draft-ietf-dnsop-5966bis-00.txt   draft-ietf-dnsop-5966bis-01.txt 
dnsop J. Dickinson dnsop J. Dickinson
Internet-Draft Sinodun Internet Technologies Internet-Draft Sinodun Internet Technologies
Updates: 5966 (if approved) R. Bellis Obsoletes: 5966 (if approved) R. Bellis
Intended status: Standards Track Nominet Intended status: Standards Track Nominet
Expires: June 7, 2015 A. Mankin Expires: September 10, 2015 A. Mankin
D. Wessels D. Wessels
Verisign Labs Verisign Labs
December 4, 2014 March 9, 2015
DNS Transport over TCP - Implementation Requirements DNS Transport over TCP - Implementation Requirements
draft-ietf-dnsop-5966bis-00 draft-ietf-dnsop-5966bis-01
Abstract Abstract
This document updates the requirements for the support of TCP as a This document specifies the requirement for support of TCP as a
transport protocol for DNS implementations. 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 Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 7, 2015. This Internet-Draft will expire on September 10, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements Terminology . . . . . . . . . . . . . . . . . . 3
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Transport Protocol Selection . . . . . . . . . . . . . . . . 4 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Connection Handling . . . . . . . . . . . . . . . . . . . . . 5 5. Transport Protocol Selection . . . . . . . . . . . . . . . . 4
6. Query Pipelining . . . . . . . . . . . . . . . . . . . . . . 6 6. Connection Handling . . . . . . . . . . . . . . . . . . . . . 5
7. Response Reordering . . . . . . . . . . . . . . . . . . . . . 6 7. Query Pipelining . . . . . . . . . . . . . . . . . . . . . . 6
8. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . . 7 8. Response Reordering . . . . . . . . . . . . . . . . . . . . . 7
9. Summary of Advantages and Disadvantages to using TCP for DNS 8 9. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . . 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
11. Security Considerations . . . . . . . . . . . . . . . . . . . 9 11. Security Considerations . . . . . . . . . . . . . . . . . . . 8
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
13.1. Normative References . . . . . . . . . . . . . . . . . . 9 13.1. Normative References . . . . . . . . . . . . . . . . . . 9
13.2. Informative References . . . . . . . . . . . . . . . . . 10 13.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Changes to RFC 5966 . . . . . . . . . . . . . . . . 11 Appendix A. Summary of Advantages and Disadvantages to using TCP
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 for DNS . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Changes -00 to -01 . . . . . . . . . . . . . . . . . 11
Appendix C. Changes to RFC 5966 . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
Most DNS [RFC1034] transactions take place over UDP [RFC0768]. TCP Most DNS [RFC1034] transactions take place over UDP [RFC0768]. TCP
[RFC0793] is always used for full zone transfers (AXFR) and is often [RFC0793] is always used for full zone transfers (AXFR) and is often
used for messages whose sizes exceed the DNS protocol's original used for messages whose sizes exceed the DNS protocol's original
512-byte limit. 512-byte limit.
Section 6.1.3.2 of [RFC1123] states: Section 6.1.3.2 of [RFC1123] states:
skipping to change at page 3, line 7 skipping to change at page 3, line 11
TCP. The primary audience for this document is those implementors TCP. The primary audience for this document is those implementors
whose failure to support TCP restricts interoperability and limits whose failure to support TCP restricts interoperability and limits
deployment of new DNS features. deployment of new DNS features.
This document therefore updates the core DNS protocol specifications This document therefore updates the core DNS protocol specifications
such that support for TCP is henceforth a REQUIRED part of a full DNS such that support for TCP is henceforth a REQUIRED part of a full DNS
protocol implementation. protocol implementation.
There are several advantages and disadvantages to the increased use There are several advantages and disadvantages to the increased use
of TCP as well as implementation details that need to be considered. of TCP as well as implementation details that need to be considered.
This document addresses these issues and updates [RFC5966], with This document addresses these issues and therefore extends the
additional considerations and lessons learned from new research and content of [RFC5966], with additional considerations and lessons
implementations [Connection-Oriented-DNS]. learned from new research and implementations
[Connection-Oriented-DNS].
Whilst this document makes no specific requirements for operators of Whilst this document makes no specific requirements for operators of
DNS servers to meet, it does offer some suggestions to operators to DNS servers to meet, it does offer some suggestions to operators to
help ensure that support for TCP on their servers and network is help ensure that support for TCP on their servers and network is
optimal. It should be noted that failure to support TCP (or the optimal. It should be noted that failure to support TCP (or the
blocking of DNS over TCP at the network layer) may result in blocking of DNS over TCP at the network layer) may result in
resolution failure and/or application-level timeouts. resolution failure and/or application-level timeouts.
2. Terminology 2. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Discussion 3. Terminology
o Connection Reuse: the sending of multiple queries and responses
over a single TCP 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 in parallel 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), 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 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 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 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 flag in the response header. When the client receives such a
response, it takes the TC flag as an indication that it should retry response, it takes the TC flag as an indication that it should retry
over TCP instead. over TCP instead.
RFC 1123 also says: RFC 1123 also says:
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size are discussed in [RFC5625]. size are discussed in [RFC5625].
The MTU most commonly found in the core of the Internet is around The MTU most commonly found in the core of the Internet is around
1500 bytes, and even that limit is routinely exceeded by DNSSEC- 1500 bytes, and even that limit is routinely exceeded by DNSSEC-
signed responses. signed responses.
The future that was anticipated in RFC 1123 has arrived, and the only The future that was anticipated in RFC 1123 has arrived, and the only
standardised UDP-based mechanism that may have resolved the packet standardised UDP-based mechanism that may have resolved the packet
size issue has been found inadequate. size issue has been found inadequate.
4. Transport Protocol Selection 5. Transport Protocol Selection
All general-purpose DNS implementations MUST support both UDP and TCP All general-purpose DNS implementations MUST support both UDP and TCP
transport. transport.
o Authoritative server implementations MUST support TCP so that they o Authoritative server implementations MUST support TCP so that they
do not limit the size of responses to what fits in a single UDP do not limit the size of responses to what fits in a single UDP
packet. packet.
o Recursive server (or forwarder) implementations MUST support TCP o Recursive server (or forwarder) implementations MUST support TCP
so that they do not prevent large responses from a TCP-capable so that they do not prevent large responses from a TCP-capable
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limit their interoperability with their own clients and with limit their interoperability with their own clients and with
upstream servers. upstream servers.
Regarding the choice of when to use UDP or TCP, Section 6.1.3.2 of Regarding the choice of when to use UDP or TCP, Section 6.1.3.2 of
RFC 1123 also says: RFC 1123 also says:
... a DNS resolver or server that is sending a non-zone-transfer ... a DNS resolver or server that is sending a non-zone-transfer
query MUST send a UDP query first. query MUST send a UDP query first.
This requirement is hereby relaxed. A resolver MAY elect to send This requirement is hereby relaxed. A resolver MAY elect to send
either TCP or UDP queries depending on local operational reasons. If either TCP or UDP queries depending on local operational reasons.
it already has an open TCP connection to the server it SHOULD reuse TCP MAY be used before sending any UDP queries. If it already has an
this connection. open TCP connection to the server it SHOULD reuse this connection.
In essence, TCP SHOULD be considered as valid a transport as UDP.
In essence, TCP SHOULD be considered as valid a transport as UDP. It
SHOULD NOT be used only for zone transfers and as a fallback.
In addition it is noted that all Recursive and Authoritative servers In addition it is noted that all Recursive and Authoritative servers
MUST send responses using the same transport as the query arrived on. MUST send responses using the same transport as the query arrived on.
In the case of TCP this MUST also be the same connection. In the case of TCP this MUST also be the same connection.
5. Connection Handling 6. Connection Handling
One perceived disadvantage to DNS over TCP is the added connection One perceived disadvantage to DNS over TCP is the added connection
setup latency, generally equal to one RTT. To amortize connection setup latency, generally equal to one RTT. To amortize connection
setup costs, both clients and servers SHOULD support connection reuse setup costs, both clients and servers SHOULD support connection reuse
by sending multiple queries and responses over a single TCP by sending multiple queries and responses over a single TCP
connection. connection.
DNS currently has no connection signaling mechanism. Clients and DNS currently has no connection signaling mechanism. Clients and
servers may close a connection at any time. Clients MUST be prepared servers may close a connection at any time. Clients MUST be prepared
to retry failed queries on broken connections. to retry failed queries on broken connections.
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made to any individual server. Similarly, servers MAY impose limits made to any individual server. Similarly, servers MAY impose limits
on the number of concurrent TCP connections being handled for any on the number of concurrent TCP connections being handled for any
particular client. It is RECOMMENDED that for any given client - particular client. It is RECOMMENDED that for any given client -
server interaction there SHOULD be no more than one connection for server interaction there SHOULD be no more than one connection for
regular queries, one for zone transfers and one for each protocol 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 that is being used on top of TCP, for example, if the resolver was
using TLS. The server MUST NOT enforce these rules for a particular using TLS. The server MUST NOT enforce these rules for a particular
client because it does not know if the client IP address belongs to a client because it does not know if the client IP address belongs to a
single client or is, for example, multiple clients behind NAT. single client or is, for example, multiple clients behind NAT.
6. Query Pipelining For reasons of efficiency, implementations SHOULD wherever possible
attempt to coalesce the two byte length field and subsequent DNS
payload data into a single packet.
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. Query Pipelining
Due to the use of TCP primarily for zone transfer and truncated Due to the use of TCP primarily for zone transfer and truncated
responses, no existing RFC discusses the idea of pipelining DNS responses, no existing RFC discusses the idea of pipelining DNS
queries over a TCP connection. queries over a TCP connection.
In order to achieve performance on par with UDP, it is therefore In order to achieve performance on par with UDP DNS clients SHOULD
RECOMMENDED that DNS clients should pipeline their queries. When a pipeline their queries. When a DNS client sends multiple queries to
DNS client sends multiple queries to a server, it should not wait for a server, it should not wait for an outstanding reply before sending
an outstanding reply before sending the next query. Clients should the next query. Clients should treat TCP and UDP equivalently when
treat TCP and UDP equivalently when considering the time at which to considering the time at which to send a particular query.
send a particular query.
DNS servers (especially recursive) SHOULD expect to receive pipelined DNS servers (especially recursive) SHOULD expect to receive pipelined
queries. The server should process TCP queries in parallel, just as queries. The server should process TCP queries in parallel, just as
it would for UDP. The handling of responses to pipelined queries is it would for UDP. The handling of responses to pipelined queries is
covered in the following section. covered in the following section.
When pipelining queries over TCP it is very easy to send more DNS When pipelining queries over TCP it is very easy to send more DNS
queries than there are DNS Message ID's. Implementations MUST take queries than there are DNS Message ID's. Implementations MUST take
care to check their list of outstanding DNS Message ID's before care to check their list of outstanding DNS Message ID's before
sending a new query over an existing TCP connection. This is sending a new query over an existing TCP connection. This is
especially important if the server could be performing out-of-order especially important if the server could be performing out-of-order
processing. In addition, when sending multiple queries over TCP it processing. In addition, when sending multiple queries over TCP it
is very easy for a name server to overwhelm its own network is very easy for a name server to overwhelm its own network
interface. Implementations MUST take care to manage buffer sizes or interface. Implementations MUST take care to manage buffer sizes or
to throttle writes to the network interface. to throttle writes to the network interface.
7. Response Reordering 8. Response Reordering
RFC 1035 is ambiguous on the question of whether TCP responses may be 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 reordered -- the only relevant text is in Section 4.2.1, which
relates to UDP: relates to UDP:
Queries or their responses may be reordered by the network, or by Queries or their responses may be reordered by the network, or by
processing in name servers, so resolvers should not depend on them processing in name servers, so resolvers should not depend on them
being returned in order. being returned in order.
For the avoidance of future doubt, this requirement is clarified. For the avoidance of future doubt, this requirement is clarified.
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In order to achieve performance on par with UDP, recursive resolvers In order to achieve performance on par with UDP, recursive resolvers
SHOULD process TCP queries in parallel and return individual SHOULD process TCP queries in parallel and return individual
responses as soon as they are available, possibly out-of-order. responses as soon as they are available, possibly out-of-order.
Since responses may arrive out-of-order, clients must take care to Since responses may arrive out-of-order, clients must take care to
match responses to outstanding queries, using the ID field, port match responses to outstanding queries, using the ID field, port
number, query name/type/class, and any other relevant protocol number, query name/type/class, and any other relevant protocol
features. features.
8. TCP Fast Open 9. TCP Fast Open
This section is non-normative. This section is non-normative.
TCP fastopen [I-D.ietf-tcpm-fastopen] (TFO) allows data to be carried TCP fastopen [RFC7413] (TFO) allows data to be carried in the SYN
in the SYN packet. It also saves up to one RTT compared to standard packet. It also saves up to one RTT compared to standard TCP.
TCP.
TFO mitigates the security vulnerabilities inherent in sending data TFO mitigates the security vulnerabilities inherent in sending data
in the SYN, especially on a system like DNS where amplification in the SYN, especially on a system like DNS where amplification
attacks are possible, by use of a server-supplied cookie. TFO attacks are possible, by use of a server-supplied cookie. TFO
clients request a server cookie in the initial SYN packet at the 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- start of a new connection. The server returns a cookie in its SYN-
ACK. The client caches the cookie and reuses it when opening ACK. The client caches the cookie and reuses it when opening
subsequent connections to the same server. subsequent connections to the same server.
The cookie is stored by the client's TCP stack (kernel) and persists The cookie is stored by the client's TCP stack (kernel) and persists
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Adding support for this to existing name server implementations is Adding support for this to existing name server implementations is
relatively easy, but does require source code modifications. On the relatively easy, but does require source code modifications. On the
client, the call to connect() is replaced with a TFO aware version of client, the call to connect() is replaced with a TFO aware version of
sendmsg() or sendto(). On the server, TFO must be switched into sendmsg() or sendto(). On the server, TFO must be switched into
server mode by changing the kernel parameter (net.ipv4.tcp_fastopen server mode by changing the kernel parameter (net.ipv4.tcp_fastopen
on Linux) to enable the server bit (Set the integer value to 2 on Linux) to enable the server bit (Set the integer value to 2
(server only) or 3 (client and server)) and setting a socket option (server only) or 3 (client and server)) and setting a socket option
between the bind() and listen() calls. between the bind() and listen() calls.
DNS services taking advantage of IP anycast [RFC4786] may need to DNS services taking advantage of IP anycast [RFC4786] may need to
take additional steps when enabling TFO. From take additional steps when enabling TFO. From [RFC7413]:
[I-D.ietf-tcpm-fastopen]:
Servers that accept connection requests to the same server IP Servers that accept connection requests to the same server IP
address should use the same key such that they generate identical address should use the same key such that they generate identical
Fast Open Cookies for a particular client IP address. Otherwise a Fast Open Cookies for a particular client IP address. Otherwise a
client may get different cookies across connections; its Fast Open client may get different cookies across connections; its Fast Open
attempts would fall back to regular 3WHS. attempts would fall back to regular 3WHS.
9. 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.
TCP does not suffer from UDP's issues with fragmentation.
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 be disrupted due to
routing changes. Clients utilizing TCP for DNS must always be
prepared to re-establish connections or otherwise retry outstanding
queries. It may also 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 all implementations.
10. IANA Considerations 10. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA.
11. Security Considerations 11. Security Considerations
Some DNS server operators have expressed concern that wider use of Some DNS server operators have expressed concern that wider use of
DNS over TCP will expose them to a higher risk of denial-of-service DNS over TCP will expose them to a higher risk of denial-of-service
(DoS) attacks. (DoS) attacks.
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Operators of recursive servers should ensure that they only accept Operators of recursive servers should ensure that they only accept
connections from expected clients, and do not accept them from connections from expected clients, and do not accept them from
unknown sources. In the case of UDP traffic, this will help protect unknown sources. In the case of UDP traffic, this will help protect
against reflector attacks [RFC5358] and in the case of TCP traffic it against reflector attacks [RFC5358] and in the case of TCP traffic it
will prevent an unknown client from exhausting the server's limits on will prevent an unknown client from exhausting the server's limits on
the number of concurrent connections. the number of concurrent connections.
12. Acknowledgements 12. Acknowledgements
The authors would like to thank Liang Zhu, Zi Hu, and John Heidemann The authors would like to thank Francis Dupont for his detailed
for extensive DNS-over-TCP discussions and code; and Lucie Guiraud review, Liang Zhu, Zi Hu, and John Heidemann for extensive DNS-over-
and Danny McPherson for reviewing early versions of this document. TCP discussions and code and Lucie Guiraud and Danny McPherson for
We would also like to thank all those who contributed to RFC 5966. reviewing early versions of this document. We would also like to
thank all those who contributed to RFC 5966.
13. References 13. References
13.1. Normative References 13.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980. August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981. 793, September 1981.
skipping to change at page 11, line 5 skipping to change at page 10, line 43
Protocol (TCP)", 2009, <http://www.cpni.gov.uk/Docs/ Protocol (TCP)", 2009, <http://www.cpni.gov.uk/Docs/
tn-03-09-security-assessment-TCP.pdf>. tn-03-09-security-assessment-TCP.pdf>.
[Connection-Oriented-DNS] [Connection-Oriented-DNS]
Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A., Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
and N. Somaiya, "T-DNS: Connection-Oriented DNS to Improve and N. Somaiya, "T-DNS: Connection-Oriented DNS to Improve
Privacy and Security (extended)", Privacy and Security (extended)",
<http://www.isi.edu/publications/trpublic/files/ <http://www.isi.edu/publications/trpublic/files/
tr-693.pdf>. tr-693.pdf>.
[I-D.ietf-tcpm-fastopen]
Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", draft-ietf-tcpm-fastopen-10 (work in
progress), September 2014.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple "TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, January 2013. Addresses", RFC 6824, January 2013.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, December 2014.
[fragmentation-considered-poisonous] [fragmentation-considered-poisonous]
Herzberg, A. and H. Shulman, "Fragmentation Considered Herzberg, A. and H. Shulman, "Fragmentation Considered
Poisonous", May 2012, <http://arxiv.org/abs/1205.4011>. Poisonous", May 2012, <http://arxiv.org/abs/1205.4011>.
Appendix A. Changes to RFC 5966 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.
TCP does not suffer from UDP's issues with fragmentation.
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 be disrupted due to
routing changes. Clients utilizing TCP for DNS must always be
prepared to re-establish connections or otherwise retry outstanding
queries. It may also 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 all implementations.
Appendix B. 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 C. Changes to RFC 5966
This document differs from RFC 5966 in four additions: This document differs from RFC 5966 in four additions:
1. DNS implementations are recommended not only to support TCP but 1. DNS implementations are recommended not only to support TCP but
to support it on an equal footing with UDP to support it on an equal footing with UDP
2. DNS implementations are recommended to support reuse of TCP 2. DNS implementations are recommended to support reuse of TCP
connections connections
3. DNS implementations are recommended to support pipelining and out 3. DNS implementations are recommended to support pipelining and out
skipping to change at page 12, line 4 skipping to change at page 12, line 39
John Dickinson John Dickinson
Sinodun Internet Technologies Sinodun Internet Technologies
Magdalen Centre Magdalen Centre
Oxford Science Park Oxford Science Park
Oxford OX4 4GA Oxford OX4 4GA
UK UK
Email: jad@sinodun.com Email: jad@sinodun.com
URI: http://sinodun.com URI: http://sinodun.com
Ray Bellis Ray Bellis
Nominet Nominet
Edmund Halley Road Edmund Halley Road
Oxford OX4 4DQ Oxford OX4 4DQ
UK UK
Phone: +44 1865 332211 Phone: +44 1865 332211
Email: ray.bellis@nominet.org.uk Email: ray.bellis@nominet.org.uk
URI: http://www.nominet.org.uk/ URI: http://www.nominet.org.uk/
Allison Mankin Allison Mankin
Verisign Labs Verisign Labs
12061 Bluemont Way 12061 Bluemont Way
Reston, VA 20190 Reston, VA 20190
US
Phone: +1 703 948-3200 Phone: +1 703 948-3200
Email: amankin@verisign.com Email: amankin@verisign.com
Duane Wessels Duane Wessels
Verisign Labs Verisign Labs
12061 Bluemont Way 12061 Bluemont Way
Reston, VA 20190 Reston, VA 20190
US
Phone: +1 703 948-3200 Phone: +1 703 948-3200
Email: dwessels@verisign.com Email: dwessels@verisign.com
 End of changes. 30 change blocks. 
89 lines changed or deleted 139 lines changed or added

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