draft-ietf-tls-ticketrequests-02.txt   draft-ietf-tls-ticketrequests-03.txt 
Network Working Group T. Pauly Network Working Group T. Pauly
Internet-Draft Apple Inc. Internet-Draft Apple Inc.
Intended status: Informational D. Schinazi Intended status: Informational D. Schinazi
Expires: March 30, 2020 Google LLC Expires: April 20, 2020 Google LLC
C. Wood C. Wood
Apple Inc. Apple Inc.
September 27, 2019 October 18, 2019
TLS Ticket Requests TLS Ticket Requests
draft-ietf-tls-ticketrequests-02 draft-ietf-tls-ticketrequests-03
Abstract Abstract
TLS session tickets enable stateless connection resumption for TLS session tickets enable stateless connection resumption for
clients without server-side, per-client state. Servers vend an clients without server-side, per-client state. Servers vend an
arbitrary number of session tickets to clients, at their discretion, arbitrary number of session tickets to clients, at their discretion,
upon connection establishment. Clients store and use tickets when upon connection establishment. Clients store and use tickets when
resuming future connections. This document describes a mechanism by resuming future connections. This document describes a mechanism by
which clients may specify the desired number of tickets needed for which clients can specify the desired number of tickets needed for
future connections. This extension aims to provide a means for future connections. This extension aims to provide a means for
servers to determine the number of tickets to generate in order to servers to determine the number of tickets to generate in order to
reduce ticket waste, while simultaneously priming clients for future reduce ticket waste, while simultaneously priming clients for future
connection attempts. connection attempts.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 30, 2020. This Internet-Draft will expire on April 20, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5 7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6 7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction 1. Introduction
As per [RFC5077], and as described in [RFC8446], TLS servers send As per [RFC5077], and as described in [RFC8446], TLS servers vend
clients an arbitrary number of session tickets at their own clients an arbitrary number of session tickets at their own
discretion in NewSessionTicket messages. There are two limitations discretion in NewSessionTicket messages. There are two limitations
with this design. First, servers choose some (often hard-coded) with this design. First, servers choose some (often hard-coded)
number of tickets vended per connection. Second, clients do not have number of tickets vended per connection. Second, clients do not have
a way of expressing their desired number of tickets, which may impact a way of expressing their desired number of tickets, which can impact
future connection establishment. For example, clients may open future connection establishment. For example, clients can open
multiple TLS connections to the same server for HTTP, or may race TLS multiple TLS connections to the same server for HTTP, or race TLS
connections across different network interfaces. The latter is connections across different network interfaces. The latter is
especially useful in transport systems that implement Happy Eyeballs especially useful in transport systems that implement Happy Eyeballs
[RFC8305]. Since clients control connection concurrency and [RFC8305]. Since clients control connection concurrency and
resumption, a standard mechanism for requesting more than one ticket resumption, a standard mechanism for requesting more than one ticket
is desirable. is desirable.
This document specifies a new TLS extension - "ticket_request" - that This document specifies a new TLS extension - "ticket_request" - that
may be used by clients to express their desired number of session can be used by clients to express their desired number of session
tickets. Servers may use this extension as a hint of the number of tickets. Servers can use this extension as a hint of the number of
NewSessionTicket messages to vend. This extension is only applicable NewSessionTicket messages to vend. This extension is only applicable
to TLS 1.3 [RFC8446], DTLS 1.3 [I-D.ietf-tls-dtls13], and future to TLS 1.3 [RFC8446], DTLS 1.3 [I-D.ietf-tls-dtls13], and future
versions thereof. versions thereof.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
[RFC2119] [RFC8174] when, and only when, they appear in all capitals, [RFC2119] [RFC8174] when, and only when, they appear in all capitals,
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o Parallel HTTP connections: To minimize ticket reuse while still o Parallel HTTP connections: To minimize ticket reuse while still
improving performance, it may be useful to use multiple, distinct improving performance, it may be useful to use multiple, distinct
tickets when opening parallel connections. Clients must therefore tickets when opening parallel connections. Clients must therefore
bound the number of parallel connections they initiate by the bound the number of parallel connections they initiate by the
number of tickets in their possession, or risk ticket re-use. number of tickets in their possession, or risk ticket re-use.
o Connection racing: Happy Eyeballs V2 [RFC8305] describes o Connection racing: Happy Eyeballs V2 [RFC8305] describes
techniques for performing connection racing. The Transport techniques for performing connection racing. The Transport
Services Architecture implementation from [TAPS] also describes Services Architecture implementation from [TAPS] also describes
how connections may race across interfaces and address families. how connections can race across interfaces and address families.
In cases where clients have early data to send and want to In cases where clients have early data to send and want to
minimize or avoid ticket re-use, unique tickets for each unique minimize or avoid ticket re-use, unique tickets for each unique
connection attempt are useful. Moreover, as some servers may connection attempt are useful. Moreover, as some servers may
implement single-use tickets (and even session ticket encryption implement single-use tickets (and even session ticket encryption
keys), distinct tickets will be needed to prevent premature ticket keys), distinct tickets will be needed to prevent premature ticket
invalidation by racing. invalidation by racing.
o Connection priming: In some systems, connections may be primed or o Connection priming: In some systems, connections can be primed or
bootstrapped by a centralized service or daemon for faster bootstrapped by a centralized service or daemon for faster
connection establishment. Requesting tickets on demand allows connection establishment. Requesting tickets on demand allows
such services to vend tickets to clients to use for accelerated such services to vend tickets to clients to use for accelerated
handshakes with early data. (Note that if early data is not handshakes with early data. (Note that if early data is not
needed by these connections, this method SHOULD NOT be used. needed by these connections, this method SHOULD NOT be used.
Fresh handshakes SHOULD be performed instead.) Fresh handshakes SHOULD be performed instead.)
o Less ticket waste: Currently, TLS servers use application- o Less ticket waste: Currently, TLS servers use application-
specific, and often implementation-specific, logic to determine specific, and often implementation-specific, logic to determine
how many tickets to issue. By moving the burden of ticket count how many tickets to issue. By moving the burden of ticket count
to clients, servers do not generate wasteful tickets for clients. to clients, servers do not generate wasteful tickets. As an
Moreover, as ticket generation may involve expensive computation, example, clients might only request one ticket during resumption.
e.g., public key cryptographic operations, avoiding waste is Moreover, as ticket generation might involve expensive
desirable. computation, e.g., public key cryptographic operations, avoiding
waste is desirable.
o Decline resumption: Clients may indicate they have no intention of o Decline resumption: Clients can indicate they have no intention of
resuming connections by sending a ticket request with count of resuming connections by sending a ticket request with count of
zero. zero.
3. Ticket Requests 3. Ticket Requests
Clients may indicate to servers their desired number of tickets for a Clients can indicate to servers their desired number of tickets for a
single connection via the following "ticket_request" extension: single connection via the following "ticket_request" extension:
enum { enum {
ticket_request(TBD), (65535) ticket_request(TBD), (65535)
} ExtensionType; } ExtensionType;
Clients may send this extension in ClientHello. It contains the Clients MAY send this extension in ClientHello. It contains the
following structure: following structure:
struct { struct {
uint8 count; uint8 count;
} TicketRequestContents; } TicketRequestContents;
count The number of tickets desired by the client. count The number of tickets desired by the client.
A supporting server MAY vend TicketRequestContents.count A supporting server MAY use TicketRequestContents.count when
NewSessionTicket messages to a requesting client, and SHOULD NOT send determining how many NewSessionTicket messages to send to a
more than TicketRequestContents.count NewSessionTicket messages to a requesting client, and SHOULD place a limit on the number of tickets
requesting client. Servers SHOULD place a limit on the number of sent. The number of NewSessionTicket messages sent SHOULD be the
tickets they are willing to vend to clients. Thus, the number of minimum of the server's self-imposed limit and
NewSessionTicket messages sent should be the minimum of the server's TicketRequestContents.count.
self-imposed limit and TicketRequestContents.count. Servers MUST NOT
send more than 255 tickets to clients.
Servers that support ticket requests MUST NOT echo "ticket_request" Servers that support ticket requests MUST NOT echo "ticket_request"
in the EncryptedExtensions message. A client MUST abort the in the EncryptedExtensions message. A client MUST abort the
connection with an "illegal_parameter" alert if the "ticket_request" connection with an "illegal_parameter" alert if the "ticket_request"
extension is present in the EncryptedExtensions message. extension is present in the EncryptedExtensions message.
Clients MUST NOT change the value of TicketRequestContents.count in If a client receives a HelloRetryRequest, the presence (or absence)
second ClientHello messages sent in response to a HelloRetryRequest. 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 4. IANA Considerations
IANA is requested to Create an entry, ticket_request(TBD), in the IANA is requested to Create an entry, ticket_request(TBD), in the
existing registry for ExtensionType (defined in [RFC8446]), with "TLS existing registry for ExtensionType (defined in [RFC8446]), with "TLS
1.3" column values being set to "CH", and "Recommended" column being 1.3" column values being set to "CH", and "Recommended" column being
set to "Yes". set to "Yes".
5. Security Considerations 5. Security Considerations
Ticket re-use is a security and privacy concern. Moreover, clients Ticket re-use is a security and privacy concern. Moreover, clients
must take care when pooling tickets as a means of avoiding or must take care when pooling tickets as a means of avoiding or
amortizing handshake costs. If servers do not rotate session ticket amortizing handshake costs. If servers do not rotate session ticket
encryption keys frequently, clients may be encouraged to obtain and encryption keys frequently, clients may be encouraged to obtain and
use tickets beyond common lifetime windows of, e.g., 24 hours. use tickets beyond common lifetime windows of, e.g., 24 hours.
Despite ticket lifetime hints provided by servers, clients SHOULD Despite ticket lifetime hints provided by servers, clients SHOULD
dispose of pooled tickets after some reasonable amount of time that dispose of pooled tickets after some reasonable amount of time that
mimics the ticket rotation period. 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 6. Acknowledgments
The authors would like to thank David Benjamin, Eric Rescorla, Nick The authors would like to thank David Benjamin, Eric Rescorla, Nick
Sullivan, Martin Thomson, and other members of the TLS Working Group Sullivan, Martin Thomson, Hubert Kario, and other members of the TLS
for discussions on earlier versions of this draft. Working Group for discussions on earlier versions of this draft.
7. References 7. References
7.1. Normative References 7.1. Normative References
[I-D.ietf-tls-dtls13] [I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-32 (work in progress), July 1.3", draft-ietf-tls-dtls13-33 (work in progress), October
2019. 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without "Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077, Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
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