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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group D. Schinazi 3 Internet-Draft Google LLC 4 Intended status: Experimental July 08, 2019 5 Expires: January 9, 2020 7 The MASQUE Protocol 8 draft-schinazi-masque-01 10 Abstract 12 This document describes MASQUE (Multiplexed Application Substrate 13 over QUIC Encryption). MASQUE is a mechanism that allows co-locating 14 and obfuscating networking applications behind an HTTPS web server. 15 The currently prevalent use-case is to allow running a proxy or VPN 16 server that is indistinguishable from an HTTPS server to any 17 unauthenticated observer. We do not expect major providers and CDNs 18 to deploy this behind their main TLS certificate, as they are not 19 willing to take the risk of getting blocked, as shown when domain 20 fronting was blocked. An expected use would be for individuals to 21 enable this behind their personal websites via easy to configure 22 open-source software. 24 This document is a straw-man proposal. It does not contain enough 25 details to implement the protocol, and is currently intended to spark 26 discussions on the approach it is taking. Discussion of this work is 27 encouraged to happen on the MASQUE IETF mailing list masque@ietf.org 28 [1] or on the GitHub repository which contains the draft: 29 https://github.com/DavidSchinazi/masque-drafts [2]. 31 Status of This Memo 33 This Internet-Draft is submitted in full conformance with the 34 provisions of BCP 78 and BCP 79. 36 Internet-Drafts are working documents of the Internet Engineering 37 Task Force (IETF). Note that other groups may also distribute 38 working documents as Internet-Drafts. The list of current Internet- 39 Drafts is at https://datatracker.ietf.org/drafts/current/. 41 Internet-Drafts are draft documents valid for a maximum of six months 42 and may be updated, replaced, or obsoleted by other documents at any 43 time. It is inappropriate to use Internet-Drafts as reference 44 material or to cite them other than as "work in progress." 46 This Internet-Draft will expire on January 9, 2020. 48 Copyright Notice 50 Copyright (c) 2019 IETF Trust and the persons identified as the 51 document authors. All rights reserved. 53 This document is subject to BCP 78 and the IETF Trust's Legal 54 Provisions Relating to IETF Documents 55 (https://trustee.ietf.org/license-info) in effect on the date of 56 publication of this document. Please review these documents 57 carefully, as they describe your rights and restrictions with respect 58 to this document. Code Components extracted from this document must 59 include Simplified BSD License text as described in Section 4.e of 60 the Trust Legal Provisions and are provided without warranty as 61 described in the Simplified BSD License. 63 Table of Contents 65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 66 1.1. Conventions and Definitions . . . . . . . . . . . . . . . 3 67 2. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . . 3 68 2.1. Protection from Network Providers . . . . . . . . . . . . 3 69 2.2. Protection from Web Servers . . . . . . . . . . . . . . . 4 70 2.3. Making a Home Server Available . . . . . . . . . . . . . 4 71 2.4. Onion Routing . . . . . . . . . . . . . . . . . . . . . . 4 72 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4 73 3.1. Invisibility of Usage . . . . . . . . . . . . . . . . . . 4 74 3.2. Invisibility of the Server . . . . . . . . . . . . . . . 5 75 3.3. Fallback to HTTP/2 over TLS over TCP . . . . . . . . . . 5 76 4. Overview of the Mechanism . . . . . . . . . . . . . . . . . . 5 77 5. Mechanisms the Server Can Advertise to Authenticated Clients 6 78 5.1. HTTP Proxy . . . . . . . . . . . . . . . . . . . . . . . 6 79 5.2. DNS over HTTPS . . . . . . . . . . . . . . . . . . . . . 6 80 5.3. UDP Proxying . . . . . . . . . . . . . . . . . . . . . . 6 81 5.4. QUIC Proxying . . . . . . . . . . . . . . . . . . . . . . 6 82 5.5. IP Proxying . . . . . . . . . . . . . . . . . . . . . . . 7 83 5.6. Path MTU Discovery . . . . . . . . . . . . . . . . . . . 7 84 5.7. Service Registration . . . . . . . . . . . . . . . . . . 7 85 6. Operation over HTTP/2 . . . . . . . . . . . . . . . . . . . . 7 86 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 87 7.1. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 8 88 7.2. Untrusted Servers . . . . . . . . . . . . . . . . . . . . 8 89 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 90 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 91 9.1. Normative References . . . . . . . . . . . . . . . . . . 9 92 9.2. Informative References . . . . . . . . . . . . . . . . . 10 93 9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11 94 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11 95 Design Justifications . . . . . . . . . . . . . . . . . . . . . . 11 96 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13 98 1. Introduction 100 This document describes MASQUE (Multiplexed Application Substrate 101 over QUIC Encryption). MASQUE is a mechanism that allows co-locating 102 and obfuscating networking applications behind an HTTPS web server. 103 The currently prevalent use-case is to allow running a proxy or VPN 104 server that is indistinguishable from an HTTPS server to any 105 unauthenticated observer. We do not expect major providers and CDNs 106 to deploy this behind their main TLS certificate, as they are not 107 willing to take the risk of getting blocked, as shown when domain 108 fronting was blocked. An expected use would be for individuals to 109 enable this behind their personal websites via easy to configure 110 open-source software. 112 This document is a straw-man proposal. It does not contain enough 113 details to implement the protocol, and is currently intended to spark 114 discussions on the approach it is taking. Discussion of this work is 115 encouraged to happen on the MASQUE IETF mailing list masque@ietf.org 116 [3] or on the GitHub repository which contains the draft: 117 https://github.com/DavidSchinazi/masque-drafts [4]. 119 MASQUE leverages the efficient head-of-line blocking prevention 120 features of the QUIC transport protocol [I-D.ietf-quic-transport] 121 when MASQUE is used in an HTTP/3 [I-D.ietf-quic-http] server. MASQUE 122 can also run in an HTTP/2 server [RFC7540] but at a performance cost. 124 1.1. Conventions and Definitions 126 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 127 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 128 "OPTIONAL" in this document are to be interpreted as described in BCP 129 14 [RFC2119] [RFC8174] when, and only when, they appear in all 130 capitals, as shown here. 132 2. Usage Scenarios 134 There are currently multiple usage scenarios that can benefit from 135 MASQUE. 137 2.1. Protection from Network Providers 139 Some users may wish to obfuscate the destination of their network 140 traffic from their network provider. This prevents network providers 141 from using data harvested from this network traffic in ways the user 142 did not intend. 144 2.2. Protection from Web Servers 146 There are many clients who would rather not establish a direct 147 connection to web servers, for example to avoid location tracking. 148 The clients can do that by running their traffic through a MASQUE 149 server. The web server will only see the IP address of the MASQUE 150 server, not that of the client. 152 2.3. Making a Home Server Available 154 It is often difficult to connect to a home server. The IP address 155 might change over time. Firewalls in the home router or in the 156 network may block incoming connections. Using a MASQUE server as a 157 rendez-vous point helps resolve these issues. 159 2.4. Onion Routing 161 Routing traffic through a MASQUE server only provides partial 162 protection against tracking, because the MASQUE server knows the 163 address of the client. Onion routing as it exists today mitigates 164 this issue for TCP/TLS. A MASQUE server could allow onion routing 165 over QUIC. 167 In this scenario, the client establishes a connection to the MASQUE 168 server, then through that to another MASQUE server, etc. This 169 creates a tree of MASQUE servers rooted at the client. QUIC 170 connections are mapped to a specific branch of the tree. The first 171 MASQUE server knows the actual address of the client, but the other 172 MASQUE servers only know the address of the previous server. To 173 assure reasonable privacy, the path should include at least 3 MASQUE 174 servers. 176 3. Requirements 178 This section describes the goals and requirements chosen for the 179 MASQUE protocol. 181 3.1. Invisibility of Usage 183 An authenticated client using MASQUE features appears to observers as 184 a regular HTTPS client. Observers only see that HTTP/3 or HTTP/2 is 185 being used over an encrypted channel. No part of the exchanges 186 between client and server may stick out. Note that traffic analysis 187 is discussed in Section 7.1. 189 3.2. Invisibility of the Server 191 To anyone without private keys, the server is indistinguishable from 192 a regular web server. It is impossible to send an unauthenticated 193 probe that the server would reply to differently than if it were a 194 normal web server. 196 3.3. Fallback to HTTP/2 over TLS over TCP 198 When QUIC is blocked, MASQUE can run over TCP and still satisfy 199 previous requirements. Note that in this scenario performance may be 200 negatively impacted. 202 4. Overview of the Mechanism 204 The server runs an HTTPS server on port 443, and has a valid TLS 205 certificate for its domain. The client has a public/private key 206 pair, and the server maintains a list of authorized MASQUE clients, 207 and their public key. (Alternatively, clients can also be 208 authenticated using a shared secret.) The client starts by 209 establishing a regular HTTPS connection to the server (HTTP/3 over 210 QUIC or HTTP/2 over TLS 1.3 [RFC8446] over TCP), and validates the 211 server's TLS certificate as it normally would for HTTPS. If 212 validation fails, the connection is aborted. At this point the 213 client can send regular unauthenticated HTTP requests to the server. 214 When it wishes to start MASQUE, the client uses HTTP Transport 215 Authentication (draft-schinazi-httpbis-transport-auth) to prove its 216 possession of its associated key. The client sends the Transport- 217 Authentication header alongside an HTTP CONNECT request for "/.well- 218 known/masque/initial" with the :protocol pseudo-header field set to 219 "masque". 221 When the server receives this CONNECT request, it authenticates the 222 client and if that fails responds with code "405 Method Not Allowed", 223 making sure its response is the same as what it would return for any 224 unexpected CONNECT request. If authentication succeeds, the server 225 responds with code "101 Switching Protocols", and from then on this 226 HTTP stream is now dedicated to the MASQUE protocol. That protocol 227 provides a reliable bidirectional message exchange mechanism, which 228 is used by the client and server to negotiate what protocol options 229 are supported and enabled by policy, and client VPN configuration 230 such as IP addresses. When using QUIC, this protocol also allows 231 endpoints to negotiate the use of QUIC extensions, such as support 232 for the DATAGRAM extension [I-D.pauly-quic-datagram]. 234 Clients MUST NOT attempt to "resume" MASQUE state similarly to how 235 TLS sessions can be resumed. Every new QUIC or TLS connection 236 requires fully authenticating the client and server. QUIC 0-RTT and 237 TLS early data MUST NOT be used with MASQUE as they are not forward 238 secure. 240 5. Mechanisms the Server Can Advertise to Authenticated Clients 242 Once a server has authenticated the client's MASQUE CONNECT request, 243 it advertises services that the client may use. 245 5.1. HTTP Proxy 247 The client can make proxied HTTP requests through the server to other 248 servers. In practice this will mean using the CONNECT method to 249 establish a stream over which to run TLS to a different remote 250 destination. The proxy applies back-pressure to streams in both 251 directions. 253 5.2. DNS over HTTPS 255 The client can send DNS queries using DNS over HTTPS (DoH) [RFC8484] 256 to the MASQUE server. 258 5.3. UDP Proxying 260 In order to support WebRTC or QUIC to further servers, clients need a 261 way to relay UDP onwards to a remote server. In practice for most 262 widely deployed protocols other than DNS, this involves many 263 datagrams over the same ports. Therefore this mechanism implements 264 that efficiently: clients can use the MASQUE protocol stream to 265 request an UDP association to an IP address and UDP port pair. In 266 QUIC, the server would reply with a DATAGRAM_ID that the client can 267 then use to have UDP datagrams sent to this remote server. Datagrams 268 are then simply transferred between the DATAGRAMs with this ID and 269 the outer server. There will also be a message on the MASQUE 270 protocol stream to request shutdown of a UDP association to save 271 resources when it is no longer needed. When running over TCP, the 272 client opens a new stream with a CONNECT request to the "masque-udp- 273 proxy" protocol and then sends datagrams encapsulated inside the 274 stream with a two-byte length prefix in network byte order. The 275 target IP and port are sent as part of the URL query. Resetting that 276 stream instructs the server to release any associates resources. 278 5.4. QUIC Proxying 280 By leveraging QUIC client connection IDs, a MASQUE server can act as 281 a QUIC proxy while only using one UDP port. The server informs the 282 client of a scheme for client connection IDs (for example, random of 283 a minimum length or vended by the MASQUE server) and then the server 284 can forward those packets to further web servers. 286 This mechanism can elide the connection IDs on the link between the 287 client and MASQUE server by negotiating a mapping between 288 DATAGRAM_IDs and the tuple (client connection ID, server connection 289 ID, server IP address, server port). 291 Compared to UDP proxying, this mode has the advantage of only 292 requiring one UDP port to be open on the MASQUE server, and can lower 293 the overhead on the link between client and MASQUE server by 294 compressing connection IDs. 296 5.5. IP Proxying 298 For the rare cases where the previous mechanisms are not sufficient, 299 proxying can be performed at the IP layer. This would use a 300 different DATAGRAM_ID and IP datagrams would be encoded inside it 301 without framing. Over TCP, a dedicated stream with two byte length 302 prefix would be used. The server can inspect the IP datagram to look 303 for the destination address in the IP header. 305 5.6. Path MTU Discovery 307 In the main deployment of this mechanism, QUIC will be used between 308 client and server, and that will most likely be the smallest MTU link 309 in the path due to QUIC header and authentication tag overhead. The 310 client is responsible for not sending overly large UDP packets and 311 notifying the server of the low MTU. Therefore PMTUD is currently 312 seen as out of scope of this document. 314 5.7. Service Registration 316 MASQUE can be used to make a home server accessible on the wide area. 317 The home server authenticates to the MASQUE server and registers a 318 domain name it wishes to serve. The MASQUE server can then forward 319 any traffic it receives for that domain name (by inspecting the TLS 320 Server Name Indication (SNI) extension) to the home server. This 321 received traffic is not authenticated and it allows non-modified 322 clients to communicate with the home server without knowing it is not 323 colocated with the MASQUE server. 325 To help obfuscate the home server, deployments can use Encrypted 326 Server Name Indication (ESNI) [I-D.ietf-tls-esni]. That will require 327 the MASQUE server sending the cleartext SNI to the home server. 329 6. Operation over HTTP/2 331 MASQUE implementations using HTTP/3 MUST support the fallback to 332 HTTP/2 to avoid incentivizing censors to block HTTP/3 or QUIC. When 333 running over HTTP/2, MASQUE uses the Extended CONNECT method to 334 negotiate the use of datagrams over an HTTP/2 stream 335 [I-D.kinnear-httpbis-http2-transport]. 337 MASQUE implementations SHOULD discover that HTTP/3 is available (as 338 opposed to only HTTP/2) using the same mechanism as regular HTTP 339 traffic. This current standardized mechanism for this is HTTP 340 Alternative Services [RFC7838], but future mechanisms such as 341 [I-D.schwartz-httpbis-dns-alt-svc] can be used if they become 342 widespread. 344 7. Security Considerations 346 Here be dragons. TODO: slay the dragons. 348 7.1. Traffic Analysis 350 While MASQUE ensures that proxied traffic appears similar to regular 351 HTTP traffic, it doesn't inherently defeat traffic analysis. 352 However, the fact that MASQUE leverages QUIC allows it to segment 353 STREAM frames over multiple packets and add PADDING frames to change 354 the observable characteristics of its encrypted traffic. The exact 355 details of how to change traffic patterns to defeat traffic analysis 356 is considered an open research question and is out of scope for this 357 document. 359 When multiple MASQUE servers are available, a client can leverage 360 QUIC connection migration to seamlessly transition its end-to-end 361 QUIC connections by treating separate MASQUE servers as different 362 paths. This could afford an additional level of obfuscation in hopes 363 of rendering traffic analysis less effective. 365 7.2. Untrusted Servers 367 As with any proxy or VPN technology, MASQUE hides some of the 368 client's private information (such as who they are communicating 369 with) from their network provider by transferring that information to 370 the MASQUE server. It is paramount that clients only use MASQUE 371 servers that they trust, as a malicious actor could easily setup a 372 MASQUE server and advertise it as a privacy solution in hopes of 373 attracting users to send it their traffic. 375 8. IANA Considerations 377 We will need to register: 379 o the "/.well-known/masque/" URI (expert review) 380 https://www.iana.org/assignments/well-known-uris/well-known- 381 uris.xhtml [5] 383 o The "masque" and "masque-udp-proxy" extended HTTP CONNECT 384 protocols 386 We will also need to define the MASQUE control protocol and that will 387 be likely to define new registries of its own. 389 9. References 391 9.1. Normative References 393 [I-D.ietf-quic-http] 394 Bishop, M., "Hypertext Transfer Protocol Version 3 395 (HTTP/3)", draft-ietf-quic-http-20 (work in progress), 396 April 2019. 398 [I-D.ietf-quic-transport] 399 Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed 400 and Secure Transport", draft-ietf-quic-transport-20 (work 401 in progress), April 2019. 403 [I-D.kinnear-httpbis-http2-transport] 404 Kinnear, E. and T. Pauly, "Using HTTP/2 as a Transport for 405 Arbitrary Bytestreams", draft-kinnear-httpbis- 406 http2-transport-01 (work in progress), March 2019. 408 [I-D.pauly-quic-datagram] 409 Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable 410 Datagram Extension to QUIC", draft-pauly-quic-datagram-03 411 (work in progress), July 2019. 413 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 414 Requirement Levels", BCP 14, RFC 2119, 415 DOI 10.17487/RFC2119, March 1997, 416 . 418 [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext 419 Transfer Protocol Version 2 (HTTP/2)", RFC 7540, 420 DOI 10.17487/RFC7540, May 2015, 421 . 423 [RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP 424 Alternative Services", RFC 7838, DOI 10.17487/RFC7838, 425 April 2016, . 427 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 428 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 429 May 2017, . 431 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 432 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 433 . 435 [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS 436 (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, 437 . 439 9.2. Informative References 441 [I-D.ietf-httpbis-http2-secondary-certs] 442 Bishop, M., Sullivan, N., and M. Thomson, "Secondary 443 Certificate Authentication in HTTP/2", draft-ietf-httpbis- 444 http2-secondary-certs-04 (work in progress), April 2019. 446 [I-D.ietf-tls-esni] 447 Rescorla, E., Oku, K., Sullivan, N., and C. Wood, 448 "Encrypted Server Name Indication for TLS 1.3", draft- 449 ietf-tls-esni-03 (work in progress), March 2019. 451 [I-D.pardue-httpbis-http-network-tunnelling] 452 Pardue, L., "HTTP-initiated Network Tunnelling (HiNT)", 453 draft-pardue-httpbis-http-network-tunnelling-01 (work in 454 progress), October 2018. 456 [I-D.schwartz-httpbis-dns-alt-svc] 457 Schwartz, B. and M. Bishop, "Finding HTTP Alternative 458 Services via the Domain Name Service", draft-schwartz- 459 httpbis-dns-alt-svc-02 (work in progress), April 2018. 461 [I-D.schwartz-httpbis-helium] 462 Schwartz, B., "Hybrid Encapsulation Layer for IP and UDP 463 Messages (HELIUM)", draft-schwartz-httpbis-helium-00 (work 464 in progress), June 2018. 466 [I-D.sullivan-tls-post-handshake-auth] 467 Sullivan, N., Thomson, M., and M. Bishop, "Post-Handshake 468 Authentication in TLS", draft-sullivan-tls-post-handshake- 469 auth-00 (work in progress), August 2016. 471 [RFC8441] McManus, P., "Bootstrapping WebSockets with HTTP/2", 472 RFC 8441, DOI 10.17487/RFC8441, September 2018, 473 . 475 [RFC8471] Popov, A., Ed., Nystroem, M., Balfanz, D., and J. Hodges, 476 "The Token Binding Protocol Version 1.0", RFC 8471, 477 DOI 10.17487/RFC8471, October 2018, 478 . 480 9.3. URIs 482 [1] mailto:masque@ietf.org 484 [2] https://github.com/DavidSchinazi/masque-drafts 486 [3] mailto:masque@ietf.org 488 [4] https://github.com/DavidSchinazi/masque-drafts 490 [5] https://www.iana.org/assignments/well-known-uris/well-known- 491 uris.xhtml 493 Acknowledgments 495 This proposal was inspired directly or indirectly by prior work from 496 many people. In particular, this work is related to 497 [I-D.schwartz-httpbis-helium] and 498 [I-D.pardue-httpbis-http-network-tunnelling]. The mechanism used to 499 run the MASQUE protocol over HTTP/2 streams was inspired by 500 [RFC8441]. Brendan Moran is to thank for the idea of leveraging 501 connection migration across MASQUE servers. 503 The author would like to thank Christophe A., an inspiration and true 504 leader of VPNs. 506 Design Justifications 508 Using an exported key as a nonce allows us to prevent replay attacks 509 (since it depends on randomness from both endpoints of the TLS 510 connection) without requiring the server to send an explicit nonce 511 before it has authenticated the client. Adding an explicit nonce 512 mechanism would expose the server as it would need to send these 513 nonces to clients that have not been authenticated yet. 515 The rationale for a separate MASQUE protocol stream is to allow 516 server-initiated messages. If we were to use HTTP semantics, we 517 would only be able to support the client-initiated request-response 518 model. We could have used WebSocket for this purpose but that would 519 have added wire overhead and dependencies without providing useful 520 features. 522 There are many other ways to authenticate HTTP, however the 523 authentication used here needs to work in a single client-initiated 524 message to meet the requirement of not exposing the server. 526 The current proposal would also work with TLS 1.2, but in that case 527 TLS false start and renegotiation must be disabled, and the extended 528 master secret and renegotiation indication TLS extensions must be 529 enabled. 531 If the server or client want to hide that HTTP/2 is used, the client 532 can set its ALPN to an older version of HTTP and then use the Upgrade 533 header to upgrade to HTTP/2 inside the TLS encryption. 535 The client authentication used here is similar to how Token Binding 536 [RFC8471] operates, but it has very different goals. MASQUE does not 537 use token binding directly because using token binding requires 538 sending the token_binding TLS extension in the TLS ClientHello, and 539 that would stick out compared to a regular TLS connection. 541 TLS post-handshake authentication 542 [I-D.sullivan-tls-post-handshake-auth] is not used by this proposal 543 because that requires sending the "post_handshake_auth" extension in 544 the TLS ClientHello, and that would stick out from a regular HTTPS 545 connection. 547 Client authentication could have benefited from Secondary Certificate 548 Authentication in HTTP/2 [I-D.ietf-httpbis-http2-secondary-certs], 549 however that has two downsides: it requires the server advertising 550 that it supports it in its SETTINGS, and it cannot be sent unprompted 551 by the client, so the server would have to request authentication. 552 Both of these would make the server stick out from regular HTTP/2 553 servers. 555 MASQUE proposes a new client authentication method (as opposed to 556 reusing something like HTTP basic authentication) because HTTP 557 authentication methods are conceptually per-request (they need to be 558 repeated on each request) whereas the new method is bound to the 559 underlying connection (be it QUIC or TLS). In particular, this 560 allows sending QUIC DATAGRAM frames without authenticating every 561 frame individually. Additionally, HMAC and asymmetric keying are 562 preferred to sending a password for client authentication since they 563 have a tighter security bound. Going into the design rationale, 564 HMACs (and signatures) need some data to sign, and to avoid replay 565 attacks that should be a fresh nonce provided by the remote peer. 566 Having the server provide an explicit nonce would leak the existence 567 of the server so we use TLS keying material exporters as they provide 568 us with a nonce that contains entropy from the server without 569 requiring explicit communication. 571 Author's Address 573 David Schinazi 574 Google LLC 575 1600 Amphitheatre Parkway 576 Mountain View, California 94043 577 United States of America 579 Email: dschinazi.ietf@gmail.com