Internet-Draft TLS Ticket Requests April 2020
Pauly, et al. Expires 26 October 2020 [Page]
Network Working Group
Intended Status:
Standards Track
T. Pauly
Apple Inc.
D. Schinazi
Google LLC
C.A. Wood

TLS Ticket Requests


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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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 26 October 2020.

Table of Contents

1. Introduction

As 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 full handshake that created the session. No static choice, whether fixed, or resumption-dependent 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 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.

Third, all tickets in the client's possession ultimately derive from some initial full handshake. 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 full handshake 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 fresh ones obtained from the just performed full handshake. The number of tickets the server should vend for a full handshake may therefore need to be larger than the number for routine resumption.

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.

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:

3. Ticket Requests

As discussed in Section 1, clients may want different numbers of tickets for fresh or resumed handshakes. Clients may indicate to servers their desired number of tickets for a single connection, in the case of a full handshake or resumption, 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 new_session_count;
    uint8 resumption_count;
} ClientTicketRequest;
The number of tickets desired by the client when the server chooses to negotiate a fresh session (full handshake).
The number of tickets desired by the client when the server is willing to resume using the presented ticket.

A client starting a fresh 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 fresh ticket, without over-provisioning the client with excess tickets. However, clients which 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 sessions they are attempting in parallel. (Clients which 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 fresh session, 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 full handshake and all tickets subsequently obtained as part of subsequent resumptions. Requesting more than one ticket in cases when servers select a full handshake helps keep the session cache primed.

Servers SHOULD NOT send more tickets than requested for the handshake type selected by the server (resumption or full handshake). Moreover, servers SHOULD place a limit on the number of tickets they are willing to send, whether for full handshakes or resumptions, 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, up to the same limit, if the tickets that are originally sent are somehow invalidated.

A server which 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;
The number of tickets the server expects to send in this connection.

Servers MUST NOT send the "ticket_request" extension in ServerHello or HelloRetryRequest messages. A client MUST abort the connection with an "illegal_parameter" alert if the "ticket_request" extension is present in either of these messages.

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 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, EE", and "Recommended" column being set to "Yes".

5. Performance Considerations

Servers can send tickets in NewSessionTicket messages any time after the server Finished message (see [RFC8446]; Section 4.6.1). A server which 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 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.

In some cases, a server may send NewSessionTicket messages immediately upon sending the server Finished message rather than waiting for the client Finished. If the server has not verified the client's ownership of its IP address, e.g., with the TLS Cookie extension (see [RFC8446]; Section 4.2.2), an attacker may take advantage of this behavior to create an amplification attack proportional to the count value toward a target by performing a 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. 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. Viktor Dukhovni contributed text allowing clients to send multiple counts in a ticket request.

8. References

8.1. Normative References

Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-dtls13-37, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <>.

8.2. Informative References

Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, , <>.
Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K., Jones, T., Tiesel, P., Perkins, C., and M. Welzl, "Implementing Interfaces to Transport Services", Work in Progress, Internet-Draft, draft-ietf-taps-impl-06, , <>.

Authors' Addresses

Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014,
United States of America
David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, California 94043,
United States of America
Christopher A. Wood
101 Townsend St
San Francisco,
United States of America