisoning [fragmentation-considered-poisonous].

   TCP setup costs an additional RTT compared to UDP queries.  Setup
   costs can be amortized by reusing connections, pipelining queries,
   and enabling TCP Fast Open.

   TCP imposes additional state-keeping requirements on clients and
   servers.  The use of TCP Fast Open reduces the cost of closing and
   re-opening TCP connections.

   Long-lived TCP connections to anycast servers may might be disrupted due
   to routing changes.  Clients utilizing TCP for DNS must need to always be
   prepared to re-establish connections or otherwise retry outstanding
   queries.  It may might also be possible for TCP Multipath [RFC6824] to
   allow a server to hand a connection over from the anycast address to
   a unicast address.

   There are many "Middleboxes" in use today that interfere with TCP
   over port 53 [RFC5625].  This document does not propose any
   solutions, other than to make it absolutely clear that TCP is a valid
   transport for DNS and must be supported by support for it is a requirement for all
   implementations.

   A more in-depth discussion of connection orientated DNS can be found
   elsewhere [Connection-Oriented-DNS].

Appendix B.  Changes between revisions

   [Note to RFC Editor: please remove this section prior to
   publication.]

B.1.  Changes -02 to -03

   o  Replaced certain lower case RFC2119 keywords to improve clarity.

   o  Updated section 6.2.2 to recognise requirements for concurrent
      zone transfers.

   o  Changed 'client IP address' to 'client IP address or subnet' when
      discussing restrictions on TCP connections from clients.

   o  Added reference to edns-tcp-keepalive draft.

   o  Added wording to introduction to reference Appendix A and state
      TCP is a valid transport alternative for DNS.

   o  Improved description of CPNI-TCP as a general reference source on
      TCP security related RFCs.

B.2.  Changes -01 to -02

   o  Added more text to Introduction as background to TCP use.

   o  Added definitions of Persistent connection and Idle session to
      Terminology section.

   o  Separated Connection Handling section into Current Practice and
      Recommendations.  Provide more detail on current practices and
      divided Recommendations up into more granular sub-sections.

   o  Add section on Idle time with new text on recommendations for
      client idle behaviour.

   o  Move TCP message field length discussion to separate section.

   o  Removed references to system calls in TFO section.

   o  Added more discussion on DoS mitigation in Security Considerations
      section.

   o  Added statement that servers MAY use 0 idle timeout.

   o  Re-stated position of TCP as an alternative to UDP in Discussion.

   o  Updated text on server limits on concurrent connections from a
      particular client.

   o  Added text that client retry logic is outside the scope of this
      document.

   o  Clarified that servers should answer all pipelined queries even if
      sent very close together.

Appendix C.

B.3.  Changes -00 to -01

   o  Changed updates to obsoletes RFC 5966.

   o  Improved text in Section 4 Transport Protocol Selection to change
      "TCP SHOULD NOT be used only for the transfers and as a fallback"
      to make the intention clearer and more consistent.

   o  Reference to TCP FASTOPEN updated now that it is an RFC.

   o  Added paragraph to say that implementations MUST NOT send the TCP
      framing 2 byte length field in a separate packet to the DNS
      message.

   o  Added Terminology section.

   o  Changed should and RECOMMENDED in reference to parallel processing
      to SHOULD in sections 7 and 8.

   o  Added text to address what a server should do when a client closes
      the TCP connection before pending responses are sent.

   o  Moved the Advantages and Disadvantages section to an appendix.

Appendix D.

B.4.  Changes to RFC 5966

   This document differs from RFC 5966 in four additions:

   1.  DNS implementations are recommended not only to support TCP but
       to support it on an equal footing with UDP

   2.  DNS implementations are recommended to support reuse of TCP
       connections

   3.  DNS implementations are recommended to support pipelining and out
       of order processing of the query stream

   4.  A non-normative discussion of use of TCP Fast Open is added

Authors' Addresses

   John Dickinson
   Sinodun Internet Technologies
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   UK

   Email: jad@sinodun.com
   URI:   http://sinodun.com

   Sara Dickinson
   Sinodun Internet Technologies
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   UK

   Email: sara@sinodun.com
   URI:   http://sinodun.com
   Ray Bellis
   Internet Systems Consortium, Inc
   950 Charter Street
   Redwood City  CA  94063
   USA

   Phone: +1 650 423 1200
   Email: ray@isc.org
   URI:   http://www.isc.org

   Allison Mankin
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190
   US

   Phone: +1 703 948-3200
   Email: amankin@verisign.com

   Duane Wessels
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190
   US

   Phone: +1 703 948-3200
   Email: dwessels@verisign.com