DNSOP Working Group R. Bellis Internet-Draft ISC Updates: RFC 7766, RFC 1035 (if S. Cheshire approved) Apple Inc. Intended status: Standards Track J. Dickinson Expires: March 17, 2018 S. Dickinson Sinodun A. Mankin Salesforce T. Pusateri Unaffiliated September 13, 2017 DNS Stateful Operations draft-ietf-dnsop-session-signal-04 Abstract This document defines a new DNS Stateful Operation OPCODE used to communicate operations within persistent stateful sessions, expressed using type-length-value (TLV) syntax, and defines an initial set of TLVs used to manage session timeouts and termination. This mechanism is intended to reduce the overhead of existing "per-packet" signaling mechanisms with "per-message" semantics as well as defining new stateful operations not defined in EDNS(0). 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 March 17, 2018. Bellis, et al. Expires March 17, 2018 [Page 1] Internet-Draft DNS Stateful Operations September 2017 Copyright Notice Copyright (c) 2017 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 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 6 4.1. DSO Session Establishment . . . . . . . . . . . . . . . . 7 4.1.1. Middle-box Considerations . . . . . . . . . . . . . . 8 4.2. Message Format . . . . . . . . . . . . . . . . . . . . . 8 4.2.1. Header . . . . . . . . . . . . . . . . . . . . . . . 9 4.2.2. DSO Data . . . . . . . . . . . . . . . . . . . . . . 10 4.2.3. EDNS(0) and TSIG . . . . . . . . . . . . . . . . . . 12 4.3. Message Handling . . . . . . . . . . . . . . . . . . . . 13 5. Keepalive Operation TLV . . . . . . . . . . . . . . . . . . . 14 5.1. Client handling of received Session Timeout values . . . 16 5.2. Relation to EDNS(0) TCP Keepalive Option . . . . . . . . 17 6. Retry Delay TLV . . . . . . . . . . . . . . . . . . . . . . . 17 6.1. Use as an Operation TLV . . . . . . . . . . . . . . . . . 18 6.2. Use as a Modifier TLV . . . . . . . . . . . . . . . . . . 19 7. Encryption Padding TLV . . . . . . . . . . . . . . . . . . . 19 8. DSO Session Lifecycle and Timers . . . . . . . . . . . . . . 20 8.1. DSO Session Initiation . . . . . . . . . . . . . . . . . 20 8.2. DSO Session Timeouts . . . . . . . . . . . . . . . . . . 20 8.3. Inactive DSO Sessions . . . . . . . . . . . . . . . . . . 20 8.4. The Inactivity Timeout . . . . . . . . . . . . . . . . . 21 8.4.1. Closing Inactive DSO Sessions . . . . . . . . . . . . 21 8.4.2. Values for the Inactivity Timeout . . . . . . . . . . 22 8.5. The Keepalive Interval . . . . . . . . . . . . . . . . . 23 8.5.1. Keepalive Interval Expiry . . . . . . . . . . . . . . 23 8.5.2. Values for the Keepalive Interval . . . . . . . . . . 23 8.6. Server-Initiated Termination on Error . . . . . . . . . . 24 8.7. Client Behaviour in Receiving an Error . . . . . . . . . 25 8.8. Server-Initiated Termination on Overload . . . . . . . . 25 Bellis, et al. Expires March 17, 2018 [Page 2] Internet-Draft DNS Stateful Operations September 2017 8.9. Retry Delay Operation TLV . . . . . . . . . . . . . . . . 26 8.9.1. Outstanding Operations . . . . . . . . . . . . . . . 26 8.9.2. Client Reconnection . . . . . . . . . . . . . . . . . 27 9. Connection Sharing . . . . . . . . . . . . . . . . . . . . . 27 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 10.1. DSO OPCODE Registration . . . . . . . . . . . . . . . . 28 10.2. DSO RCODE Registration . . . . . . . . . . . . . . . . . 28 10.3. DSO Type Codes Registry . . . . . . . . . . . . . . . . 28 11. Security Considerations . . . . . . . . . . . . . . . . . . . 29 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 13.1. Normative References . . . . . . . . . . . . . . . . . . 29 13.2. Informative References . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31 1. Introduction The use of transports for DNS other than UDP is being increasingly specified, for example, DNS over TCP [RFC1035][RFC7766] and DNS over TLS [RFC7858]. Such transports can offer persistent, long-lived sessions and therefore when using them for transporting DNS messages it is of benefit to have a mechanism that can establish parameters associated with those sessions, such as timeouts. In such situations it is also advantageous to support server initiated messages. The existing EDNS(0) Extension Mechanism for DNS [RFC6891] is explicitly defined to only have "per-message" semantics. Whilst EDNS(0) has been used to signal at least one session related parameter (the EDNS(0) TCP Keepalive option [RFC7828]) the result is less than optimal due to the restrictions imposed by the EDNS(0) semantics and the lack of server-initiated signalling. For example, a server cannot arbitrarily instruct a client to close a connection because the server can only send EDNS(0) options in responses to queries that contained EDNS(0) options. This document defines a new DNS Stateful Operation OPCODE used to carry operations within persistent stateful connections, expressed using type-length-value (TLV) syntax, and defines an initial set of TLVs including ones used to manage session timeouts and termination. This new format has distinct advantages over an RR based format because it is more explicit and more compact. Each TLV definition is specific to the use case, and as a result contains no redundant or overloaded fields. Importantly, it completely avoids conflating DNS Stateful Operations in anyway with normal DNS operations or with existing EDNS(0) based functionality. A goal of this approach is to avoid the operational issues that have befallen EDNS(0), particularly relating to middle-box behaviour. Bellis, et al. Expires March 17, 2018 [Page 3] Internet-Draft DNS Stateful Operations September 2017 With EDNS(0), multiple options may be packed into a single OPT pseudo-RR, and there is no generalized mechanism for a client to be able to tell whether a server has processed or otherwise acted upon each individual option within the combined OPT RR. The specifications for each individual option need to define how each different option is to be acknowledged, if necessary. With DNS Stateful Operations, in contrast, there is no compelling motivation to pack multiple operations into a single message for efficiency reasons. Each Stateful operation is communicated in its own separate DNS message, and the transport protocol can take care of packing separate DNS messages into a single IP packet if appropriate. For example, TCP can pack multiple small DNS messages into a single TCP segment. The RCODE in each response message indicates the success or failure of the operation in question. It should be noted that the message format for DNS Stateful Operations (see Section 4.2) differs from the traditional DNS packet format used for standard queries and responses. The standard twelve- octet header is used, but the four count fields (QDCOUNT, ANCOUNT, NSCOUNT, ARCOUNT) are set to zero and their corresponding sections are not present. The actual data pertaining to DNS Stateful Operations is appended to the end of the DNS message header. When displayed using today's packet analyzer tools that have not been updated to recognize the DNS Stateful Operations format, this will result in the Stateful Operations data being displayed as unknown additional data after the end of the DNS message. It is likely that future updates to these tools will add the ability to recognize, decode, and display the Stateful Operations data. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119]. "DSO" is used to mean DNS Stateful Operation. The term "connection" means a bidirectional byte stream of reliable, in-order messages, such as provided by using DNS over TCP [RFC1035][RFC7766] or DNS over TLS [RFC7858]. The unqualified term "session" in the context of this document means the exchange of DNS messages over a connection where: Bellis, et al. Expires March 17, 2018 [Page 4] Internet-Draft DNS Stateful Operations September 2017 o The connection between client and server is persistent and relatively long-lived (i.e., minutes or hours, rather than seconds). o Either end of the connection may initiate messages to the other. A "DSO session" is established between two endpoints that acknowledge persistent DNS state via the exchange of DSO messages over the connection. This is distinct from, for example a DNS-over-TCP session as described in RC7766. A "DSO session" is terminated when the underlying connection is closed. The term "server" means the software with a listening socket, awaiting incoming connection requests. The term "client" means the software which initiates a connection to the server's listening socket. The terms "initiator" and "responder" correspond respectively to the initial sender and subsequent receiver of a DSO request message, regardless of which was the "client" and "server" in the usual DNS sense. The term "sender" may apply to either an initiator (when sending a DNS Stateful Operation request message) or a responder (when sending a DNS Stateful Operation response message). Likewise, the term "receiver" may apply to either a responder (when receiving a DNS Stateful Operation request message) or an initiator (when receiving a DNS Stateful Operation response message). DNS Stateful Operations are expressed using type-length-value (TLV) syntax. Two timers (elapsed time since an event) are defined in this document: o an inactivity timer (see Section 5 and Section 8.3) o a keepalive timer (see Section 5 and Section 8.5) The timeouts associated with these timers are called the inactivity timeout and the keepalive interval respectively. The term "Session Timeouts" is used to refer to this pair of timeout values. Bellis, et al. Expires March 17, 2018 [Page 5] Internet-Draft DNS Stateful Operations September 2017 Reseting a timer means resetting the timer value to zero and starting the timer again. Clearing a timer means resetting the timer value to zero but NOT starting the time again. 3. Discussion There are several use cases for DNS Stateful operations that can be described here. Firstly, establishing session parameters such as server defined timeouts is of great use in the general management of persistent connections. For example, using DSO sessions for stub to recursive DNS-over-TLS [RFC7858] is more flexible for both the client and the server than attempting to manage sessions using just the EDNS(0) TCP Keepalive option [RFC7828]. The simple set of TLVs defined in this document is sufficient to greatly enhance connection management for this use case. Secondly, DNS-SD has evolved into a naturally session based mechanism where, for example, long-lived subscriptions lend themselves to 'push' mechanisms as opposed to polling. Long-lived stateful connections and server initiated messages align with this use case as described in [I-D.ietf-dnssd-push]. A general use case is that DNS traffic is often bursty but session establishment can be expensive. One challenge with long-lived connections is to maintain sufficient traffic to maintain NAT and firewall state. To mitigate this issue this document introduces a new concept for the DNS, that is DSO "Keepalive traffic". This traffic carries no DNS data and is not considered 'activity' in the classic DNS sense but serves to reset a keepalive timer in order to avoid re-cycling a DSO session. There are a myriad of other potential use cases for DSO given the versatility and extensibility of this specification. Section 4 of this document first describes the protocol details of DNS Stateful Operations including definitions of three TLVs for session management and encryption padding. Section 8 then presents a detailed discussion of the DSO Session lifecycle including an in- depth discussion of keepalive traffic and session termination. 4. Protocol Details Bellis, et al. Expires March 17, 2018 [Page 6] Internet-Draft DNS Stateful Operations September 2017 4.1. DSO Session Establishment DSO messages MUST only be carried in protocols and in environments where a session may be established according to the definition above. Standard DNS over TCP [RFC1035][RFC7766], and DNS over TLS [RFC7858] are suitable protocols. DNS over plain UDP [RFC0768] is not appropriate since it fails on the requirement for in-order message delivery, and, in the presence of NAT gateways and firewalls with short UDP timeouts, it fails to provide a persistent bi-directional communication channel unless an excessive amount of keepalive traffic is used. DSO messages relate only to the specific "DSO session" in which they are being carried. A "DSO session" is established over a connection when either side of the connection sends the first DSO TLV and it is acknowledged by the other side. The DSO message format Section 4.2.2 includes an option to specify that a DSO request does not require a response acknowledgement. Session establishment can only be performed using a DSO message that requires a response acknowledgement. While this specification defines an initial set of three TLVs, additional TLVs may be defined in additional specifications. All three of the TLVs defined here are mandatory to implement. A client MAY attempt to initiate DSO messages at any time on a connection; receiving a NOTIMP response in reply indicates that the server does not implement DSO, and the client SHOULD NOT issue further DSO messages on that connection. A server SHOULD NOT initiate DSO messages until a client-initiated DSO message is received first, unless in an environment where it is known in advance by other means that the client supports DSO. This requirement is to ensure that the clients that do not support DSO do not receive unsolicited inbound DSO messages that they would not know how to handle. On a session between a client and server that support DSO, once the client has sent at least one DSO message (or it is known in advance by other means that the client supports DSO) either end may unilaterally send DSO messages at any time, and therefore either client or server may be the initiator of a message. From this point on it is considered that a "DSO session" is in progress. Clients and servers should behave as described in this specification with regard to inactivity timeouts and connection close, not as prescribed in [RFC7766]. Bellis, et al. Expires March 17, 2018 [Page 7] Internet-Draft DNS Stateful Operations September 2017 4.1.1. Middle-box Considerations Where an application-layer middle box (e.g., a DNS proxy, forwarder, or session multiplexer) is in the path the middle box MUST NOT blindly forward DSO messages in either direction, and MUST treat the inbound and outbound connections as separate sessions. This does not preclude the use of DSO messages in the presence of an IP-layer middle box such as a NAT that rewrites IP-layer and/or transport- layer headers, but otherwise preserves the effect of a single session between the client and the server. To illustrate the above, consider a network where a middle box terminates one or more TCP connections from clients and multiplexes the queries therein over a single TCP connection to an upstream server. The DSO messages and any associated state are specific to the individual TCP connections. A DSO-aware middle box MAY in some circumstances be able to retain associated state and pass it between the client and server (or vice versa) but this would be highly TLV- specific. For example, the middle box may be able to maintain a list of which clients have made Push Notification subscriptions [I-D.ietf-dnssd-push] and make its own subscription(s) on their behalf, relaying any subsequent notifications to the client (or clients) that have subscribed to that particular notification. 4.2. Message Format A DSO message begins with the standard twelve-octet DNS message header [RFC1035] with the OPCODE field set to the DSO OPCODE (tentatively 6). However, unlike standard DNS messages, the question section, answer section, authority records section and additional records sections are not present. The corresponding count fields (QDCOUNT, ANCOUNT, NSCOUNT, ARCOUNT) MUST be set to zero on transmission. If a DSO message is received where any of the count fields are not zero, then a FORMERR MUST be returned. Bellis, et al. Expires March 17, 2018 [Page 8] Internet-Draft DNS Stateful Operations September 2017 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | MESSAGE ID | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ |QR | OPCODE | Z | RCODE | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | QDCOUNT (MUST be zero) | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | ANCOUNT (MUST be zero) | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | NSCOUNT (MUST be zero) | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | ARCOUNT (MUST be zero) | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | / DSO Data / / / +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 4.2.1. Header In a request the MESSAGE ID field MUST be set to a unique value, that the initiator is not currently using for any other active operation on this connection. For the purposes here, a MESSAGE ID is in use in this DSO session if the initiator has used it in a request for which it has not yet received a response, or if the client has used it to setup state that it has not yet ready to delete. For example, state could be a subscription as defined in [I-D.ietf-dnssd-push]. In a response the MESSAGE ID field MUST contain a copy of the value of the MESSAGE ID field in the request being responded to. In a request the DNS Header QR bit MUST be zero (QR=0). If the QR bit is not zero the message is not a request. In a response the DNS Header QR bit MUST be one (QR=1). If the QR bit is not one the message is not a response. The DNS Header OPCODE field holds the DSO OPCODE value (tentatively 6). The Z bits are currently unused, and in both requests and responses the Z bits MUST be set to zero (0) on transmission and MUST be silently ignored on reception, unless a future document specifies otherwise. Bellis, et al. Expires March 17, 2018 [Page 9] Internet-Draft DNS Stateful Operations September 2017 In a request message (QR=0) the RCODE is generally set to zero on transmission, and silently ignored on reception, except where specified otherwise (for example, the Retry Delay operation (see Section 6), where the RCODE indicates the reason for termination). The RCODE value in a response may be one of the following values: +------+-----------+------------------------------------------------+ | Code | Mnemonic | Description | +------+-----------+------------------------------------------------+ | 0 | NOERROR | Operation processed successfully | | | | | | 1 | FORMERR | Format error | | | | | | 2 | SERVFAIL | Server failed to process request due to a | | | | problem with the server | | | | | | 3 | NXDOMAIN | TLV dependent | | | | | | 4 | NOTIMP | DSO not supported | | | | | | 5 | REFUSED | Operation declined for policy reasons | | | | | | 9 | NOTAUTH | Not Authoritative (TLV dependent) | | | | | | 11 | DSONOTIMP | DSO type code not supported | +------+-----------+------------------------------------------------+ Use of the above RCODE's is likely to be common in DSO but does not preclude the definition and use of other codes in future documents that make use of DSO. If a document describing a DSO makes use of either NXDOMAIN or NOTAUTH then that document MUST explain the meaning. 4.2.2. DSO Data The standard twelve-octet DNS message header is followed by the DSO Data. The first TLV in a DSO request message is called the Operation TLV. Any subsequent TLVs after this initial Operation TLV are called Modifier TLVs. Depending on the operation a DSO response can contain: o No TLVs Bellis, et al. Expires March 17, 2018 [Page 10] Internet-Draft DNS Stateful Operations September 2017 o Only an Operation TLV o An Operation TLV followed by one or more Modifier TLVs o Only Modifier TLVs 4.2.2.1. TLV Format Operation and modifier TLVs both use the same encoding format. Operation TLVs SHOULD normally require a response and, therefore, set the TLV Acknowledgement bit in a request. However, for some Operation TLVs, this may be undesirable and the TLV Acknowledgement bit MAY be cleared in the request. Each Operation TLV definition should stipulate whether an acknowledgement is REQUIRED. If the TLV Acknowledgement bit is cleared in a request, a response MUST NOT be sent. Modifier TLVs MUST NEVER set the Acknowledgement bit. The Acknowledgement bit is NEVER set in the response to an Operation TLV. 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | A | DSO-TYPE | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | DSO DATA LENGTH | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | / TYPE-DEPENDENT DATA / / / +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ A: A 1 bit TLV Response flag indicating whether or not an Operation TLV requires a response Acknowledgement. DSO-TYPE: A 15 bit field in network order giving the type of the current DSO TLV per the IANA DSO Type Codes Registry. DSO DATA LENGTH: A 16 bit field in network order giving the size in octets of the TYPE-DEPENDENT DATA. TYPE-DEPENDENT DATA: Type-code specific format. Where domain names appear within TYPE-DEPENDENT DATA, they MAY be compressed using standard DNS name compression. However, the compression MUST NOT point outside of the TYPE-DEPENDENT DATA section and offsets MUST be from the start of the TYPE-DEPENDENT DATA. Bellis, et al. Expires March 17, 2018 [Page 11] Internet-Draft DNS Stateful Operations September 2017 4.2.2.2. Operation TLVs An "Operation TLV" specifies the operation to be performed. A DSO message MUST contain at most one Operation TLV. In all cases a DSO request message MUST contain exactly one Operation TLV, indicating the operation to be performed. Depending on the operation, a DSO response message MAY contain no Operation TLV, because it is simply a response to a previous request message, and the MESSAGE ID in the header is sufficient to identify the request in question. Or it may contain a single corresponding response Operation TLV, with the same DSO-TYPE as in the request message. The specification for each DSO type determines whether a response for that operation type is required to carry the Operation TLV. If a DSO response is received for an operation which requires that the response carry an Operation TLV, and the required Operation TLV is not the first DSO TLV in the response message, then this is a fatal error and the recipient of the defective response message MUST immediately terminate the connection with a TCP RST (or equivalent for other protocols). 4.2.2.3. Modifier TLVs A "Modifier TLV" specifies additional parameters relating to the operation. Immediately following the Operation TLV, if present, a DSO message MAY contain one or more Modifier TLVs. 4.2.2.4. Unrecognized TLVs If a DSO request is received containing an unrecognized Operation TLV, the receiver MUST send a response with matching MESSAGE ID, and RCODE DSONOTIMP (tentatively 11). The response MUST NOT contain an Operation TLV. If a DSO message (request or response) is received containing one or more unrecognized Modifier TLVs, the unrecognized Modifier TLVs MUST be silently ignored, and the remainder of the message is interpreted and handled as if the unrecognized parts were not present. 4.2.3. EDNS(0) and TSIG Since the ARCOUNT field MUST be zero, a DSO message MUST NOT contain an EDNS(0) option in the additional records section. If functionality provided by current or future EDNS(0) options is Bellis, et al. Expires March 17, 2018 [Page 12] Internet-Draft DNS Stateful Operations September 2017 desired for DSO messages, an Operation TLV or Modifier TLV needs to be defined to carry the necessary information. For example, the EDNS(0) Padding Option [RFC7830] used for security purposes is not permitted in a DSO message, so if message padding is desired for DSO messages then the Encryption Padding TLV described in Section 7 MUST be used. Similarly, a DSO message MUST NOT contain a TSIG record. A TSIG record in a conventional DNS message is added as the last record in the additional records section, and carries a signature computed over the preceding message content. Since DSO data appears after the additional records section, it would not be included in the signature calculation. If use of signatures with DSO messages becomes necessary in the future, an explicit Modifier TLV needs to be defined to perform this function. Note however that, while DSO _messages_ cannot include EDNS(0) or TSIG records, a DSO _session_ is typically used to carry a whole series of DNS messages of different kinds, including DSO messages, and other DNS message types like Query [RFC1034] [RFC1035] and Update [RFC2136], and those messages can carry EDNS(0) and TSIG records. This specification explicitly prohibits use of the EDNS(0) TCP Keepalive Option [RFC7828] in _any_ messages sent on a DSO session (because it duplicates the functionality provided by the DSO Keepalive operation), but messages may contain other EDNS(0) options as appropriate. 4.3. Message Handling The initiator MUST set the value of the QR bit in the DNS header to zero (0), and the responder MUST set it to one (1). Every DSO request message (QR=0) MUST elicit a response (QR=1), which MUST have the same MESSAGE ID in the DNS message header as in the corresponding request. DSO request messages sent by the client elicit a response from the server, and DSO request messages sent the server elicit a response from the client. With most TCP implementations, the TCP data acknowledgement (generated because data has been received by TCP), the TCP window update (generated because TCP has delivered that data to the receiving software) and the DSO response (generated by the receiving software itself) are all combined into a single packet, so in practice the requirement that every DSO request message MUST elicit a DSO response incurs minimal extra cost on the network. Requiring that every request elicit a corresponding response also avoids Bellis, et al. Expires March 17, 2018 [Page 13] Internet-Draft DNS Stateful Operations September 2017 performance problems caused by interaction between Nagle's Algorithm and Delayed Ack [NagleDA]. The namespaces of 16-bit MESSAGE IDs are disjoint in each direction. For example, it is _not_ an error for both client and server to send a request message with the same ID. In effect, the 16-bit MESSAGE ID combined with the identity of the initiator (client or server) serves as a 17-bit unique identifier for a particular operation on a DSO session. As described in Section 4.2.1 An initiator MUST NOT reuse a MESSAGE ID that is already in use for an outstanding request, unless specified otherwise by the relevant specification for the DSO in question. At the very least, this means that a MESSAGE ID MUST NOT be reused in a particular direction on a particular DSO session while the initiator is waiting for a response to a previous request on that DSO session, unless specified otherwise by the relevant specification for the DSO in question. (For a long-lived state the MESSAGE ID for the operation MUST NOT be reused whilst that state remains active.) If a client or server receives a response (QR=1) where the MESSAGE ID does not match any of its outstanding operations, this is a fatal error and it MUST immediately terminate the connection with a TCP RST (or equivalent for other protocols). 5. Keepalive Operation TLV The Keepalive Operation TLV (DSO-TYPE=1) performs two functions: to reset the keepalive timer for the DSO session and to establish the values for the Session Timeouts. The Keepalive Operation TLV resets only the keepalive timer, not the inactivity timer. The reason for this is that periodic Keepalive Operation TLVs are sent for the sole purpose of keeping a DSO session alive because that DSO session has current or recent activity that warrants keeping the DSO session alive. If sending keepalive traffic itself were to reset the inactivity timer, then that would create a circular livelock where keepalive traffic would be sent indefinitely to keep a DSO session alive, where the only activity on that DSO session would be keepalive traffic keeping the DSO session alive so that further keepalive traffic can be sent. Sending keepalive traffic is considered a maintenance activity that is performed in service of other client activities. Sending keepalive traffic itself is not considered a client activity. For a DSO session to be considered active, it must be carrying something more than just keepalive traffic. This is why merely sending a Keepalive Operation TLV does not reset the inactivity timer. Bellis, et al. Expires March 17, 2018 [Page 14] Internet-Draft DNS Stateful Operations September 2017 When sent by a client, the Keepalive Operation TLV resets a DSO session's keepalive timer, and at the same time requests what the Session Timeout values should be from this point forward in the DSO session. An acknowledgement is always required for a Keepalive Operation TLV and the TLV Acknowledgement bit MUST be set in the request when originated by either the client or the server. Once a DSO session is in progress (see Section 4) the Keepalive TLV also MAY be initiated by a server. When sent by a server, it resets a DSO session's keepalive timer, and unilaterally informs the client of the new Session Timeout values to use from this point forward in this DSO session. It is not required that the Keepalive TLV be used in every DSO session. While many DNS Stateful operations will be used in conjunction with a long-lived session state, not all DNS Stateful operations require long-lived session state, and in some cases the default 15-second value for both the inactivity timeout and keepalive interval may be perfectly appropriate. However, it can be noted that for clients that implement only the TLVs defined in this document it is the only way for a client to initiate a DSO session. The TYPE-DEPENDENT DATA for the the Keepalive TLV is as follows: 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | INACTIVITY TIMEOUT (32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | KEEPALIVE INTERVAL (32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ INACTIVITY TIMEOUT: the inactivity timeout for the current DSO session, specified as a 32 bit word in network (big endian) order in units of milliseconds. This is the timeout at which the client MUST close an inactive DSO session. If the client does not gracefully close an inactive DSO session then after twice this interval the server will forcibly terminate the connection with a TCP RST (or equivalent for other protocols). KEEPALIVE INTERVAL: the keepalive interval for the current DSO session, specified as a 32-bit word, in network (big endian) order, in units of milliseconds. This is the interval at which a client MUST generate keepalive traffic to maintain connection state. If the client does not generate the necessary keepalive traffic then after twice this interval the server will forcibly Bellis, et al. Expires March 17, 2018 [Page 15] Internet-Draft DNS Stateful Operations September 2017 terminate the connection with a TCP RST (or equivalent for other protocols). In a client-initiated DSO Keepalive message, the Session Timeouts contain the client's requested values. In a server response to a client-initiated message, the Session Timeouts contain the server's chosen values, which the client MUST respect. This is modeled after the DHCP protocol, where the client requests a certain lease lifetime using DHCP option 51 [RFC2132], but the server is the ultimate authority for deciding what lease lifetime is actually granted. In a server-initiated DSO Keepalive message, the Session Timeouts unilaterally inform the client of the new values from this point forward in this DSO session. The client MUST generate a response to the server-initiated DSO Keepalive message. The MESSAGE ID in the response message MUST match the ID from the server-initiated DSO Keepalive message, and the response message MUST NOT contain any Operation TLV. When a client is sending its second and subsequent Keepalive DSO requests to the server, the client SHOULD continue to request its preferred values each time. This allows flexibility, so that if conditions change during the lifetime of a DSO session, the server can adapt its responses to better fit the client's needs. 5.1. Client handling of received Session Timeout values When a client receives a response to its client-initiated DSO Keepalive message, or receives a server-initiated DSO Keepalive message, the client has then received Session Timeout values dictated by the server. The two timeout values contained in the DSO Keepalive TLV from the server may each be higher, lower, or the same as the respective Session Timeout values the client previously had for this DSO session. In the case of the keepalive timer, the handling of the received value is straightforward. The act of receiving the message containing the DSO Keepalive TLV itself resets the keepalive timer and updates the keepalive interval for the DSO session. The new keepalive interval indicates the maximum time that may elapse before another message must be sent or received on this DSO session, if the DSO session is to remain alive. In the case of the inactivity timeout, the handling of the received value superficially appears a little more subtle, though the meaning of the inactivity timeout is unchanged - it still indicates the maximum permissible time allowed without activity on a DSO session. The act of receiving the message containing the DSO Keepalive TLV Bellis, et al. Expires March 17, 2018 [Page 16] Internet-Draft DNS Stateful Operations September 2017 does not itself reset the inactivity timer. The time elapsed since the last useful activity on this DSO session is unaffected by exchange of DSO Keepalive messages. The act of receiving the message containing the DSO Keepalive TLV does update the timeout associated with the running inactivity timer; that becomes the new maximum permissible time without activity on a DSO session. o If the inactivity timer value is not greater than the new inactivity timeout, then the DSO session may remain open for now. When the inactivity timer value exceeds the new inactivity timeout, the client MUST then close the DSO session, as described above. o If the inactivity timer value is already greater than the new inactivity timeout, then this DSO session has already been inactive for longer than the server permits, and the client MUST immediately close this DSO session. o If the inactivity timer value is more than twice the new inactivity timeout, then this DSO session is eligible to be forcibly terminated by the server and and the client MUST immediately close this DSO session. However if a server abruptly reduces the inactivity timeout in this way the server SHOULD give the client a grace period of one quarter of the new inactivity timeout, to give the client time to close the connection gracefully before the server resorts to terminating it forcibly. 5.2. Relation to EDNS(0) TCP Keepalive Option The inactivity timeout value in the Keepalive TLV (DSO-TYPE=1) has similar intent to the EDNS(0) TCP Keepalive Option [RFC7828]. A client/server pair that supports DSO MUST NOT use the EDNS(0) TCP KeepAlive option within any message after a DSO session has been established. Once a DSO session has been established, if either client or server receives a DNS message over the DSO session that contains an EDNS(0) TCP Keepalive option, this is an error and the receiver of the EDNS(0) TCP Keepalive option MUST immediately terminate the connection with a TCP RST (or equivalent for other protocols). 6. Retry Delay TLV The Retry Delay TLV (DSO-TYPE=0) can be used as an Operation TLV or as a Modifier TLV. The TYPE-DEPENDENT DATA for the the Retry Delay TLV is as follows: Bellis, et al. Expires March 17, 2018 [Page 17] Internet-Draft DNS Stateful Operations September 2017 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RETRY DELAY (32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ RETRY DELAY: a time value, specified as a 32 bit word in network order in units of milliseconds, within which the client MUST NOT retry this operation, or retry connecting to this server. The RECOMMENDED value is 10 seconds. 6.1. Use as an Operation TLV When sent in a DSO request message, from server to client, the Retry Delay TLV (0) is considered an Operation TLV. It is used by a server to request that a client close the DSO session and underlying connection, and not to reconnect for the indicated time interval. In this case it applies to the DSO session as a whole, and the client MUST close the DSO session, as described in section Section 8.9. The RCODE in the message header MUST indicate the reason for the termination: o NOERROR indicates a routine shutdown. o SERVFAIL indicates that the server is overloaded due to resource exhaustion. o REFUSED indicates that the server has been reconfigured and is no longer able to perform one or more of the functions currently being performed on this DSO session (for example, a DNS Push Notification server could be reconfigured such that is is no longer accepting DNS Push Notification requests for one or more of the currently subscribed names). This document specifies only these three RCODE values for Retry Delay request. Servers sending Retry Delay requests SHOULD use one of these three values. However, future circumstances may create situations where other RCODE values are appropriate in Retry Delay requests, so clients MUST be prepared to accept Retry Delay requests with any RCODE value. An acknowledgement is not desired for a Retry Delay Operation TLV and the TLV Acknowledgement bit MUST be cleared in the request. Bellis, et al. Expires March 17, 2018 [Page 18] Internet-Draft DNS Stateful Operations September 2017 6.2. Use as a Modifier TLV When appended to a DSO response message for some client request, the Retry Delay TLV (0) is considered a Modifier TLV. The indicated time interval during which the client SHOULD NOT retry applies only to the failed operation, not to the DSO session as a whole. In the case of a client request that returns a nonzero RCODE value, the server MAY append a Retry Delay TLV (0) to the response, indicating the time interval during which the client SHOULD NOT attempt this operation again. 7. Encryption Padding TLV The Encryption Padding TLV (DSO-TYPE=2) can only be used as a Modifier TLV. It is only applicable when the DSO Transport layer uses encryption such as TLS. The TYPE-DEPENDENT DATA for the the Padding TLV is optional and is a variable length field containing non-specified values. A DATA LENGTH of 0 essentially provides for 4 octets of padding (the minimum amount). 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ / / / VARIABLE NUMBER OF OCTETS / / / +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ As in [RFC7830] the PADDING octets SHOULD be set to 0x00. Other values MAY be used, for example, in cases where there is a concern that the padded message could be subject to compression before encryption. PADDING octets of any value MUST be accepted in the messages received. The Encryption Padding TLV may be included in either a DSO request, response, or both. As in [RFC7830] if a request is received with a Encryption Padding TLV, then the response MUST also include an Encryption Padding TLV. The length of padding is intentionally not specified in this document and is a function of current best practices with respect to the type and length of data in the preceding TLVs. See [I-D.ietf-dprive-padding-policy] Bellis, et al. Expires March 17, 2018 [Page 19] Internet-Draft DNS Stateful Operations September 2017 8. DSO Session Lifecycle and Timers 8.1. DSO Session Initiation A session begins when a client makes a new connection to a server. A DSO session begin as described in Section 4.1. The client may perform as many DNS operations as it wishes using the newly created DSO session. Operations SHOULD be pipelined (i.e., the client doesn't need wait for a response before sending the next message). The server MUST act on messages in the order they are transmitted, but responses to those messages MAY be sent out of order, if appropriate. 8.2. DSO Session Timeouts Two timeout values are associated with a DSO session: the inactivity timeout, and the keepalive interval. The first timeout value, the inactivity timeout, is the maximum time for which a client may speculatively keep a DSO session open in the expectation that it may have future requests to send to that server. The second timeout value, the keepalive interval, is the maximum permitted interval between client messages to the server if the client wishes to keep the DSO session alive. The two timeout values are independent. The inactivity timeout may be lower, the same, or higher than the keepalive interval, though in most cases the inactivity timeout is expected to be shorter than the keepalive interval. Only when the client has a very long-lived low-traffic state does the keepalive interval come into play, to ensure that a sufficient residual amount of traffic is generated to maintain NAT and firewall state. On a new DSO session, if no explicit DSO Keepalive message exchange has taken place, the default value for both timeouts is 15 seconds. For both timeouts, lower values of the timeout result in higher network traffic and higher CPU load on the server. 8.3. Inactive DSO Sessions At both servers and clients, the generation or reception of any complete DNS message, including DNS requests, responses, updates, or DSO messages, resets both timers for that DSO session, with the Bellis, et al. Expires March 17, 2018 [Page 20] Internet-Draft DNS Stateful Operations September 2017 exception that a DSO Keepalive message resets only the keepalive timer, not the inactivity timeout timer. In addition, for as long as the client has an outstanding operation in progress, the inactivity timer remains cleared, and an inactivity timeout cannot occur. For short-lived DNS operations like traditional queries and updates, an operation is considered in progress for the time between request and response, typically a period of a few hundred milliseconds at most. At the client, the inactivity timer is cleared upon transmission of a request and remains cleared until reception of the corresponding response. At the server, the inactivity timer is cleared upon reception of a request and remains cleared until transmission of the corresponding response. For long-lived DNS Stateful operations, an operation is considered in progress for as long as the state is active, until it is cancelled. This means that a DSO session can exist, with a state active, with no messages flowing in either direction, for far longer than the inactivity timeout, and this is not an error. This is why there are two separate timers: the inactivity timeout, and the keepalive interval. Just because a DSO session has no traffic for an extended period of time does not automatically make that DSO session "inactive", if it has an active state that is awaiting for events. 8.4. The Inactivity Timeout The purpose of the inactivity timeout is for the server to balance its trade off between the costs of setting up new DSO sessions and the costs of maintaining inactive DSO sessions. A server with abundant DSO session capacity can offer a high inactivity timeout, to permit clients to keep a speculative DSO session open for a long time, to save the cost of establishing a new DSO session for future communications with that server. A server with scarce memory resources can offer a low inactivity timeout, to cause clients to promptly close DSO sessions whenever they have no outstanding operations with that server, and then create a new DSO session later when needed. 8.4.1. Closing Inactive DSO Sessions A client is NOT required to wait until the inactivity timeout expires before closing a DSO session. A client MAY close a DSO session at any time, at the client's discretion. If a client determines that it has no current or reasonably anticipated future need for an inactive DSO session, then the client SHOULD close that connection. Bellis, et al. Expires March 17, 2018 [Page 21] Internet-Draft DNS Stateful Operations September 2017 If, at any time during the life of the DSO session, the inactivity timeout value (i.e., 15 seconds by default) elapses without there being any operation active on the DSO session, the client MUST gracefully close the connection with a TCP FIN (or equivalent for other protocols). If, at any time during the life of the DSO session, twice the inactivity timeout value (i.e., 30 seconds by default) elapses without there being any operation active on the DSO session, the server SHOULD consider the client delinquent, and forcibly abort the DSO session. For DSO sessions over TCP (or over TLS over TCP), to avoid the burden of having a connection in TIME-WAIT state, instead of closing the connection gracefully with a TCP FIN the server SHOULD abort the connection with a TCP RST (or equivalent for other protocols). (In the BSD Sockets API this is achieved by setting the SO_LINGER option to zero before closing the socket.) In this context, an operation being active on a DSO session includes a query waiting for a response, an update waiting for a response, or active state, but not a DSO Keepalive message exchange itself. A DSO Keepalive message exchange resets only the keepalive interval timer, not the inactivity timeout timer. If the client wishes to keep an inactive DSO session open for longer than the default duration without having to send traffic every 15 seconds, then it uses the DSO Keepalive message to request longer timeout values, as described in Section 5. 8.4.2. Values for the Inactivity Timeout For the inactivity timeout value, lower values result in more frequent DSO session teardown and re-establishment. Higher values result in lower traffic and CPU load on the server, but a larger memory burden to maintain state for inactive DSO sessions. A shorter inactivity timeout with a longer keepalive interval signals to the client that it should not speculatively keep inactive DSO sessions open for very long for no reason, but when it does have an active reason to keep a DSO session open, it doesn't need to be sending an aggressive level of keepalive traffic. A longer inactivity timeout with a shorter keepalive interval signals to the client that it may speculatively keep inactive DSO sessions open for a long time, but it should be sending a lot of keepalive traffic on those inactive DSO sessions. This configuration is expected to be less common. Bellis, et al. Expires March 17, 2018 [Page 22] Internet-Draft DNS Stateful Operations September 2017 To avoid excessive traffic the server MUST NOT send a Keepalive message (either a response to a client-initiated request, or a server-initiated message) with an inactivity timeout value less than ten seconds. If a client receives a Keepalive message specifying an inactivity timeout value less than ten seconds this is an error and the client MUST immediately terminate the connection with a TCP RST (or equivalent for other protocols). 8.5. The Keepalive Interval The purpose of the keepalive interval is to manage the generation of sufficient messages to maintain state in middle-boxes (such at NAT gateways or firewalls) and for the client and server to periodically verify that they still have connectivity to each other. This allows them to clean up state when connectivity is lost, and attempt re- connection if appropriate. 8.5.1. Keepalive Interval Expiry If, at any time during the life of the DSO session, the keepalive interval value (i.e., 15 seconds by default) elapses without any DNS messages being sent or received on a DSO session, the client MUST take action to keep the DSO session alive. To keep the DSO session alive the client MUST send a DSO Keepalive message (see Section 5). A DSO Keepalive message exchange resets only the keepalive timer, not the inactivity timer. If a client disconnects from the network abruptly, without cleanly closing its DSO session, leaving long-lived state uncanceled, the server learns of this after failing to receive the required keepalive traffic from that client. If, at any time during the life of the DSO session, twice the keepalive interval value (i.e., 30 seconds by default) elapses without any DNS messages being sent or received on a DSO session, the server SHOULD consider the client delinquent, and forcibly abort the connection with a TCP RST (or equivalent for other protocols). 8.5.2. Values for the Keepalive Interval For the keepalive interval value, lower values result in higher volume keepalive traffic. Higher values of the keepalive interval reduce traffic and CPU load, but have minimal effect on the memory burden at the server, because clients keep a DSO session open for the same length of time (determined by the inactivity timeout) regardless of the level of keepalive traffic required. Bellis, et al. Expires March 17, 2018 [Page 23] Internet-Draft DNS Stateful Operations September 2017 It may be appropriate for clients and servers to select different keepalive interval values depending on the nature of the network they are on. A corporate DNS server that knows it is serving only clients on the internal network, with no intervening NAT gateways or firewalls, can impose a higher keepalive interval, because frequent keepalive traffic is not required. A public DNS server that is serving primarily residential consumer clients, where it is likely there will be a NAT gateway on the path, may impose a lower keepalive interval, to generate more frequent keepalive traffic. A smart client may be adaptive to its environment. A client using a private IPv4 address [RFC1918] to communicate with a DNS server at an address that is not in the same IPv4 private address block, may conclude that there is likely to be a NAT gateway on the path, and accordingly request a lower keepalive interval. For environments where there is a NAT gateway or firewalls on the path, it is RECOMMENDED that clients request, and servers grant, a keepalive interval of 15 minutes. In other environments it is RECOMMENDED that clients request, and servers grant, a keepalive interval of 60 minutes. Note that the lower the keepalive interval value, the higher the load on client and server. For example, an keepalive interval value of 100ms would result in a continuous stream of at least ten messages per second, in both directions, to keep the DSO session alive. And, in this extreme example, a single packet loss and retransmission over a long path could introduce a momentary pause in the stream of messages, long enough to cause the server to overzealously abort the connection. Because of this concern, the server MUST NOT send a Keepalive message (either a response to a client-initiated request, or a server- initiated message) with an keepalive interval value less than ten seconds. If a client receives an Keepalive message specifying an keepalive interval value less than ten seconds this is an error and the client MUST immediately terminate the connection with a TCP RST (or equivalent for other protocols). 8.6. Server-Initiated Termination on Error After sending an error response to a client, the server MAY close the DSO session, or may allow the DSO session to remain open. For error conditions that only affect the single operation in question, the Bellis, et al. Expires March 17, 2018 [Page 24] Internet-Draft DNS Stateful Operations September 2017 server SHOULD return an error response to the client and leave the DSO session open for further operations. For error conditions that are likely to make all operations unsuccessful in the immediate future, the server SHOULD return an error response to the client and then close the DSO session by sending a Retry Delay request message, as described in Section 6. 8.7. Client Behaviour in Receiving an Error Upon receiving an error response from the server, a client SHOULD NOT automatically close the DSO session. An error relating to one particular operation on a DSO session does not necessarily imply that all other operations on that DSO session have also failed, or that future operations will fail. The client should assume that the server will make its own decision about whether or not to close the DSO session, based on the server's determination of whether the error condition pertains to this particular operation, or would also apply to any subsequent operations. If the server does not close the DSO session then the client SHOULD continue to use that DSO session for subsequent operations. 8.8. Server-Initiated Termination on Overload Apart from the cases where: o Session Timeouts expire (see Section 8.2) o On error (see {Section Section 8.7) o When under load (see below) a server MUST NOT close a DSO session with a client, except in extraordinary error conditions. Closing the DSO session is the client's responsibility, to be done at the client's discretion, when it so chooses. A server only closes a DSO session under exceptional circumstances, such as when the server application software or underlying operating system is restarting, the server application terminated unexpectedly (perhaps due to a bug that makes it crash), or the server is undergoing maintenance procedures. When possible, a server SHOULD send a Retry Delay message informing the client of the reason for the DSO session being closed, and allow the client five seconds to receive it before the server resorts to forcibly aborting the connection. Bellis, et al. Expires March 17, 2018 [Page 25] Internet-Draft DNS Stateful Operations September 2017 8.9. Retry Delay Operation TLV There may be rare cases where a server is overloaded and wishes to shed load. If a server is low on resources it MAY simply terminate a client connection with a TCP RST (or equivalent for other protocols). However, the likely behavior of the client may be simply to to treat this as a network failure and connect immediately, putting more burden on the server. Therefore to avoid this reconnection implosion, a server SHOULD instead choose to shed client load by sending a Retry Delay request message, with an RCODE of SERVFAIL, to inform the client of the overload situation. After sending a Retry Delay request message, the server MUST NOT send any further messages on that DSO session. After sending the Retry Delay request the server SHOULD allow the client five seconds to close the connection, and if the client has not closed the connection after five seconds then the server SHOULD abort the connection with a TCP RST (or equivalent for other protocols). Upon receipt of a Retry Delay request from the server, the client MUST make note of the reconnect delay for this server, and then immediately close the connection. This is to place the burden of TCP's TIME-WAIT state on the client. A Retry Delay request message MUST NOT be initiated by a client. If a server receives a Retry Delay request message this is an error and the server MUST immediately terminate the connection with a TCP RST (or equivalent for other protocols). 8.9.1. Outstanding Operations At the moment a server chooses to initiate a Retry Delay request message there may be DNS requests already in flight from client to server on this DSO session, which will arrive at the server after its Retry Delay request message has been sent. The server MUST silently ignore such incoming requests, and MUST NOT generate any response messages for them. When the Retry Delay request message from the server arrives at the client, the client will determine that any DNS requests it previously sent on this DSO session, that have not yet received a response, now will certainly not be receiving any response. Such requests should be considered failed, and should be retried at a later time, as appropriate. In the case where some, but not all, of the existing operations on a DSO session have become invalid (perhaps because the server has been reconfigured and is no longer authoritative for some of the names), Bellis, et al. Expires March 17, 2018 [Page 26] Internet-Draft DNS Stateful Operations September 2017 but the server is terminating all DSO sessions en masse with a REFUSED (5) RCODE, the RECONNECT DELAY MAY be zero, indicating that the clients SHOULD immediately attempt to re-establish operations. It is likely that some of the attempts will be successful and some will not. In the case where a server is terminating a large number of DSO sessions at once (e.g., if the system is restarting) and the server doesn't want to be inundated with a flood of simultaneous retries, it SHOULD send different RECONNECT delay values to each client. These adjustments MAY be selected randomly, pseudorandomly, or deterministically (e.g., incrementing the time value by one tenth of a second for each successive client, yielding a post-restart reconnection rate of ten clients per second). 8.9.2. Client Reconnection After a DSO session is closed by the server, the client SHOULD try to reconnect, to that server, or to another suitable server, if more than one is available. If reconnecting to the same server, the client MUST respect the indicated delay before attempting to reconnect. If a particular server does not want a client to reconnect (it is being de-commissioned), it SHOULD set the retry delay to the maximum value (which is approximately 497 days). If the server will only be out of service for a maintenance period, it should use a value closer to the expected maintenance window and not default to a very large delay value or clients may not attempt to reconnect after it resumes service. 9. Connection Sharing As in [RFC7766], 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). However, it is noted that certain primary/ secondary configurations with many busy zones might need to use more than one TCP connection for zone transfers for operational reasons (for example, to support concurrent transfers of multiple zones). A single server may support multiple services, including DNS Updates [RFC2136], DNS Push Notifications [I-D.ietf-dnssd-push], and other services, for one or more DNS zones. When a client discovers that Bellis, et al. Expires March 17, 2018 [Page 27] Internet-Draft DNS Stateful Operations September 2017 the target server for several different operations is the same target hostname and port, the client SHOULD use a single shared DSO session for all those operations. A client SHOULD NOT open multiple connections to the same target host and port just because the names being operated on are different or happen to fall within different zones. This is to reduce unnecessary connection load on the DNS server. However, server implementers and operators should be aware that connection sharing may not be possible in all cases. A single client device may be home to multiple independent client software instances that don't coordinate with each other. Similarly, multiple independent client devices behind the same NAT gateway will also typically appear to the DNS server as different source ports on the same client IP address. Because of these constraints, a DNS server MUST be prepared to accept multiple connections from different source ports on the same client IP address. 10. IANA Considerations 10.1. DSO OPCODE Registration IANA are directed to assign a value (tentatively 6) in the DNS OPCODEs Registry for the DSO OPCODE. 10.2. DSO RCODE Registration IANA are directed to assign a value (tentatively 11) in the DNS RCODE Registry for the DSONOTIMP error code. 10.3. DSO Type Codes Registry IANA are directed to create the DSO Type Codes Registry, with initial values as follows: Bellis, et al. Expires March 17, 2018 [Page 28] Internet-Draft DNS Stateful Operations September 2017 +-----------+--------------------------------+----------+-----------+ | Type | Name | Status | Reference | +-----------+--------------------------------+----------+-----------+ | 0x0000 | RetryDelay | Standard | RFC-TBD | | | | | | | 0x0001 | KeepAlive | Standard | RFC-TBD | | | | | | | 0x0002 | Encryption Padding | Standard | RFC-TBD | | | | | | | 0x0003 - | Unassigned, reserved for DSO | | | | 0x003F | session management TLVs | | | | | | | | | 0x0040 - | Unassigned | | | | 0xF7FF | | | | | | | | | | 0xF800 - | Reserved for local / | | | | 0xFBFF | experimental use | | | | | | | | | 0xFC00 - | Reserved for future expansion | | | | 0xFFFF | | | | +-----------+--------------------------------+----------+-----------+ Registration of additional DSO Type Codes requires publication of an appropriate IETF "Standards Action" or "IESG Approval" document [RFC5226]. 11. Security Considerations If this mechanism is to be used with DNS over TLS, then these messages are subject to the same constraints as any other DNS over TLS messages and MUST NOT be sent in the clear before the TLS session is established. The data field of the "Encryption Padding" TLV could be used as a covert channel. 12. Acknowledgements Thanks to Tim Chown, Ralph Droms, Jan Komissar, and Manju Shankar Rao for their helpful contributions to this document. 13. References 13.1. Normative References [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, . Bellis, et al. Expires March 17, 2018 [Page 29] Internet-Draft DNS Stateful Operations September 2017 [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, . [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, DOI 10.17487/RFC2136, April 1997, . [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, DOI 10.17487/RFC5226, May 2008, . [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013, . [RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and D. Wessels, "DNS Transport over TCP - Implementation Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016, . [RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The edns-tcp-keepalive EDNS0 Option", RFC 7828, DOI 10.17487/RFC7828, April 2016, . [RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830, DOI 10.17487/RFC7830, May 2016, . Bellis, et al. Expires March 17, 2018 [Page 30] Internet-Draft DNS Stateful Operations September 2017 13.2. Informative References [I-D.ietf-dnssd-push] Pusateri, T. and S. Cheshire, "DNS Push Notifications", draft-ietf-dnssd-push-12 (work in progress), July 2017. [I-D.ietf-dprive-padding-policy] Mayrhofer, A., "Padding Policy for EDNS(0)", draft-ietf- dprive-padding-policy-01 (work in progress), July 2017. [NagleDA] Cheshire, S., "TCP Performance problems caused by interaction between Nagle's Algorithm and Delayed ACK", May 2005, . [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, . [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., and P. Hoffman, "Specification for DNS over Transport Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 2016, . Authors' Addresses Ray Bellis Internet Systems Consortium, Inc. 950 Charter Street Redwood City CA 94063 USA Phone: +1 650 423 1200 Email: ray@isc.org Stuart Cheshire Apple Inc. 1 Infinite Loop Cupertino CA 95014 USA Phone: +1 408 974 3207 Email: cheshire@apple.com Bellis, et al. Expires March 17, 2018 [Page 31] Internet-Draft DNS Stateful Operations September 2017 John Dickinson Sinodun Internet Technologies Magadalen Centre Oxford Science Park Oxford OX4 4GA United Kingdom Email: jad@sinodun.com Sara Dickinson Sinodun Internet Technologies Magadalen Centre Oxford Science Park Oxford OX4 4GA United Kingdom Email: sara@sinodun.com Allison Mankin Salesforce Email: allison.mankin@gmail.com Tom Pusateri Unaffiliated Phone: +1 919 867 1330 Email: pusateri@bangj.com Bellis, et al. 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