Network Working Group C. Wood Internet-Draft T. Pauly Intended status: Informational D. Schinazi Expires: October 14, 2018 Apple Inc. April 12, 2018 TLS Ticket Requests draft-wood-tls-ticketrequests-00 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 request tickets as needed during a connection. 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 October 14, 2018. 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 Wood, et al. Expires October 14, 2018 [Page 1] Internet-Draft TLS Ticket Requests April 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. Negotiation . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 8. Normative References . . . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction As per [RFC5077], and as described in [I-D.ietf-tls-tls13], 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 mechanism to request tickets on demand is desirable. This document specifies a new TLS post-handshake message - TicketRequest - that may be used to request tickets via NewSessionTicket messages in TLS 1.3. Ticket requests may carry optional application-specific contexts to define the ways in which tickets may be used. NewSessionTicket responses reciprocate this application context in an extension. 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. Wood, et al. Expires October 14, 2018 [Page 2] Internet-Draft TLS Ticket Requests April 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.brunstrom-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. 3. Ticket Requests TLS tickets may be requested via a TicketRequest post-handshake message, ticket_request(TBD). Its structure is shown below. struct { opaque identifier<0..255>; opaque context<0..2^16-1>; } TicketRequest; o identifier: A unique value for this ticket request. Clients SHOULD fill this in with a monotonically increasing counter. Wood, et al. Expires October 14, 2018 [Page 3] Internet-Draft TLS Ticket Requests April 2018 o context: An opaque context to be used when generating the ticket request. Clients and servers may use this context to implement or exchange data to be included in the ticket computation. Clients SHOULD make this field empty if it is not needed. Upon receipt of a TicketRequest message, servers MAY reply with a NewSessionTicket message, as defined in [I-D.ietf-tls-tls13]. The latter message MUST carry two extensions, ticket_identifer and ticket_context, defined below. enum { ... ticket_identifier(TBD), ticket_context(TBD+1), (65535) } ExtensionType; The value of ticket_identifier MUST match that of the corresponding TicketRequest identifier field. The value of ticket_context MAY be used by servers to convey ticket context to clients. Its value MUST be empty if the corresponding TicketRequest context field is empty. Servers SHOULD place a limit on the number of tickets they are willing to vend to clients. Servers MUST NOT send more than 255 tickets to clients, as this is the limit imposed by the request and response identifier size. Servers SHOULD NOT send unsolicited NewSessionTickets to clients that express support for TicketRequests. When operated over an unreliable transport, e.g., DTLS, servers may not always be able to identify retransmissions of ticket requests. In this case, when a server S receives a TicketRequest with new identifier N it MUST generate a new ticket and SHOULD cache it locally for some period of time T. If S receives a TicketRequest with identifier N within time period T, S SHOULD reply with the same ticket previously generated (and cached). If S receives a TicketRequest with identifier N outside time period T, S SHOULD reply with an empty NewSessionTicket, i.e., a NewSessionTicket with extension ticket_identifier carrying N, appropriate ticket_context extension, and empty ticket field. 4. Negotiation Clients negotiate use of ticket requests via a new ExtensionType, ticket_request(TBD). The extension_data for this extension MUST be empty, i.e., have length of 0. Servers that support ticket requests MAY echo this extension in the EncryptedExtensions. Clients MUST NOT send ticket requests to servers that do not signal support for this Wood, et al. Expires October 14, 2018 [Page 4] Internet-Draft TLS Ticket Requests April 2018 message. If absent from a ClientHello, servers MUST NOT generate responses to TicketRequests issued by the client. 5. IANA Considerations ((TODO: codepoint for post-handshake message type and extensions)) 6. Security Considerations Ticket re-use is a security and privacy concern. Moreover, pre- fetching 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 pre- fetched tickets after some reasonable amount of time that mimics the ticket rotation period. 7. Acknowledgments The authors would like to thank Eric Rescorla, Martin Thomson, and Nick Sullivan for discussions on earlier versions of this draft. 8. Normative References [I-D.brunstrom-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- brunstrom-taps-impl-00 (work in progress), March 2018. [I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", draft-ietf-tls-tls13-28 (work in progress), March 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, . Wood, et al. Expires October 14, 2018 [Page 5] Internet-Draft TLS Ticket Requests April 2018 [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, . Authors' Addresses Christopher A. Wood Apple Inc. One Apple Park Way Cupertino, California 95014 United States of America Email: cawood@apple.com Tommy Pauly Apple Inc. One Apple Park Way Cupertino, California 95014 United States of America Email: tpauly@apple.com David Schinazi Apple Inc. One Apple Park Way Cupertino, California 95014 United States of America Email: dschinazi@apple.com Wood, et al. Expires October 14, 2018 [Page 6]