| 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: July 30, 2018 | S. Dickinson |
| Sinodun | |
| A. Mankin | |
| Salesforce | |
| T. Pusateri | |
| Unaffiliated | |
| January 26, 2018 |
DNS Stateful Operations
draft-ietf-dnsop-session-signal-05
This document defines a new DNS OPCODE for DNS Stateful Operations (DSO). DSO messages communicate operations within persistent stateful sessions, using type-length-value (TLV) syntax. Three TLVs are defined that manage session timeouts, termination, and encryption padding, and a framework is defined for extensions to enable new stateful operations.
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This Internet-Draft will expire on July 30, 2018.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 OPCODE, DSO (tentatively 6), for DNS Stateful Operations. DSO messages are used to communicate operations within persistent stateful sessions, expressed using type-length-value (TLV) syntax. This document defines an initial set of three TLVs, used to manage session timeouts, termination, and encryption padding.
All three of the TLVs defined here are mandatory for all implementations of DSO. Further TLVs may be defined in additional specifications.
The format for DSO messages (see Section 4.2) differs somewhat from the traditional DNS message 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 accordingly their corresponding sections are not present. The actual data pertaining to DNS Stateful Operations (expressed in TLV syntax) is appended to the end of the DNS message header. When displayed using packet analyzer tools that have not been updated to recognize the DSO format, this will result in the DSO 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 DSO data.
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 its use case, and as a result contains no redundant or overloaded fields. Importantly, it completely avoids conflating DNS Stateful Operations in any way 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 middlebox behaviour.
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.
In contrast to EDNS(0), with DSO there is no compelling motivation to pack multiple operations into a single message for efficiency reasons, because DSO always operates using a connection-oriented transport protocol. 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. This simplification allows for clearer semantics. Each DSO request message communicates just one primary operation, and the RCODE in the corresponding response message indicates the success or failure of that operation.
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:
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 a DNS-over-TCP session as described in the previous specification for DNS over TCP [RFC7766].
A “DSO Session” is terminated when the underlying connection is closed. The underlying connection can be closed in two ways.
Where this specification says, “close gracefully,” that means sending a TLS close_notify followed by a TCP FIN, or the equivalents for other protocols. Where this specification requires a connection to be closed gracefully, the requirement to initiate that graceful close is placed on the client, to place the burden of TCP’s TIME-WAIT state on the client rather than the server.
Where this specification says, “forcibly abort,” that means sending a TCP RST, or the equivalent for other protocols. In the BSD Sockets API this is achieved by setting the SO_LINGER option to zero before closing the socket.
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 DSO request message) or a responder (when sending a DSO response message).
Likewise, the term “receiver” may apply to either a responder (when receiving a DSO request message) or an initiator (when receiving a DSO response message).
The term “long-lived operations” refers to operations such as Push Notification subscriptions [I-D.ietf-dnssd-push], Discovery Relay interface subscriptions [I-D.sctl-dnssd-mdns-relay], and other future long-lived DNS operations that choose to use DSO as their basis, that establish state that persists beyond the lifetime of a traditional brief request/response transaction. This document, the base specification for DNS Stateful Operations, defines a framework for supporting long-lived operations, but does not itself define any long-lived operations. Nonetheless, to appreciate the design rationale behind DNS Stateful Operations, it is helpful to understand the long-lived operations that it is intended to support.
DNS Stateful Operations uses “DSO request messages” and “DSO response messages”. DSO request messages are further subdivided into two variants, “acknowledged request messages” (which generate a corresponding response message) and “unacknowledged request messages” (which do not generate any corresponding response message).
The content of DSO messages is expressed using type-length-value (TLV) syntax.
In a DSO request message the first TLV is referred to as the “Primary TLV” and determines the nature of the operation being performed, including whether it is an acknowledged or unacknowledged operation; any other TLVs in a DSO request message are referred to as “Additional TLVs” and serve additional non-primary purposes, which may be related to the primary purpose, or not, as in the case of the encryption padding TLV.
A DSO response message may contain no TLVs, or it may contain one or more TLVs as appropriate to the information being communicated. In the context of DSO response messages, one or more TLVs with the same DSO-TYPE as the Primary TLV in the corresponding DSO request message are referred to as “Response Primary TLVs”. Any other TLVs with different DSO-TYPEs are referred to as “Response Additional TLVs”. The Response Primary TLV(s), if present, MUST occur first in the response message, before any Response Additional TLVs.
Two timers (elapsed time since an event) are defined in this document:
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.
Resetting 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 timer again.
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 [RFC6763] 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 [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 maintain state in middleboxes, and to assure client and server that they still have connectivity to each other.
There are a myriad of other potential use cases for DSO given the versatility and extensibility of this specification.
Section 4 of this document describes the protocol details of DNS Stateful Operations including definitions of three TLVs for session management and encryption padding. Section 5 presents a detailed discussion of the DSO Session lifecycle including an in-depth discussion of keepalive traffic and session termination.
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.
In some environments it may be known in advance by external means that both client and server support DSO, and in these cases either client or server may initiate DSO messages at any time.
However, in the typical case a server will not know in advance whether a client supports DSO, so in general, unless it is known in advance by other means that a client does support DSO, a server MUST NOT initiate DSO request messages until a DSO Session has been mutually established, as described below. Similarly, unless it is known in advance by other means that a server does support DSO, a client MUST NOT initiate non-response-requiring DSO request messages until after a DSO Session has been mutually established.
Whether or not a given DSO request message elicits a response is determined by whether or not the first DSO TLV (see Section 4.2.2.1) in the message (the Primary TLV) is one that is specified to generate a response. Whether a Primary TLV will be specified to elicit a response will depend on the intended use pattern for that particular TLV.
A DSO Session is established over a connection by the client sending a DSO request message of a kind that requires a response, such as the DSO Keepalive TLV (see Section 6.1), and receiving a response, with matching MESSAGE ID, and RCODE set to NOERROR (0), indicating that the DSO request was successful.
If the RCODE is set to DSONOTIMP (tentatively 11) this indicates that the server does support DSO, but does not support the particular operation the client requested. A server MUST NOT return DSONOTIMP for the DSO Keepalive TLV, but a DSONOTIMP response could happen in the future, if a client attempts to establish a DSO Session using a future response-requiring DSO TLV that the server does not understand. If the server returns DSONOTIMP then a DSO Session is not considered established, but the client is permitted to continue sending DNS messages on the connection, including other response-requiring DSO messages such as the DSO Keepalive, which may result in a successful NOERROR response, yielding the establishment of a DSO Session.
If the RCODE is set to any value other than NOERROR (0) or DSONOTIMP (tentatively 11), then the client should assume that the server does not support DSO. In this case the client is permitted to continue sending DNS messages on that connection, but the client SHOULD NOT issue further DSO messages on that connection.
When the server receives a response-requiring DSO request message from a client, and transmits a successful NOERROR response to that request, the server considers the DSO Session established.
When the client receives the server’s NOERROR response to its DSO request message, the client considers the DSO Session established.
Once a DSO Session has been established, either end may unilaterally send DSO messages at any time, and therefore either client or server may be the initiator of a message.
Once a DSO Session has been established, clients and servers should behave as described in this specification with regard to inactivity timeouts and session termination, not as previously prescribed in the earlier specification for DNS over TCP [RFC7766].
As previously specified for DNS over TCP [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 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 host 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.
There is increased awareness today of the performance benefits of eliminating round trips in session establishment. Technologies like TCP Fast Open [RFC7413] and TLS 1.3 [I-D.ietf-tls-tls13] provide mechanisms to reduce or eliminate round trips in session establishment.
Similarly, DSO supports zero round-trip operation.
Having initiated a connection to a server, possibly using zero round-trip TCP Fast Open and/or zero round-trip TLS 1.3, a client MAY send multiple response-requiring DSO request messages to the server in succession without having to wait for a response to the first request message to confirm successful establishment of a DSO session.
However, a client MUST NOT send non-response-requiring DSO request messages until after a DSO Session has been mutually established.
Similarly, a server MUST NOT send DSO request messages until it has received a response-requiring DSO request message from a client and transmitted a successful NOERROR response for that request.
Where an application-layer middlebox (e.g., a DNS proxy, forwarder, or session multiplexer) is in the path the middlebox 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 middlebox, 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 middlebox 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 middlebox 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 middlebox 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.
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, unless a future IETF Standard specifies otherwise.
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 /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
In an unacknowledged request message the MESSAGE ID field MUST be set to zero. In an acknowledged request message the MESSAGE ID field MUST be set to a unique nonzero 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 is still awaiting a response, or if the client has used it to setup a long-lived operation that has not yet been cancelled. For example, a long-lived operation could be a Push Notification subscription [I-D.ietf-dnssd-push] or a Discovery Relay interface subscription [I-D.sctl-dnssd-mdns-relay].
Whether a message is acknowledged or unacknowledged is determined only by the specification for the Primary TLV. An acknowledgment cannot be requested by including a nonzero message ID in a message the primary TLV of which is specified to be unacknowledged, nor can an acknowledgment be prevented by sending a message ID of zero in a message with a primary TLV that is specified to be acknowledged. A responder that receives either such malformed message MUST treat it as a programming error and terminate the connection.
In a request message the DNS Header QR bit MUST be zero (QR=0).
If the QR bit is not zero the message is not a request message.
In a response message the DNS Header QR bit MUST be one (QR=1).
If the QR bit is not one the message is not a response message.
In a response message (QR=1) the MESSAGE ID field MUST contain a copy of the value of the MESSAGE ID field in the acknowledged request message being responded to. In a response message (QR=1) the MESSAGE ID field MUST NOT be zero. If a response message (QR=1) is received where the MESSAGE ID is zero this is a fatal error and the receiver MUST forcibly abort the connection immediately.
The DNS Header OPCODE field holds the DSO OPCODE value (tentatively 6).
The Z bits are currently unused in DSO messages, and in both DSO requests and DSO responses the Z bits MUST be set to zero (0) on transmission and MUST be silently ignored on reception, unless a future IETF Standard specifies otherwise.
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 request message (see Section 5.6.3), where the RCODE indicates the reason for termination).
The RCODE value in a response message (QR=1) 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 | Name Error — Named entity does not exist (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 RCODEs 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 defining a new DSO TLV makes use of NXDOMAIN (Name Error) or NOTAUTH (Not Authoritative) then that document MUST specify the specific interpretation of these RCODE values in the context of that new DSO TLV.
The standard twelve-octet DNS message header with its zero-valued count fields is followed by the DSO Data, expressed using TLV syntax, as described below Section 4.2.2.1.
A DSO message may be either a request message or a response message, as indicated by the QR bit in the DNS message header. DSO request messages are further subdivided into two variants, acknowledged request messages (which generate a corresponding response message) and unacknowledged request messages (which do not generate any corresponding response message).
A DSO request message MUST contain at least one TLV. The first TLV in a DSO request message is referred to as the “Primary TLV” and determines the nature of the operation being performed, including whether it is an acknowledged or unacknowledged operation. In some cases it may be appropriate to include other TLVs in a request message, such as the Encryption Padding TLV (Section 6.3), and these extra TLVs are referred to as the “Additional TLVs”.
A DSO response message may contain no TLVs, or it may be specified to contain one or more TLVs appropriate to the information being communicated.
A DSO response message may contain one or more TLVs with DSO-TYPE the same as the Primary TLV from the corresponding DSO request message, in which case those TLV(s) are referred to as “Response Primary TLVs”. A DSO response message is not required to carry Response Primary TLVs. The MESSAGE ID field in the DNS message header is sufficient to identify to which DSO request message this response message relates.
A DSO response message may contain one or more TLVs with DSO-TYPEs different from the Primary TLV from the corresponding DSO request message, in which case those TLV(s) are referred to as “Response Additional TLVs”.
Response Primary TLV(s), if present, MUST occur first in the response message, before any Response Additional TLVs.
It is anticipated that by default most DSO request messages will be specified to be acknowledged request messages, which generate corresponding responses. In some specialized high-traffic use cases, it may be appropriate to specify unacknowledged request messages. Unacknowledged request messages can be more efficient on the network, because they don’t generate a stream of corresponding reply messages. Using unacknowledged request messages can also simplify software in some cases, by removing need for an initiator to maintain state while it waits to receive replies it doesn’t care about. When the specification for a particular TLV states that, when used as a Primary TLV (i.e., first) in a request message, that request message is to be unacknowledged, the MESSAGE ID field MUST be set to zero and the receiver MUST NOT generate any response message corresponding to this unacknowledged request message.
The previous point, that the receiver MUST NOT generate responses to unacknowledged request messages, applies even in the case of errors. When a DSO request message is received with the MESSAGE ID field set to zero, the receiver MUST NOT generate any response. For example, if the DSO-TYPE in the Primary TLV is unrecognized, then a DSONOTIMP error MUST NOT be returned; instead the receiver MUST forcibly abort the connection immediately.
Unacknowledged request messages MUST NOT be used “speculatively” in cases where the sender doesn’t know if the receiver supports the Primary TLV in the message, because there is no way to receive any response to indicate success or failure of the request message (the request message does not contain a unique MESSAGE ID with which to associate a response with its corresponding request). Unacknowledged request messages are only appropriate in cases where the sender already knows that the receiver supports and wishes to receive these messages.
For example, after a client has subscribed for Push Notifications [I-D.ietf-dnssd-push], the subsequent event notifications are then sent as unacknowledged messages, and this is appropriate because the client initiated the message stream by virtue of its Push Notification subscription, thereby indicating its support of Push Notifications, and its desire to receive those notifications.
Similarly, after an mDNS Relay client has subscribed to receive inbound mDNS traffic from an mDNS Relay, the subsequent stream of received packets is then sent using unacknowledged messages, and this is appropriate because the client initiated the message stream by virtue of its mDNS Relay link subscription, thereby indicating its support of mDNS Relay, and its desire to receive inbound mDNS packets over that DSO session [I-D.sctl-dnssd-mdns-relay].
All TLVs, whether used as “Primary”, “Additional”, “Response Primary”, or “Response Additional”, use the same encoding syntax.
The specification for a TLV determines whether, when used as the Primary (i.e., first) TLV in a request message, that request message is to be acknowledged. If the request message is to be acknowledged, the specification also states which TLVs, if any, are to be included in the response. The Primary TLV may or may not be contained in the response, depending on what is stated in the specification for that TLV.
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| DSO-TYPE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| DSO DATA LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| |
/ TYPE-DEPENDENT DATA /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Where domain names appear within TYPE-DEPENDENT DATA, they MAY
be compressed using standard DNS name compression [RFC1035].
However, the compression offsets MUST be relative to the start of the TYPE-DEPENDENT DATA and MUST NOT extend beyond the end of the TYPE-DEPENDENT DATA.
The first TLV in a DSO request message is the “Primary TLV” and indicates the operation to be performed. A DSO request message MUST contain at at least one TLV, the Primary TLV.
Immediately following the Primary TLV, a DSO request message MAY contain one or more “Additional TLVs”, which specify additional parameters relating to the operation.
Depending on the operation, a DSO response message MAY contain no TLVs, 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 response TLV, with the same DSO-TYPE as the Primary TLV in the request message. Alternatively it may contain one or more TLVs of other types, or a combination of the above, as appropriate for the information that needs to be communicated. The specification for each DSO TLV determines what TLVs are required in a response to a request using that TLV.
If a DSO response is received for an operation where the specification requires that the response carry a particular TLV or TLVs, and the required TLV(s) are not present, then this is a fatal error and the recipient of the defective response message MUST forcibly abort the connection immediately.
If DSO request is received containing an unrecognized Primary TLV, with a nonzero MESSAGE ID (indicating that a response is expected), then the receiver MUST send a response with matching MESSAGE ID, and RCODE DSONOTIMP (tentatively 11). The response MUST NOT contain a copy of the unrecognized Primary TLV.
If DSO request is received containing an unrecognized Primary TLV, with a zero MESSAGE ID (indicating that no response is expected), the receiver MUST silently ignore the message. A response MUST NOT be sent.
If a DSO request message is received where the Primary TLV is recognized, containing one or more unrecognized Additional TLVs, the unrecognized Additional TLVs MUST be silently ignored, and the remainder of the message is interpreted and handled as if the unrecognized parts were not present.
Similarly, if a DSO response message is received containing one or more unrecognized TLVs, the unrecognized TLVs MUST be silently ignored, and the remainder of the message is interpreted and handled as if the unrecognized parts were not present.
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 desired for DSO messages, one or more new DSO TLVs need 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 6.3 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, a new DSO 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 is obsoleted by the functionality provided by the DSO Keepalive operation), but messages may contain other EDNS(0) options as appropriate.
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) with a nonzero MESSAGE ID field MUST elicit a corresponding 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 with a nonzero MESSAGE ID field elicit a response from the server, and DSO request messages sent by the server with a nonzero MESSAGE ID field elicit a response from the client.
A DSO request message (QR=0) with a zero MESSAGE ID field MUST NOT elicit a response.
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 using that MESSAGE ID on that DSO Session, unless specified otherwise by the relevant specification for the DSO in question. (For a long-lived operation the MESSAGE ID for the operation MUST NOT be reused whilst that operation remains active.)
If a client or server receives a response (QR=1) where the MESSAGE ID is zero, or any other value that does not match the MESSAGE ID of any of its outstanding operations, this is a fatal error and the recipient MUST forcibly abort the connection immediately.
With most TCP implementations, for DSO requests that generate a response, 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 application-layer software itself) are all combined into a single IP packet. Combining these three elements into a single IP packet gives a potentially significant improvement in network efficiency.
For DSO requests that do not generate a response, the TCP implementation generally doesn’t have any way to know that no response will be forthcoming, so it waits fruitlessly for the application-layer software to generate a response, until the Delayed ACK timer fires [RFC1122] (typically 200 milliseconds) and only then does it send the TCP ack and window update. In conjunction with Nagle’s Algorithm at the sender, this can delay the sender’s transmission of its next (non-full-sized) TCP segment, while the sender is waiting for its previous (non-full-sized) TCP segment to be acknowledged, which won’t happen until the Delayed ACK timer fires. Nagle’s Algorithm exists to combine multiple small application writes into more efficient large TCP segments, to guard against wasteful use of the network by applications that would otherwise transmit a stream of small TCP segments, but in this case Nagle’s Algorithm (created to improve network efficiency) can interact badly with TCP’s Delayed ACK feature (also created to improve network efficiency) [NagleDA] with the result of delaying some messages by up to 200 milliseconds.
Possible mitigations for this problem include:
This document, the base specification for DNS Stateful Operations, does not itself define any long-lived operations, but it defines a framework for supporting long-lived operations such as Push Notification subscriptions [I-D.ietf-dnssd-push] and Discovery Relay interface subscriptions [I-D.sctl-dnssd-mdns-relay].
Generally speaking, a long-lived operation is initiated by the initiator, and, if successful, remains active until the initiator terminates the operation.
However, it is possible that a long-lived operation may be valid at the time it was initiated, but then a later change of circumstances may render that previously valid operation invalid.
For example, a long-lived client operation may pertain to a name that the server is authoritative for, but then the server configuration is changed such that it is no longer authoritative for that name.
In such cases, instead of terminating the entire session it may be desirable for the responder to be able to cancel selectively only those operations that have become invalid.
The responder performs this selective cancellation by sending a new response message, with the MESSAGE ID field containing the MESSAGE ID of the long-lived operation that is to be terminated (that it had previously acknowledged with a NOERROR RCODE), and the RCODE field of the new response message giving the reason for cancellation.
After a response message with nonzero RCODE has been sent, that operation has been terminated from the responder’s point of view, and the responder sends no more messages relating to that operation.
After a response message with nonzero RCODE has been received by the initiator, that operation has been terminated from the initiator’s point of view, and its MESSAGE ID is now free for reuse.
A DSO Session begins 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 SHOULD be sent out of order when appropriate.
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 operation does the keepalive interval come into play, to ensure that a sufficient residual amount of traffic is generated to maintain NAT and firewall state and to assure client and server that they still have connectivity to each other.
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.
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 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 (such as a Push Notification subscription [I-D.ietf-dnssd-push] or a Discovery Relay interface subscription [I-D.sctl-dnssd-mdns-relay]), an operation is considered in progress for as long as the operation is active, until it is cancelled. This means that a DSO Session can exist, with active operations, 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 operation that is awaiting events.
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.
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.
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 close the connection gracefully.
If, at any time during the life of the DSO Session, twice the inactivity timeout value (i.e., 30 seconds by default), or five seconds, if twice the inactivity timeout value is less than five seconds, elapses without there being any operation active on the DSO Session, the server SHOULD consider the client delinquent, and SHOULD forcibly abort the DSO Session.
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 an active long-lived operation, 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 6.1.
For the inactivity timeout value, lower values result in more frequent DSO Session teardown and re-establishment. Higher values result in lower traffic and lower CPU load on the server, but higher memory burden to maintain state for inactive DSO Sessions.
A server may dictate (in a server-initiated Keepalive message, or in a response to a client-initiated Keepalive request message) any value it chooses for the inactivity timeout. When a connection’s inactivity timeout is reached the client MUST begin closing the idle connection, but a client is NOT REQUIRED to keep an idle connection open until the inactivity timeout is reached — a client SHOULD begin closing the connection sooner if it has no reason to expect future operations with that server before the inactivity timeout is reached.
A shorter inactivity timeout with a longer keepalive interval signals to the client that it should not speculatively keep an inactive DSO Session open for very long without 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 to maintain that session.
A longer inactivity timeout with a shorter keepalive interval signals to the client that it may speculatively keep an inactive DSO Session open for a long time, but to maintain that inactive DSO Session it should be sending a lot of keepalive traffic. This configuration is expected to be less common.
A server may dictate any value it chooses for the inactivity timeout (either in a response to a client-initiated request, or in a server-initiated message) including values under one second, or even zero.
An inactivity timeout of zero informs the client that it should not speculatively maintain idle connections at all, and as soon as the client has completed the operation or operations relating to this server, the client should immediately begin closing this session.
An inactivity timeout of 0xFFFFFFFF (2^32-1 milliseconds, approximately 49.7 days) informs the client that it may keep an idle connection open as long as it wishes. Note that after granting an unlimited inactivity timeout in this way, at any point the server may revise that inactivity timeout by sending a new Keepalive TLV dictating new Session Timeout values to the client.
A server will abort an idle client session after twice the inactivity timeout value, or five seconds, whichever is greater. In the case of a zero inactivity timeout value, this means that if a client fails to close an idle client session then the server will forcibly abort the idle session after five seconds.
The purpose of the keepalive interval is to manage the generation of sufficient messages to maintain state in middleboxes (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.
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, by sending a DSO Keepalive message (see Section 6.1). 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 a long-lived operation 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 SHOULD forcibly abort the DSO Session.
For the keepalive interval value, lower values result in a higher volume of 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.
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 outside that 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.
By default it is RECOMMENDED that clients request, and servers grant, a keepalive interval of 60 minutes. This keepalive interval provides for reasonably timely detection if a client abruptly disconnects without cleanly closing the session, and is sufficient to maintain state in firewalls and NAT gateways that follow the IETF recommended Best Current Practice that the “established connection idle-timeout” used by middleboxes be at least 2 hours 4 minutes [RFC5382].
Note that the lower the keepalive interval value, the higher the load on client and server. For example, a hypothetical 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 a keepalive interval value less than ten seconds. If a client receives a Keepalive message specifying a keepalive interval value less than ten seconds this is an error and the client MUST forcibly abort the connection immediately.
A keepalive interval value of 0xFFFFFFFF (2^32-1 milliseconds, approximately 49.7 days) informs the client that it should generate no keepalive traffic. Note that after signaling that the client should generate no keepalive traffic in this way, at any point the server may revise that keepalive traffic requirement by sending a new Keepalive TLV dictating new Session Timeout values to the client.
In addition to cancelling individual operations selectively (see Section 4.5) there are also occasions where a server may need to terminate one or more entire sessions wholesale. An entire session may need to be terminated if the client is defective in some way, or departs from the network without closing its session. Sessions may also need to be terminated if the server becomes overloaded, or if the server is reconfigured and lacks the ability to be selective about which operations need to be cancelled.
This section discusses various reasons a session may be terminated, and the mechanisms for doing so.
After sending an error response to a client, the server MAY end the DSO Session, or may allow the DSO Session to remain open. For error conditions that only affect the single operation in question, the 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 end the DSO Session by sending a Retry Delay request message, as described in Section 5.6.3.
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 end 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 end the DSO Session by sending the client a Retry Delay message (see Section 5.6.3) then the client SHOULD continue to use that DSO Session for subsequent operations.
A server MUST NOT close a DSO Session with a client, except in certain exceptional circumstances, as outlined below. In normal operation, closing a DSO Session is the client’s responsibility. The client makes the determination of when to close a DSO Session based on an evaluation of both its own needs, and the inactivity timeout value dictated by the server.
Some exceptional situations where a server may terminate a DSO Session include:
When a server has to close a DSO Session with a client (because of exceptional circumstances such as those outlined above) the server SHOULD, whenever possible, send a Retry Delay request message (see below) 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.
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 by forcibly aborting it. However, the likely behavior of the client may be simply to to treat this as a network failure and reconnect 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. The format of the Retry Delay TLV is described in Section 6.2. After sending a Retry Delay request message, the server MUST NOT send any further messages on that DSO Session.
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 gracefully.
After sending a Retry Delay request message 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 forcibly abort the connection.
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 forcibly abort the connection immediately.
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), 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, depending on the nature of the reconfiguration.
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).
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 (the server is being de-commissioned), it SHOULD set the retry delay to the maximum value (which is approximately 49.7 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.
This section describes the three base TLVs for DNS Stateful Operations: Keepalive, Retry Delay, and Encryption Padding.
The Keepalive 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 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The transmission or reception of DSO Keepalive messages (i.e., messages where the Keepalive TLV is the first TLV) reset only the keepalive timer, not the inactivity timer. The reason for this is that periodic Keepalive messages are sent for the sole purpose of keeping a DSO Session alive, when that DSO Session has current or recent non-maintenance activity that warrants keeping that DSO Session alive. Sending keepalive traffic itself is not considered a client activity; it is considered a maintenance activity that is performed in service of other client activities. If 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 the keepalive traffic keeping the DSO Session alive so that further keepalive traffic can be sent. For a DSO Session to be considered active, it must be carrying something more than just keepalive traffic. This is why merely sending or receiving a Keepalive message does not reset the inactivity timer.
When sent by a client, the Keepalive request message MUST be sent as an acknowledged request, with a nonzero MESSAGE ID. If a server receives a Keepalive request message with a zero MESSAGE ID then this is a fatal error and the server MUST forcibly abort the connection immediately. The Keepalive request message resets a DSO Session’s keepalive timer, and at the same time communicates to the server the the client’s requested Session Timeout values. In a server response to a client-initiated Keepalive request message, the Session Timeouts contain the server’s chosen values from this point forward in the DSO Session, 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.
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.
Once a DSO Session is in progress (see Section 4) a Keepalive request message MAY be initiated by a server. When sent by a server, the Keepalive request message MUST be sent as an unacknowledged request, with the MESSAGE ID set to zero. The client MUST NOT generate a response to a server-initiated DSO Keepalive message. If a client receives a Keepalive request message with a nonzero MESSAGE ID then this is a fatal error and the client MUST forcibly abort the connection immediately. The Keepalive request message from the server resets a DSO Session’s keepalive timer, and at the same time unilaterally informs the client of the new Session Timeout values to use from this point forward in this DSO Session. No client DSO response message to this unilateral declaration is required or allowed.
The Keepalive TLV is not used as a request message Additional TLV.
In response messages the Keepalive TLV is used only as a Response Primary TLV, replying to a Keepalive request message from the client. A Keepalive TLV MUST NOT be added as to other responses a Response Additional TLV. If the server wishes to update a client’s Session Timeout values other than in response to a Keepalive request message from the client, then it does so by sending an unacknowledged Keepalive request message of its own, as described above.
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, note 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.
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 is a little more subtle, though the meaning of the inactivity timeout is unchanged — it still indicates the maximum permissible time allowed without useful activity on a DSO Session. The act of receiving the message containing the DSO Keepalive TLV 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 new inactivity timeout value in the DSO Keepalive TLV in the received message does update the timeout associated with the running inactivity timer; that becomes the new maximum permissible time without activity on a DSO Session.
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 forcibly abort the connection immediately.
The Retry Delay TLV (DSO-TYPE=2) can be used as a Primary TLV (unacknowledged) in a server-to-client message, or as a Response Additional TLV in a server-to-client response to a client-to-server request message.
The TYPE-DEPENDENT DATA for the the Retry Delay 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RETRY DELAY (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RECOMMENDED value is 10 seconds.
When sent in a DSO request message, from server to client, the Retry Delay TLV (0) is used as a Primary TLV. It is used by a server to instruct a client to 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 begin closing the DSO Session, as described in Section 5.6.3. The RCODE in the message header MUST indicate the reason for the termination:
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.
A Retry Delay request is an unacknowledged request message; the MESSAGE ID MUST be set to zero in the request and the client MUST NOT send a response.
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.
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.
A client MUST NOT send a Retry Delay TLV to a server, either in a DSO request message, or in a DSO response message. If a server receives a DSO message containing a Retry Delay TLV, this is a fatal error and the server MUST forcibly abort the connection immediately.
The Encryption Padding TLV (DSO-TYPE=3) can only be used as an Additional or Response Additional 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 specified for the EDNS(0) Padding Option [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 specified for the EDNS(0) Padding Option [RFC7830] if a request is received with an 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 [I-D.ietf-dprive-padding-policy].
This section summarizes some noteworthy highlights about various components of the DSO protocol.
In DSO Request Messages the MESSAGE ID may be either nonzero (signaling that the responder MUST generate a response) or zero (signaling that the responder MUST NOT generate a response).
In DSO Response Messages the MESSAGE ID MUST NOT be zero (since this would be a response to a request that had indicated that a response is not allowed).
The table below illustrates the legal combinations:
+--------------------+-------------------+
| Nonzero MESSAGE ID | Zero MESSAGE ID |
+----------------------+--------------------+-------------------+
| DSO Request Message | X | X |
+----------------------+--------------------+-------------------+
| DSO Response Message | X | |
+----------------------+--------------------+-------------------+
The table below indicates, for each of the three TLVs defined in this document, whether they are valid in each of ten different contexts.
The first five contexts are requests from client to server, and the corresponding responses from server back to client:
The second five contexts are the reverse: requests from server to client, and the corresponding responses from client back to server.
+-------------------------+-------------------------+
| C-P C-U C-A CRP CRA | S-P S-U S-A SRP SRA |
+------------+-------------------------+-------------------------+
| KeepAlive | X X | X |
+------------+-------------------------+-------------------------+
| RetryDelay | X | X |
+------------+-------------------------+-------------------------+
| Padding | X X | X X |
+------------+-------------------------+-------------------------+
It is recommended that definitions of future TLVs include a similar table summarizing the contexts where the new TLV is valid.
The Inactivity Timeout may have any 32-bit unsigned integer value.
The value zero informs the client that it should not speculatively maintain idle connections at all, and as soon as the client has completed the operation or operations relating to this server, the client should immediately begin closing this session.
The maximum possible value, 0xFFFFFFFF (2^32-1 milliseconds, approximately 49.7 days), informs the client that it may keep an idle connection open as long as it wishes.
The Inactivity timer is reset by any message except the Keepalive TLV, and remains cleared any time that an operation is outstanding.
The Keepalive Interval is a 32-bit unsigned integer value, with a minimum value of 10,000 milliseconds (10 seconds).
The maximum possible value, 0xFFFFFFFF (2^32-1 milliseconds, approximately 49.7 days), informs the client that it should generate no keepalive traffic.
Any message exchange (including the Keepalive TLV) resets the Keepalive timer.
The IANA is directed to record the value (tentatively) 6 for the DSO OPCODE in the DNS OPCODE Registry.
The IANA is directed to record the value (tentatively) 11 for the DSONOTIMP error code in the DNS RCODE Registry.
The IANA is directed to create the 16-bit DSO Type Code Registry, with initial (hexadecimal) values as shown below:
| Type | Name | Status | Reference |
|---|---|---|---|
| 0000 | Reserved | Standard | RFC-TBD |
| 0001 | KeepAlive | Standard | RFC-TBD |
| 0002 | RetryDelay | Standard | RFC-TBD |
| 0003 | EncryptionPadding | Standard | RFC-TBD |
| 0004-003F | Unassigned, reserved for DSO session-management TLVs | ||
| 0040-F7FF | Unassigned | ||
| F800-FBFF | Reserved for experimental/local use | ||
| FC00-FFFF | Reserved for future expansion |
DSO Type Code zero is reserved and is not currently intended for allocation.
Registrations of new DSO Type Codes in the “Reserved for DSO session-management” range 0004-003F and the “Reserved for future expansion” range FC00-FFFF require publication of an IETF Standards Action document [RFC5226].
Requests to register additional new DSO Type Codes in the “Unassigned” range 0040-F7FF are to be recorded by IANA after consultation with the registry’s Designated Expert [RFC5226] at that time. At the time of publication of this document, the Designated Expert for the newly created DSO Type Code registry is [TBD].
DSO Type Codes in the “experimental/local” range F800-FBFF may be used as Experimental Use or Private Use values [RFC5226] and may be used freely for development purposes, or for other purposes within a single site. No attempt is made to prevent multiple sites from using the same value in different (and incompatible) ways. There is no need for IANA to review such assignments (since IANA does not record them) and assignments are not generally useful for broad interoperability. It is the responsibility of the sites making use of “experimental/local” values to ensure that no conflicts occur within the intended scope of use.
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.
Thanks to Tim Chown, Ralph Droms, Jan Komissar, Manju Shankar Rao, and Ted Lemon for their helpful contributions to this document.
| [I-D.ietf-dnssd-push] | Pusateri, T. and S. Cheshire, "DNS Push Notifications", Internet-Draft draft-ietf-dnssd-push-13, October 2017. |
| [I-D.ietf-dprive-padding-policy] | Mayrhofer, A., "Padding Policy for EDNS(0)", Internet-Draft draft-ietf-dprive-padding-policy-03, January 2018. |
| [I-D.ietf-tls-tls13] | Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-23, January 2018. |
| [I-D.sctl-dnssd-mdns-relay] | Cheshire, S. and T. Lemon, "Multicast DNS Discovery Relay", Internet-Draft draft-sctl-dnssd-mdns-relay-02, November 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. |
| [RFC1122] | Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989. |
| [RFC6763] | Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013. |
| [RFC7413] | Cheng, Y., Chu, J., Radhakrishnan, S. and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014. |
| [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. |