dnsop                                                         P. Wouters
Internet-Draft                                                   Red Hat
Intended status: Standards Track                                J. Abley
Expires: April 30, 2015 January 4, 2016                                       Dyn, Inc.
                                                        October 27, 2014
                                                            S. Dickinson
                                                               R. Bellis
                                                            July 3, 2015

                  The edns-tcp-keepalive EDNS0 Option


   DNS messages between clients and servers may be received over either
   UDP or TCP.  UDP transport involves keeping less state on a busy
   server, but can cause truncation and retries over TCP.  Additionally,
   UDP can be exploited for reflection attacks.  Using TCP would reduce
   retransmits and amplification.  However, clients are currently
   limited in their commonly use of the TCP transport as RFC 5966 suggests
   only for fallback and servers typically use idle TCP sessions "in timeouts on the
   order of seconds", making use of
   TCP only suitable for individual queries generated as a fallback
   protocol for truncated UDP answers. seconds.

   This document defines an EDNS0 option ("edns-tcp-keepalive") that
   allows DNS clients and servers to signal their respective readiness
   to conduct multiple DNS transactions over individual TCP sessions. a variable idle timeout.  This
   signalling facilitates a better balance of UDP and TCP transport
   between individual clients and servers, reducing the impact of
   problems associated with UDP transport and allowing the state
   associated with TCP transport to be managed effectively with minimal
   impact on the DNS transaction time.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 30, 2015. January 4, 2016.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   3
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  The edns-tcp-keepalive Option . . . . . . . . . . . . . . . .   4
     3.1.  Option Format . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Use by DNS Clients  . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Sending Queries . . . . . . . . . . . . . . . . . . .   5
       3.2.2.  Receiving Responses . . . . . . . . . . . . . . . . .   6   5
     3.3.  Use by DNS Servers  . . . . . . . . . . . . . . . . . . .   6
       3.3.1.  Receiving Queries . . . . . . . . . . . . . . . . . .   6
       3.3.2.  Sending Responses . . . . . . . . . . . . . . . . . .   6
     3.4.  TCP Session Management  . . . . . . . . . . . . . . . . .   7   6
     3.5.  Non-Clean Paths . . . . . . . . . . . . . . . . . . . . .   7
     3.6.  Anycast Considerations  . . . . . . . . . . . . . . . . .   7
   4.  Intermediary Considerations . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Editors' Notes . . . . . . . . . . . . . . . . . . .   9  10
     A.1.  Venue . . . . . . . . . . . . . . . . . . . . . . . . . .   9  10
     A.2.  Abridged Change History . . . . . . . . . . . . . . . . .   9  10
       A.2.1.  draft-ietf-dnsop-edns-tcp-keepalive-01  draft-ietf-dnsop-edns-tcp-keepalive-02  . . . . . . .   9  10
       A.2.2.  draft-ietf-dnsop-edns-tcp-keepalive-00  draft-ietf-dnsop-edns-tcp-keepalive-01  . . . . . . .   9  11
       A.2.3.  draft-ietf-dnsop-edns-tcp-keepalive-00  . . . . . . .  11
       A.2.4.  draft-wouters-edns-tcp-keepalive-01 . . . . . . . . .   9
       A.2.4.  11
       A.2.5.  draft-wouters-edns-tcp-keepalive-00 . . . . . . . . .   9  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10  11

1.  Introduction

   DNS messages between clients and servers may be received over either
   UDP or TCP [RFC1035].  Historically, DNS clients used API's that only
   facilitated sending and receiving a single query over either UDP or
   TCP.  New APIs and deployment of DNSSEC validating resolvers on hosts
   that in the past were using stub resolving only is increasing the DNS
   client base that prefer using long lived TCP connections.  Long-lived
   TCP connections can result in lower request latency than the case
   where UDP transport is used and truncated responses are received,
   since clients that have fallen back to TCP transport in response to a
   truncated response typically only uses the TCP session for a single
   (request, response) pair, continuing with UDP transport for
   subsequent queries.  Clients wishing to use other stream-based
   transport protocols for DNS would also benefit from the set-up
   amortisation afforded by long lived connections.

   UDP transport is stateless, and hence presents a much lower resource
   burden on a busy DNS server than TCP.  An exchange of DNS messages
   over UDP can also be completed in a single round trip between
   communicating hosts, resulting in optimally-short transaction times.
   UDP transport is not without its risks, however.

   A single-datagram exchange over UDP between two hosts can be
   exploited to enable a reflection attack on a third party.  Mitigation
   of such attacks on authoritative-only servers is possible using an
   approach known as Response Rate-Limiting [RRL], an approach designed
   to minimise the frequency at which legitimate responses are discarded
   by truncating responses that appear to be motivated by an attacker,
   forcing legitimate clients to re-query using TCP transport.

   [RFC1035] specified a maximum DNS message size over UDP transport of
   512 bytes.  Deployment of DNSSEC [RFC4033] and other protocols
   subsequently increased the observed frequency at which responses
   exceed this limit.  EDNS0 [RFC6891] allows DNS messages larger than
   512 bytes to be exchanged over UDP, with a corresponding increased
   incidence of fragmentation.  Fragmentation is known to be problematic
   in general, and has also been implicated in increasing the risk of
   cache poisoning attacks.

   The use of TCP transport does not suffer from the risks of
   fragmentation nor reflection attacks.  However, TCP transport as
   currently deployed has expensive overhead.

   The overhead of the three-way TCP handshake for a single DNS
   transaction is substantial, increasing the transaction time for a
   single (request, response) pair of DNS messages from 1 x RTT to 2 x
   RTT.  There is no such overhead for a session that is already
   established, however, and the overall impact of the TCP setup
   handshake when the resulting session is used to exchange N DNS
   message pairs over a single session, (1 + N)/N, approaches unity as N

   (It should perhaps be noted that the overhead for a DNS transaction
   over UDP truncacated truncated due to RRL is 3x RTT, higher than the overhead
   imposed on the same transaction initiated over TCP.)

   With increased deployment of DNSSEC and new RRtypes containing
   application specific cryptographic material, there is an increase in
   the prevalence of truncated responses received over UDP with fallback
   to TCP.

   The use of TCP transport requires considerably more state to be
   retained on DNS servers.  If a server is to perform adequately with a
   significant query load received over TCP, it must manage its
   available resources to ensure that all established TCP sessions are
   well-used, and that those which are unlikely to be used that remain idle for the
   exchange of multiple DNS messages long periods are closed

   This document proposes a signalling mechanism between DNS clients and
   servers that provides a means to better balance the use of UDP and
   TCP transport, reducing the impact of problems associated with UDP
   whilst constraining the impact of TCP on response times and server
   resources to a manageable level.

   The reduced overhead of this extension adds up significantly when
   combined with other edns extensions, such as [CHAIN-QUERY].  The [CHAIN-QUERY] and
   [STARTTLS].  For example, the combination of these two EDNS extensions
   make it possible for hosts on high-latency mobile networks to
   natively perform DNSSEC
   validation. validation and encrypt queries.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  The edns-tcp-keepalive Option

   This document specifies a new EDNS0 [RFC6891] option, edns-tcp-
   keepalive, which can be used by DNS clients and servers to signal a
   willingness to keep an (idle) idle TCP session open for a certain amount of
   time to conduct future DNS transactions.  This specification does not
   distinguish between different types of DNS client and server in the
   use of this option.

3.1.  Option Format

   The edns-tcp-keepalive option is encoded as follows:

                        1                   2                   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
   !         OPTION-CODE           !         OPTION-LENGTH         !
   |                            TIMEOUT                            |


   OPTION-CODE:   the EDNS0 option code assigned to edns-tcp-keepalive,

   OPTION-LENGTH:   the value 2; 0 if the TIMEOUT is omitted, the value 2
      if it is present;

   TIMEOUT:   a   an idle timeout value for the TCP connection, specified in
      units of 100 milliseconds, encoded in network byte order.

3.2.  Use by DNS Clients

3.2.1.  Sending Queries

   DNS clients MAY MUST NOT include the edns-tcp-keepalive option in queries
   sent using UDP transport to signal their general ability to use individual
   TCP sessions for multiple DNS transactions with a particular server. transport.

   DNS clients MAY include the edns-tcp-keepalive option in the first
   query sent to a server using TCP transport to signal their desire
   that that specific TCP session be used for multiple DNS transactions. to
   keep the connection open when idle.

   Clients MAY MUST specify a TIMEOUT value that is representative OPTION-LENGTH of 0 and omit the
   minimum expected time an individual TCP session should remain
   established for it to be used by multiple TIMEOUT value.

3.2.2.  Receiving Responses

   A DNS transactions.

   In the case where there are multiple candidate servers available to
   service a particular transaction, clients MAY include data associated
   with all servers when computing a TIMEOUT value to be signalled to
   any one of those servers.

   If the client has insufficient data to be able to provide a
   meaningful estimate, the TIMEOUT value MUST be set to zero.

3.2.2.  Receiving Responses

   A DNS client that receives client that receives a response using UDP transport that
   includes the edns-tcp-keepalive option MAY record the presence of the
   option and the associated TIMEOUT value, and use that information as
   part of its server selection algorithm in MUST ignore the case where multiple
   candidate servers are available to service a particular query. option.

   A DNS client that receives a response using TCP transport that
   includes the edns-tcp-keepalive option MAY keep the existing TCP
   session open. open when it is idle.  It SHOULD honour the timeout and
   initiate close of the connection before the timeout expires.

   A DNS client that receives a response that includes the edns-tcp-
   keepalive option with a TIMEOUT value of 0 is allowed to keep should send no more
   queries on that connection and initiate closing the TCP connection open indefinately. as
   soon as it has received all outstanding responses.

   A DNS client that sent a query containing the edns-keepalive-option
   but receives a response that does not contain the edns-keepalive-
   option should assume the server does not support keepalive and behave
   following the guidance in [DRAFT-5966bis].  This holds true even if a
   previous edns-keepalive-option exchange occurred on the existing TCP

3.3.  Use by DNS Servers

3.3.1.  Receiving Queries

   A DNS server that receives a query using UDP transport that includes
   the edns-tcp-keepalive option MAY record the presence of MUST ignore the option
   for statistical purposes, but should not otherwise modify its usual
   behaviour in sending a response. option.

   A DNS server that receives a query using TCP transport that includes
   the edns-tcp-keepalive option SHOULD extend MAY modify the local idle timeout normally
   associated with that TCP sessions session if resources permit, using the TIMEOUT
   value supplied by the client as a guide. permit.

3.3.2.  Sending Responses

   DNS servers MAY include the edns-tcp-keepalive option in responses
   sent using UDP transport to signal their general ability to use
   individual TCP sessions for multiple DNS transactions with a
   particular server.  The TIMEOUT value should be indicative of what a
   client might expect if it was to open a TCP session with the server
   and receive a response with the edns-tcp-keepalive option present.
   The DNS server MAY omit including the edns-tcp-keepalive option if it
   is running too low on resources to service more TCP keepalive

   DNS servers MAY include the edns-tcp-keepalive option in responses
   sent using TCP transport to signal their ability to use that specific
   session to exchange multiple DNS transactions. expected idle timeout on a
   connection.  Servers MUST specify the TIMEOUT value that is currently
   associated with the TCP session.  It is reasonable for this value to
   change according to local resource
   constraints. constraints or in consideration of
   intermediary behaviour (for example TCP middleboxes or NATs).  The
   DNS server MAY omit including the edns-tcp-
   keepalive SHOULD send a edns-tcp-keepalive option with a timeout of
   0 if it deems its local resources are too low to service more TCP
   keepalive sessions.

3.4.  TCP Session Management

   Both DNS clients and servers are subject to resource constraints
   which will limit the extent to which TCP sessions can persist.
   Effective limits for the number of active sessions that can be
   maintained on individual clients and servers should be established,
   either as configuration options or by interrogation of process limits
   imposed by the operating system.  Servers that implement keepalive
   should also engage in TCP connection management by recycling existing
   connections when appropriate, closing connections gracefully and
   managing request queues to enable fair use.

   In the event that there is greater demand for TCP sessions than can
   be accommodated, servers may reduce the TIMEOUT value signalled in
   successive DNS messages to avoid abrupt termination of a session. minimise idle time on existing sessions.

   This also allows, for example, clients with other candidate servers
   to query to establish new TCP sessions with different servers in
   expectation that an existing session is likely to be closed, or to
   fall back to UDP.

   Based on TCP session resources servers may signal a TIMEOUT value of
   0 to request clients to close connections as soon as possible.  This
   is useful when server resources become very low or a denial-of-
   service attack is detected and further maximises the shifting of
   TIME_WAIT state to well-behaved clients.

   However it should be noted that RCF6891 states:

      Lack of presence of an OPT record in a request MUST be taken as an
      indication that the requestor does not implement any part of this
      specification and that the responder MUST NOT include an OPT
      record in its response.

   Since servers must be faithful to this specification even on a
   persistent TCP connection it means that (following the initial
   exchange of timeouts) a server may not be presented with the
   opportunity to signal a change in the idle timeout associated with a
   connection if the client does not send any further requests
   containing EDNS0 OPT RRs.  This limitation makes persistent
   connection handling via an initial idle timeout signal more
   attractive than a mechanism that establishes default persistence and
   then uses a connection close signal (in a similar manner to HTTP 1.1

   DNS clients and servers MAY close a TCP session at any time in order
   to manage local resource constraints.  The algorithm by which clients
   and servers rank active TCP sessions in order to determine which to
   close is not specified in this document.

3.5.  Non-Clean Paths

   Many paths between DNS clients and servers suffer from poor hygiene,
   limiting the free flow of DNS messages that include particular EDNS0
   options, or messages that exceed a particular size.  A fallback
   strategy similar to that described in [RFC6891] section 6.2.2 SHOULD
   be employed to avoid persistent interference due to non-clean paths.

3.6.  Anycast Considerations

   DNS servers of various types are commonly deployed using anycast

   Successive DNS transactions between a client and server using UDP
   transport may involve responses generated by different anycast nodes,
   and the use of anycast in the implementation of a DNS server is
   effectively undetectable by the client.  The edns-tcp-keepalive
   option SHOULD NOT be included in responses using UDP transport from
   be employed to avoid persistent interference due to non-clean paths.

3.6.  Anycast Considerations

   DNS servers provisioned of various types are commonly deployed using anycast unless all anycast server nodes are
   capable of processing the edns-tcp-keepalive option.

   Changes in network topology between clients and anycast servers may
   cause disruption to TCP sessions making use of edns-tcp-keepalive
   more often than with TCP sessions that omit it, since the TCP
   sessions are expected to be longer-lived.  Anycast servers MAY make
   use of TCP multipath [RFC6824] to anchor the server side of the TCP
   connection to an unambiguously-unicast address in order to avoid
   disruption due to topology changes.

4.  Intermediary Considerations

   It is RECOMMENDED that DNS intermediaries which terminate TCP
   connections implement keepalive.  Should a non-keepalive-aware
   intermediary sit between a client and server that both support
   keepalive a potential side effect will be an increased frequency of
   the proxy closing idle client connections.

5.  Security Considerations

   The edns-tcp-keep-alive option can potentially be abused to request
   large numbers of sessions in a quick burst.  When a Nameserver
   detects abusive behaviour, it SHOULD immediately close the TCP
   connection and free all buffers used.

   Readers are advised to familiarise themselves with the security
   considerations outlined in [DRAFT-5966bis]

   This section needs more work.  As usual.


6.  IANA Considerations

   The IANA is directed to assign an EDNS0 option code for the edns-tcp-
   keepalive option from the DNS EDNS0 Option Codes (OPT) registry as

        | Value | Name               | Status   | Reference       |
        | [TBA] | edns-tcp-keepalive | Optional | [This document] |


7.  Acknowledgements

   The authors acknowledge the contributions of Ray Bellis, Jinmei TATUYA and Mark


8.  References


8.1.  Normative References

              Wouters, P., "chain Query requests in DNS", draft-ietf-
              dnsop-edns-chain-query (work in progress), October 2014.

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

   [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.

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

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

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


   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190, RFC
              7320, July 2014.

8.2.  Informative References

              Wouters, P., "Chain Query requests in DNS", draft-ietf-
              dnsop-edns-chain-query (work in progress), October 2014.

              Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", draft-ietf-dnsop-5966bis-02 (work in
              progress), July 2015.

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

              Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "TLS for DNS: Initiation and Performance
              Considerations", draft-ietf-dprive-start-tls-for-dns-02
              (work in progress), May 2015.

Appendix A.  Editors' Notes

A.1.  Venue

   An appropriate venue for discussion of this document is

A.2.  Abridged Change History

A.2.1.  draft-ietf-dnsop-edns-tcp-keepalive-02

   Changed timeout value to idle timeout and re-phrased document around

   Changed units of timeout to 100ms to allow values less than 1 second.

   Change specification to remove use of the option over UDP.  This is
   potentially confusing, could cause issues with ALG's and adds only
   limited value.

   Changed semantics so the client no longer sends a timeout.  The
   client timeout is of limited value as servers should be managing
   connections based on their view of their resources, not on client
   requests as this is open to abuse.  Additionally this identifies
   cases were the option is simply being reflected back.

   Changed semantics for the meaning of a server sending a timeout of 0.
   The maximum timeout value of 6553.5s (~1.8h) is already large and a
   distinct 'connection close'-like signal is potentially more useful.

   Added more detail on server side requirements when supporting
   keepalive in terms of resource and connection management.

   Added discussion of EDNS0 per-message limitation and implications of

   Added reference to STARTTLS draft and RFC7320.

A.2.2.  draft-ietf-dnsop-edns-tcp-keepalive-01

   Version bump with no changes


A.2.3.  draft-ietf-dnsop-edns-tcp-keepalive-00

   Clarifications, working group adoption.


A.2.4.  draft-wouters-edns-tcp-keepalive-01

   Also allow clients to specify KEEPALIVE timeout values, clarify
   motivation of document.


A.2.5.  draft-wouters-edns-tcp-keepalive-00

   Initial draft.

Authors' Addresses

   Paul Wouters
   Red Hat

   Email: pwouters@redhat.com

   Joe Abley
   Dyn, Inc.
   470 Moore Street
   London, ON  N6C 2C2

   Phone: +1 519 670 9327
   Email: jabley@dyn.com

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

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

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