Network Working Group T. Pauly Internet-Draft D. Schinazi Intended status: Informational C. Wood Expires: April 16, 2019 Apple Inc. October 13, 2018 TLS Ticket Requests draft-wood-tls-ticketrequests-01 Abstract TLS session tickets enable stateless connection resumption for clients without server-side per-client state. Servers vend session tickets to clients, at their discretion, upon connection establishment. Clients store and use tickets when resuming future connections. Moreover, clients should use tickets at most once for session resumption, especially if such keying material protects early application data. Single-use tickets bound the number of parallel connections a client may initiate by the number of tickets received from a given server. To address this limitation, this document describes a mechanism by which clients may specify the desired number of tickets needed for future connections. 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 http://datatracker.ietf.org/drafts/current/. 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 April 16, 2019. Copyright Notice Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents Pauly, et al. Expires April 16, 2019 [Page 1] Internet-Draft TLS Ticket Requests October 2018 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Ticket Requests . . . . . . . . . . . . . . . . . . . . . . . 3 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . . . . 4 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 4 7. Normative References . . . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction As per [RFC5077], and as described in [RFC8446], TLS servers send clients session tickets at their own discretion in NewSessionTicket messages. Clients are in complete control of how many tickets they may use when establishing future and subsequent connections. For example, clients may open multiple TLS connections to the same server for HTTP, or may race TLS connections across different network interfaces. The latter is especially useful in transport systems that implement Happy Eyeballs [RFC8305]. Since connection concurrency and resumption is controlled by clients, a standard mechanism to request more than one ticket is desirable. This document specifies a new TLS extension - ticket_request - that may be used by clients to express their desired number of session tickets. Servers may use this extension as a hint of the number of NewSessionTicket messages to vend. This extension is only applicable to TLS 1.3 [RFC8446] and future versions of 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Pauly, et al. Expires April 16, 2019 [Page 2] Internet-Draft TLS Ticket Requests October 2018 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 [I-D.ietf-taps-impl] also describes how connections may 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 may 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 for clients. Moreover, as ticket generation may involve expensive computation, e.g., public key cryptographic operations, avoiding waste is desirable. 3. Ticket Requests Clients may indicate to servers their desired number of tickets via the following "ticket_request" extension: enum { ticket_request(TBD), (65535) } ExtensionType; Pauly, et al. Expires April 16, 2019 [Page 3] Internet-Draft TLS Ticket Requests October 2018 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 vend TicketRequestContents.count NewSessionTicket messages to a requesting client, and SHOULD NOT send more than TicketRequestContents.count NewSessionTicket messages to a requesting client. Servers SHOULD place a limit on the number of tickets they are willing to vend to clients. Thus, the number of NewSessionTicket messages sent should be the minimum of the server's 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" in the EncryptedExtensions. 4. IANA Considerations IANA is requested to Create an entry, ticket_requests(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". 5. Security Considerations Ticket re-use is a security and privacy concern. Moreover, ticket pooling as a means of avoiding or amortizing handshake costs must be used carefully. 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. 6. Acknowledgments The authors would like to thank David Benjamin, Eric Rescorla, Nick Sullivan, and Martin Thomson for discussions on earlier versions of this draft. Pauly, et al. Expires April 16, 2019 [Page 4] Internet-Draft TLS Ticket Requests October 2018 7. Normative References [I-D.ietf-taps-impl] 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-01 (work in progress), July 2018. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, December 2017, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . Authors' Addresses Tommy Pauly Apple Inc. One Apple Park Way Cupertino, California 95014 United States of America Email: tpauly@apple.com Pauly, et al. Expires April 16, 2019 [Page 5] Internet-Draft TLS Ticket Requests October 2018 David Schinazi Apple Inc. One Apple Park Way Cupertino, California 95014 United States of America Email: dschinazi@apple.com Christopher A. Wood Apple Inc. One Apple Park Way Cupertino, California 95014 United States of America Email: cawood@apple.com Pauly, et al. Expires April 16, 2019 [Page 6]