DNSOP Working Group R. Bellis
Internet-Draft ISC
Intended status: Standards Track S. Cheshire
Expires: February 15, 2017 Apple Inc.
J. Dickinson
S. Dickinson
Sinodun
A. Mankin
Salesforce
T. Pusateri
Unaffiliated
August 14, 2016
DNS Session Signaling
draft-ietf-dnsop-session-signal-00
Abstract
The EDNS(0) Extension Mechanism for DNS is explicitly defined to only
have "per-message" semantics. This document defines a new Session
Signaling Opcode used to carry persistent "per-session" type-length-
values (TLVs), and defines an initial set of TLVs used to manage
session timeouts and termination.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 15, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Session Lifecycle . . . . . . . . . . . . . . . . . . . . 4
3.2. Message Format . . . . . . . . . . . . . . . . . . . . . 5
3.3. Message Handling . . . . . . . . . . . . . . . . . . . . 6
3.4. TLV Format . . . . . . . . . . . . . . . . . . . . . . . 7
4. Mandatory TLVs . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Session Management Support TLVs . . . . . . . . . . . . . 7
4.1.1. "Not Implemented" . . . . . . . . . . . . . . . . . . 8
4.2. Session Management TLVs . . . . . . . . . . . . . . . . . 8
4.2.1. Idle Timeout . . . . . . . . . . . . . . . . . . . . 8
4.2.2. Terminate Session . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. DNS Session Signaling Opcode Registration . . . . . . . . 10
5.2. DNS Session Signaling Type Codes Registry . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The use of transports other than UDP for DNS is being increasingly
specified, for example, DNS-over-TCP [RFC7766], DNS-over-TLS
[RFC7858] and recent work on DNS Push Notifications
[I-D.ietf-dnssd-push]. Such transports frequently use persistent,
long-lived sessions and therefore when using them for transporting
DNS messages it is of benefit to have a mechanism that can establish
parameters associated with those sessions, such as timeouts. In such
situations it is also advantageous to support server initiated
messages.
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The EDNS(0) Extension Mechanism for DNS [RFC6891] is explicitly
defined to only have "per-message" semantics. Whilst EDNS(0) has
been used to signal at least one session related parameter (the EDNS
TCP KeepAlive option [RFC7828]) the result is less than optimal due
to the restrictions imposed by the EDNS(0) semantics and the lack of
server initiated signalling. This document defines a new Session
Signaling Opcode used to carry persistent "per-session" type-length-
values (TLVs), and defines an initial set of TLVs used to manage
session timeouts and termination.
With EDNS(0), multiple options may be packed into a single OPT RR,
and there is no generalized mechanism for a client to be able to tell
whether a server has processed or otherwise acted upon each
individual option within the combined OPT RR. The specifications for
each individual option need to define how each different option is to
be acknowledged, if necessary.
With Session Signaling, in contrast, each Session Signaling operation
is communicated in its own separate DNS message, and the RCODE in the
response indicates the success or failure of that operation.
It should be noted that the message format for Session Signaling
operations (see Section 3.2) differs from the DNS packet format used
for standard queries and responses, in that it has a shorter header
(four octets instead of usual twelve octets).
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
The terms "initiator" and "responder" correspond respectively to the
initial sender and subsequent receiver of a Session Signaling TLV,
regardless of which was the "client" and "server" in the usual DNS
sense.
The term "sender" may apply to either an initiator or responder.
The term "session" in the context of this document means the exchange
of DNS messages using an end-to-end transport protocol where:
o The connection between client and server is persistent and
relatively long-lived (i.e. minutes or hours, rather than
seconds).
o Either end of the connection may initiate messages to the other
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o Messages are delivered in order
3. Protocol Details
Session Signaling messages MUST only be carried in protocols and in
environments where a session may be established according to the
definition above. Standard DNS over TCP [RFC1035], and DNS over TLS
[RFC7858] are suitable protocols. DNS over plain UDP is not
appropriate since it fails on the requirement for in-order message
delivery, and, in the presence of NAT gateways and firewalls with
short UDP timeouts, it fails to provide a persistent bi-directional
communication channel unless an excessive amount of keepalive traffic
is used.
Session Signaling messages relate only to the specific session in
which they are being carried. Where a middle box (e.g. a DNS proxy,
forwarder, or session multiplexer) is in the path the message MUST
NOT be blindly forwarded in either direction by that middle box.
This does not preclude the use of these messages in the presence of a
NAT box that rewrites IP-layer or transport-layer headers but
otherwise maintains the effect of a single session.
A client MAY attempt to initiate Session Signaling messages at any
time on a connection; receiving a NOTIMP response in reply indicates
that the server does not implement Session Signaling, and the client
SHOULD NOT issue further Session Signaling messages on that
connection.
A server SHOULD NOT initiate Session Signaling messages until a
client-initiated Session Signaling message is received first, unless
in an environment where it is known in advance by other means that
both client and server support Session Signaling. This requirement
is to ensure that the clients that do not support Session Signaling
do not receive unsolicited inbound Session Signaling messages that
they would not know how to handle.
3.1. Session Lifecycle
A session begins when a client makes a new connection to a server.
The client may perform as many DNS operations as it wishes on the
newly created connection. Operations SHOULD be pipelined (i.e., the
client doesn't need wait for a reply before sending the next
message). The server MUST act on messages in the order they are
received, but responses to those messages MAY be sent out of order,
if appropriate.
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When a server implementing this specification receives a new
connection from a client, it MUST begin by internally assigning an
initial idle timeout of 30 seconds to that connection. At both
servers and clients, the generation or reception of any complete DNS
message, including DNS requests, responses, updates, or Session
Signaling messages, resets the idle timer for that connection (see
[RFC7766]).
If, at any time during the life of the connection, half the idle-
timeout value (i.e., 15 seconds by default) elapses without any DNS
messages being sent or received on a connection, then the connection
is considered stale and the client MUST take action. When this
happens the client MUST either send at least one new message to reset
the idle timer - such as a Session Signaling Idle Timeout message
(see Section 4.2.1), or any other valid DNS message - or close the
connection.
If, at any time during the life of the connection, the full idle-
timeout value (i.e., 30 seconds by default) elapses without any DNS
messages being sent or received on a connection, then the connection
is considered delinquent and the server SHOULD forcibly terminate the
connection. For sessions over TCP (or over TLS-over-TCP), to avoid
the burden of having a connection in TIME-WAIT state, instead of
closing the connection gracefully with a TCP FIN the server SHOULD
abort the connection with a TCP RST. (In the Sockets API this is
achieved by setting the SO_LINGER option to zero before closing the
socket.)
If the client wishes to keep an idle connection open for longer than
the default duration without having to send traffic every 15 seconds,
then it uses the Session Signaling Idle Timeout message to request a
longer idle timeout (see Section 4.2.1).
A client is not required to wait until half of the idle-timeout value
before closing a connection. A client MAY close a connection at any
time, at the client's discretion, if it has no further need for the
connection at that time.
3.2. Message Format
A Session Signaling message begins with the first 4 octets of the
standard DNS message header [RFC1035], with the Opcode field set to
the Session Signaling Opcode. A Session Signaling message does not
contain the QDCOUNT, ANCOUNT, NSCOUNT and ARCOUNT fields fields used
in standard DNS queries and responses. This 4-octet header is
followed by a single Session Signaling TLV.
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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 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| |
/ TLV-DATA /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
The MESSAGE ID, QR, Opcode and RCODE fields have their usual meanings
[RFC1035].
The Z bits are currently unused, and SHOULD be set to zero (0) in
requests and responses unless re-defined in a later specification.
3.3. Message Handling
On a connection between a client and server that support Session
Signaling, either end may unilaterally send Session Signaling
messages at any point in the lifetime of a session, and therefore
either client or server may be the initiator of a message. The
initiator MUST set the value of the QR bit in the DNS header to zero
(0), and the responder MUST set it to one (1).
<<TODO: Specify behaviour on receipt of a message from an initiator
with the QR bit set to 1, etc.>>
Every Session Signaling request message MUST elicit a response (which
MUST have the same ID in the DNS message header as in the request).
<< RB: should the presence of a SS message create a "sequencing
point", such that all pending responses must be answered? SC: I do
not believe so. We can define an explicit SS "sequencing point"
opcode for this if necessary. >>
The RCODE value in a response uses a subset of the standard (non-
extended) RCODE values from the IANA DNS RCODEs registry, interpreted
as follows:
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+------+----------+---------------------------------+
| Code | Mnemonic | Description |
+------+----------+---------------------------------+
| 0 | NOERROR | TLV processed successfully |
| | | |
| 1 | FORMERR | TLV format error |
| | | |
| 4 | NOTIMP | Session Signaling not supported |
| | | |
| 5 | REFUSED | TLV declined for policy reasons |
+------+----------+---------------------------------+
3.4. TLV Format
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| SSOP-TYPE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| SSOP-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| |
/ SSOP-DATA /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
SSOP-TYPE: A 16 bit field in network order giving the type of the
current Session Signaling TLV per the IANA DNS Session Signaling
Type Codes Registry.
<< SC: I changed SESSION-TYPE to SSOP-TYPE because to me SESSION-TYPE
read as "type of session" which it is not. It is Session Signaling
Operation Type, Session Signaling Operation Length, Session Signaling
Operation Data. >>
SSOP-LENGTH: A 16 bit field in network order giving the size in
octets of SSOP-DATA.
SSOP-DATA: Type-code specific.
4. Mandatory TLVs
4.1. Session Management Support TLVs
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4.1.1. "Not Implemented"
Since the "NOTIMP" RCODE in the DNS message header is used to
indicate lack of support for the Session Signaling Opcode itself, a
different mechanism is used to indicate lack of support of a specific
SSOP-TYPE. If a server that supports Session Signaling receives a
Session Signaling query message (QR bit zero) with an SSOP-TYPE it
does not support, it returns a response message (QR bit one)
containing a single Session Signaling SSOP-NOTIMP TLV (0). The
MESSAGE ID in the message header serves to tell the client which of
its requests was not understood.
The SSOP-NOTIMP TLV has no SSOP-DATA.
4.2. Session Management TLVs
4.2.1. Idle Timeout
The Idle Timeout TLV (1) is be used by a client to reset a
connection's idle timer, and at the same time to request what the
idle timeout should be from this point forward in the connection.
The Idle Timeout TLV also MAY be initiated by a server, to
unilaterally inform the client of a new idle timeout this point
forward in this connection.
It is not required that the Idle Timeout TLV be used in every
session. While many Session Signaling operations (such as DNS Push
Notifications [I-D.ietf-dnssd-push]) will be used in conjunction with
a long-lived connection, this is not required, and in some cases the
default 30-second timeout may be perfectly appropriate.
The SSOP-DATA for the the Idle Timeout TLV is as follows:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| IDLE TIMEOUT |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
IDLE TIMEOUT: the idle timeout for the current session, specified as
a 16 bit word in network order in units of 100 milliseconds. This
is the timeout at which the server will forcibly terminate the
connection with a TCP RST (or equivalent for other protocols);
after half this interval the client MUST take action to either
preserve the connection, or close it if it is no longer needed.
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In a client-initiated Session Signaling Idle Timeout message, the
IDLE TIMEOUT contains the client's requested value for the idle
timeout.
In a server response to a client-initiated message, the IDLE TIMEOUT
contains the server's chosen value for the idle timeout, which the
client MUST respect. This is modeled after the DHCP protocol, where
the client requests a certain lease lifetime, but the server is the
ultimate authority for deciding what lease lifetime is actually
granted.
In a server-initiated Session Signaling Idle Timeout message, the
IDLE TIMEOUT unilaterally informs the client of the new idle timeout
this point forward in this connection.
In a client response to a server-initiated message, there is no SSOP-
DATA. SSOP-LENGTH is zero.
<<TODO: Specify the behaviour when a server sends a 0 IDLE TIMEOUT.>>
The Idle Timeout TLV (3) has similar intent to the EDNS TCP Keepalive
Option [RFC7828]. A client/server pair that supports Session
Signaling MUST NOT use the EDNS TCP KeepAlive option within any
message once bi-directional Session Signaling support has been
confirmed.
<< SC: And if client or server does use EDNS TCP Keepalive, then
other end should... close connection? Silently ignore? >>
4.2.2. Terminate Session
There may be rare cases where a server is overloaded and wishes to
shed load. If the server simply closes connections, the likely
behaviour of clients is to detect this as a network failure, and
reconnect.
To avoid this reconnection implosion, the server sends a Terminate
Session TLV (2) to inform the client of the overload situation.
After sending a Terminate Session TLV the server MUST NOT send any
further messages on the connection.
The Terminate Session TLV (2) MUST NOT be initiated by a client.
<<TODO: Specify behaviour if the Terminate Session TLV is initiated
by a client.>>
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Upon receipt of the Terminate Session TLV (2) the client MUST make
note of the reconnect delay for this server, and then immediately
close the connection.
The SSOP-DATA is as follows:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| RECONNECT DELAY |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
RECONNECT DELAY: a time value, specified as a 16 bit word in network
order in units of 100 milliseconds, within which the client MUST
NOT establish a new session to the current server.
The RECOMMENDED value is 10 seconds. << RB: text required here about
default values for load balancers, etc >>
5. IANA Considerations
5.1. DNS Session Signaling Opcode Registration
IANA are directed to assign the value TBD for the Session Signaling
Opcode in the DNS OpCodes Registry.
5.2. DNS Session Signaling Type Codes Registry
IANA are directed to create the DNS Session Signaling Type Codes
Registry, with initial values as follows:
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+-----------+--------------------------------+----------+-----------+
| Type | Name | Status | Reference |
+-----------+--------------------------------+----------+-----------+
| 0 | SSOP-NOTIMP | Standard | RFC-TBD1 |
| | | | |
| 1 | SSOP-Keepalive | Standard | RFC-TBD1 |
| | | | |
| 2 | SSOP-Terminate | Standard | RFC-TBD1 |
| | | | |
| 3 - 63 | Unassigned, reserved for | | |
| | session management TLVs | | |
| | | | |
| 64 - | Unassigned | | |
| 63487 | | | |
| | | | |
| 63488 - | Reserved for local / | | |
| 64511 | experimental use | | |
| | | | |
| 64512 - | Reserved for future expansion | | |
| 65535 | | | |
+-----------+--------------------------------+----------+-----------+
Registration of additional Session Signaling Type Codes requires
Expert Review. << RB: definition of process required? >>
6. Security Considerations
If this mechanism is to be used with DNS over TLS, then these
messages are subject to the same constraints as any other DNS over
TLS messages and MUST NOT be sent in the clear before the TLS session
is established.
7. Acknowledgements
TBW
8. References
8.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<http://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,
<http://www.rfc-editor.org/info/rfc7766>.
[RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", RFC 7828,
DOI 10.17487/RFC7828, April 2016,
<http://www.rfc-editor.org/info/rfc7828>.
8.2. Informative References
[I-D.ietf-dnssd-push]
Pusateri, T. and S. Cheshire, "DNS Push Notifications",
draft-ietf-dnssd-push-08 (work in progress), July 2016.
[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, <http://www.rfc-editor.org/info/rfc7858>.
Authors' Addresses
Ray Bellis
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City CA 94063
USA
Phone: +1 650 423 1200
Email: ray@isc.org
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino CA 95014
USA
Phone: +1 408 974 3207
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
Allison Mankin
Salesforce
Email: allison.mankin@gmail.com
Tom Pusateri
Unaffiliated
Email: pusateri@bangj.com
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