Internet DRAFT - draft-ietf-dnsop-session-signal
draft-ietf-dnsop-session-signal
DNSOP Working Group R. Bellis
Internet-Draft ISC
Updates: 1035, 7766 (if approved) S. Cheshire
Intended status: Standards Track Apple Inc.
Expires: June 9, 2019 J. Dickinson
S. Dickinson
Sinodun
T. Lemon
Nibbhaya Consulting
T. Pusateri
Unaffiliated
December 06, 2018
DNS Stateful Operations
draft-ietf-dnsop-session-signal-20
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. This document updates RFC 1035 by adding a new
DNS header opcode which has different message semantics, and a new
result code. This document updates RFC 7766 by redefining a session,
providing new guidance on connection re-use, and providing a new
mechanism for handling session idle timeouts.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 9, 2019.
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1.1. Session Management . . . . . . . . . . . . . . . . . 9
4.1.2. Long-lived Subscriptions . . . . . . . . . . . . . . 9
4.2. Applicable Transports . . . . . . . . . . . . . . . . . . 10
5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 11
5.1. DSO Session Establishment . . . . . . . . . . . . . . . . 12
5.1.1. Session Establishment Failure . . . . . . . . . . . . 13
5.1.2. Session Establishment Success . . . . . . . . . . . . 14
5.2. Operations After Session Establishment . . . . . . . . . 14
5.3. Session Termination . . . . . . . . . . . . . . . . . . . 15
5.3.1. Handling Protocol Errors . . . . . . . . . . . . . . 15
5.4. Message Format . . . . . . . . . . . . . . . . . . . . . 16
5.4.1. DNS Header Fields in DSO Messages . . . . . . . . . . 17
5.4.2. DSO Data . . . . . . . . . . . . . . . . . . . . . . 19
5.4.3. TLV Syntax . . . . . . . . . . . . . . . . . . . . . 21
5.4.4. EDNS(0) and TSIG . . . . . . . . . . . . . . . . . . 24
5.5. Message Handling . . . . . . . . . . . . . . . . . . . . 25
5.5.1. Delayed Acknowledgement Management . . . . . . . . . 26
5.5.2. MESSAGE ID Namespaces . . . . . . . . . . . . . . . . 27
5.5.3. Error Responses . . . . . . . . . . . . . . . . . . . 28
5.6. Responder-Initiated Operation Cancellation . . . . . . . 29
6. DSO Session Lifecycle and Timers . . . . . . . . . . . . . . 30
6.1. DSO Session Initiation . . . . . . . . . . . . . . . . . 30
6.2. DSO Session Timeouts . . . . . . . . . . . . . . . . . . 31
6.3. Inactive DSO Sessions . . . . . . . . . . . . . . . . . . 32
6.4. The Inactivity Timeout . . . . . . . . . . . . . . . . . 33
6.4.1. Closing Inactive DSO Sessions . . . . . . . . . . . . 33
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6.4.2. Values for the Inactivity Timeout . . . . . . . . . . 34
6.5. The Keepalive Interval . . . . . . . . . . . . . . . . . 35
6.5.1. Keepalive Interval Expiry . . . . . . . . . . . . . . 35
6.5.2. Values for the Keepalive Interval . . . . . . . . . . 35
6.6. Server-Initiated Session Termination . . . . . . . . . . 37
6.6.1. Server-Initiated Retry Delay Message . . . . . . . . 38
6.6.2. Misbehaving Clients . . . . . . . . . . . . . . . . . 39
6.6.3. Client Reconnection . . . . . . . . . . . . . . . . . 39
7. Base TLVs for DNS Stateful Operations . . . . . . . . . . . . 41
7.1. Keepalive TLV . . . . . . . . . . . . . . . . . . . . . . 41
7.1.1. Client handling of received Session Timeout values . 43
7.1.2. Relationship to edns-tcp-keepalive EDNS0 Option . . . 44
7.2. Retry Delay TLV . . . . . . . . . . . . . . . . . . . . . 45
7.2.1. Retry Delay TLV used as a Primary TLV . . . . . . . . 45
7.2.2. Retry Delay TLV used as a Response Additional TLV . . 47
7.3. Encryption Padding TLV . . . . . . . . . . . . . . . . . 48
8. Summary Highlights . . . . . . . . . . . . . . . . . . . . . 49
8.1. QR bit and MESSAGE ID . . . . . . . . . . . . . . . . . . 49
8.2. TLV Usage . . . . . . . . . . . . . . . . . . . . . . . . 50
9. Additional Considerations . . . . . . . . . . . . . . . . . . 52
9.1. Service Instances . . . . . . . . . . . . . . . . . . . . 52
9.2. Anycast Considerations . . . . . . . . . . . . . . . . . 53
9.3. Connection Sharing . . . . . . . . . . . . . . . . . . . 54
9.4. Operational Considerations for Middlebox . . . . . . . . 55
9.5. TCP Delayed Acknowledgement Considerations . . . . . . . 56
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 59
10.1. DSO OPCODE Registration . . . . . . . . . . . . . . . . 59
10.2. DSO RCODE Registration . . . . . . . . . . . . . . . . . 59
10.3. DSO Type Code Registry . . . . . . . . . . . . . . . . . 59
11. Security Considerations . . . . . . . . . . . . . . . . . . . 60
11.1. TLS 0-RTT Considerations . . . . . . . . . . . . . . . . 61
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 62
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 62
13.1. Normative References . . . . . . . . . . . . . . . . . . 62
13.2. Informative References . . . . . . . . . . . . . . . . . 63
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 65
1. Introduction
This document specifies a mechanism for managing stateful DNS
connections. DNS most commonly operates over a UDP transport, but
can also operate over streaming transports; the original DNS RFC
specifies DNS over TCP [RFC1035] and a profile for DNS over TLS
[RFC7858] has been specified. These 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
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situations it is also advantageous to support server-initiated
messages (such as DNS Push Notifications [I-D.ietf-dnssd-push]).
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 (edns-tcp-keepalive EDNS0 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 ([TBA1], tentatively 6),
for DNS Stateful Operations. DSO messages are used to communicate
operations within persistent stateful sessions, expressed using type-
length-value (TLV) syntax. This document defines an initial set of
three TLVs, used to manage session timeouts, termination, and
encryption padding.
All three TLVs defined here are mandatory for all implementations of
DSO. Further TLVs may be defined in additional specifications.
DSO messages may or may not be acknowledged; this is signalled by
providing a non-zero message ID for messages that must be
acknowledged (DSO request messages) and a zero message ID for
messages that are not to be acknowledged (DSO unidirectional
messages), and is also specified in the definition of a particular
DSO message type. Messages are pipelined; answers may appear out of
order when more than one answer is pending.
The format for DSO messages (Section 5.4) 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
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. Just
as in traditional DNS over TCP [RFC1035] [RFC7766] the stream
protocol carrying DSO messages (which are just another kind of DNS
message) frames them by putting a 16-bit message length at the start,
so the length of the DSO message is determined from that length,
rather than from any of the DNS header counts.
When displayed using packet analyzer tools that have not been updated
to recognize the DSO format, this will result in the DSO data being
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displayed as unknown additional data after the end of the DNS
message.
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 (see for example
[I-D.ietf-dnsop-no-response-issue] sections 3.2 and 4).
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.
2. Requirements Language
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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3. Terminology
DSO: DNS Stateful Operations.
connection: 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].
session: The unqualified term "session" in the context of this
document refers to a persistent network connection between two
endpoints which allows for the exchange of DNS messages over a
connection where either end of the connection can send messages to
the other end. (The term has no relationship to the "session
layer" of the OSI "seven-layer model".)
DSO Session: a session 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].
close gracefully: a normal session shutdown, where the client closes
the TCP connection to the server using a graceful close, such that
no data is lost (e.g., using TCP FIN, see Section 5.3).
forcibly abort: a session shutdown as a result of a fatal error,
where the TCP connection is unilaterally aborted without regard
for data loss (e.g., using TCP RST, see Section 5.3).
server: the software with a listening socket, awaiting incoming
connection requests, in the usual DNS sense.
client: the software which initiates a connection to the server's
listening socket, in the usual DNS sense.
initiator: the software which sends a DSO request message or a DSO
unidirectional message during a DSO session. Either a client or
server can be an initiator
responder: the software which receives a DSO request message or a
DSO unidirectional message during a DSO
session. Either a client or server can be a responder.
sender: the software which is sending a DNS message, a DSO message,
a DNS response, or a DSO response.
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receiver: the software which is receiving a DNS message, a DSO
message, a DNS response, or a DSO response.
service instance: a specific instance of server software running on
a specific host (Section 9.1).
long-lived operation: a long-lived operation is an outstanding
operation on a DSO session where either the client or server,
acting as initiator, has requested that the responder send new
information regarding the request, as it becomes available.
Early Data: A TLS 1.3 handshake containing early data that begins a
DSO session ([RFC8446] section 2.3). TCP Fast Open is only permitted
when using TLS.
DNS message: any DNS message, including DNS queries, response,
updates, DSO messages, etc.
DNS request message: any DNS message where the QR bit is 0.
DNS response message: any DNS message where the QR bit is 1.
DSO message: a DSO request message, DSO unidirectional message, or a
DSO response to a DSO request message. If the QR bit is 1 in a
DSO message, it is a DSO response message. If the QR bit is 0 in
a DSO message, it is a DSO request message or DSO unidirectional
message, as determined by the specification of its primary TLV.
DSO response message: a response to a DSO request message.
DSO request message: a DSO message that requires a response.
DSO unidirectional message: a DSO message that does not require and
cannot induce a response.
Primary TLV: The first TLV in a DSO message or DSO response; in the
DSO message this determines the nature of the operation being
performed.
Additional TLV: Any TLVs in a DSO message response that follow the
primary TLV.
Response Primary TLV: The (optional) first TLV in a DSO response.
Response Additional TLV: Any TLVs in a DSO response that follow the
(optional) Response Primary TLV.
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inactivity timer: the time since the most recent non-keepalive DNS
message was sent or received. (see Section 6.4)
keepalive timer: the time since the most recent DNS message was sent
or received. (see Section 6.5)
session timeouts: the inactivity timer and the keepalive timer.
inactivity timeout: the maximum value that the inactivity timer can
have before the connection is gracefully closed.
keepalive interval: the maximum value that the keepalive timer can
have before the client is required to send a keepalive. (see
Section 7.1)
resetting a timer: setting the timer value to zero and restarting
the timer.
clearing a timer: setting the timer value to zero but not restarting
the timer.
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4. Applicability
DNS Stateful Operations are applicable to several known use cases and
are only applicable on transports that are capable of supporting a
DSO Session.
4.1. Use Cases
There are several use cases for DNS Stateful operations that can be
described here.
4.1.1. Session Management
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-tcp-
keepalive EDNS0 Option [RFC7828]. The simple set of TLVs defined in
this document is sufficient to greatly enhance connection management
for this use case.
4.1.2. Long-lived Subscriptions
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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4.2. Applicable Transports
DNS Stateful Operations are applicable in cases where it is useful to
maintain an open session between a DNS client and server, where the
transport allows such a session to be maintained, and where the
transport guarantees in-order delivery of messages, on which DSO
depends. Examples of transports that can support DNS Stateful
Operations are DNS-over-TCP [RFC1035] [RFC7766] and DNS-over-TLS
[RFC7858].
Note that in the case of DNS over TLS, there is no mechanism for
upgrading from DNS-over-TCP to DNS-over-TLS mid-connection (see
[RFC7858] section 7). A connection is either DNS-over-TCP from the
start, or DNS-over-TLS from the start.
DNS Stateful Operations are not applicable for transports that cannot
support clean session semantics, or that do not guarantee in-order
delivery. While in principle such a transport could be constructed
over UDP, the current DNS specification over UDP transport [RFC1035]
does not provide in-order delivery or session semantics, and hence
cannot be used. Similarly, DNS-over-HTTP
[I-D.ietf-doh-dns-over-https] cannot be used because HTTP has its own
mechanism for managing sessions, and this is incompatible with the
mechanism specified here.
No other transports are currently defined for use with DNS Stateful
Operations. Such transports can be added in the future, if they meet
the requirements set out in the first paragraph of this section.
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5. Protocol Details
The overall flow of DNS Stateful Operations goes through a series of
phases:
Connection Establishment: A client establishes a connection to a
server. (Section 4.2)
Connected but sessionless: A connection exists, but a DSO session
has not been established. DNS messages can be sent from the
client to server, and DNS responses can be sent from servers to
clients. In this state a client that wishes to use DSO can
attempt to establish a DSO session (Section 5.1). Standard DNS-
over-TCP inactivity timeout handling is in effect [RFC7766] (see
Section 7.1.2).
DSO Session Establishment in Progress: A client has sent a DSO
request, but has not yet received a DSO response. In this phase,
the client may send more DSO requests and more DNS requests, but
MUST NOT send DSO unidirectional messages (Section 5.1).
DSO Session Establishment Failed: The attempt to establish the DSO
session did not succeed. At this point, the client is permitted
to continue operating without a DSO session (Connected but
Sessionless) but does not send further DSO messages (Section 5.1).
DSO Session Established: Both client and server may send DSO
messages and DNS messages; both may send replies in response to
messages they receive (Section 5.2). The inactivity timer
(Section 6.4) is active; the keepalive timer (Section 6.5) is
active. Standard DNS-over-TCP inactivity timeout handling is no
longer in effect [RFC7766] (see Section 7.1.2).
Server Shutdown: The server has decided to gracefully terminate the
session, and has sent the client a Retry Delay message
(Section 6.6.1). There may still be unprocessed messages from the
client; the server will ignore these. The server will not send
any further messages to the client (Section 6.6.1.1).
Client Shutdown: The client has decided to disconnect, either
because it no longer needs service, the connection is inactive
(Section 6.4.1), or because the server sent it a Retry Delay
message (Section 6.6.1). The client closes the connection
gracefully Section 5.3.
Reconnect: The client disconnected as a result of a server shutdown.
The client either waits for the server-specified Retry Delay to
expire (Section 6.6.3), or else contacts a different server
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instance. If the client no longer needs service, it does not
reconnect.
Forcibly Abort: The client or server detected a protocol error, and
further communication would have undefined behavior. The client
or server forcibly aborts the connection (Section 5.3).
Abort Reconnect Wait: The client has forcibly aborted the
connection, but still needs service. Or, the server forcibly
aborted the connection, but the client still needs service. The
client either connects to a different service instance
(Section 9.1) or waits to reconnect (Section 6.6.3.1).
5.1. DSO Session Establishment
In order for a session to be established between a client and a
server, the client must first establish a connection to the server,
using an applicable transport (see Section 4).
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. In this
case, the session is established as soon as the connection is
established; this is referred to as implicit session establishment.
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 unidirectional 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. This is referred to as explicit session
establishment.
Until a DSO session has been implicitly or explicitly established, a
client MUST NOT initiate DSO unidirectional messages.
A DSO Session is established over a connection by the client sending
a DSO request message, such as a DSO Keepalive request message
(Section 7.1), and receiving a response, with matching MESSAGE ID,
and RCODE set to NOERROR (0), indicating that the DSO request was
successful.
Some DSO messages are permitted as early data (Section 11.1). Others
are not. Unidirectional messages are never permitted as early data
unless an implicit session exists.
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If a server receives a DSO message in early data whose primary TLV is
not permitted to appear in early data, the server MUST forcibly abort
the connection. If a client receives a DSO message in early data,
and there is no implicit DSO session, the client MUST forcibly abort
the connection. This can only be enforced on TLS connections;
therefore, servers MUST NOT enable TFO when listening for a
connection that does not require TLS.
5.1.1. Session Establishment Failure
If the response RCODE is set to NOTIMP (4), or in practise any value
other than NOERROR (0) or DSOTYPENI (defined below), 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 MUST NOT issue further DSO messages on
that connection.
If the RCODE in the response is set to DSOTYPENI ("DSO-TYPE Not
Implemented", [TBA2] 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. 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.
Two other possibilities exist: the server might drop the connection,
or the server might send no response to the DSO message.
In the first case, the client SHOULD mark that service instance as
not supporting DSO, and not attempt a DSO connection for some period
of time (at least an hour) after the failed attempt. The client MAY
reconnect but not use DSO, if appropriate (Section 6.6.3.2).
In the second case, the client SHOULD wait 30 seconds, after which
time the server will be assumed not to support DSO. If the server
doesn't respond within 30 seconds, the client MUST forcibly abort the
connection to the server, since the server's behavior is out of spec,
and hence its state is undefined. The client MAY reconnect, but not
use DSO, if appropriate (Section 6.6.3.1).
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5.1.2. Session Establishment Success
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.
5.2. Operations After Session Establishment
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].
Because a server that supports DNS Stateful Operations MUST return an
RCODE of NOERROR when it receives a Keepalive TLV DSO request
message, the Keepalive TLV is an ideal candidate for use in
establishing a DSO session. Any other option that can only succeed
when sent to a server of the desired kind is also a good candidate
for use in establishing a DSO session. For clients that implement
only the DSO-TYPEs defined in this base specification, sending a
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 they MAY elect to always send an
initial DSO Keepalive request message as their way of initiating a
DSO Session. A future definition of a new response-requiring DSO-
TYPE gives implementers the option of using that new DSO-TYPE if they
wish, but does not change the fact that sending a Keepalive TLV
remains a valid way of initiating a DSO Session.
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5.3. Session Termination
A "DSO Session" is terminated when the underlying connection is
closed. Sessions are "closed gracefully" as a result of the server
closing a session because it is overloaded, the client closing the
session because it is done, or the client closing the session because
it is inactive. Sessions are "forcibly aborted" when either the
client or server closes the connection because of a protocol error.
o 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.
o Where this specification says, "forcibly abort," that means
sending a TCP RST, or the equivalent for other protocols. In the
BSD Sockets API this is achieved by setting the SO_LINGER option
to zero before closing the socket.
5.3.1. Handling Protocol Errors
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 compliant DSO implementation. 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".
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5.4. Message Format
A DSO message begins with the standard twelve-byte DNS message header
[RFC1035] with the OPCODE field set to the DSO OPCODE. 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.
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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5.4.1. DNS Header Fields in DSO Messages
In a DSO unidirectional message the MESSAGE ID field MUST be set to
zero. In a DSO 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 DSO request message 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.ietf-dnssd-mdns-relay].
Whether a message is a DSO request message or a DSO unidirectional
message is determined only by the specification for the Primary TLV.
An acknowledgment cannot be requested by including a nonzero message
ID in a message that is required according to its primary TLV to be
unidirectional. Nor can an acknowledgment be prevented by sending a
message ID of zero in a message that is required to be a DSO request
message according to its primary TLV. A responder that receives
either such malformed message MUST treat it as a fatal error and
forcibly abort the connection immediately.
In a DSO request message or DSO unidirectional message the DNS Header
QR bit MUST be zero (QR=0). If the QR bit is not zero the message is
not a DSO request or DSO unidirectional message.
In a DSO 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 DSO response message (QR=1) the MESSAGE ID field MUST contain a
copy of the value of the MESSAGE ID field in the DSO request message
being responded to. In a DSO response message (QR=1) the MESSAGE ID
field MUST NOT be zero. If a DSO 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.
The Z bits are currently unused in DSO messages, and in both DSO
request messages and DSO responses the Z bits MUST be set to zero (0)
on transmission and MUST be ignored on reception.
In a DSO request message (QR=0) the RCODE is set according to the
definition of the request. For example, in a Retry Delay message
(Section 6.6.1) the RCODE indicates the reason for termination.
However, in most cases, except where clearly specified otherwise, in
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a DSO 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 DSO request message |
| | | due to a problem with the server |
| | | |
| 4 | NOTIMP | DSO not supported |
| | | |
| 5 | REFUSED | Operation declined for policy reasons |
| | | |
| [TBA2] | DSOTYPENI | Primary TLV's DSO-Type is not implemented |
| 11 | | |
+--------+-----------+----------------------------------------------+
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 response codes not
defined here, then that document MUST specify the specific
interpretation of those RCODE values in the context of that new DSO
TLV.
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5.4.2. 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 in Section 5.4.3.
A DSO request message or DSO unidirectional message MUST contain at
least one TLV. The first TLV in a DSO request message or DSO
unidirectional message is referred to as the "Primary TLV" and
determines the nature of the operation being performed, including
whether it is a DSO request or a DSO unidirectional operation. In
some cases it may be appropriate to include other TLVs in a DSO
request message or DSO unidirectional message, such as the Encryption
Padding TLV (Section 7.3), and these extra TLVs are referred to as
the "Additional TLVs" and are not limited to what is defined in this
document. New "Additional TLVs" may be defined in the future and
those definitions will describe when their use is appropriate.
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. This includes "Primary TLVs" and "Additional TLVs"
defined in this document as well as in future TLV definitions. It
may be permissible for an additional TLV to appear in a response to a
primary TLV even though the specification of that primary TLV does
not specify it explicitly. See Section 8.2 for more information.
A DSO response message may contain one or more TLVs with the Primary
TLV DSO-TYPE the same as the Primary TLV from the corresponding DSO
request message or it may contain zero or more Additional TLVs only.
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
DSO request messages, which generate corresponding DSO responses. In
some specialized high-traffic use cases, it may be appropriate to
specify DSO unidirectional messages. DSO unidirectional messages can
be more efficient on the network, because they don't generate a
stream of corresponding reply messages. Using DSO unidirectional
messages can also simplify software in some cases, by removing need
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for an initiator to maintain state while it waits to receive replies
it doesn't care about. When the specification for a particular TLV
states that, when used as a Primary TLV (i.e., first) in an outgoing
DSO request message (i.e., QR=0), that message is to be
unidirectional, the MESSAGE ID field MUST be set to zero and the
receiver MUST NOT generate any response message corresponding to this
DSO unidirectional message.
The previous point, that the receiver MUST NOT generate responses to
DSO unidirectional 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.
DSO unidirectional 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. DSO unidirectional 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 DSO unidirectional 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 a Discovery Relay client has subscribed to receive
inbound mDNS (multicast DNS, [RFC6762]) traffic from a Discovery
Relay, the subsequent stream of received packets is then sent using
DSO unidirectional messages, and this is appropriate because the
client initiated the message stream by virtue of its Discovery Relay
link subscription, thereby indicating its support of Discovery Relay,
and its desire to receive inbound mDNS packets over that DSO session
[I-D.ietf-dnssd-mdns-relay].
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5.4.3. TLV Syntax
All TLVs, whether used as "Primary", "Additional", "Response
Primary", or "Response Additional", use the same encoding syntax.
Specifications that define new TLVs must specify whether the DSO-TYPE
can be used as the Primary TLV, used as an Additional TLV, or used in
either context, both in the case of requests and of responses. The
specification for a TLV must also state whether, when used as the
Primary (i.e., first) TLV in a DSO message (i.e., QR=0), that DSO
message is unidirectional or is a request message which requires a
response. If the DSO message requires a response, the specification
must also state 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 specified 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-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| |
/ DSO-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-LENGTH: A 16-bit unsigned integer, in network (big endian) byte
order, giving the size in bytes of the DSO-DATA.
DSO-DATA: Type-code specific format. The generic DSO machinery
treats the DSO-DATA as an opaque "blob" without attempting to
interpret it. Interpretation of the meaning of the DSO-DATA for a
particular DSO-TYPE is the responsibility of the software that
implements that DSO-TYPE.
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5.4.3.1. Request TLVs
The first TLV in a DSO request message or DSO unidirectional message
is the "Primary TLV" and indicates the operation to be performed. A
DSO request message or DSO unidirectional message MUST contain at at
least one TLV-the Primary TLV.
Immediately following the Primary TLV, a DSO request message or DSO
unidirectional message MAY contain one or more "Additional TLVs",
which specify additional parameters relating to the operation.
5.4.3.2. Response TLVs
Depending on the operation, a DSO response message MAY contain no
TLVs, because it is simply a response to a previous DSO request
message, and the MESSAGE ID in the header is sufficient to identify
the DSO 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 DSO
request message 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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5.4.3.3. 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. The error response MUST
NOT contain a copy of the unrecognized Primary TLV.
If DSO unidirectional 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 DSO unidirectional 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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5.4.4. EDNS(0) and TSIG
Since the ARCOUNT field MUST be zero, a DSO message cannot contain a
valid 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 7.3 MUST be used.
A DSO message can't contain a TSIG record, because a TSIG record is
included in the additional section of the message, which would mean
that ARCOUNT would be greater than zero. DSO messages are required
to have an ARCOUNT of zero. Therefore, if use of signatures with DSO
messages becomes necessary in the future, a new DSO TLV would have 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-tcp-keepalive
EDNS0 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-tcp-keepalive EDNS0 Option this is a fatal error and the
recipient of the defective message MUST forcibly abort the connection
immediately.
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5.5. Message Handling
As described above in Section 5.4.1, whether an outgoing DSO message
with the QR bit in the DNS header set to zero is a DSO request or DSO
unidirectional message 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.
Every DSO message with the QR bit in the DNS header set to zero and a
nonzero MESSAGE ID field is a DSO request message, and MUST elicit a
corresponding response, with the QR bit in the DNS header set to one
and the MESSAGE ID field set to the value given in the corresponding
DSO request message.
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.
Every DSO message with both the QR bit in the DNS header and the
MESSAGE ID field set to zero is a DSO unidirectional message, and
MUST NOT elicit a response.
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5.5.1. Delayed Acknowledgement Management
Generally, most good TCP implementations employ a delayed
acknowledgement timer to provide more efficient use of the network
and better performance.
With a bidirectional exchange over TCP, as for example with a DSO
request message, the operating system TCP implementation waits for
the application-layer client software to generate the corresponding
DSO response message. It can then send a single combined packet
containing the TCP acknowledgement, the TCP window update, and the
application-generated DSO response message. This is more efficient
than sending three separate packets, as would occur if the TCP packet
containing the DSO request were acknowledged immediately.
With a DSO unidirectional message or DSO response message, there is
no corresponding application-generated DSO response message, and
consequently, no hint to the transport protocol about when it should
send its acknowledgement and window update.
Some networking APIs provide a mechanism that allows the application-
layer client software to signal to the transport protocol that no
response will be forthcoming (in effect it can be thought of as a
zero-length "empty" write). Where available in the networking API
being used, the recipient of a DSO unidirectional message or DSO
response message, having parsed and interpreted the message, SHOULD
then use this mechanism provided by the networking API to signal that
no response for this message will be forthcoming, so that the TCP
implementation can go ahead and send its acknowledgement and window
update without further delay. See Section 9.5 for further discussion
of why this is important.
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5.5.2. MESSAGE ID Namespaces
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 DSO 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 5.4.1, an initiator MUST NOT reuse a
MESSAGE ID that it already has in use for an outstanding DSO request
message (unless specified otherwise by the relevant specification for
the DSO-TYPE in question). At the very least, this means that a
MESSAGE ID can't be reused in a particular direction on a particular
DSO Session while the initiator is waiting for a response to a
previous DSO request message 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 can't 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.
If a responder receives a DSO request message (QR=0) where the
MESSAGE ID is not zero, and the responder tracks request MESSAGE IDs,
and the MESSAGE ID matches the MESSAGE ID of a DSO request message it
received for which a response has not yet been sent, it MUST forcibly
abort the connection immediately. This behavior is required to
prevent a hypothetical attack that takes advantage of undefined
behavior in this case. However, if the responder does not track
MESSAGE IDs in this way, no such risk exists, so tracking MESSAGE IDs
just to implement this sanity check is not required.
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5.5.3. Error Responses
When a DSO unidirectional message type is received (MESSAGE ID field
is zero), the receiver should already be expecting this DSO message
type. Section 5.4.3.3 describes the handling of unknown DSO message
types. Parsing errors MUST also result in the receiver forcibly
aborting the connection. When a DSO unidirectional message of an
unexpected type is received, the receiver SHOULD forcibly abort the
connection. Whether the connection should be forcibly aborted for
other internal errors processing the DSO unidirectional message is
implementation dependent, according to the severity of the error.
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 6.6.1.
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 6.6.1)
then the client SHOULD continue to use that DSO Session for
subsequent operations.
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5.6. 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.ietf-dnssd-mdns-relay].
Long-lived operations, if successful, will remain 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 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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6. DSO Session Lifecycle and Timers
6.1. DSO Session Initiation
A DSO Session begins as described in Section 5.1.
The client may perform as many DNS operations as it wishes using the
newly created DSO Session. When the client has multiple messages to
send, it SHOULD NOT wait for each response before sending the next
message.
The server MUST act on messages in the order they are received, but
SHOULD NOT delay sending responses to those messages as they become
available in order to return them in the order the requests were
received.
Section 6.2.1.1 of the DNS-over-TCP specification [RFC7766] specifies
this in more detail.
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6.2. 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 Keepalive TLV (Section 7.1).
The first timeout value, the inactivity timeout, is the maximum time
for which a client may speculatively keep an inactive 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 DSO keepalive traffic to maintain that
session. An example of this would be a client that has subscribed to
DNS Push notifications: in this case, the client is not sending any
traffic to the server, but the session is not inactive, because there
is a active request to the server to receive push notifications.
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 DSO 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 long-lived, low-
traffic, operation that the keepalive interval comes into play, to
ensure that a sufficient residual amount of traffic is generated to
maintain NAT and firewall state and to assure client and server that
they still have connectivity to each other.
On a new DSO Session, if no explicit DSO Keepalive message exchange
has taken place, the default value for both timeouts is 15 seconds.
For both timeouts, lower values of the timeout result in higher
network traffic, and higher CPU load on the server.
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6.3. Inactive DSO Sessions
At both servers and clients, the generation or reception of any
complete DNS message (including DNS requests, responses, updates, DSO
messages, etc.) resets both timers for that DSO Session, with the one
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.ietf-dnssd-mdns-relay]), an operation is considered
in progress for as long as the operation is active, i.e. 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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6.4. The Inactivity Timeout
The purpose of the inactivity timeout is for the server to balance
the 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.
6.4.1. 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 MUST consider the client delinquent, and MUST
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 7.1.
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6.4.2. 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 forcibly 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 DSO Keepalive message dictating new Session Timeout
values to the client.
The largest *finite* inactivity timeout supported by the current
Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
days).
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6.5. 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.
6.5.1. 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 7.1). 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 DSO 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.
6.5.2. Values for the Keepalive Interval
For the keepalive interval value, lower values result in a higher
volume of DSO 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 DSO 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 DSO 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 DSO
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]
[RFC7857].
Note that the lower the keepalive interval value, the higher the load
on client and server. Moreover for a keep-alive value that is
smaller than the time needed for the transport to retransmit, a
single packet loss would cause a server to overzealously abort the
connect. For example, a (hypothetical and unrealistic) keepalive
interval value of 100 ms would result in a continuous stream of ten
messages per second or more (if allowed by the current congestion
control window), in both directions, to keep the DSO Session alive.
And, in this extreme example, a single retransmission over a path
with, e.g., 100ms RTT would introduce a momentary pause in the stream
of messages, long enough to cause the server to abort the connection.
Because of this concern, the server MUST NOT send a DSO 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 DSO 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 DSO keepalive traffic.
Note that after signaling that the client should generate no DSO
keepalive traffic in this way, at any point the server may revise
that DSO keepalive traffic requirement by sending a new DSO Keepalive
message dictating new Session Timeout values to the client.
The largest *finite* keepalive interval supported by the current
Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
days).
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6.6. Server-Initiated Session Termination
In addition to cancelling individual long-lived operations
selectively (Section 5.6) 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.
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. A server only
causes a DSO Session to be ended in the exceptional circumstances
outlined below. 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, causing the underlying operating
system to send a TCP RST).
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
DSO 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 6.2).
o The client sends a grossly invalid or malformed request that is
indicative of a seriously defective client implementation.
o The server is over capacity and needs to shed some load.
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6.6.1. 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. The format of the Retry
Delay TLV, and the interpretations of the various RCODE values, are
described in Section 7.2. After sending a Retry Delay message, the
server MUST NOT send any further messages on that DSO Session.
The server MAY randomize retry delays in situations where many retry
delays are sent in quick succession, so as to avoid all the clients
attempting to reconnect at once. In general, implementations should
avoid using the Retry Delay message in a way that would result in
many clients reconnecting at the same time, if every client attempts
to reconnect at the exact time specified.
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.
6.6.1.1. 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
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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).
6.6.2. Misbehaving Clients
A server may determine that a client is not following the protocol
correctly. There may be no way for the server to recover the
session, in which case the server forcibly terminates the connection.
Since the client doesn't know why the connection dropped, it may
reconnect immediately. If the server has determined that a client is
not following the protocol correctly, it may terminate the DSO
session as soon as it is established, specifying a long retry-delay
to prevent the client from immediately reconnecting.
6.6.3. 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
service instance, or to another suitable service instance, if more
than one is available. If reconnecting to the same service instance,
the client MUST respect the indicated delay, if available, before
attempting to reconnect. Clients should not attempt to randomize the
delay; the server will randomly jitter the retry delay values it
sends to each client if this behavior is desired.
If the service instance will only be out of service for a short
maintenance period, it should use a value a little longer that the
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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 service instance does not want a client to reconnect
ever (perhaps the service 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 service 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
service instance the client should use.
6.6.3.1. Reconnecting After a Forcible Abort
If a connection was forcibly aborted by the client, the client SHOULD
mark that service instance as not supporting DSO. The client MAY
reconnect but not attempt to use DSO, or may connect to a different
service instance, if applicable.
6.6.3.2. Reconnecting After an Unexplained Connection Drop
It is also possible for a server to forcibly terminate the
connection; in this case the client doesn't know whether the
termination was the result of a protocol error or a network outage.
When the client notices that the connection has been dropped, it can
attempt to reconnect immediately. However, if the connection is
dropped again without the client being able to successfully do
whatever it is trying to do, it should mark the server as not
supporting DSO.
6.6.3.3. Probing for Working DSO Support
Once a server has been marked by the client as not supporting DSO,
the client SHOULD NOT attempt DSO operations on that server until
some time has elapsed. A reasonable minimum would be an hour. Since
forcibly aborted connections are the result of a software failure,
it's not likely that the problem will be solved in the first hour
after it's first encountered. However, by restricting the retry
interval to an hour, the client will be able to notice when the
problem has been fixed without placing an undue burden on the server.
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7. Base TLVs for DNS Stateful Operations
This section describes the three base TLVs for DNS Stateful
Operations: Keepalive, Retry Delay, and Encryption Padding.
7.1. Keepalive TLV
The Keepalive TLV (DSO-TYPE=1) performs two functions. Primarily it
establishes the values for the Session Timeouts. Incidentally, it
also resets the keepalive timer for the DSO Session, meaning that it
can be used as a kind of "no-op" message for the purpose of keeping a
session alive. The client will request the desired session timeout
values and the server will acknowledge with the response values that
it requires the client to use.
DSO messages with the Keepalive TLV as the primary TLV may appear in
early data.
The DSO-DATA for 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 DSO keepalive traffic to
maintain connection state. The keepalive interval MUST NOT be
less than ten seconds. If the client does not generate the
mandated DSO 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 DSO keepalive traffic is twenty seconds.
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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 DSO Keepalive messages are sent for the sole purpose of
keeping a DSO Session alive, when that DSO Session has current or
recent non-maintenance activity that warrants keeping that DSO
Session alive. Sending DSO 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 DSO
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 DSO Keepalive message does not reset the
inactivity timer.
When sent by a client, the DSO Keepalive request message MUST be sent
as an DSO request message, with a nonzero MESSAGE ID. If a server
receives a DSO Keepalive message with a zero MESSAGE ID then this is
a fatal error and the server MUST forcibly abort the connection
immediately. The DSO Keepalive request message resets a DSO
Session's keepalive timer, and at the same time communicates to the
server the client's requested Session Timeout values. In a server
response to a client-initiated DSO 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 DSO Keepalive
request messages 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 5.1) a DSO Keepalive
message MAY be initiated by a server. When sent by a server, the DSO
Keepalive message MUST be sent as a DSO unidirectional 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
DSO Keepalive request message with a nonzero MESSAGE ID then this is
a fatal error and the client MUST forcibly abort the connection
immediately. The DSO Keepalive unidirectional message from the
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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 to this unilateral declaration is required or allowed.
In DSO Keepalive response messages, the Keepalive TLV is REQUIRED and
is used only as a Response Primary TLV sent as a reply to a DSO
Keepalive request message from the client. A Keepalive TLV MUST NOT
be added to other responses as a Response Additional TLV. If the
server wishes to update a client's Session Timeout values other than
in response to a DSO Keepalive request message from the client, then
it does so by sending an DSO Keepalive unidirectional 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
DSO Keepalive request message is the only way for a client to
initiate a DSO Session.
7.1.1. 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 Keepalive TLV
from the server may each be higher, lower, or the same as the
respective Session Timeout values the client previously had for this
DSO Session.
In the case of the keepalive timer, the handling of the received
value is straightforward. The act of receiving the message
containing the DSO Keepalive TLV itself resets the keepalive timer,
and updates the keepalive interval for the DSO Session. The new
keepalive interval indicates the maximum time that may elapse before
another message must be sent or received on this DSO Session, if the
DSO Session is to remain alive.
In the case of the inactivity timeout, the handling of the received
value is a little more subtle, though the meaning of the inactivity
timeout 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 Keepalive TLV does
not itself reset the inactivity timer. The time elapsed since the
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last useful activity on this DSO Session is unaffected by exchange of
DSO Keepalive messages. The new inactivity timeout value in the
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.
7.1.2. Relationship to edns-tcp-keepalive EDNS0 Option
The inactivity timeout value in the Keepalive TLV (DSO-TYPE=1) has
similar intent to the edns-tcp-keepalive EDNS0 Option [RFC7828]. A
client/server pair that supports DSO MUST NOT use the edns-tcp-
keepalive EDNS0 Option within any message after a DSO Session has
been established. A client that has sent a DSO message to establish
a session MUST NOT send an edns-tcp-keepalive EDNS0 Option from this
point on. Once a DSO Session has been established, if either client
or server receives a DNS message over the DSO Session that contains
an edns-tcp-keepalive EDNS0 Option, this is a fatal error and the
receiver of the edns-tcp-keepalive EDNS0 Option MUST forcibly abort
the connection immediately.
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7.2. Retry Delay TLV
The Retry Delay TLV (DSO-TYPE=2) can be used as a Primary TLV
(unidirectional) in a server-to-client message, or as a Response
Additional TLV in either direction. DSO messages with a Relay Delay
TLV as their primary TLV are not permitted in early data.
The DSO-DATA for 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 6.6.1.
7.2.1. 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 a DSO unidirectional 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 6.6.1.
The RCODE in the message header SHOULD indicate the principal reason
for the termination:
o NOERROR indicates a routine shutdown or restart.
o FORMERR indicates that a client request was too badly malformed
for the session to continue.
o SERVFAIL indicates that the server is overloaded due to resource
exhaustion and needs to shed load.
o REFUSED indicates that the server has been reconfigured, and at
this time it is now unable to perform one or more of the long-
lived client operations that were previously being performed on
this DSO Session.
o NOTAUTH indicates that the server has been reconfigured and at
this time it is now unable to perform one or more of the long-
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lived client operations that were previously being performed on
this DSO Session because it does not have authority over the names
in question (for example, a DNS Push Notification server could be
reconfigured such that is is no longer accepting DNS Push
Notification requests for one or more of the currently subscribed
names).
This document specifies only these RCODE values for the Retry Delay
message. Servers sending Retry Delay messages SHOULD use one of
these values. However, future circumstances may create situations
where other RCODE values are appropriate in Retry Delay messages, so
clients MUST be prepared to accept Retry Delay messages with any
RCODE value.
In some cases, when a server sends a Retry Delay message to a client,
there may be more than one reason for the server wanting to end the
session. Possibly the configuration could have been changed such
that some long-lived client operations can no longer be continued due
to policy (REFUSED), and other long-lived client operations can no
longer be performed due to the server no longer being authoritative
for those names (NOTAUTH). In such cases the server MAY use any of
the applicable RCODE values, or RCODE=NOERROR (routine shutdown or
restart).
Note that the selection of RCODE value in a Retry Delay message is
not critical, since the RCODE value is generally used 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 service 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 or restart).
A Retry Delay message from server to client is a DSO unidirectional
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 message to a server. If a
server receives a DSO message where the Primary TLV is the Retry
Delay TLV, this is a fatal error and the server MUST forcibly abort
the connection immediately.
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7.2.2. Retry Delay TLV used as a Response Additional TLV
In the case of a DSO request message that results in 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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7.3. 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 DSO-DATA for the Padding TLV is optional and is a variable length
field containing non-specified values. A DSO-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
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
/ /
/ PADDING -- 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
message, response, or both. As specified for the EDNS(0) Padding
Option [RFC7830] if a DSO request message is received with an
Encryption Padding TLV, then the DSO 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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8. Summary Highlights
This section summarizes some noteworthy highlights about various
aspects of the DSO protocol.
8.1. 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 Unidirectional 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 | DSO unidirectional Message | DSO Request Message |
+--------+------------------------------+------------------------+
| QR=1 | Invalid - Fatal Error | DSO Response Message |
+--------+------------------------------+------------------------+
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8.2. 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 DSO requests or DSO unidirectional
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 Unidirectional 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 a DSO request message or
DSO unidirectional message from client to server.
o CRP - Response Primary TLV, included in response message sent back
to 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
back to 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: DSO requests or DSO unidirectional messages from server to
client, and the corresponding responses from client back to server.
o S-P - Primary TLV, sent in DSO Request message, from server to
client, with nonzero MESSAGE ID indicating that this request MUST
generate response message.
o S-U - Primary TLV, sent in DSO Unidirectional message, from server
to client, with zero MESSAGE ID indicating that this request MUST
NOT generate response message.
o S-A - Additional TLV, optionally added to a DSO request message or
DSO unidirectional message from server to client.
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o SRP - Response Primary TLV, included in response message sent back
to the server (in response to a server "S-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 SRA - Response Additional TLV, included in response message sent
back to the server (in response to a server "S-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.
+-------------------------+-------------------------+
| C-P C-U C-A CRP CRA | S-P S-U S-A SRP SRA |
+------------+-------------------------+-------------------------+
| KeepAlive | X X | X |
+------------+-------------------------+-------------------------+
| RetryDelay | X | 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
is recommended that definitions of future TLVs include a similar
table summarizing the contexts where the new TLV is valid.
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9. Additional Considerations
9.1. Service Instances
We use the term service instance to refer to software running on a
host which can receive connections on some set of IP address and port
tuples. What makes the software an instance is that regardless of
which of these tuples the client uses to connect to it, the client is
connected to the same software, running on the same node (but see
Section 9.2), and will receive the same answers and the same keying
information.
Service instances are identified from the perspective of the client.
If the client is configured with IP addresses and port number tuples,
it has no way to tell if the service offered at one tuple is the same
server that is listening on a different tuple. So in this case, the
client treats each such tuple as if it references a separate service
instance.
In some cases a client is configured with a hostname and a port
number (either implicitly, where the port number is omitted and
assumed, or explicitly, as in the case of DNS SRV records). In these
cases, the (hostname, port) tuple uniquely identifies the service
instance (hostname comparisons are case-insensitive [RFC1034].
It is possible that two hostnames might point to some common IP
addresses; this is a configuration error which the client is not
obliged to detect. The effect of this could be that after being told
to disconnect, the client might reconnect to the same server because
it is represented as a different service instance.
Implementations SHOULD NOT resolve hostnames and then perform
matching of IP address(es) in order to evaluate whether two entities
should be determined to be the "same service instance".
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9.2. Anycast Considerations
When an anycast service is configured on a particular IP address and
port, it must be the case that although there is more than one
physical server responding on that IP address, each such server can
be treated as equivalent. What we mean by "equivalent" here is that
both servers can provide the same service and, where appropriate, the
same authentication information, such as PKI certificates, when
establishing connections.
If a change in network topology causes packets in a particular TCP
connection to be sent to an anycast server instance that does not
know about the connection, the new server will automatically
terminate the connection with a TCP reset, since it will have no
record of the connection, and then the client can reconnect or stop
using the connection, as appropriate.
If after the connection is re-established, the client's assumption
that it is connected to the same service is violated in some way,
that would be considered to be incorrect behavior in this context.
It is however out of the possible scope for this specification to
make specific recommendations in this regard; that would be up to
follow-on documents that describe specific uses of DNS stateful
operations.
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9.3. 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
service instance (see Section 9.1), the client SHOULD use a single
shared DSO Session for all those operations.
This requirement has two benefits. First, it reduces unnecessary
connection load on the DNS server. Second, it avoids paying the TCP
slow start penalty when making subsequent connections to the same
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.
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9.4. Operational Considerations for Middlebox
Where an application-layer middlebox (e.g., a DNS proxy, forwarder,
or session multiplexer) is in the path, care must be taken to avoid a
configuration in which DSO traffic is mis-handled. The simplest way
to avoid such problems is to avoid using middleboxes. When this is
not possible, middleboxes should be evaluated to make sure that they
behave correctly.
Correct behavior for middleboxes consists of one of:
o The middlebox does not forward DSO messages, and responds to DSO
messages with a response code other than NOERROR or DSOTYPENI.
o The middlebox acts as a DSO server and follows this specification
in establishing connections.
o There is a 1:1 correspondence between incoming and outgoing
connections, such that when a connection is established to the
middlebox, it is guaranteed that exactly one corresponding
connection will be established from the middlebox to some DNS
resolver, and all incoming messages will be forwarded without
modification or reordering. An example of this would be a NAT
forwarder or TCP connection optimizer (e.g. for a high-latency
connection such as a geosynchronous satellite link).
Middleboxes that do not meet one of the above criteria are very
likely to fail in unexpected and difficult-to-diagnose ways. For
example, a DNS load balancer might unbundle DNS messages from the
incoming TCP stream and forward each message from the stream to a
different DNS server. If such a load balancer is in use, and the DNS
servers it points implement DSO and are configured to enable DSO, DSO
session establishment will succeed, but no coherent session will
exist between the client and the server. If such a load balancer is
pointed at a DNS server that does not implement DSO or is configured
not to allow DSO, no such problem will exist, but such a
configuration risks unexpected failure if new server software is
installed which does implement DSO.
It is of course possible to implement a middlebox that properly
supports DSO. It is even possible to implement one that implements
DSO with long-lived operations. This can be done either by
maintaining a 1:1 correspondence between incoming and outgoing
connections, as mentioned above, or by terminating incoming sessions
at the middlebox, but maintaining state in the middlebox about any
long-lived that are requested. Specifying this in detail is beyond
the scope of this document.
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9.5. TCP Delayed Acknowledgement Considerations
Most modern implementations of the Transmission Control Protocol
(TCP) include a feature called "Delayed Acknowledgement" [RFC1122].
Without this feature, TCP can be very wasteful on the network. For
illustration, consider a simple example like remote login, using a
very simple TCP implementation that lacks delayed acks. When the
user types a keystroke, a data packet is sent. When the data packet
arrives at the server, the simple TCP implementation sends an
immediate acknowledgement. Mere milliseconds later, the server
process reads the one byte of keystroke data, and consequently the
simple TCP implementation sends an immediate window update. Mere
milliseconds later, the server process generates the character echo,
and sends this data back in reply. The simple TCP implementation
then sends this data packet immediately too. In this case, this
simple TCP implementation sends a burst of three packets almost
instantaneously (ack, window update, data).
Clearly it would be more efficient if the TCP implementation were to
combine the three separate packets into one, and this is what the
delayed ack feature enables.
With delayed ack, the TCP implementation waits after receiving a data
packet, typically for 200 ms, and then send its ack if (a) more data
packet(s) arrive (b) the receiving process generates some reply data,
or (c) 200 ms elapses without either of the above occurring.
With delayed ack, remote login becomes much more efficient,
generating just one packet instead of three for each character echo.
The logic of delayed ack is that the 200 ms delay cannot do any
significant harm. If something at the other end were waiting for
something, then the receiving process should generate the reply that
the thing at the end is waiting for, and TCP will then immediately
send that reply (and the ack and window update). And if the
receiving process does not in fact generate any reply for this
particular message, then by definition the thing at the other end
cannot be waiting for anything, so the 200 ms delay is harmless.
This assumption may be true, unless the sender is using Nagle's
algorithm, a similar efficiency feature, created to protect the
network from poorly written client software that performs many rapid
small writes in succession. Nagle's algorithm allows these small
writes to be combined into larger, less wasteful packets.
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Unfortunately, Nagle's algorithm and delayed ack, two valuable
efficiency features, can interact badly with each other when used
together [NagleDA].
DSO request messages elicit responses; DSO unidirectional messages
and DSO response messages do not.
For DSO request messages, which do elicit responses, Nagle's
algorithm and delayed ack work as intended.
For DSO messages that do not elicit responses, the delayed ack
mechanism causes the ack to be delayed by 200 ms. The 200 ms delay
on the ack can in turn cause Nagle's algorithm to prevent the sender
from sending any more data for 200 ms until the awaited ack arrives.
On an enterprise GigE backbone with sub-millisecond round-trip times,
a 200 ms delay is enormous in comparison.
When this issues is raised, there are two solutions that are often
offered, neither of them ideal:
1. Disable delayed ack. For DSO messages that elicit no response,
removing delayed ack avoids the needless 200 ms delay, and sends
back an immediate ack, which tells Nagle's algorithm that it
should immediately grant the sender permission to send its next
packet. Unfortunately, for DSO messages that *do* elicit a
response, removing delayed ack removes the efficiency gains of
combining acks with data, and the responder will now send two or
three packets instead of one.
2. Disable Nagle's algorithm. When acks are delayed by the delayed
ack algorithm, removing Nagle's algorithm prevents the sender
from being blocked from sending its next small packet
immediately. Unfortunately, on a network with a higher round-
trip time, removing Nagle's algorithm removes the efficiency
gains of combining multiple small packets into fewer larger ones,
with the goal of limiting the number of small packets in flight
at any one time.
For DSO messages that elicit a response, delayed ack and Nagle's
algorithm do the right thing.
The problem here is that with DSO messages that elicit no response,
the TCP implementation is stuck waiting, unsure if a response is
about to be generated, or whether the TCP implementation should go
ahead and send an ack and window update.
The solution is networking APIs that allow the receiver to inform the
TCP implementation that a received message has been read, processed,
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and no response for this message will be generated. TCP can then
stop waiting for a response that will never come, and immediately go
ahead and send an ack and window update.
For implementations of DSO, disabling delayed ack is NOT RECOMMENDED,
because of the harm this can do to the network.
For implementations of DSO, disabling Nagle's algorithm is NOT
RECOMMENDED, because of the harm this can do to the network.
At the time that this document is being prepared for publication, it
is known that at least one TCP implementation provides the ability
for the recipient of a TCP message to signal that it is not going to
send a response, and hence the delayed ack mechanism can stop
waiting. Implementations on operating systems where this feature is
available SHOULD make use of it.
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10. IANA Considerations
10.1. DSO OPCODE Registration
The IANA is requested to record the value [TBA1] (tentatively 6) for
the DSO OPCODE in the DNS OPCODE Registry. DSO stands for DNS
Stateful Operations.
10.2. DSO RCODE Registration
The IANA is requested to record the value [TBA2] (tentatively 11) for
the DSOTYPENI error code in the DNS RCODE Registry. The DSOTYPENI
error code ("DSO-TYPE Not Implemented") indicates that the receiver
does implement DNS Stateful Operations, but does not implement the
specific DSO-TYPE of the primary TLV in the DSO request message.
10.3. 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 | Early | Status | Reference |
| | | Data | | |
+-----------+------------------------+-------+----------+-----------+
| 0000 | Reserved | NO | Standard | RFC-TBD |
| | | | | |
| 0001 | KeepAlive | OK | Standard | RFC-TBD |
| | | | | |
| 0002 | RetryDelay | NO | Standard | RFC-TBD |
| | | | | |
| 0003 | EncryptionPadding | NA | Standard | RFC-TBD |
| | | | | |
| 0004-003F | Unassigned, reserved | NO | | |
| | for DSO session- | | | |
| | management TLVs | | | |
| | | | | |
| 0040-F7FF | Unassigned | NO | | |
| | | | | |
| F800-FBFF | Experimental/local use | NO | | |
| | | | | |
| FC00-FFFF | Reserved for future | NO | | |
| | expansion | | | |
+-----------+------------------------+-------+----------+-----------+
The meanings of the fields are as follows:
Type: the 16-bit DSO type code
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Name: the human-readable name of the TLV
Early Data: If OK, this TLV may be sent as early data in a TLS 0-RTT
([RFC8446] Section 2.3) initial handshake. If NA, the TLV may
appear as a secondary TLV in a DSO message that is send as early
data.
Status: IETF Document status (or "External" if not documented in an
IETF document.
Reference: A stable reference to the document in which this TLV is
defined.
DSO Type Code zero is reserved and is not currently intended for
allocation.
Registrations of new DSO Type Codes in the "Reserved for DSO session-
management" range 0004-003F and the "Reserved for future expansion"
range FC00-FFFF require publication of an IETF Standards Action
document [RFC8126].
Any document defining a new TLV which lists a value of "OK" in the
0-RTT column must include a threat analysis for the use of the TLV in
the case of TLS 0-RTT. See Section 11.1 for details.
Requests to register additional new DSO Type Codes in the
"Unassigned" range 0040-F7FF are to be recorded by IANA after Expert
Review [RFC8126]. The expert review should validate that the
requested type code is specified in a way that conforms to this
specification, and that the intended use for the code would not be
addressed with an experimental/local assignment.
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.
11. Security Considerations
If this mechanism is to be used with DNS over TLS, then these
messages are subject to the same constraints as any other DNS-over-
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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 unidirectional DSO
Retry Delay Message into the data stream, specifying an unreasonably
large RETRY DELAY, causing a denial-of-service attack against the
client.
The establishment of DSO sessions has an impact on the number of open
TCP connections on a DNS server. Additional resources may be used on
the server as a result. However, because the server can limit the
number of DSO sessions established and can also close existing DSO
sessions as needed, denial of service or resource exhaustion should
not be a concern.
11.1. TLS 0-RTT Considerations
DSO permits zero round-trip operation using TCP Fast Open [RFC7413]
with TLS 1.3 [RFC8446] 0-RTT to reduce or eliminate round trips in
session establishment. TCP Fast Open is only permitted in
combination with TLS 0-RTT. In the rest of this section we refer to
TLS 1.3 early data in a TLS 0-RTT initial handshake message, whether
or not it is included in a TCP SYN packet with early data using the
TCP Fast Open option, as "early data."
A DSO message may or may not be permitted to be sent as early data.
The definition for each TLV that can be used as a primary TLV is
required to state whether or not that TLV is permitted as early data.
Only response-requiring messages are ever permitted as early data,
and only clients are permitted to send any DSO message as early data,
unless there is an implicit session (see Section 5.1).
For DSO messages that are permitted as early data, a client MAY
include one or more such messages as early data without having to
wait for a DSO response to the first DSO request message to confirm
successful establishment of a DSO session.
However, unless there is an implicit session, a client MUST NOT send
DSO unidirectional messages until after a DSO Session has been
mutually established.
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Similarly, unless there is an implicit session, 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 as early data
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.
It would be possible to add a TLV that requires the server to do some
significant work, and send that to the server as initial data in a
TCP SYN packet. A flood of such packets could be used as a DoS
attack on the server. None of the TLVs defined here have this
property.
If a new TLV is specified that does have this property, that TLV must
be specified as not permitted in 0-RTT messages. This prevents work
from being done until a round-trip has occurred from the server to
the client to verify that the source address of the packet is
reachable.
Documents that define new TLVs must state whether each new TLV may be
sent as early data. Such documents must include a threat analysis in
the security considerations section for each TLV defined in the
document that may be sent as early data. This threat analysis should
be done based on the advice given in [RFC8446] Section 2.3, 8 and
Appendix E.5.
12. Acknowledgements
Thanks to Stephane Bortzmeyer, Tim Chown, Ralph Droms, Paul Hoffman,
Jan Komissar, Edward Lewis, Allison Mankin, Rui Paulo, David
Schinazi, Manju Shankar Rao, Bernie Volz and Bob Harold for their
helpful contributions to this document.
13. References
13.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<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>.
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[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>.
[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>.
[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>.
[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>.
13.2. Informative References
[I-D.ietf-dnsop-no-response-issue]
Andrews, M. and R. Bellis, "A Common Operational Problem
in DNS Servers - Failure To Respond.", draft-ietf-dnsop-
no-response-issue-12 (work in progress), November 2018.
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[I-D.ietf-dnssd-mdns-relay]
Lemon, T. and S. Cheshire, "Multicast DNS Discovery
Relay", draft-ietf-dnssd-mdns-relay-01 (work in progress),
July 2018.
[I-D.ietf-dnssd-push]
Pusateri, T. and S. Cheshire, "DNS Push Notifications",
draft-ietf-dnssd-push-16 (work in progress), November
2018.
[I-D.ietf-doh-dns-over-https]
Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", draft-ietf-doh-dns-over-https-14 (work in
progress), August 2018.
[I-D.ietf-dprive-padding-policy]
Mayrhofer, A., "Padding Policy for EDNS(0)", draft-ietf-
dprive-padding-policy-06 (work in progress), July 2018.
[NagleDA] Cheshire, S., "TCP Performance problems caused by
interaction between Nagle's Algorithm and Delayed ACK",
May 2005,
<http://www.stuartcheshire.org/papers/nagledelayedack/>.
[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>.
[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>.
[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>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[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>.
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[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>.
[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>.
[RFC7857] Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar,
S., and K. Naito, "Updates to Network Address Translation
(NAT) Behavioral Requirements", BCP 127, RFC 7857,
DOI 10.17487/RFC7857, April 2016,
<https://www.rfc-editor.org/info/rfc7857>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Authors' Addresses
Ray Bellis
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City CA 94063
USA
Phone: +1 (650) 423-1200
Email: ray@isc.org
Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino CA 95014
USA
Phone: +1 (408) 996-1010
Email: cheshire@apple.com
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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
Nibbhaya Consulting
P.O. Box 958
Brattleboro VT 05302-0958
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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