DNSOP Working Group R. Bellis
Internet-Draft ISC
Updates: RFC 1035, RFC 7766 (if S. Cheshire
approved) Apple Inc.
Intended status: Standards Track J. Dickinson
Expires: September 6, 2018 S. Dickinson
Sinodun
T. Lemon
Barefoot Consulting
T. Pusateri
Unaffiliated
March 05, 2018
DNS Stateful Operations
draft-ietf-dnsop-session-signal-06
Abstract
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.
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
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This Internet-Draft will expire on September 6, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 10
4.1. DSO Session Establishment . . . . . . . . . . . . . . . . 10
4.1.1. Connection Sharing . . . . . . . . . . . . . . . . . 12
4.1.2. Zero Round-Trip Operation . . . . . . . . . . . . . . 12
4.1.3. Middlebox Considerations . . . . . . . . . . . . . . 13
4.2. Message Format . . . . . . . . . . . . . . . . . . . . . 14
4.2.1. DNS Header Fields in DSO Messages . . . . . . . . . . 15
4.2.2. DSO Data . . . . . . . . . . . . . . . . . . . . . . 17
4.2.3. EDNS(0) and TSIG . . . . . . . . . . . . . . . . . . 22
4.3. Message Handling . . . . . . . . . . . . . . . . . . . . 23
4.3.1. Error Responses . . . . . . . . . . . . . . . . . . . 24
4.4. DSO Response Generation . . . . . . . . . . . . . . . . . 25
4.5. Responder-Initiated Operation Cancellation . . . . . . . 26
5. DSO Session Lifecycle and Timers . . . . . . . . . . . . . . 27
5.1. DSO Session Initiation . . . . . . . . . . . . . . . . . 27
5.2. DSO Session Timeouts . . . . . . . . . . . . . . . . . . 27
5.3. Inactive DSO Sessions . . . . . . . . . . . . . . . . . . 28
5.4. The Inactivity Timeout . . . . . . . . . . . . . . . . . 29
5.4.1. Closing Inactive DSO Sessions . . . . . . . . . . . . 29
5.4.2. Values for the Inactivity Timeout . . . . . . . . . . 30
5.5. The Keepalive Interval . . . . . . . . . . . . . . . . . 31
5.5.1. Keepalive Interval Expiry . . . . . . . . . . . . . . 31
5.5.2. Values for the Keepalive Interval . . . . . . . . . . 31
5.6. Server-Initiated Session Termination . . . . . . . . . . 33
5.6.1. Server-Initiated Retry Delay Message . . . . . . . . 34
6. Base TLVs for DNS Stateful Operations . . . . . . . . . . . . 37
6.1. Keepalive TLV . . . . . . . . . . . . . . . . . . . . . . 37
6.1.1. Client handling of received Session Timeout values . 39
6.1.2. Relation to EDNS(0) TCP Keepalive Option . . . . . . 41
6.2. Retry Delay TLV . . . . . . . . . . . . . . . . . . . . . 42
6.2.1. Retry Delay TLV used as a Primary TLV . . . . . . . . 42
6.2.2. Retry Delay TLV used as a Response Additional TLV . . 43
6.3. Encryption Padding TLV . . . . . . . . . . . . . . . . . 44
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7. Summary Highlights . . . . . . . . . . . . . . . . . . . . . 45
7.1. QR bit and MESSAGE ID . . . . . . . . . . . . . . . . . . 45
7.2. TLV Usage . . . . . . . . . . . . . . . . . . . . . . . . 46
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
8.1. DSO OPCODE Registration . . . . . . . . . . . . . . . . . 48
8.2. DSO RCODE Registration . . . . . . . . . . . . . . . . . 48
8.3. DSO Type Code Registry . . . . . . . . . . . . . . . . . 48
9. Security Considerations . . . . . . . . . . . . . . . . . . . 49
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 49
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
11.1. Normative References . . . . . . . . . . . . . . . . . . 50
11.2. Informative References . . . . . . . . . . . . . . . . . 51
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52
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. While
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.
The three TLVs defined here are all mandatory for all implementations
of DSO. Further TLVs may be defined in additional specifications.
The format for DSO messages (Section 4.2) differs somewhat from the
traditional DNS message format used for standard queries and
responses. The standard twelve-byte header is used, but the four
count fields (QDCOUNT, ANCOUNT, NSCOUNT, ARCOUNT) are set to zero and
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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 pseudo-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 DSO operation is communicated in its own
separate DNS message, and the transport protocol can take care of
packing several 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.
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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", when, and
only when, they appear in all capitals, as shown here [RFC2119]
[RFC8174].
"DSO" is used to mean DNS Stateful Operation.
The term "connection" means a bidirectional byte (or message) stream,
where the bytes (or messages) are delivered reliably and in-order,
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:
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.
In this document the term "session" is used exclusively as described
above, and is in no way related to the anachronistic term "session"
as used in the obsolete "seven-layer model" that shaped the design of
the failed OSI protocols of the 1980s.
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 (if TLS is in use) 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
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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 or
unacknowledged 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 unacknowledged 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 unacknowledged message) or an
initiator (when receiving a DSO response message).
In protocol implementation there are generally two kinds of errors
that software writers have to deal with. The first is situations
that arise due to factors in the environment, such as temporary loss
of connectivity. While undesirable, these situations do not indicate
a flaw in the software, and they are situations that software should
generally be able to recover from. The second is situations that
should never happen when communicating with a correctly-implemented
peer. If they do happen, they indicate a serious flaw in the
protocol implementation, beyond what it is reasonable to expect
software to recover from. This document describes this latter form
of error condition as a "fatal error" and specifies that an
implementation encountering a fatal error condition "MUST forcibly
abort the connection immediately". Given that these fatal error
conditions signify defective software, and given that defective
software is likely to remain defective for some time until it is
fixed, after forcibly aborting a connection, a client SHOULD refrain
from automatically reconnecting to that same server instance for at
least one hour.
This document uses the term "same server instance" as follows:
o In cases where a server is specified or configured using an IP
address and TCP port number, two different configurations are
referring to the same server instance if they contain the same IP
address and TCP port number.
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o In cases where a server is specified or configured using a
hostname and TCP port number, such as in the content of a DNS SRV
record [RFC2782], two different configurations (or DNS SRV
records) are considered to be referring to the same server
instance if they contain the same hostname (subject to the usual
case insensitive DNS name matching rules [RFC1034] [RFC1035]) and
TCP port number. In these cases, configurations with different
hostnames are considered to be referring to different server
instances, even if those different hostnames happen to be aliases,
or happen to resolve to the same IP address(es). Implementations
are not expected to resolve hostnames and then perform matching of
IP address(es) in order to evaluate whether two entities should be
determined to be the "same server instance".
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 kind of long-lived operations that it is intended to support.
DNS Stateful Operations uses three kinds of message: "DSO request
messages", "DSO response messages", and "DSO unacknowledged
messages". A DSO request message elicits a DSO response message.
DSO unacknowledged messages are unidirectional messages and do not
generate any response.
Both DSO request messages and DSO unacknowledged messages are
formatted as DNS request messages (the header QR bit is set to zero,
as described in Section 4.2). One difference is that in DSO request
messages the MESSAGE ID field is nonzero; in DSO unacknowledged
messages it is zero.
The content of DSO messages is expressed using type-length-value
(TLV) syntax.
In a DSO request message or DSO unacknowledged 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 or
unacknowledged 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.
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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:
o an inactivity timer (see Section 5.4 and Section 6.1)
o a keepalive timer (see Section 5.5 and Section 6.1)
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.
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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 [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.
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4. Protocol Details
4.1. DSO Session Establishment
DSO messages MUST be carried in only protocols and in environments
where a session may be established according to the definition given
above in the Terminology section (Section 2).
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.
At the time of publication, DSO is specified only for DNS over TCP
[RFC1035] [RFC7766], and for DNS over TLS over TCP [RFC7858]. Any
use of DSO over some other connection technology needs to be
specified in an appropriate future document.
Determining whether a given connection is using DNS over TCP, or DNS
over TLS over TCP, is outside the scope of this specification, and
must be determined using some out-of-band configuration information.
There is no provision within the DSO specification to turn TLS on or
off during the lifetime of a connection. For service types where the
service instance is discovered using a DNS SRV record [RFC2782], the
specification for that service type SRV name [RFC6335] will state
whether the connection uses plain TCP, or TLS over TCP. For example,
the specification for the "_dns-push-tls._tcp" service
[I-D.ietf-dnssd-push], states that it uses TLS. It is a common
convention that protocols specified to run over TLS are given IANA
service type names ending in "-tls".
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 or DSO unacknowledged messages
until a DSO Session has been mutually established by at least one
successful DSO request/response exchange initiated by the client, as
described below. Similarly, unless it is known in advance by other
means that a server does support DSO, a client MUST NOT initiate DSO
unacknowledged messages until after a DSO Session has been mutually
established.
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A DSO Session is established over a connection by the client sending
a DSO request message, such as a DSO Keepalive request message
(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 in the response is set to DSOTYPENI ("DSO-TYPE Not
Implemented", tentatively RCODE 11) this indicates that the server
does support DSO, but does not implement the DSO-TYPE of the primary
TLV in this DSO request message. A server implementing DSO MUST NOT
return DSOTYPENI for a DSO Keepalive request message, because the
Keepalive TLV is mandatory to implement, so all implementations of
DSO are required to implement the Keepalive TLV. But in the future,
if a client attempts to establish a DSO Session using a response-
requiring DSO request message using some newly-defined DSO-TYPE that
the server does not understand, that would result in a DSOTYPENI
response. If the server returns DSOTYPENI then a DSO Session is not
considered established, but the client is permitted to continue
sending DNS messages on the connection, including other 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 DSOTYPENI
(tentatively 11), then the client MUST assume that the server does
not implement DSO at all. 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 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 appropriate 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].
Note that for clients that implement only the DSO-TYPEs defined in
this base specification, sending a DSO Keepalive TLV is the only DSO
request message they have available to initiate a DSO Session. Even
for clients that do implement other future DSO-TYPEs, for simplicity
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they MAY elect to always send an initial DSO Keepalive request
message as their way of initiating a DSO Session.
4.1.1. Connection Sharing
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 requirement 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.
4.1.2. Zero Round-Trip Operation
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.
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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.
Caution must be taken to ensure that DSO messages sent before the
first round-trip is completed are idempotent, or are otherwise immune
to any problems that could be result from the inadvertent replay that
can occur with zero round-trip operation.
4.1.3. Middlebox Considerations
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.
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4.2. Message Format
A DSO message begins with the standard twelve-byte 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 /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
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4.2.1. DNS Header Fields in DSO Messages
In an unacknowledged 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 set up 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 fatal error and forcibly abort the connection immediately.
In a request or unacknowledged message the DNS Header QR bit MUST be
zero (QR=0). If the QR bit is not zero the message is not a request
or unacknowledged 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 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 recipient 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.
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In a DNS request message (QR=0) the RCODE is set according to the
definition of the request. For example, in a Retry Delay message
(Section 5.6.1) the RCODE indicates the reason for termination.
However, in most cases, except where clearly specified otherwise, in
a DNS request message (QR=0) the RCODE is set to zero on
transmission, and silently ignored on reception.
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 | DSOTYPENI | Primary TLV's DSO-Type is not implemented |
+------+-----------+------------------------------------------------+
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-TYPE 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.
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4.2.2. DSO Data
The standard twelve-byte 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 a request message, a response message, or an
unacknowledged message.
A DSO request message or DSO unacknowledged message MUST contain at
least one TLV. The first TLV in a DSO request message or DSO
unacknowledged 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 or unacknowledged 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 the DSO request message to which 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 most DSO operations will be specified to use
request messages, which generate corresponding responses. In some
specialized high-traffic use cases, it may be appropriate to specify
unacknowledged messages. Unacknowledged messages can be more
efficient on the network, because they don't generate a stream of
corresponding reply messages. Using unacknowledged 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
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used as a Primary TLV (i.e., first) in an outgoing DNS request
message (i.e., QR=0), that 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
message.
The previous point, that the receiver MUST NOT generate responses to
unacknowledged messages, applies even in the case of errors. When a
DSO message is received where both the QR bit and the MESSAGE ID
field are zero, the receiver MUST NOT generate any response. For
example, if the DSO-TYPE in the Primary TLV is unrecognized, then a
DSOTYPENI error MUST NOT be returned; instead the receiver MUST
forcibly abort the connection immediately.
Unacknowledged 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 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].
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4.2.2.1. TLV Syntax
All TLVs, whether used as "Primary", "Additional", "Response
Primary", or "Response Additional", use the same encoding syntax.
The specification for a TLV states whether that DSO-TYPE may be used
in "Primary", "Additional", "Response Primary", or "Response
Additional" TLVs. The specification for a TLV also states whether,
when used as the Primary (i.e., first) TLV in a DNS request message
(i.e., QR=0), that DSO message is to be acknowledged. If the DSO
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 /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
DSO-TYPE: A 16-bit unsigned integer, in network (big endian) byte
order, giving the DSO-TYPE of the current DSO TLV per the IANA DSO
Type Code Registry.
DSO DATA LENGTH: A 16-bit unsigned integer, in network (big endian)
byte order, giving the size in bytes of the TYPE-DEPENDENT DATA.
TYPE-DEPENDENT DATA: Type-code specific format. The generic DSO
machinery treats the TYPE-DEPENDENT DATA as an opaque "blob"
without attempting to interpret it. Interpretation of the meaning
of the TYPE-DEPENDENT DATA for a particular DSO-TYPE is the
responsibility of the software that implements that DSO-TYPE.
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4.2.2.2. Request TLVs
The first TLV in a DSO request message or unacknowledged message is
the "Primary TLV" and indicates the operation to be performed. A DSO
request message or unacknowledged message MUST contain at at least
one TLV, the Primary TLV.
Immediately following the Primary TLV, a DSO request message or
unacknowledged message MAY contain one or more "Additional TLVs",
which specify additional parameters relating to the operation.
4.2.2.3. Response TLVs
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.
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4.2.2.4. Unrecognized TLVs
If DSO request message is received containing an unrecognized Primary
TLV, with a nonzero MESSAGE ID (indicating that a response is
expected), then the receiver MUST send an error response with
matching MESSAGE ID, and RCODE DSOTYPENI (tentatively 11). The error
response MUST NOT contain a copy of the unrecognized Primary TLV.
If DSO unacknowledged message is received containing an unrecognized
Primary TLV, with a zero MESSAGE ID (indicating that no response is
expected), then this is a fatal error and the recipient MUST forcibly
abort the connection immediately.
If a DSO request message or unacknowledged 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.
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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
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.
Although messages may contain other EDNS(0) options as appropriate,
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). If any message sent on a DSO Session contains
an EDNS(0) TCP Keepalive Option this is a fatal error and the
recipient of the defective message MUST forcibly abort the connection
immediately.
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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).
As described above in Section 4.2.1 whether an outgoing message with
QR=0 is unacknowledged or acknowledged is determined by the
specification for the Primary TLV, which in turn determines whether
the MESSAGE ID field in that outgoing message will be zero or
nonzero.
A DSO unacknowledged message has both the QR bit and the MESSAGE ID
field set to zero, and MUST NOT elicit a response.
Every DSO request message (QR=0) with a nonzero MESSAGE ID field is
an acknowledged DSO request, and MUST elicit a corresponding response
(QR=1), which MUST have the same MESSAGE ID in the DNS message header
as in the corresponding request.
Valid DSO request messages sent by the client with a nonzero MESSAGE
ID field elicit a response from the server, and Valid DSO request
messages sent by the server with a nonzero MESSAGE ID field elicit a
response from the client.
The namespaces of 16-bit MESSAGE IDs are independent in each
direction. This means it is *not* an error for both client and
server to send request messages at the same time as each other, using
the same MESSAGE ID, in different directions. This simplification is
necessary in order for the protocol to be implementable. It would be
infeasible to require the client and server to coordinate with each
other regarding allocation of new unique MESSAGE IDs. It is also not
necessary to require the client and server to coordinate with each
other regarding allocation of new unique MESSAGE IDs. The value of
the 16-bit MESSAGE ID combined with the identity of the initiator
(client or server) is sufficient to unambiguously identify the
operation in question. This can be thought of as a 17-bit message
identifier space, using message identifiers 0x00001-0x0FFFF for
client-to-server DSO request messages, and message identifiers
0x10001-0x1FFFF for server-to-client DSO request messages. The
least-significant 16 bits are stored explicitly in the MESSAGE ID
field of the DSO message, and the most-significant bit is implicit
from the direction of the message.
As described above in Section 4.2.1, an initiator MUST NOT reuse a
MESSAGE ID that it already has in use for an outstanding request
(unless specified otherwise by the relevant specification for the
DSO-TYPE in question). At the very least, this means that a MESSAGE
ID MUST NOT be reused in a particular direction on a particular DSO
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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-TYPE in
question), and for a long-lived operation the MESSAGE ID for the
operation MUST NOT be reused while that operation remains active.
If a client or server receives a response (QR=1) where the MESSAGE ID
is zero, or is 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.
4.3.1. Error Responses
When a DSO unacknowledged message is unsuccessful for some reason,
the responder immediately aborts the connection.
When a DSO request message is unsuccessful for some reason, the
responder returns an error code to the initiator.
In the case of a server returning an error code to a client in
response to an unsuccessful DSO request message, the server MAY
choose to end the DSO Session, or MAY choose to 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 message, as described in Section 5.6.1.
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 (Section 5.6.1)
then the client SHOULD continue to use that DSO Session for
subsequent operations.
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4.4. DSO Response Generation
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 can give a 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:
o Disable Nagle's Algorithm at the sender. This is not great,
because it results in less efficient use of the network.
o Disable Delayed ACK at the receiver. This is not great,
because it results in less efficient use of the network.
o Use a networking API that lets the receiver signal to the TCP
implementation that the receiver has received and processed a
client request for which it will not be generating any immediate
response. This allows the TCP implementation to operate
efficiently in both cases; for requests that generate a response,
the TCP ACK, window update, and DSO response are transmitted
together in a single TCP segment, and for requests that do not
generate a response, the application-layer software informs the
TCP implementation that it should go ahead and send the TCP ACK
and window update immediately, without waiting for the Delayed ACK
timer. Unfortunately it is not known at this time which (if any)
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of the widely-available networking APIs currently include this
capability.
4.5. Responder-Initiated Operation Cancellation
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 the cancelled operation's MESSAGE ID is now free
for reuse.
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5. DSO Session Lifecycle and Timers
5.1. DSO Session Initiation
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.
5.2. DSO Session Timeouts
Two timeout values are associated with a DSO Session: the inactivity
timeout, and the keepalive interval. Both values are communicated in
the same TLV, the DSO Keepalive TLV (Section 6.1).
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 messages 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.
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.
In the usual case where the inactivity timeout is shorter than the
keepalive interval, it is only when a client has a very long-lived,
low-traffic, operation that the keepalive interval comes into play,
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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.
5.3. 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
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.
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5.4. 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.
5.4.1. Closing Inactive DSO Sessions
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 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 a currently inactive DSO
Session, then the client SHOULD gracefully 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 then it uses the DSO Keepalive message to
request longer timeout values, as described in Section 6.1.
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5.4.2. 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 lower CPU load on the server, but higher
memory burden to maintain state for inactive DSO Sessions.
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.
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.
An inactivity timeout of 0xFFFFFFFF represents "infinity" and 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 message dictating new Session Timeout
values to the client.
The largest *finite* inactivity timeout supported by the current DSO
Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
days).
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5.5. The Keepalive Interval
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 to establish a
new session if appropriate.
5.5.1. 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, by sending a DSO Keepalive
message (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, perhaps leaving a long-lived operation
uncancelled, 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.
5.5.2. Values for the Keepalive Interval
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,
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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 a fatal error
and the client MUST forcibly abort the connection immediately.
A keepalive interval value of 0xFFFFFFFF represents "infinity" and
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 message
dictating new Session Timeout values to the client.
The largest *finite* keepalive interval supported by the current DSO
Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
days).
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5.6. Server-Initiated Session Termination
In addition to cancelling individual long-lived operations
selectively (Section 4.5) there are also occasions where a server may
need to terminate one or more entire sessions. 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.
Normally a server MUST NOT close a DSO Session with a client. A
server only causes a DSO Session to be ended in the exceptional
circumstances 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 of the exceptional situations in which a server may terminate a
DSO Session include:
o The server application software or underlying operating system is
shutting down or restarting.
o The server application software terminates unexpectedly (perhaps
due to a bug that makes it crash).
o The server is undergoing a reconfiguration or maintenance
procedure, that, due to the way the server software is
implemented, requires clients to be disconnected. For example,
some software is implemented such that it reads a configuration
file at startup, and changing the server's configuration entails
modifying the configuration file and then killing and restarting
the server software, which generally entails a loss of network
connections.
o The client fails to meets its obligation to generate the required
keepalive traffic, or to close an inactive session by the
prescribed time (twice the time interval dictated by the server,
or five seconds, whichever is greater, as described in
Section 5.2).
o The client sends a grossly invalid or malformed request that is
indicative of a seriously defective client implementation.
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o The server is over capacity and needs to shed some load.
5.6.1. Server-Initiated Retry Delay Message
In the cases described above where a server elects to terminate a DSO
Session, it could do so simply by forcibly aborting the connection.
However, if it did this the likely behavior of the client might 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 message,
with an appropriate RCODE value informing the client of the reason
the DSO Session needs to be terminated. For example, an RCODE value
of SERVFAIL indicates that the server is overloaded due to resource
exhaustion, or is restarting. An RCODE value of NOTAUTH 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 because it no longer has authority over the names in
question. An RCODE value of REFUSED indicates a policy change
regarding this session. An RCODE value of FORMERR indicates that the
client requests are too badly malformed for the session to continue.
The format of the Retry Delay TLV is described in Section 6.2. After
sending a Retry Delay message, the server MUST NOT send any further
messages on that DSO Session.
Upon receipt of a Retry Delay message 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 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 message MUST NOT be initiated by a client. If a server
receives a Retry Delay message this is a fatal error and the server
MUST forcibly abort the connection immediately.
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5.6.1.1. Outstanding Operations
At the instant a server chooses to initiate a Retry Delay 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 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 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 affected DSO Sessions en masse by
sending them all a Retry Delay message, 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).
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5.6.1.2. Client Reconnection
After a DSO Session is ended by the server (either by sending the
client a Retry Delay message, or by forcibly aborting the underlying
transport connection) the client SHOULD try to reconnect, to that
server instance, or to another suitable server instance, if more than
one is available. If reconnecting to the same server instance, the
client MUST respect the indicated delay, if available, before
attempting to reconnect.
If the server instance will only be out of service for a short
maintenance period, it should use a value a little longer that the
expected maintenance window. It should not default to a very large
delay value, or clients may not attempt to reconnect after it resumes
service.
If a particular server instance does not want a client to reconnect
ever (perhaps the server instance is being de-commissioned), it
SHOULD set the retry delay to the maximum value 0xFFFFFFFF (2^32-1
milliseconds, approximately 49.7 days). It is not possible to
instruct a client to stay away for longer than 49.7 days. If, after
49.7 days, the DNS or other configuration information still indicates
that this is the valid server instance for a particular service, then
clients MAY attempt to reconnect. In reality, if a client is
rebooted or otherwise lose state, it may well attempt to reconnect
before 49.7 days elapses, for as long as the DNS or other
configuration information continues to indicate that this is the
server instance the client should use.
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6. Base TLVs for DNS Stateful Operations
This section describes the three base TLVs for DNS Stateful
Operations: Keepalive, Retry Delay, and Encryption Padding.
6.1. Keepalive TLV
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
INACTIVITY TIMEOUT: The inactivity timeout for the current DSO
Session, specified as a 32-bit unsigned integer, in network (big
endian) byte order, in units of milliseconds. This is the timeout
at which the client MUST begin closing an inactive DSO Session.
The inactivity timeout can be any value of the server's choosing.
If the client does not gracefully close an inactive DSO Session,
then after twice this interval, or five seconds, whichever is
greater, the server will forcibly abort the connection.
KEEPALIVE INTERVAL: The keepalive interval for the current DSO
Session, specified as a 32-bit unsigned integer, in network (big
endian) byte order, in units of milliseconds. This is the
interval at which a client MUST generate keepalive traffic to
maintain connection state. The keepalive interval MUST NOT be
less than ten seconds. If the client does not generate the
mandated keepalive traffic, then after twice this interval the
server will forcibly abort the connection. Since the minimum
allowed keepalive interval is ten seconds, the minimum time at
which a server will forcibly disconnect a client for failing to
generate the mandated keepalive traffic is twenty seconds.
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
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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 DSO 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 (Section 4.1) a Keepalive message
MAY be initiated by a server. When sent by a server, the Keepalive
message MUST be sent as an unacknowledged message, 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 unacknowledged 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 an Additional TLV.
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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 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 DSO-TYPEs defined in this document, a
Keepalive request message is the only way for a client to initiate a
DSO Session.
6.1.1. 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.
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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 remains as specified -- 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.
o If the current inactivity timer value is less than the new
inactivity timeout, then the DSO Session may remain open for now.
When the inactivity timer value reaches the new inactivity
timeout, the client MUST then begin closing the DSO Session, as
described above.
o If the current inactivity timer value is equal to the new
inactivity timeout, then this DSO Session has been inactive for
exactly as long as the server will permit, and now the client MUST
immediately begin closing this DSO Session.
o If the current 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 begin closing this DSO Session.
o If the current inactivity timer value is already more than twice
the new inactivity timeout, then the client is immediately
considered delinquent (this DSO Session is immediately eligible to
be forcibly terminated by the server) and the client MUST
immediately begin closing this DSO Session. However if a server
abruptly reduces the inactivity timeout in this way, then, to give
the client time to close the connection gracefully before the
server resorts to forcibly aborting it, the server SHOULD give the
client an additional grace period of one quarter of the new
inactivity timeout, or five seconds, whichever is greater.
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6.1.2. 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 a fatal error and
the receiver of the EDNS(0) TCP Keepalive option MUST forcibly abort
the connection immediately.
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6.2. Retry Delay TLV
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 either direction.
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RETRY DELAY: A time value, specified as a 32-bit unsigned integer,
in network (big endian) byte order, in units of milliseconds,
within which the initiator MUST NOT retry this operation, or retry
connecting to this server. Recommendations for the RETRY DELAY
value are given in Section 5.6.1.
6.2.1. Retry Delay TLV used as a Primary TLV
When sent from server to client, the Retry Delay TLV is used as the
Primary TLV in an unacknowledged message. 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 5.6.1.
The RCODE in the message header MUST indicate the reason for the
termination:
o NOERROR indicates a routine shutdown.
o FORMERR indicates that the client requests are too badly malformed
for the session to continue.
o SERVFAIL indicates that the server is overloaded due to resource
exhaustion, or is restarting.
o REFUSED indicates that the server has been reconfigured and due to
a policy change is no longer able to perform one or more of the
functions currently being performed on this DSO Session
o NOTAUTH 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 because it no longer has
authority over the names in question (for example, a DNS Push
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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 RCODE values for Retry Delay
request. Servers sending Retry Delay requests SHOULD use one of
these 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.
The RCODE value received in the message header of a Retry Delay
unacknowledged message is generally usually useful only for
information purposes, such as writing to a log file for future human
analysis regarding the nature of the disconnection. Generally
clients do not modify their behavior depending on the RCODE value.
The RETRY DELAY in the message tells the client how long it should
wait before attempting a new connection to this server instance.
For clients that do in some way modify their behavior depending on
the RCODE value, they should treat unknown RCODE values the same as
RCODE=NOERROR (routine shutdown).
A Retry Delay message from server to client is an unacknowledged
message; the MESSAGE ID MUST be set to zero in the outgoing message
and the client MUST NOT send a response.
A client MUST NOT send a Retry Delay DSO request message or DSO
unacknowledged message to a server. If a server receives a DNS
request message (i.e., QR=0) where the Primary TLV is the Retry Delay
TLV, this is a fatal error and the server MUST forcibly abort the
connection immediately.
6.2.2. Retry Delay TLV used as a Response Additional TLV
In the case of a request that returns a nonzero RCODE value, the
responder MAY append a Retry Delay TLV to the response, indicating
the time interval during which the initiator SHOULD NOT attempt this
operation again.
The indicated time interval during which the initiator SHOULD NOT
retry applies only to the failed operation, not to the DSO Session as
a whole.
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6.3. Encryption Padding TLV
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 bytes 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 BYTES /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
As specified for the EDNS(0) Padding Option [RFC7830] the PADDING
bytes 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 bytes 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].
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7. Summary Highlights
This section summarizes some noteworthy highlights about various
components of the DSO protocol.
7.1. QR bit and MESSAGE ID
In DSO Request Messages the QR bit is 0 and the MESSAGE ID is
nonzero.
In DSO Response Messages the QR bit is 1 and the MESSAGE ID is
nonzero.
In DSO Unacknowledged Messages the QR bit is 0 and the MESSAGE ID is
zero.
The table below illustrates which combinations are legal and how they
are interpreted:
+--------------------------+------------------------+
| MESSAGE ID zero | MESSAGE ID nonzero |
+--------+--------------------------+------------------------+
| QR=0 | Unacknowledged Message | Request Message |
+--------+--------------------------+------------------------+
| QR=1 | Invalid - Fatal Error | Response Message |
+--------+--------------------------+------------------------+
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7.2. TLV Usage
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 or unacknowledged messages from
client to server, and the corresponding responses from server back to
client:
o C-P - Primary TLV, sent in DSO Request message, from client to
server, with nonzero MESSAGE ID indicating that this request MUST
generate response message.
o C-U - Primary TLV, sent in DSO Unacknowledged message, from client
to server, with zero MESSAGE ID indicating that this request MUST
NOT generate response message.
o C-A - Additional TLV, optionally added to request message or
unacknowledged message from client to server.
o CRP - Response Primary TLV, included in response message sent to
back the client (in response to a client "C-P" request with
nonzero MESSAGE ID indicating that a response is required) where
the DSO-TYPE of the Response TLV matches the DSO-TYPE of the
Primary TLV in the request.
o CRA - Response Additional TLV, included in response message sent
to back the client (in response to a client "C-P" request with
nonzero MESSAGE ID indicating that a response is required) where
the DSO-TYPE of the Response TLV does not match the DSO-TYPE of
the Primary TLV in the request.
The second five contexts are their counterparts in the opposite
direction: requests or unacknowledged messages 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 |
+------------+-------------------------+-------------------------+
Note that some of the columns in this table are currently empty. The
table provides a template for future TLV definitions to follow. It
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is recommended that definitions of future TLVs include a similar
table summarizing the contexts where the new TLV is valid.
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8. IANA Considerations
8.1. DSO OPCODE Registration
The IANA is requested to record the value (tentatively) 6 for the
DSO OPCODE in the DNS OPCODE Registry. DSO stands for DNS Stateful
Operations.
8.2. DSO RCODE Registration
The IANA is requested to record the value (tentatively) 11 for the
DSOTYPENI error code in the DNS RCODE Registry. The DSOTYPENI error
code ("DSO-TYPE Not Implemented") signifies that the DSO-TYPE of the
primary TLV in the DSO request message is not implemented by the
receiver.
8.3. DSO Type Code Registry
The IANA is requested 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"
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range FC00-FFFF require publication of an IETF Standards Action
document [RFC8126].
Requests to register additional new DSO Type Codes in the
"Unassigned" range 0040-F7FF are to be recorded by IANA after Expert
Review [RFC8126]. 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 [RFC8126] 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.
9. 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.
When designing new DSO TLVs, the potential for data in the TLV to be
used as a tracking identifier should be taken into consideration, and
should be avoided when not required.
When used without TLS or similar cryptographic protection, a
malicious entity maybe able to inject a malicious Retry Delay
Unacknowledged Message into the data stream, specifying an
unreasonably large RETRY DELAY, causing a denial-of-service attack
against the client.
10. Acknowledgements
Thanks to Stephane Bortzmeyer, Tim Chown, Ralph Droms, Paul Hoffman,
Jan Komissar, Edward Lewis, Allison Mankin, Rui Paulo, David
Schinazi, Manju Shankar Rao, and Bernie Volz for their helpful
contributions to this document.
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11. References
11.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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,
<https://www.rfc-editor.org/info/rfc1918>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/info/rfc2132>.
[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,
<https://www.rfc-editor.org/info/rfc2136>.
[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC5382, October 2008,
<https://www.rfc-editor.org/info/rfc5382>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.
[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,
<https://www.rfc-editor.org/info/rfc7766>.
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[RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", RFC 7828,
DOI 10.17487/RFC7828, April 2016,
<https://www.rfc-editor.org/info/rfc7828>.
[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016,
<https://www.rfc-editor.org/info/rfc7830>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11.2. Informative References
[I-D.ietf-dnssd-push]
Pusateri, T. and S. Cheshire, "DNS Push Notifications",
draft-ietf-dnssd-push-13 (work in progress), October 2017.
[I-D.ietf-dprive-padding-policy]
Mayrhofer, A., "Padding Policy for EDNS(0)", draft-ietf-
dprive-padding-policy-04 (work in progress), February
2018.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-24 (work in progress),
February 2018.
[I-D.sctl-dnssd-mdns-relay]
Cheshire, S. and T. Lemon, "Multicast DNS Discovery
Relay", draft-sctl-dnssd-mdns-relay-02 (work in progress),
November 2017.
[NagleDA] Cheshire, S., "TCP Performance problems caused by
interaction between Nagle's Algorithm and Delayed ACK",
May 2005,
<http://www.stuartcheshire.org/papers/nagledelayedack/>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
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[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[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, <https://www.rfc-editor.org/info/rfc7858>.
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
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Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino CA 95014
USA
Phone: +1 408 974 3207
Email: cheshire@apple.com
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
Ted Lemon
Barefoot Consulting
Brattleboro
VT 05301
USA
Email: mellon@fugue.com
Tom Pusateri
Unaffiliated
Raleigh NC 27608
USA
Phone: +1 919 867 1330
Email: pusateri@bangj.com
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