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Versions: (draft-wood-tls-ticketrequests) 00
01 02 03 04
Network Working Group T. Pauly
Internet-Draft Apple Inc.
Intended status: Standards Track D. Schinazi
Expires: May 7, 2020 Google LLC
C. Wood
Apple Inc.
November 04, 2019
TLS Ticket Requests
draft-ietf-tls-ticketrequests-04
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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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 May 7, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Ticket Requests . . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
As per [RFC5077], and as described in [RFC8446], TLS servers vend
clients an arbitrary number of session tickets at their own
discretion in NewSessionTicket messages. There are two limitations
with this design. First, servers choose some (often hard-coded)
number of tickets vended per connection. 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
multiple TLS connections to the same server for HTTP, or 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.
This document specifies a new TLS extension - "ticket_request" - that
can be used by clients to express their desired number of session
tickets. Servers can use this extension as a hint of the number of
NewSessionTicket messages to vend. This extension is only applicable
to TLS 1.3 [RFC8446], DTLS 1.3 [I-D.ietf-tls-dtls13], and future
versions thereof.
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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
[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:
o Parallel HTTP connections: To minimize ticket reuse while still
improving performance, it may be useful to use multiple, distinct
tickets when opening parallel connections. Clients must therefore
bound the number of parallel connections they initiate by the
number of tickets in their possession, or risk ticket re-use.
o Connection racing: Happy Eyeballs V2 [RFC8305] describes
techniques for performing connection racing. The Transport
Services Architecture implementation from [TAPS] also describes
how connections can race across interfaces and address families.
In cases where clients have early data to send and want to
minimize or avoid ticket re-use, unique tickets for each unique
connection attempt are useful. Moreover, as some servers may
implement single-use tickets (and even session ticket encryption
keys), distinct tickets will be needed to prevent premature ticket
invalidation by racing.
o Connection priming: In some systems, connections can be primed or
bootstrapped by a centralized service or daemon for faster
connection establishment. Requesting tickets on demand allows
such services to vend tickets to clients to use for accelerated
handshakes with early data. (Note that if early data is not
needed by these connections, this method SHOULD NOT be used.
Fresh handshakes SHOULD be performed instead.)
o 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.
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o Decline resumption: Clients can indicate they have no intention of
resuming connections by sending a ticket request with count of
zero.
3. Ticket Requests
Clients can indicate to servers their desired number of tickets for a
single connection via the following "ticket_request" extension:
enum {
ticket_request(TBD), (65535)
} ExtensionType;
Clients MAY send this extension in ClientHello. It contains the
following structure:
struct {
uint8 count;
} TicketRequestContents;
count The number of tickets desired by the client.
A supporting server MAY use TicketRequestContents.count when
determining how many NewSessionTicket messages to send to a
requesting client, and SHOULD place a limit on the number of tickets
sent. The number of NewSessionTicket messages sent SHOULD be the
minimum of the server's self-imposed limit and
TicketRequestContents.count.
Servers that support ticket requests MUST NOT echo "ticket_request"
in the EncryptedExtensions message. A client MUST abort the
connection with an "illegal_parameter" alert if the "ticket_request"
extension is present in the EncryptedExtensions 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 TicketRequestContents.count in
the second ClientHello message.
4. IANA Considerations
IANA is requested to Create an entry, ticket_request(TBD), in the
existing registry for ExtensionType (defined in [RFC8446]), with "TLS
1.3" column values being set to "CH", and "Recommended" column being
set to "Yes".
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5. Security Considerations
Ticket re-use 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 pooled tickets after some reasonable amount of time that
mimics the ticket rotation period.
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.
6. Acknowledgments
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 versions of this draft.
7. References
7.1. Normative References
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-33 (work in progress), October
2019.
[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>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[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>.
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[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>.
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., Pauly, T., Enghardt, T., Grinnemo, K.,
Jones, T., Tiesel, P., Perkins, C., and M. Welzl,
"Implementing Interfaces to Transport Services", draft-
ietf-taps-impl-04 (work in progress), July 2019.
Authors' Addresses
Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: tpauly@apple.com
David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, California 94043
United States of America
Email: dschinazi.ietf@gmail.com
Christopher A. Wood
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: cawood@apple.com
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