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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-43) exists of draft-ietf-tls-dtls13-33 ** Obsolete normative reference: RFC 5077 (Obsoleted by RFC 8446) == Outdated reference: A later version (-18) exists of draft-ietf-taps-impl-04 Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group T. Pauly 3 Internet-Draft Apple Inc. 4 Intended status: Standards Track D. Schinazi 5 Expires: May 7, 2020 Google LLC 6 C. Wood 7 Apple Inc. 8 November 04, 2019 10 TLS Ticket Requests 11 draft-ietf-tls-ticketrequests-04 13 Abstract 15 TLS session tickets enable stateless connection resumption for 16 clients without server-side, per-client state. Servers vend an 17 arbitrary number of session tickets to clients, at their discretion, 18 upon connection establishment. Clients store and use tickets when 19 resuming future connections. This document describes a mechanism by 20 which clients can specify the desired number of tickets needed for 21 future connections. This extension aims to provide a means for 22 servers to determine the number of tickets to generate in order to 23 reduce ticket waste, while simultaneously priming clients for future 24 connection attempts. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at https://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on May 7, 2020. 43 Copyright Notice 45 Copyright (c) 2019 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (https://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 62 2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 3. Ticket Requests . . . . . . . . . . . . . . . . . . . . . . . 4 64 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 65 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 66 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 67 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 68 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 69 7.2. Informative References . . . . . . . . . . . . . . . . . 6 70 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 72 1. Introduction 74 As per [RFC5077], and as described in [RFC8446], TLS servers vend 75 clients an arbitrary number of session tickets at their own 76 discretion in NewSessionTicket messages. There are two limitations 77 with this design. First, servers choose some (often hard-coded) 78 number of tickets vended per connection. Second, clients do not have 79 a way of expressing their desired number of tickets, which can impact 80 future connection establishment. For example, clients can open 81 multiple TLS connections to the same server for HTTP, or race TLS 82 connections across different network interfaces. The latter is 83 especially useful in transport systems that implement Happy Eyeballs 84 [RFC8305]. Since clients control connection concurrency and 85 resumption, a standard mechanism for requesting more than one ticket 86 is desirable. 88 This document specifies a new TLS extension - "ticket_request" - that 89 can be used by clients to express their desired number of session 90 tickets. Servers can use this extension as a hint of the number of 91 NewSessionTicket messages to vend. This extension is only applicable 92 to TLS 1.3 [RFC8446], DTLS 1.3 [I-D.ietf-tls-dtls13], and future 93 versions thereof. 95 1.1. Requirements Language 97 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 98 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 99 "OPTIONAL" in this document are to be interpreted as described in 100 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, 101 as shown here. 103 2. Use Cases 105 The ability to request one or more tickets is useful for a variety of 106 purposes: 108 o Parallel HTTP connections: To minimize ticket reuse while still 109 improving performance, it may be useful to use multiple, distinct 110 tickets when opening parallel connections. Clients must therefore 111 bound the number of parallel connections they initiate by the 112 number of tickets in their possession, or risk ticket re-use. 114 o Connection racing: Happy Eyeballs V2 [RFC8305] describes 115 techniques for performing connection racing. The Transport 116 Services Architecture implementation from [TAPS] also describes 117 how connections can race across interfaces and address families. 118 In cases where clients have early data to send and want to 119 minimize or avoid ticket re-use, unique tickets for each unique 120 connection attempt are useful. Moreover, as some servers may 121 implement single-use tickets (and even session ticket encryption 122 keys), distinct tickets will be needed to prevent premature ticket 123 invalidation by racing. 125 o Connection priming: In some systems, connections can be primed or 126 bootstrapped by a centralized service or daemon for faster 127 connection establishment. Requesting tickets on demand allows 128 such services to vend tickets to clients to use for accelerated 129 handshakes with early data. (Note that if early data is not 130 needed by these connections, this method SHOULD NOT be used. 131 Fresh handshakes SHOULD be performed instead.) 133 o Less ticket waste: Currently, TLS servers use application- 134 specific, and often implementation-specific, logic to determine 135 how many tickets to issue. By moving the burden of ticket count 136 to clients, servers do not generate wasteful tickets. As an 137 example, clients might only request one ticket during resumption. 138 Moreover, as ticket generation might involve expensive 139 computation, e.g., public key cryptographic operations, avoiding 140 waste is desirable. 142 o Decline resumption: Clients can indicate they have no intention of 143 resuming connections by sending a ticket request with count of 144 zero. 146 3. Ticket Requests 148 Clients can indicate to servers their desired number of tickets for a 149 single connection via the following "ticket_request" extension: 151 enum { 152 ticket_request(TBD), (65535) 153 } ExtensionType; 155 Clients MAY send this extension in ClientHello. It contains the 156 following structure: 158 struct { 159 uint8 count; 160 } TicketRequestContents; 162 count The number of tickets desired by the client. 164 A supporting server MAY use TicketRequestContents.count when 165 determining how many NewSessionTicket messages to send to a 166 requesting client, and SHOULD place a limit on the number of tickets 167 sent. The number of NewSessionTicket messages sent SHOULD be the 168 minimum of the server's self-imposed limit and 169 TicketRequestContents.count. 171 Servers that support ticket requests MUST NOT echo "ticket_request" 172 in the EncryptedExtensions message. A client MUST abort the 173 connection with an "illegal_parameter" alert if the "ticket_request" 174 extension is present in the EncryptedExtensions message. 176 If a client receives a HelloRetryRequest, the presence (or absence) 177 of the "ticket_request" extension MUST be maintained in the second 178 ClientHello message. Moreover, if this extension is present, a 179 client MUST NOT change the value of TicketRequestContents.count in 180 the second ClientHello message. 182 4. IANA Considerations 184 IANA is requested to Create an entry, ticket_request(TBD), in the 185 existing registry for ExtensionType (defined in [RFC8446]), with "TLS 186 1.3" column values being set to "CH", and "Recommended" column being 187 set to "Yes". 189 5. Security Considerations 191 Ticket re-use is a security and privacy concern. Moreover, clients 192 must take care when pooling tickets as a means of avoiding or 193 amortizing handshake costs. If servers do not rotate session ticket 194 encryption keys frequently, clients may be encouraged to obtain and 195 use tickets beyond common lifetime windows of, e.g., 24 hours. 196 Despite ticket lifetime hints provided by servers, clients SHOULD 197 dispose of pooled tickets after some reasonable amount of time that 198 mimics the ticket rotation period. 200 Servers that do not enforce a limit on the number of NewSessionTicket 201 messages sent in response to a "ticket_request" extension could leave 202 themselves open to DoS attacks, especially if ticket creation is 203 expensive. 205 6. Acknowledgments 207 The authors would like to thank David Benjamin, Eric Rescorla, Nick 208 Sullivan, Martin Thomson, Hubert Kario, and other members of the TLS 209 Working Group for discussions on earlier versions of this draft. 211 7. References 213 7.1. Normative References 215 [I-D.ietf-tls-dtls13] 216 Rescorla, E., Tschofenig, H., and N. Modadugu, "The 217 Datagram Transport Layer Security (DTLS) Protocol Version 218 1.3", draft-ietf-tls-dtls13-33 (work in progress), October 219 2019. 221 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 222 Requirement Levels", BCP 14, RFC 2119, 223 DOI 10.17487/RFC2119, March 1997, 224 . 226 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 227 "Transport Layer Security (TLS) Session Resumption without 228 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 229 January 2008, . 231 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 232 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 233 May 2017, . 235 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 236 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 237 . 239 7.2. Informative References 241 [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: 242 Better Connectivity Using Concurrency", RFC 8305, 243 DOI 10.17487/RFC8305, December 2017, 244 . 246 [TAPS] Brunstrom, A., Pauly, T., Enghardt, T., Grinnemo, K., 247 Jones, T., Tiesel, P., Perkins, C., and M. Welzl, 248 "Implementing Interfaces to Transport Services", draft- 249 ietf-taps-impl-04 (work in progress), July 2019. 251 Authors' Addresses 253 Tommy Pauly 254 Apple Inc. 255 One Apple Park Way 256 Cupertino, California 95014 257 United States of America 259 Email: tpauly@apple.com 261 David Schinazi 262 Google LLC 263 1600 Amphitheatre Parkway 264 Mountain View, California 94043 265 United States of America 267 Email: dschinazi.ietf@gmail.com 269 Christopher A. Wood 270 Apple Inc. 271 One Apple Park Way 272 Cupertino, California 95014 273 United States of America 275 Email: cawood@apple.com