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The edns-tcp-keepalive EDNS0 Option

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7828.
Authors Paul Wouters , Joe Abley , Sara Dickinson , Ray Bellis
Last updated 2018-12-20 (Latest revision 2016-02-22)
Replaces draft-wouters-edns-tcp-keepalive
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Tim Wicinski
Shepherd write-up Show Last changed 2015-12-20
IESG IESG state Became RFC 7828 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Joel Jaeggli
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
dnsop                                                         P. Wouters
Internet-Draft                                                   Red Hat
Intended status: Standards Track                                J. Abley
Expires: August 25, 2016                                       Dyn, Inc.
                                                            S. Dickinson
                                                               R. Bellis
                                                       February 22, 2016

                  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 commonly use TCP
   only for retries and servers typically use idle timeouts on the order
   of seconds.

   This document defines an EDNS0 option ("edns-tcp-keepalive") that
   allows DNS servers to signal a variable idle timeout.  This
   signalling encourages the use of long-lived TCP connections by
   allowing the state associated with TCP transport to be managed
   effectively with minimal impact on the DNS transaction time.

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 August 25, 2016.

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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  The edns-tcp-keepalive Option . . . . . . . . . . . . . . . .   5
     3.1.  Option Format . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Use by DNS Clients  . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Sending Queries . . . . . . . . . . . . . . . . . . .   5
       3.2.2.  Receiving Responses . . . . . . . . . . . . . . . . .   6
     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
     3.5.  Non-Clean Paths . . . . . . . . . . . . . . . . . . . . .   8
     3.6.  Anycast Considerations  . . . . . . . . . . . . . . . . .   8
   4.  Intermediary Considerations . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Editors' Notes . . . . . . . . . . . . . . . . . . .  11
     A.1.  Abridged Change History . . . . . . . . . . . . . . . . .  11
       A.1.1.  draft-ietf-dnsop-edns-tcp-keepalive-06  . . . . . . .  11
       A.1.2.  draft-ietf-dnsop-edns-tcp-keepalive-05  . . . . . . .  11
       A.1.3.  draft-ietf-dnsop-edns-tcp-keepalive-04  . . . . . . .  12
       A.1.4.  draft-ietf-dnsop-edns-tcp-keepalive-03  . . . . . . .  12
       A.1.5.  draft-ietf-dnsop-edns-tcp-keepalive-02  . . . . . . .  12
       A.1.6.  draft-ietf-dnsop-edns-tcp-keepalive-01  . . . . . . .  13
       A.1.7.  draft-ietf-dnsop-edns-tcp-keepalive-00  . . . . . . .  13
       A.1.8.  draft-wouters-edns-tcp-keepalive-01 . . . . . . . . .  13
       A.1.9.  draft-wouters-edns-tcp-keepalive-00 . . . . . . . . .  13

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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.
   This is because clients that retry over TCP following a truncated UDP
   response typically only use the TCP session for a single (request,
   response) pair, continuing with UDP transport for subsequent queries.

   The use of TCP transport requires 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 idle
   connections are closed after an appropriate amount of time.

   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.  Response
   Rate Limiting [RRL] is designed to help mitigate such attacks against
   authoritative-only servers.  One feature of RRL is to let some amount
   of responses "slip" through the rate limiter.  These are returned
   with the TC (truncation) bit set, which causes 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 [fragmentation-considered-poisonous].

   TCP transport is less susceptible to the risks of fragmentation and
   reflection attacks.  However, TCP transport for DNS as currently

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   deployed has expensive setup overhead, compared to using UDP (when no
   retry is required).

   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 therefore the overhead of the initial TCP handshake is
   minimised when the resulting session is used to exchange multiple DNS
   message pairs over a single session.  The extra RTT time for session
   setup can be represented as the equation (1 + N)/N, where N
   represents the number of DNS message pairs that utilize the session
   and the result approaches unity as N increases.

   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 retries
   over TCP.  The overhead for a DNS transaction over UDP truncated due
   to RRL is 3x RTT, higher than the overhead imposed on the same
   transaction initiated over TCP.

   This document proposes a signalling mechanism between DNS clients and
   servers that encourages the use of long-lived TCP connections by
   allowing the state associated with TCP transport to be managed
   effectively with minimal impact on the DNS transaction time.

   This mechanism will be of benefit both for stub-resolver and
   resolver-authoritative TCP connections.  In the latter case the
   persistent nature of the TCP connection can provide improved defence
   against attacks including DDoS.

   The reduced overhead of this extension adds up significantly when
   combined with other EDNS0 extensions, such as [CHAIN-QUERY] and
   [DNS-over-TLS].  For example, the combination of these EDNS0
   extensions make it possible for hosts on high-latency mobile networks
   to natively and efficiently perform DNSSEC validation and encrypt

2.  Requirements Notation

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

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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 TCP session open to conduct future DNS
   transactions, with the idle timeout being specified by the server.
   This specification does not distinguish between different types of
   DNS client and server in the use of this option.

   [DRAFT-5966bis] defines an 'idle' DNS-over-TCP session from both the
   client and server perspective.  The idle timeout described here
   begins when the idle condition is met per that definition and should
   be reset when that condition is lifted i.e. when a client sends a
   message or when a server receives a message on an idle connection.

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 0 if the TIMEOUT is omitted, the value 2
      if it is present;

   TIMEOUT:   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 MUST NOT include the edns-tcp-keepalive option in queries
   sent using UDP 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 to
   keep the connection open when idle.

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   DNS clients MAY include the edns-tcp-keepalive option in subsequent
   queries sent to a server using TCP transport to signal their
   continued desire to keep the connection open when idle.

   Clients MUST specify an OPTION-LENGTH of 0 and omit the TIMEOUT

3.2.2.  Receiving Responses

   A DNS client that receives a response using UDP transport that
   includes the edns-tcp-keepalive option MUST ignore the 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 when it is idle.  It SHOULD honour the timeout received
   in that response (overriding any previous 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 SHOULD send no more
   queries on that connection and initiate closing the connection 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 MUST ignore the option.

   A DNS server that receives a query using TCP transport that includes
   the edns-tcp-keepalive option MAY modify the local idle timeout
   associated with that TCP session if resources permit.

3.3.2.  Sending Responses

   A DNS server that receives a query sent using TCP transport that
   includes an OPT RR (with or without the edns-tcp-keepalive option)
   MAY include the edns-tcp-keepalive option in the response to signal
   the expected idle timeout on a connection.  Servers MUST specify the
   TIMEOUT value that is currently associated with the TCP session.  It

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   is reasonable for this value to change according to local resource
   constraints.  The DNS server 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, or if it wants clients to close
   currently open connections.

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 edns-tcp-
   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 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

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   then uses a connection close signal (in a similar manner to HTTP 1.1

   If a client includes the edns-tcp-keepalive option in the first
   query, it SHOULD include an EDNS0 OPT RR periodically in any further
   messages it sends during the TCP session.  This will increase the
   chance of the client being notified should the server modify the
   timeout associated with a session.  The algorithm for choosing when
   to do this is out of scope of this document and is left up to the
   implementor and/or operator.

   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

   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.  It might be possible for
   anycast servers to avoid disruption due to topology changes by making
   use of TCP multipath [RFC6824] to anchor the server side of the TCP
   connection to an unambiguously-unicast address.

4.  Intermediary Considerations

   It is RECOMMENDED that DNS intermediaries which terminate TCP
   connections implement edns-tcp-keepalive.  An intermediary that does
   not implement edns-tcp-keepalive but sits between a client and server
   that both support edns-tcp-keepalive might close idle connections

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5.  Security Considerations

   The edns-tcp-keepalive option can potentially be abused to request
   large numbers of long-lived sessions in a quick burst.  When a DNS
   Server detects abusive behaviour, it SHOULD immediately close the TCP
   connection and free the resources used.

   Servers could choose to monitor client behaviour with respect to the
   edns-tcp-keepalive option to build up profiles of clients that do not
   honour the specified timeout.

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

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       |
        | TBD1  | edns-tcp-keepalive | Standard | [This document] |

7.  Acknowledgements

   The authors acknowledge the contributions of Jinmei TATUYA and Mark
   Andrews.  Thanks to Duane Wessels for detailed review and the many
   others who contributed to the mailing list discussion.

8.  References

8.1.  Normative References

              Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", draft-ietf-dnsop-5966bis (work in
              progress), January 2016.

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

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 7320, DOI 10.17487/RFC7320, July 2014,

8.2.  Informative References

              Wouters, P., "Chain Query requests in DNS", draft-ietf-
              dnsop-edns-chain-query (work in progress), January 2016.

              Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "TLS for DNS: Initiation and Performance
              Considerations", draft-ietf-dprive-dns-over-tls (work in
              progress), January 2016.

              Herzberg, A. and H. Shulman, "Fragmentation Considered
              Poisonous", arXiv 1205.4011, May 2012,

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,

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

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Appendix A.  Editors' Notes

A.1.  Abridged Change History

   [Note to RFC Editor: please remove this section prior to

A.1.1.  draft-ietf-dnsop-edns-tcp-keepalive-06

   Introduction: Moved paragraph 8 to paragraph 2 for readability.

   Introduction: clarified that TCP has expensive setup overhead
   compared to UDP.

   Section 3: Add explicit description of the idle timeout.

   Section 3.3.2, 1st para: make explicit that query may or may not
   contain edns-tcp-keepalive option.

   Section 3.3.2: remove discussion of intermediary behaviour.

A.1.2.  draft-ietf-dnsop-edns-tcp-keepalive-05

   Reword Abstract and paragraph 9 in Introduction to remove discussion
   on balancing UDP/TCP and talk about encouraging use of long-lived TCP

   Section 3.2.2: should -> SHOULD

   Changed draft-ietf-dnsop-5966bis to be a normative reference,
   therefore adding a dependancy on publication of that as RFC.

   Reword sentence referring to RFC6824 since it is informational.

   Update IANA option to Standard.

   Remove last sentence from 1st paragraph of introduction.

   Reword paragraph 6 in Introduction, merge paragraph 7 and 8.

   Reword Section 3, first sentence to clarify the timeout is specified
   by the server.

   Correct missing URIs in 2 references.

   Clarify statement in Section 3.2.2 as how clients should handle
   updating the timeout when receiving a response.

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   Reworded first paragraph of Introduction discussing TCP vs (UDP +
   retry over TCP).  Changed 'fallback' to 'retry' in 2 places.

A.1.3.  draft-ietf-dnsop-edns-tcp-keepalive-04

   Adding wording to sections 3.2.1 and 3.4 to clarify client behaviour
   on subsequent queries on a TCP connection.

   Changed the should to a SHOULD in section 3.2.2

   Changed Nameserver to DNS server in section 5.

   Updated references.

   Changed reference to RFC6824 to be informative.

   Corrected reference to requested EDNS0 option code to be 'TBD1'.

A.1.4.  draft-ietf-dnsop-edns-tcp-keepalive-03

   Clarified that a response to a query with any OPT RR may contain the
   ends-tcp-keepalive option.

   Corrected TIMEOUT length from 4 to 2 in the diagram.

   Updated references, including name change of STARTTLS -> DNS-over-TLS
   and adding reference for cache poisoning.

   Updated wording in section on Intermediary Considerations.

   Updated wording describing RRL.

   Added paragraph to security section describing client behaviour

   Added wording to introduction on use case for stub/resolver/

A.1.5.  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.

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   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.1.6.  draft-ietf-dnsop-edns-tcp-keepalive-01

   Version bump with no changes

A.1.7.  draft-ietf-dnsop-edns-tcp-keepalive-00

   Clarifications, working group adoption.

A.1.8.  draft-wouters-edns-tcp-keepalive-01

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

A.1.9.  draft-wouters-edns-tcp-keepalive-00

   Initial draft.

Authors' Addresses

   Paul Wouters
   Red Hat


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

   Phone: +1 519 670 9327

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


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

   Phone: +1 650 423 1200

Wouters, et al.          Expires August 25, 2016               [Page 14]