Internet Engineering Task Force (IETF) T. Pauly
Request for Comments: 9149 Apple Inc.
Category: Standards Track D. Schinazi
ISSN: 2070-1721 Google LLC
C.A. Wood
Cloudflare
April 2022
TLS Ticket Requests
Abstract
TLS session tickets enable stateless connection resumption for
clients without server-side, per-client state. Servers vend an
arbitrary number of session tickets to clients, at their discretion,
upon connection establishment. Clients store and use tickets when
resuming future connections. This document describes a mechanism by
which clients can specify the desired number of tickets needed for
future connections. This extension aims to provide a means for
servers to determine the number of tickets to generate in order to
reduce ticket waste while simultaneously priming clients for future
connection attempts.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9149.
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Table of Contents
1. Introduction
1.1. Requirements Language
2. Use Cases
3. Ticket Requests
4. IANA Considerations
5. Performance Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
As described in [RFC8446], TLS servers vend clients an arbitrary
number of session tickets at their own discretion in NewSessionTicket
messages. There are at least three limitations with this design.
First, servers vend some (often hard-coded) number of tickets per
connection. Some server implementations return a different default
number of tickets for session resumption than for the initial
connection that created the session. No static choice, whether fixed
or dependent upon resumption, is ideal for all situations.
Second, clients do not have a way of expressing their desired number
of tickets, which can impact future connection establishment. For
example, clients can open parallel TLS connections to the same server
for HTTP, or they can race TLS connections across different network
interfaces. The latter is especially useful in transport systems
that implement Happy Eyeballs [RFC8305]. Since clients control
connection concurrency and resumption, a standard mechanism for
requesting more than one ticket is desirable for avoiding ticket
reuse. See Appendix C.4 of [RFC8446] for discussion of ticket reuse
risks.
Third, all tickets in the client's possession ultimately derive from
some initial connection. Especially when the client was initially
authenticated with a client certificate, that session may need to be
refreshed from time to time. Consequently, a server may periodically
force a new connection even when the client presents a valid ticket.
When that happens, it is possible that any other tickets derived from
the same original session are equally invalid. A client avoids a
full handshake on subsequent connections if it replaces all stored
tickets with new ones obtained from the just-performed full
handshake. The number of tickets the server should vend for a new
connection may therefore need to be larger than the number for
routine resumption.
This document specifies a new TLS extension, "ticket_request", that
clients can use to express their desired number of session tickets.
Servers can use this extension as a hint for the number of
NewSessionTicket messages to vend. This extension is only applicable
to TLS 1.3 [RFC8446], DTLS 1.3 [RFC9147], and future versions of
(D)TLS.
1.1. 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.
2. Use Cases
The ability to request one or more tickets is useful for a variety of
purposes:
Parallel HTTP connections: To improve performance, a client may open
parallel connections. To avoid ticket reuse, the client may use
distinct tickets on each connection. Clients must therefore bound
the number of parallel connections they initiate by the number of
tickets in their possession or risk ticket reuse.
Connection racing: Happy Eyeballs V2 [RFC8305] describes techniques
for performing connection racing. The Transport Services
Implementation document [TAPS] also describes how connections can
race across interfaces and address families. In such cases,
clients may use more than one ticket while racing connection
attempts in order to establish one successful connection. Having
multiple tickets equips clients with enough tickets to initiate
connection racing while avoiding ticket reuse and ensuring that
their cache of tickets does not empty during such races.
Moreover, as some servers may implement single-use tickets,
distinct tickets prevent premature ticket invalidation by racing.
Less ticket waste: Currently, TLS servers use application-specific,
and often implementation-specific, logic to determine how many
tickets to issue. By moving the burden of ticket count to
clients, servers do not generate wasteful tickets. As an example,
clients might only request one ticket during resumption.
Moreover, as ticket generation might involve expensive
computation, e.g., public key cryptographic operations, avoiding
waste is desirable.
Decline resumption: Clients can indicate they do not intend to
resume a connection by sending a ticket request with count of
zero.
3. Ticket Requests
As discussed in Section 1, clients may want different numbers of
tickets for new or resumed connections. Clients may indicate to
servers their desired number of tickets to receive on a single
connection, in the case of a new or resumed connection, via the
following "ticket_request" extension:
enum {
ticket_request(58), (65535)
} ExtensionType;
Clients MAY send this extension in ClientHello. It contains the
following structure:
struct {
uint8 new_session_count;
uint8 resumption_count;
} ClientTicketRequest;
new_session_count: The number of tickets desired by the client if
the server chooses to negotiate a new connection.
resumption_count: The number of tickets desired by the client if the
server is willing to resume using a ticket presented in this
ClientHello.
A client starting a new connection SHOULD set new_session_count to
the desired number of session tickets and resumption_count to 0.
Once a client's ticket cache is primed, a resumption_count of 1 is a
good choice that allows the server to replace each ticket with a new
ticket without over-provisioning the client with excess tickets.
However, clients that race multiple connections and place a separate
ticket in each will ultimately end up with just the tickets from a
single resumed session. In that case, clients can send a
resumption_count equal to the number of connections they are
attempting in parallel. (Clients that send a resumption_count less
than the number of parallel connection attempts might end up with
zero tickets.)
When a client presenting a previously obtained ticket finds that the
server nevertheless negotiates a new connection, the client SHOULD
assume that any other tickets associated with the same session as the
presented ticket are also no longer valid for resumption. This
includes tickets obtained during the initial (new) connection and all
tickets subsequently obtained as part of subsequent resumptions.
Requesting more than one ticket when servers complete a new
connection helps keep the session cache primed.
Servers SHOULD NOT send more tickets than requested for the
connection type selected by the server (new or resumed connection).
Moreover, servers SHOULD place a limit on the number of tickets they
are willing to send, whether for new or resumed connections, to save
resources. Therefore, the number of NewSessionTicket messages sent
will typically be the minimum of the server's self-imposed limit and
the number requested. Servers MAY send additional tickets, typically
using the same limit, if the tickets that are originally sent are
somehow invalidated.
A server that supports and uses a client "ticket_request" extension
MUST also send the "ticket_request" extension in the
EncryptedExtensions message. It contains the following structure:
struct {
uint8 expected_count;
} ServerTicketRequestHint;
expected_count: The number of tickets the server expects to send in
this connection.
Servers MUST NOT send the "ticket_request" extension in any handshake
message, including ServerHello or HelloRetryRequest messages. A
client MUST abort the connection with an "illegal_parameter" alert if
the "ticket_request" extension is present in any server handshake
message.
If a client receives a HelloRetryRequest, the presence (or absence)
of the "ticket_request" extension MUST be maintained in the second
ClientHello message. Moreover, if this extension is present, a
client MUST NOT change the value of ClientTicketRequest in the second
ClientHello message.
4. IANA Considerations
IANA has added the following entry to the "TLS ExtensionType Values"
registry [RFC8446] [RFC8447]:
+=======+================+=========+===========+=============+
| Value | Extension Name | TLS 1.3 | DTLS-Only | Recommended |
+=======+================+=========+===========+=============+
| 58 | ticket_request | CH, EE | N | Y |
+-------+----------------+---------+-----------+-------------+
Table 1: Addition to TLS ExtensionType Values Registry
5. Performance Considerations
Servers can send tickets in NewSessionTicket messages any time after
the server Finished message (see Section 4.6.1 of [RFC8446]). A
server that chooses to send a large number of tickets to a client can
potentially harm application performance if the tickets are sent
before application data. For example, if the transport connection
has a constrained congestion window, ticket messages could delay
sending application data. To avoid this, servers should prioritize
sending application data over tickets when possible.
6. Security Considerations
Ticket reuse is a security and privacy concern. Moreover, clients
must take care when pooling tickets as a means of avoiding or
amortizing handshake costs. If servers do not rotate session ticket
encryption keys frequently, clients may be encouraged to obtain and
use tickets beyond common lifetime windows of, e.g., 24 hours.
Despite ticket lifetime hints provided by servers, clients SHOULD
dispose of cached tickets after some reasonable amount of time that
mimics the session ticket encryption key rotation period.
Specifically, as specified in Section 4.6.1 of [RFC8446], clients
MUST NOT cache tickets for longer than 7 days.
In some cases, a server may send NewSessionTicket messages
immediately upon sending the server Finished message rather than
waiting for the client Finished message. If the server has not
verified the client's ownership of its IP address, e.g., with the TLS
cookie extension (see Section 4.2.2 of [RFC8446]), an attacker may
take advantage of this behavior to create an amplification attack
proportional to the count value toward a target by performing a
(DTLS) key exchange over UDP with spoofed packets. Servers SHOULD
limit the number of NewSessionTicket messages they send until they
have verified the client's ownership of its IP address.
Servers that do not enforce a limit on the number of NewSessionTicket
messages sent in response to a "ticket_request" extension could leave
themselves open to DoS attacks, especially if ticket creation is
expensive.
7. References
7.1. Normative References
[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>.
[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>.
[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>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
7.2. Informative References
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[TAPS] Brunstrom, A., Ed., Pauly, T., Ed., Enghardt, T., Tiesel,
P., and M. Welzl, "Implementing Interfaces to Transport
Services", Work in Progress, Internet-Draft, draft-ietf-
taps-impl-12, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-taps-
impl-12>.
Acknowledgements
The authors would like to thank David Benjamin, Eric Rescorla, Nick
Sullivan, Martin Thomson, Hubert Kario, and other members of the TLS
Working Group for discussions on earlier draft versions of this
document. Viktor Dukhovni contributed text allowing clients to send
multiple counts in a ticket request.
Authors' Addresses
Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, CA 95014
United States of America
Email: tpauly@apple.com
David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: dschinazi.ietf@gmail.com
Christopher A. Wood
Cloudflare
101 Townsend St
San Francisco, CA 94107
United States of America
Email: caw@heapingbits.net