dnsop J. Dickinson Internet-Draft S. Dickinson Obsoletes: 5966 (if approved) Sinodun Intended status: Standards Track R. Bellis Expires: January 7, 2016 ISC A. Mankin D. Wessels Verisign Labs July 6, 2015 DNS Transport over TCP - Implementation Requirements draft-ietf-dnsop-5966bis-02 Abstract This document specifies the requirement for support of TCP as a transport protocol for DNS implementations and provides guidelines towards DNS-over-TCP performance on par with that of DNS-over-UDP. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 7, 2016. Copyright Notice Copyright (c) 2015 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 (http://trustee.ietf.org/license-info) 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 Dickinson, et al. Expires January 7, 2016 [Page 1] Internet-Draft DNS over TCP July 2015 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Terminology . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Transport Protocol Selection . . . . . . . . . . . . . . . . 5 6. Connection Handling . . . . . . . . . . . . . . . . . . . . . 6 6.1. Current practices . . . . . . . . . . . . . . . . . . . . 6 6.1.1. Clients . . . . . . . . . . . . . . . . . . . . . . . 6 6.1.2. Servers . . . . . . . . . . . . . . . . . . . . . . . 7 6.2. Recommendations . . . . . . . . . . . . . . . . . . . . . 7 6.2.1. Connection Re-use . . . . . . . . . . . . . . . . . . 7 6.2.1.1. Query Pipelining . . . . . . . . . . . . . . . . 8 6.2.2. Concurrent connections . . . . . . . . . . . . . . . 8 6.2.3. Idle Timeouts . . . . . . . . . . . . . . . . . . . . 8 6.2.4. Tear Down . . . . . . . . . . . . . . . . . . . . . . 9 7. Response Reordering . . . . . . . . . . . . . . . . . . . . . 9 8. TCP Message Length Field . . . . . . . . . . . . . . . . . . 10 9. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . . 10 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 11. Security Considerations . . . . . . . . . . . . . . . . . . . 11 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 13.1. Normative References . . . . . . . . . . . . . . . . . . 12 13.2. Informative References . . . . . . . . . . . . . . . . . 13 Appendix A. Summary of Advantages and Disadvantages to using TCP for DNS . . . . . . . . . . . . . . . . . . . . . . 13 Appendix B. Changes -01 to -02 . . . . . . . . . . . . . . . . . 14 Appendix C. Changes -00 to -01 . . . . . . . . . . . . . . . . . 15 Appendix D. Changes to RFC 5966 . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 1. Introduction Most DNS [RFC1034] transactions take place over UDP [RFC0768]. TCP [RFC0793] is always used for full zone transfers (AXFR) and is often used for messages whose sizes exceed the DNS protocol's original 512-byte limit. The growing deployment of DNSSEC and IPv6 has increased response sizes and therefore the use of TCP. The need for increased TCP use has also been driven by the protection it provides against address spoofing and therefore exploitation of DNS in reflection/amplification attacks. It is now widely used in Response Rate Limiting [RRL] Response Rate Limiting [RRL]. Dickinson, et al. Expires January 7, 2016 [Page 2] Internet-Draft DNS over TCP July 2015 Section 6.1.3.2 of [RFC1123] states: DNS resolvers and recursive servers MUST support UDP, and SHOULD support TCP, for sending (non-zone-transfer) queries. However, some implementors have taken the text quoted above to mean that TCP support is an optional feature of the DNS protocol. The majority of DNS server operators already support TCP and the default configuration for most software implementations is to support TCP. The primary audience for this document is those implementors whose limited support for TCP restricts interoperability and hinders deployment of new DNS features. This document therefore updates the core DNS protocol specifications such that support for TCP is henceforth a REQUIRED part of a full DNS protocol implementation. There are several advantages and disadvantages to the increased use of TCP as well as implementation details that need to be considered. This document addresses these issues and therefore extends the content of [RFC5966], with additional considerations and lessons learned from research, developments and implementation in DNS and in other internet protocols. Whilst this document makes no specific requirements for operators of DNS servers to meet, it does offer some suggestions to operators to help ensure that support for TCP on their servers and network is optimal. It should be noted that failure to support TCP (or the blocking of DNS over TCP at the network layer) may result in resolution failure and/or application-level timeouts. 2. Requirements Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. Terminology o Persistent connection: a TCP connection that is not closed either by the server after sending the first response nor by the client after receiving the first response. o Connection Reuse: the sending of multiple queries and responses over a single TCP connection. Dickinson, et al. Expires January 7, 2016 [Page 3] Internet-Draft DNS over TCP July 2015 o Idle DNS-over-TCP session: Clients and servers view application level idleness differently. A DNS client considers a DNS-over-TCP session to be idle when it has no pending queries to send and there are no outstanding responses. A DNS server considers a DNS- over-TCP session to be idle when it has sent responses to all the queries it has received on that connection. o Pipelining: the sending of multiple queries and responses over a single TCP connection but not waiting for any outstanding replies before sending another query. o Out-Of-Order Processing: The processing of queries concurrently and the returning of individual responses as soon as they are available, possibly out-of-order. This will most likely occur in recursive servers, however it is possible in authoritative servers that, for example, have different backend data stores. 4. Discussion In the absence of EDNS0 (Extension Mechanisms for DNS 0) (see below), the normal behaviour of any DNS server needing to send a UDP response that would exceed the 512-byte limit is for the server to truncate the response so that it fits within that limit and then set the TC flag in the response header. When the client receives such a response, it takes the TC flag as an indication that it should retry over TCP instead. RFC 1123 also says: ... it is also clear that some new DNS record types defined in the future will contain information exceeding the 512 byte limit that applies to UDP, and hence will require TCP. Thus, resolvers and name servers should implement TCP services as a backup to UDP today, with the knowledge that they will require the TCP service in the future. Existing deployments of DNS Security (DNSSEC) [RFC4033] have shown that truncation at the 512-byte boundary is now commonplace. For example, a Non-Existent Domain (NXDOMAIN) (RCODE == 3) response from a DNSSEC-signed zone using NextSECure 3 (NSEC3) [RFC5155] is almost invariably larger than 512 bytes. Since the original core specifications for DNS were written, the Extension Mechanisms for DNS (EDNS0 [RFC6891]) have been introduced. These extensions can be used to indicate that the client is prepared to receive UDP responses larger than 512 bytes. An EDNS0-compatible server receiving a request from an EDNS0-compatible client may send Dickinson, et al. Expires January 7, 2016 [Page 4] Internet-Draft DNS over TCP July 2015 UDP packets up to that client's announced buffer size without truncation. However, transport of UDP packets that exceed the size of the path MTU causes IP packet fragmentation, which has been found to be unreliable in many circumstances. Many firewalls routinely block fragmented IP packets, and some do not implement the algorithms necessary to reassemble fragmented packets. Worse still, some network devices deliberately refuse to handle DNS packets containing EDNS0 options. Other issues relating to UDP transport and packet size are discussed in [RFC5625]. The MTU most commonly found in the core of the Internet is around 1500 bytes, and even that limit is routinely exceeded by DNSSEC- signed responses. The future that was anticipated in RFC 1123 has arrived, and the only standardised UDP-based mechanism that may have resolved the packet size issue has been found inadequate. 5. Transport Protocol Selection All general-purpose DNS implementations MUST support both UDP and TCP transport. o Authoritative server implementations MUST support TCP so that they do not limit the size of responses to what fits in a single UDP packet. o Recursive server (or forwarder) implementations MUST support TCP so that they do not prevent large responses from a TCP-capable server from reaching its TCP-capable clients. o Stub resolver implementations (e.g., an operating system's DNS resolution library) MUST support TCP since to do otherwise would limit their interoperability with their own clients and with upstream servers. Regarding the choice of when to use UDP or TCP, Section 6.1.3.2 of RFC 1123 also says: ... a DNS resolver or server that is sending a non-zone-transfer query MUST send a UDP query first. This requirement is hereby relaxed. A resolver MAY elect to send either TCP or UDP queries depending on local operational reasons. TCP MAY be used before sending any UDP queries. If it already has an Dickinson, et al. Expires January 7, 2016 [Page 5] Internet-Draft DNS over TCP July 2015 open TCP connection to the server it SHOULD reuse this connection. In essence, TCP should be considered a valid alternative transport to UDP, not purely a fallback option. In addition it is noted that all Recursive and Authoritative servers MUST send responses using the same transport as the query arrived on. In the case of TCP this MUST also be the same connection. 6. Connection Handling 6.1. Current practices Section 4.2.2 of [RFC1035] says: o The server should assume that the client will initiate connection closing, and should delay closing its end of the connection until all outstanding client requests have been satisfied. o If the server needs to close a dormant connection to reclaim resources, it should wait until the connection has been idle for a period on the order of two minutes. In particular, the server should allow the SOA and AXFR request sequence (which begins a refresh operation) to be made on a single connection. Since the server would be unable to answer queries anyway, a unilateral close or reset may be used instead of graceful close. Other more modern protocols (e.g., HTTP/1.1 [RFC7230]) have support by default for persistent TCP connections for all requests. Connections are then normally closed via a 'connection close' signal from one party. The description in [RFC1035] is clear that servers should view connections as persistent (particularly after receiving an SOA), but unfortunately does not provide enough detail for an unambiguous interpretation of client behaviour for queries other than a SOA. Additionally, DNS does not yet have a signalling mechanism for connection timeout or close, although some have been proposed. 6.1.1. Clients There is no clear guidance today in any RFC as to when a DNS client should close a TCP connection, and there are no specific recommendations with regard to DNS client idle timeouts. However it is common practice for clients to close the TCP connection after sending a single request (apart from the SOA/AXFR case). Dickinson, et al. Expires January 7, 2016 [Page 6] Internet-Draft DNS over TCP July 2015 6.1.2. Servers Many DNS server implementations use a long fixed idle timeout and default to a small number of TCP connections. They also offer little by the way of TCP connection management options. The disadvantages of this include: o Operational experience has shown that long server timeouts can easily cause resource exhaustion and poor response under heavy load. o Intentionally opening many connections and leaving them idle can trivially create a TCP "denial-of-service" attack as many DNS servers are poorly equipped to defend against this by modifying their idle timeouts or other connection management policies. o A modest number of clients that all concurrently attempt to use persistent connections with non-zero idle timeouts to such a server could unintentionally cause the same "denial-of-service" problem. Note that this denial-of-service is only on the TCP service. However, in these cases it affects not only clients wishing to use TCP for their queries for operational reasons, but all clients who must fall back to TCP from UDP after receiving a TC=1 flag. 6.2. Recommendations The following sections include recommendations that are intended to result in more consistent and scalable implementations of DNS-over- TCP. 6.2.1. Connection Re-use One perceived disadvantage to DNS over TCP is the added connection setup latency, generally equal to one RTT. To amortize connection setup costs, both clients and servers SHOULD support connection reuse by sending multiple queries and responses over a single persistent TCP connection. When sending multiple queries over a TCP connection clients MUST take care to avoid Message ID collisions. In other words, they MUST not re-use the DNS Message ID of an in-flight query. This is especially important if the server could be performing out-of-order processing (see Section 7). Dickinson, et al. Expires January 7, 2016 [Page 7] Internet-Draft DNS over TCP July 2015 6.2.1.1. Query Pipelining Due to the historical use of TCP primarily for zone transfer and truncated responses, no existing RFC discusses the idea of pipelining DNS queries over a TCP connection. In order to achieve performance on par with UDP DNS clients SHOULD pipeline their queries. When a DNS client sends multiple queries to a server, it should not wait for an outstanding reply before sending the next query. Clients should treat TCP and UDP equivalently when considering the time at which to send a particular query. DNS clients should note that DNS servers that do not both process pipelined queries concurrently and send out-of-order responses will likely not provide performance on a par with UDP. IF TCP performance is of importance, clients may find it useful to use server processing times as input to server and transport selection algorithms. DNS servers (especially recursive) SHOULD expect to receive pipelined queries. The server should process TCP queries concurrently, just as it would for UDP. The server SHOULD answer all pipelined queries, even if they are sent in quick succession. The handling of responses to pipelined queries is covered in Section 7. 6.2.2. Concurrent connections To mitigate the risk of unintentional server overload, DNS clients MUST take care to minimize the number of concurrent TCP connections made to any individual server. It is RECOMMENDED that for any given client/server interaction there SHOULD be no more than one connection for regular queries, one for zone transfers and one for each protocol that is being used on top of TCP, for example, if the resolver was using TLS. Similarly, servers MAY impose limits on the number of concurrent TCP connections being handled for any particular client. These limits SHOULD be much looser than the client guidelines above, because the server does not know if the client IP address belongs to a single client or is, for example, multiple resolvers on a single machine, or multiple clients behind NAT. 6.2.3. Idle Timeouts To mitigate the risk of unintentional server overload, DNS clients MUST take care to minimize the idle time of DNS-over-TCP sessions made to any individual server. DNS clients SHOULD close the TCP connection of an idle session, unless an idle timeout has been established using some other signalling mechanism. Dickinson, et al. Expires January 7, 2016 [Page 8] Internet-Draft DNS over TCP July 2015 To mitigate the risk of unintentional server overload it is RECOMMENDED that the default server application-level idle period be of the order of seconds, but no particular value is specified. In practice, the idle period may vary dynamically, and servers MAY allow idle connections to remain open for longer periods as resources permit. A timeout of at least a few seconds is advisable for normal operations to support those clients that expect the SOA and AXFR request sequence to be made on a single connection as originally specified in [RFC1035]. Servers MAY use zero timeouts when experiencing heavy load or are under attack. 6.2.4. Tear Down Under normal operation clients should initiate connection closing on idle connections however servers may close the connection if their local idle timeout policy is exceeded. Connections may be also closed by either end under unusual conditions such as defending against an attack or system failure/reboot. Clients SHOULD retry unanswered queries if the connection closes before receiving all outstanding responses. No specific retry algorithm is specified in this document. If a server finds that a client has closed a TCP session, or if the session has been otherwise interrupted, before all pending responses have been sent then the server MUST NOT attempt to send those responses. Of course the server MAY cache those responses. 7. Response Reordering RFC 1035 is ambiguous on the question of whether TCP responses may be reordered -- the only relevant text is in Section 4.2.1, which relates to UDP: Queries or their responses may be reordered by the network, or by processing in name servers, so resolvers should not depend on them being returned in order. For the avoidance of future doubt, this requirement is clarified. Authoritative servers and recursive resolvers are RECOMMENDED to support the sending of responses in parallel and/or out-of-order, regardless of the transport protocol in use. Stub and recursive resolvers MUST be able to process responses that arrive in a different order to that in which the requests were sent, regardless of the transport protocol in use. Dickinson, et al. Expires January 7, 2016 [Page 9] Internet-Draft DNS over TCP July 2015 In order to achieve performance on par with UDP, recursive resolvers SHOULD process TCP queries in parallel and return individual responses as soon as they are available, possibly out-of-order. Since pipelined responses may arrive out-of-order, clients must take care to match responses to outstanding queries, using the ID field, port number, query name/type/class, and any other relevant protocol features. Failure by clients to properly match responses to outstanding queries can have serious consequences for inter- operability. 8. TCP Message Length Field For reasons of efficiency, DNS clients and servers SHOULD transmit the two-octet length field, and the message described by that length field, in a single TCP segment. This additionally avoids problems due to some DNS servers being very sensitive to timeout conditions on receiving messages (they may abort a TCP session if the first TCP segment does not contain both the length field and the entire message). 9. TCP Fast Open This section is non-normative. TCP fastopen [RFC7413] (TFO) allows data to be carried in the SYN packet. It also saves up to one RTT compared to standard TCP. TFO mitigates the security vulnerabilities inherent in sending data in the SYN, especially on a system like DNS where amplification attacks are possible, by use of a server-supplied cookie. TFO clients request a server cookie in the initial SYN packet at the start of a new connection. The server returns a cookie in its SYN- ACK. The client caches the cookie and reuses it when opening subsequent connections to the same server. The cookie is stored by the client's TCP stack (kernel) and persists if either the client or server processes are restarted. TFO also falls back to a regular TCP handshake gracefully. DNS services taking advantage of IP anycast [RFC4786] may need to take additional steps when enabling TFO.From [RFC7413]: Servers that accept connection requests to the same server IP address should use the same key such that they generate identical Fast Open Cookies for a particular client IP address. Otherwise a Dickinson, et al. Expires January 7, 2016 [Page 10] Internet-Draft DNS over TCP July 2015 client may get different cookies across connections; its Fast Open attempts would fall back to regular 3WHS. 10. IANA Considerations This memo includes no request to IANA. 11. Security Considerations Some DNS server operators have expressed concern that wider use of DNS over TCP will expose them to a higher risk of denial-of-service (DoS) attacks. Although there is a higher risk of such attacks against TCP-enabled servers, techniques for the mitigation of DoS attacks at the network level have improved substantially since DNS was first designed. Readers are advised to familiarise themselves with [CPNI-TCP]. To mitigate the risk of DoS attacks, DNS servers should engage in TCP connection management. This may include maintaining state on existing connections, re-using existing connections and controlling request queues to enable fair use. It is likely to be advantageous to provide configurable connection management options, for example: o total number of TCP connections o maximum TCP connections per source IP address o TCP connection idle timeout o maximum DNS transactions per TCP connection o maximum TCP connection duration No specific values are recommended for these parameters. Operators are advised to familiarise themselves with the configuration and tuning parameters available in the operating system TCP stack. However detailed advice on this is outside the scope of this document. Operators of recursive servers should ensure that they only accept connections from expected clients, and do not accept them from unknown sources. In the case of UDP traffic, this will help protect against reflector attacks [RFC5358] and in the case of TCP traffic it will prevent an unknown client from exhausting the server's limits on the number of concurrent connections. Dickinson, et al. Expires January 7, 2016 [Page 11] Internet-Draft DNS over TCP July 2015 12. Acknowledgements The authors would like to thank Francis Dupont and Paul Vixie for detailed review, Andrew Sullivan, Tony Finch, Stephane Bortzmeyer and the many others who contributed to the mailing list discussion. Also Liang Zhu, Zi Hu, and John Heidemann for extensive DNS-over-TCP discussions and code. Lucie Guiraud and Danny McPherson for reviewing early versions of this document. We would also like to thank all those who contributed to RFC 5966. 13. References 13.1. Normative References [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast Services", BCP 126, RFC 4786, December 2006. [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS Security (DNSSEC) Hashed Authenticated Denial of Existence", RFC 5155, March 2008. [RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive Nameservers in Reflector Attacks", BCP 140, RFC 5358, October 2008. Dickinson, et al. Expires January 7, 2016 [Page 12] Internet-Draft DNS over TCP July 2015 [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP 152, RFC 5625, August 2009. [RFC5966] Bellis, R., "DNS Transport over TCP - Implementation Requirements", RFC 5966, August 2010. [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, April 2013. [RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 2014. 13.2. Informative References [CPNI-TCP] CPNI, "Security Assessment of the Transmission Control Protocol (TCP)", 2009, . [Connection-Oriented-DNS] Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A., and N. Somaiya, "Connection-Oriented DNS to Improve Privacy and Security", . [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, January 2013. [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP Fast Open", RFC 7413, December 2014. [RRL] Vixie, P. and V. Schryver, "DNS Response Rate Limiting (DNS RRL)", ISC-TN 2012-1-Draft1, April 2012. [fragmentation-considered-poisonous] Herzberg, A. and H. Shulman, "Fragmentation Considered Poisonous", May 2012, . 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 Dickinson, et al. Expires January 7, 2016 [Page 13] Internet-Draft DNS over TCP July 2015 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. A more in-depth discussion of connection orientated DNS can be found elsewhere [Connection-Oriented-DNS]. Appendix B. Changes -01 to -02 o Added more text to Introduction as background to TCP use. o Added definitions of Persistent connection and Idle session to Terminology section. o Separated Connection Handling section into Current Practice and Recommendations. Provide more detail on current practices and divided Recommendations up into more granular sub-sections. o Add section on Idle time with new text on recommendations for client idle behaviour. o Move TCP message field length discussion to separate section. o Removed references to system calls in TFO section. o Added more discussion on DoS mitigation in Security Considerations section. Dickinson, et al. Expires January 7, 2016 [Page 14] Internet-Draft DNS over TCP July 2015 o Added statement that servers MAY use 0 idle timeout. o Re-stated position of TCP as an alternative to UDP in Discussion. o Updated text on server limits on concurrent connections from a particular client. o Added text that client retry logic is outside the scope of this document. o Clarified that servers should answer all pipelined queries even if sent very close together. Appendix C. Changes -00 to -01 o Changed updates to obsoletes RFC 5966. o Improved text in Section 4 Transport Protocol Selection to change "TCP SHOULD NOT be used only for the transfers and as a fallback" to make the intention clearer and more consistent. o Reference to TCP FASTOPEN updated now that it is an RFC. o Added paragraph to say that implementations MUST NOT send the TCP framing 2 byte length field in a separate packet to the DNS message. o Added Terminology section. o Changed should and RECOMMENDED in reference to parallel processing to SHOULD in sections 7 and 8. o Added text to address what a server should do when a client closes the TCP connection before pending responses are sent. o Moved the Advantages and Disadvantages section to an appendix. Appendix D. Changes to RFC 5966 This document differs from RFC 5966 in four additions: 1. DNS implementations are recommended not only to support TCP but to support it on an equal footing with UDP 2. DNS implementations are recommended to support reuse of TCP connections Dickinson, et al. Expires January 7, 2016 [Page 15] Internet-Draft DNS over TCP July 2015 3. DNS implementations are recommended to support pipelining and out of order processing of the query stream 4. A non-normative discussion of use of TCP Fast Open is added Authors' Addresses John Dickinson Sinodun Internet Technologies Magdalen Centre Oxford Science Park Oxford OX4 4GA UK Email: jad@sinodun.com URI: http://sinodun.com Sara Dickinson Sinodun Internet Technologies Magdalen Centre Oxford Science Park Oxford OX4 4GA UK Email: sara@sinodun.com URI: http://sinodun.com Ray Bellis Internet Systems Consortium, Inc 950 Charter Street Redwood City CA 94063 USA Phone: +1 650 423 1200 Email: ray@isc.org URI: http://www.isc.org Allison Mankin Verisign Labs 12061 Bluemont Way Reston, VA 20190 US Phone: +1 703 948-3200 Email: amankin@verisign.com Dickinson, et al. Expires January 7, 2016 [Page 16] Internet-Draft DNS over TCP July 2015 Duane Wessels Verisign Labs 12061 Bluemont Way Reston, VA 20190 US Phone: +1 703 948-3200 Email: dwessels@verisign.com Dickinson, et al. Expires January 7, 2016 [Page 17]