One document matched: draft-ietf-dnsop-edns-tcp-keepalive-00.xml
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docName="draft-ietf-dnsop-edns-tcp-keepalive-00">
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<front>
<title>The edns-tcp-keepalive EDNS0 Option</title>
<author initials='P.' surname="Wouters" fullname='Paul Wouters'>
<organization>Red Hat</organization>
<address>
<email>pwouters@redhat.com</email>
</address>
</author>
<author initials='J.' surname="Abley" fullname='Joe Abley'>
<organization>Dyn, Inc.</organization>
<address>
<postal>
<street>470 Moore Street</street>
<city>London</city>
<region>ON</region>
<code>N6C 2C2</code>
<country>Canada</country>
</postal>
<phone>+1 519 670 9327</phone>
<email>jabley@dyn.com</email>
</address>
</author>
<date month="April" day="11" year="2014" />
<area>ops</area>
<workgroup>dnsop</workgroup>
<abstract>
<t>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 use of the
TCP transport as RFC 5966 suggests closing idle TCP sessions
"in the order of seconds", making use of TCP only suitable for
individual queries generated as a fallback protocol for truncated
UDP answers.</t>
<t>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. 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.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>DNS messages between clients and servers may be received
over either UDP or TCP <xref target="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.</t>
<t>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.</t>
<t>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 <xref target="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.</t>
<t><xref target="RFC1035"/> specified a maximum DNS message
size over UDP transport of 512 bytes. Deployment of DNSSEC
<xref target="RFC4033"/> and other protocols subsequently
increased the observed frequency at which responses exceed
this limit. EDNS0 <xref target="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.</t>
<t>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.</t>
<t>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 increases.</t>
<t>(It should perhaps be noted that the overhead for a DNS
transaction over UDP truncacated due to RRL is 3x RTT, higher
than the overhead imposed on the same transaction initiated
over TCP.)</t>
<t>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.</t>
<t>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 for the exchange of multiple DNS
messages are closed promptly.</t>
<t>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.</t>
<t>The reduced overhead of this extension adds up significantly
when combined with other edns extensions, such as
<xref target="CHAIN-QUERY"/>. The combination of
these two EDNS extensions make it possible for hosts on
high-latency mobile networks to natively perform DNSSEC validation.
</t>
</section>
<section title="Requirements Notation">
<t>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 <xref target="RFC2119"/>.</t>
</section>
<section title="The edns-tcp-keepalive Option" anchor="overview">
<t>This document specifies a new <xref target="RFC6891">EDNS0</xref>
option, edns-tcp-keepalive, which can be used by DNS clients and
servers to signal a willingness to keep an (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.</t>
<t>
<vspace blankLines="12"/>
</t>
<section title="Option Format" anchor="format">
<t>The edns-tcp-keepalive option is encoded as follows:</t>
<figure><artwork align="left"><![CDATA[
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 |
+---------------------------------------------------------------+
]]></artwork></figure>
<t>where:
<list style="hanging">
<t hangText="OPTION-CODE: ">the EDNS0 option code
assigned to edns-tcp-keepalive, [TBD]</t>
<t hangText="OPTION-LENGTH: ">the value 2;</t>
<t hangText="TIMEOUT: ">a timeout value for the TCP connection, specified in
seconds, encoded in network byte order.</t>
</list>
</t>
</section>
<section title="Use by DNS Clients">
<section title="Sending Queries">
<t>DNS clients MAY 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.</t>
<t>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.</t>
<t>Clients MAY specify a TIMEOUT value that is representative
of the minimum expected time an individual TCP session
should remain established for it to be used by multiple
DNS transactions.</t>
<t>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.</t>
<t>If the client has insufficient data to be able to
provide a meaningful estimate, the TIMEOUT value MUST
be set to zero.</t>
</section>
<section title="Receiving Responses">
<t>A DNS 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 the case where multiple candidate
servers are available to service a particular query.</t>
<t>A DNS client that receives a response using TCP transport
that includes the edns-tcp-keepalive option MAY keep the
existing TCP session open.</t>
<t>A DNS client that receives a response that includes the edns-tcp-keepalive option
with a TIMEOUT value of 0 is allowed to keep the TCP connection open indefinately.</t>
</section>
</section>
<section title="Use by DNS Servers">
<section title="Receiving Queries">
<t>A DNS server that receives a query using UDP transport
that includes the edns-tcp-keepalive option MAY record
the presence of the option for statistical purposes, but should not otherwise
modify its usual behaviour in sending a response.</t>
<t>A DNS server that receives a query using TCP transport that includes
the edns-tcp-keepalive option SHOULD extend the timeout normally
associated with TCP sessions if resources permit, using the TIMEOUT
value supplied by the client as a guide.</t>
</section>
<section title="Sending Responses">
<t>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
sessions. </t>
<t>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. 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. The DNS server MAY omit including the
edns-tcp-keepalive option if it deems its local resources
are too low to service more TCP keepalive sessions.</t>
</section>
</section>
<section title="TCP Session Management">
<t>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.</t>
<t>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. This 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.</t>
<t>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.</t>
</section>
<section title="Non-Clean Paths">
<t>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 <xref target="RFC6891"/> section 6.2.2 SHOULD
be employed to avoid persistent interference due to
non-clean paths.</t>
</section>
<section title="Anycast Considerations">
<t>DNS servers of various types are commonly deployed using
anycast <xref target="RFC4786"/>.</t>
<t>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 servers
provisioned using anycast unless all anycast server nodes are capable
of processing the edns-tcp-keepalive option.</t>
<t>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
<xref target="RFC6824"/> to anchor the server side of the
TCP connection to an unambiguously-unicast address in
order to avoid disruption due to topology changes.</t>
</section>
</section>
<section title="Security Considerations">
<t>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.</t>
<t>This section needs more work. As usual.</t>
</section>
<section title="IANA Considerations">
<t>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 follows:</t>
<texttable>
<ttcol>Value</ttcol>
<ttcol>Name</ttcol>
<ttcol>Status</ttcol>
<ttcol>Reference</ttcol>
<c>[TBA]</c>
<c>edns-tcp-keepalive</c>
<c>Optional</c>
<c>[This document]</c>
</texttable>
</section>
<section title="Acknowledgements">
<t>The authors acknowledge the contributions of Ray Bellis,
Jinmei TATUYA and Mark Andrews.</t>
</section>
</middle>
<back>
<references title='Normative References'>
&rfc1035;
&rfc2119;
&rfc4033;
&rfc4786;
&rfc5966;
&rfc6824;
&rfc6891;
<reference anchor='CHAIN-QUERY'>
<front>
<title>chain Query requests in DNS</title>
<author initials='P' surname='Wouters' fullname='P. Wouters'>
<organization>Red Hat</organization>
</author>
<date month='April' day='11' year='2014' />
<abstract><t>
This document defines an EDNS0 extension that can be used by a DNSSEC
enabled Recursive Nameserver configured as a forwarder to send a
single DNS query requesting to receive a complete validation path
along with the regular DNS answer, without the need to rapid-fire
many UDP requests in an attempt to attain a low latency.
</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-dnsop-edns-chain-query' />
<format type='TXT'
target='http://www.ietf.org/internet-drafts/draft-ietf-dnsop-edns-chain-query-00.txt' />
</reference>
</references>
<references title="Informative References">
<reference anchor="RRL">
<front>
<title>DNS Response Rate Limiting (DNS RRL)</title>
<author initials="P." surname="Vixie" fullname="Paul Vixie">
<organization>ISC</organization>
<address>
<email>vixie@isc.org</email>
</address>
</author>
<author initials="V." surname="Schryver" fullname="Vernon Schryver">
<organization>Rhyolite</organization>
<address>
<email>vjs@rhyolite.com</email>
</address>
</author>
<date year="2012" month="April"/>
</front>
<seriesInfo name="ISC-TN" value="2012-1-Draft1"/>
<format type="TXT" target="http://ss.vix.su/~vixie/isc-tn-2012-1.txt"/>
</reference>
</references>
<section title="Editors' Notes">
<section title="Venue">
<t>An appropriate venue for discussion of this document is
dnsop@ietf.org.</t>
</section>
<section title="Abridged Change History">
<section title="draft-ietf-dnsop-edns-tcp-keepalive-00">
<t>Clarifications, working group adoption.</t>
</section>
<section title="draft-wouters-edns-tcp-keepalive-01">
<t>Also allow clients to specify KEEPALIVE timeout values, clarify motivation of document.</t>
</section>
<section title="draft-wouters-edns-tcp-keepalive-00">
<t>Initial draft.</t>
</section>
</section>
</section>
</back>
</rfc>
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